Register to reply

White dwarf v. sun magnitudes

by hfitzgerald
Tags: luminosity, magnitude, sun, white dwarf
Share this thread:
May7-11, 05:01 PM
P: 2
1. The problem statement, all variables and given/known data
This is a past exam question that I am trying to get my head around but keep getting stuck
Show that if a white dwarf were at the centre of the solar system instead of the sun, it would have an apparent visual brightness of -18.6mag.
Assume that the white dwarf has surface temperature 3 times that of the sun and a radius 0.01 times that of the sun.
The sun has absolute bolometric magnitude 4.75mag and the visual magnitude of the white dwarf will be 3 times smaller than it's bolometric magnitude.

2. Relevant equations
Mbol=-2.5log(Lbol)+5+constant (absolute bolometric magnitude)
m-M=-2.5log(d) 5 (M=absolute magnitude, m=apparent magnitude, d=distance in parsecs)

3. The attempt at a solution
L(dwarf)= 4π(0.01R)2σ(3T)4=34 *0.012 *Lsun
M(dwarf)-M(sun)=- 2.5log(L(dwarf)/L(sun))=-2.5log(34*0.012)=5.23
M(dwarf)=5.23 +M(sun)=-5.23+ 4.75= 9.98
m=5log(d) 5 +M
d=1AU =4.84813681 10-6 Parsecs
m=5log(4.84813681 10-6) 5 + 9.98=-21.6
Phys.Org News Partner Science news on
Climate change increases risk of crop slowdown in next 20 years
Researcher part of team studying ways to better predict intensity of hurricanes
New molecule puts scientists a step closer to understanding hydrogen storage

Register to reply

Related Discussions
White Dwarf cooling times Astronomy & Astrophysics 0
Density of the sun as a white dwarf. Introductory Physics Homework 6
White dwarf accretion Astronomy & Astrophysics 3
White dwarf Astronomy & Astrophysics 1
A white dwarf formed by a supernova, possible? Astronomy & Astrophysics 2