 #1
astroYEEET
 12
 1
 Homework Statement:

A galaxy is located at a distance D = 20Mpc from the Sun.
The shape of the galaxy was observed to look like an ellipse with a semi major axis a = 22.6 ”and
a / b = 0.85 ratio
Half of the total light from the galaxy comes from this ellipse and the average brightness of the surface in the ellipse is about 24.7 mag / arcsec2.
A) Calculate the absolute magnitude (mag) that corresponds to the whole galaxy.
B) Determine the total mass of the stars in the galaxy (in solar masses) based on this relation:
mass / brightness = (M / M⊙) / (L / L⊙) = 2
 Relevant Equations:

1)S=m+2.5logA where S surface brightness, m apparent magnitude, A area of arcsec^2
2)Mm=55logD where M absolute magnitude and D distance
3)(I am not sure if this can be used because it is about two stars not one star(the sun here) and one galaxy)L1/L2=(M1/M2)^3.5
So i first calculated the angular area of the galaxy. a=22.6'' and a/b=0.85 => b = 26.588''( btw I do not know why a/b =0.85 since b is the semi minor axis). Then the area of the elipse is a*b*π =1887.745''. Then using the first equation we get an apparent magnitude of m=16.511.
Using the second equation we get M=14.994. So if i am correct this is the answer for A and this is how far i have gone.
Any ideas about the second question will be appreciated. Thanks in advance for your help
Using the second equation we get M=14.994. So if i am correct this is the answer for A and this is how far i have gone.
Any ideas about the second question will be appreciated. Thanks in advance for your help