How do we know how far away stars are?

[Jack123456]

hi everyone,

I know there are some cleaver people on here, and i was hoping someone might be able to explain to me how long distances in space are measured.

I am sure there is a lot of mathematics involved, but i would still love to know how it is worked out.

I have been thinging about it for a while, and can't come to a reasonable conclusion.

Could someone please help me!

Thanks,

Jack

[NGC 1502]

Hi Jack, good question, with not a simple answer I'm afraid, but

here is a start -

Cosmic distance ladder - Wikipedia, the free encyclopedia

Regards, Ed.

[BournemouthJ]

Thanks for posting Jack, and cheers for replying Ed!

[Karlos]

I know that there are several ways this has been done and it does have its limitations when directly measuring, anything further than 400 lights years from Earth has to be indirectly calculated (Using brightness usually), so here are the ways I know of: (Anyone feel free to expand these)

1) There is a way of working out how far away a object is by viewing it from two different angles but with a singular object close to you that doesn't move. Obviously there is a precise way of using this method (I think this is to do with geometry or something) After researching this it is called triangulation so my guess was correct

2) It has been proven that also that there is a relationship between colour and brightness, and tested on objects <400LY away which we know the distance of. From the colour they can work out an objects actual brightness, compare this to apparent brightness seen from the Earth giving a measure of distance.

3) The final method is if the star is variable. Some stars change in brightness periodically and the length of that period is also related to the average luminosity. By comparing the calculated figure with how bright it appears to us we can again find the distance in the same manner as before. This is a slightly more accurate method but obviously it only works for stars that vary in brightness.

So just like telescopes, there really isn't ONE for it ALL. I'm sure there are many more ways, I hope this was helpful as I'm unable to provide the actual mathematics of it, but maybe will enlightened you on some of the method

[Big Jim Slade]

Redshift (Wikipedia). There's a reason the Hubble ST is called the Hubble.

[palebluedot]

Not entirely relevant, but very cool if you have not seen it

Scale of Universe - Interactive Scale of the Universe Tool

[ollypenrice]

There are lots of tools but only the nearby stellar distances can be 'measured' in any kind of normal sense using trigonometry. We are able to measure the position of a star, move to a point nearly 200 million miles away (by waiting six months) measure the position again, draw a triangle and derive the distance.

After that, we have many ways of estimating how bright an object really is (in Watts, like a light bulb.) We then measure how bright it appears and from Newton's inverse square law we work out the distance; if the distance to an object doubles, its brightness falls to a quarter of what it was. By the time you get to huge distances the only thing you can see are supernovae (exploding stars) but fortunately we can estimate their true brightness in (lots of!) Watts and again estimate a distance.

Great stuff!

Olly

[Byz1453]

On top of all that has been said above Prof Jim Al-Khalili explains all in an informative and interesting way in his recent series.

BBC iPlayer - Everything and Nothing: Everything

[FraserClarke]

I am sure there is a lot of mathematics involved, but i would still love to know how it is worked out.

Put your arm out infront of you and stick a finger up (try not to insult anyone in the process). Close your left eye, and line up your finger with something in the background. Now open your left eye and close your right eye... does your finger still line up??

That simple experiment shows "parallax", which is what Olly described in his post. As he says -- it's the fundamental measurement all of the others are based on. The maths in very simple -- as usual, the simplier the better

[Jack123456]

Thanks for the relpies. It is really interesting!!

I guess the measurement on earth for triangulation have to be incredably accurate because the errors will get exagerated.

[GazOC]

edit

[ollypenrice]

Thanks for the relpies. It is really interesting!!

I guess the measurement on earth for triangulation have to be incredably accurate because the errors will get exagerated.

Yes, but precision and astronopmy are not always bedfellows; on a course once the guy next to me and I got very different answers to a calculation. We called the lecturer and asked her if either of us had got it right. I had one point something followed by nineteen zeros and he had nine and a bit followed by nineteen zeros. She wandered off saying, 'Yes, well done, both right. Orders of magnitude...'

It's like the dinosaur joke. Museum visitor says to caretaker, how old is that dinosaur? Well, says he, it is 70 million and three years, twelve days old because it was already 70 million years old when I started this job.

Olly

[Red Coffee]

Not entirely relevant, but very cool if you have not seen it

Scale of Universe - Interactive Scale of the Universe Tool

Cool link.

[Big Jim Slade]

Yes, but precision and astronopmy are not always bedfellows;

Very good point. I was reading up on M104 the other day, and it's somewhere around 29 mil ly away, give or take 1.6 ly.

What's a light year or two between friends?