X-ray astronomy is opening a new window on the universe

• With X-ray astronomy, "we can observe most of the universe over most of cosmic time," says Grant Tremblay, an astrophysicist at the Center for Astrophysics who works on X-ray astronomy.

What's happening: Several X-ray missions — large and small — are active today, including NASA's Chandra X-ray Observatory and the European Space Agency's (ESA) XMM-Newton. Both launched in 1999.

• Since then, China's HXMT mission, NASA's NuSTAR and Swift missions and a handful of others have launched and remain active today.
• The newest addition is IXPE — a small NASA mission launched in December 2021. It has three identical X-ray telescopes, each with a detector to measure the polarization of X-rays emitted from neutron stars, supermassive black holes and other sources.
• A new study of Chandra data accepted to be published in the Monthly Notices of the Royal Astronomical Society found the physics of two massive colliding galaxy clusters is similar to what can be seen in our own smaller solar system.

Background: When stars are born, they emit X-rays that can move through dust and gas carrying information about the young stars and their development.

• And when stars die and explode into supernovas or collapse into black holes or leave a neutron star corpse in their wake, they can release X-rays that tell scientists about their structure and evolution.
• When stars interact, they form winds and jets that can change the shapes of galaxies and spark star formation. In the process, they produce X-rays that offer clues about those events.
• X-rays also create a "fossil record" of the activity of clusters of bright active galaxies, which often produce radio jets, says Belinda Wilkes, an astrophysicist at the Smithsonian Astrophysical Observatory and former director of the Chandra X-ray Center. That data is then used to test models of the origin and development of the universe.
• "We think X-rays thread through ... all of the universe and all kinds of sources," Wilkes says.

How it works: Unlike optical telescopes that focus incoming photons that bounce nearly head-on off mirrors, X-ray telescopes focus the high-energy X-ray photons to a detector at an angle similar to "skipping stones off the surface of a pond," says Tremblay.

• The result is a single observation will result in just a few X-ray photons landing on a detector, whereas optical telescopes can sometimes receive trillions.

The big picture: X-ray astronomy has existed for more than two decades but the science is now "coming of age," Wilkes writes in a review this week in Nature.

• The power of X-ray astronomy is being unlocked by combining it with other data collected in different wavelengths of light and even gravitational wave signals.
• That suite of information can give a fuller view of the universe that's more accurate than any one source of data could provide.

Yes, but: Large X-ray observatories take years, if not decades, to develop and build, and right now, the future of X-ray astronomy at NASA and ESA is uncertain.

• Chandra is aging, and a proposed plan for NASA to build a new X-ray observatory is still years away.
• The future of the European Space Agency's Athena mission — a large X-ray observatory — hangs in the balance and could see its projected budget significantly cut.
• Meanwhile, China is expected to launch its own X-ray observatory next year.

What to watch: The first results from the IXPE mission will be presented next week at the American Astronomical Society's annual meeting.

• The telescopes' first targets included a magnetar, or neutron star with a strong magnetic field, a supernova remnant, an X-ray source beyond the Milky Way and the Crab Nebula. Astronomers first measured the nebula's X-ray polarization in the 1970s.
• "It was a relief to measure that and see we got the same result as 50 years ago," says Brian Ramsey, the principal investigator for IXPE.