# help with question (mass-luminosity relation law)Watch

#1
..
0
3 years ago
#2
(Original post by iamspiderman)
Here's the Question:

Sirius A is 2.3 times the mass of the sun and about 20 times as bright. Is this consistent with the mass-luminosity law?

Need help.
Hey again :-)

So this is like the other question with getting you to think about the mass-luminosity relation/law.

As they haven't told you what type of star it is and asked you if it is consistent with the mass-luminosity relation which works for main sequence stars, you can assume its a main sequence star. [fyi Its an Am type star so it is main sequence ]

A stars luminosity is a measure of how 'bright' it is, so they are telling you Sirius A has a luminosity that is about 20 times greater than that of the Suns.

The mass-luminosity equation for main sequence stars is:

(L_star/L_sun) = (M_star/M_sun)^3.5

where M_star=mass of the star, M_sun=mass of the Sun

If you plug the masses they gave you into the equation do you find that Sirius A has a luminosity about 20 times that of the Sun?

Explanation of the mass-luminosity relation here: http://hyperphysics.phy-astr.gsu.edu...ro/herrus.html
0
#3
..
0
3 years ago
#4
(Original post by iamspiderman)
Hey again!

I've filled in the stars mass (2.3) but they didn't give the sun's mass?

L star/ L sun = (2.3/ ? )^3.5
np

'they didn't give the sun's mass'
Its kinda like the other question where you’ll end up not needing to know its value

The question said that Sirius A has a mass 2.3 times the suns, not '2.3' i.e.

M_star=2.3*M_sun

if you substitute that into the equation instead of just '2.3' do you find any terms cancel?
0
X

new posts
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

### University open days

• University of Birmingham
Wed, 20 Mar '19
• King's College London
Wed, 20 Mar '19
• University of East Anglia
All Departments Open 13:00-17:00. Find out more about our diverse range of subject areas and career progression in the Arts & Humanities, Social Sciences, Medicine & Health Sciences, and the Sciences. Postgraduate
Wed, 20 Mar '19

### Poll

Join the discussion

#### Where do you need more help?

Which Uni should I go to? (35)
14.29%
How successful will I become if I take my planned subjects? (21)
8.57%
How happy will I be if I take this career? (50)
20.41%
How do I achieve my dream Uni placement? (36)
14.69%
What should I study to achieve my dream career? (30)
12.24%
How can I be the best version of myself? (73)
29.8%