Astronomy fanatica
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Astronomy Picture of the Day
2005 May 1
Planetary Nebula Mz3: The Ant Nebula
Explanation: Why isn't this ant a big sphere? Planetary nebula Mz3 is being cast off by a star similar to our Sun that is, surely, round. Why then would the gas that is streaming away create an ant-shaped nebula that is distinctly not round? Clues might include the high 1000-kilometer per second speed of the expelled gas, the light-year long length of the structure, and the magnetism of the star visible above at the nebula's center. One possible answer is that Mz3 is hiding a second, dimmer star that orbits close in to the bright star. A competing hypothesis holds that the central star's own spin and magnetic field are channeling the gas. Since the central star appears to be so similar to our own Sun, astronomers hope that increased understanding of the history of this giant space ant can provide useful insight into the likely future of our own Sun and Earth.
Credit: R. Sahai (JPL) et al., Hubble Heritage Team, ESA, NASA -
SETI - Cont'd
Optical SETI experiments
While most SETI sky searches have studied the radio spectrum, some SETI researchers have considered the possibility that alien civilizations might be using powerful lasers for interstellar communications at optical wavelengths. The idea was first suggested in a paper published in the British journal Nature in 1961, and in 1983 Charles Townes, one of the inventors of the laser, published a detailed study of the idea in the US journal Proceedings of the National Academy of Sciences.
Most SETI researchers were cool to the idea. The 1971 Cyclops study discounted the possibility of optical SETI, reasoning that construction of a laser system that could outshine the bright central sun of a remote star system would be too difficult. Now some SETI advocates, such as Frank Drake, have suggested that such a judgement was too conservative.
There are two problems with optical SETI, one of which is easy to deal with, the second of which is troublesome. The first problem is that lasers are highly "monochromatic", that is, they only emit light on one frequency, making it troublesome to figure out what frequency to look for. However, according to Fourier analysis, emitting light in narrow pulses results in a broad spectrum of emission, with the frequencies becoming higher as the pulse width becomes narrower, and an interstellar communications system could use pulsed lasers.
The other problem is that while radio transmissions can be broadcast in all directions, lasers are highly directional. This means that a laser beam could be easily blocked by clouds of interstellar dust, and more to the point, we could only pick it up if we happened to cross its line of fire. As it is unlikely an alien civilization would focus an interstellar laser communications beam on Earth deliberately, we would have to cross such a beam by accident.
However, as discussed earlier, the power requirements for omnidirectional interstellar radio broadcasts are tremendous, and narrow-beam radio communications are technically more plausible. As SETI researchers have adjusted to the idea that interstellar radio communications may be over narrow beams, the idea of hunting for interstellar laser beams has become no more troublesome.
In the 1980s, two Soviet researchers conducted a short optical SETI search, but turned up nothing. During much of the 1990s, the optical SETI cause was kept alive through searches by Stuart Kingsley, a British dedicated amateur living in the US state of Ohio.
Now the SETI old-timers have warmed to the concept of optical SETI. Paul Horowitz of Harvard and researchers with the SETI institute have conducted simple optical SETI searches using a telescope and a photon pulse detection system, and are considering further searches. Horowitz says: "Everyone's been mesmerized by radio, but we've done that experiment a lot and we're a little tired of it."
Optical SETI enthusiasts have conducted paper studies of the effectiveness of using contemporary high-energy lasers and a ten-meter focus mirror as an interstellar beacon. The analysis shows that an infrared pulse from a laser, whose light output is not bound by the inverse-square law of light emitted from a hot body like the Sun, would appear thousands of times brighter than the Sun to a distant civilization in the beam's line of fire. The Cyclops study proved incorrect in suggesting a laser beam would be inherently hard to see.
Such a system could be made to automatically steer itself through a target list, sending a pulse to each target at a rate, say, of once a second. This would allow targeting of all Sun-like stars within a distance of 100 light-years. The studies have also described an automatic laser pulse detector system with a low-cost, two-meter mirror made of carbon composite materials, focusing on an array of light detectors. This automatic detector system could perform sky surveys to detect laser flashes from civilizations attempting to contact us.
Several optical SETI experiments are now in progress. A Harvard-Smithsonian group that includes Paul Horowitz designed a laser detector and mounted it on Harvard's 155 centimeter (61 inch) optical telescope. This telescope is currently being used for a more conventional star survey, and the optical SETI survey is "piggybacking" on that effort.
Between October 1998 and November 1999, the survey inspected about 2,500 stars. Nothing that resembled an intentional laser signal was detected, but efforts continue. The Harvard-Smithsonian group is now working with Princeton to mount a similar detector system on Princeton's 91-centimeter (36-inch) telescope. The Harvard and Princeton telescopes will be "ganged" to track the same targets at the same time, with the intent being to detect the same signal in both locations as a means of reducing errors from detector noise.
The Harvard-Smithsonian group is now building a dedicated all-sky optical survey system along the lines of that described above, featuring a 1.8-meter (72-inch) telescope. The new optical SETI survey telescope is being set up at the Oak Ridge Observatory in Harvard, Massachusetts.
The University of California, Berkeley, home of SERENDIP and SETI@home, is also conducting optical SETI searches. One is being directed by Geoffrey Marcy, the well-known extrasolar planet hunter, and involves examination of records of spectra taken during extrasolar planet hunts for a continuous, rather than pulsed, laser signal.
The other Berkeley optical SETI effort is more like that being pursued by the Harvard-Smithsonian group and is being directed by Dan Wertheimer of Berkeley, who built the laser detector for the Harvard-Smithsonian group. The Berkeley survey uses a 76-centimeter (30-inch) automated telescope and an older laser detector built by Wertheimer.
Issue #69 -
Astronomy Picture of the Day
2005 May 2
Methusaleh Outcrop on Mars
Explanation: What the history of the outcropping of rock called Methuselah? The unusual rock group is visible on the left of the above image taken by the robot Spirit rover current exploring Mars. Methuselah was discovered while maneuvering over hilly terrain and shows unusual multiple layering that caught the attention of the rover science team. Since the above representative color image was taken about three weeks ago, Spirit has moved in to get a closer look. Also visible in the above image are another rock outcrop dubbed Larry's Lookout on the upper right and a larger Clark Hill in the left background. On the far right is a more distant peak of the Columbia Hills.
Credit: Mars Exploration Rover Mission, JPL, NASA -
SETI - Cont'd
Probe SETI and SETA Experiments
The possibility of using interstellar messenger probes in the search for extraterrestrial intelligence was first suggested by Ronald N. Bracewell in 1960, and the technical feasibility of this approach was demonstrated by the British Interplanetary Society's starship study Project Daedalus in 1978. Starting in 1979, Robert Freitas advanced arguments for the proposition that physical space-probes are a superior mode of interstellar communication to radio signals, which, if true, would favor a more solarcentric Search for Extraterrestrial Artifacts (SETA). Much like the "preferred frequency" concept in SETI radio beacon theory, the Earth-Moon or Sun-Earth libration orbits might therefore constitute the most universally convenient parking places for automated extraterrestrial spacecraft exploring arbitrary stellar systems. A viable long-term SETI program may be founded upon a search for these objects.
In 1979, Freitas and Valdes conducted a photographic search of the vicinity of the Earth-Moon triangular libration points L4 and L5, and of the solar-synchronized positions in the associated halo orbits, seeking possible orbiting extraterrestrial interstellar probes, but found nothing to a detection limit of about 14th magnitude. The authors conducted a second more comprehensive photographic search for probes in 1982 that examined the five Earth-Moon Lagrangian positions and included the solar-synchronized positions in the stable L4/L5 libration orbits, the potentially stable nonplanar orbits near LI/L2, Earth-Moon L3, and also L2 in the Sun-Earth system. Again no extraterrestrial probes were found to limiting magnitudes of 17-19th magnitude near L3/L4/L5, 10-18th magnitude for L1/L2, and 14-16th magnitude for Sun-Earth L2.
In June 1983, Valdes and Freitas used the 26-m radiotelescope at Hat Creek Radio Observatory to search for the tritium hyperfine line at 1516 MHz from 108 assorted astronomical objects, with emphasis on 53 nearby stars including all visible stars within a 20 light-year radius. The tritium frequency was deemed highly attractive for SETI work because (1) the isotope is cosmically rare, (2) the tritium hyperfine line is centered in the SETI waterhole region of the terrestrial microwave window, and (3) in addition to beacon signals, tritium hyperfine emission may occur as a byproduct of extensive nuclear fusion energy production by extraterrestrial civilizations. The wideband- and narrowband-channel observations achieved sensitivities of 5-14 x 10^(-21) W/m²/channel and 0.7-2 x 10^(-24) W/m²/channel, respectively, but no detections were made.
Issue #70 -
Astronomy Picture of the Day
2005 May 3
Solar System Rising Over Fire Island
Explanation: If you wait long enough, the entire Solar System will rise before you. To see such a sight, however, you will need to look in the direction of the ecliptic. All of the planets and their moons orbit the Sun in nearly the same plane, the ecliptic plane. From the Earth, this means that each day they will all rise in nearly the same direction - and later set in the opposite direction. Ten years ago, a series of time exposures caught, left to right, the Sun, Venus, the Moon, and Jupiter, all rising in the ecliptic plane behind Fire Island, New York, USA. Exposures were taken every six minutes and digitally superposed on an image taken from the same location at sunrise. Smaller members of our Solar System, including most comets and many asteroids, do not always move along the ecliptic plane. The picturesque Fire Island Lighthouse, visible in the foreground, was built in 1826 and is still in use today.
Credit & Copyright: Larry Landolfi (Landolfi Photography) -
SETI - Cont'd
Where are they? / The interstellar Internet
SETI experiments performed so far have not found anything that resembles an interstellar communications signal. Says Frank Drake of the SETI Institute: "All we know for sure is that the sky is not littered with powerful microwave transmitters."
The great Italian physicist Enrico Fermi suggested in the 1950s that if there was an interstellar civilization, its presence would be obvious once we bothered to look. This is known as the Fermi paradox.
The paradox can be illustrated as follows:
While faster than light, or "superluminal", flight is ruled out by contemporary physics, no law of physics absolutely rules out interstellar flight at "subluminal" speeds, though the physical requirements are formidable. Assuming that stars are on the average about ten light-years apart; that an interstellar mission can be conducted at a speed of 10% of the speed of light; and that it takes four centuries for an interstellar colony to grow to the point where it can launch a pair of interstellar missions, then the "doubling time" of the interstellar colonies created by this advanced civilization would be 500 years. This would allow colonization of the entire galaxy in five million years.
Even limiting an interstellar mission to 1% of the speed of light and assuming it takes a millennium for a society to get to the point where it can mount two interstellar missions, this still means the galaxy would be completely populated in 20 million years. That is a very short interval on a cosmic time scale.
Given the lack of observable signals, as well as the lack of any persuasive evidence that extra-terrestrials have ever visited this planet, Fermi's argument suggests that there is no such interstellar civilization. This argument, depressing for many SETI enthusiasts, is called the "Fermi paradox".
The fact that radio-based SETI searches have not come up with anything very interesting so far is not cause to rule out the existence of contactable alien intelligence. As the previous sections of this document show, trying to find another civilization in space is a difficult proposition, and we have only searched a small fraction of the entire "parameter space" of targets, frequencies, power levels, and so on.
The negative results do place limits on the proximity of certain "classes" of alien civilizations, as specified in a scheme proposed by Soviet SETI researcher Nikolai S. Kardashev in the early 1960s called the Kardashev scale and later extended by Carl Sagan. In this scheme, a "Type I" civilization is one capable of using all the sunlight falling on the surface of an Earthlike planet for an interstellar signal; a "Type II" civilization is capable of harnessing the power of an entire star; and a "Type III" civilization is capable of making use of an entire galaxy. Intermediate civilizations can be numerically defined on a logarithmic scale.
Assuming that an alien civilization is actually transmitting a signal that we could pick up, the searches so far rule out a Type I civilization within a spherical radius of 1,000 light-years, though there may be many civilizations comparable to our own within a few hundred light years that have remained undetected.
A similar analysis using the same assumption shows that there is no detectable Type II civilization in our Galaxy. In the early days of SETI, researchers assumed that such advanced civilizations were very common in our Galaxy. It is discouraging that this does not seem to be so.
However, it is important to emphasize that our SETI hunts have been based on assumptions on communications frequencies and technologies that may be laughable to alien societies, if they have the concept of humor. It is possible that intelligent species abandon radio when new technologies are discovered, making the length of time a world is transmitting on conventional radio extremely short. The lack of results do not say that alien civilizations don't exist. They only say that if they do, our most optimistic assumptions for getting in touch with them have proven unrealistic.
There is another issue that provides another possible explanation as to why we don't see evidence of a large number of alien societies. That issue is time.
Our Sun is not a first-generation star. All first-generation stars are either very small and dim, or have exploded, or have burned out. This first generation synthesized the heavy elements needed to create planets and lifeforms. Later generations of stars, including our Sun, have been born and have died or will die in their turn.
Our galaxy is more than 10 billion years old. Intelligent life and technological societies may have arisen and died out many times during this ten billion years. Assuming that an intelligent species survives for ten million years, that means that only 0.1% of all societies that have arisen during the history of our galaxy are in existence now.
Science writer Timothy Ferris has suggested that since galactic societies could be only transitory, then if there is in fact an interstellar communications network, it consists mostly of automated systems that store the cumulative knowledge of vanished civilizations and communicate that knowledge through the galaxy. Ferris calls this the "Interstellar Internet", with the various automated systems acting as network "servers".
Ferris suspects that if such an Interstellar Internet exists, communications between servers are mostly through narrow-band, highly directional radio or laser links. Intercepting such signals is, as discussed earlier, very difficult. However, the network could maintain some broadcast nodes in hopes of making contact with new civilizations. The Interstellar Internet may be out there, waiting for us to figure out how to link up with it.
Another theory which has been proposed to explain the apparent lack of interstellar communication is the suggestion that the galaxy may contain predatory (or otherwise aggressive) species. Those species smart enough to maintain radio silence are those that survive such predation. Yet another theory proposed by physicist Arnon Dar, and described in the PBS Nova show 'Death Star' (see link), proposes that gamma-ray burst events are sufficiently frequent to sterilize vast swaths of galactic real-estate.
Issue #71 -
Astronomy Picture of the Day
2005 May 4
Cassini Spacecraft Crosses Saturn's Ring Plane
Explanation: If this is Saturn, where are the rings? When Saturn's "appendages" disappeared in 1612, Galileo did not understand why. Later that century, it became understood that Saturn's unusual protrusions were rings and that when the Earth crosses the ring plane, the edge-on rings will appear to disappear. This is because Saturn's rings are confined to a plane many times thinner, in proportion, than a razor blade. In modern times, the robot Cassini spacecraft orbiting Saturn now also crosses Saturn's ring plane. A series of plane crossing images from late February was dug out of the vast online Cassini raw image archive by interested Spanish amateur Fernando Garcia Navarro. Pictured above, digitally cropped and set in representative colors, is the striking result. Saturn's thin ring plane appears in blue, bands and clouds in Saturn's upper atmosphere appear in gold, and dark shadows of the rings curve across the top of the gas giant planet. Moons appear as bumps in the rings.
Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA -
SETI - Cont'd
Criticism of SETI
Not all scientists think that SETI is proper science, labelling it pseudoscience. While many of the arguments against SETI are themselves often viewed as unscientific, there is a line of attack on SETI which has credibility and has yet to be adequately answered in the opinions of many scientists.
The core issue in this line of attack is falsifiability. Karl Popper's criterion for distinguishing science from pseudoscience is short and succinct:
"Scientific methodology exists wherever theories are subjected to rigorous empirical testing, and it is absent wherever the practice is to protect a theory rather than to test it."
It is argued that SETI, by this criterion, is not science. (Extreme versions of this criticism label it a religion, an accusation which is aided somewhat by nigh-spiritual levels of importance attached to the finding of extraterrestrial intelligence by prominent SETI advocates present and past.) The key problem with SETI, according to its detractors, is that it lacks falsifiability. Predictions of positive results of the SETI experiments are explained at great length in a wide variety of ways and places, but a definitive failure condition is not provided.
Ironically, the need for such a failure condition can even be found in a former SETI advocate's own writing. Dr. Carl Sagan, a famous scientist, prominent skeptic and affable science populist, in his book The Demon-Haunted World: Science as a Candle in the Dark, provides his so-called "Baloney Detection Kit". SETI critics have sometimes charged that the following entries of this mental kit are violated by SETI:
- Arguments from authority carry little weight (in science there are no "authorities").
- "Occam's razor" - if there are two hypotheses that explain the data equally well, choose the simpler.
- Ask whether the hypothesis can, at least in principle, be falsified (shown to be false by some unambiguous test). In other words, is it testable? Can others duplicate the experiment and get the same result?
Aside from the falsifiability issue, the first item is also often violated--ironically enough, by invoking Dr. Sagan as an authority whose belief in SETI vindicates the SETI programmes. The second is violated by the very conception of SETI. All available data from astronomy research is adequately explained by an assumption that there is no detectable intelligent life in our (or any) galaxy. SETI is, by this criticism, asking a question which does not arise from available data.
SETI advocates respond to this criticism by pointing out that no mainstream scientist claims proof of intelligent life. The advocates merely claim that it is possible or likely that it exists. By such reasoning, any search for anything not yet discovered can be called unscientific. In terms of Occam's Razor, it is debatable whether it is indeed a simple hypothesis that Earth is the only planet in the entire universe that harbors intelligent life.
Given SETI's popularity in the public arena, it is unlikely that its critics will prevail. Nevertheless, the criticisms associated with SETI have made it difficult for the project to acquire public funding, so it relies primarily on private donors.
Issue #72 -
Astronomy Picture of the Day
2005 May 5
Mira: The Wonderful Star
Explanation: To seventeenth century astronomers, Omicron Ceti or Mira was known as a wonderful star - a star whose brightness could change dramatically in the course of about 11 months. Modern astronomers now recognize an entire class of long period Mira-type variables as cool, pulsating, red giant stars, 700 or so times the diameter of the Sun. Only 420 light-years away, red giant Mira (Mira A, right) itself co-orbits with a companion star, a small white dwarf (Mira B). Mira B is surround by a disk of material drawn from the pulsating giant and in such a double star system, the white dwarf star's hot accretion disk is expected to produce some x-rays. But this sharp, false-color image from the Chandra Observatory also captures the cool giant star strongly flaring at x-ray energies, clearly separated from the x-ray emission of its companion's accretion disk. Clicking the Chandra x-ray image of Mira will reveal an artist's vision of this still wonderful interacting binary star system.
Credit:
X-ray Image: M. Karovska (Harvard-Smithsonian CfA) et al., CXC / NASA
Illustration: M.Weiss(CXC) -
SETI@home
SETI@home ("SETI at home") is a distributed computing project for Internet-connected home computers, hosted by the University of California, Berkeley, in the United States. SETI is an acronym for the Search for Extra-Terrestrial Intelligence.
SETI@home under classic client (version 3.08)
SETI@home version 4.18
The purpose of SETI@home is to analyze data incoming from the Arecibo radio telescope, searching for possible evidence of radio transmissions from extraterrestrial intelligence. With over five million users worldwide, the project is the most successful example of distributed computing to date. Anybody can participate by running a free program that downloads and analyzes radio telescope data.
It performs three main tests:
- searching for Gaussian rises and falls in transmission power, possibly representing the antenna passing over a radio source
- searching for pulses possibly representing a narrowband digital-style transmission
- searching for triplets, three pulses in a row
While the project has not found any conclusive signs of extraterrestrial intelligence, it has identified several candidate spots for further analysis. On September 1, 2004, an interesting signal SHGb02+14a was announced.
Figures
Since its launch on May 17, 1999, the project has logged over two million years of aggregate computing time. On September 26, 2001, SETI@home had performed a total of 10^21 floating point operations. It is acknowledged by the Guinness Book of World Records as the largest computation in history. With about 500,000 in the system, SETI@home has, by conservative estimates, the ability to compute 100 TeraFLOPS.
How does the system work?
Data is recorded on high density tapes at the Arecibo Observatory in Puerto Rico, about one 35 Gbyte tape per day, then mailed to Berkeley. It is then divided into 0.25 Mbyte chunks which get sent from the SETI@home server over the internet to people around the world to analyze. Arecibo does not have a high bandwidth internet connection, so data must go by postal mail to Berkeley at first.
SETI@home searches for strong narrow band signals. The process is somewhat like tuning a radio set to various channels, and looking at the signal strength meter. If the strength meter goes up, that gets the attention. More technically, it involves a lot of digital signal processing, mostly Fourier transforms at various chirp rates and durations.
The project also searches for pulsing and drifting signals, and signals which match the antenna beam pattern as the telescope slews across the sky. The analysis software can search for signals with about one-tenth the strength of those sought in previous surveys, because it makes use of a computationally intensive algorithm called coherent integration that no one else has had the computing power to implement.
Data are merged in a database using SETI@home computers in Berkeley. Interference is rejected, and various pattern-detection algorithms are applied to search for the most interesting signals.
Software
The SETI@home distributed computing software, available for all major operating systems, runs either as a screensaver or continuously while a user works, making use of otherwise wasted processor power for research. SETI@home was the first popular distributed computing application. After six years of fruitless searching, however, some critics now believe that unused computer cycles could be better spent on projects that have more direct benefits to the human race, such as Folding@home.
SETI@home, in addition to its altruistic use to aid SETI, is quite useful as a stress testing tool for computer workstations. Since it uses error-correction algorithms to verify the results of the computations, SETI@home is often used to check on the reliability of a computer configuration when overclocking.
There are future plans to get data from the Parkes Observatory in Australia to analyse the southern hemisphere. SETI@home is in the process of transferring to a new software platform called Berkeley Open Infrastructure for Network Computing (BOINC) that will allow testing for more types of signals as well as let users to contribute to other distributed computing projects running on the BOINC platform.
There are minor technical requirements in order to participate in the project. Systems running Windows OS need at least 32 MB of RAM, the ability to display 8-bit graphics in 800x600 resolution, 10 MB of disk space, and an Internet connection. For Macintosh, a PowerPC processor and Mac OS 7.5.5 or later is required. SETI@home software is also supported by UNIX and Linux.
The results of the data processing can be sent automatically, next time the users are on the internet, or it can be set to ask permission before logging on to the internet.
Competitive aspect and SETI@home farms
SETI@home users quickly started to compete with one another in an effort to process the maximium number of work units. Teams were formed to combine the efforts of individual users. The Ars Technica 'Team Lamb Chop' (see link) in the US led the statistics for many years but has recently been overtaken by both SETI.Germany and OcUK – OverClockers UK.
Some users were able to run the application on PCs they had access to at work (an act known as Borging, after the assimilation-driven Borg of Star Trek: The Next Generation. Others simply collected large quantities of equipment together at home and created "SETI farms" (typically consisting of motherboard, CPU, RAM and PSU only) arranged on shelves as diskless workstations running either LINUX or Windows 98se "headless" (without a video card).
As with any competition, numerous attempts have been made to 'cheat' the system and claim credit for work that has not been performed. To combat cheats, the organisers send out each work unit multiple times and only stop sending out the same work unit when they receive back results from two or more different users that exactly agree.
Issue #73 -
Astronomy Picture of the Day
2005 May 6
Fred Espenak's Solar Eclipse
Stephan Heinsius' Solar Eclipse
Hybrid Solar Eclipse
Explanation: April's spectacular geocentric celestial event was a rare hybrid eclipse of the Sun - a total or an annular eclipse could be seen depending on the observer's location. For Fred Espenak, aboard a gently swaying ship within the middle of the Moon's shadow track about 2,200 kilometers west of the Galapagos, the eclipse was total, the lunar silhouette exactly covering the bright solar disk for a few brief moments. His camera captured a picture of totality revealing the extensive solar corona and prominences rising above the Sun's edge. But for Stephan Heinsius, near the end of the shadow track at Penonome Airfield, Panama, the Moon's apparent size had shrunk enough to create an annular eclipse, showing a complete annulus of the Sun's bright disk as a dramatic ring of fire. Pictures from the two locations are compared above. How rare is such a hybrid eclipse? Calculations show that during the 21st century just 3.1% (7 out of 224) of solar eclipses are hybrid while hybrids comprise about 5% of all solar eclipses over the period 2,000 BC to AD 4,000.
Credit & Copyright: Fred Espenak and Stephan Heinsius -
Wow! signal
The Wow! signal was a strong narrowband radio signal detected by the astrophysicist Jerry R. Ehman on August 15, 1977 by a SETI project at the Big Ear radio telescope of Ohio State University. The signal was apparently of non-terrestrial and non-solar system origin. It lasted for 72 seconds and has not been detected thereafter. All searches so far for the signal were unsuccessful, both by J. R. Ehman and others. The nature of the signal is therefore unknown and only a few possibilities can be ruled out.
The bandwidth of the signal is less than 10 kHz. Two different values for its frequency have been given, 1420.356 MHz (J. D. Kraus) and 1420.456 MHz (J. R. Ehman), but both very close to the frequency of 1420.405 MHz produced by the hyperfine transition neutral hydrogen in the universe. Also two possible equatorial coordinates are given: R.A. = 19h22m22s ± 5s or 19h25m12s ± 5s, and both dec. = -27°03´ ± 20´ (epoch B1950.0).
Amazed by the signal, how the narrowband it was, and the exact match to the intensity variation a localized signal would produce in the antenna used, J. R. Ehman circled on the computer printout the "6EQUJ5" letter code (the intensities detected are coded by space = intensity 0, "1"-"9" = intensities 1-9, "A" = intensity 10, "B" = intensity 11, and so on) of the intensity variation and wrote the comment "Wow!" on its side. This comment became the name of the signal.
It has been speculated whether interstellar scintillation of a weaker continuous signal — similar to the effect of atmospheric twinkling — could be a possible explanation (although this still would not exclude the possibility for the signal being artificial in its nature). However, even by using the significantly more sensitive Very Large Array, such a signal could not be detected, while the likelihood of a signal below the Very Large Array level could be detected by the Big Ear radio telescope due to interstellar scintillation is extremely low, less than 1E(-40).
Issue #74 -
Astronomy Picture of the Day
2005 May 7
NGC 3314: When Galaxies Overlap
Explanation: NGC 3314 consists of two large spiral galaxies which just happen to almost exactly line-up. The foreground spiral is viewed nearly face-on, its pinwheel shape defined by young bright star clusters. But against the glow of the background galaxy, dark swirling lanes of interstellar dust are also seen to echo the face-on spiral's structure. The dust lanes are surprisingly pervasive, and this remarkable pair of overlapping galaxies is one of a small number of systems in which absorption of visible light can be used to directly explore the distribution of dust in distant spirals. NGC 3314 is about 140 million light-years away in the multi-headed constellation Hydra. This color composite was constructed from Hubble Space Telescope images made in 1999 and 2000.
Credit: W. Keel & R. White (Univ. Alabama, Tuscaloosa), Hubble Heritage Team (STScI/ AURA), ESA, NASA -
Archaeoastronomy
Archaeoastronomy (also spelled Archeoastronomy) is, as the name implies, the combination of astronomical and archaeological studies. Specifically, it covers the intersection between the two. One aspect of this is the use of the historical record prior to the genesis of the modern astronomical discipline in order to study past astronomical events. It also may involve the use of astronomical knowledge and extrapolation to further our understanding of the historical record. The two disciplines interact constructively. Cultural anthropologists have contributed to the field of archaeoastronomy by studying the astronomy of more modern, historical cultures; this study is termed, ethnoastronomy.
An example of the first sort of archaeoastronomy is the study of the extensive records of ancient China for references to "guest stars". "Guest stars," or star-like objects which appeared in the night sky, were of great interest to the observers of ancient China and were often dutifully recorded. These events have been associated with many transitory phenomena, such as comets and, particularly, supernovae.
An example of the second sort of archeoastronomy is the study of solar, lunar, and stellar alignments of historical monuments. Many claims have been made that the megalithic monument, Stonehenge, represents an "ancient observatory," although the extent of its use in that regard is in dispute. Certainly Stonehenge - and many other ancient monuments - is aligned with particular significance to the solstice and equinox points. Similar claims have been made that the Great Pyramids of Egypt are aligned with the stars in the belt of Orion, in reflection of the significance invested in that constellation by the ancient Egyptians.
During the 1960s, Alexander Thom did a thorough research on megalithic monuments in Britain, and published the results is Megalithic sites in Britain (Oxford, 1967). Apart from arguing for his theory of the megalithic yard, he also argues with statistical methods that a large part of the monuments in Britain are oriented in a way so they can be used as calendars. His theory proposes that the monuments mark points on the horizon where the sun, moon and principal stars rises at seasonal extremes like midsummer, midwinter and the equinoxes.
Some Old World sites where archaeoastronomy is being explored
- Stonehenge and other structures apparently aligned to astronomical bodies and/or events
- Newgrange
- Ballochroy
- Kintraw
- Minard, Brainport Bay, raising methodological issues
- Carnac, the Grand Menhir Brisé
- Sarmizegetusa Regia, Romania
Stonehenge
Some New World sites where archaeoastronomy is being explored
- Chichen Itza, the caracol
- Monte Alban, zenith tube
- Xochicalco, zenith tube
- Uxmal, Venus allignment of the "Governor's Palace"
- Teotihuacan, the pecked-cross circles as survey-markers
- Cahokia, Mirror of the cosmos
Some artifacts that throw light on archaeoastronomy
- Ancient calendars based on astronomical observations
- Antikythera mechanism
- Nebra skydisk
Issue #75 -
Astronomy Picture of the Day
2005 May 8
CMBR Dipole: Speeding Through the Universe
Explanation: Our Earth is not at rest. The Earth moves around the Sun. The Sun orbits the center of the Milky Way Galaxy. The Milky Way Galaxy orbits in the Local Group of Galaxies. The Local Group falls toward the Virgo Cluster of Galaxies. But these speeds are less than the speed that all of these objects together move relative to the cosmic microwave background radiation (CMBR). In the above all-sky map, radiation in the Earth's direction of motion appears blueshifted and hence hotter, while radiation on the opposite side of the sky is redshifted and colder. The map indicates that the Local Group moves at about 600 kilometers per second relative to this primordial radiation. This high speed was initially unexpected and its magnitude is still unexplained. Why are we moving so fast? What is out there?
Credit: DMR, COBE, NASA, Four-Year Sky Map -
Antikythera mechanism
The Antikythera mechanism is an ancient artifact believed to be an early clockwork mechanism. It was discovered in a shipwreck off the Greek island of Antikythera, between Kythera and Crete, and has been dated to about 87 BC.
The Antikythera mechanism
The wreck was discovered in 1900 at a depth of about 40 m (140 feet), and many statues and other works were retrieved from it by sponge divers. On May 17, 1902 archaeologist Spyridon Stais noticed that one of the pieces of rock had a gear wheel embedded in it.
The mechanism is the oldest known surviving geared mechanism, made from bronze in a wooden frame, and has puzzled and intrigued historians of science and technology since its discovery. The most commonly accepted theory of its function is that it was an analog computer designed to track the movements of heavenly objects. Recent working reconstructions of the device support this analysis. The device is all the more impressive for its use of a differential gear, which was previously believed to have been invented in the 16th century AD.
The late Professor Derek J. de Solla Price, a science historian working at Yale University, published an article on the mechanism in Scientific American in June 1959 while the device was still only partially inspected. In 1973 or 1974 he published an analysis based on gamma ray imaging by Greek archaeologists. He claimed that the device had been built by a Greek astronomer, Geminus of Rhodes. His conclusion was not accepted by experts at the time, who believed that the Ancient Greeks had the theoretical knowledge but not the necessary practical skills.
A partial reconstruction was built by Australian computer scientist Allan George Bromley (1947-2002) of the University of Sydney and Sydney clockmaker Frank Percival. This project led Bromley to review Price's X-ray analysis and to make new, more accurate X-ray images that were studied by Bromley's student, Bernard Gardner, in 1993.
Later, a British orrery maker named John Gleave constructed a working replica of the mechanism. According to his reconstruction, the front dial shows the annual progress of the Sun and Moon through the zodiac against the Egyptian calendar. The upper rear dial displays a four-year period and has associated dials showing the Metonic cycle of 235 synodic months, which equals 19 solar years. A synodic month is the period between two new moons. The lower rear dial plots the cycle of a single synodic month, with a secondary dial showing the lunar year of 12 synodic months.
Another reconstruction was made in 2002 by Michael Wright, mechanical engineering curator for the Science Museum in London, working with Allan Bromley. He analyzed the mechanism using linear tomography, which can create images of a narrow focal plane and thus visualized the gears in great detail.
In Wright's reconstruction, the device not only models the motions of the sun and moon, but those of every celestial body known to the Ancient Greeks: Mercury, Venus, Mars, Jupiter, and Saturn.
This new reconstruction gives credence to ancient mentions of such devices. Cicero, writing in the first century BC, mentions an instrument "recently constructed by our friend Poseidonius, which at each revolution reproduces the same motions of the sun, the moon and the five planets." Such devices are mentioned elsewhere as well.
It also adds support to the idea that there was an ancient Greek tradition of complex mechanical technology which was transmitted via the Arab world, where similar devices are found later, and which formed the foundation of European clockmaking techniques.
Some scientists believe that not only was the device used to track celestial bodies, but to calculate their positions for events or births.
The original mechanism is kept in the National Archaeological Museum in Athens. There is a replica on display at the American Computer Museum in Bozeman, MT.
The Antikythera mechanism, not described in any surviving source, shows that our knowledge of ancient technology is very incomplete. In 1996 the Italian physicist Lucio Russo (professor at Università di Roma "Tor Vergata") published an essay putting new light on the issue. The book has been translated and published in English in 2004 under the title "The Forgotten Revolution : How Science Was Born in 300 BC and Why it Had to Be Reborn".
Issue #76 -
Astronomy Picture of the Day
2005 May 9
Stars, Dust and Nebula in NGC 6559
Explanation: When stars form, pandemonium reigns. A textbook case is the star forming region NGC 6559. Visible above are red glowing emission nebulas of hydrogen, blue reflection nebulas of dust, dark absorption nebulas of dust, and the stars that formed from them. The first massive stars formed from the dense gas will emit energetic light and winds that erode, fragment, and sculpt their birthplace. And then they explode. The resulting morass can be as beautiful as it is complex. After tens of millions of years, the dust boils away, the gas gets swept away, and all that is left is a naked open cluster of stars.
Credit & Copyright: Jean-Charles Cuillandre (CFHT), Hawaiian Starlight, CFHT -
Armillary sphere
An armillary sphere (also known as spherical astrolabe) is a model of the celestial sphere, invented by Eratosthenes in 255 BC. Its name comes from the Latin armilla (circle, bracelet), since it has a skeleton made of graduated metal circles linking the poles and representing the equator, the ecliptic, meridians and parallels. Usually a ball representing the Earth or, later, the Sun is placed in its center. It is used to demonstrate the motion of the stars around the Earth.
Armillary sphere
Armillary spheres were developed by the Greeks and were used as teaching tools already in the 3rd century B.C.. In larger and more precise forms they were also used as observational instruments, being preferred by Ptolemy. Armillary spheres became popular again in the late middle ages; the Danish astronomer Tycho Brahe (1546-1601) constructed several of such instruments.
Chinese Armillary sphere
Renaissance scientists and public figures often had their portraits painted showing them with one hand on an armillary sphere, which represented the height of wisdom and knowledge.
Armillary spheres were among the first complex mechanical devices. Their development led to many improvements in techniques and design of all mechanical devices.
A representation of an armillary sphere is present in the modern Portuguese flag and has been a national symbol since the reign of Manuel I.
Issue #77 -
Astronomy Picture of the Day
2005 May 10
The First Image of an Extra Solar Planet
Explanation: It's the faint red object, not the bright white one that might be a historic find. The white object is surely a brown dwarf star. Quite possibly, however, the red object is the first direct image of a planet beyond our Solar System. The intriguing possibility was first reported last year, but many astronomers weren't then convinced that the "planet" was not just a background star. Earlier this year, the 2M1207 star system was imaged twice more in an effort to resolve the issue. To the delight of the scientific team, the objects kept the same separation, indicating that they are gravitationally bound. The faint red object 2M1207b is therefore 100 times fainter, intrinsically, than the bright white brown dwarf 2M1207b -- a characteristic well explained by a planet roughly five times the mass of Jupiter. The discovery - still subject to further conformation - is considered a step toward the more ambitious goal of imaging Earth-like planets orbiting distant stars. The above image was taken with the high-resolution adaptive-optic NaCo camera attached to the 8-meter Very Large Telescope Yepun in Chile.
Credit: NaCo, VLT, ESO -
Astrolabe
The astrolabe was invented by Hypatia of Alexandria and was the chief navigational instrument until the 16th century (when the sextant was invented). Some historians credit the invention of the astrolabe to Hipparchus (2nd century BC) while others credit the Persian Fazari (Richard Nelson Frye: Golden Age of Persia. p163). The 15th Century metal highly precise Astrolabe was developed by Abraham Zacuto in Lisbon, from its quite different and imprecise wood arab-used precursor.
A 16th century astrolabe
An astrolabe consists of a circle marked in degrees (similar to a protractor) with a rotating arm attached at its center. When the 0° mark on the circle is aligned with the horizon, and a star (or other celestial body) "sighted" at the end of the movable arm, the position (in degrees) of the star can be read ("taken") off the calibrated circle (hence, "astro" = star + "labe" = to take).
On the plate (mater) are engraved coordinate lines which represent a stereographic projection of the celestial sphere (climate), valid for places at a specific geographic latitude. Over this coordinate grid rotates the rete, which is a framework with spikes whose points represent fixed stars. After adjusting the instrument for the current time, the position of a star can be read from the coordinate grid. Conversely, the instrument can be adjusted to fit the measured position, and the time can be read off the scale.
The astrolabe therefore is a predecessor of the modern planisphere.
An 18th Century Persian astrolabe, maker unknown, from the Whipple Museum of the History of Science in Cambridge. The points of the curved spikes on the front rete plate, mark the positions of the brightest stars. The name of each star being labeled at the base of each spike. The back plate, or mater is engraved with projected coordinate lines.
Stereographical projection was first described by Ptolemy (2nd century). The astrolabe reached the Islamic world in the 8th or 9th century, and it was re-introduced to Europe via Islamic Spain in the 11th century (early Christian recipients of Arab astronomy included Gerbert of Aurillac and Hermannus Contractus). The mathematical background was established by Al-Battani in his treatise Kitab az-Zij (ca. 920), which was translated into Latin by Plato Tiburtinus (De Motu Stellarum).
The English author Geoffrey Chaucer (ca. 1343 - 1400) compiled a treatise on the astrolabe for his son, mainly based on Messahalla. The same source was translated by the French astronomer and astrologer Pelerin de Prusse and others. The first printed book on the astrolabe was Composition and Use of Astrolabe by Cristannus de Prachaticz, also using Messahalla, but relatively original.
In the 15th century, the French instrument-maker Jean Fusoris (ca. 1365 - 1436) started selling astrolabes in his shop in Paris, along with portable sundials and other popular scientific gadgets of the day.
Issue #78 -
Astronomy Picture of the Day
2005 May 11
The Sombrero Galaxy in Infrared
Explanation: This floating ring is the size of a galaxy. In fact, it is part of the photogenic Sombrero Galaxy, one of the largest galaxies in the nearby Virgo Cluster of Galaxies. The dark band of dust that obscures the mid-section of the Sombrero Galaxy in optical light actually glows brightly in infrared light. The above image shows the infrared glow, recently recorded by the orbiting Spitzer Space Telescope, superposed in false-color on an existing image taken by NASA's Hubble Space Telescope in optical light. The Sombrero Galaxy, also known as M104, spans about 50,000 light years across and lies 28 million light years away. M104 can be seen with a small telescope in the direction of the constellation of Virgo.
Credit: R. Kennicutt (Steward Obs.) et al., SSC, JPL, Caltech, NASA -
Astronomical clock
An astronomical clock is a clock with special mechanisms and dials to display the relative positions of the sun, moon, zodiacal constellations, and sometimes major planets.
Prague Town Hall astronomical clock
One of the most famous of this type of clock is the Town Hall clock in Prague, Czech Republic. It is also known as the Prague Orloj.
Prague Town Hall astronomical clock. The clock, built in 1410, shows the phases of the moon and the signs of the zodiac. Later additions include a procession of the apostles above and an 1870 calendar below.
The central portion was completed in 1410. The four figures are set in motion at the hour, with Death (represented by a skeleton) striking the time.
On the hour there is a presentation of statues of the Apostles at the doorways above the clock, with all twelve presented at noon.
In 1870 a calendar display was added below the clock.
Astronomical clock face and animated figures of the Prague Town Hall clock
During World War II the clock was nearly destroyed by Nazi fire. The townspeople are credited with heroic efforts in saving most of the parts. It was gradually renovated till 1948. In 1979 the clock was once more cleaned and renovated. According to local legend the city will suffer if the clock is neglected and its good operation is placed in jeopardy.
Other astronomical clocks
Olomouc, the former capital of Moravia in the eastern part of the Czech Republic, also has an impressive exterior astronomical clock on the main town square.
Astronomical clocks of lesser size are found in other countries around the world:
The Science Museum (London) has a scale model of the 'Cosmic Engine', which Su Sung constructed in 1092. This great astronomical clock was about ten meters high (about 30 feet) and was indirectly powered by falling water and Mercury.
The Copenhagen city hall has what is billed as a complete astronomical clock, set in an interior cabinet. The clock was designed over a period of 50 years by amateur astronomer and professional clockmaker Jens Olsen. It was assembled from 1948 to 1955. From 1995 to 1997 the clock underwent complete restoration.
The Palace of Versailles near Paris has a sumptuous rococo table top astronomical clock which took 12 years for a clockmaker and an engineer to build. It was presented to Louis XV in 1754.
The Gros Horloge in Rouen is a famous astronomical clock (14th century), located in the Gros Horloge street.
The Zytglogge in Bern is a famous astronomical clock from the 15th century situated in the Capital of Switzerland.
Issue #79 -
Astronomy Picture of the Day
2005 May 12
Stars, Galaxies, and Comet Tempel 1
Explanation: Faint comet Tempel 1 sports a fuzzy blue-tinted tail, just right of center in this lovely field of stars. Recorded on May 3rd slowly sweeping through the constellation Virgo, periodic comet Tempel 1 orbits the Sun once every 5.5 years. Also caught in the skyview are two galaxies at the upper left - NGC 4762 and NGC 4754 - both members of the large Virgo Cluster of galaxies. Classified as a lenticular galaxy, NGC 4762 presents an edge-on disk as a narrow gash of light while NGC 4754 is a football-shaped elliptical galaxy. Similar in apparent size, the galaxies and comet make for an intriguing visual comparison, but Tempel 1 is only about 3 light-minutes from planet Earth. The two Virgo cluster galaxies are 50 million light-years away. NASA's Deep Impact spacecraft is scheduled to encounter Tempel 1 on July 4th, launching a probe to impact the comet's nucleus.
Credit & Copyright: Johannes Schedler (Panther Observatory) -
Orrery
An orrery is a mechanical device that illustrates the relative positions and motions of the planets and moons in the solar system in heliocentric model. They are typically driven by a large clockwork mechanism with a globe representing the Sun at the center, and with a planet at the end of each of the arms.
A normal mechanical clock could be used to produce an extremely simple orrery with the Sun in the center, Earth on the minute hand and Jupiter on the hour hand; Earth would make 12 revolutions around the Sun for every 1 revolution of Jupiter. Note however that Jupiter's actual year is 11.86 Earth years long, so this particular example would lose accuracy rapidly. A real orrery would be more accurate and include more planets, and would perhaps make the planets rotate as well.
Orrerys are sometimes referred to as planetariums, although generally, planetariums are hemispherical theaters in which images of the night sky are projected onto an overhead surface. Orreries can range widely in size from hand-held to room-sized.
The first orrery was built in 1713 by George Graham. He was supported (as a patron) by Charles Boyle, 4th Earl of Orrery, and thus the name.
Orrerys are usually not built to scale. Some fixed Solar system scale models have been built and are often many kilometers in size.
Issue #80 -
Astronomy Picture of the Day
2005 May 13
When the Moon Was Young
Explanation: Remember when the Moon was young? It was just last Monday. On May 9th, this slender crescent Moon was recorded at a tender age of 34 hours and 18 minutes. Well, OK ... when calculating the lunar age during a lunation or complete cycle of phases - from New Moon to Full Moon and back to New Moon again - the Moon never gets more than 29.5 days old. Still, a young Moon can be a rewarding sight, even for casual skygazers, though the slim crescent is relatively faint and only easy to see low in the west as the sky grows dark after sunset. Sighting this young Moon last Monday, lucky astronomer Stefan Seip was also treated to a very dramatic telescopic view of an airliner flying in front of the distant sunlit crescent. At a high altitude, the jet's stunning contrails reflect the strongly reddened light of the Sun setting below the western horizon.
Credit & Copyright: Stefan Seip
