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## A supernova in Messier 82

A few night ago, on the 21st of January, at 19:20 Universal Time (UT), a supernova was discovered in the relatively nearby galaxy Messier 82. This is one of the closest supernovae to explode in the last few decades, and provides an opportunity for people to see it with binoculars and small telescopes.

A “before” and “after” image of Messier 82. The supernova is circled in the image on the right, just to the East (right) of the centre of the galaxy.

From analysis of its spectrum, we have determined that this supernova is what we call a Type Ia supernova. This type of supernova happens when a white dwarf star, the remains of a relatively low-mass star like the Sun, accretes material from a companion. White dwarfs are prevented from collapsing by something called electron degeneracy pressure, but it was calculated in the 1930s by the Indian astrophysicist Subrahmanyan Chandrasekhar that if a white dwarf exceeds a mass of 1.4 times the mass of the Sun it will collapse into a neutron star. This limit is now called the Chandrasekhar limit, and a Type Ia supernova happens when the white dwarf accretes enough mass to exceed this limit. The white dwarf explodes, brightening millions of times in the process.

Type Ia supernovae are produced when a white dwarf exceeds to Chandrasekhar limit and explodes.

If you get the chance over the next few nights, and you live in the Northern Hemisphere, Messier 82 is not difficult to find. You can use the bright stars of The Plough (Big Dipper) and the chart below to find it.

Messier 82 and its companion Messier 81 are to be found near the stars which form the right hand part of the bowl of the Plough (Big Dipper).

Because of the way Type Ia supernova are produced, they all reach a very similar maximum absolute (intrinsic) magnitude of $M_{V} = -19.3$. This has led to their being used as a way of determining the distance of galaxies, because using a relatively simple formula one can determine the galaxy’s distance by observing how bright the supernova’s maximum brightness appears to be, something called its apparent magnitude. The formula is

$m-M = 5log_{10}(d) - 5$

where $M$ is the absolute magnitude, $m$ is the apparent magnitude, and $d$ is the distance expressed in parsecs. This equation is known as the distance modulus equation. The known distance of Messier 82 is $d=3.5 \text{ Mpc}$, so if we plug these numbers into the above equation we find that the predicted maximum apparent magnitude $m$ of this supernova is

$m = -19.3 + 5log(3.5) + 5log(10^{6}) - 5 = +8.4$

This is sadly below the limiting magnitude that can be seen with the naked eye, which is about $m=+6$ in a dark place. But it is well within the capabilities of a small telescope or a pair of binoculars. So, good luck in seeing it, it is not very common to be able to see a supernova through such rudimentary equipment.

### 4 Responses

1. This galaxy has always looked somewhat messy. Now it looks even messier. 😀

• Starburst galaxies do mess up their morphology 😛

2. […] der Galaxie ertrinkt. Ferner ein visueller Bericht, ein Aufruf an Schulen, ein Video, Artikel hier, hier, hier, hier, hier, hier, hier, hier, hier, hier, hier, hier, hier, hier, hier, hier und hier und […]

3. […] know the star is at 2 parsecs, if we measure an angle of then the star is at 10 parsecs. Notice in my blog on the supernova in Messier 82, I quoted the distance to M82 in Mega parsecs (Mpc), and in my blog on the most distant galaxy yet […]