spectrum of M31 to find its velocity">

# National Aeronautics and Space Administration

## Using the spectrum of M31 to find its velocity

### Observing the Spectrum of M31

Recap: Your astronomy professor has tasked the class with determining the velocity of Andromeda with respect to the Milky Way. You thought of three possible ways to do this, one of which will give you the right answer. You've decided to try using the Doppler shift of emission lines from M31's spectrum to find the velocity of M31.

Below is the spectrum of M31. The locations of laboratory-measured lines of calcium K and H lines are indicated in red, and the laboratory wavelengths for them are given below. The corresponding calcium H and K lines from M31 can be seen in absorption. Look carefully in making your comparison. You may find it easier to print the image and estimate the Doppler shifted wavelengths using a ruler.

Possibly helpful quantities:

• Calcium K laboratory wavelength, λ0, Ca K = 3934 angstrom
• Calcium H laboratory wavelength, λ0, Ca H = 3969 angstrom
• Speed of light, c0 = 3×105 km/s

km/sec.

 A service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC