Violent Events in the Cosmos
Astronomers once pictured the cosmos as an arena of calm, as planets moved serenely in their orbits around stars, stars orbited within galaxies, and galaxies moved through galaxy clusters. The past four decades, however, have revealed cosmic arenas of extreme violence. Crucial news about these violent locales reaches Earth in the form of high-energy x-ray and gamma-ray photons, typically created in enormous outbursts. Earth's atmosphere blocks all x-rays and gamma rays, protecting life on Earth's surface but seriously restricting our ability to study the most violent cosmic phenomena.
The Chandra X-ray Observatory, launched in 1999, has yielded superbly detailed images of objects that generate copious amounts of x rays. In many cases, these images reveal the sites of tremendous outbursts such as supernovas, dying stars that blast hot gas into space at velocities of many thousands of miles per second. More sensitive x-ray observations will lead to understanding of the mechanisms through which these explosions occur. Among these mechanisms are the collapse of stellar cores, the sudden destruction of white dwarf stars, and the creation of neutron stars. Neutron stars are made entirely of neutrons and they pack into a region only a few miles across a mass equal to the Sun's.
X-ray-observing satellites have also found hot gas floating among the stars and galaxies. Observations of the spectrum of the x rays from this gas will make it possible to determine the temperature and relative amounts of its chemical elements. This information will provide crucial evidence for understanding the processes by which the elements were formed. It will help to establish the history of the creation of the elements throughout the universe. X-rays also arise from high-temperature gas close to the surfaces of neutron stars, the collapsed remnant cores of exploded stars.
A new generation of x-ray satellites will be able to make superior observations of another class of objects: black holes. Supermassive black holes, each with millions of times the Sun’s mass, appear at the centers of giant galaxies such as our Milky Way. Much closer to our solar system, black holes with masses similar to the Sun’s have been created by dying stars. By studying the motions of matter close to these objects, astronomers have deduced the existence of these star-mass black holes within our galaxy. X-ray telescopes with high angular resolution and good spectral sensitivity can map the motions of the extremely hot, x-ray-emitting gas that swirls around and into the black holes. Such maps will verify that these black holes do exist and will test theories of how black holes bend nearby space.
Still greater mysteries arise from observations made in the highest-energy domain of the electromagnetic spectrum. During the past few years, astronomers have identified a new class of objects, gamma-ray bursts, that suddenly release enormous outflows of gamma rays. The nature and origin of these objects remain largely a mystery. We know that they lie far outside the Milky Way, but in all but a very few cases, astronomers have not yet been able to match the locations of these outbursts with any object visible in the sky. Because gamma-ray astronomy remains in relative infancy, we must look to the next generation of gamma- ray-detecting satellites to determine the nature of the gamma-ray bursts.
Read More About New Instruments For X-Ray And Gamma-Ray Astronomy