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## Homework Statement

I've been measuring hubble's constant by calcultaing the redshift on a star and it's distance from earth using a simple method of assuming galaxies have a standard size, then measuring their apparent size to gauge their distance away.

To refine the model i have been asked to include a deceleration parameter q

_{0}. I have found an equation relating q

_{0}to Hubble's constant but i'm confused by one of the terms, the 'angular diameter distance'. What is it and what is it actually measuring?

## Homework Equations

[tex]d_a= \frac{c}{q_0h_0} \frac{zq_0+(q_0-1)(\sqrt{(2zq_0+1)}-1)}{(1+z)^2}[/tex]

## The Attempt at a Solution

it tells me to use q

_{0}=0.1 or 0.5 or 0.8 and plot d

_{a}against z and check which value of H

_{0}fits the data. q

_{0}seems to just be an arbitrary number? is it just an expansion coefficient for the universe expansion?

Now i can see that as i increase q i increase the "bend" of the line produced, but as i change H

_{0}i just change the gradient, which i'm not sure really helps me at all?

questions in summary:

What is "angular diameter distance"?

surely there is only one value for H0 so how do i "see which fits the data"?

what is q

_{0}? it seems to just be arbitrary?

Sorry for a lot of confusion, but i've not taken any astronomy before but have changed courses and have been set this as "catch-up" over, i also have no astronomy textbooks since i hadn't been doing it until recently.