Unit 8: Beyond Earth Chapter 31: Galaxies and the Universe     Gamma Rays On June 4, 2000, the Compton Gamma Ray Observatory (CGRO), the second of NASA's powerful telescopes in space, re-entered Earth's atmosphere after a successful nine-year mission. As its name indicates, CGRO was placed in space to record images created by gamma rays, which are absorbed by Earth's atmosphere and thus cannot be studied directly by Earth-based telescopes. One of the instruments aboard CGRO was the Burst and Transient Source Experiment (BATSE), which gathered crucial information about one of the greatest mysteries in modern astronomy-gamma-ray bursts. Gamma-Ray Bursts Like supernovae, gamma-ray bursts are extremely violent, brief explosions that can outshine the brightest galaxy. But unlike supernovae, scientists don't know what causes gamma-ray bursts. Although these powerful flashes occur roughly once a day, no one can predict when or where they will occur. Astronomers are not even sure if they originate from within the Milky Way Galaxy or from the edge of the observable universe. The Facts The first gamma-ray bursts were observed in 1967 by scientists at the Los Alamos National Laboratory in New Mexico. These scientists had a clear mission-to see if other countries were violating an international nuclear test ban. The scientists relied on satellites to gather much of the necessary data. Inexplicably, the data sometimes indicated a sharp increase in gamma rays originating from outer space. This occurred on 16 separate occasions and was detected by widely-spaced satellites. The scientists did not report their discovery until 1973. In the years that followed, other gamma-ray bursts were detected-BATSE alone recorded more than 2500 bursts. Data from BATSE also showed that the bursts generally lasted from about 30 ms to more than 1000 s. They are evenly distributed across the sky and rarely occur twice in the exact same location. LINK UP: Find out more information about gamma-ray bursts. The Hypotheses Astronomers first theorized that gamma-ray bursts came from stars within the Milky Way Galaxy. This was a logical assumption because if the opposite were true-if the bursts occurred in distant galaxies-they would have to release a mind-boggling amount of energy to be detected with such incredible brightness. Data from BATSE, however, punched a hole in this theory. If the bursts originated from stars within our galaxy, then the bursts would be concentrated in the same region where most stars are found-along the galactic plane. BATSE data showed conclusively that the bursts occur throughout space in no particular region. In response, scientists have put forth two new hypotheses about gamma-ray bursts. The first hypothesis states that the bursts come from neutron stars that surround the outer edges of the Milky Way Galaxy. This region of space cannot be seen from Earth and remains largely a mystery. If the hypothesis is correct, then gamma rays are our only clue thus far to some of the processes that take place in the most distant parts of our galaxy. The second theory states that gamma-ray bursts are emitted by undefined objects that, for reasons unknown, are common in all galaxies throughout the universe, except for the Milky Way. If this hypothesis is correct, then very powerful objects exist in the universe which we have yet to discover. Additional research may lead scientists to discard these hypotheses and replace them with new ones. Regardless, solving the mystery of gamma-ray bursts will give astronomers a better grasp of the evolution and structure of the universe. Activity Gamma rays are part of the electromagnetic spectrum, which also includes radio waves, microwaves, infrared radiation, visible light, and X rays. Draw a diagram of the electromagnetic spectrum in your science journal that shows the arrangement of electromagnetic radiation according to wavelength and frequency. Use research materials to include an example of each type of radiation.