The brightest supernova, ever.
Published on May 7, 2007 at 2:46 pm.
15 Comments.
Filed under supernova.
At a NASA press conference today, astronomers released some information about the most luminous supernova ever seen. This supernova, SN 2006gy, was not the brightest in the sky, but rather it shone with the most light. The brightest appearing supernovae are the ones in our own galaxy. This supernova occurred in the galaxy NGC 1260, almost 250,000,000 light years away. This supernova was studied using the Chandra X-ray satellite.
Supernova SN 2006gy was shockingly about 100 times brighter than the brightest type of supernova previously known, Type Ia supernovae. A supernova this bright can not result from the normal models of supernovae. In fact, this may be the first recorded example of a type of supernova predicted by theorists, but never actually seen before.
Typically, there are two types of supernovae. In one type, the core of a massive star gets too big to support itself, and it collapses into a neutron star, or even a black hole. The energy released then blows the rest of the star apart. Another type of supernova occurs when a white dwarf gets too large (larger than about 1.4 solar masses) and collapses. In that case, the white dwarf goes into a frenzy of nuclear fusion that releases so much energy that the white dwarf itself explodes, and nothing is left but a cloud of debris. This second type of supernova in much brighter than that from a massive star collapsing.
But, theorists have held that there may be another type of supernova. A truly massive star has a core that is fusing furiously. As the star goes through the stages of nuclear fusion, it needs to fuse at higher and higher temperatures to keep up with the press of gravity. The photons produced in this fusion help support the star. But, if the temperature gets high enough, these photons can actually have enough energy to produce pairs of particles and antiparticles. Matter and energy are interchangeable, by Einstein’s famous equation: E = mc2 . But, if the photons are making particles and antiparticles, they are not supporting the star, and the core begins to collapse. Theorists have predicted that such a collapse would do something very similar to what happens in a Type Ia supernova. The core would undergo a massive burst of fusion, and it would completely detonate and explode outward, leaving behind no neutron star or black hole. In essence, the entire star would just explode, leaving only a big cloud of gas. Such an explosion would release far more energy than a typical supernova.
But, in order for one of these hypernovae to occur, you need a really massive star. Such as star would need to be over 100 times the mass of the Sun. Such stars are exceedingly rare. Only a few exist in our galaxy, the nearest of which is the star Eta Carinae, 7500 light years away.
For a typical supernova, some of the light is generated by simply the heat from the explosion, and some of the light is generated by heat released from radioactive decay in the expanding cloud of gas. In the runaway fusion processes going on inside the star during the explosion, pretty much any imaginable isotopes of all possible elements are produced. Many of these are radioactive, and their decay adds to the light of the supernova, often causing the supernova to peak in luminosity some time after the initial explosion.
SN 2006gy was discovered in September 2006. At discovery, it was already as bright as a Type Ia supernova at its peak. But, instead of getting dimmer, SN 2006gy continued to get brighter for several weeks. The peak brightness seldom comes much more than a week after the explosion. Theoretical models suggest that SN 2006gy gets its light from both the expanding cloud of gas and a shock front as the cloud of gas expands into very dense gasses surrounding the progenitor star. But, the expanding gas cloud is so bright that it requires substantially more radioactive decay to heat it that would be present in almost any other supernova. The best way to get that much radioactive material, according to the model that the theorists have come up with, is for basically the whole core to be thrown out into the supernova, leaving little or nothing behind to form a neutron star or black hole. So much material is thrown out, that the supernova continues to be heated long after the explosion itself. In fact, even months later, SN 2006gy has faded in brightness only to as bright as the peak brightness of a Type Ia supernova!
Now, all of this is very exciting. We may have the first observations of something that has been predicted by theory. But, it gets even more interesting. It takes a star of 100 to 150 solar masses to yield an explosion like this. But, we think that the very first stars that formed in the universe, what we call Population III stars, probably exploded in supernovae of this same type. If so, then this may be where many of the metals came from that give rise to planetary systems.
But, there is something else even more interesting about this supernova. The Chandra observations suggest a truly massive star that has shed massive amounts of material shortly before the supernova explosion. It also suggests a star that has not yet shed its entire outer hydrogen envelope. Theorists have been divided on whether these really massive stars shed all of their outer layers before the core collapses. This star apparently did not. But, this description also fits the star Eta Carinae. Eta Carinae has shed material in massive explosions, the latest of which was in the middle of the 19th Century, when Eta Carinae temporarily became of of the brightest stars in the sky. That’s quite a feat, considering that it is 7500 light years away! Eta Carinae’s mass is estimated at about 120 solar masses. It still has a hydrogen envelope. Could it also go supernova in a massive hypernova type event like SN 2006gy? The team of astronomers investigating SN 2006gy think this likely. If so, when would that happen? Well, it could happen just about any time: today, tomorrow, next year, in a hundred years, or in a thousand years.
So, what effect would a supernova of this type have on Earth if Eta Carinae were to blow up in the same manner. Well, we are comfortably far enough away that the blast would not affect us. Such an explosion may produce highly collimated beams of hard gamma rays extending outward from the exploding star. Should those beams hit Earth, then we would all be dead soon afterwards. But, those beams are very highly collimated, and they would likely extend outward along the same axis as the expanding lobes of gas in the photo of Eta Carinae above. Clearly, those lobes are not pointed at Earth. Even if the beams came out perpendicular to the lobes (about the next likely direction) then they’d also miss Earth. It would be highly unlikely that they’d be aimed right at us. But, a monster supernova of Eta Carinae would be very bright. In fact, you’d be able to see it in the daytime (if you were in the right part of the Earth, that is). Also, the supernova would outshine the Moon at night. And, it would stay bright for months, and perhaps even be bright enough to read by at night for a year or longer.
And that could happen at any time, if the theorists are right.
-Astroprof
Images courtesy of NASA
Dave Ross on May 8, 2007 at 10:04 am: 1
If we see Eta Carinae’s explosion next year, wouldn’t it actually have exploded 7,499 years ago? Is it standard convention for scientists to refer to distant stellar events as contemporary?
Astroprof on May 8, 2007 at 10:34 am: 2
You raise a good point, and this is something that tends to confuse my students. Simultaneity becomes something that you have to think about when dealing with either very fast or very large things (and the galaxy and universe are large things!). It is common practice for astronomers to refer to astronomical events as happening when they are observed, even though we know that they really happened long ago. For example, we talk about Supernova 1987A as happening in 1987, knowing full well that it really happened some 168,000 years ago. Likewise, we talk about the supernova that occured in 1054, knowing that it really happened almost 6300 years before that, and the light only just arrived in the year 1054.
It is easiest to refer to these events as occurring when they are observed, in part because the exact distances, and hence time it took the light to arrive, are not always known. And, since we are stuck on this planet, the soonest that any distant event can interact with us is when the light arrives. So, we might as well talk about it happening then.
Matt Schur on May 8, 2007 at 2:42 pm: 3
Will this have any impact on the use of supernovae as standard candles, and specifically on measurements which seem to indicate an accelerating cosmic expansion?
Astroprof on May 8, 2007 at 2:51 pm: 4
The type of supernovae used for standard candles are Type Ia supernovae, and they have a much different spectrum and light curve, so they would not be confused with this type of supernova. Thus, this should not have any effect on our standard candles.
Sapna Isotupa on May 8, 2007 at 3:15 pm: 5
Is there any way of measuring when a certain star would explode with any degree of confidence?
Astroprof on May 8, 2007 at 4:32 pm: 6
No. There is no way to tell for sure when a star will go supernova.
Astronomy Buff - The King of all Supernovae on May 8, 2007 at 9:53 pm: 7
[…] The big news out of NASA today is that the Chandra Observatory recorded the brightest supernova seen so far. AstroProf has a really good post about it here. He points out out that by bright, we don’t mean the brightest in the sky (those are usually within our own galaxy), rather we mean that this supernova was among the most energetic, it gave off the most radiation (light) we’ve ever seen. […]
Astrolink [Global Edition] » The King of all Supernovae | Latest astronomy news in 11 languages on May 8, 2007 at 10:10 pm: 8
[…] The big news out of NASA today is that the Chandra Observatory recorded the brightest supernova seen so far. AstroProf has a really good post about it here. He points out out that by bright, we don’t mean the brightest in the sky (those are usually within our own galaxy), rather we mean that this supernova was among the most energetic, it gave off the most radiation (light) we’ve ever seen. […]
Sapna Isotupa on May 9, 2007 at 8:53 am: 9
What is the difference between a supernova and a hypernova? I have heard some scietists say that Eta carinae will go hypernova rather than supernova. What does that mean?
Also if Eta carinae does go supernova, won’t it be visible only from the southern hemisphere?
Astroprof on May 9, 2007 at 9:54 am: 10
A hypernova is a stupendously big supernova. The term originally was coined by theorists to explain just this sort of super supernova, but the term has come into somewhat more general use for extremely large supernova.
Eta Carinae is so far south that it can not be seen north of 30° north latitude. So, you can see in from the northern hemisphere, but only in the very far southern part of the northern hemisphere. Most of us are left out.
Mike Plaiss on May 11, 2007 at 3:09 pm: 11
I came across your site when I googled “SN 2006gy”. I am seeking a more thorough explanation of the process involved and was hoping you could either provide one or direct me to a site that details the process more extensively.
I am an amateur astronomer (past president of the Louisville Astronomical Society) and am reasonably familiar with the process in which supernovae explode.
SN 2006gy, however, has caught me off-guard. I did not know there were other theoretical types of supernovae. Specifically, I want to know more about the process in which high energy photons create the particle/anti-particle pairs. Why does this happen? And why at a certain energy threshold - in other words, what triggers it?
It makes perfect sense to me that if this process did occur that there would no longer be enough energy to support the star. Thus, I understand the collapse, but I still don’t understand why the core would explode, and I certainly don’t understand why further fusion would ensue. Wouldn’t the core of such a massive star be made of iron? I have always understood that once you get to iron, fusion can no longer occur no matter how much energy is applied. Obviously I’m missing something.
Sorry to have gotten so long winded. Any help you could give me would be greatly appreciated.
Astroprof on May 11, 2007 at 5:43 pm: 12
Fusion does not stop with iron. Rather, fusion past about iron becomes endothermic rather than exothermic, so it can not support the core of a star. But, the core of such a supermassive star might not make it to iron before it goes supernova. It might well trigger early if the core temperature reaches the point that thermal radiation is hot enough to produce electron-positron pairs. The energy threshold is when the energy of the gamma rays reache the energy equivalent to the mass of a positron and electron, a bit over 1.02 MeV. (When electrons and positrons meet, they produce two 0.511 MeV gamma rays).
Wikipedia has an entry that provides a nice not too technical explanation of this type of supernova.
Mike Plaiss on May 11, 2007 at 9:52 pm: 13
Thank you. That helps a lot. It occurred to me while I was waiting on your response that the collapse could occur before the core became iron.
Absolutely fascinating that such a complex process could be predicted first and observed later. It seems astronomers have leared A LOT about how stars form, live and die.
You have a great site. I’m sure I’ll be visiting often. Thanks again.
Peter Koval on October 26, 2007 at 12:12 pm: 14
Can’t a supernova create gold, well isn’t it the only thing that can?
Astroprof on October 26, 2007 at 1:46 pm: 15
Correct. Supernovae can create gold and the rest of the elements beyond iron, and it is about the only thing that does that in nature.