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Project 3: "Adopt a Galaxy--The Search for Supernovae"
by Blake Bartosh
Level: All
Objective
The student/amateur is to choose galaxies of proper location for time of observation, and image these galaxies periodically. The images are examined for signs of supernovae. Images that show possibilities for supernovae are validated by comparison methods. Findings are publishable. Observing and recording the behavior of supernovae adds much needed information to our understanding of stellar makeup and evolution.
Background
A supernova is a star which is cataclysmically shedding its stellar material, resulting in a tremendous increase in its radiant energy. Early astronomers referred to such a "celestial guest" as "stella nova," or "new star," perhaps first coined by Tycho Brahe in the sixteenth century. Such stars can be bright enough to be seen during the daytime.
It is heat and pressure at a star's core that keeps its stellar material stable. In order for a star to "go supernova" it must be old, or cooling off, so that the stellar material begins to fall into the core. Then, nuclear reactions occur at the atomic level of the stellar material, and the outer layers are ejected out at huge velocities. The resulting energy created by the nuclear reactions result in a dramatically quick increase in the star's light output.
The most famous supernova is probably that recorded by the Chinese in A.D. 1054. This erupting star, which was luminous for 21 months, has been hypothesized to have appeared near the star known to the Chinese as T'ien kuan, which modern astronomers believe to be Zeta Tauri. Construed from historical records, the guest star brightened to about magnitude -5, bright enough to be seen during the day, and remained a daylight object for three weeks.
Can we see remnants of this supernova of A.D. 1054 today? Remember that the telescope was not to be invented for about 500 years, and so no one could track the progress of this celestial guest for 5 centuries. It was 900 years after the explosion that Edwin Hubble, in examining the rate of expansion of one particular nebulae, known as The Crab, suggested that it may be related to the event. Astronomers have used many techniques to determine that the Crab Nebula is most likely the "smoking gun," including sensitive radio surveys.
We can view the Crab Nebula today. It is M1, the first object recorded by Messier in his famous catalog of objects he developed to prevent confusion with his attempts at discovering new comets. M1 is a beautiful tangle of threads and luminous gas located in the constellation Taurus.
Another believed remnant of a supernova is Cassiopeia A, whose glowing knots of plasma emit strong radio waves and are visible today in optical telescopes.
The search for supernovae has typically been conducted using photographic techniques, in which portions of the sky are photographed, and painstakingly compared to other photographs of the same area to see if any unusual star brightening or dimming have occurred. The pioneer of this method is Fritz Zwicky, who started searching for supernovae with a camera and 80 mm lens mounted to the roof of the physics laboratory at the California Institute of Technology in Pasadena, California in 1934. By 1949, Zwicky had identified over 50 supernovae with the help of a new invention, the Schmidt camera. Finally, in 1959, Zwicky was granted access to a very large camera at Mount Palomar, California, the 48 inch Schmidt camera which had been used for the famous Palomar Sky Survey.
Zwicky did not go at this alone. He established an international "supernova patrol" and as a result, over 400 extra galactic supernovae have been catalogued.
Discussion of Work
Students can contribute to the search for supernovae by diligence and consistent observation, like Fritz Zwicky. Extra galactic supernovae (occurring outside of our Milky Way galaxy) are to be searched for in the arms of spiral galaxies. Studies have shown that supernovae are most commonly found in spiral galaxies with arms widely spread out (type Sc in Hubble's classification scheme for galaxies). Examples of such spirals include M33, the Pinwheel Galaxy in Triangulum, and M101 in Ursa Major. These galaxies are seen nearly face-on, and so their arms are easily examined.
Students should choose several galaxies which fit the proper classification (to increase the likelihood of finding a supernova), and which can be repeatably imaged over the course of a season or semester. SkyPro allows viewing several images on screen simultaneously, so students can tile images of one galaxy, taken over several sessions, and examine them for differences.
SkyPro also has a feature called "Blink Comparator" which alternately displays one of two images in rapid succession. If the images of the galaxy under scrutiny are similarly framed, the Blink Comparator is very helpful in identifying star magnitude variation from one image to another.
Upon discovery of a supernovae candidate, students should first confirm that the finding is repeatable by taking another image of the galaxy in question. If the candidate supernova still appears in a subsequent image, then the students should report the finding. In addition, the students now should plan to image the galaxy several times over two to three weeks and create a "light curve" which shows the variation of intensity of the suspected supernova over time.
The light curve of the supernova is extremely important to astronomers, who need this type of information to increase our understanding of how stars evolve. For help in constructing light curves, see Project 9.
References
Vehrenberg,
Atlas of Deep Sky Splendors
Mitton, The Crab Nebula, 1978
"Supernova Patrol," CCD Astronomy, Fall 1994
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