# need help w/ "You find a very distant star cluster and measure its mean velocity."

#### iamalivenow

##### New member
need help w/ "You find a very distant star cluster and measure its mean velocity."

3 You find a very distant star cluster and measure its mean velocity. You measure the velocities for 12 stars and find a mean velocity of 427.5 km/s for the cluster. You also add up the light, use that to estimate the mass, and from the formulas from Lab 6 you compute the population standard deviation to be 9.4 km/s.

A confidence interval for a measurement is specified by a probability. If we choose 68%, then the 68% confidence interval for a measurement is the range of values that will include the true population value 68% of the time.

3a What is the 95% confidence interval for your measurement? What range of velocities will contain the true population value 95% of the time?

3b You measure the velocities of 10 stars in a dwarf galaxy to be 395.2 km/s. The standard deviation of the velocities is 30 km/s, and that is the best guess you have for the population standard deviation. What is the 99.7% confidence interval for your measurement?

#### j-astron

##### Junior Member
Hi iamalivenow, welcome to the forum!

First, a note on rules: we don't do your homework for you here, because you don't learn any math (or astronomy!) that way. We are able to guide you through the steps of working through a solution and help you in places where you are stuck. But you have to show us what thoughts you've had on solving the problem at the very least, and more ideally, what steps you've taken already towards solving the problem. Please see the rules here: