Supernova Sn2011b and Galaxy Ngc2655

[Hypernova]

Looks like tonight as well as the next few nights are going to be clear!!! I must get some more observation of this SN done before the clouds return and it starts to fade.

[michael.h.f.wilkinson]

The absolute magnitude of a type 1a supernova is -19.3, using a brightest value of 12.7 (for ease of calculation, 12.8 and 12.5 have been reported) we have a magnitude difference of 32. The ratio in brightness is 100^(32/5) = 6.31 x 10^12. Because light falls off quadratically with distance, and the absolute magnitude is the apparent magnitude at 10 parsecs, the distance comes to sqrt(6.31 x 10^12)x 10 pc = 25.1 Mpc = 81 million light years. However, this calculation does not take extinction due to interstellar dust into account. We are in the right ballpark.

[DarkerSky]

The absolute magnitude of a type 1a supernova is -19.3, using a brightest value of 12.7 (for ease of calculation, 12.8 and 12.5 have been reported) we have a magnitude difference of 32. The ratio in brightness is 100^(32/5) = 6.31 x 10^12. Because light falls off quadratically with distance, and the absolute magnitude is the apparent magnitude at 10 parsecs, the distance comes to sqrt(6.31 x 10^12)x 10 pc = 25.1 Mpc = 81 million light years. However, this calculation does not take extinction due to interstellar dust into account. We are in the right ballpark.

This is awesome!! Will be interesting to see what peak brightness we reach before it fades.

[mdstuart]

Thank you for that. Interesting calculation.

Mark

[michael.h.f.wilkinson]

I got it!! With some perseverance I star-hopped to the right spot (always awkward with an equatorial mount so close to the pole). Even without the UHC filter I spotted the galaxy easily. The supernova was tricky, but with averted vision it was distinctly there.

I confirmed it using Mick's sketch. Thanks for that!

[mdstuart]

Well done Michael. Its a nice galaxy to see at any time but the little point of light makes it so special at the moment.

Mark

[michael.h.f.wilkinson]

It is a very nice galaxy which I had not seen before. I had not realized the surface brightness was so high.

[michael.h.f.wilkinson]

I redid my distance calculations for a peak magnitude of 12.5, which is 0.2 mag brighter, which means the galaxy is about 9% closer by, yielding an estimated distance of 73.9 million light years.

[mdstuart]

Out if interest how do they work out the absolute magnitude of -19.5? Let me guess someone compared the brightness of SN against distance using red shift and came up with an average. Then this is used to check the distance using red shift...but that would be a circular argument?

Mark

[michael.h.f.wilkinson]

Not quite: you observe supernova brightness in galaxies in which Cepheid variables have been observed. These in turn have been observed in our galaxy, and we know we can work out absolute magnitude of these from the period.

Ultimately, most distance measures stem from measuring the distance to the Hyades. All these stars formed together, and travel through space in more-or-less parallel orbits. By observing the proper motion, we can estimate the vanishing point, which gives us the orientation of their motion vector. We then measure the radial component of their velocity in km/s from redshift, and combined with the true direction of the motion it gives us the velocity in km/s of the RA and Dec components of their proper motion. Because we have already measured the proper motion in arc seconds, we can work out the distance. This calibrates the Hertzsprung Russel diagram in absolute magnitude terms. Thus, given a spectral type and visual magnitude, we can work out the distance.

Distance measurement in astronomy is a long chain of reasoning, so it is difficult to be accurate. Assuming just a 0.2 magnitude error in the brightness estimate, my estimate for the distance to SN2011b should be given as 73.9 + 9 million light years.