# How far are stars?

1. Feb 5, 2009

### ElectroBurger

How do people calculate the distnce between earth and a star?

2. Feb 5, 2009

### mgb_phys

Hold out your finger at arms length
Now close one eye
Open it and close the other eye instead
You can see the finger appear to move against whats behind it.
If you measure the angle between the finger and some background mark and know the distance between your eyes you can work out the distance to your finger

In the case of stars you make measurements 6 months apart and use the earths movement around the sun for the baseline. This lets you measure the nearest stars very accurately. It's called stellar parallax.

Then for more distant stars we use the fact that 2 stars with the same real (absolute) brightness will appear different (apparent) brightnesses if they are at different distances. We use this to find the distances to more distant stars by comparing their brightness to the ones we measured close to us.

3. Feb 5, 2009

### pixel01

Hi Mgb-phys, can we use redshift index to estimate the distance of far away galaxies?

4. Feb 5, 2009

### mgb_phys

There are a whole series of techniques as you get further away.
So beyond the techniques above for stars you can use Cepheid variables to give you the distance to nearby galaxies, then type I supernova, and then finally cosmological redshift for the most distant objects.

5. Feb 5, 2009

### Nabeshin

Expanding on what mgb_phys said, the method of stellar parallax currently works out to a distance of approximately 1,500ly. This, however, will be expanded within several years to around 10,000ly. Stellar parallax is really the only "direct" method that we have of measuring distances to star in so far as it relies solely on the assumptions of geometry. Everything else is based on inference.

As mgb_phys said anything else relies on the principle that there are objects in our universe which are esentially the same (put out the same amount of light), so by measuring how much we receive, we can compute the distance to the object.

6. Feb 10, 2009

### ElectroBurger

Thank you all for the help,

I've never thought of using parallax to find distant stars. I suppose that it's quite easy to calculate distance once we know the way we travel through space.

But wouldn't these distances between us and the stars change since the universe is expanding?

Making several measurements to the same star over time would tell us the rate of accelerated expansion of the universe, correct?

7. Feb 10, 2009

### mgb_phys

No the galaxy (and the local cluster of galaxies) is held together by gravity.
The distance to nearby stars does change slightly as the galaxy rotates, each star is moving in a slightly different orbit and affected by the motion of other nearby stars.

Not on the timescales we live for!
You can measure the expansion rate from measuring the speed and distance to very distant objects. It's possible to measure the speed directly from the redshift of the spectral lines.

8. Feb 12, 2009

### Chronos

Nearby stars can be distanced based on parallax - the measured difference between their position relative to distant stars and the position of earth at six month intervals. This kind of measurement is accurate for any star within about 20 light years of earth. Indirect methods must be used at greater distances.

9. Feb 12, 2009

### mgb_phys

The Hipparchos mission pushed that out to around 200pc ( 650ly) for high accuracy results and further for the 10-20% accuracy measurements.

10. Feb 12, 2009

### ElectroBurger

you used the measurment of a parsec, exactly how is that defined? I know that a light year is not an SI unit, and the au is, but is the pc?

11. Feb 12, 2009

### Janus

Staff Emeritus
Parsec is short for "parallax second" it is the distance at which the stellar parallax would be 1 arc-second (an arc second is 1/3600 of a degree). It is equal to 3.262 light years.

12. Feb 12, 2009

### mgb_phys

Astronomers don't like writing down long number so make up their own units.

If the base of the triangle is one au (the earth sun distance) and the angle of the star is one arc-sec (about the smallest angle you can see through a blurry atmosphere) then the distance is one parsec. The nearest star is around 1.2parsec