Hey guys, Im really hoping somebody can help me out with a question. I have an astronomy test this friday and there are going to be several questions which require finding the distance and radii of stars, I worked on this all day yesterday and I just cant seem to figure it out. Ok here is the info that I have: star A, spectral type M0, apparant magnitude 8.0, absolute magnitude 13.0, surface temp. 3000K, luminosity 1/6400. How can I get the distance with this info using this equation, d=1/P, (p = parsec). Is this not the right equation, and if it is how can I find the distance if this info does not include that parsec value????
OMG hope somebody can help me I Just dont get it!!!! thanks

From the Book Stars by Kaler.

Magnitude= (apparenet mag)+5-5*log d
Where d is distance in parsecs..

13=8+5-5*log d
solve for D
13-8-5=-5*log d

0=-5 log d.. (how can this be?, A special
situation if D=1 parsec..log d=0
-5*0= zero...it's ok)

d=1 parsec away.

hope this helps

hey thanks for trying but this is so confusing that I need specific step by step instructions, I realize I can ask my prof. tomorrow but I really need to be working on some problems to get use to doing this equation. Thanks so much for the help

The equation you have will give you the distance, but not with the information you have.

You do have an error in your interpretation of the equation, though. d is distance (in parsecs), and p is parallax (in arc seconds). I won't go into that more because it's not what you're looking for.



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Peteeo gave the right procedure for calculating using the information you have, and got the right answer. I'll see if I can give you a walk-through.

The apparent magnitude (m) is a measure of how bright a star looks from Earth. The absolute magnitude (M) is a measure of how bright the star actually is, and is equal to what the apparent magnitude would be if the star were 10 parsecs away. Those are the only values you need to calculate the distance. All the rest is either their to confuse you or trhere for other parts of the question.

The equation relating the two is (as Peteeo said)

m - M = -5 + 5log₁₀(d)

Plugging in your values

8 - 13 = -5 + 5log₁₀(d)

Rearranging

8 - 13 + 5 = 5log₁₀(d)
(8 -13 + 5)/5 = log₁₀(d)

simplifying

0 = log₁₀(d)

taking the anti-log of both sides (which is the same to raising 10 to whatever is on each side)

10⁰ = d

simplifying

1 = d

Since this equation gives an answer in units of parsecs, the distance is 1 parsec.

This message has been edited. Last edited by: methos,

OK I think Ive got it now. I DID mean to put parallax, its just so confusing to someone like me who can barely add 2+2 OY. Thank you both so much for helping I will let you know how I do on the test