## Light Years Away

If something is supposedly 1 billion light years away from us, how are we able to calculate this..? Wouldn't we have to send a light out and/or x/-ray out to that point and have it bounce back, calculating the time it takes to bounce back to us..? If so, it will take 1 billion years to get there and 1 billion years for the light and/or x/-ray to return, therefore rendering this process too long for us to even calculate.

Having said that, if we can truely even see something that far away, how big must that object be compared to our sun..? From the estimations I've read about, that the closest star, Alpha Centuri, which is supposedly 4.3 light years away, only has the size of it as about the same as our sun. That seems to be grossly erroneous, imo..!!

If we can see a star and it's that far away, my guestamation is that Alpha Centuri has to be X(umteen) times bigger than our sun.

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 There are various ways of calculating astronomical distances. For stars within a few parsecs, photos taken at opposite sides of the earth's orbit (six months apart) show how much the position of the star has shifted compared to the background stars much farther away. Farther than that, Cepheid variable stars are used. These are variable stars whose brightness varies in a regular cycle and the period of that cycle is closely related to the stars brightness. Knowing the absolute brightness of the star allows the astronomers to calculate its distance. Cepheid variables have been seen in nearby galaxies. Using these two methods, astronomers (Hubble) noticed an approximately linear relationship between distance and red shift. Beyond the distance where Cepheid variables can be observed, red shift is used. Recently a relationship between a type of supernova and its brightness was observed allowing the distance between those supernovas to be calculated by that method also. It was discovered the red shift versus distance was not as linear as believed, leading to the conclusion that the expansion of the universe is accelerating.
 Blog Entries: 27 Recognitions: Gold Member Homework Help Science Advisor hi buzzdiamond! welcome to pf! we see alpha centauri as a dot, not a disc we know its distance from using parallax … we measure the difference in its position at two opposite points on the earth's orbit (ie, 6 months apart), and use a bit of trig for galaxies, we can't use parallax because the difference in position is too small instead we can use "standard candles" … particular types of star which always have exactly the same brightness

## Light Years Away

 Quote by buzzdiamond Having said that, if we can truely even see something that far away, how big must that object be compared to our sun..? From the estimations I've read about, that the closest star, Alpha Centuri, which is supposedly 4.3 light years away, only has the size of it as about the same as our sun. That seems to be grossly erroneous, imo..!! If we can see a star and it's that far away, my guestamation is that Alpha Centuri has to be X(umteen) times bigger than our sun. Your thoughts...
The size of a star isn't as important as its brightness. Generally the size of a star can only be determined with an interferometer. Perhaps the Hubble can resolve the disks of closer stars.

How much more is the apparent brightness of the sun than Alpha Centauri? If you were to take the square root of that ratio and multiply it by the distance from the earth to the sun, you would have the distance to Alpha Centauri.

Thanks Tiny, glad to be here. I am very perplexed by the universe and have some doubts about how things are being calculated and/or what we're really seeing. Feel free to chime in refuting my questions and/or thoughts, as I will be trying to tear apart what is currently accepted. Hope you don't mind hearing my position and theories..?
 Quote by skeptic2 The size of a star isn't as important as its brightness.
If we're trying to get a grasp on the size of the universe, the surrounding planets and such, then yes, the size of the star would be important or at least very interesting to know.

I'm willing to bet that it's not possible to differentiate between a star that's less bright or one that's farther away, as a brighter star farther away will look the same as a closer star that's less bright. Therefore, we can't accurately calculate the distance of a star. Correct..?

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hi buzzdiamond!

(just got up )
 Quote by buzzdiamond Thanks Tiny, glad to be here. I am very perplexed by the universe and have some doubts about how things are being calculated and/or what we're really seeing. Feel free to chime in refuting my questions and/or thoughts, as I will be trying to tear apart what is currently accepted. Hope you don't mind hearing my position and theories..?
if you want to try to "tear apart what is currently accepted" here, you need to provide links to what you want to criticise, with a brief quotation of something you disagree with

 I'm willing to bet that it's not possible to differentiate between a star that's less bright or one that's farther away, as a brighter star farther away will look the same as a closer star that's less bright. Therefore, we can't accurately calculate the distance of a star. Correct..?
for most stars, that's exactly correct

we can only tell the distance of most stars by finding another star in the same galaxy whose intrinsic brightness, or actual size, we do know

 Quote by tiny-tim hi buzzdiamond! (just got up ) if you want to try to "tear apart what is currently accepted" here, you need to provide links to what you want to criticise, with a brief quotation of something you disagree with
Sounds good, will do. I'm headed out for the weekend so have a good one yourself.

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 Quote by buzzdiamond I'm willing to bet that it's not possible to differentiate between a star that's less bright or one that's farther away, as a brighter star farther away will look the same as a closer star that's less bright. Therefore, we can't accurately calculate the distance of a star. Correct..?
If you have a method to find the size of the object (with a model, or with a comparison to other objects with known size), there is a direct way to measure its distance: The total (thermal) light emission of the object just depends on the surface and its temperature. You can measure the temperature with spectroscopy, and compare the total power output with the intensity here on earth to calculate the distance.

If you have a double-star system, it is even better, as you get an additional constraint based on the orbital period and the angle between the position of the stars.

And for stars nearby, the parallax is useful, too.

By the way: A star with the diameter of sun in a distance of 4 light years appears with a visual angle of ~10 milliarcseconds, the best resolution of VLT is ~1 milliarcsecond. In theory, it should be possible to get a two-dimensional image of a star.

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 Quote by buzzdiamond I'm willing to bet that it's not possible to differentiate between a star that's less bright or one that's farther away, as a brighter star farther away will look the same as a closer star that's less bright. Therefore, we can't accurately calculate the distance of a star. Correct..?
You need to study the concept of red shift. In the meanwhile, don't put actually money on that bet.

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 Quote by buzzdiamond I'm willing to bet that it's not possible to differentiate between a star that's less bright or one that's farther away, as a brighter star farther away will look the same as a closer star that's less bright.
Just by looking at it, or even measuring its brightness? No. But astronomers do more than that.

 Quote by buzzdiamond Therefore, we can't accurately calculate the distance of a star. Correct..?
Incorrect. As many people have already explained, there are various techniques for measuring distances in astronomy, each of which is applicable out to different maximum range. One that hasn't been mentioned (EDIT: mfb mentioned it in post #8) is one that can be done for individual stars, even if they are too far away for a parallax to be determined (although I should mention that the Gaia mission, a European satellite scheduled for launch next year, will be able to measure parallax angles as small as 0.000000006 degrees, allowing us to compute distances for stars all the way out to the edge of the Galactic disc). But I digress. As I was saying, even if a star is too far away to measure its parallax currently, another technique involves measuring the spectrum of that star. The thing you have to realize is that we understand a lot about the physics of stars, especially during the longest portion of their lifetime, when they are happily fusing hydrogen into helium in their cores. We call this portion of a stellar lifetime the "Main Sequence", because if you make a graph of luminosity vs. surface temperature for stars in this portion of their lifetime, they will all lie along a line or "sequence" on the diagram. In other words, there is a well defined relation between luminosity and surface temperature for stars when they are in this hydrogen-fusing portion of their lifetime. I should define some terms. The diagram I mentioned above is called a Hertzsprung-Russell or H-R diagram. The luminosity of a star is its power output: how much light energy it outputs every second. So you can think of luminosity as a measure of the intrinsic brightness of a star (how much brighter or dimmer than other stars it would look if it were right next to them) as opposed to the apparent brightness, which is how bright that star appears to us. As you've correctly pointed out, the apparent brightness of a star depends not only on luminosity, but also on distance.

What if you could figure out how luminous a star was? I.e. what if you could figure out its intrinsic brightness? Well then, you could determine its distance by comparing the luminosity to the apparent brightness. This works because of the inverse-square law for dimming: the amount of light you receive from an object varies inversely with the square of the distance to that object. So if you take a given object, and double the distance to it, you'll receive 1/4 the light, and if you triple the distance, then the brightness will go down by a factor of 9, etc. So, by comparing the luminosity to the apparent brightness, you can determine distance.

How do you get the luminosity? From the Main Sequence: as I mentioned before, stars on the Main Sequence have a well-defined relationship between their surface temperature and their luminosity. So if you can determine the surface temperature of a star, you can figure out how luminous it is. How do you determine the surface temperature of a star? Spectroscopy. We categorize stars by spectral type (which is determined from the measured properties of their spectra), and the spectral type depends on surface temperature. Roughly speaking, stars at different temperatures will be different colours, meaning that their emission will peak at different wavelengths. The hottest stars are blue or blueish-white, and then we go down a sequence to white, yellow, orange, and red. It's more than that: stars of different spectral types will have different absorption lines in their spectra, because the chemical compositions of stellar atmospheres vary with temperature. Anyway, the Main Sequence tells us that this sequence in spectral type/colour is also a sequence in luminosity: the hot blue stars are much more luminous than the cool red stars. If you can measure the spectrum of a star accurately enough to determine its spectral type, you can determine its luminosity, and hence the distance to it: This technique is known as Spectroscopic Parallax: http://en.wikipedia.org/wiki/Spectroscopic_parallax

(The "parallax" part is a misnomer.) The only limit to this technique is that you need a lot of light to get an accurate spectrum, and at about 10,000 parsecs, things start becoming too faint for this method to be useful.

So: Q. can you tell the difference between a close and moderately bright star and a distant and extremely luminous star? A. Just by looking at them, no. By actually analyzing the light from them scientifically? Yes.

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 Quote by phinds You need to study the concept of red shift. In the meanwhile, don't put actually money on that bet.
Phinds, can you give some indication of how exactly you think redshift would be applicable to measuring the distances to individual stars?

 Quote by cepheid Just by looking at it, or even measuring its brightness? No. But astronomers do more than that. Incorrect. As many people have already explained, there are various techniques for measuring distances in astronomy, each of which is applicable out to different maximum range. One that hasn't been mentioned (EDIT: mfb mentioned it in post #8) is one that can be done for individual stars, even if they are too far away for a parallax to be determined (although I should mention that the Gaia mission, a European satellite scheduled for launch next year, will be able to measure parallax angles as small as 0.000000006 degrees, allowing us to compute distances for stars all the way out to the edge of the Galactic disc). But I digress. As I was saying, even if a star is too far away to measure its parallax currently, another technique involves measuring the spectrum of that star. The thing you have to realize is that we understand a lot about the physics of stars, especially during the longest portion of their lifetime, when they are happily fusing hydrogen into helium in their cores. We call this portion of a stellar lifetime the "Main Sequence", because if you make a graph of luminosity vs. surface temperature for stars in this portion of their lifetime, they will all lie along a line or "sequence" on the diagram. In other words, there is a well defined relation between luminosity and surface temperature for stars when they are in this hydrogen-fusing portion of their lifetime. I should define some terms. The diagram I mentioned above is called a Hertzsprung-Russell or H-R diagram. The luminosity of a star is its power output: how much light energy it outputs every second. So you can think of luminosity as a measure of the intrinsic brightness of a star (how much brighter or dimmer than other stars it would look if it were right next to them) as opposed to the apparent brightness, which is how bright that star appears to us. As you've correctly pointed out, the apparent brightness of a star depends not only on luminosity, but also on distance. What if you could figure out how luminous a star was? I.e. what if you could figure out its intrinsic brightness? Well then, you could determine its distance by comparing the luminosity to the apparent brightness. This works because of the inverse-square law for dimming: the amount of light you receive from an object varies inversely with the square of the distance to that object. So if you take a given object, and double the distance to it, you'll receive 1/4 the light, and if you triple the distance, then the brightness will go down by a factor of 9, etc. So, by comparing the luminosity to the apparent brightness, you can determine distance. How do you get the luminosity? From the Main Sequence: as I mentioned before, stars on the Main Sequence have a well-defined relationship between their surface temperature and their luminosity. So if you can determine the surface temperature of a star, you can figure out how luminous it is. How do you determine the surface temperature of a star? Spectroscopy. We categorize stars by spectral type (which is determined from the measured properties of their spectra), and the spectral type depends on surface temperature. Roughly speaking, stars at different temperatures will be different colours, meaning that their emission will peak at different wavelengths. The hottest stars are blue or blueish-white, and then we go down a sequence to white, yellow, orange, and red. It's more than that: stars of different spectral types will have different absorption lines in their spectra, because the chemical compositions of stellar atmospheres vary with temperature. Anyway, the Main Sequence tells us that this sequence in spectral type/colour is also a sequence in luminosity: the hot blue stars are much more luminous than the cool red stars. If you can measure the spectrum of a star accurately enough to determine its spectral type, you can determine its luminosity, and hence the distance to it: This technique is known as Spectroscopic Parallax: http://en.wikipedia.org/wiki/Spectroscopic_parallax (The "parallax" part is a misnomer.) The only limit to this technique is that you need a lot of light to get an accurate spectrum, and at about 10,000 parsecs, things start becoming too faint for this method to be useful. So: Q. can you tell the difference between a close and moderately bright star and a distant and extremely luminous star? A. Just by looking at them, no. By actually analyzing the light from them scientifically? Yes.
Cepheid, great response..!! My argument.to that.would go back to what skeptic said, that size isn't important. If you have a large star with the same surface temperature as a small star, it will be more luminus than the smaller star. Therefore, it will seem closer, but in actuality, it may not be. Having said this, how accurate are we when distances are being given for stars and/or the size and dimensions of galaxies?

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 Quote by buzzdiamond Cepheid, great response..!! My argument.to that.would go back to what skeptic said, that size isn't important. If you have a large star with the same surface temperature as a small star, it will be more luminus than the smaller star. Therefore, it will seem closer, but in actuality, it may not be. Having said this, how accurate are we when distances are being given for stars and/or the size and dimensions of galaxies?
Before I answer your question, let me briefly describe spectral types. We assign letters to spectral types, and these are O B A F G K and M. O-type stars are the hottest and bluest (surface temperatures of 30,000 to 50,000 kelvins), and M-type stars are the coolest and reddest (surface temps of 3000 K). Our sun is a G-type star (yellowish-white and surface temp of 6000 kelvins). For more info, just Google stellar spectral types.

On to your question:

There are many stellar properties that vary in a progression across the main sequence. I already mentioned surface temperature and luminosity. Another is stellar radius. Where a star lies on the main sequence entirely determines its radius. So, two main sequence stars of the same spectral type cannot have significantly different radii. Take M-type main sequence stars as an example. These are often called M-dwarfs or red dwarfs. What about a red giant? It's cool enough for it to be red. In fact it has the same spectral type (it's an M-giant). However, it is significantly larger and therefore it will be significantly more luminous. So much so, that it won't lie on the main sequence of the HR diagram. Indeed, the red giant stage of stellar evolution is the stage that many stars enter after their main sequence lifetime is over. (They have fused all of the hydrogen in their cores, and the absence of an internal energy source causes them to evolve off the main sequence). Check out this HR diagram to see where stars in different phases of stellar evolution end up: http://en.m.wikipedia.org/wiki/File:HRDiagram.png

The method of spectroscopic parallax only applies to main sequence stars, which have well defined relations amongst their various stellar properties. Some other method of distance determination would have to be used for a more evolved star like a giant.

You might be wondering why the placement of a star on the main sequence determines so many of its properties uniquely. It's because the fundamental stellar parameter that determines everything else is mass. Where a star will end up on the main sequence is determined by the mass that it has when it forms. A star forms from a cloud of interstellar gas that collapses under its own gravity. But as it collapses, it heats up. This heat (especially after fusion ignites in the core), creates an outward pressure that fights against the inward force of gravity. A star is said to have formed when a balance is achieved between these two forces, and the star is stable (a condition called hydrostatic equilibrium). The more mass that is present, the higher the internal temperature that will be reached before equilibrium. The higher the core temp, the higher the rate of nuclear fusion, and the more luminous the star, and the hotter its surface temp will be. And of course the radius of the star at hydrostatic equilibrium is determined by its mass as well. In this sense, the main sequence can really be thought of as a sequence from high-mass to low-mass stars.

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 If you have a large star with the same surface temperature as a small star
Well, you do not have this in the main sequence.

 how accurate are we when distances are being given for stars
It depends on the star. For stars nearby, parallax measurements can give an uncertainty of less than 1%. For other methods (mainly outside the galaxy), the uncertainty is a bit larger (something like ~10%, but depending on the method), you can find some numbers at wikipedia, for example.

If you have a value for the distance of a galaxy, the size is easy to measure.

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 Quote by buzzdiamond Cepheid, great response..!! My argument.to that.would go back to what skeptic said, that size isn't important. If you have a large star with the same surface temperature as a small star, it will be more luminus than the smaller star. Therefore, it will seem closer, but in actuality, it may not be. Having said this, how accurate are we when distances are being given for stars and/or the size and dimensions of galaxies?
Your mention of other galaxies here makes me think you havent understood what others have said in this thread. Every distance measurement technique has a maximum distance out to which it works. The method I described is only relevant for distance measurement of stars within our own Galaxy and has no relevance to extragalactic distance measurements. For all but the most nearby galaxies, we can't even resolve individual stars in them. For these, we have to use "standard candles" like Cepheid variables or Type Ia supernovae. I think other people have described these methods for you.
 What im getting at is, because measuring distance of so vast a scale is physically impossible, are we really outside our galaxy when we're supposedly seeing other stars and/or galaxies?

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