Gamma rays from novae detected
August 12, 2010 7:10 PM   Subscribe

Astronomers using NASA's Fermi Gamma-ray Space Telescope have detected gamma-rays from a nova for the first time, a finding that stunned observers and theorists alike. (via)
posted by Joe Beese (18 comments total) 5 users marked this as a favorite
+1 for the Bruce Banner tag.
posted by Babblesort at 7:15 PM on August 12, 2010

a finding that stunned observers and theorists alike.

Is it something like the ending of The Crying Game?
posted by KokuRyu at 7:26 PM on August 12, 2010 [1 favorite]

"In human terms, this was an immensely powerful eruption, equivalent to about 1,000 times the energy emitted by the sun every year,"

*sets-up lawn chair, puts on sun glasses, applies SPF 365k*
posted by ZenMasterThis at 7:26 PM on August 12, 2010 [1 favorite]

I don't understand why this is such a surprise. The shock wave/magnetic field formation of gamma rays is nothing new. Is it because they thought this would only happen with supernovae? Heck, it happens in fission explosions...

Do we have an astrophysicist in the house?
posted by warbaby at 7:28 PM on August 12, 2010

Warbaby, that's an interesting question, I too wasn't aware that these kinds of events weren't supposed to be able to produce gamma rays. Note that this is not the first time that the V407 Cygni symbiotic binary has produced an outburst (see, e.g, the March 2010 event, also here).
posted by RichardP at 7:58 PM on August 12, 2010 [1 favorite]

[gamma rays happen] in fission explosions

Not 100 MeV gamma rays.
posted by ryanrs at 8:00 PM on August 12, 2010

The reporter doesn't do a very good job of explaining why this is surprising.

Stars start out fusing hydrogen. It fuses and becomes helium, which accumulates in the center of the star. It just sits there, not doing anything, because there's not enough heat and pressure to start it fusing.

If it's a very small star, it just slowly goes out, never having the mass required to ignite its helium. These small stars last a very, VERY long time, possibly hundreds of billions of years.

But if they're about half the mass of the Sun or higher, the helium in the center will eventually be compressed enough by its own gravity to start a new fusion reaction, sort of a star-within-a-star. This new energy pressure causes the outer layer (still fusing hydrogen) to expand a great deal, and cool off a bunch because its density drops. This is called a 'red giant', and it will happen to our sun in a few billion years.

The helium reaction generates carbon, which will in turn eventually ignite, and that makes neon, and on up the chain. You can end up with a bunch of 'nested stars', in a sense, each layer fusing a heavier element than the layer above. Each new layer is smaller and burns faster. That continues until the star gets to iron. Iron is actually energy-consumptive when it fuses, so even though some of it will probably fuse from the temperatures and pressures involved, it would be a self-cooling and self-limiting process.

Now, this is where Wikipedia diverges from my knowledge; I assume the science has improved over the thirty years since I learned all this. Thirty years ago, the explanation was that when the iron accumulated past a certain point, the cooling would put out the layer above it, and the subsequent layers would also stop fusing because they needed the heat input from below them. The star would, over a very short period, simply go out, and collapse inward from gravity.

If the star was small, under 1.4 solar masses, the collapse would force most of the matter in the star into a semi-degenerate state, glowing white-hot. It would continue to glow for a very, very long time. I don't remember, anymore, what the energy source was... I'm not sure if it's normal fusion, or if it's just emitting energy somehow from being compressed.

If the star was big enough, the huge amounts of heat and pressure involved would "re-light" the iron, powering it through its energy-sucking fusion, and up into heavier elements. Those elements would then start their own fusion, causing the whole center of the star to go off like a giant bomb, blowing it to smithereens, and scattering heavy elements all over space. This is why, for instance, the Earth has uranium -- we're living on the remnants of a dead star. That was called a 'nova'.

Supernovas, by that explanation, were just novas from really big stars. And if there was enough dead star left over after the explosion, you could end up with a white dwarf, a quasar, or a black hole.

It appears, however, that the explanation for the end of a star's life has changed somewhat in the last generation. The progressive fusion and shells are still the same as they were, but they seem to be using nova and supernova differently.

According to the Wiki article, novas are what happens when you get a layer of hydrogen or helium on top of a white dwarf... when enough of it accumulates to ignite, it blows off in a big explosion. And that means that novas shouldn't normally be that energetic, and that's why the astronomers are surprised at all the gamma rays from this one. (this post is about a pair of stars -- a red giant outgassing, and a companion white dwarf that's been accumulating its effluent, and just went through a nova.)

Supernovas happen when the iron core in the center of a star exceeds 1.4 solar masses, and can no longer support itself. It doesn't, apparently, put the outer layers out, it collapses and then explodes all by itself. The shockwave blows the rest of the star apart. I assume that this must involve fusion and a huge amount of energy being released, since there needs to be a source for heavy elements, and supernovae can outshine whole galaxies, but Wikipedia barely touches on this at all, or at least not in language I recognize.

Pesky science, always changing. I liked the old novas and supernovas better. More drama.
posted by Malor at 8:42 PM on August 12, 2010 [20 favorites]

As shocking discoveries which stun observers go, this is kind of boring. Even after Malor's comment this strikes me as more of a "moderately surprised" kind of event. Maybe astronomers are easily impressed.

Tomorrow's headline: "Observers and theorists alike shocked and appalled when restaurant serves slightly overcooked steak."
posted by Justinian at 8:56 PM on August 12, 2010

Malor: AIUI, supernovas can be either the accretion-onto-a-dwarf type or the core-collapse type (type I or type II), and novas are always accretion-type (not sure where the nova/supernova distinction comes from there). I guess this is one of those terminological warts in astronomy which come from having observed and named a phenomenon before figuring out its cause, like Population II stars being an earlier population than Population I stars.
posted by hattifattener at 1:14 AM on August 13, 2010

"Novae are thermonuclear explosions on a white dwarf surface fueled by mass accreted from a companion star." [from the abstract]

'shocking' is a pun:

a 'shockwave' of an exploding white dwarf interacts with the red giant from which its accreting mass.

its also 'surprising' to see high energies (above x-ray) from a white dwarf exploding (<>Gamma ray bursts (GRBs) are thought to come from massive (> 100 solar mass) stars collapsing and exploding as Hypernovae---beaming Gamma rays out in two slender beams from the axis of rotation.

for white dwarf V407 Cygni, the Fermi-LAT authors suggest the Gamma radiation is the result of Pion formation and decay. Also, "inverse Compton scattering ... is not ruled out" as the source of gamma rays. [from the abstract].

anyone behind Nature's paywall have any more details?
posted by dongolier at 5:30 AM on August 13, 2010

Gamma ray bursts should have been linked. PBS did a cool NOVA show on the phenomena in 2002 ("Death Stars") although unfortunately the last segment interviews this Chicken Little ranting about the remote possibility that Earth might get blasted by a near GRB and the end-of-the-human-race-so-write-your-congressman, etc....
posted by dongolier at 5:37 AM on August 13, 2010

sorry, i mangled it above:

[surprising to see Gamma rays from] white dwarfs (<>100 solar mass) stars collapsing....
posted by dongolier at 6:16 AM on August 13, 2010

trying a 4th time... forgot the old &ampgt and &amplt trick

[surprising to see Gamma rays from] white dwarfs (&lt 1.4 solar mass). Typically GRBs originate from massive stars (&gt 100 solar mass) stars collapsing....
posted by dongolier at 6:25 AM on August 13, 2010

i give up....
posted by dongolier at 6:26 AM on August 13, 2010

Hmmmm. Since the gamma ray telescope has been in operation for only two years, this would be the sort of thing it was supposed to do - find new stuff not predicted by existing theory.

So they thought it took a much more massive star to trigger this sort of accretionary/intermittent fusion, eh?

There are implications that a lower energy threshold fusion reaction means they haven't got it all mapped out yet. Which might have some sort of long term implication for fusion power if this was a science fiction story.

So the unexpected part was the spectrum of the gamma rays? Didn't expect them to be so hot, or didn't expect them to be possible with a dwarf?
posted by warbaby at 7:27 AM on August 13, 2010

I apologize if the article is overselling this. I'm generally credulous towards science reporting - especially if published on a .org domain.

I assumed that astronomers aren't "stunned" that often. But maybe Justinian is right.
posted by Joe Beese at 8:01 AM on August 13, 2010

Eh, well, maybe the journalist oversold it a bit, but it's still interesting.
posted by Malor at 8:47 AM on August 13, 2010

to the contrary Joe Beese, this is a great post.

i think astrophysicists are used to---as well as unable to answer----"who cares?" type questions---and we are seeing some of this in the thread. but when it comes to Gamma rays, the backstory is also intriguing:

in the 60s the army was concerned that the USSR was testing nukes on the backside of the moon in contravention of an above ground ban so they put a detector into an orbit much higher than typically orbits and outside the Van Allen radiation belt. Within a few years the first GRB is detected and people speculated as to the sources---mostly that they were nearby sources but somehow invisible at longer wavelengths. Subsequent innovation and collaboration across the iron curtain yielded directional estimates of the Gamma ray bursts and eventually in 90s it became clear that GRBs are massive Hypernovae and that they are incredibly far away from us: in 2009 one was observed that was 15 billion light years away, calculated from its red-shift. that is to say this event occurred 670 million years after the big bang and energy from it is just now reaching our detectors.

and that these same detectors have now detected Gamma radiation from small supposedly well-understood white dwarf nova is really cool, another accidental discovery just like GRBs and cosmic background radiation.
posted by dongolier at 2:04 AM on August 14, 2010 [1 favorite]

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