A European Space Agency telescope celebrated 20 years of exploring the secrets of the X-ray universe this week.

The XMM-Newton telescope, which launched on Dec. 10, 1999, has made contributions in various fields of science and astronomy, and has observed objects ranging from galaxy clusters to star flares. But in a press release celebrating the anniversary, scientists zeroed in on the observatory's black hole discoveries.

Black holes are areas in space that are so dense that no other object can escape their pull after passing beyond a point of no return known as the "event horizon." Even light can't escape, which means the black holes can't be seen. But when black holes munch on nearby gas, dust or objects, they produce a distinctive glow that can be mapped out in X-rays.

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Although XMM-Newton can't see black holes directly in fact, the first-ever image of a black hole was just produced this year using data from the Event Horizon Telescope, a collaboration of observatories from around the world (that does not include XMM-Newton).

What XMM-Newton is good at is seeing X-rays produced by iron molecules. These molecules are heated to high temperatures and ionized, or stripped of their electrons, on their death plunge toward the black hole.

The observatory has made several discoveries in the field of supermassive black holes, which are thousands of times the mass of the sun and which tend to be embedded in galaxies. XMM-Newton made a key find using iron molecules in a supermassive black hole in 2013.

"The X-rays given out from the iron contain information about the geometry and dynamics of the black hole," ESA said in a statement. "XMM-Newton was used to measure such emission in order to study the rotation rate of the supermassive black hole at the center of the spiral galaxy NGC 1365."

XMM-Newton also spotted flashes from a black hole embedded in the galaxy GSN 069, emanating about once every nine hours. "These eruptions are thought to be coming from the matter caught in the black holes gravitational grip, or from a less massive black hole circling the more massive one," ESA said in the statement.

XMM-Newton is still working well, and the observatory will focus on supermassive black holes and the galaxies in which they are hosted in the coming years.

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Europe's XMM-Newton Telescope Celebrates 20 Years of X-Ray Astronomy - Space.com