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Homework Help: Properties of a Star Homework help

  1. Dec 11, 2015 #1
    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)
    T=2.898*10^3/(lamda max)

    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. jcsd
  3. Dec 12, 2015 #2


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    Staff: Mentor

    Courtesy bump.
  4. Dec 13, 2015 #3
    bump , would also like to know
  5. Jan 16, 2016 #4

    Buzz Bloom

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    Gold Member

    Hi Haydn:

    The page
    "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.

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