If the Hubble Space Telescope set the standard for what a jaw-dropping space picture should look like, then the Chandra X-Ray Observatory, launched 20 years ago this week, perfected it.
The Chandra X-Ray Observatory went to orbit onboard the Space Shuttle Columbia on July 23, 1999. Unlike Hubble, which measures ultraviolet, visible, and near-infrared radiation emitted by stars and galaxies, Chandra captures X-rays. Various things in space emit X-rays, like jets of matter released by the centres of galaxies, stellar winds interacting with cool gas, black holes and neutron stars, and leftovers from supernovae. In other words, Chandra specialises in helping us understand the most mind-blowing stuff.
But unlike many Hubble images, which typically contain lots of visible light and are colour shifted for aesthetic or explanatory reasons, X-rays aren’t visible to our eyes, so all of Chandra’s images must be presented in false colour. Typically, these images are superimposed onto visible-light images to illustrate the whole scene. It’s not what the area would look like to our eyes, but it’s an additional filter through which to look at the universe, one that captures important data left out by other telescopes.
Twenty years is old for a telescope, and Chandra has begun showing signs of age. Last fall, the telescope experienced a glitch in one of the gyroscopes that help keep it oriented. Astronomers have since resolved that issue, but questions remain as to what the telescope’s successor will be—unlike the James Webb Space Telescope that will cover much of the wavelengths that Hubble observes today, there’s no upgraded version of Chandra on the horizon. Scientists have proposed the Lynx X-ray telescope for the 2020 decadal survey, but that’s just one of four potential flagship concepts.
But looking back, Chandra has made a tonne of important scientific discoveries. Here are some of the coolest, as well as some of its most important images. Generally, these pictures combine several wavelengths, so I’ll notes what you’re looking at and what Chandra added—the additional colour that Chandra contributed to these images also means a better understanding of how these celestial objects work.
Galaxy cluster 1E 0657-56, aka the “Bullet Cluster.”
The Bullet ClusterImage: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.
This 2004 observation is among Chandra’s most famous images, as it provided scientists with evidence of the existence of dark matter, a mysterious mass that seems to far exceed the amount of regular matter in the universe but that doesn’t seem to interact with regular matter. The cluster itself is the result of two large clusters of galaxies colliding with one another, with the galaxies’ visible light shown in white and orange. The X-rays at the centre of the image are shown in pink and were emitted by regular matter in the centre.
But the shocking find for astronomers was the blue bits. The blue represents where most of the mass of the cluster is located, based on the principle of gravitational lensing, where heavy objects warp the light from stars and galaxies shining behind them. Basically, the image shows that most of the “stuff” in the image isn’t where the regular matter is (pink), but where the blue is. Scientists think that when the two clusters collided, the dark matter kept moving without interacting, while much of the regular matter slowed down from the collision and stayed in the centre. Scientists have since repeated this study on dozens of other galaxy clusters.
The Crab Nebula
The Crab Nebula, with X-rays in pink, optical light in green, infrared in yellow, radio in red, and ultraviolet in blue. Image: X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA/JPL/Caltech; Radio: NSF/NRAO/VLA; Ultraviolet: ESA/XMM-Newton
The Crab Nebula is among the most studied objects in the night sky, and is the result of a supernova that astronomers observed here on Earth back in 1054. The outer region with the tendrils is the optical and infrared light emitted by lower-energy electrons farther from the centre. Chandra’s 2009 contribution is the swirly pink component with the bright spot in the centre, the higher-energy electrons. That region represents a striking image of a neutron star, the remnants of a collapsed star made of incredibly dense matter.
Given how much humans have studied it, the Crab Nebula has revealed a number of important scientific finds as a persistent source of high-energy radiation in the sky. It helped scientists refine their understanding of supernovae and rotating neutron stars, released the highest-energy radiation ever observed, and lets astronomers measure the solar corona and other celestial objects when they pass in front of it.
Sag A*. Image: NASA/CXC/Univ. of Wisconsin/Y.Bai, et al.
Chandra’s X-ray observations have generated a tonne of data about Sagittarius A*, the black hole at the centre of our galaxy and the high-energy environment surrounding it. In this image, the different colours represent different energies of X-rays, with the highest-energy X-rays in blue. Sag A* would sit in the centre of the brightest part of the red area.
The image doesn’t show the black hole itself, but the region of hot gas around the black hole. And though this is just a snapshot of one observation, this area is quite chaotic. Scientists operating Chandra will occasionally measure X-ray flares from the region, thought to be the result of the black hole gulping up matter or the reconfiguration of magnetic fields in the region.
SN 1987A. Image: X-ray: NASA/CXC/PSU/S.Park & D.Burrows.; Optical: NASA/STScI/CfA/P.Challis
Thirty-two years ago, a new flash appeared in the sky, coming from our neighbouring galaxy, the Large Magellanic Cloud. Telescopes around the world, including Chandra, were able to take images of the supernova and have continued to observe it to this day, watching how it changes over time. This was the first example of “multimessenger astronomy,” as astronomers measured both electromagnetic radiation and neutrinos emanating from this stellar explosion.
Chandra’s observations here are in blue (not the white spots). The whole image shows the matter ejected by the supernova, illuminated by a passing shockwave. Today, scientists think that the shockwave has now left the ring of matter, and is moving even faster as it passes to a less-dense region of space.
M87. Image: X-ray: NASA/CXC/Villanova University/J. Neilsen
Earlier this year, observatories around the world announced that they’d created the first image of a black hole’s shadow. The Chandra X-ray Telescope released an accompanying image of M87 that isn’t quite as dramatic, bur provides some important context for the black hole’s behaviour.
Most importantly, in the inset you can see a bright line blasting from the plus symbol. This is a jet of high-energy stuff emitted by the black hole that extends for a thousand light-years. Scientists study this galaxy’s X-rays in order to better understand the environment around M87's black hole, as well as how and why its jets form.
Image: X-ray: NASA/CXC/SAO/J. Drake et al; H-alpha: Univ. of Hertfordshire/INT/IPHAS; Infrared: NASA/JPL-Caltech/Spitzer
Chandra also released some new images to celebrate its 20th birthday, and though most Chandra images are cool, I liked this one especially because it looks like an abstract painting, though as I explained before, the colours here are mostly arbitrary. The image shows Cygnus OB2 which, according to a 2014 paper, is “the largest concentration of young and massive stars within 2 [kiloparsecs] of the Sun,” where 2 kiloparsecs is about 6,500 light-years. The image shows these young stars emitting high-energy winds, producing shock waves that excite the surrounding stars.