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Black hole swallows star in GALACTIC SUPER-GUZZLE!

Astroid-gazing telescope catches distant star flare-out

By Richard Chirgwin, 3 May 2012

With billions of galaxies to choose from, it shouldn’t be hard to catch, but it is. Astronomers are celebrating after capturing the whole sequence of a star falling into a supermassive black hole.

The event happens roughly once every 10,000 years per galaxy*, but you still have to be looking in the right place at the right time – and as a result, astronomers usually only catch the end of the event.

This time, however, Johns Hopkins University astronomer Suvi Gezari and her colleagues hit the jackpot, garnering enough information from the two billion light year-distant galactic flare to determine both the black hole mass and the type of star it consumed.

Finding mostly helium left behind by the event, Gezari’s paper in Nature (abstract) suggests that the doomed star was a red giant with a helium core. Hydrogen – the other key stellar fuel – is nearly absent from the sample, suggesting that the lighter element was long-ago stripped by the black hole, leaving helium to make up most of the ejected gas.

Before/after Flare (GALEX/Pan-STARRS1)
Source: - the flare-up from 2009 to 2010

Also important to the astronomers is that they had the good fortune to catch both the rising and falling emissions on multiple instruments, which let them gather radio, X-Ray, ultraviolet and spectroscopic data of the event.

Gezari’s group discovered the flare in May 2010, a transient dubbed PS1-10jh, using the visible Pan-STARRS 1 telescope in Hawaii; and at the same time, the NASA’s space-borne Galaxy Evolution Explorer saw a UV flare from the same region.

Pan-STARRS conducts a whole-sky survey multiple times per month – mostly to watch for near-Earth asteroids, but also handy for catching transients like PS1-10jh. The observing the rising emissions were also able to call in spectroscopic data from the Arizona-based Multiple Mirror Telescope (MMT).

“The glowing helium was a tracer for an extraordinarily hot accretion event,” Gezari says in this release.

“So that set off an alarm for us. And, the fact that no hydrogen was found set off a big alarm that this was not typical gas. You can't find gas like that lying around near the center of a galaxy. It's processed gas that has to have come from a stellar core. There's nothing about this event that could be easily explained by any other phenomenon.”

The Chandra X-Ray telescope was also used, to check the characteristics of the gas and make sure it wasn’t a flare-up in the galaxy. The speed of the gas is also a marker: the black hole had accelerated the helium to more than 32 million kilometers per hour.

The flare – and therefore the final death of the star – took around a month, with the glowing gases taking around another year to fade. ®

Bootnote: The “per galaxy” distinction is important. Various news outlets have stumbled on this point, calling the star-black hole approach a “once in 10,000 years” event. ®

Note: Thanks to the commenters who picked me up on an editing error that mis-identified "hydrogen" as a gas left behind, in the original version of this story.

Update: About the image: some commenters have had trouble working out what's going on. The top two images are UV, with more magnification on the right, showing the flare emerging. The bottom two are visual, showing the same sequence.

El Reg doesn't agree that they show different regions of space (although they're in slightly different field of view). What they do show, for example in object in the bottom-left corner, is that something quite faint in the visual spectrum can be quite loud in the UV. ®

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