Absolute Magnitudes
Published on Jun 14, 2007 at 12:09 am.
3 Comments.
Filed under astronomy.
A few of days ago, I posted about magnitudes in astronomy. Magnitudes are a measure of how bright a star appears in the sky. But, it doesn’t really explain how bright stars really are. For many years, astronomers simply assumed that all stars are nearly the same brightness. But, by the Nineteenth Century, it became readily apparent to most that such an assumption was probably not valid. By the early Twentieth Century, astronomers Ejnar Hertzsprung and Henry Norris Russell had independently of one another learned that stars can have vast differences in luminosity.
The luminosity of a star is given by the amount of light energy that the star emits per second. But, this is a different measure of stellar brightness than the magnitude system. For various reasons, it would be useful to have a magnitude number that would be related to the actual luminosity of a star. Well, such a system exists. It is called the absolute magnitude of the star. The absolute magnitude is a theoretical magnitude that a star would have if it were located at a set distance of ten parsecs. If the apparent magnitude of a star, m, and its distance in parsecs, d, are known, then the absolute magnitude of the star can be computed using the formula:
The Sun is so bright because it is nearby. It blazes away in the sky at a staggering -26.7 magnitude. However, it has an absolute magnitude of 4.85. That is just a few times brighter than the dimmest star visible to the naked eye. Sirius, the brightest star in the night time sky has a visual magnitude of -1.44, but an absolute magnitude of +1.45. So, Sirius is a much brighter star, intrinsically, than the Sun. Deneb, the brightest star in the constellation Cygnus, has a visual magnitude of 1.25, quite respectable. But, Deneb’s absolute magnitude is a whopping -7.5! That means that at a distance of ten parsecs, Deneb would be brighter than anything in the sky from Earth other than the Sun and Moon!
So, what is the parsec distance measure that I am talking about? A parsec is defined to be the distance at which a star would have a parallax of one arcsecond. An arcsecond is 1/3600 of a degree. The parallax is a measure of the apparent shift of the star due to Earth’s motion around the Sun. The distance in parsecs can be determined by using the formula
where the term Ï€ is the parallax angle measured in arcseconds (not the value 3.14, even though that has the same symbol). There is a whole story about why we measure distances in parsecs. But, I’ll save that for another day.
-Astroprof
A Ler…-- Rastos de Luz on June 15, 2007 at 9:04 am: 1
[…] “Absolute Magnitudes“, no Astroprof’s Page. Este artigo é a continuação do anterior sobre magnitudes; […]
Lab Lemming on June 16, 2007 at 8:43 am: 2
Do parsec measurements take our orbital eccentricity into account, or is it negligible compared to other sources of error?
Astroprof on June 16, 2007 at 11:43 pm: 3
Parsec measurements are based upon 1 AU, which is defined as the semimajor axis of Earth’s orbit. In the early days, I think that perhaps eccentricity was a minor error, but it can be corrected for.