X-Ray Astronomy - astronomy.

« Some neutron stars have weaker magnetic fields that allow incoming material to settle onto the entire surface of the neutron star.

Eventually, so much material buildsup that the surface layer becomes dense enough to set off a vast thermonuclear explosion, called an outburst.

The explosion heats gas to produce X rays.

Such aneutron star—called an X-ray burster—can increase its X-ray production by a million times during an outburst.

The X-ray glow fades over time, and the binary systementers a long, quiet period as material from the companion star again begins to accumulate on the neutron star’s surface. Some very massive stars explode into supernovas at the end of their lives.

The star begins to use up its nuclear fuel and begins to collapse in on itself.

Eventually thestar’s increasing density triggers one final, huge nuclear explosion.

The shock waves that this explosion generates crash into interstellar gas and heat it so much thatthe gas radiates X-ray light for thousands of years. B Extragalactic Sources Most galaxies contain the same types of X-ray-emitting objects that the Milky Way does, but the other galaxies are so distant that these sources usually cannot bedetected from Earth.

Only extremely energetic objects in other galaxies, such as large supernovas and supermassive black holes, are detectable from Earth.

Thecenters of many galaxies are powerful X-ray emitters.

Astronomers believe that this is because almost every galaxy has a supermassive black hole at its center.

Theseblack holes are a million to a billion times the mass of the Sun and produce intense gravitational and magnetic fields.

Galaxies that are especially active in the X-rayspectrum (X-ray galaxies) often have large amounts of material surrounding the black hole in the galactic nucleus.

As matter swirls toward the black hole, friction heatsthe gas and dust enough to produce X rays.

Electrons also get caught and accelerated in the black hole’s magnetic field and produce X rays.

Astronomers do not knowexactly what quasars are, but many believe that quasars may be galaxies that appear at strange angles from Earth. Clusters of galaxies are among the most luminous X-ray sources in the sky.

Huge amounts of very hot hydrogen gas in the clusters produce the X-ray radiation.Astronomers have found that the amount of visible matter in the cluster is not sufficient to explain how so much gas gets squeezed and heated to such hightemperatures.

They have concluded that a vast amount of dark matter—matter that does not emit electromagnetic radiation and is therefore invisible to observers fromEarth—must be present to provide the gravitational energy to hold the gas in the cluster. The universe also glows in X rays.

The background X-ray glow is strong and uniform, or the same in all directions.

This uniformity is one of the reasons astronomersbelieve that nearly every galaxy in the universe contains an X-ray-producing black hole at its center.

The distribution of galaxies in the universe is uniform on a largescale, so X rays from galactic black holes would explain the uniformity of the X-ray background radiation. IV HISTORY OF X-RAY ASTRONOMY German physicist Wilhelm Roentgen accidentally discovered X rays during an 1895 laboratory experiment.

Roentgen dubbed the radiation X rays because x is a common mathematical symbol for the unknown.

Roentgen and other scientists quickly discovered the ability of X rays to pass through many substances and applied theirresearch on X rays to medicine and industry. Astronomical X rays were not discovered until after World War II (1939-1945).

Scientists at the United States Naval Research Laboratory equipped a captured GermanV-2 rocket with an X-ray detector and sent it through Earth’s atmosphere.

The X-ray detector registered X-ray radiation from the Sun. In 1962 a group of American scientists—including Riccardo Giacconi, Herbert Gursky, Frank Paolini, and Bruno Rossi—sent a rocket-mounted X-ray detector to study theSun’s effect on the Moon’s X-ray emission.

Instead, the detector picked up X rays from a bright X-ray source in the constellation Scorpius.

This object, called ScorpiusX-1 or Sco X-1, was the first black hole discovered. Encouraged by this unexpected discovery, Giacconi led development of the first Earth-orbiting observatory, named Uhuru (“freedom” in Swahili).

Launched in 1970, Uhuru mapped the entire sky and discovered about 300 X-ray sources—far more than anyone had ever imagined. In the late 1970s the U.S.

National Aeronautics and Space Administration (NASA) launched a series of remarkably successful X-ray satellites, called the High EnergyAstronomy Observatories (HEAO).

HEAO 2—also called the Einstein Observatory—was the first space observatory capable of making X-ray images comparable to theimages produced by optical telescopes.

The Einstein telescope revealed that many active galaxies and quasars are strong X-ray emitters. During the 1980s the European, Japanese, and Russian space agencies continued to launch successful X-ray astronomy missions largely directed toward in-depthstudies of phenomena such as X-ray bursters and X-ray pulsars.

In the 1990s a German X-ray satellite, called the Roentgen Satellite (ROSAT), provided a sharper,more sensitive, and wider view of the X-ray sky.

ROSAT’s all-sky survey has identified nearly 60,000 X-ray sources across the universe.

In addition, an armada of othersatellites now in operation are contributing to important new breakthroughs, including the U.S.

Advanced Satellite for Cosmology and Astrophysics (ASCA), the U.S.Rossi X-ray Timing Explorer (RXTE), and the Italian Satellite for X-ray Astronomy (Beppo SAX). In 1999 NASA launched the most sensitive X-ray satellite to date, the Chandra X-Ray Observatory.

Named for American astrophysicist Subrahmanyan Chandrasekhar,the telescope aboard Chandra has eight times the resolution of any previous X-ray telescope.

Also in 1999, the European Space Agency launched the X-ray Multi-Mirror(XMM) Newton satellite, whose unique configuration of nested mirrors was designed to help scientists discover many more astronomical sources of X rays. Contributed By:Ray VillardMicrosoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation.

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