How to measure distance to stars

[paul1966]

Hi

Have just watched stargazing on the BBC and finally going to get a telescope after years of thinking about it....but I have a couple of science questions that I find hard to explain to friends. Not sure if this is the right place, if not please let me know as only joined 5 minutes ago.

I understand the principles of stars that are less that 400 light years away and the triangulation process, but in simple terms can't quite get my head around how the dating process works for a galaxy say 1 million light years away is calculated. If somebody could explain in terms that sceptical non scientist could understand that would very much be appreciated (not me).

Thanks

Paul

[crabbiT Scot]

Hi mate welcome!!

I am very new to all of this, but I believe it has something to do with the doppler effect and redshift.

I've posted a link that may be useful?

Astronomy For Beginners...Astronomy Basics...How Do they Know That...Part 2

Hope it helps!!

Scott

[GazOC]

Hi Paul,

By "dating" do you mean how many light years away the galaxy is? If so there are a number of techniques, one of the most widely used is to look for certain types of star in the galaxy that vary in brightness over time (called, not surprisingly "variable stars").

There are classes of this type of star that behave in a predicable manner so of you know the period of time of the variation it tells you the luminosity (brightness) of the star.

As soon as you have the actual luminosity of the star the distance of the star can be calculated by how bright the star seems from Earth compared with how bright the star actually is "close up". Giving you the distance the galaxy that contains it.

[FraserClarke]

^^ Wot Gaz said.

There are different distance measures that work well at different distances. We put these all together and come up with a thing called "the distance ladder", which allows you measure distances all the way across the Universe. The first rung of the ladder is geometric triangulation process you know (often called "parallax").

[themos]

That is a fascinating subject with a very interesting history. The "Cepheid" law that relates how often a particular kind of star fades and brightens (which is down to its internal dynamics, our sun doesn't vary much) to how big and bright it is overall was discovered by a woman astronomer, Henrietta Swan Leavitt. This is great for establishing distances as it is easy to observe a star over a long period of time and log its apparent brightness fluctuations (called a "light curve"). If you have that and you know that the star is of the right type (you look at the "spectrum" for that, the particular composition of colours in its light) then the law tells you how intrinsically bright it is. You then look at how bright it appears and that tells you how far away it must be. Other astronomers then found such stars in other galaxies (that's where you need the big telescopes!) and established a distance to them.

[Dave S]

There is also the Type Ia supernova which also has a constant luminosity and so as mentioned measuring how bright they appear allows us to calculate its distance.

Dave...

[Casus belli]

When astronomers say a galaxy is say 30 million light years away it is a very rough estimate. What they really mean is its between 20 and 40 million light years away. (And even then they're not really sure)

[BobTheSaxon]

Hubble's law - Wikipedia, the free encyclopedia

This article will tell you about using spectroscopic method to measure the larger astronomical distances, a light version of the Doppler shift.

Clear skies

Bob