Archive for March, 2012

PostHeaderIcon How bright is bright

I just got through doing this month’s university assignment that was amongst other things about magnitude. So here is my whole assignment in half a page.

Magnitude is the measure of how bright a star or other celestial object appears. Fairly easy to understand eh? Well as I found out its not quite that simple.

The problem is that we look at things from the point of view of our little planet and that screws everything up. I mean, the sun is what we call a spectral class G2V star (more about that next week) with an apparent visual magnitude of -26.74. The lower the magnitude the brighter the object appears so -26.74 is very bright. In a big city the highest magnitude object visible to the naked eye is about +4.5 anything else is drowned out by light pollution.

So back to the sun and its enormous magnitude. The problem here is that the sun is a pretty average star of average brightness in a pretty ordinary galaxy (excepting the fact that you and I live in the same galaxy which makes it somewhat special!) so why such a large magnitude?Well the response is because its near to us duh! So to even things out we developed a system of absolute magnitude. This is a measure of brightness based not upon our point of view but on the basis that if every star was viewed from the same distance away then how bright would it appear. Using this other method the magnitude of the sun is about +4.83 which means that in large city on a distant planet on the other side of the milky way galaxy (say about 25,000 Parsecs away [multiply by 3.26 to get light years]) the sun would not even be visible!

But it does not end there. We use something called bolometric magnitude to further refine absolute magnitude by taking account of the effects earth’s atmosphere, effecs of clouds of interstellar dust, gravity and the entire electromagnetic spectrum and not only visible light.

Well that’s enough for now so I say “bolometrics to it all”- I am going to the pub for a beer!

R