The last hurrah of a star wrenched apart by a supermassive black hole tells astronomers what the stellar crumbs are doing.

I generally think of black holes as friendly. (Yes, I know I’m nuts.) The supermassive ones loll around in galactic centers, sloppily eating gas like big Labrador puppies — but even less efficiently.

star eaten by black hole
This illustration of the tidal disruption ASASSN-14li (top) shows a disk of stellar debris around the black hole. A long tail of ejected stellar debris extends to the right, far from the black hole -- a black hole generally ejects half of the star it shreds. The X-ray spectrum (inset) shows dips in X-ray intensity over a narrow range of wavelengths. These dips are shifted toward bluer wavelengths than expected, and the shift changed over time. That suggests the gas is moving toward us, providing evidence for a wind blowing away from, or debris orbiting, the black hole.
NASA / CXC / M. Weiss

Stars are more likely to have a dour perspective of these spacetime beasts. If a star comes too close to an average supermassive black hole — a couple times the Earth-Sun distance or less — it will be ripped apart in a tidal disruption event (TDE). This gravitational tug of war is an intense version of the process by which the Moon creates tides on Earth.

We’ve caught several of these star shredders in action. In the October 22nd Nature, Jon Miller (University of Michigan) and colleagues report another, ASASSN-14li (named after the All-Sky Automated Survey for Supernovae that detected it), at the center of the galaxy PGC 043234. This one is the closest so far — it has a redshift of a mere 0.02, so the light has only been traveling about 290 million years to reach us. Since it’s so close, the astronomers could see what the star’s taffy-fied gas was doing.

As gas falls in toward the black hole, friction heats it so much that it emits X-rays. ASASSN-14li’s glow was variable, and its glow can only vary as fast as the photons can travel from one side of the emitting object to the other — else the variation would be smeared out. In other words, the variability tells you how big the source object is. ASASSN-14li’s variability indicates that the gas formed an accretion disk right near the event horizon of a black hole “weighing” a couple million solar masses.

There’s also, however, some sort of wind or filamentary debris: the X-ray spectra are blueshifted, meaning the wavelengths are shorter than we’d expect because the glowing stuff is moving toward us. The amount of shifting also changed over time. This gas isn’t moving fast enough to escape the black hole. It could be gas still stuck on the star’s elliptical orbit, swinging out the farthest it can get from the hole. Computer simulations do predict that leftover gas will take a while for its orbit to circularize, and that flows should be filamentary, even while gas much closer to the black hole quickly forms a disk that siphons material in past the event horizon. So these observations match what we’d expect.

You can read more about the study in the press release from the University of Maryland and the one put out jointly by NASA Marshall and the Chandra X-ray mission. Also, read up on TDEs in our June 2013 cover story, “Star-Shredding Black Holes.”


Reference: J. M. Miller et al. “Flows of X-ray gas reveal the disruption of a star by a massive black hole.” Nature. October 22, 2015.


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