Astrophysics, high-energy
The study of the universe as revealed by high-energy, invisible forms of light: x-rays and gamma rays. These radiations are produced in the cosmos when gas is heated to millions of degrees Kelvin or electrons have been accelerated to near the speed of light by violent and extreme conditions. Exploding stars, neutron stars, black holes, and galaxy clusters, the most massive objects in the universe, are among the objects studied.
The high energies of x-rays and gamma rays have two important consequences for astronomical research. First, these forms of light are absorbed by the atmosphere, so telescopes to detect them must be placed on spacecraft above the atmosphere. Second, the telescopes must be constructed differently. Gamma rays have such high energy that they cannot be focused by traditional techniques, although indirect methods can give a rough estimate of their direction. See also Satellite astronomy.
X-rays will reflect off mirrors, but only if they strike at grazing angles, like a stone skipping across a pond. For this reason, x-ray mirrors have to be carefully shaped and aligned nearly parallel to the incoming x-rays. These barrel-shaped mirrors are nested one inside the other to increase the collection area, and therefore the sensitivity, of the telescope.
The Chandra X-ray Observatory, launched by the National Aeronautics and Space Administration (NASA) in July 1999, is the premier focusing x-ray telescope. It is an assembly of four pairs of mirrors. Chandra's mirrors are the smoothest mirrors ever constructed. The largest of the mirrors is almost 4 feet (1.2 m) in diameter and 3 ft (0.9 m) long. See also X-ray telescope.
The European Space Agency's XMM, a powerful telescope launched in December 1999, has 58 mirrors. These mirrors are not as smooth as Chandra's mirrors, so XMM cannot make images of the same crispness, but it can detect fainter sources and measure the energies of x-rays very accurately.
The new era of gamma-ray astronomy was inaugurated by the launches of NASA's Compton Gamma-Ray Observatory, and Granat, a Russian-French mission. Granat, launched in 1989, has two instruments that cover the x-ray through the gamma-ray region. Compton was launched in 1991 as one of NASA's Great Observatories, along with the Hubble Space Telescope and the Chandra X-ray Observatory. It has four instruments on board that go from high-energy x-ray to high-energy gamma-ray energies. The capabilities of the instruments aboard these observatories are more than ten times that of any previous gamma-ray mission. See also Gamma-ray astronomy.
Supernovae
A massive star explodes about once every 50 years in the Milky Way Galaxy. The shell of matter thrown off by the supernova creates a bubble of multimillion-degree gas called a supernova remnant. The hot gas expands and produces x-rays for thousands of years. Gamma rays from radioactive elements have also been detected from supernova remnants by gamma-ray telescopes such as those on the Compton Gamma-Ray Observatory and the SIGMA telescope on board Granat.
The study of remnants of exploded stars, or supernovae, is essential for understanding the origin of life on Earth. The cloud of gas and dust that collapsed to form the Sun, Earth, and other planets was composed mostly of hydrogen and helium, with a small amount of heavier elements such as carbon, nitrogen, oxygen, and iron. The only place where these and other heavy elements necessary for life are made is deep in the interior of a massive star. There they remain until a supernova explosion spreads them throughout space. See also Nucleosynthesis; Supernova.
Neutron stars
When a massive star explodes, most of it is flung into space, but the core of the star is compressed to form a rapidly rotating dense ball of neutrons that is about 12 mi (20 km) in diameter. The collapse and rapid rotation of the neutron star cause it to become highly magnetized. A magnetized, rapidly rotating neutron star can produce electric voltages of 1016 V.
Neutron star gravity, which is more than 1011 times stronger than gravity on Earth, is overwhelmed by the electric field, and particles are pulled off the neutron star and accelerated to speeds near the speed of light. An intense shower of electrons and antimatter electrons, or positrons, is produced by these particles. The pulsed emission from the Crab Nebula, observed at all wavelengths from radio through gamma rays, is thought to be caused by this process (see illustration). See also Crab Nebula; Pulsar.
pulsar in the constellation Taurus. The image shows the central pulsar, a rapidly spinning neutron star, or pulsar that emits pulses of radiation 30 times a second, surrounded by tilted rings of high-energy particles that appear to have been flung outward over a distance of more than a light-year from the pulsar. (NASA/Chandra X-ray Observatory Center/Smithsonian Astrophysical Observatory)">
Chandra X-ray Observatory image of the Crab Nebula, a supernova remnant and pulsar in the constellation Taurus. The image shows the central pulsar, a rapidly spinning neutron star, or pulsar that emits pulses of radiation 30 times a second, surrounded by tilted rings of high-energy particles that appear to have been flung outward over a distance of more than a light-year from the pulsar. (NASA/Chandra X-ray Observatory Center/Smithsonian Astrophysical Observatory)
Black holes and quasars
When a very massive star collapses, it forms a black hole. A black hole does not have a surface in the usual sense of the word. There is simply a region in space around a black hole beyond which nothing can be seen, because nothing can escape from inside this region. This region is called the event horizon.
Anything that passes beyond the event horizon is doomed to be crushed as it descends ever deeper into the gravitational well of the black hole. Neither visible light, nor x-rays, nor any other form of electromagnetic radiation given off by the particle can escape.
A black hole cannot be seen directly. The only way to find one is by observing the energy released by matter that is falling toward the black hole. As gas and dust particles swirl toward a black hole, they speed up and form a flattened disk. Friction caused by collisions between the particles heats them to extreme temperatures. Just before the particles pass beyond the event horizon, they produce x-rays and gamma rays as their temperatures approach 108 K.
Black holes grow when matter falls into them. A black hole in the center of a galaxy where stars are densely packed may grow to the mass of 109 suns. Energy released from large clouds of gas as they fall into these supermassive black holes can be stupendous. This is the accepted explanation for quasars, sources in which the power output at the center of a galaxy is a thousand times greater than an entire galaxy of 1011 stars. See also Quasar.
One of the most intriguing features of supermassive black holes is that they do not suck up all the matter that falls within their sphere of influence. Some of the matter falls inexorably toward the black hole, and some explodes away from the black hole in high-energy jets that move at near the speed of light. These jets produce radio, optical, x-ray, and gamma radiation. The matter swirling around the black hole must somehow be producing enormous electric and magnetic fields that accelerate electrons to extremely high energies. Exactly how this happens is unknown and is a major focus of research. See also Black hole.
Galaxy clusters and dark matter
More than half of all galaxies in the universe are members of groups of galaxies or larger collections of galaxies, called clusters. X-ray observations have shown that most clusters of galaxies are filled with vast clouds of multimillion-degree gas. The mass of this gas, which was heated when it collapsed from a much larger size, is greater than all the stars in all the galaxies in a cluster of a thousand galaxies. Galaxy clusters are the largest and most massive gravitationally bound objects in the universe.
The x-ray-producing hot gas found in a typical cluster of galaxies presents a great mystery. Over time this extremely hot gas should escape the cluster, since the galaxies and gas do not provide enough gravity to hold it in. Yet the gas remains in clusters of all ages. Scientists have concluded that some unobserved form of matter, called dark matter, is providing the gravity needed to hold the hot gas in the cluster. An enormous amount of dark matter is needed—about three to ten times as much matter as that observed in the gas and galaxies. This means that most of the matter in the universe may be dark matter.
The dark matter could be collapsed stars, planet-like objects, black holes, or exotic subatomic particles that produce no light, and can be detected only through their gravity. Detailed measurements of the size and temperature of the hot gas clouds in galaxy clusters with x-ray telescopes could help solve the dark matter mystery. See also
