How X-Ray Astronomy Works

A Chandra image of M51 contains nearly a million seconds of observing time. X-ray: NASA/CXC/Wesleyan Univ./R.Kilgard, et al; Optical: NASA/STScI

There's a hidden universe out there—one that radiates in wavelengths of light that humans can't sense. One of these radiation types is the x-ray spectrum. X-rays are given off by objects and processes that are extremely hot and energetic, such as superheated jets of material near black holes and the explosion of a giant star called a supernova. Closer to home, our own Sun emits x-rays, as do comets as they encounter the solar wind. The science of x-ray astronomy examines these objects and processes and helps astronomers understand what's happening elsewhere in the cosmos.

The X-Ray Universe

A pulsar in the galaxy M82.
A very luminous object called a pulsar emanates incredible energy in the form of x-ray radiation in the galaxy M82. Two x-ray-sensitive telescopes called Chandra and NuSTAR focused on this object to measure the energy output of the pulsar, which is the rapidly rotating remnant of a supermassive star that blew up as a supernova. Chandra's data appears in blue; NuSTAR's data is in purple. The background image of the galaxy was taken from the ground in Chile. X-ray: NASA/CXC/Univ. of Toulouse/M.Bachetti et al, Optical: NOAO/AURA/NSF

X-ray sources are scattered throughout the universe. The hot outer atmospheres of stars are prodigious sources of x-rays, particularly when they flare (as our Sun does). X-ray flares are incredibly energetic and contain clues to the magnetic activity in and around a star's surface and lower atmosphere. The energy contained in those flares also tells astronomers something about the evolutionary activity of the star. Young stars are also busy emitters of x-rays because they're much more active in their early stages.

When stars die, particularly the most massive ones, they explode as supernovae. Those catastrophic events give off huge amounts of x-ray radiation, which provide clues to the heavy elements that form during the explosion. That process creates elements such as gold and uranium. The most massive stars can collapse to become neutron stars (which also give off x-rays) and black holes.

The x-rays emitted from black hole regions don't come from the singularities themselves. Instead, the material that is gathered in by the black hole's radiation forms an "accretion disk" that spins material slowly into the black hole. As it spins, magnetic fields are created, which heat the material. Sometimes, material escapes in the form of a jet that is funneled ​by the magnetic fields. Black hole jets also emit heavy amounts of x-rays, as do supermassive black holes at the centers of galaxies. 

Galaxy clusters often have superheated gas clouds in and around their individual galaxies. If they get hot enough, those clouds can emit x-rays. Astronomers observe those regions to better understand the distribution of gas in clusters, as well as the events that heat the clouds. 

Detecting X-Rays from Earth

The Sun in x-rays.
The Sun in x-rays, as seen by the NuSTAR observatory. Active regions are the brightest in x-rays. NASA

X-ray observations of the universe and the interpretation of x-ray data comprise a relatively young branch of astronomy. Since x-rays are largely absorbed by Earth's atmosphere, it wasn't until scientists could send sounding rockets and instrument-laden balloons high in the atmosphere that they could make detailed measurements of x-ray "bright" objects. The first rockets went up in 1949 aboard a V-2 rocket captured from Germany at the end of World War II. It detected x-rays from the Sun. 

Balloon-borne measurements first uncovered such objects as the Crab Nebula supernova remnant (in 1964). Since that time, many such flights have been made, studying a range of x-ray-emitting objects and events in the universe.

Studying X-Rays from Space

Chandra X-ray Observatory
Artist's conception of the Chandra X-Ray Observatory on orbit around Earth, with one of its targets in the background. NASA/CXRO

The best way to study x-ray objects in the long term is to use space satellites. These instruments don't need to fight the effects of Earth's atmosphere and can concentrate on their targets for longer periods of time than balloons and rockets. The detectors used in x-ray astronomy are configured to measure the energy of the x-ray emissions by counting the numbers of x-ray photons. That gives astronomers an idea of the amount of energy being emitted by the object or event. There have been at least four dozen x-ray observatories sent to space since the first free-orbiting one was sent, called the Einstein Observatory. It was launched in 1978.

Among the best-known x-ray observatories are the Röntgen Satellite (ROSAT, launched in 1990 and decommissioned in 1999), EXOSAT (launched by the European Space Agency in 1983, decommissioned in 1986), NASA's Rossi X-ray Timing Explorer, the European XMM-Newton, the Japanese Suzaku satellite, and the Chandra X-Ray Observatory. Chandra, named for Indian astrophysicist Subrahmanyan Chandrasekhar, was launched in 1999 and continues to give high-resolution views of the x-ray universe.

The next generation of x-ray telescopes includes NuSTAR (launched in 2012 and still operating), Astrosat (launched by the Indian Space Research Organisation), the Italian AGILE satellite (which stands for Astro-rivelatore Gamma ad Imagini Leggero), launched in 2007. Others are in planning which will continue astronomy's look at the x-ray cosmos from near-Earth orbit.

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Petersen, Carolyn Collins. "How X-Ray Astronomy Works." ThoughtCo, Jan. 26, 2018, Petersen, Carolyn Collins. (2018, January 26). How X-Ray Astronomy Works. Retrieved from Petersen, Carolyn Collins. "How X-Ray Astronomy Works." ThoughtCo. (accessed February 18, 2018).