Science Questions

How do we measure distances across the universe?

Sun, 22nd Feb 2009

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Richard, Australia asked:

How do we measure distances across the universe?


Chris - This is a very difficult question o answer, or at least it was.  The problem is that if you’re  looking at stars in the night sky; if a star is at a certain distance from you its brightness can’t really be used as a measure of how far away it  is because a bigger star will be brighter and because light  gets dimmer the farther it is from you a big star can be a lot  farther away than a small star and yet they’ll both  appear exactly the same brightness.

How do you solve that one?

This kept astronomers guessing for a very long time until about the turn of last century.  A woman in contact with Hubble, after whom the Hubble Space Telescope is named, solved the problem.  Her name was Henrietta Levitt and she was looking at star charts.  She noticed that some star appeared to get bigger and brighter and then dimmer and weaker.  They did it with a regular period.  These have now become known as the stellar yardsticks.  They’re called Cepheid variables.  They’re stars that swell up and shrink down.  Because the period at which they do that varies with the size of the star you therefore know, if you look at how often a star like that is blinking on and off, you know how big it is.  Therefore you know how bright it is.  Because light follows an inverse square law you can work backwards to work out how bright that star must be and therefore how far away it is.

Scientists now use these Cepheid variables when they look at a distant star structure they can use the period of any Cepheid variables that are there to work out how far away those particular entities are.  That’s a stellar yardstick and it was solved by a lady at Harvard a hundred years ago.


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Richard Nirui asked the Naked Scientists: I am addicted to your show and absolutely love it. Thank you all for such a wonderful and unique programme. I have three questions: I)How can astronomers tell how far the light from a star had to travel to reach us? In other words how do they determine how far a star is from us? And how accurate is this measurement,a few light years or thousands of light years? II)where did the big bang took place relative to the present universe? Did it happen in the center of the universe as we know it, exploding matter and energy into the space and time symmetrically? III) Big bang happened some 13.7 billion years ago. The light and energy from the big bang should have been travelling faster than the matter that has given rise to stars, planets, etc. In other words the light from the big bang is moving away from us not towards us. Does that mean no matter how strong a telescope we ever make, we will never be able to see what happened in the beginning of the universe? What do you think? Richard Nirui, Sun, 27th Jul 2008

1.Measuring distances in space relies on a hierarchy of methods - starting with simple 'range finder' methods for nearby objects to very speculative methods based on average brightnesses of  very distant objects.
This summarises the topic.

2. The accepted view is that, during the 'big bang' space, time and matter all came into being. The Universe is everything that was created in the BB, so you can say it was always 'full'. Space expanded (not into anything which is part of our Universe - it just expanded) so the distance between objects just kept increasing.  Hence, there wasn't a place 'in the middle' from which things moved. This, of course, introduces the idea of the Universe having no middle and no edge. If you have a problem with that, then you are not alone. Many contributors on these forums have ranted and ranted that it is a stupid idea but people who are much cleverer than you and me seem to reckon it's ok . I accepted some while ago and, once you start thinking around that particular model,  it makes sense. You needn't lose sleep over it!

3. What I wrote in {2} shows that energy from the BB was not traveling 'outwards' in particular; it was (and still is) traveling in all directions within the Universe.  The Cosmic Background Microwave Radiation which we can detect is the nearest thing to seeing the start of things. lyner, Mon, 28th Jul 2008

Just to add to answers to your question 1, the Wikipedia summary really only covers extra-galactic bodies. There are other methods that are used within our galaxy which are not mentioned. One such method is able to estimate the distances of pulsars to quite good accuracy, although does depend on an assumed knowledge about the density of interstellar material and how this changes within the galaxy. This is based on the knowledge that electromagnetic radiation is emitted from Pulsars at all wavelengths simultaneously (Bremstrahlung radiation) and then "disperses" in time as a result of the small but significant insterstellar medium which slows the light down (depending on wavelegth) to below its speed in a vacuum. The amount of dispersion (for example how long blue light arrives after red light, although this feature extends well into the radio spectrum) can be correlated well with distance. The whole method has been correlated quite well with other methods of distance measurement, which also helps to confirm assumptions about the galactic model.  graham.d, Mon, 28th Jul 2008

Yes - using Cepheid Variable stars is also a very smart method, I think. Some stars oscillate in brightness (they are very large and can't decide whether to blow up or collapse and keep oscillating for most of their lives).
Their Absolute Magnitude (i.e their mass, which governs how much light they produce) determines their period of oscillation. Their Relative Magnitude (how bright they actually appear to us) also depends upon how far away they are (inverse square law). So, if you measure the relative magnitude of a star which is pulsing at a certain rate you know how much light it is producing so you can tell far away it must be because of how much fainter it is than it started off. Of course, it isn't quite as simple as that because there are different types of Cepheids but you get the idea.
You calibrate the method by using alternative methods for measuring the distances to nearby Cephieds and use it to infer distances to Cepheids which are too far away for those simpler methods. The Cepheids in a distant galaxy tell you where the other stars are.
As far as the question of accuracy is concerned - it is fairly crude in terms of distance but, as a percentage, it's good enough to infer things like expansion and gravitational effects.

Using parallax, which is the relative movement of nearby stars against the very distant stars as the Earth goes in its orbit, is a sort of 'rangefinder' method, which uses a huge baseline, and so it can measure large distances.  If you take two photographs of the sky, six months apart, some stars can be seen to have 'moved' due to parallax. The more apparent the movement, the nearer they are. This is why they use Parsecs (Parallax Arc Seconds) to describe distances, sometimes. lyner, Mon, 28th Jul 2008

I think that we cannot says accurace what is distance,,even near accurace,,when we speak long distance.
Why so,,i tell.

Few thoughts;
1. Light speed is not constant.
2. Speed change depend of where light goes.
3. Light goes only limited distance,,,not forever.
4. Light start to travell very fast,,,get maximun speed,,and after that speed decrease.
5. Light source power effect that distance(and speed?) also.

I suggest if someone is interesting that light speed issue,,,make test method.
Example light-cable,,,long light-cable,,,put light-impulse to travel in the cable and measure that how long it travell (km) and what is the speed-change (km/s/km) to it's travell distance.

My thought is few point;
1. Many light source light dont come to earth through space.
2. And long distance,,we cannot says what is the distance to that light source-to us.
(We dont know even is this light source exist any more,,,hmm,,if we think that light can travell after light source is end.)

ps. This speed(km/s)/km change all moving hmm. things,,,like air, radiowaves, etc.

I dont see any reasonable natural reasons why speeds is constant.

Of cource constant speed value km/s is many cases enough accurace to normal life,,like air 330m/s and etc,,,but science need accurace answer.

Hmm,,wrong or right,,,i dont say,,i say only what i think.

Hei-Tai, Wed, 25th Feb 2009

Hei Tai

Have you read the standard works on relativity or is this an inspired view? lyner, Wed, 25th Feb 2009

"I dont see any reasonable natural reasons why speeds is constant."
Because you can calculate it from Maxwell's equations.
The two factors in the equation are the permittivity of a vacuum and the permeabillity of a vacuum.
Now, for the speed of light to be different in different places the value for one or other of these properties of the vacuum would need to change.
How does a vacuum know where it is so it can have the right value for these two quantities? Bored chemist, Wed, 25th Feb 2009

Means,,every object(travelles) has same natural motion-law,,,acceleration period m/s,, max speed period m/s  decelaration period m/s.

Hmm,,, i dont see any natural reasons why this motion-law must be different even vacuum?

If something is in vacuum and round of it is nothing,,then i dont see any motion-power vector any direction and how then object can go any direction?

What is the light-speed in the air-surface?

Or in the water?
How deep light goes to water,,,,example ocean,,,speed decrease(-m/s2).

What is the light speed through 1m thick glass?

What is the light speed when this glass surface is painted 10mm paint-layer?
How deep light goes this paint-layer?

Where is vacuum?

I sit dark room and it is not vacuum,,i match fire and light-particles start to fly this wood burning-process to room,,and i can see my room.

But,,anypath,,like i have mention many times,,this is only my opinions,,,thoughts,,i dont know is this right or wrong.

Hei-Tai, Thu, 26th Feb 2009

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