# Help calculating The mass of a Black hole (homework help)

1. Oct 27, 2014

### JDenise

on this lab worksheet i need to fill out the bottom table (the top one is already filled out). Im having trouble understanding The numbers in the first part of the chart. I know RA is right accession and DEC is declination but what I'm not understanding is why there are two different numbers for the mass and why there are 2 numbers in one column. if someone could help me fill out the first column in the bottom chart for Cygnus X-1 that would be amazing, then if you could, explain. i already know the formula for R(s) is 2GM/c^2, but I'm not quite sure how to get the mass for the above numbers. thank you very much !!(:

P.S. sorry for the dark picture. I know i already have numbers in two boxes but I'm 100% sure they are incorrect so please ignore.

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2. Oct 27, 2014

### Drakkith

Staff Emeritus
What's the 4th column of the top chart labeled as at the top? I'm having difficulty reading the symbol next to the first M.

3. Oct 28, 2014

### JDenise

M(BH)

4. Oct 28, 2014

### Drakkith

Staff Emeritus
The 4th column. M(BH) is in the 5th column.

5. Oct 28, 2014

### JDenise

oh, sorry ! its actually an asterisk. thats where i got confused, I've never seen it in a formula like that before .

6. Oct 28, 2014

### Drakkith

Staff Emeritus
Are these all binary systems? Cygnus x-1 appears to be a binary system. Perhaps M* is the mass of the other stellar object.

7. Oct 28, 2014

### JDenise

i believe so, do you know which values I'd use to plug into the bottom table? I'm not sure why there are two numbers in a column instead of one.

8. Oct 28, 2014

### Drakkith

Staff Emeritus
Well, why do you think there are two numbers? If I measured the mass of something, would I get an exact value?

9. Oct 28, 2014

### JDenise

I'd think you'd get something like 14.5 or 16.3 but i know that measuring something that large won't get you an exact number . I'm also guessing thats the reason why is says estimate at the bottom. Thank you, though . I'll need to look at the numbers some more .

10. Nov 6, 2014

### |Glitch|

I do not believe the purpose of this exercise is to determine the mass of the black holes. The fourth column is the mass of the black hole's companion, and the fifth column is the mass of the black hole.

In order to determine the mass of any orbiting object, you need more information. Such as the distance the object is from its companion, and its orbital velocity. Since the masses of the black holes are already given on your worksheet, I can only conclude that calculating the mass of the black holes is not the point of the exercise.

What is not clear is the Rs columns. If the instructor had wanted you to calculate the Schwarzschild Radius of the black hole it should have said RBH on the worksheet. However, in order to estimate the radius of the companion star, more information is also required, such as the surface temperature of the companion star and its bolometric luminosity.

Therefore, I would fill out the worksheet as you had originally intended - calculate the Schwarzschild Radius in both kilometers and miles based upon the given masses of the black holes.