# 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.

### 12 thoughts on “A Not-So-Far-Away Type Ia Supernova”

1. Matt – a great article, clearly written as always. I don’t necessarily understand details but it helps to show a landscape – e.g. your prior article on Quantum Fields and String Theory – but I don’t understand at all where you get time to write it all. Do you ever sleep? Are a mythical person like the French mathematician Bourbaki?

2. ” Type Ia supernovas … 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.) ”
Prof. Matt, this quote of yours returned my memory back to the 60-s, when I was a student, and as the story went then ( if I got it correctly ), the fate of the Universe depends on the amount of matter in it.

Three variants are possible:
1) if the amount of matter in our Universe is larger than a certain critical mass, then we live in a “closed Universe” and it will end up with a Big Crunch, possibly followed by the next Big Bang ( oscillating Universe ). BTW, Dr. Penrose still ?! thinks it’s a likely scenario based on his analysis of the relic radiation from the Bing Bang ( he noticed some privileged direction in it )
2) if it’s smaller than a critical mass, the Universe will dissipate and eventually “freeze” to death ( we live in ever expanding “open” Universe )
3) if the amount of matter is exactly equal ( which was considered to be very unlikely, how possible ? ) to a critical mass, then the expansion will be even and the Universe has a flat geometry.
Today, as I understand, the general consensus is that our Universe is !! flat, and the amount of matter in it is known ( I wonder what is it and how we found it out). But doesn’t it contradict to both evenly expanding and accelerating Universes ??
I am sure my lines sound to you as a childish bla-bla-bla, but it bothers me anyway.

• The expansion of the universe is always changing. Traditionally it was believed it was a maximum when the universe was young and gravity acted to slow it. There were two possibilities, either the expansion stopped and reversed or it didn’t. (A ‘flat’ universe was a special case where the expansion only just kept going but very nearly stopped.)

This would depend on the density, not total amount, of matter in the universe. (Twice as much matter in twice the volume has the same effect.) This can be estimated by simply looking at how much stuff is out there. (Along with gravitational observations that allow us to calculate the amount of stuff that we can’t see, dark matter.)

We can also check the geometry of space in essence by drawing large triangles on it; if space is curved one way or the other then the angles in a triangle whose corners are the earth and two distant points will be less or greater than 180 degrees (In the same way that they are when drawn on the surface of a ball. This allowed us to state that the universe was quite flat as (to my knowledge) we have yet to see such discrepancies.

Of course while the universe is flat today that need not remain the case in the future, a collapsing universe becomes more and more curved in on itself for example and one with ‘dark energy’ also becomes more curved (in the opposite manner). This gives us a whole new scenario the ‘big rip’

We live in an interesting period in our universe’s history, were we around earlier we would not have been able to discern dark energy and would assume that our universe would keep expanding, but ever slower, for eternity.

3. Not-so-long-ago?!? 🙂
12 million years but 0.1% of the age of the universe.
Oh… the mirage of the Hubble constant!
Back to business: … to the Poisson’s equation))

4. Professor Strassler,

You mentioned the single-degenerate ‘path’ to a Type Ia. However, the astronomers have tried to find the surviving star after the supernova explosions without much success. They now estimate that this scenario occurs in only 1-in-5 Type Ia events!

Recent research suggests that the more heavily trodden path to Type Ia is the merger of two white dwarves. This puts into question the assumption that Type Ia is a “standard candle”, since the amount of CO to be burned is no longer very close to the Chandrasekhar limit, but could anywhere from 1 to 2x the mass limit.

How certain are we, then, that the universe is undergoing accelerated expansion if Type Ia events aren’t the standard candles we thought them to be?

Interesting times in astronomy too!

Mark

• Good point, Mark.
I´d like to add that there are also indications that some Type Ia supernovae, such as seen in the remnants G1.9+0.3, seem to explode in an asymmetrical manner. (I saw a citation of an article of Borkowski et al., but I am not sure if it has been published so far.) This also adds to some variability to the brightness of the explosion, and means that the observed brightness depends on the perspective of the observer.
Of course, it does not mean that these events are useless, only that the variance in the processes, and therefore the “error bars” for deriving a distance from the observed brightness, might be larger than previously expected.

• Addendum: The article I mentioned has been published, the reference is:
Borkowski et al., SUPERNOVA EJECTA IN THE YOUNGEST GALACTIC SUPERNOVA REMNANT G1.9+0.3, The Astrophysical Journal Letters, 771:L9 (6pp), 2013 July 1, doi:10.1088/2041-8205/771/1/L9.
Abstract at http://iopscience.iop.org/2041-8205/771/1/L9/article

5. Mark – most cosmological parameter studies now show results for microwave background + supernova data, and separately microwave background with baryon acoustic oscillations. e.g. the Planck collaboration http://arxiv.org/abs/1303.5076 (section 6.5). The evidence for acceleration is strong even if you ignore the SN entirely.

• Interesting.
In section 6.5 they conclude that the dark energy is nothing but a cosmological constant
I would also quote section 3.2 where they note that even small changes in model assumptions can change the Hubble constant noticeably.

🙁 oh… the heavy use of fortran