# Homework Help: Finding an absolute magnitude

1. May 24, 2015

### shanepitts

1. The problem statement, all variables and given/known data

2. Relevant equations
m1-m2[/SB]=2.5log(ι21)

m-M=2.5log (d/10)2

3. The attempt at a solution

Not sure if my approach and answers are correct

2. May 24, 2015

### nrqed

First question: In your very first line with an equation, you changed the factor of 2.5 to a factor of 5 in front of the log. Why did you do this? This seems to be a mistake.

3. May 24, 2015

### shanepitts

I forgot the exponential: m-M=2.5log(d/10)2

4. May 24, 2015

### nrqed

AH yes, Ok.

EDIT: you seem to have made a sign mistake. In the exponential for the calculation of the luminosity, you should have

M_1 - M_2 = M_1 - ( m +1.99) = 5 -m - 1.99

Then your work looks good. You just need to plug in the value of m=2. The absolute magnitude of the star is smaller than the Sun's absolute magnitude (3.99 versus 5) so the star has a larger luminosity than the Sun's and your final expression agrees with this. All the steps look good.

5. May 24, 2015

### shanepitts

Thanks a bunch and sorry for the typo

6. May 24, 2015

### nrqed

You are welcome. And no problem about the typo, I make typos all the time :-)

Patrick

Last edited: May 24, 2015