Expert: Ed - 1/15/2010

Question
Salutations Ed

I have multiple questions.  For example, how did Shapley determine that the sun was NOT at the center of our galaxy?

What evidence do we have that the center of our galaxy is a powerful source of energy?

How can one differentiate between a galactic and globular cluster?
Does the cluster method tell us the mass of galaxies?

Answer
Antoniece,

A.) Using the relation between the period of Cepheids and their observed brightness, discovered in 1912 by Henrietta Leavitt, he was able to map the relative distances of clusters from us and from each other. To his surprise he found that they were widely and randomly distributed both above and below the plane of the Milky Way and appeared to be concentrated in one smallish area in the direction of the constellation Sagittarius. He argued that such a distribution would make sense if the Galaxy had the shape of a flattened disk with the clusters grouped around the galactic center. This required that the solar system be displaced from its accepted central position by a considerable distance.


B.) our instruments tell us that there is lots of radio and infrared energy coming from that direction.  The energy isn't from the big black hole in the center of our galaxy, but from the huge swirling whirlpool of hot gas, dust, etc falling into it.  The whirlpool is called an accretion disk.
http://en.wikipedia.org/wiki/Sagittarius_A*

C.) basically, by redshift.  Clusters of galaxies are always moving away from us much, much faster than globular clusters within our galaxy.  In fact, some globular clusters in our galaxy aren't moving away from us at all, but rather WITH us, as they are travelling in the same direction as our arm of our galaxy.  These differences in velocity, toward or away from us, show up in redshift measurements.

D.) the moving cluster methid isn't really about measuring mass, so basically the answer is no.
http://physics.uoregon.edu/~soper/Stars/movingcluster.html

Keep Looking Up!
-Ed