Astronomy fanatica
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Spiral galaxy
A spiral galaxy is a type of galaxy in the Hubble sequence which is characterized by the following physical properties:
- A considerable total angular momentum
- Composed of a central bulge surrounded by a disk
-- The bulge resembles an elliptical galaxy, containing many old, so-called "Population II" stars, and usually a supermassive black hole at its center.
-- The disk is a flat, rotating assembly consisting of interstellar matter, young "Population I" stars and open star clusters.
Spiral Galaxy M74 presents a face-on view of its spiral arms. It contains about 100 billion stars, 30 million light-years away toward the constellation Pisces.
Spiral galaxies are so named due to the bright arms of star formation within the disk that extend—roughly logarithmically—from the bulge. Though sometimes difficult to discern, such as in flocculent spirals, these arms distinguish spiral galaxies from their lenticular counterparts, which exhibit a disk structure but no evident spiral.
The disks of spiral galaxies tend to be surrounded by large spheroid halos of Population II stars, many of which are concentrated in globular clusters that orbit the galactic center.
Our galaxy, the Milky Way, is a spiral, with a Hubble sequence classification of Sbc (possibly SBb).
Origin of the spiral structure
The early pioneer of studies on the formation of the spiral arms was Bertil Lindblad. He realised that the idea of stars arranged permanently in a spiral shape was untenable due to the "winding dilemma". Since the speed of rotation of the galactic disk varies with distance from the centre of the galaxy, a radial arm (like a spoke) would quickly become curved as the galaxy rotates. The arm would, after a few galactic rotations, become increasingly curved and wind around the galaxy ever tighter. This is not what is observed.
Diagram of orbits that form the appearance of spiral arms
The first acceptable theory was devised by C. C. Lin and Frank Shu in 1964. They suggested that the spiral arms were manifestations of spiral density waves. They assumed that the stars travel in slightly elliptical orbits and that the orientations of their orbits is correlated i.e. the ellipses vary in their orientation (one to another) in a smooth way with increasing distance from the galactic centre. This is illustrated in the diagram. It is clear that the elliptical orbits come close together in certain areas to give the effect of arms. Stars therefore do not remain forever in the position that we now see them in, but pass through the arms as they travel in their orbits.
Issue #10 -
Astronomy Picture of the Day
2005 March 3
Still Life with NGC 2170
Explanation: In this beautiful celestial still life composed with a cosmic brush, dusty nebula NGC 2170 shines at the upper left. Reflecting the light of nearby hot stars, NGC 2170 is joined by other bluish reflection nebulae and a compact red emission region against a backdrop of stars. Like the common household items still life painters often choose for their subjects, the clouds of gas, dust, and hot stars pictured here are also commonly found in this setting - a massive, star-forming molecular cloud in the constellation Monoceros. The giant molecular cloud, Mon R2, is impressively close, estimated to be only 2,400 light-years or so away. At that distance, this canvas would be about 15 light-years across.
Credit & Copyright: Stefan Seip -
Elliptical galaxy
An elliptical galaxy is a type of galaxy in the Hubble sequence characterized by the following physical properties:
- No or at most inconsiderable amount of angular momentum.
- No or at most inconsiderable amount of interstellar matter, no young stars, no open star clusters.
- Consists of old, so-called Population II stars.
- Larger elliptical galaxies typically have a system of globular clusters.
The giant elliptical galaxy NGC 4881 (the spherical glow at upper left) lies at the edge of the Coma Cluster of Galaxies.
This traditional portrait of elliptical galaxies paints them as galaxies where the star formation was over after the initial burst, now shining only with their aging stars. No significant evolution was thought to happen.
Some recent observations have found young, blue star clusters inside a few elliptical galaxies, along with other structures that can be explained by galaxy mergers. In the new vision (still quite experimental), an elliptical galaxy is the result of a long process where one or more smaller galaxies, of any type, collide and fuse in a larger one.
This fusion process may at time carry on until recent or contemporary ages, and is not limited to giant ellipticals. For example, our own Milky Way is known to be "digesting" a couple of small galaxies right now.
Issue #11 -
Astronomy Picture of the Day
2005 March 4
NGC 1427A: Galaxy in Motion
Explanation: In this tantalizing image, young blue star clusters and pink star-forming regions abound in NGC 1427A, a galaxy in motion. The small irregular galaxy's swept back outline points toward the top of this picture from the Hubble Space Telescope - and that is indeed the direction NGC 1427A is moving as it travels toward the center of the Fornax cluster of galaxies, some 62 million light-years away. Over 20,000 light-years long and similar to the nearby Large Magellanic Cloud, NGC 1427A is speeding through the Fornax cluster's intergalactic gas at around 600 kilometers per second. The resulting pressure is giving the galaxy its arrowhead outline, triggering the beautiful but violent episodes of star formation. Still, it is understood that interactions with cluster gas and the other cluster galaxies during its headlong flight will ultimately disrupt galaxy NGC 1427A. Many unrelated background galaxies are visible in the sharp Hubble image, including a striking face-on spiral galaxy at the upper left.
Credit: Hubble Heritage Team (AURA / STScI), ESA, NASA -
Galaxy
A galaxy is a large gravitationally bound system of stars, interstellar gas and dust, unseen dark matter, and possibly dark energy. Typical galaxies contain 10 million to one trillion (10^7 to 10^12) or more stars, all orbiting a common center of gravity. In addition to single stars and a tenuous interstellar medium, most galaxies contain a large number of multiple star systems and star clusters as well as various types of nebulae. Most galaxies are several thousand to several hundred thousand light years in diameter and are usually separated from one another by distances on the order of millions of light years.
Although so called dark matter and dark energy appear to account for well over 90% of the mass of most galaxies, the nature of these unseen components is not well understood. There is some evidence that supermassive black holes may exist at the center of many, if not all, galaxies.
Intergalactic space, the space between galaxies, is a near vacuum with an average density of less than one atom per cubic meter of gas or dust. There are probably more than 10^11 galaxies in the visible universe.
The word galaxy was derived from the name of our own galaxy, the Milky Way, using the Greek word gala (gen. galaktos) meaning milk.
NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 56,000 light years in diameter and approximately 60 million light years distant.
Types of galaxies
Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types is given by the Hubble sequence. Our own galaxy, the Milky Way, sometimes simply called the Galaxy (with uppercase), is a large barred spiral galaxy about 30 kiloparsecs or 100,000 light years in diameter and 3,000 light years in width, contains about 300 billion stars and has a total mass of about a trillion times the mass of the Sun.
In spiral galaxies, the spiral arms have the shape of approximate logarithmic spirals, a pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars.
Like the stars, the spiral arms also rotate around the center, but they do so with constant angular velocity. That means that stars pass in and out of spiral arms. The spiral arms are thought to be areas of high density or density waves. As stars move into an arm, they slow down, thus creating a higher density; this is akin to a "wave" of slowdowns moving along a highway full of moving cars. The arms are visible because the high density facilitates star formation and they therefore harbor many bright and young stars.
A new type of galaxy, Ultra Compact Dwarf Galaxies was discovered in 2003 by Dr Michael Drinkwater of the University of Queensland et al.
Larger scale structures
The space between galaxies is relatively empty, except for intergalactic gas clouds.
Only few galaxies exist by themselves; these are known as field galaxies. Most galaxies are gravitationally bound to a number of other galaxies. Structures containing up to about 50 galaxies are called groups of galaxies, and larger structures containing many thousands of galaxies packed into an area a few megaparsecs across are called clusters. Superclusters are giant collections containing tens of thousands of galaxies, found in clusters, groups and sometimes individually; as far as we can tell the universe is uniform at scales above this.
Our galaxy is a member of the Local Group, and together with the Andromeda Galaxy dominates it; overall the Local Group contains about 30 galaxies in a space about one megaparsec across. The Local Group is part of the Local Supercluster, also known as the Virgo Supercluster.
History
In 1610, Galileo Galilei used a telescope to study the bright band on the night sky known as the Milky Way and discovered that it was composed of a huge number of faint stars. In a treatise in 1755, Immanuel Kant, drawing on earlier work by Thomas Wright, speculated (correctly) that the galaxy might be a rotating body of a huge number of stars, held together by gravitational forces akin to the solar system but on much larger scales. The resulting disk of stars would be seen as a band on the sky from our perspective inside the disk. Kant also conjectured that some of the nebulae visible in the night sky might be separate galaxies.
Towards the end of the 18th century, Charles Messier compiled a catalog containing the 109 brightest nebulae, later followed by a catalog of 5,000 nebulae assembled by William Herschel. In 1845, Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture. However, the nebulae were not universally accepted as distant separate galaxies until the matter was settled by Edwin Hubble in the early 1920s using a new telescope. He was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables, thus allowing to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way. In 1936, Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence.
The first attempt to describe the shape of the Milky Way and the position of the Sun within it was carried out by William Herschel in 1785 by carefully counting the number of stars in different regions of the sky. Using a refined approach, Kapteyn in 1920 arrived at the picture of a small (diameter ~15 kiloparsecs) ellipsoid galaxy with the Sun close to the center. A different method by Harlow Shapley based on the cataloging of globular clusters lead to a radically different picture: a flat disk with diameter ~70 kiloparsecs and the Sun far from the center. Both analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane; once Robert Julius Trumpler had quantified this effect in 1930 by studying open clusters, the present picture of our galaxy as described above emerged.
In 1944, Hendrik van de Hulst predicted microwave radiation at a wave length of 21 centimetres, resulting from interstellar atomic hydrogen gas; this radiation was observed in 1951. This radiation allowed for much improved study of the Galaxy, since it is not affected by dust absorption and its Doppler shift can be used to map the motion of the gas in the Galaxy. These observations led to the postulation of a rotating bar structure in the center of the Galaxy. With improved radio telescopes, hydrogen gas could also be traced in other galaxies. In the 1970s it was realized that the total visible mass of galaxies (from stars and gas) does not properly account for the speed of the rotating gas, thus leading to the postulation of dark matter.
Beginning in the 1990s, the Hubble Space Telescope yielded improved observations. Among other things, it established that the missing dark matter in our galaxy cannot solely consist of inherently faint and small stars. It photographed the Hubble Deep Field, providing evidence for hundreds of billions of galaxies in existence in the visible universe alone.
In 2004, the galaxy Abell 1835 IR1916 became the most distant galaxy ever seen by humans.
Galaxy Abell 1835 IR1916 is seen as a tiny dot in this photo of distant galaxies.
Issue #12 -
Astronomy Picture of the Day
2005 March 5
Tycho and Copernicus: Lunar Ray Craters
Explanation: Dazzling in binoculars or a small telescope, the Moon is pocked with impact craters. During partial lunar phases, the craters along the terminator are cast in dramatic relief by strong shadows. But when the Moon is full some craters seem to sprout systems of bright radial lines or rays. This detailed close-up of the full Moon features two prominent ray craters, Copernicus (upper left) and Tycho (lower right), each with extensive ray systems of light colored debris blasted out by the crater-forming impacts. In general, ray craters are relatively young as their rays overlay the lunar terrain. In fact, at 85 kilometers wide, Tycho, with its far reaching rays, is the youngest large crater on the nearside. Crater Copernicus, surrounded by dark mare which contrast nicely with its bright rays, is 93 kilometers in diameter.
Credit & Copyright: Steve Mandel, Hidden Valley Observatory -
Milky Way
The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Galaxia Kuklos (meaning milky way)) is a hazy band of white light across the celestial sphere, formed by stars within the disc of its namesake galaxy. It is also simply known as the Galaxy, as our Solar System is a part of it. The Milky Way appears brightest in the direction of Sagittarius, where the galactic center lies. Relative to the celestial equator, the Milky Way passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux. This reflects the fact that the Earth's equatorial plane is highly inclined to the galactic plane, as is the Sun's equator and the ecliptic (the plane in which the Earth and the other significant planets orbit the Sun). The galactic pole lies at right ascension 12 h 51,42 m, declination 27° 7,8' (epoch 2000.0; this is a conventional value adopted by the IAU in 1959). The fact that the Milky Way divides our night sky into two roughly equal hemispheres reflects the fact that the solar system lies close to the galactic plane.
Structure
The Milky Way galaxy is a large spiral galaxy of Hubble type SBbc (loosely wound barred spiral) with a total mass of about 10^12 solar masses, comprising 200-400 billion stars. The galactic disk has a diameter of about 100,000 light-years. The distance from the Sun to the galactic center is about 27,700 light-years.
The approximate shape of the Milky Way Galaxy and the position within it of our solar system.
The stars in the Galaxy's disk rotate around the Galaxy's center, which is suspected to harbour a supermassive black hole. Sagittarius A* is thought to be the most plausible candidate for the location of this supermassive black hole. It takes the solar system about 226 million years to complete one orbit ("galactic year"), and so has completed about 25 orbits during its lifetime. The orbital speed is 217 km/s, i.e. 1 light-year in ca. 1400 years, and 1 AU in 8 days. The orbital speed of stars in the Milky Way does not depend much on the distance to the center: it is always between 200 and 250 km/s for the Sun's neighbours. Hence the orbital period is approximately proportional to the distance from the star to the Galaxy's center (without the power 1.5 which applies in the case of a central mass). The disk has a bulge at the center.
There are believed to be four major spiral arms and at least two smaller ones which all start at the Galaxy's center. These are named as follows, counting outwards from the centre along a radius through our solar system:
- Norma Arm, or 3 kpc Arm
- Scutum-Crux or Centaurus Arm
- Sagittarius or Sagittarius-Carina Arm
- Orion or Local Arm - labeled "0"; a minor spiral arm. The Earth's solar system may be found close to the inner rim of the Local Arm, in the Local Fluff, 8.0±0.5 kpc from the galactic center.
- Perseus Arm
- Cygnus or Outer Arm
The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years. Each spiral arm describes a logarithmic spiral (as do the arms of all spiral galaxies) with a pitch of approximately 12 degrees.
The disk is surrounded by a spheroid halo of old stars and globular clusters. While the disk contains gas and dust obscuring the view in some wavelengths, the halo does not. Active star formation takes place in the disk (especially in the spiral arms, which represent areas of high density), but not in the halo. Open clusters also occur primarily in the disk.
X-ray image of Milky Way taken by Chandra X-ray Observatory
Age
In 2004, a team of astronomers estimated the age of the Galaxy. (The team consisted of Luca Pasquini, Piercarlo Bonifacio, Sofia Randich, Daniele Galli, and Raffaele G. Gratton.) The team used the UV-Visual Echelle Spectrograph of the Very Large Telescope to measure, for the first time, the Beryllium content in two stars in Globular Cluster NGC 6397. This allowed them to deduce the time elapsed between the rise of first generation of stars in the entire Galaxy and the first generation of stars in the Globular Cluster, at 200 million to 300 million years. They added that estimate to an estimated age of the stars in the Globular Cluster: 13,400 ± 800 million years. The sum is their estimated age of the Milky Way Galaxy: 13,600 ± 800 million years.
The galactic neighborhood
The Milky Way, the Andromeda Galaxy and the Triangulum Galaxy are the major members of the Local Group, a group of some 35 closely bound galaxies; The Local Group is part of the Virgo Supercluster.
The Milky Way is orbited by a number of dwarf galaxies in the Local Group. The largest of these is the Large Magellanic Cloud with a diameter of 20,000 light years. The smallest, Carina Dwarf, Draco Dwarf, and Leo II are only 500 light years in diameter. The other dwarfs orbiting our galaxy are the Small Magellanic Cloud; Canis Major Dwarf, the closest; Sagittarius Dwarf Elliptical Galaxy, previously thought to be the closest; Ursa Minor Dwarf; Sculptor Dwarf, Sextans Dwarf, Fornax Dwarf, and Leo I.
Mythology
There are numerous legends in many traditions around the world regarding the creation of the Milky Way. In particular, there are two similar ancient Greek stories, that explain the etymology of the name Galaxias and its association with milk. One legend describes the Milky Way as a smear of milk, created when the baby Herakles suckled from the goddess Hera. When Hera realized that the suckling infant was not her own but the illegitimate son of Zeus and another woman, she pushed it away and the spurting milk became the Milky Way.
Another story tells that the milk came from the goddess Rhea, the wife of Cronus, and the suckling infant was Zeus himself. Cronus swallowed his children to ensure his position as head of the Pantheon and sky god, and so Rhea conceived a plan to save her newborn son Zeus: She wrapped a stone in infant's clothes and gave it to Cronus to swallow. Cronus asked her to nurse the child once more before he swallowed it, and the milk that spurted when she pressed her nipple against the rock eventually became the Milky Way.
Older mythology associates the constellation with a herd of dairy cows/cattle, whose milk gives the blue glow, and where each cow is a star. As such, it is intimately associated with legends concerning the constellation of Gemini, which it is in contact with. Firstly, with Gemini, it may form the origin of the myth of Castor and Polydeuces, concerning cattle raiding. Secondly, again with Gemini, but also with other features of the Zodiac sign of Gemini (i.e. Canis Major, Orion, Auriga, and the deserted area now regarded as Camelopardalis), it may form the origin of the myth of the Cattle of Geryon, one of The Twelve Labours of Herakles.
Issue #13 -
Astronomy Picture of the Day
2005 March 6
The View from Everest
Click link above for picture
Explanation: What would it be like to stand atop the tallest mountain on Earth? To see a full panoramic vista from there, scroll right. Visible are snow peaked mountains near and far, tremendous cliffs, distant plateaus, the tops of clouds, and a dark blue sky. Mt. Everest stands 8.85 kilometers above sea level, roughly the maximum height reached by international airplane flights, but much less than the 300 kilometers achieved by a space shuttle. Hundreds of people have tried and failed to climb the behemoth by foot, a feat first accomplished successfully in 1953. About 1000 people have now made it to the summit. Roddy Mackenzie, who climbed the mountain in 1989, captured the above image. Mt. Everest lies in the Himalayan mountains in the country of Nepal. In the native language of Nepal, the mountain's name is "Sagarmatha" which means "forehead of the sky."
Credit & Copyright: Roddy Mackenzie -
Local Group
The Local Group is the group of galaxies that includes our galaxy, the Milky Way. The group comprises over 30 galaxies, with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy. The galaxies of the Local Group cover a 10 million light year diameter. The group belongs to the Virgo Supercluster.
Map of the Local Group
The two most massive members of the group are the Milky Way and the Andromeda Galaxy. These two spirals each have a system of satellite galaxies. The Milky Way's satellite system consists of Sag DEG, Large Magellanic Cloud, Small Magellanic Cloud, Canis Major, Ursa Minor, Draco, Carina, Sextans, Sculptor, Fornax, Leo I, Leo II and Tucana. Andromeda's system comprises M32, M110, NGC 147, NGC 185, And I, And II, And III and And IV. M33, the third largest galaxy in the Local Group may or may not be a companion to the Andromeda galaxy but probably has LGS 3 as a satellite. The other members of the group are gravitationally secluded from these large subgroups.
- Spiral galaxies
-- Milky Way
-- Andromeda Galaxy (M31, NGC 224) - type Sb in Andromeda
-- Triangulum Galaxy (M33, NGC 598) - type Sc in Triangulum
- Elliptical galaxies
-- M110 (NGC 205) - type E6p in Andromeda
-- M32 (NGC 221) - type E2 in Andromeda
- Irregular galaxies
-- Wolf-Lundmark-Melotte (WLM, DDO221) - type Ir+ in Cetus
-- IC 10 - type KBm or Ir+ in Cassiopeia
-- Small Magellanic Cloud (SMC, NGC 292) - SB(s)m pec in Tucana
-- Pisces Dwarf (LGS3) - type Irr in Pisces
-- IC 1613 (DDO 8) - type IAB(s)m V in Cetus
-- Phoenix Dwarf - type Irr in Phoenix
-- Large Magellanic Cloud (LMC) - type Irr/SB(s)m in Dorado
-- Leo A (Leo III) - type IBm V in Leo
-- Sextans B (DDO 70) - type Ir+IV-V in Sextans
-- NGC 3109 - type Ir+IV-V in Hydra
-- Sextans A (DDO 75) - type Ir+V in Sextans
A member of the local group of galaxies which includes the massive spirals Andromeda and our own Milky Way, Sextans A is about 10 million light years distant. The bright Milky Way foreground stars appear yellowish in this view. Beyond them lie the stars of Sextans A with young blue star clusters clearly visible.
- Dwarf elliptical galaxies
-- NGC 147 (DDO 3) - type dE5 pec in Cassiopeia
-- GR 8 (DDO 155) - type Im V
-- SagDIG (Sagittarius Dwarf Irregular Galaxy) - type IB(s)m V in Sagittarius
-- Barnard's Galaxy (NGC 6822) - type IB(s)m IV-V in Sagittarius
-- Aquarius Dwarf (DDO 210) - type Im V in Aquarius
-- IC 5152 - type IAB(s)m IV in Indus
-- Pegasus Dwarf or Pegasus Dwarf Irregular (DDO 216) - type Irr in Pegasus
- Dwarf spheroidal galaxies
-- Cetus Dwarf - type dSph/E4 in Cetus
-- Andromeda III - type dE2 in Andromeda
-- NGC 185 - (type dE3 pec) - Cassiopeia
-- Andromeda I - type dE3 pec? in Andromeda
-- Sculptor Dwarf (E351-G30) - type dE3 in Sculptor
-- Andromeda V - type dSph in Andromeda
-- Andromeda II - type dE0 in Andromeda
-- Fornax Dwarf (E356-G04) - type dSph/E2 in Fornax
-- Carina Dwarf (E206-G220) - type dE3 in Carina
-- Antlia Dwarf - type dE3 in Antlia
-- Leo I (DDO 74) - type dE3 in Leo
-- Sextans Dwarf - type dE3 in Sextans
-- Leo II (Leo B) - type dE0 pec in Leo
-- Ursa Minor Dwarf - type dE4 in Ursa Minor
-- Draco Dwarf (DDO 208) - type dE0 pec in Draco
-- Sag DEG (Sagittarius Dwarf Elliptical Galaxy) - type dSph/E7 in Sagittarius
-- Tucana Dwarf - type dE5 in Tucana
-- Cassiopeia Dwarf (Andromeda VII) - type dSph in Cassiopeia
-- Pegasus Dwarf Spheroidal Galaxy (Andromeda VI) - type dSph in Pegasus
- Indentification Unclear
-- Andromeda IV - Irregular Galaxy or Star Cloud (Irr?) in Andromeda
-- UGC-A 86 (0355+66) - Irregular, Elliptical or Lenticular Galaxy (Irr, dE or S0) in Camelopardalis
-- UGC-A 92 (EGB0427+63) - Irregular or Lenticular Galaxy (Irr or S0) in Camelopardalis
- Probable non-members
-- NGC 404 - Elliptical or Lenticular Galaxy (E0 or SA(s)0-) in Andromeda
-- NGC 1569 - Irregular Galaxy (Irp+ III-IV) in Camelopardalis
-- NGC 1560 (IC 2062) - Spiral Galaxy (Sd) in Camelopardalis
-- Camelopardalis A - Irregular Galaxy (Irr) in Camelopardalis
-- Argo Dwarf - Irregular Galaxy (Irr) in Carina
-- 2318-42 - Irregular Galaxy (Irr) in Grus
-- UKS 2323-326 - Irregular Galaxy (Irr) in Sculptor
-- UGC 9128 (DDO 187) - Irregular Galaxy (Irp+) in Boötes
-- Capricornus Dwarf is a Milky Way Globular Cluster formerly classified as a Dwarf Spheroidal Galaxy
-- Ursa Major Dwarf
-- Sextans C
Issue #14 -
Astronomy Picture of the Day
2005 March 7
Mercury Over Leeds
Explanation: Have you ever seen the planet Mercury? This week might be a good time. Because Mercury orbits so close to the Sun, it never wanders far from the Sun in Earth's sky. If trailing the Sun, Mercury will be visible low on the horizon for only a short while after sunset. If leading the Sun, Mercury will be visible only shortly before sunrise. So at certain times of the year an informed skygazer with a little determination can usually pick Mercury out from a site with an clear horizon. Above, a lot of determination has been combined with a little digital trickery to show Mercury's successive positions during March of 2004. Each picture was taken from the same location in Leeds, England exactly 33 minutes after sunset. Over the next two weeks, Mercury will again be well placed for viewing above the western horizon at sunset, but by the third week in March it will have faded and dropped into the twilight.
Credit & Copyright: Tony Cook -
Galactic and Extragalactic astronomy
Galactic astronomy
Galactic astronomy is the study of galaxies, their formation, structure, components, dynamics, interactions, and the range of forms they take.
Our own Milky Way galaxy, where our solar system belongs, is in some ways the best studied, although important parts of it are obscured from view in visible wavelengths by regions of galactic dust. Modern instruments, and particularly the Hubble Space Telescope, have allowed us to study galaxies other than our own in great detail.
Major topics include:
- Stellar populations
-- Globular clusters
-- Open clusters
- Interstellar medium
-- Interstellar clouds
Extragalactic astronomy
Extragalactic astronomy is the branch of astronomy concerned with objects outside of our own Milky Way Galaxy.
As instrumentation has improved, more distant objects can now be examined in detail. It is therefore often useful to sub-divide this branch into Near-Extragalactic Astronomy and Far-Extragalactic Astronomy. The former deals with objects, such as the galaxies of our Local Group, which are close enough to allow very detailed analyses of their contents (e.g. supernova remnants, stellar associations). The latter describes the study of objects sufficiently far away that only the brightest phenomena are observable.
Some topics include:
- Supernovae
- Quasars
- Radio galaxies
- Groups and clusters of galaxies
Issue #15 -
Astronomy Picture of the Day
2005 March 8
Crater on Mimas
Explanation: Whatever hit Mimas nearly destroyed it. What remains is one of the largest impact craters on one of Saturn's smallest moons. The crater, named Herschel after the 1789 discoverer of Mimas, Sir William Herschel, spans about 130 kilometers and is pictured above in the dramatic light of its terminator. Mimas' low mass produces a surface gravity just strong enough to create a spherical body but weak enough to allow such relatively large surface features. Mimas is made of mostly water ice with a smattering of rock - so it is accurately described as a big dirty snowball. The above image was taken during the 2005 January flyby of the robot spacecraft Cassini now in orbit around Saturn.
Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA -
Globular cluster
A globular cluster is a spherical bundle of stars (star cluster) that orbits a galaxy as a satellite. Globular clusters are very tightly gravitationally bound, which gives them their spherical shape, and extremely dense (in relative terms) towards their core.
The Globular Cluster M80.
Globular clusters are usually composed of hundreds of thousands of old stars, similar to the bulge of a spiral galaxy but confined to a volume of only a few cubic parsecs. Globular clusters are fairly numerous; there are about 150 currently known globulars of the Milky Way (with perhaps 10-20 more undiscovered), and larger galaxies like Andromeda have more (Andromeda may have as many as 500). Some giant elliptical galaxies (e.g., M87) may have as many as 10 thousand globular clusters. These globular clusters orbit the galaxy out to large radii, 100 kiloparsecs or more.
With a few notable exceptions, each globular cluster appears to have a definite age. That is, all the stars in the cluster formed at virtually the same time. It was the recognition of this fact, studying Hertzsprung-Russell diagrams of globulars, that led to the earliest understanding of stellar evolution.
Some globular clusters (like Omega Centauri in our Milky Way, and G1 in M31) are truly massive clusters, with several million times the mass of our Sun. Such globular clusters may be the former nuclei of galaxies that once orbited their host galaxy, but were totally engulfed and tidally stripped of their stars save for the dense nucleus. However, most globular clusters are much smaller, having on the order of a few hundred thousand stars.
In many galaxies (especially massive elliptical galaxies) there appear to be two populations of globular clusers, which appear to be of similar ages (nearly as old as the universe itself) but of different metal abundances. These subpopulations are generally known as "metal-poor" and "metal-rich", although the metallicities of the metal-rich clusters are generally less than that of the Sun. Many scenarios have been suggested to explain these subpopulations, including violent gas-rich galaxy mergers, the accretion of dwarf galaxies, and multiple phases of star formation in a single galaxy. In our Milky Way, the metal-poor clusters are associated with the halo and the metal-rich clusters with the Bulge.
It was through the study of globular clusters that the Sun's position in the Milky Way galaxy became known. Until the 1930s, it was thought that the Sun was near the middle of the galaxy because the distribution of stars in the observable Milky Way appeared uniform. However, the distribution of globular clusters was strongly asymmetric. Assuming a roughly spherical distribution of globular clusters around the galaxy's center, one can estimate the direction from the Sun to the galactic center. By further estimating the distances to the clusters, the distance of the Sun to the galactic center can be estimated as well. It thus became clear that the part of the Milky Way seen from Earth was only a small part of the total galaxy, most of which was obscured by gas and dust.
Although most globular clusters are very old, some young clusters appear to form in a variety of star-forming events at the present day. These include major spiral-spiral galaxy mergers, starbursts in dwarf galaxies, and even in quiescent disk galaxies.
Globular clusters have a very high star density, and therefore close interactions and near-collisions of stars do sometimes occur. Some exotic classes of stars, such as blue stragglers, millisecond pulsars and low-mass X-ray binaries are much more common in globulars.
Issue #16 -
Astronomy Picture of the Day
2005 March 9
A Sun Halo Over Tennessee
Explanation: Sometimes it looks like the Sun is being viewed through a large lens. In the above case, however, there are actually millions of lenses: ice crystals. As water freezes in the upper atmosphere, small, flat, six-sided, ice crystals might be formed. As these crystals flutter to the ground, much time is spent with their faces flat, parallel to the ground. An observer may pass through the same plane as many of the falling ice crystals near sunrise or sunset. During this alignment, each crystal can act like a miniature lens, refracting sunlight into our view. The above image was taken near sunset last month Winter Solstice near Nashville, Tennessee, USA. Dramatically visible behind neighborhood houses and trees and above the cloud deck is the 22 degree halo created by sunlight reflecting off of atmospheric ice crystals.
Credit & Copyright: Vydor -
Open cluster
An open cluster is a group of stars (star cluster) that were born at the same time from a molecular cloud, and are still near to each other. They are also called galactic clusters since they exist within the galaxy's disk.
Open clusters are usually young (in astronomical terms), and so contain many hot and luminous stars. This makes open clusters visible from large distances and one of the preferred objects by amateur astronomers. The "parent" molecular cloud is sometimes still associated with the cluster, which illuminates parts of the cloud that are then visible as one or more nebulae.
The Pleiades is one of the most famous open clusters.
All the stars in an open cluster are more or less the same age and have the same chemical composition, so any difference between them is solely due to their mass. Most open clusters are dominated by their O-type and B-type giant blue stars, which are very luminous but short-lived. An analysis of the light from an open cluster can lead to an estimation of its age by looking at the ratio between blue, yellow and red stars; the more blue stars there are, the younger the cluster is. The uniformity of a cluster's stars makes them a perfect test for stellar evolution models, because when comparing one star to another, most of the variable parameters are now fixed. Testing the model is therefore easier.
The closest open cluster is in Ursa Major, or to be more correct, it is Ursa Major. Most of the stars in this famous constellation are members of an old and mostly dispersed open cluster. Sirius is a former member of this cluster and our sun is in the outskirts of what is called The Ursa Major Stream, a group of stars that are all ex-members of the Ursa Major Cluster spanning over a thousand light years in space. Our Sun is not a member, however; it is just passing through. Considering its location in the galaxy, our Sun has a very strange velocity; it probably had a close encounter with another star a few billion years ago which gravitationally accelerated the Sun and the solar system.
Stars inside an open cluster are at first tighly packed, moving at the same speed around the center of the Galaxy. Every half-billion years or so, a classic open cluster such as the Pleiades or the Hyades (both in Taurus) tends to be disturbed by external factors (such as molecular clouds passing by), setting its stars moving at slightly different speeds and so causing them to drift apart exactly like the ones in Ursa Major have done. When this happens, the cluster becomes a stream of stars, not close enough to be a cluster but all related and moving in very similar directions at similar speeds.
After a billion or so years, the cluster is totally lost. Some stars will be on the far side of the galaxy, some on the near side. The sun's original cluster met the same fate; there is no way to tell which are former members and which just happened to have been created at the same time but in a different location.
The exact timeline of this evolution is believed to vary according to the cluster's initial density: more tighly packed clusters will survive for longer, but no known cluster has lasted more than a few billion years.
Issue #17 -
Astronomy Picture of the Day
2005 March 10
NGC 1499: California Nebula
Explanation: Drifting through the Orion Arm of the spiral Milky Way Galaxy, this cosmic cloud by chance echoes the outline of California on the west coast of the United States. Our own Sun also lies within the Milky Way's Orion Arm, only about 1,500 light-years from the California Nebula. Also known as NGC 1499, the classic emission nebula is around 100 light-years long. It glows with the red light characteristic of hydrogen atoms recombining with long lost electrons, stripped away (ionized) by energetic starlight. In this case, the star most likely providing the energetic starlight is the bright, hot, bluish Xi Persei, just right of the nebula and above picture center. Fittingly, this composite picture was made with images from a telescope in California - the 48-inch (1.2-meter) Samuel Oschin Telescope - taken as a part of the second National Geographic Palomar Observatory Sky Survey.
Credit: Caltech, Palomar Observatory, Digitized Sky Survey
Courtesy Scott Kardel -
Interstellar medium
The interstellar medium (or ISM) is a term used in astronomy to describe the rarefied gas and dust that exists between the stars (or their immediate "circumstellar" environment) within a galaxy. The matter normally consists of about 99% gas particles and usually 1% of dust.
This compound is usually extremely tenuous, with typical densities ranging from a few single to a few hundreds of a particles per cubic centimeter. Generally the gas is roughly 90% hydrogen and 10% helium, with additional elements ("metals" in astronomical parlance) present in trace amounts.
The medium is also responsible for cosmic extinction, namely the decreasing light intensity of a star as the light travels through the medium. This extinction is caused by refraction and absorption of photons in certain wavelengths.
For example, a typical absorption wavelength of atomic hydrogen lies at ca. 121.5 nm, the Lyman-alpha transition. Therefore, it is nearly impossible to see light emitted at that wavelength from a star, because most of it is absorbed during the trip to Earth by Lyman-alpha absorption.
The interstellar medium is usually divided into three phases, depending on the temperature of the gas: hot (millions of kelvins), warm (thousands of kelvins), and cold (tens of kelvins). This "three-phase" model of the ISM was initially developed by McKee and Ostriker in a 1977 paper, which has formed the basis for further study over the past quarter-century. The relative proportions of the phases is still a matter of considerable contention in scientific circles.
Features prominent in the study of the interstellar medium include molecular clouds, interstellar clouds, supernova remnants, planetary nebulae, and similar diffuse structures.
Issue #18 -
Astronomy Picture of the Day
2005 March 11
Infrared Ring Nebula
Explanation: The classic appearance of the popular Ring Nebula (aka M57) is understood to be due to perspective - our view from planet Earth looks down the center of a roughly barrel-shaped cloud of gas. But graceful looping structures are seen to extend even beyond the Ring Nebula's familiar central regions in this false-color infrared image from the Spitzer Space Telescope. Of course in this well-studied example of a planetary nebula, the glowing material does not come from planets. Instead, the gaseous shroud represents outer layers expelled from a dying, sun-like star. By chance, spiral galaxy IC 1296 is also visible in the upper right of this Spitzer view toward the constellation Lyra. The central ring of the Ring Nebula is about one light-year across and 2,000 light-years away. However, galaxy IC 1296 much bigger and hence farther away ... about 200 million light-years distant.
Credit: J. Hora (Harvard-Smithsonian CfA) (SSC/Caltech), JPL-Caltech, NASA -
Interstellar cloud
Interstellar cloud is the generic name given to accumulations of gas and dust in our galaxy. Depending on the density, size and temperature of a given cloud, the hydrogen in it can be neutral (H I regions), ionized, (H II regions), or molecular (molecular clouds). When insufficiently dense to be called a cloud, it may be referred to as interstellar dust or simply dust. Dust in an astronomical context typically has a different composition from the household meaning of dust.
The nebula cataloged as NGC 604 is a giant star forming region, 1,500 light years across, in the nearby spiral galaxy, M33. Over 200 newly formed, hot, massive, stars are scattered within a cavern-like, gaseous, interstellar cloud. The stars irradiate the gas with energetic ultraviolet light stripping electrons from atoms and exciting them - producing a characteristic nebular glow.
Chemical compositions
Analysing the composition of interstellar clouds is achieved by studying electromagnetic radiation that we receive from them. Large radio telescopes scan the intensity in the sky of particular frequencies of electromagnetic radiation which are characteristic of certain molecules' spectra. Some interstellar clouds are cold and tend to give out EM radiation of large wavelengths. We can produce a map of the abundance of these molecules to produce an understanding of the varying composition of the clouds. In hot clouds, there are often ions of many elements, whose spectra can be seen in visible and ultraviolet light.
Radio telescopes can also scan over the frequencies from one point in the map, recording the intensities of each type of molecule. Peaks of frequencies mean that an abundance of that molecule or atom is present in the cloud. The height of the peak is proportional to the relative percentage that it makes up.
Unexpected chemicals detected in interstellar clouds
Until recently the rates of reactions in interstellar clouds were expected to be very slow, with minimal products being produced due to the low temperature and density of the clouds. However, large organic molecules were observed in the spectra that scientists would not have expected to find under these conditions. The reactions needed to create them normally only occur at much higher temperatures and pressures. The fact that they were found indicates that these chemical reactions in interstellar clouds take place faster than suspected. These reactions are studied in the CRESU experiment.
Issue #19 -
Astronomy Picture of the Day
2005 March 12
Accretion Disk Simulation
Explanation: Don't be fooled by the familiar pattern. The graceful spiral structure seen in this computer visualization does not portray winding spiral arms in a distant galaxy of stars. Instead, the graphic shows spiral shock waves in a three dimensional simulation of an accretion disk -- material swirling onto a compact central object that could represent a white dwarf star, neutron star, or black hole. Such accretion disks power bright x-ray sources within our own galaxy. They form in binary star systems which consist of a donor star (not shown above), supplying the accreting material, and a compact object whose strong gravity ultimately draws the material towards its surface. For known x-ray binary systems the size of the accretion disk itself might fall somewhere between the diameter of the Sun (about 1,400,000 kilometers) and the diameter of the Moon's orbit (800,000 kilometers). One interesting result of the virtual reality astrophysics illustrated here is that the simulated disk develops instabilities which tend to smear out the pronounced spiral shocks.
Credit: Michael Owen, John Blondin (North Carolina State Univ.) -
Supernova
A supernova is a type of stellar explosion which appears to result in the creation of a new star upon the celestial sphere. ("Nova" is Latin for "new"). The "super" prefix distinguishes this from a nova, which also involves a star increasing in brightness, though to a lesser extent and through a different mechanism. Supernovae involve the expulsion of a star's outer layers; filling the surrounding space with hydrogen and helium (along with other elements); the debris eventually forms clouds of dust and gas. When the explosion of a supernova compresses nearby clouds, it can induce their gravitational collapse to form new stars, and enrich those new stars in heavy elements.
Remnant of Kepler's Supernova, SN 1604.
Supernovae can release several times 10^44 joules of energy, roughly equivalent to the output of the Sun over its entire lifetime.
Classification
As part of the attempt to understand supernova explosions, astronomers have classified them according to the lines of different chemical elements that appear in their spectra. See "Optical Spectra of Supernovae" by Filippenko (Annual Review of Astronomy and Astrophysics, Volume 35, 1997, pp. 309-355) for a good description of the classes.
The first element for division is the presence or absence of a line from hydrogen. If a supernova's spectrum contains a hydrogen line, it is classified Type II, otherwise it is Type I.
Among those groups, there are subdivisions according to the presence of other lines and the shape of its light curve.
Summary
Type I
-- No Balmer lines
-- Type Ia: Si II line at 615.0 nm
-- Type Ib: He I line at 587.6 nm
-- Type Ic: Weak or no Helium lines
Type II
-- Has Balmer lines
-- Type II-P: Plateau
-- Type II-L: Linear
Type Ia
Type Ia supernovae lack helium and present a silicon absorption line in their spectra near peak light. The most commonly accepted theory of these type of supernovae is that they are the result of a carbon-oxygen white dwarf accreting matter from a nearby companion star, typically a red giant, until it reaches the Chandrasekhar limit. The increase in pressure raises the temperature near the center, and a period of convection, lasting ~100 years begins. At some point in this simmering phase, a fusion flame is born, although the details of the ignition---the location and number of points where the flame begins---is still unknown. This flame accelerates dramatically, through the Rayleigh-Taylor instability and interactions with turbulence. It is still a matter of considerable debate as to whether this flame transitions into a detonation. The energy release from the thermonuclear burning (~10^44 joules) causes the star to explode violently and to release a shock wave in which matter is typically ejected at speeds on the order of 10,000 km/s. The energy released in the explosion also causes an extreme increase in luminosity.
The theory of these type of supernovae is similar to that of novae, in which a white dwarf accretes matter more slowly and does not reach the Chandrasekhar limit. In the case of a nova, the infalling matter causes a fusion reaction of material near its surface but does not cause the star to collapse.
Type Ia supernovae have a characteristic light curve (graph of luminosity as a function of time after the explosion). Near the time of maximum luminosity, the spectrum contains lines of intermediate-mass elements from oxygen to calcium; these are the main constituents of the outer layers of the star. Months after the explosion, when the outer layers have expanded to the point of transparency, the spectrum is dominated by light emitted by material near the core of the star: heavy elements synthesized during the explosion, most prominently iron-group elements. The radioactive decay of Nickel-56 through Cobalt-56 to Iron-56 produces high-energy photons which dominate the energy output of the ejecta at intermediate to late times.
Unlike the other types of supernove, Type Ia supernovae are generally found in all types of galaxies, including ellipticals. They show no preference for regions of current star formation.
The similarity in the shapes of the luminosity profiles of all known Type Ia supernovae has led to their use as a standard candle in extragalactic astronomy. The cause of this similarity in the luminosity curve is still an open question mark. In the late 1990s, observations of type Ia supernovae produced the unexpected result that the universe seems to undergo an accelerating expansion.
The Type Ia supernova releases the highest amounts of energy amongst all known classifications of supernovae. The farthest single object ever detected in the universe (galaxies or globular clusters do not count) was a Type Ia supernova located billions of light-years (tens of yottameters) away.
Issue #20 -
Supernova - Cont'd
Type Ib and Ic
The early spectra of Types Ib and Ic do not show lines of hydrogen, nor the strong silicon absorption feature near 615 nanometers. These events, like supernovae of Type II, are probably massive stars running out of fuel at their centers; however, the progenitors of Types Ib and Ic have lost most of their envelopes due to strong stellar winds or interaction with a companion. Type Ib supernovae are thought to be the result of a Wolf-Rayet star collapsing.
Type II
Stars far more massive than our sun evolve in far more complex fashions. In the core of our sun, 589 teragrams of hydrogen fuse into 584 teragrams of helium every second, the extra 4.3 teragrams of mass converted into pure energy which then radiates outwards. The helium produced in the core accumulates there since temperatures in the core are not yet high enough to cause it to fuse. Eventually, as the hydrogen at the core is exhausted, having been either fused to helium or progressively diluted by the ongoing build-up of helium "ash", fusion begins to slow down and gravity begins to cause the core to contract. This contraction spikes the temperature high enough to initiate a shorter phase of helium burning, this accounts for less than 10% of the star's total lifetime. In stars with less than about 10 solar masses, the carbon produced by helium fusion does not ignite, and the star gradually cools to being a "white dwarf". White dwarf stars can become Type I supernovae as described above.
A much larger star, however, has the kind of gravity needed to create temperatures and pressures sufficient to cause the carbon in the core to begin to fuse once the star contracts. The cores of these massive stars become layered like onions as progressively heavier atomic nuclei build up at the center, with an outermost layer of hydrogen gas, sinking down on a layer of hydrogen fusing into helium, with the helium sinking down into a layer of helium fusing into carbon, with the carbon sinking down to fuse into ever heavier elements. These stars go through progressive stages where the core will shrink, built-up atomic nuclei which were previously unfusable begin to fuse, and the core springs back into equilibrium with gravity. This causes them to be irregular variables—as each new burst of fusion pushes elements out of the fusing core into what is called the "stellar envelope", and dims the star, causing gravity to pull mass back into the fusing core and begin the cycle over again.
The limiting factor in this process is the amount of energy that is released through fusion, which is dependent on the binding energy of these atomic nuclei. Each additional step produces progressively and progressively heavier nuclei, which is also more and more tightly bound by the strong force, this means it releases less energy per fusion reaction than lighter elements fusing.
The most tightly bound of all nuclei is iron, chemical symbol Fe. It represents the "bottom of the hill" for lighter elements to fuse, and for heavier elements to fission. Lighter elements release energy when they fuse and heavier elements release energy when they fission. As iron "ash" begins to accumulate in the core of the star, gravity pulls more and more mass into the area of fusion, which, in turn, goes through all of the steps of fusion: Hydrogen to helium by the Proton chain, helium to carbon by the triple-alpha process, carbon and helium combine into oxygen, oxygen fuses into neon, neon into magnesium, magnesium into silicon and silicon into iron.
The iron (Fe) core is under huge gravitational pressure, and since there is no fusion and cannot be supported by ordinary gas pressure, it is supported by electron degeneracy pressure, the electrons pushing against other electrons. If it builds up to the Chandrasekhar limit at which electron degeneracy pressure cannot sustain it, the iron core begins to collapse. The collapsing core produces high energy gamma rays, which decompose some iron nuclei into 13 He plus 4 neutrons, a process known as photodisassociation. However, no nuclear reaction of an iron nucleus can create energy; it can only absorb it. Thus, where reactions in the core have for millions of years been radiating energy outward, balancing the star against gravity, they suddenly begin sucking energy inwards, joining hands with gravity to cause the core, a massive structure the size of our sun, to collapse within a fraction of a second.
Issue #20 -
Supernova - Cont'd
Type II - Cont'd
As the density in the collapsing core skyrockets, electrons and protons are pushed together until their electrical attraction overcomes their inherent nuclear repulsion from each other. This combination, a process called "electron capture", creates a neutron and releases a neutrino. The neutrinos escape from the core, carrying away energy and further accelerating the collapse, which proceeds in milliseconds as the core detaches from the outer layers of the star and reaches the density of nuclear matter, where the neutrons press against each other and the entire core is the density of an atomic nucleus. This is the core collapse. At this point neutron degeneracy pressure is sufficient to balance gravity; however the core has actually overshot the equilibrium point and undergoes a slight bounce, creating a shock wave which slams into the collapsing outer layers of the star. A "proto-neutron star" begins to form at the core, though if it is massive enough, it will continue collapsing to form a black hole.
Currently it is not understood how this shock wave completes the process of pressuring the outer layer of the star. This is the "supernova problem", and various models suggest different ways that the huge amounts of energy present are formed into a single explosion. One such model is the convective overturnmodel, which suggests that convection, either from neutrinos from below, or infalling matter from above, completes the process of destroying the progenitor star. Heavier elements than iron are formed during this explosion by neutron capture, and from the pressure of the neutrinos pressing into the boundary of the "neutrinosphere", seeding the surrounding space with a cloud of gas and dust which is richer in heavy elements than the one the star originally formed from.
Neutrino physics, which is not well understood, is crucial to the understanding of this process, since half of the energy of the core collapse is radiated out in the form of neutrinos. The other crucial area of investigation is the "hydrology" of the plasma that makes up the dying star, how it behaves during the core collapse determines when and how the "shock wave" forms and when and how it "stalls" and is re-energized.
The remaining core of the star may become a neutron star or a black hole, depending on its mass, although because the processes of supernova collapse are poorly understood, it is unknown what the cutoff mass is.
Type II supernovae can be further classified based on the shape of their light curves into Type II-P and Type II-L. Type II-P reach a "plateau" in their light curve while II-Ls have a "linear" decrease in their light curve ("linear" in magnitude versus time, or exponential in luminosity versus time). This is believed to result from differences in the envelope of the stars. II-Ps have a large hydrogen envelope that traps energy released in the form of gamma rays and releases it slowly, while II-Ls are believed to have much smaller envelopes converting less of the gamma ray energy into visible light.
One can also sub-divide supernovae of Type II based on their spectra. While most Type II supernova show very broad emission lines which indicate expansion velocities of many thousands of km/s, some have relatively narrow features which may be produced by the interaction of the ejecta with circumstellar material; these are called Type IIn, where the "n" stands for "narrow".
A few supernovae, such as SN 1987K and 1993J, appear to change types: they show lines of hydrogen at early times, but, over a period of weeks to months, become dominated by lines of helium. The term "Type IIb" is used to describe the combination of features normally associated with Types II and Ib. These are likely massive stars which have lost most, but not all, of their hydrogen envelopes. As the ejecta expand, the hydrogen layer quickly becomes optically thin and reveals the deeper layers.
Some exceptionally large stars may instead produce a "hypernova" when they die, a theoretical type of explosion. In the hypernova mechanism, the core of a very massive star collapses directly into a black hole and two extremely energetic jets of plasma are emitted from its rotational poles at nearly light speed. These jets emit intense gamma rays, and are one of many candidate explanations for gamma ray bursts.
Naming of Supernovae
Supernova discoveries are reported to the International Astronomical Union's Central Bureau for Astronomical Telegrams, which sends out a circular with the name it assigns to it. The name is formed by the year of discovery, and a one- or two-letter designation. The first 26 supernovae of the year get a letter from A to Z. After Z, they start with aa, ab, and so on.
Notable supernovae
* 1006 – SN 1006 – Extremely bright supernova; accounts found in Egypt, Iraq, Italy, Switzerland, China, Japan, and possibly France and Syria
* 1054 – SN 1054 – the formation of the Crab Nebula, recorded by Chinese astronomers and possibly by Native Americans
* 1181 – SN 1181 – Recorded by Chinese and Japanese astronomers, supernova in Cassiopeia most likely left as its remnant the strange star 3C 58.
* 1572 – SN 1572 – Supernova in Cassiopeia, observed by Tycho Brahe, whose book De Nova Stella on the subject gives us the word "nova"
* 1604 – SN 1604 – Supernova in Ophiuchus, observed by Johannes Kepler; last supernova to be observed in the Milky Way
* 1885 – S Andromedae in the Andromeda Galaxy, discovered by Ernst Hartwig
* 1987 – Supernova 1987A in the Large Magellanic Cloud, observed within hours of its start, it was the first opportunity for modern theories of supernova formation to be tested against observations.
The Crab Nebula is an expanding cloud of gas created by the 1054 supernova.
The 1604 supernova was used by Galileo as evidence against the Aristotelian dogma of his period, that the heavens never changed.
Supernovae often leave behind supernova remnants; the study of these objects has helped to increase our knowledge of supernovae.
Role of supernovae in stellar evolution
Supernovae tend to enrich the surrounding interstellar medium with metals (for astronomers, metals are all the elements after helium). Thus, each stellar generation has a slightly different composition, going from an almost pure mixture of hydrogen and helium to a more metal-rich composition. The different chemical abundances have important influences on the star's life, and may decisively influence the possibility of having planets orbiting it.
Issue #20 -
Astronomy Picture of the Day
2005 March 13
A Message From Earth
Explanation: What are these Earthlings trying to tell us? The above message was broadcast from Earth towards the globular star cluster M13 in 1974. During the dedication of the Arecibo Observatory - still the largest radio telescope in the world - a string of 1's and 0's representing the above diagram was sent. This attempt at extraterrestrial communication was mostly ceremonial - humanity regularly broadcasts radio and television signals out into space accidentally. Even were this message received, M13 is so far away we would have to wait almost 50,000 years to hear an answer. The above message gives a few simple facts about humanity and its knowledge: from left to right are numbers from one to ten, atoms including hydrogen and carbon, some interesting molecules, DNA, a human with description, basics of our Solar System, and basics of the sending telescope. Several searches for extraterrestrial intelligence are currently underway, including one where you can use your own home computer.
Credit: Frank Drake (UCSC) et al., Arecibo Observatory (Cornell, NAIC) -
Quasar
A quasar (from quasi-stellar radio source) is an astronomical object that looks like a star in optical telescopes (i.e. it is a point source), and has a very high redshift. The general consensus is that this high redshift is cosmological, the result of Hubble's law, which implies that quasars must be very distant and must emit more energy than dozens of normal galaxies.
This view, taken with infrared light, is a false-color image of a quasar-starburst tandem with the most luminous starburst ever seen in such a combination. The quasar-starburst was found by a team of researchers from six institutions.
Some quasars display rapid changes in luminosity, which implies that they are small (an object cannot change faster than the time it takes light to travel from one end to the other). The highest redshift currently known for a quasar is 6.4, which is significant because it implies a maximum distance—more distant quasars should be easily observable if they existed. This is taken to mean that the oldest observed quasars correspond to the beginning of galaxy formation. The first quasars marked the end of the dark age, the period after the emission of the cosmic microwave background radiation where there are no observable sources of radiation.
Properties of Quasars
Of the several hundred quasars observed, all spectra have shown considerable redshifts, ranging from 0.06 to the recent maximum of 6.4. Therefore, all known quasars lie at great distances from us, the closest being 240 Mpc away and the farthest being 5500 Mpc away. Most quasars are known to lie above 1000 Mpc in distance; since light takes such a long time to cover these great distances, we are seeing quasars as they existed long ago - the universe as it was in the distant past. Although faint when seen optically, their high redshift and great distance imply that quasars are the brightest objects in the known universe. In a general sense, quasars range in luminosity from 10^38 W (the luminosity of the brightest radio galaxy) to 10^42 W. The average quasar is found to be 10^40 W, which is comparable to 1000 Milky Way galaxies, or 10 trillion Suns.
When compared to active galaxies, quasars exhibit much of the same properties. Radiation is nonthermal and some are shown to have emission jets and lobes. Quasars can be observed in many parts of the electromagnetic spectrum including radio, infrared, optical, ultraviolet, X-ray and even gamma rays while most quasars are found to emit in the infrared.
Quasars are also found to vary in luminosity in differing time periods. Some vary in brightness every few months, weeks, days, or hours. This recent evidence has allowed scientists to theorize that quasars exhibit energy in a very small region, since each part of the quasar would have to be in contact with other parts on such a timescale to coordinate the luminosity variations. As such, a quasar varying on the time scale of a few weeks cannot be larger than a few light weeks across.
Quasar Emission Generation
Since quasars exhibit properties of all active galaxies, many scientists have compared the emissions from quasars to those of small active galaxies due to their likeness. The best explanation for quasars is that they are powered by supermassive black holes. Scientists theorize to create the luminosity of 10^40 W (average brightness of a quasar), a super-massive black hole would have to consume 10 stars per year. The brightest known quasars are thought to devour 1000 solar masses of material every year. Quasars are thought to 'turn on' and off depending on their surroundings. One implication is that a quasar would not, for example, continue to feed at that rate for 10 billion years, which nicely explains why there are no nearby quasars. In this framework, after a quasar finishes eating up gas and dust, it becomes an ordinary, normal galaxy.
Quasars also provide some clues as to the end of the Big Bang's reionization. The oldest quasars (z > 4) display a Gunn-Peterson trough and clearly have absorption regions in front of them indicating that the intergalactic medium at that time was neutral gas. More recent quasars show no absorption region but rather their spectra contain a spiky area known as the Lyman-alpha forest. This indicates that the intergalactic medium has undergone reionization into plasma, and that neutral gas exists only in small clouds.
One other interesting characteristic of quasars is that they show evidence of elements heavier than helium. This is taken to mean that galaxies underwent a massive phase of star formation creating population III stars between the time of the Big Bang and the first observed quasars. However, this prediction has the problem in that, as of 2004, no evidence for such stars have been found, and it may seriously undermine our current views of the universe if no such stars are found in the next few years, and alternate mechanisms for producing heavy elements cannot be found.
Issue #21
