Black Hole Eats Stripped Star

Distant leviathan peeled star before swallowing half.

A closely-watched flare from a gargantuan black hole in a distant galaxy has revealed to astronomers not only the mass of the black hole that ate the snack but the type of star that met its end as the meal.

Black hole tears star apart
This computer-simulated image shows gas from a tidally shredded star (orange) falling into a black hole (itsy-bitsy greenish dot in upper left, inside the accreting material). Some of the gas also is being ejected at high speeds into space (stream stretching right).
NASA / S. Gezari (JHU) / J. Guillochon (UCSC)
Astronomers have seen other stars ripped apart by supermassive black holes before now. Two separate events reported last year were found thanks to associated relativistic jets, high-powered beams that shot out as the black hole scarfed down the star. But while previous cases have matched observers’ expectations, not one provided enough information to determine what kind of star had died.

The new result, reported online this week in Nature, is “fantastic” in that respect, says Andrew Levan (University of Warwick, England), who worked on both of last year’s discoveries but was not involved with the current study. “The challenge of finding this sort of event is to find it young,” he says. “The relativistic versions had this because they triggered gamma-ray detectors, but this is the first time we’ve found a non-relativistic event early enough to do all the analysis [the study authors] are able to do.”

The team first found the flare in May 2010 in optical wavelengths using the Pan-STARRS1 survey, a wide-field imaging system attached to a 1.8-meter scope on Mount Haleakala in Hawaii. It was discovered independently a month later in ultraviolet with the Galaxy Evolution Explorer (GALEX) satellite. But figuring out just what the flare was took a long time, explains study coauthor Suvi Gezari (Johns Hopkins University). Because the light from the flare was so bright, observers had to wait a whole year for it to fade before they could see the host galaxy. Once the flare did fade, they could determine things such as the galaxy’s distance (2.7 billion light-years) and the chemical composition of the material.

With detailed data in hand, the international team scrutinized the flare’s behavior. The flare’s rise and decay times matched just what are expected of material falling onto a black hole from a disrupted star. Even though the flare didn’t start until the black hole began feeding, the astronomers could backtrack and determine that the star had been torn apart about 76 days before the flare peaked.

But the really cool part is what the leftovers revealed. A black hole tears a star apart via gravitational influence, a souped-up version of the tidal force that the Moon and Earth exert on each other. This tidal process stretches the star out into a pseudo-banana-shape as it nears the black hole, Levan explains. Once the star swings through its closest approach to the central leviathan, the black hole’s gravity overpowers it, ripping the star apart. Due to the nature of the tidal pull on the star, half the star is thrown out, ejected at high velocity. The other half spreads out into a disk of material that then accretes onto the black hole. It's this accreting material that produces the radiation seen as a flare.

Spectroscopic observations of the ejected debris, made with the 6.5-meter MMT in Arizona, showed ionized helium emission but no sign of hydrogen. Normally, stars are mostly hydrogen. But an evolved star called a red giant has fused all its core hydrogen into helium, leaving the outer layers hydrogen. If a red giant lost its outer hydrogen envelope and was reduced to its helium-rich core, it would exactly match the observations.

Losing the hydrogen envelope is probably part of the process of being gobbled, Gezari says. “It’s a consequence of being in the environment of the black hole,” she says. “The stars that actually have orbits that will bring them close enough to be disrupted are going to get stripped on their way in.” It’s kind of like going through an airport: you probably won’t get to the gate without first having to take off all your outer layers in security.

The way the black hole tears the star apart depends on its mass. Pairing the detailed flare behavior with estimates of the stellar core’s mass and radius, the astronomers were able to “weigh” the beast and came up with about 3 million solar masses. That’s in keeping with indirect mass measurements using the galaxy’s properties, which put the black hole between 2 and 8 million solar masses.

There’s one major question left unanswered in this observational knockout. The accreted material is about one-tenth as hot as expected, and its temperature doesn’t decline with time, both of which should happen. “What we’re seeing is more complicated than what you would expect from just a simple accretion disk,” Gezari says. “We don’t know exactly what’s going on.” It could be the radiation is being absorbed and reemitted by intervening material, but at present that’s just a guess.

If you want to see the black hole in action — at least in a simulation — check out the video below.

The computer simulation shows a star being shredded by the gravity of a massive black hole. The areas in white are regions of highest density, with progressively redder colors corresponding to lower-density regions. The elapsed time corresponds to the amount of time it takes for a Sun-like star to be ripped apart by a black hole a million times more massive than the Sun.

Credit: NASA / S. Gezari (JHU) / J. Guillochon (UCSC)

8 thoughts on “Black Hole Eats Stripped Star

  1. Bruce

    Very interesting article Camille. It made me want to "read more about it." Sadly, however, the only link in this piece is to another S@T article? Were’s the links to the papers of the sited research?

  2. Bruce

    Thanks Actualit. Didn’t think I’d have to make such a long distance call to get an answer, glad posting is free. Where did you find the "detailed data in hand?" In other words, name your source, please. Where’d the Bangladesh newspaper find this story?

  3. Anthony Barreiro

    Recently "Actualit" has been posting random comments on S&T stories. The comments are always a paragraph lifted directly from the story, with the phrase "Bangladesh Newspaper" inserted in the middle. This seems to be the work of an errant ‘bot with some bad code. Is there a way to prevent this from happening in the future?

  4. Bruce

    Thanks for the heads up Anthony. No one likes to get snookered. I’ve wondered about the strange "Bangladesh Newspaper" insertions myself. His or it’s comment in this thread sure seemed like a response to my request for more info. But Anthony, have you seen the "Actualit" post starting the current "Asteroid Mining for Fun and Profit" thread? Check that comment against your non-actuality of Actualit hypothesis. And Actualit, your reality is being called into question by a well respected member of the amater astronomical community. If your real, post a comment to prove it. And Sky&Tel, while your building an anti ‘bot filter a simple dubble post filter would be usefull too.

  5. jiminboulder

    “We would have to look at 100 million galaxies every year to see at least one event (like this) a year," according to researcher Andrew Levan, quoted in this article and another about last year’s sighting. What was actually seen was the GRB — everything else is conjecture. No one saw the star or the black hole. Makes for an exciting story, but please.

    May we have some investigative journalism on these subjects? The most accurate statement in the article is by the study’s coauthor, who said: “We don’t know exactly what’s going on.”

  6. Jon Hanford

    Bruce,

    A copy of the Nature paper by Gezari et al is available at the Hubble website: http://hubblesite.org/pubinfo/pdf/2012/18/pdf.pdf

    Jiminboulder, I thought the article by Camille did a pretty good job of laying out the evidence collected as well as the current best interpretation of that evidence. We may not have all the answers concerning this event at this time, but it looks like we have a pretty good idea of what was being observed (check out the paper I mentioned for details, if you’re really interested). BTW, this was NOT a GRB event, which occur in a matter of seconds. This event was observed for over a year.

  7. Bruce

    Jon, thanks for the reference. In it I found what I was looking for: the peak luminosity of this event was calculated to have been 2.2 x 10^44 ergs/sec. Since the Sun shines at 3.839 x 10^33 ergs/sec that works out to a peak of 5.73 x 10^10 L(sun), or 57.3 billion times the sun’s average luminosity. Not bad for a non-supernova flash, I’d say. To Jiminboulder, Mr. Hanson looks to be correct. The researchers seem to have a good handle on what happened 2.7 billion years ago. One other point, while were anthropomorphizing super massive black holes; they’re extremely messy eaters. The simulation looks the SMBH lost it’s lunch, or at least a great chunk of it. Is this “banana-shaped" material destined to fall back into the hole? Was it realy launched to escape velocity as is suggested in the article? Escape velocity would have to be enormous this near the SMBH.

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