# Homework Help: Properties of a Star Homework help

1. Dec 11, 2015

### Haydn Ellis

1. The problem statement, all variables and given/known data
A star cluster contains star HD1, which has an apparent V magnitude of 7.3 mag and a B − V colour of 0.5 mag. Its continuum emission peaks at a wavelength of 557.3 nm, and this star is known to have a bolometric correction of −0.4.
(a): Calculate the distance to the star cluster.
(b): For star HD1, calculate: i) the temperature, ii) the luminosity.
A spectroscopic binary, composed by star A and B, is found within this star cluster. An observer monitors the variation of the Hα line, with restframe wavelength of 656.3 nm, finding a maximum wavelength of 656.5 nm for star A and 656.4 nm for star B.
(c): Assuming circular orbits, calculate: i) the maximum radial velocity for star A and B; ii) the mass ratio of the two stars in the binary system.
(d): Compute the luminosity ratio for star A and B.

2. Relevant equations
m_u - m_v=2.5log(fv/fu)
m-M=5log(D)-5
T=2.898*10^3/(lamda max)
BC=M_bol-M_v
L=4piR^2(sigma)T^4

3. The attempt at a solution
I don't understand what the B-V colour is for part (a)
Part (b) I think I've done using the 3rd equation I stated, and got a value of 5200K
No idea for (c) and (d)

2. Dec 12, 2015

### Staff: Mentor

Courtesy bump.

3. Dec 13, 2015

### john smith529

bump , would also like to know

4. Jan 16, 2016

### Buzz Bloom

Hi Haydn:

The page
http://www.astro.ucla.edu/~wright/magcolor.htm
says
"When astronomers measure the flux of an object at two or more wavelengths, they can take ratios of fluxes. Since the logarithm of a ratio is the difference in logarithms, these flux ratios are defined by subtracting the magnitudes in different filter bands: such as U-B or B-V. In the UBV system, the zeroth magnitudes fluxes are defined for a bright nearby star with a temperature of 10,000 K [Vega]. Thus B-V = 0 corresponds to a temperature of 10,000 K, while a star with the temperature of the Sun (5,770 K) has a B-V color of 0.65."

Hope this helps.

Regards,
Buzz