Expert: Philip Stahl - 6/5/2008

Question
Hello...
To compute the "brightness" of a star the formula is --
B = L / 4*PI*D^2
"L" I think is bolometric luminosity
"D" is in meters

I've done this equation over and over and over again only to come up with ridiculous values for "B" (brightness) as 4.9^ minus 35

How can such a negative brightness solution happen if, say, some star is 450 light years away?  

Answer
Hello,

I agree what you have here looks whacky. But then there is no context, so it cannot be checked. Is this part of a problem? Is there other information given, e.g.

log (L(*)/ Lo)  for ratio of stellar to solar luminosities for this star at 450 LY

Are other parameters for the star given, e.g. m_v (apparent visual magnitude)?

All I know is you are dealing with trying to find the brightness B for some star (no other data given) at 450 LY.

You say you "think" L is the bolometric luminosity so clearly you don't know.

If you can provide other data, information for the problem we might be able to get somewhere.

As it is posed, it appears to me to be a mystery wrapped in an enigma. The "enigma" created by a dearth of data, supplemental information.

At the very least, we need to know what L is (e.g. in watts),or how defined, and this is hard to do (for the star in question) unless we have *more info* - say like the apparent visual magnitude. (So then the absolute magnitude can be obtained from the distance)

Hope you can fill in the details, then we can go from there!