Friday, April 08, 2011

New discoveries on odd stellar explosions

Image Credit: NASA/Swift/Penn State/J. Kennea
Over the last week and a half, there have been a couple of news releases about stellar explosions.  In the first story, astronomers have spotted a puzzling blast of gamma ray and X-ray emission that could be a star being ripped apart by a massive black hole.  In the other story, astronomers have made substantial progress in understanding the brightest supernovae ever observed.

First, the puzzling gamma rays.  For decades, astronomers have seen sudden, short bursts of gamma rays coming from all over the sky.  About ten years ago, after a lot of hard work (and a little luck) by many different researchers, most astronomers came to believe that many of these "gamma-ray bursts" are the birthing cry of new black holes formed at the centers of massive, exploding stars. 

Several space missions have been studying these gamma-ray bursts, including the currently-operating Swift satellite.  These satellites automatically detect the few-second long burst of gamma rays, locate where in the sky they are coming from, and send emails and instant messages to astronomers around the globe alerting them to the event.  Especially interesting events can get rapid observations from large telescopes and major satellites such as the Chandra X-ray Observatory and the Hubble Space Telescope.

On March 28, the Swift satellite detected a burst of gamma rays in the direction of the constellation Draco.  Since gamma-ray bursts are seen every few days, this burst started the normal response.  Automated messages went out, a team analyzed the data and put out some standard preliminary analysis.  But just 43 minutes later, Swift detected another burst at exactly the same place.  This is very rare, though not unheard of - but it is rare enough that additional resources started swinging into action.  Over the next few days, many additional bursts of both gamma rays and X-rays were seen coming from the same object. 

Finally, data from the Chandra X-ray Observatory and the Hubble Space Telescope came in.  The source of the gamma rays and X-rays lies very close to the center of an otherwise normal-looking galaxy.  In fact, as far as astronomers can tell, the source lies directly in the center of that galaxy.

This discovery, that the weird source lies at the center of a galaxy, casts suspicion squarely on the type of object that lives in the center of most galaxies: a supermassive black hole.  Now unlike what many people think, a black hole is not some sort of cosmic vacuum cleaner, sucking in everything around it.  A black hole can only eat anything that wanders too close. 

How close is too close?  The diameter of a black hole can be found by multiplying its mass (in terms of the sun's mass) by 3.7 miles.  So, if the sun were to collapse into a black hole, the black hole would be 3.7 miles across.  Typical black holes that form from dying stars are about 10 times the mass of the sun, and so are a few dozen miles in diameter.  The black hole at the center of our Milky Way galaxy is about 4 million times the mass of the sun, and so it is about 15 million miles in diameter.

The really weird stuff that happens around black holes due to Einstein's general relativity (time slowing way down, space highly distorted, light being highly bent, and unfortunate space explorers being turned into spaghetti) only happens when you get closer than a few times this distance.  So, if the sun were to be magically transformed into a black hole, really weird things would only happen if you happened to get within a dozen miles or so of the black hole.  The Earth, 93 million miles away, would be unharmed.

The black holes at the centers of galaxies are much larger, but compared to the distances between stars, they are still tiny.  The black hole at the center of the Milky Way has many stars orbiting it, including one star that gets within 10 billion miles (about three times the average Sun-Pluto distance) every 16 years.  That star passed by the black hole in 2002 with no ill effects.

Still, if a star were to somehow wander within a hundred million miles or so of a supermassive black hole in the center of a galaxy, it would get ripped to shreds.  This shredding would release a lot of energy in the form of gamma rays and X-rays.  A press release from NASA suggests that this is precisely what caused the multiple gamma ray bursts from the otherwise normal galaxy in Draco last week.

This explanation makes sense, but it's important to emphasize that it is just a hypothesis right now.  More data continues to come in, and as news of the discovery spreads, more astronomers will begin to compare these data to simulations of what happens when a star is shredded by a black hole.  Perhaps they will agree, and perhaps they won't.  Time will tell.

This leads us to the second story, which was announced this week by McDonald Observatory.  This story is based on a journal article that has been published in the Astrophysical Journal, one of the main astronomy journals, so the science has already passed significant vetting by peer reviewers.  It doesn't mean the science is absolutely, positively right, but it does mean the science has met some substantial level of quality control.

About four years ago, astronomers announced the discovery of what was then the most energetic supernova ever detected.  The initial discovery was made by Robert Quimby, then a graduate student at the University of Texas in Austin, and now a postdoctoral researcher at Caltech. 

Many people initially speculated that this supernovae, and a few others like it, was a new kind of exploding star.  Some models of really massive stars suggest that, as the star ages, it becomes unstable, manages to create large amounts of antimatter, and rips itself apart in the ensuing explosion, called a pair instability supernova.

However, new studies by Emmanouil "Manos" Chatzopoulos, a graduate student at the University of Texas at Austin, and his advisor, Dr. Craig Wheeler, seem to show that these very luminous explosions are not a pair instability supernova.  The stars are, alas, not being torn asunder by the explosive mixture of matter and antimatter.   Instead, the evidence suggests that these are normal supernova explosions, but as the blast wave from the star travels outwards at high speeds, it rams into shells of matter thrown off by the star decades or centuries before the supernova. This violent collision releases tremendous amounts of energy in the form of visible light, and makes the supernova appear much more luminous than it otherwise would.

These shells of matter are known to exist around a type of star called a Luminous Blue Variable (LBV).  These stars sometimes shed huge amounts of material into space via dramatic eruptions from the surface of the star.  In our own Milky Way, the LBV Eta Carinae had just such an eruption back in the 1840s.  The Hubble Space Telescope has taken amazing images of the material blown off the star during that eruption:

Eta Carina and its lobes of material erupted in 1840
Image Credit: Jon Morse and NASA

If Eta Carina were to explode as a supernova now (and it almost certainly will explode within the next million years), the blast wave from the supernova would smash into those large lobes of material, brightening in a very similar way to the very luminous supernovae Manos has been studying.

So, it looks as if Manos's work may have changed the explanation of  these ultra-bright supernovae from some exciting and exotic antimatter-driven explosion mechanism to a slightly more mundane "giant outer space train wreck" explanation.    But this is so often how science works, and how it should work: explanations for observed phenomena must be tested, re-tested, and then scrutinized some more.  Only then can we be reasonably sure we understand what is happening in the depths of space.

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