Tag Archives: supernova

A Not-So-Far-Away Type Ia Supernova

As many of you have already read, there is a supernova that has gone off in a relatively nearby galaxy, and with a rather small telescope, you can see it.  And if you can find the host galaxy, M82 [often called the “cigar”, not because it is really shaped like a cigar but because it looks like one from the angle at which we see it], you can’t miss the supernova.  Like most supernovas, it’s as bright as the entire galaxy that it’s sitting in.  It will probably get a bit brighter for the next week before gradually dimming.

This supernova is of Type Ia. (There was a similar one, just a little further away, two years ago, in the galaxy M101.) This is not to be confused with a Type II supernova, in which the core of a big star, at the end of its life, runs out of fuel, collapses and explodes.  In a Type 1a, two stars, one a white dwarf (a very old star which has run out of fuel and ceased to burn, but not big enough to collapse and explode), the other a red giant (a bit younger but also old, cool and large), orbit one another.  Over time the white dwarf accumulates material from the red giant, and eventually the temperature and pressure on the white dwarf reach a critical point that causes a nuclear explosion, destroying the star in an explosion we can see well across the universe.  Or so the story goes; it’s a very plausible story, but there are details still needing clarification.

Importantly, Type Ia supernovas are quite regular (though precisely how regular is under study, and I’m sure this one will provide us with more information how about these objects work) and can therefore be used to figure out, on average, roughly how far away a host galaxy is.  This information was critical in the discovery that the universe’s expansion is accelerating rather than slowing down, i.e. in the definitive discovery of “dark `energy’ ”, also known as the cosmological constant (if it’s really in fact constant.)

M82 is about 12 million light years away, so that’s how long ago this supernova exploded; the light’s been traveling out from M82, in all directions, for 12 million years, and just reached Earth this month.  For scale, that’s about 0.1% of the age of the universe.  And it also means that this supernova is about 70 times further away than was Supernova 1987a, the bright one visible with the naked eye in the Large Magellanic Cloud (one of the satellite galaxies of our own galaxy, the Milky Way.)

A nice post which tells you more about the discovery and where to find M82 in the sky (it’s not far from the Big Dipper) can be found here.  While you’re looking, check out M81 too; no supernova there, but it’s a notable and photogenic galaxy right next to M82.

Neutrinos From That Recent Gamma-Ray Burst?

[NOTE ADDED: A reader forwarded a message that IceCube did not see any neutrinos with energies above 1 TeV = 1000 GeV from this GRB. Maybe this is not quite the final word (there would still be sensitivity, with some effort, to neutrinos in the 100 GeV – 1000 GeV range) but clearly the neutrino signal isn’t striking, and it is probably not there at all.  But as I’ve suggested below, even a non-observation might have significant implications for the science; the question is, how many neutrinos would the standard speculations about how GRB’s work have led you to expect at IceCube?  If a reader can provide that info, I’d appreciate that.]

The very recent report of a powerful and long-lived gamma-ray burst (GRB), and questions and remarks by my readers (thank you!), have motivated me, both as a scientist and a blogger, to try to understand whether we should have observed neutrinos from this GRB. This is forcing me to catch up on the related subjects of GRB’s, searches for high-energy neutrinos, and the highest-energy cosmic rays. I’m certainly not caught up yet; there are decades of research out there, and I’m quite far behind on developments over the past three or four years. But here are some of the basics that I believe I understand. Still, be cautious with the content of this post, both because I’m not an expert and because this is a very active area of research in which some fraction of the more speculative stuff will surely turn out to be wrong.   I will try to refine this post with a more detailed and corrected article sometime later, perhaps once we know whether neutrinos from this GRB were or were not observed.

GRBs that last more than a few seconds are widely believed to be associated with an exceptional form of Type II (or “core-collapse”) supernova, though this is not known for certain. In these types of GRBs, there are (at least) two sources of photons (everything from gamma-rays to visible light to radio waves) and two sources of neutrinos. It is important not to confuse the different sources! Continue reading

Big Bright Burst

This is hot off the stellar press: as NASA announced today (with cool pictures), a brilliant, long, and rather nearby GRB, or “gamma-ray burster”, was observed on April 27th, initially by the Fermi and Swift satellites.  Gamma-rays are just an old name for photons (i.e. particles of light) which have lots more energy (per photon) than the photons of visible light.   And a GRB is a distant astronomical explosion that produces an enormously bright flare of these high-energy photons, typically for a short time (seconds or minutes), though this one lasted for hours.   It is believed that a narrow jet of high-energy particles produced in a supernova (a powerful explosion of a star) is behind these flares, but they are still poorly understood and are under active study.

Everything about last week’s GRB is on the exceptional side.  The most energetic photon detected had somewhat more energy than the photons produced in the decays of Higgs particles, a bit less than the energy of the photons that Fermi might be seeing from dark matter, and more than three times more energy than any GRB photon previously detected by Fermi. Its gamma rays were produced for many hours, setting another record.  It lasted so long that several other types of telescopes were able to observe it, including those that look at visible light (it was even seen by an amateur astronomer), and those that look at radio waves (which are made from photons with vastly lower energy).  And it was relatively close… well, relatively compared to most GRB’s.  It occured in a galaxy 3.6 billion light years away.  Now that is still a good fraction of the distance across the visible part of our universe, but still, it puts this GRB in the top 5% as far as proximity to Earth.

With such a vast amount of data to work with, it seems very likely that astronomers will learn qualitatively new things about GRBs by studying this blast.  In astronomy, it sometimes takes just one spectacular event to change the scientific landscape!  The next phase of the process will involve directly detecting the lesser (but still intense) glow from the (presumed) supernova that produced the GRB flare.  Stay tuned!  It should be a matter of a week or so…