Please help- astronomy question (real answers only please!)?

Question

The stars in the disk of the Milky Way Galaxy near the Sun are all orbiting the center of the Galaxy at roughly the same velocity and in the same direction. The stars in the halo of the Milky Way Galaxy orbit the Galactic center in different directions and at different speeds.

Picture the stars in the disk behaving like the planets orbiting the Sun or like runners on a track. If the Sun and a nearby star are both moving in the same direction at nearly the same speed, how will that other star's velocity appear when measured by us? That is, what will we measure its relative velocity to be? Now picture a halo star moving in a direction perpendicular to the Sun with a velocity of 100 kilometers per second. From our point of view, will this halo star appear to be moving quickly or slowly?

Can you use the information that you provided to answer these questions to propose a method for separating halo stars from disk stars?

Answer 1

OK, so it's a little more complicated than that. We get to measure the radial velocity and the proper motion. Measuring the radial velocity is easy.  You use a spectroscope, and you measure the doppler shift of some element, and compare to the spectral line of that element on the ground.  For example, you measure an iodine absorbtion line.  That gets you the speed the object is coming toward us, or away.

But proper motion, motion across the sky, is harder to measure. You measure the position of the object on the sky, and then later measure it again.  If it hasn't moved, then you only know that it is going slower than your lower bound of sensitivity.  There are lots of even nearby stars for which no proper motion as been detected.  This is going to make this project harder.

Perhaps there are directions for more distant objects, like those in the halo, where measuring the radial velocity is enough.  Not everywhere, but some directions.  Then the problem becomes easier.

Answer 2

i was going to answer but plaster has it right

Answer 3

A galaxy is made up of satellite [solar] systems.

  The satellite systems all orbit the galactic center at the same speed .

  Each system itself has planets orbiting it's sun at different speeds and distances.

  You have a complicated array of velocities in the individual systems but an orderly set of velocities overall.

Answer 4

The stars are in constant motion, but because of the vast distances between each one, and us, they would appear to stand still.  For example, the Big Dipper will be unrecognizable as a constellation several thousand years from now.  From the side, the Big Dipper as a constellation disappears only because the vantage point is different, say from the side.