Astronomy fanatica
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Astronomy Picture of the Day
2005 March 26
Composite Crab
Explanation: The Crab Pulsar, a city-sized, magnetized neutron star spinning 30 times a second, lies at the center of this composite image of the inner region of the well-known Crab Nebula. The spectacular picture combines optical data (red) from the Hubble Space Telescope and x-ray images (blue) from the Chandra Observatory, also used in the popular Crab Pulsar movies. Like a cosmic dynamo the pulsar powers the x-ray and optical emission from the nebula, accelerating charged particles and producing the eerie, glowing x-ray jets. Ring-like structures are x-ray emitting regions where the high energy particles slam into the nebular material. The innermost ring is about a light-year across. With more mass than the Sun and the density of an atomic nucleus, the spinning pulsar is the collapsed core of a massive star that exploded, while the nebula is the expanding remnant of the star's outer layers. The supernova explosion was witnessed in the year 1054.
Credit: J. Hester (ASU) et al., CXC, HST, NASA -
Asteroid
An asteroid is a small, solid object in our Solar System, orbiting the Sun. An asteroid is an example of a minor planet (or planetoid), which are much smaller than planets. Most asteroids are believed to be remnants of the protoplanetary disc which were not incorporated into planets during the system's formation. Some asteroids have moons. The vast majority of the asteroids are within the asteroid belt, with elliptical orbits between those of Mars and Jupiter.
This picture of Eros, taken on February 14, 2000, shows the view looking from one end of the asteroid across the gouge on its underside and toward the opposite end. In this mosaic, constructed from two images taken after the NEAR spacecraft was inserted into orbit, features as small as 120 feet (35 meters) across can be seen. House-sized boulders are present in several places; one lies on the edge of the giant crater separating the two ends of the asteroid. A bright patch is visible on the asteroid in the top left-hand part of this image, and shallow troughs can be see just below this patch. The troughs run parallel to the asteroid's long dimension.
Definition
The term "asteroid", meaning star-like (from the Greek asteroeides, aster "star" + -eidos "form, shape"), was coined in 1802 by Sir William Herschel shortly after Olbers discovered the second one, 2 Pallas, in late March of the same year, to describe their star-like appearance; the other then-known planets all show discs, by comparison. He also applied that term to the small moons of the giant planets. The first scientific paper to use the word in its title was published in 1840 by Erman.
The exact definition of an asteroid is unsettled. The term "minor planet" (or "planetoid") carries no strong suggestion about the composition of the object or its general location in the solar system, and some argue that not every minor planet should be called an "asteroid".
One way to classify asteroids is in terms of size. A working definition is that asteroids are larger than 50 m in diameter, distinguishing them from meteoroids, which are typically boulder-sized or smaller. The distinction is made because asteroids are large enough to survive passage through Earth's atmosphere and strike Earth largely intact while the smaller meteoroids generally break up high in Earth's atmosphere.
Thus, it would be safest to use the term "asteroid" for Solar System objects that are bigger than meteoroids, smaller than planets, and made out of rock, not ice.
The term artificial asteroid is sometimes used to designate man-made objects which have ended up in solar orbits, such as the Mariner IV probe.
Asteroids in Earth's Solar System
Hundreds of thousands of asteroids have been discovered within Earth's solar system. As of February 24, 2005, from a total of 277,090 minor planets with calculated orbits 99,906 asteroids had been calculated well enough to be given official numbers and 12,198 of these had been officially named (598 of which have names requiring diacriticals). The first numbered but unnamed minor planet was (3360) 1981 VA; the last numbered and named minor planet was 95959 Covadonga.
Current estimates put the total number of asteroids in the solar system at several million. The largest asteroid in our inner solar system is 1 Ceres, with a diameter of 900-1000 km. Two other large asteroids are 2 Pallas and 4 Vesta; both have diameters of ~500 km. 4 Vesta is the only main belt asteroid that is sometimes visible to the naked eye (in some very rare occasions, a near-Earth asteroid may be visible without technical aid; see 2004 MN4).
4 Vesta, one of the largest asteroids. NASA Hubble Space Telescope image of the asteroid 4 Vesta, taken in May 1996 when it was 177 Gm from Earth.
Issue #34 -
Astronomy Picture of the Day
2005 March 27
The Einstein Cross Gravitational Lens
Explanation: Most galaxies have a single nucleus -- does this galaxy have four? The strange answer leads astronomers to conclude that the nucleus of the surrounding galaxy is not even visible in this image. The central cloverleaf is rather light emitted from a background quasar. The gravitational field of the visible foreground galaxy breaks light from this distant quasar into four distinct images. The quasar must be properly aligned behind the center of a massive galaxy for a mirage like this to be evident. The general effect is known as gravitational lensing, and this specific case is known as the Einstein Cross. Stranger still, the images of the Einstein Cross vary in relative brightness, enhanced occasionally by the additional gravitational microlensing effect of specific stars in the foreground galaxy.
Credit & Copyright: J. Rhoads (STScI) et al., WIYN, AURA, NOAO, NSF -
Asteroid - Cont'd
Asteroid classification
Asteroids are commonly classified into groups based on the characteristics of their orbits and on the details of the spectrum of sunlight they reflect.
Orbit groups and families
Asteroids are divided into groups and families based on their orbital characteristics. It is customary to name a group of asteroids after the first member of that group to be discovered. Groups are relatively loose dynamical associations, whereas families are much "tighter" and result from the catastrophic break-up of a large parent asteroid sometime in the past.
Spectral classification
In 1975, an asteroid taxonomic system based on colour, albedo, and spectral shape was developed by Clark R. Chapman, David Morrison, and Ben Zellner. These properties are thought to correspond to the composition of the asteroid's surface material. Originally, they classified only three types of asteroids:
-- C-type asteroids - carbonaceous, 75% of known asteroids
-- S-type asteroids - silicaceous, 17% of known asteroids
-- M-type asteroids - metallic, most of the remaining asteroids
This list has since been expanded to include a number of other asteroid types. The number of types continues to grow as more asteroids are studied.
253 Mathilde, a C-type asteroid.
Note that the proportion of known asteroids falling into the various spectral types does not necessarily reflect the proportion of all asteroids that are of that type; some types are easier to detect than others, biasing the totals.
Problems with spectral classification
Originally, spectral designations were based on inferences of an asteroid's composition:
-- C - Carbonaceous
-- S - Silicaceous
-- M - Metallic
This has led to great confusion though in that an asteroid's type is not indicative of its composition. While asteroids of different spectral classifications are likely to be composed of different materials, there are no assurances that asteroids within the same taxonomic class are composed of similar materials.
However, scientists have been unable to agree on a new taxonomic system for asteroids and as a result, the spectral classification has stuck.
Issue #35 -
Astronomy Picture of the Day
2005 March 28
A Tether in Space
Explanation: One of the greatest unrequited legends of outer space is the tether. Tethers, long strands of material, hold the promise of stabilizing satellites, generating electricity, and allowing easy transportation. Possibly the most extreme vision of the space tether is the space elevator popularized by Arthur C. Clarke, where a tether is constructed that connects the ground to geosynchronous orbit. One problem is strength - it is difficult to make a long useful tether that does not snap. To help realize more of the tremendous potential of space tethers, NASA and the SpaceWard Foundation have issued the Centennial Tether Challenge, promising a monetary reward for the best space tether that exceeds a minimum standard. Pictured above is the deployment of the Tethered Satellite System 1 (TSS-1) by the space shuttle Altantis in 1992. Like other tested tethers, TSS-1 failed to live up to its promise, although many valuable lessons were learned.
Credit: TSS-1, STS-46 Crew, NASA -
Asteroid - Cont'd
Asteroid discovery
Historical discovery methods
Asteroid discovery methods have drastically improved over the past two centuries.
In the last years of the 18th century, Baron Franz Xaver von Zach organised a group of 24 astronomers to search the sky for the "missing planet" predicted at about 2.8 AU from the Sun by the Titius-Bode law, partly as a consequence of the discovery, by Sir William Herschel in 1781, of the planet Uranus at the distance "predicted" by the law. This task required that hand-drawn sky charts be prepared for all stars in the zodiacal band down to an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again and any moving object would, hopefully, be spotted. The expected motion of the missing planet was about 30 seconds of arc per hour, readily discernable by observers.
Ironically, the first asteroid, 1 Ceres, was not discovered by a member of the group, but rather by accident in 1801 by Giuseppe Piazzi, director, at the time, of the observatory of Palermo, in Sicily. He discovered a new star-like object in Taurus and followed the displacement of this object during several nights. His colleague, Carl Friedrich Gauss, used these observations to determine the exact distance from this unknown object to the Earth. Gauss' calculations placed the object between the planets Mars and Jupiter. Piazzi named it after Ceres, the Greek goddess of agriculture.
Three other asteroids (2 Pallas, 3 Juno, 4 Vesta) were discovered over the next few years, with Vesta found in 1807. After eight more years of fruitless searches, most astronomers assumed that there were no more and abandoned any further searches.
However, Karl Ludwig Hencke persisted, and began searching for more asteroids in 1830. Fifteen years later, he found 5 Astraea, the first new asteroid in 38 years. He also found 6 Hebe less than two years later. After this, other astronomers joined in the search and at least one new asteroid was discovered every year after that (except the wartime year 1945). Notable asteroid hunters of this early era were J. R. Hind, Annibale de Gasparis, Robert Luther, H. M. S. Goldschmidt, Jean Chacornac, James Ferguson, Norman Robert Pogson, E. W. Tempel, J. C. Watson, C. H. F. Peters, A. Borrelly, J. Palisa, Paul Henry and Prosper Henry and Auguste Charlois.
In 1891, however, Max Wolf pioneered the use of astrophotography to detect asteroids, which appeared as short streaks on long-exposure photographic plates. This drastically increased the rate of detection compared with previous visual methods: Wolf alone discovered 248 asteroids, beginning with 323 Brucia, whereas only slightly more than 300 had been discovered up to that point. Still, a century later, only a few thousand asteroids were identified, numbered and named. It was known that there were many more, but most astronomers did not bother with them, calling them "vermin of the skies".
Modern discovery methods
Until 1998, asteroids were discovered by a four-step process. First, a region of the sky was photographed by a wide-field telescope. Pairs of photographs were taken, typically one hour apart. Multiple pairs could be taken over a series of days. Second, the two films of the same region were viewed under a stereoscope. Any body in orbit around the Sun would move slightly between the pair of films. Under the stereoscope, the image of the body would appear to float slightly above the background of stars. Third, once a moving body was identified, its location would be measured precisely using a digitizing microscope. The location would be measured relative to known star locations (link).
These first three steps do not constitute asteroid discovery: the observer has only found an apparition, which gets a provisional designation, made up of the year of discovery, a code of two letters representing the week of discovery, and of a number so more than the one discovered one took place in this week (example: 1998 FJ74).
The final step of discovery is to send the locations and time of observations to Brian Marsden of the Minor Planet Center. Dr. Marsden has computer programs that compute whether an apparition tied together previous apparitions into a single orbit. If so, the object gets a number. The observer of the final apparition is declared a discoverer, and he gets the honour of naming the asteroid (subject to the approval of the International Astronomical Union) once it is numbered.
Latest technology: detecting hazardous asteroids
There is increasing interest in identifying asteroids whose orbits cross Earth's orbit, and that could, given enough time, collide with Earth (see Earth-crosser asteroids). The three most important groups of near-Earth asteroids are the Apollos, Amors, and the Atens. Various asteroid deflection strategies have been proposed.
The near-Earth asteroid 433 Eros had been discovered as long ago as 1898, and the 1930s brought a flurry of similar objects. In order of discovery, these were: 1221 Amor, 1862 Apollo, 2101 Adonis, and finally 69230 Hermes, which approached within 0.005 AU of the Earth in 1937. Astronomers began to realize the possibilities of Earth impact.
Two events in later decades increased the level of alarm: the increasing acceptance of Walter Alvarez' theory of dinosaur extinction being due to an impact event, and the 1994 observation of Comet Shoemaker-Levy 9 crashing into Jupiter. The U.S. military also declassified the information that its military satellites, built to detect nuclear explosions, had detected hundreds of upper-atmosphere impacts by objects ranging from one to 10 metres across.
All of these considerations helped spur the launch of highly efficient automated systems that consist of Charge-Coupled Device (CCD) cameras and computers directly connected to telescopes. Since 1998, a large majority of the asteroids have been discovered by such automated systems. A list of teams using such automated systems include (link):
-- The Lincoln Near-Earth Asteroid Research (LINEAR) team
-- The Near-Earth Asteroid Tracking (NEAT) team
-- Spacewatch
-- The Lowell Observatory Near-Earth-Object Search team
-- The Catalina Sky Survey
-- The Campo Imperatore Near-Earth Objects Survey (CINEOS) team
-- The Japanese Spaceguard Association
-- The Asiago-DLR Asteroid Survey
The LINEAR system alone has discovered over 40,500 asteroids as of May, 2004 (link). Between all of the automated systems, 2701 near-Earth asteroids have been discovered (link) including over 600 more than 1 km in diameter.
Issue #36 -
Astronomy Picture of the Day
2005 March 29
Crescents of Titan and Dione
Explanation: What would it be like to see a sky with many moons? Such is the sky above Saturn. When appearing close to each other, moons will show a similar phase. A view with two of the more famous moons of Saturn in crescent phase was captured last month by the robot spacecraft Cassini now orbiting Saturn. Titan, on the lower left, is among the largest moons in the Solar System and is perpetually shrouded in clouds. Recently, the Huygens probe landed on Titan and gave humanity its first view of its unusual surface. Dione, on the upper right, has less than a quarter of Titan's diameter and has no significant atmosphere. Dione, although appearing smaller, was only half the distance to Titan when the above image was taken.
Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA -
Asteroid - Cont'd
Naming asteroids
The naming format
Newly discovered asteroids are given a provisional designation consisting of the year of discovery and an alphanumeric code, such as 2001 FH. When its orbit is confirmed, it is given a number, and later may also be given a name (e.g. 1 Ceres). The formal naming convention uses parentheses around the number (e.g. (433) Eros).
Unnamed asteroids may be given a number, in which case they keep their provisional designation, e.g. (29075) 1950 DA.
Unnamed asteroids
As modern discovery techniques have discovered vast numbers of new asteroids, they are increasingly being left unnamed. The first asteroid to be left unnamed was (3708) 1974 FV1. On rare occasions, an asteroid's provisional designation may become used as a name in itself: the still unnamed (15760) 1992 QB1 gave its name to a group of asteroids which became known as cubewanos.
Sources for names
The first few asteroids were named after figures from Graeco-Roman mythology, but as such names started to run out, others were used —famous people, literary characters, the names of the discoverer's wives, children, and even television characters.
The first asteroid to be given a non-mythological name was 20 Massalia, named after the city of Marseilles. For some time only female (or feminized) names were used; Alexander von Humboldt was the first man to have an asteroid named after him, but his name was feminized to 54 Alexandra. This unspoken tradition lasted until 334 Chicago was named; even then, oddly feminised names show up in the list for years afterward.
As the number of asteroids began to run into the hundreds, and eventually the thousands, discoverers began to give them increasingly frivolous names. The first hints of this were 482 Petrina and 483 Seppina, named after the discoverer's pet dogs. However, there was little controversy about this until 1971, upon the naming of 2309 Mr. Spock (which was not even named after the Star Trek character, but after the discoverer's cat who supposedly bore a resemblance to him). Although the IAU subsequently banned pet names as sources (even though there had been others before 2309 Mr. Spock, such as 482 Petrina), increasingly eccentric asteroid names have been accepted, including 3494 Purple Mountain, 6042 Cheshirecat, 9007 James Bond and 26858 Misterrogers.
Special naming rules
Asteroid naming is not always a free-for-all: there are some types of asteroid for which rules have developed about the sources of names. For instance Centaurs (asteroids orbiting between Saturn and Neptune) are all named after mythological centaurs, Trojans after heroes from the Trojan War, and trans-Neptunian objects after underworld spirits.
Asteroid exploration
Until the age of space travel, asteroids were merely pinpricks of light in even the largest telescopes and their shapes and terrain remained a mystery.
The first close-up photographs of asteroid-like objects were taken in 1971 when the Mariner 9 probe imaged Phobos and Deimos, the two small moons of Mars, which are probably captured asteroids. These images revealed the irregular, potato-like shapes of most asteroids, as did subsequent images from the Voyager probes of the small moons of the gas giants.
The first true asteroid to be photographed in close-up was 951 Gaspra in 1991, followed in 1993 by 243 Ida and its moon Dactyl, all of which were imaged by the Galileo probe en route to Jupiter.
951 Gaspra, the first asteroid to be imaged in close up.
The first dedicated asteroid probe was NEAR Shoemaker, which photographed 253 Mathilde in 1997, before entering into orbit around 433 Eros, finally landing on its surface in 2001.
Other asteroids briefly visited by spacecraft en route to other destinations include 9969 Braille (by Deep Space 1 in 1999), and 5535 Annefrank (by Stardust in 2002).
In July 2005, the Japanese Hayabusa probe will study in detail 25143 Itokawa in June 2005 and return samples of its surface to earth. Following that, the next asteroid encounters will involve the European Rosetta probe (launched in 2004), which will study 2867 Šteins and 21 Lutetia in 2008 and 2010. NASA is planning to launch the Dawn Mission in 2006, which will orbit both 1 Ceres and 4 Vesta in 2010-2014.
Asteroids in fiction and film
Understandably, most fictional depictions of asteroids focus on their potential risk of striking Earth. Representations of the asteroid belt in film tend to make it unrealistically cluttered with dangerous rocks; in reality asteroids, even in the main belt, are spaced extremely far apart.
-- In The Little Prince, a 1943 novel by Antoine de Saint-Exupéry, the title character lives on an asteroid. The asteroid moon Petit-Prince was named after the character.
-- 'Catch that Rabbit', one of the short stories in Isaac Asimov's collection I, Robot (1950), takes place on an asteroid.
-- In Green Slime (1968), a masterpiece of B-movies, a rogue asteroid hurtles toward Earth. The astronauts leave Space Station Gamma 3 and place bombs on the asteroid, finding it inhabited by strange blobs of glowing slime that are drawn to the equipment. Unfortunately for everyone some of the slime was carried back on a space suit and soon evolves into tentacled creatures! See the review here. The movie inspired the classic board game Awful Green Things from Outer Space.
-- In the classic science-fiction movie 2001: A Space Odyssey (1968), the Discovery has a scientifically accurate "close approach" by an asteroid whilst en route to Jupiter. The scene simply cuts briefly to a lone rock passing by the ship, with tens of thousands of kilometres to spare.
-- The disaster movie Meteor (1979) depicts an asteroid named Orpheus hurtling toward Earth after its orbit is deflected by a comet.
-- In The Empire Strikes Back (1980), Han Solo escapes Empire spacecraft by hiding the Millennium Falcon on an asteroid, but is then attacked by a vast monster that lives (for some unexplained reason) within the asteroid.
-- Arthur C. Clarke's novel 2061: Odyssey Three (1986) depicts a journey through the asteroid belt and its ominous parallels with the journey of the RMS Titanic.
-- Arthur C. Clarke's novel The Hammer of God (1993) depicts mankind's efforts to stop an asteroid named Kali from hitting the Earth. The film Deep Impact (1998) was based on Clarke's novel, although in the movie, the asteroid becomes a comet.
-- In the LucasArts game The Dig (originally released in 1995) and its novelization, the impact-threatening asteroid Attila turns out to be an alien probe.
-- The film Armageddon (1998) is also about efforts to stop an asteroid hitting Earth. Its representation of an asteroid (and of space travel in general) is deeply unrealistic.
-- Ben Bova's novel series The Asteroid Wars (2001-2004) focuses on a war over the mining of the asteroid belt.
-- An episode of the political television drama, The West Wing entitled "Impact Winter" included a subplot in which the White House staff prepared for a possible asteroid strike on the Earth. (First broadcast on December 15, 2004).
-- The Japanese science fiction film The Mysterians reveals the solar system's asteroid belt as the remnants of the Mysterian's home planet after a nuclear war broke out.
Issue #37 -
Astronomy Picture of the Day
2005 March 30
ULXs in M74
Explanation: In visual appearance, M74 is a nearly perfect face-on spiral galaxy, about 30 million light-years away toward the constellation Pisces. The red blotches seen in this composite view are ultraluminous x-ray sources (ULXs) mapped by the Chandra X-ray Observatory. The ULXs are so called because they actually do radiate 10 to 1,000 times more x-ray power than "ordinary" x-ray binary stars, which harbor a neutron star or stellar mass black hole. In fact, watching these ULXs change their x-ray brightness over periods of 2 hours or so, astronomers conclude that ULXs could well be intermediate mass black holes -- black holes with masses 10,000 times or so greater than the Sun, but still much less than the million solar mass black holes which lurk in the centers of large spiral galaxies. How did these intermediate mass black holes get there? One intriguing suggestion is that they are left over from the cores of much smaller galaxies that are merging with spiral galaxy M74.
Credit: X-ray; J. Liu (U.Mich.) et al., CXC, NASA - Optical; T. Boroson (NOAO), AURA, NOAO, NSF -
Brown dwarf
Brown dwarfs are sub-stellar objects (~5 to 90 Jupiter masses) that do not fuse hydrogen into helium in their cores, as do stars on the main sequence, but have fully convective surfaces and interiors, with no chemical differentiation by depth. There is some question as to whether brown dwarfs are required to have experienced fusion at some point in their history; in any event, brown dwarfs heavier than 13 Jupiter masses do fuse deuterium.
History
Brown dwarfs were originally called black dwarfs, a classification for dark substellar objects floating freely in space which were too light to sustain stable hydrogen fusion (black dwarfs currently refer to something slightly different).
Early stellar models suggested that a true star requires a mass at least 80 times that of Jupiter to support such fusion. Dense star-like objects with smaller masses, or "brown dwarfs," were hypothesized by the early 1960s -- formed much the way stars are formed, they would however be hard to find in the sky, as they would emit almost no light. Their strongest emissions would be in the infrared [IR] spectrum, and ground-based IR detectors were too imprecise for a few decades after that to firmly identify any brown dwarfs.
More recently, it has been hypothesized that, depending on the compounds that make up a growing stellar object, the critical mass for star-like hydrogen fusion could be as large as 90 Jupiter masses; and on the other end of the spectrum, that substellar objects formed quickly from a collapsing nebula could produce brown dwarfs smaller than 13 Jupiter masses, which nevertheless experience no fusion at all.
Since 1995, when the first brown dwarf was confirmed, hundreds have been identified. They are now believed to be the most numerous type of body in the Milky Way. Brown dwarfs close to Earth including Epsilon Indi Ba and Bb, a pair of dwarfs around 12 light-years from the Sun.
In theory
Distinguishing heavy brown dwarfs from light stars
Lithium: Lithium is generally present in brown dwarfs and not in low-mass stars. Stars, which achieve the high temperature necessary for fusing hydrogen, rapidly deplete their lithium. This occurs by a collision of Li7 and a proton producing two He4 nuclei. The temperature necessary for this reaction is just below the temperature necessary for hydrogen fusion. Convection in the stars ensures that lithium in the whole volume of the star is depleted, resulting in an observable change in the star spectral lines relative to those of brown dwarfs.
- However, lithium is also seen in very young stars, which have not yet had a chance to burn it off. Heavier stars like our sun can retain lithium in their outer atmospheres, which never get hot enough for lithium depletion, but those are distinguishable from brown dwarfs by their size.
- Contrariwise, brown dwarfs at the high end of their mass range can be hot enough to deplete their lithium when they are young. Dwarfs massing 65-80 MJ can burn off their lithium by the time they are half a billion years old[Kulkarni]. So this test is not perfect.
Methane: Unlike stars, older brown dwarfs are sometimes cool enough that over very long periods of time their atmospheres can gather observable quantities of methane. Dwarfs confirmed in this fashion include Gliese 229B.
Luminosity: Mainline stars cool, but eventually reach a minimum luminosity which they can sustain through steady fusion. This varies from star to star, but is generally at least 0.01% the luminosity of our Sun. Brown dwarfs cool and darken steadily over their lifetimes: sufficiently old dwarfs will be too faint to be a star.
Distinguishing light brown dwarfs from large planets
A remarkable property of brown dwarfs is that they are all roughly the same radius, more or less the radius of Jupiter. At the high end of their mass range [60-90 Jupiter masses], the volume of a brown dwarf is governed primarily by electron degeneracy pressure, as it is in white dwarfs; at the low end of the range [1-10 Jupiter masses], their volume is governed primarily by Coulomb pressure, as it is in planets. The net result is that the radii of brown dwarfs vary by only 10-15% over the range of possible masses. This can make distinguishing them from planets difficult.
In addition, many brown dwarfs undergo no fusion; those at the low end of the mass range [under 13 Jupiter masses] are never hot enough to fuse even deuterium, and even those at the high end of the mass range [over 60 Jupiter masses] cool quickly enough that they no longer undergo fusion after something on the order of 10 million years. However, there are other ways to distinguish dwarfs from planets:
Density is a clear giveaway. Brown dwarfs are all about the same radius and volume; so anything that size with over 10 Jupiter masses is unlikely to be a planet.
X-ray and infrared spectra are telltale signs. Some brown dwarfs emit X-rays; and all "warm" dwarfs continue to glow tellingly in the red and infrared spectra until they cool to planet like temperatures (under 1000K).
In practice
Typical atmospheres of known brown dwarfs range in temperature from 300 to over 3000 K, in comparison with stars, which cool to minimum temperatures of around 4000 K. Compared to stars, which warm themselves with steady internal fusion, brown dwarfs cool quickly over time; more massive dwarfs cool slower than less massive ones.
Observational techniques
Coronographs have recently been used to detect faint objects orbiting bright visible stars, including Gliese 229B.
Sensitive telescopes such as charge-coupled devices [CCDs] have been used to search distant star clusters for faint objects, including Teide 1.
Wide-field searches have identified individual faint objects, such as Kelu-1 (30 ly away).
Milestones
-- 1995: First brown dwarf verified. Tiede 1, an M8 object in the Pleiades cluster, is picked out with a CCD
First methane brown dwarf verified. Gliese 229B is discovered orbiting red dwarf Gliese 229A (20 ly away) using an Adaptive Optics Chronograph to sharpen images from the 60 inch (1.5 m) reflecting telescope at Palomar Observatory on southern California's Mt. Palomar; followup infrared spectroscopy made with their 200 inch (5 m) Hale telescope shows an abundance of methane.
-- 1998: First X-ray-emitting brown dwarf found. Cha Halpha 1, an M8 object in the Chamaeleon I dark cloud, is determined to be an X-ray source, similar to convective late-type stars.
-- December 15, 1999: First X-ray flare detected from a brown dwarf. A team at the University of California monitoring LP 944-20 (60 Jupiter masses, 16 ly away) via the Chandra X-ray observatory, catches a 2-hour flare.
Recent developments
Recent observations of known brown dwarf candidates have revealed a pattern of brightening and dimming of infrared emissions that suggests relatively cool, opaque cloud patterns obscuring a hot interior that is stirred by extreme winds. The weather on such bodies is thought to be extremely violent, comparable to but far exceeding Jupiter's famous storms.
X-ray flares detected from brown dwarfs since late 1999 suggest changing magnetic fields within them, similar to those in very low-mass stars.
Issue #38 -
Astronomy Picture of the Day
2005 March 31
Gamma-Ray Earth
Explanation: The pixelated planet above is actually our own planet Earth seen in gamma rays - the most energetic form of light. In fact, the gamma rays used to construct this view pack over 35 million electron volts (MeV) compared to a mere two electron volts (eV) for a typical visible light photon. The Earth's gamma-ray glow is indeed very faint, and this image was constructed by combining data from seven years of exposure during the life of the Compton Gamma Ray Observatory, operating in Earth orbit from 1991 to 2000. Brightest near the edge and faint near the center, the picture indicates that the gamma rays are coming from high in Earth's atmosphere. The gamma rays are produced as the atmosphere interacts with high energy cosmic rays from space, blocking the harmful radiation from reaching the surface. Astronomers need to understand Earth's gamma-ray glow well as it can interfere with observations of cosmic gamma-ray sources like pulsars, supernova remnants, and distant active galaxies powered by supermassive black holes.
Credit: Dirk Petry (GLAST Science Support Center), EUD, EGRET, NASA -
Comet
A comet is a small astronomical object similar to an asteroid but composed largely of ice. Comets typically move in highly elliptical orbits, the aphelia of which may be many times more distant than Pluto's orbit. Often described as "dirty snowballs", comets are composed largely of frozen carbon dioxide, methane and water with dust and various mineral aggregates mixed in.
Comets are believed to originate in a cloud (the Oort cloud) at large distances from the sun consisting of debris left over from the condensation of the solar nebula; the outer edges of such nebulae are cool enough that water exists in a solid (rather than gaseous) state. Asteroids originate via a different process, but very old comets which have lost all their volatile materials may come to resemble asteroids.
Comet Hale-Bopp, showing a white dust tail and blue gas tail (February 1997)
Physical characteristics
Comets are believed to originate in a distant cloud known as the Oort cloud, after the astronomer Jan Hendrik Oort who hypothesised its existence. They are sometimes perturbed from their distant orbits by gravitational interactions, falling into extremely elliptical orbits that bring them very close to the Sun. When a comet approaches the inner solar system, radiation from the Sun causes its outer layers of ice to evaporate. The streams of dust and gas this releases form a huge but extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the sun's radiation pressure and solar wind cause an enormous tail to form pointing away from the sun. The dust and gas each form their own distinct tail, pointed in slightly different directions — dust being left behind in the comet's orbit (so that it often forms a curved tail) and the ion tail (gas) always pointing directly away from the Sun, since the gas is more strongly affected by the solar wind than dust is, and follows magnetic field lines rather than an orbital trajectory. While the solid body of the comet (called the nucleus) is generally less than 50km across, the coma may be larger than the Sun, and the tails can extend over 150 million km (1 Astronomical unit) or more.
Both coma and tail are illuminated by the Sun, and may become visible from the Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly and the gases glowing due to ionization. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible with the naked eye. Before the invention of the telescope, comets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad omens of deaths of kings or noble men, or coming catastrophes. From ancient sources, such as Chinese oracle bones, it is known that their appearance have been noticed by humans for millennia. One very famous old recording of a comet is the appearance of Halley's Comet on the Bayeux Tapestry, which records the Norman conquest of England in 1066.
Comets have highly elliptical orbits. Note the two distinct tails.
Surprisingly, cometary nuclei are among the blackest objects known to exist in the solar system. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.4% to 3% of the light that falls on it; by comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.
In 1996, comets were found to emit X-rays (see link). These X-rays surprised researchers, because their emission by comets had not previously been predicted. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules. In these collisions, the ions will capture one or more electrons leading to emission of X-rays and far ultraviolet photons (see link).
Issue #39 -
Astronomy Picture of the Day
2005 April 1
Water On Mars
Explanation: Can you help discover water on Mars? Finding water on different regions on Mars has implications for understanding its complex geologic history, the possible existence of past life and the sustenance of potential future astronauts. Many space missions have taken photographs of the surface of the red planet, and some of them might show a subtle clue pointing to water on Mars that has been missed. By close inspection of images, following curiosity, applying scientific principles, applying knowledge about features on the Martian surface, and applying principles of planetary geology, such clues might be brought to light. In the meantime, happy April Fool's Day from the folks at APOD!
Credit & Copyright: Ellen Roper (GCC) -
Comet - Cont'd
Orbital characteristics
Comets are classified according to their orbital periods. Short period comets have orbits of less than 200 years, while Long period comets have longer orbits but remain gravitationally bound to the Sun. Single-apparition comets have parabolic and hyperbolic orbits which will cause them to permanently exit the solar system after one pass by the Sun. On the other extreme, the short period Comet Encke has an orbit which never places it farther from the Sun than Jupiter. Short-period comets are thought to originate in the Kuiper belt, whereas the source of long-period comets is thought to be the Oort cloud. A variety of mechanisms have been proposed to explain why comets get perturbed into highly elliptical orbits, including close approaches to other stars as the Sun follows its orbit through the Milky Way Galaxy; the Sun's hypothetical companion star Nemesis; or an unknown Planet X.
Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun.
Because of their low masses, and their elliptical orbits which frequently take them close to the giant planets, cometary orbits are often perturbed. Short period comets display a strong tendency for their aphelia to coincide with a giant planet's orbital radius, with the Jupiter family of comets being the largest. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined.
Also because of gravitational interactions, a number of periodic comets discovered in earlier decades or previous centuries are now lost, since their orbits were never known well enough to know where to look for their future appearances. However, occasionally a "new" comet will be discovered and upon calculation of its orbit it turns out to be an old "lost" comet. An example is Comet 11P/Tempel-Swift-LINEAR, which was discovered in 1869 but became unobservable after 1908 due to perturbations by Jupiter, and was not found again until accidentally rediscovered by LINEAR in 2001.
Comet names
The names given to comets have followed several different conventions over the past two centuries. Before any systematic naming convention was adopted, comets were named in a variety of ways. Halley's Comet was named after Edmund Halley, who had calclated its orbit. Similarly, the second known periodic comet, Comet Encke, was named after the astronomer who had calculated its orbit rather than the original discoverer of the comet. Most bright comets were referred to as 'The Great Comet Of...' the year in which they appeared.
In the early 20th century, the convention of naming comets after their discoverers became common, and this remains today. A comet is named after up to the first three independent discoverers of it. In recent years, many comets have been discovered by instruments operated by large teams of astronomers, and in this case, comets may be named for the instrument (for example, Comet IRAS-Araki-Alcock was discovered independently by the IRAS satellite and amateur astronomers Genichi Araki and George Alcock).
Until 1994, the systematic naming of comets involved first giving them a provisional designation of the year of their discovery followed by a lower case letter indicating its order of discovery in that year (eg Comet Bennett 1969 i was the 9th comet discovery in 1969). Once an orbit had been established, the comet was given a permanent designation of the year of its perihelion, followed by a Roman numeral, so that Comet Bennett 1969 i became Comet Bennett 1970 II.
Increasing numbers of comet discoveries made this procedure difficult to operate, and in 1994 the International Astronomical Union approved a new naming system. Comets are now designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery, so that the fourth comet discovered in the second half of February 2006 would be designated 2006 D4. Prefixes are also added to indicate the nature of the comet, with P/ indicating a periodic comet, C/ indicating a non-periodic comet, X/ indicating a comet for which no reliable orbit could be calculated, and D/ indicating a comet which has broken up or been lost. Periodic comets also have a number indicating the order of their discovery. So Halley's Comet, the first comet to be identified as periodic, has the systemic name 1P/1682 Q1. Comet Hale-Bopp's systemic name is C/1995 O1.
Issue #40 -
Astronomy Picture of the Day
2005 April 2
Cyg X-1: Can Black Holes Form in the Dark?
Explanation: The formation of a black hole from the collapsing core of a massive star is thought to be heralded by a spectacular supernova explosion. Such an extremely energetic collapse is also a leading explanation for the mysterious cosmic gamma-ray bursts. But researchers now suggest that the Milky Way's most famous black hole, Cygnus X-1, was born when a massive star collapsed -- without any supernova explosion at all. Their dynamical evidence is summarized in this color image of a gorgeous region in Cygnus, showing Cyg X-1 and a cluster of massive stars (yellow circles) known as Cygnus OB3. Arrows compare the measured direction and speed of Cyg X-1 and the average direction and speed of the massive stars of Cyg OB3. The similar motions indicate that Cyg X-1's progenitor star was itself a cluster member and that its path was not altered at all when it became a black hole. In contrast, if Cyg X-1 were born in a violent supernova it would have likely received a fierce kick, changing its course. If not a supernova, could the formation of the Cyg X-1 black hole have produced a dark gamma-ray burst in the Milky Way?
Credit: I. F. Mirabel and I. Rodrigues (IAFE, SAp/CEA) -
Comet - Cont'd
History of comet study
Early observations and thought
Historically, comets were thought to be unlucky, or even interpreted as attacks by heavenly beings against terrestrial inhabitants. Some authorities interpret references to "falling stars" in Gilgamesh, Revelation and the Book of Enoch as references to comets, or possibly bolides.
In the first book of his Meteorology (see link), Aristotle propounded the view of comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that comets were planets, or at least a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of the Zodiac, comets could appear in any part of the sky. Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way.
A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky, humanity's knowledge of celestial things being very limited. However, the Aristotelean viewpoint proved more influential, and it was not until the 16th century that it was demonstrated that comets must exist outside the earth's atmosphere.
In 1577, a bright comet was visible for several months. The Danish astronomer Tycho Brahe used measurements of the comet's position taken by himself and other, geographically separated observers to determine that the comet had no measureable parallax. Within the precision of the measurements, this implied the comet must be at least four times more distant from the earth than the moon.
Orbital studies
Although comets had now been demonstrated to be in the heavens, the question of how they moved through the heavens would be debated for most of the next century. Even after Johannes Kepler had determined in 1609 that the planets moved about the sun in elliptical orbits, he was reluctant to believe that the laws that governed the motions of the planets should also influence the motion of other bodies—he believed that comets travel among the planets along straight lines. Galileo Galilei, although a staunch Copernicanist, rejected Tycho's parallax measurements and held to the Aristotelean notion of comets moving on straight lines through the upper atmosphere.
The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia.
The first suggestion that Kepler's laws of planetary notion should also apply to the comets was made by William Lower in 1610. In the following decades, other astronomers, including Pierre Petit, Giovanni Borelli, Adrien Auzout, Robert Hooke, and Jean-Dominique Cassini, all argued for comets curving about the sun on elliptical or parabolic paths, while others, such as Christian Huygens and Johannes Hevelius, supported comets' linear motion.
The matter was resolved by the bright comet that was discovered by Gottfried Kirch on November 14, 1680. Astronomers throughout Europe tracked its position for several months. In his Principia Mathematica of 1687, Isaac Newton proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.
In 1705, Edmond Halley applied Newton's method to twenty-four cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758-9. (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618, while Jean-Dominique Cassini had suspected the identity of the comets of 1577, 1665, and 1680. Both were incorrect.) Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy. When the comet returned as predicted, it became known as Comet Halley or Halley's Comet (its official designation is 1P/Halley). Its next appearance is due in 2061.
Among the comets with short enough periods to have been observed several times in the historical record, Comet Halley is unique in consistently being bright enough to be visible to the naked eye. Since the confirmation of Comet Halley's periodicity, many other periodic comets have been discovered through the telescope. The second comet to be discovered to have a periodic orbit was Comet Encke (official designation 2P/Encke). Over the period 1819-1821 the German mathematician and physicist Johann Franz Encke computed orbits for a series of cometary apparitions observed in 1786, 1795, 1805, and 1818, concluded they were same comet, and successfully predicted its return in 1822. By 1900, seventeen comets had been observed at more than one perihelion passage and recognized as periodic comets. As of January 2005, 164 comets have achieved this distinction, though several have since been destroyed or lost.
Issue #41 -
Comet - Cont'd
Studies of physical characteristics
Hast thou ne'er seen the Comet's flaming flight?
Isaac Newton described comets as compact, solid, fixed, and durable bodies: in one word, a kind of planets, which move in very oblique orbits, every way, with the greatest freedom, persevering in their motions even against the course and direction of the planets; and their tail as a very thin, slender vapour, emitted by the head, or nucleus of the comet, ignited or heated by the sun. Comets also seemed to Newton absolutely requisite for the conservation of the water and moisture of the planets; from their condensed vapours and exhalations all that moisture which is spent on vegetations and putrefactions, and turned into dry earth, might be resupplied and recruited; for all vegetables were thought to increase wholly from fluids, and turn by putrefaction into earth. Hence the quantity of dry earth must continually increase, and the moisture of the globe decrease, and at last be quite evaporated, if it have not a continual supply. Newton suspected that the spirit which makes the finest, subtilest, and best part of our air, and which is absolutely requisite for the life and being of all things, came principally from the comets.
Another use which he conjectured comets might be designed to serve, is that of recruiting the sun with fresh fuel, and repairing the consumption of his light by the streams continually sent forth in every direction from that luminary-
"From his huge vapouring train perhaps to shake
Reviving moisture on the numerous orbs,
Thro' which his long ellipsis winds; perhaps
To lend new fuel to declining suns,
To light up worlds, and feed th' ethereal fire."
As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion. In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor in the 1835 apparition of Comet Halley, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit and argued that the non-gravitational movements of Comet Encke resulted from this mechanism.
However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 1864-1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852. A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.
By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock. This "dirty snowball" model soon became accepted. It was confirmed when an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) flew through the coma of Halley's comet in 1986 to photograph the nucleus and observed the jets of evaporating material. The American probe Deep Space 1 flew past the nucleus of Comet Borrelly on September 21, 2001 and confirmed that the characteristics of Comet Halley are common on other comets as well.
Forthcoming space missions will add greater detail to our understanding of what comets are made of. The Stardust spacecraft, launched in February 1999, has already collected particles from the coma of Comet Wild 2 in January 2004, and will return the samples to Earth in a capsule in 2006. In 2005, the Deep Impact probe will blast a crater on Comet Tempel 1 to study its interior. And in 2014, the European Rosetta space probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface.
Issue #41 -
Comet - Cont'd
Great comets
While hundreds of tiny comets pass through the inner solar system every year, only a very few comets make any impact on the general public. About every decade or so, a comet will become bright enough to be noticed by a casual observer — such comets are often designated Great Comets. In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, while the appearance of Comet Hale-Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult. To most people, however, a great comet is simply a beautiful spectacle.
Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it will have a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular, but failed to do so. Comet West, which appeared three years later, had much lower expectations (perhaps because scientists were much warier of glowing predictions after the Kohoutek fiasco), but became an extremely impressive comet.
The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession — Comet Hyakutake in 1996, followed by Hale-Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. As yet, the 21st century has not seen the arrival of any great comets.
Peculiar comets
Of the thousands of known comets, some are very unusual. Comet Encke orbits from inside the orbit of Jupiter to inside the orbit of Mercury while Comet 29P/Schwassmann-Wachmann orbits in an unstable orbit entirely between Jupiter and Saturn. 2060 Chiron, whose unstable orbit keeps it between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed. Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL3. Some near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
Some comets have been observed to break up. Comet Biela was one significant example, breaking into two during its 1846 perihelion passage. The two comets were seen separately in 1852, but never again after that. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A lesser meteor shower, the Andromedids, occurs annually in November, and is caused by the Earth crossing Biela's orbit (see link).
Several other comets have been seen to break up during their perihelion passage, including great comets West and Comet Ikeya-Seki. Some comets, such as the Kreutz Sungrazers, orbit in groups and are thought to be pieces of a single object that has previously broken apart.
Another very significant cometary disruption was that of Comet Shoemaker-Levy 9, which was discovered in 1993. At the time of its discovery, the comet was in orbit around Jupiter, having been captured by the planet during a very close approach in 1992. This close approach had already broken the comet into hundreds of pieces, and over a period of 6 days in July 1994, these pieces slammed into Jupiter's atmosphere — the first time astronomers had observed a collision between two objects in the solar system. However, it has been suggested that the object responsible for the Tunguska event in 1908 was a fragment of Comet Encke.
Comets in fiction
Comets are popular subjects for science fiction authors and filmmakers although they are often misrepresented as fiery objects, rather than icy.
-- Jules Verne's Off on a Comet (1877) is a deeply implausible Victorian vision of touring the solar system via a handy comet.
-- H. G. Wells' In the Days of the Comet (1905) is a account of how the vapors of a comet's tail cause an instantaneous worldwide utopian society.
-- Tove Jansson's Comet in Moominland depicts the world of the Moomins threatened by a fiery comet.
-- Arthur C. Clarke's novel 2061: Odyssey Three includes a detailed description of a manned mission to Halley's Comet.
-- In Heart of the Comet, a novel by Gregory Benford and David Brin (1987), a multinational team colonizes Halley's Comet, building a habitat within the ice.
-- Lucifer's Hammer, a novel by Larry Niven, is an apocalyptic survival story featuring a comet impact on Earth.
Issue #42 -
Astronomy Picture of the Day
2005 April 3
The Galactic Center Radio Arc
Explanation: What causes this unusual structure near the center of our Galaxy? The long parallel rays slanting across the top of the above radio image are known collectively as the Galactic Center Radio Arc and jut straight out from the Galactic plane. The Radio Arc is connected to the Galactic center by strange curving filaments known as the Arches. The bright radio structure at the bottom right likely surrounds a black hole at the Galactic center and is known as Sagittarius A*. One origin hypothesis holds that the Radio Arc and the Arches have their geometry because they contain hot plasma flowing along lines of constant magnetic field. Recent images from the Chandra X-ray Observatory appear to show this plasma colliding with a nearby cloud of cold gas.
Credit: Farhad Yusef-Zadeh et al. (Northwestern), VLA, NRAO -
Astronomy Picture of the Day
2005 April 4
NGC 1316: After Galaxies Collide
Explanation: How did this strange-looking galaxy form? Astronomers turn detectives when trying to figure out the cause of unusual jumbles of stars, gas, and dust like NGC 1316. A preliminary inspection indicates that NGC 1316 is an enormous elliptical galaxy that includes dark dust lanes usually found in a spiral. The above image taken by the Hubble Space Telescope shows details, however, that help in reconstructing the history of this gigantic jumble. Close inspection finds fewer low mass globular clusters of stars toward NGC 1316's center. Such an effect is expected in galaxies that have undergone collisions or merging with other galaxies in the past few billion years. After such collisions, many star clusters would be destroyed in the dense galactic center. The dark knots and lanes of dust indicate that one or more of the devoured galaxies were spiral galaxies. NGC 1316 spans about 60,000 light years and lies about 75 million light years away toward the constellation of the Furnace.
Credit: P. Goudfrooij (STScI), Hubble Heritage Team, (STScI/AURA), ESA, NASA -
Equatorial bulge
An equatorial bulge is a planetological term which describes a bulge which a planet may have around its equator, distorting it into an oblate spheroid. The Earth has an equatorial bulge of 42.72 km due to the centrifugal force of its rotation. That is, its diameter measured across the equatorial plane (12,756.28 km) is 42.72 km more than that measured between the poles (12,713.56 km). Earth's atmosphere is distorted as well, causing low air pressure at the South Pole, which has an atmospheric pressure consistent with an altitude around 3,300 to 4,000 meters rather than the actual 2,900 m; this has led to illness among those working at the pole, especially immediately after arrival. Ironically, the pole itself is technically at the center of a high pressure zone due to Hadley circulation
The planet with the largest known equatorial bulge (11,808 km) is Saturn.
Because of a planet's equatorial bulge, its gravitational field is weaker at the equator than its poles. In the 17th century, following the invention of the pendulum clock, French scientists found that clocks sent to French Guiana, on the northern coast of South America, were slower than their exact counterparts in Paris. However, measurements of the acceleration due to gravity at the equator must also take into account the centrifugal force due to the planet's rotation. On Earth, 35% of the difference in acceleration from pole to equator is due to the equatorial bulge, and the other 65% is due to the centrifugal effect.
The differing gravitational field also affects the orbits of satellites and changes their orbits from pure ellipses.
Many rotating astronomical bodies other than planets also exhibit equatorial bulges.
Issue #43 -
Astronomy Picture of the Day
2005 April 5
Light From A Distant Planet
Explanation: Light emitted by a planet far beyond our Solar System has been identified for the first time. The planet, illustrated in the above drawing, had its light detected by comparing the brightness of only the parent star, when the planet was behind the star, to the light emitted when both the planet and its parent star were visible. The Earth-orbiting Spitzer Space Telescope made the observation in infrared light, where the intrinsic glow of the planet outshines the light it reflects from its central star. The direct observation of light allowed a measurement of both the temperature and size of the planet: HD 209458b. Planet HD 209458b was confirmed to be larger than expected for its mass and on an orbit around its parent star that was unexpectedly close to a circle.
Illustration Credit: R. Hurt (SSC-Caltech), JPL-Caltech, NASA -
Extrasolar planet
An extrasolar planet (or exoplanet) is a planet which orbits a star other than the Sun, and therefore belongs to a planetary system other than our solar system.
Infrared image of the star GQ Lupi (A) orbited by a planet (b) at a distance of approximately 20 times the distance between Jupiter and our Sun. GQ Lupi is 400 light years from our Solar System and the star itself has approximately 70% of our Sun's mass. It is unclear what the mass of the planet is, but it is estimated to be between 1 and 42 times the mass of Jupiter.
Although extrasolar planets were long posited, no planets orbiting main sequence stars were discovered until the 1990s. The discovery of extrasolar planets raises the question of whether they support extraterrestrial life.
History of detection
Discoveries regarding extrasolar planets were first published in 1989, when variations in the radial velocities of HD 114762 and Alrai (Gamma Cephei) were explained as being caused by sub-brown dwarf masses, possibly giant planets (11 MJ & 2-3 MJ respectively). Alrai had been the subject of a paper (see link) the year before, but the question of a planetary companion as the cause was left open. Subsequent work in 1992 however concluded that no planet was likely present (see link). The case for HD 114762 has yet to be disproven.
The Polish astronomer Aleksander Wolszczan also claimed to have found the first extrasolar planets in 1993, later confirmed, orbiting the pulsar PSR 1257+12. They are believed to be formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation; or the rocky cores that remain of gas giants that survived the supernova, and spiralled in to their current orbits.
Extrasolar planets around solar-type stars began to be discovered in large numbers during the late 1990s as a result of improved telescope technology, such as CCD and computer-based image processing. Such advances allowed for more accurate measurements of stellar motion, allowing astronomers to detect planets, not visually (the luminosity of a planet being too low for such detection), but by measuring gravitational influences upon stars (see astrometrics and radial velocity method). In addition, extrasolar planets can be detected by measuring the variance in a star's apparent luminosity, as a planet passes in front of it (see eclipse).
The first definitive extrasolar planet around a main sequence star was announced on October 6, 1995 by Michel Mayor and Didier Queloz; the primary star was 51 Pegasi. Since then dozens of planets have been detected, and original claims from the late 1980s confirmed, many by a team led by Geoffrey Marcy at the University of California's Lick and Keck Observatories. The first system to have more than one planet detected was Upsilon Andromedae. The majority of the detected planets have highly elliptical orbits. All of the planets so far discovered are high-mass and most are larger than Jupiter.
As of mid 2004, there were 108 known planetary systems around main sequence stars, containing at least 152 known planets. In July, 2004, it was announced that Hubble had been used to detect an additional 100 planets, but the presence of these planets could not yet be confirmed. Besides this, many observations point to the existence of millions of comets also in extrasolar systems.
Issue #44 -
Astronomy Picture of the Day
2005 April 6
The M7 Open Star Cluster in Scorpius
Explanation: M7 is one of the most prominent open clusters of stars on the sky. The cluster, dominated by bright blue stars, can be seen with the naked eye in a dark sky in the tail of the constellation of Scorpius. M7 contains about 100 stars in total, is about 200 million years old, spans 25 light-years across, and lies about 1000 light-years away. This color picture was taken recently at the Kitt Peak National Observatory in Arizona, USA as part of the Advanced Observers Program. The M7 star cluster has been known since ancient times, being noted by Ptolemy in the year 130 AD. Also visible is a dark dust cloud near the bottom of the frame, and literally millions of unrelated stars towards the Galactic center.
Credit & Copyright: Allan Cook & Adam Block, NOAO, AURA, NSF -
Astronomy Picture of the Day
2005 April 7
Solar Eclipse in View
Explanation: Friday's solar eclipse will be a rare hybrid - briefly appearing as either an annular eclipse or a total eclipse when viewed from along the narrow track of the Moon's shadow. Unfortunately that track, never more than about 30 kilometers wide, lies mostly across the Pacific Ocean, beginning south of New Zealand and just ending in Venezuela. Skywatchers along the beginning and end of the shadow track will see an annular eclipse of the Sun, with the Moon's silhouette briefly surrounded by a bright ring of fire, while observers along the middle of the track will witness a total eclipse phase. But the good news is that over a much broader region of the globe, including New Zealand and much of South and North America, a partial eclipse can be seen as the Moon appears to take a bite out of the Sun. If you want to view the eclipse, take care to do it safely, and check the times for your specific location. So, what location is this solar eclipse view from? The picture above was recorded in November of 2003 from within the track of the Moon's shadow across Antarctica, of course.
Credit & Copyright: Fred Bruenjes (link)
