Dr. Vera C. Rubin's Article

There is a wide variety of observational techniques and tools used in astronomy. Yet, in essence, they all do the same thing -- collect light (electromagnetic radiation). The way in which the light is interpreted is what is important.

Rubin+, 1962^[4]

In Rubin's article, Distances to the stars were computed the distance modulus. The distance modulus is an equation that relates the brightness observed of a star to the distance we must be from that star, which looks like

where m is the apparent magnitude (of brightness), M is the absolute magnitude, A_λ is the interstellar absorption (reduction of brightness by gas and dust) in magnitudes at wavelength λ, and d is the distance away to the star. The absolute magnitude of the star is determined by the type of star, which can be found by looking at the color of the star.

The color of a star is computed by looking at the color index of a star. For example, The B-V color index is found by looking at just the magnitude of the Blue, B, range of wavelengths and comparing it to the magnitude of the Visual, V (peaking in green), wavelengths. Once you have the color index you can look up the type of star and absolute magnitude for that star. In Rubin's paper, all the stars were of OB-5 type stars.

Radial velocities were computed using the Doppler shift. A star that is moving away from us will have the wavelength of it's light shifted toward the red side of the spectrum, where as, if the star is moving towards us, the shift will be towards the blue. The equation describing this is

where Δλ is the change in wavelength, λ in the initial wavelength, v is the velocity of the star, and c is the speed of light.

One way astronomers measure this is by taking the light from a telescope and sending it to a spectrometer, which separates the light out into its spectrum. By looking at the shift in spectral lines from where they would be at rest, astronomers can find the velocity with the equation above.

Since I just threw out the Doppler shift equation, I am including a derivation of the Doppler shift on the next page as it is important for both Rubin and Clemens observations.

Next: Doppler Shift

_{If you are already comfortable with Doppler shift, feel free to move on to: Clemens}