Absolute Magnitude Imagine that you can move closer to a star or further from a
ID: 1752630 • Letter: A
Question
Absolute Magnitude
Imagine that you can move closer to a star or further from a starin a very fast spaceship. It is obvious that as you get closer tothe star it will appear brighter and as you move further from thestar it will appear less bright. The absolute magnitude of a staris the magnitude that a star would have if you stopped thespaceship at a distance of 10 pc from the star and measured thebrightness at this point. Remember that the magnitude scale ininverted so that a smaller number corresponds to a brighter star,and also a change in magnitude of 1 corresponds to an intensitychange of about 2.5. These facts can be combined into amathematical expression to give the absolute magnitude,M
,
where m is the apparent magnitude measured on Earth andd is the distance to the star from the Earth in pc. Forexample, if and then the absolute magnitude of thisstar is
.
Notice that this value for M is less than the value ofm. This is because the distance to the star increases asyou go to the standard distance of 10 pc so the brightness of thestar must decrease or m must increase.
The absolute magnitude is related to the actual luminosity of astar. For example, a star having is less luminous than a starhaving .
1. For the following list of stars the parallax, p and themagnitude, m are given. Find the distance to the star andthe absolute magnitude of each star.
Star p d(pc) m M
Arcturus 0.09 -0.05
Procyon 0.29 0.34
Hadar 0.006 0.61
Rigel 0.004 0.18
Sirius 0.378 -1.46
Altair 0.194 0.77
Betelgeuse 0.009 0.58
2. Which star in this list has the most intrinsic luminosity?
3. Which star is least luminous?
Explanation / Answer
the 2 equations you refer to are missing.