# Astronomy Math

#### liamwpope

I'm having some difficulty with my astronomy homework. I've submitted several answers using different formulas but none of them are correct.

1. What is the luminosity of a star having 3 times the radius of the Sun and a surface temperature of 10,000 Kelvin?

2. A certain star has a temperature twice that of the Sun and a luminosity 55 times greater than the solar value. What is its radius, in solar units?

As always, thanks for the help!

#### Debsta

MHF Helper
Do you have a "formula" for luminosity?

• topsquark

#### liamwpope

Do you have a "formula" for luminosity?
Here is the formula provided by my professor.
Luminosity = (sigma x T^4) x (4 x pi x r^2)
T being temperature and r being radius.

#### Debsta

MHF Helper
Is T measures in Kelvin?
and what's sigma?
and what's the radius of the sun?
and what's the temperature of the sun?

#### Debsta

MHF Helper

• topsquark

#### Debsta

MHF Helper
Ok, so for Q1, what is the radius of the star in question? What is the temperature?
So where are you stuck?

• topsquark

#### liamwpope

Ok, so for Q1, what is the radius of the star in question? What is the temperature?
So where are you stuck?
Debsta,
Thank you for all your help! I solved it. The solution is 79.5 solar luminosities.

• Debsta

#### Debsta

MHF Helper
Ok, are you ok with Q2?

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