# How do we know how far away stars are? **URL:** \ **Category:** Factual Questions\ **Created:** [April 3, 2006, 9:47pm UTC](https://boards.straightdope.com/t/how-do-we-know-how-far-away-stars-are/351147 "2006-04-03T21:47:12Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![George\_Kaplin](https://avatars.discourse-cdn.com/v4/letter/g/3e96dc/32.png) [@George\_Kaplin](https://boards.straightdope.com/u/George_Kaplin)\ **Post date:** [April 3, 2006, 9:47pm UTC](https://boards.straightdope.com/t/how-do-we-know-how-far-away-stars-are/351147/1 "2006-04-03T21:47:12Z")
Apparently, [Betelgeuse](http://en.wikipedia.org/wiki/Betelgeuse) is 427 light years away from earth. How was this figure arrived at? After all, It’s not like we can use a tape measure :). How is the distance between the earth and the stars calculated? [sub]P.S. If possible, I would appreciate it if all explanations were kept simple (like Ladybird simple) as I am a complete mathematical moron. Thanks[/sub] ---
**Author:** ![Miller](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/miller/32/481_2.png) [@Miller](https://boards.straightdope.com/u/Miller)\ **Post date:** [April 3, 2006, 9:52pm UTC](https://boards.straightdope.com/t/how-do-we-know-how-far-away-stars-are/351147/2 "2006-04-03T21:52:36Z")
Parallax. Extremely simplified, you look at the star from an observatory on one side of the Earth, and again from an observatory on the other side, and the amount of change in the star’s apparent position can tell you how far away it is. ---
**Author:** ![Alive\_At\_Both\_Ends](https://avatars.discourse-cdn.com/v4/letter/a/73ab20/32.png) [@Alive\_At\_Both\_Ends](https://boards.straightdope.com/u/Alive_At_Both_Ends)\ **Post date:** [April 3, 2006, 9:53pm UTC](https://boards.straightdope.com/t/how-do-we-know-how-far-away-stars-are/351147/3 "2006-04-03T21:53:54Z")
In the case of the closer stars, it was done by measuring parallax. You measure the position of a star as accurately as possible, then do it again six months later when the Earth is at the opposite point of its orbit (186 million miles away). The star will have shifted very slightly against the background of more distant stars, and you can then work out how far away it is by trigonometry. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [April 3, 2006, 10:05pm UTC](https://boards.straightdope.com/t/how-do-we-know-how-far-away-stars-are/351147/4 "2006-04-03T22:05:35Z")
Here’s a rather comprehensive discussion of [methods of distance measurement to stars](http://www.astro.ucla.edu/~wright/distance.htm). Some of it is rather technical. Parallax will only work for closer stars. Distances to more distant objects such globular clusters and closer galaxies can be determined by figuring out the true luminosity of [Cepheid variables](http://zebu.uoregon.edu/~soper/MilkyWay/cepheid.html). For really distant galaxies, the [redshift](http://en.wikipedia.org/wiki/Redshift) due to the Doppler effect is proportional to distance and can be used to estimate it. ---
**Author:** ![drewbert](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drewbert/32/15890_2.png) [@drewbert](https://boards.straightdope.com/u/drewbert)\ **Post date:** [April 3, 2006, 10:06pm UTC](https://boards.straightdope.com/t/how-do-we-know-how-far-away-stars-are/351147/5 "2006-04-03T22:06:30Z")
For stars too far away to use the parallax method, you can judge distances to certain kinds of stars which have predictable brightnesses. If you know how much light a star emits, you can measure how bright it appears from Earth. Then you can calculate how far away it is. “Type Ia” supernovas have very predictable brightnesses too, and can be used to judge distances to very faraway galaxies. ---
**Author:** ![chrisk](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@chrisk](https://boards.straightdope.com/u/chrisk)\ **Post date:** [April 3, 2006, 10:07pm UTC](https://boards.straightdope.com/t/how-do-we-know-how-far-away-stars-are/351147/6 "2006-04-03T22:07:31Z")
I’ll add that for stars too distant to use parallax, it can be done by comparing their brightness to the brightness of comparable stars that we’re reasonably sure are the same size and temperature. (which would mean that we can work out their relative distance by the inverse-square law.) ---
**Author:** ![Alive\_At\_Both\_Ends](https://avatars.discourse-cdn.com/v4/letter/a/73ab20/32.png) [@Alive\_At\_Both\_Ends](https://boards.straightdope.com/u/Alive_At_Both_Ends)\ **Post date:** [April 3, 2006, 10:08pm UTC](https://boards.straightdope.com/t/how-do-we-know-how-far-away-stars-are/351147/7 "2006-04-03T22:08:31Z")
Having just re-read the OP, I ought to mention that Betelgeuse is too far away for its parallax to be measurable. I think the estimates for stars as far away as Betelgeuse are very unreliable. They’re mostly based on theoretical calculations of how bright a star is in absolute terms. If you know how much light a star is emitting, and you know how bright it appears to us on Earth, you can get some idea of how far away it is. But the margin for error is quite big. ---
**Author:** ![Anne\_Neville](https://avatars.discourse-cdn.com/v4/letter/a/b9e5f3/32.png) [@Anne\_Neville](https://boards.straightdope.com/u/Anne_Neville)\ **Post date:** [April 3, 2006, 10:15pm UTC](https://boards.straightdope.com/t/how-do-we-know-how-far-away-stars-are/351147/8 "2006-04-03T22:15:51Z")
> [@Miller](#): > > Parallax. Extremely simplified, you look at the star from an observatory on one side of the Earth, and again from an observatory on the other side, and the amount of change in the star’s apparent position can tell you how far away it is. Close, but not quite. Stars don’t show visible parallax shifts between one side of the earth and the other. What you do is, you observe the star’s position, then you do the same thing six months later. Even so, the shifts are tiny even for the nearest stars- Alpha Centauri, one of the closest stars, shifts position only by one second of arc (the width of a full moon is 30 minutes of arc, or half a degree). ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [April 3, 2006, 10:22pm UTC](https://boards.straightdope.com/t/how-do-we-know-how-far-away-stars-are/351147/9 "2006-04-03T22:22:18Z")
[Hipparcos](http://en.wikipedia.org/wiki/Hipparcos), the High Precision Parallax Collecting Satellite. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 3, 2006, 10:33pm UTC](https://boards.straightdope.com/t/how-do-we-know-how-far-away-stars-are/351147/10 "2006-04-03T22:33:01Z")
If I may? [Here you go](http://www.straightdope.com/mailbag/mstardistance.html). ---
**Author:** ![The\_Controvert](https://avatars.discourse-cdn.com/v4/letter/t/dec6dc/32.png) [@The\_Controvert](https://boards.straightdope.com/u/The_Controvert)\ **Post date:** [April 3, 2006, 10:38pm UTC](https://boards.straightdope.com/t/how-do-we-know-how-far-away-stars-are/351147/11 "2006-04-03T22:38:44Z")
A related question. Can a star be so far away as to be red-shifted out of the visible light spectrum? If so, how far away does it need to be? ---
**Author:** ![Nic2004](https://avatars.discourse-cdn.com/v4/letter/n/4491bb/32.png) [@Nic2004](https://boards.straightdope.com/u/Nic2004)\ **Post date:** [April 3, 2006, 10:50pm UTC](https://boards.straightdope.com/t/how-do-we-know-how-far-away-stars-are/351147/12 "2006-04-03T22:50:06Z")
I remember once having a conversation with a Fundamentalist that believed the 6000 year old Earth/Universe dogma. Me: If so, then we would only be able to see stars at 6000 light years or closer. Them: Right. They’re only that far away! Me: Huh? There’s stuff billions of LY away. Them: They can’t really tell because paralax only can be done with the diameter of the Earth; less than 8000 miles. Not enough to measure more than 6000 years away Me: What about the diameter of the Earths orbit around the Sun. You’d have that diameter and twice the Earths combined. And what about Red Shift/Doppler Effect? Them: Doppler who? It went progessively downhill from there. ---
**Author:** ![Bill\_Door](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@Bill\_Door](https://boards.straightdope.com/u/Bill_Door)\ **Post date:** [April 3, 2006, 11:01pm UTC](https://boards.straightdope.com/t/how-do-we-know-how-far-away-stars-are/351147/13 "2006-04-03T23:01:53Z")
OK, then that’s how they tell how far away they are, and I can figure out how they tell how big the stars are, and how hot they are, and how fast they are moving, and all that, but how the hell do they find out what the stars names are? ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [April 3, 2006, 11:07pm UTC](https://boards.straightdope.com/t/how-do-we-know-how-far-away-stars-are/351147/14 "2006-04-03T23:07:24Z")
> [@Bill Door](#): > > but how the hell do they find out what the stars names are? Oddly enough, almost all of them are Arabs. ---
**Author:** ![Miller](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/miller/32/481_2.png) [@Miller](https://boards.straightdope.com/u/Miller)\ **Post date:** [April 3, 2006, 11:48pm UTC](https://boards.straightdope.com/t/how-do-we-know-how-far-away-stars-are/351147/15 "2006-04-03T23:48:41Z")
> [@Anne Neville](#): > > Close, but not quite. Stars don’t show visible parallax shifts between one side of the earth and the other. What you do is, you observe the star’s position, then you do the same thing six months later. Even so, the shifts are tiny even for the nearest stars- Alpha Centauri, one of the closest stars, shifts position only by one second of arc (the width of a full moon is 30 minutes of arc, or half a degree). [Don Adams]Missed it by that much.[/Don Adams] ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [April 4, 2006, 12:53am UTC](https://boards.straightdope.com/t/how-do-we-know-how-far-away-stars-are/351147/16 "2006-04-04T00:53:16Z")
> [@Nic2004](#): > > I remember once having a conversation with a Fundamentalist that believed the 6000 year old Earth/Universe dogma. It never goes well from here. > [@Colibri](#): > > Oddly enough, almost all of them are Arabs. [True](http://www.jas.org.jo/star.html). Why is that, exactly? The Arabs were reasonably advanced practical astronomers, but they were neither the first nor the most advanced of that art. Stranger ---
**Author:** ![clairobscur](https://avatars.discourse-cdn.com/v4/letter/c/839c29/32.png) [@clairobscur](https://boards.straightdope.com/u/clairobscur)\ **Post date:** [April 4, 2006, 2:49am UTC](https://boards.straightdope.com/t/how-do-we-know-how-far-away-stars-are/351147/17 "2006-04-04T02:49:43Z")
> [@Stranger On A Train](#): > > It never goes well from here. > > [True](http://www.jas.org.jo/star.html). Why is that, exactly? The Arabs were reasonably advanced practical astronomers, but they were neither the first nor the most advanced of that art. > > Stranger Simply because they were more advanced that their european contemporaries who adopted the names, I would assume? ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [April 4, 2006, 2:55am UTC](https://boards.straightdope.com/t/how-do-we-know-how-far-away-stars-are/351147/18 "2006-04-04T02:55:46Z")
> [@Stranger On A Train](#): > > It never goes well from here. > > [True](http://www.jas.org.jo/star.html). Why is that, exactly? The Arabs were reasonably advanced practical astronomers, but they were neither the first nor the most advanced of that art. > > Stranger However, they were much more advanced than Europeans during the early Renaissance. Islamic civilization had preserved a lot of the classical learning on astronomy that had been lost in the west, and also had made very significant strides on its own. When Europeans began to be interested in scientific knowledge again, much of it was picked up through Islamic (Arabic) scholars, especially via Cordoba in Spain. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [April 4, 2006, 2:59am UTC](https://boards.straightdope.com/t/how-do-we-know-how-far-away-stars-are/351147/19 "2006-04-04T02:59:19Z")
As an addendum, a number of other terms used in astronomy and navigation are also from Arabic, including almanac, azimuth, nadir, and zenith. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [April 4, 2006, 3:37am UTC](https://boards.straightdope.com/t/how-do-we-know-how-far-away-stars-are/351147/20 "2006-04-04T03:37:11Z")
> [@Colibri](#): > > However, they were much more advanced than Europeans during the early Renaissance. Islamic civilization had preserved a lot of the classical learning on astronomy that had been lost in the west, and also had made very significant strides on its own. When Europeans began to be interested in scientific knowledge again, much of it was picked up through Islamic (Arabic) scholars, especially via Cordoba in Spain. I was vaguely aware of this but never really thought about it. Serves me right for my Percodan-aided naps through Basic Astronomy. Thanks, Stranger [Next page](https://boards.straightdope.com/t/how-do-we-know-how-far-away-stars-are/351147.md?page=2)