Unit 8: Beyond Earth Chapter 30: Stars     Stellar Remnants The galaxy is littered with the remnants of supernovae and stars, such as white dwarfs, neutron stars, and black holes. Some of these remnants are readily observable; however, others can be difficult to detect. Regardless, these dead cinders play a large role in the universe-their total mass may be comparable to the total mass of living stars. Supernova Remnants A supernova is one of the brightest phenomena to occur in the universe. Thus, astronomers can observe the spectacular explosions even when they take place in faraway galaxies. At its peak, the luminosity of a supernova increases by a factor of 108, so for one brief moment, it shines as bright as an entire galaxy. Supernovae are rare events, occurring about once every 30 to 50 years in a galaxy such as the Milky Way. Scientists can determine the composition of supernova remnants by observing the spectra that they produce. During a supernova explosion, matter that has been enriched by heavy elements, such as iron, is ejected into interstellar space. Some of these elements are formed in the star before it explodes. Other elements, however-including those heavier than iron-are formed in rapid reactions that take place during the brief outburst. The heavy elements are then dispersed back into interstellar gas and are recycled in new stars. Most of the elements that make up Earth originated in massive stars that came to a violent end. LINK UP: Find out more information about supernovae. Detecting Stellar Remnants White dwarfs are very dim. Still, a moderately large telescope can spot one from up to 1000 ly away. As a white dwarf evolves, however, it cools off very gradually and becomes dimmer until eventually, it can no longer be seen by a telescope. How do astronomers detect these dim stellar remnants? In certain cases, a white dwarf can become bright again if it receives new matter and flares up in nuclear reactions. This has happened in some binary-star systems wherein a white dwarf and a red giant orbit a common center of mass. Some of the gas lost by the red giant may land on the surface of the white dwarf with enough energy to start nuclear reactions, which result in a quick flare-up called a nova. Sometimes, a white dwarf may gain so much new matter that it exceeds the 1.4 solar-mass limit for electron-gas pressure support. When this happens to a white dwarf made of carbon, the entire star undergoes an instantaneous nuclear reaction. The white dwarf is totally vaporized and its matter is converted from carbon to iron in the blink of an eye. This explosion, known as a Type I supernova, is different from the implosion of a massive star core, but similar in total energy and luminosity. Neutron Stars A neutron star is much dimmer and therefore harder to see than a white dwarf. However, in some cases, a neutron star can be easily detected by the radiation emitted by energetic particles near the star. The very strong magnetic field of a neutron star can cause the energetic particles to emit beams of radiation from the star's magnetic poles. Often, the magnetic poles are not aligned with the rotational poles. Thus as the star, called a pulsar, rapidly rotates, the beams of radiation sweep across the sky much like the flashes of light from a lighthouse. If Earth happens to be in the path of the beams, the rapid flashes of radiation can be observed by astronomers. Activity No one has ever directly seen a black hole because light cannot escape its strong gravity. Research and write about the evidence that leads astronomers to theorize that black holes exist.