Question

Edwin Hubble’s discovery of Cepheid variable stars in the Andromeda Nebula demonstrated that spiral galaxies were actually distant galaxies. What might have happened had Andromeda not contained any Cepheid variables? Imagine that you are an astronomer living at that time, and you are trying to find some other evidence that would decide between the “Nebular Hypothesis” and the “Island Universe Hypothesis.” You have been granted a small amount of observing time on the world’s largest telescope, with no possibility of asking for more time.

A. Describe the research program that you would carry out with your precious telescope time. What data would it collect? How would this discriminate between hypotheses?

B. Why do you feel this program has the best chance of success?

Answer 1

Through observations of Cepheid variables, astronomers have determined the distances to other galaxies. They compare the Cepheid variable's apparent brightness with its intrinsic brightness. The difference between observed and actual brightness yields the distance. Had there been no cepheid variable in a galaxies their distance measurement would not have been possible.

A. My experiment would essentially be to to find out the red shift in the wavelength. Knowing it's redshift, you can calculate the galaxy's velocity. For small redshifts this is simply, velocity = redshift x speed of light. Knowing the speed of light to be 3 x 10^5 km/s, calculate the galaxy velocity and enter the answer here : 6.83 x 10^4 km/s.
Knowing the galaxy's velocity, you can now find it's distance from the Hubble law.
velocity (km/s) = H x distance (Mpc),
where H is the Hubble constant, which you can take to be 65 km/s/Mpc. Mpc is short for Mega-parsec,A parsec is a measure of distance that astronomers use that is equal to about 3.2 light-years, and mega means a million, so: 1 Mpc=3.2 x 10^6 light-years.

The value of distance obtained would be a conclusive proof about the galaxy's position. We already know the span of our Milky way galaxy so comparing with the distance at the edge of our galaxy and that with the distance of the galaxy we can conclude whether the galaxy is contained within ( Island Universe Hypothesis) or without ( Nebular hypothesis) our galaxy.

B. Doppler's idea of red shift and Hubble law have both been tested and verified so the chances of my experiment being successful is quite high.