In the medical and dental fields, x-rays are often used for diagnosing what ails us. There are also x-rays in space. Where do they come from?

When we think of "light" we usually think of visible light - light our eyes detect. X-rays are also a form of light (otherwise known as electromagnetic radiation). Wilhelm Conrad Roentgen, a German physicist, discovered X-rays in the late 1800s. After that discovery, x-rays began to be used to "see" bones, tumors, and other things normally hidden within the body. In this process, the body is placed between an x-ray source and a detector.

In space there are many objects, which could be seen if we had x-ray vision. In the world of astronomy, if something is emitting x-rays, it usually means there are extremely high temperatures involved - like millions of degrees hot! What can make matter so hot? Very strong magnetic or gravitational fields can heat particles up (make them more energetic). Another way to heat particles is in large explosions such as a supernova. The gaseous remains of an exploded star (a supernova remnant) can shine brightly in x-rays for thousands of years! Astronomers have detected x-rays from many types of objects including stars, interstellar and intergalactic gas, and galaxies. X-rays are produced from our Sun's hot atmosphere, disks of gas swirling around black holes, and rapidly spinning pulsars. Astronomers can also detect x-rays from our Moon, and some planets. These x-rays are caused by the interaction of energetic particles (like electrons in the solar wind) with the Moon's surface, or a planet's atmosphere. And that is to name just a few x-ray sources.

X-ray image of an elliptical galaxy Credit: NASA/CXC/UVa/C.Sarazin et al.	Optical image of same galaxy Credit: Digital Sky Survey

The Earth's atmosphere serves as a really good blocker of x-rays originating from objects in the universe. In order to "see" these objects in x-rays, we must put our x-ray detectors in space, above the Earth's atmosphere. One such detector is found in the Chandra X-ray Observatory. Chandra was launched in 1999, and is the most sophisticated X-ray observatory built to date. Other current X-ray missions include the Rossi X-ray Timing Explorer (RXTE) and XMM-Newton.

For more information on x-rays and astronomy, see:

Dr. Beth Brown is an astrophysicist in the National Space Science Data Center at NASA Goddard Space Flight Center. She is involved with data acquisition and archiving, and her research focuses on the x-ray emission from elliptical galaxies.