How Do We Measure the Distance to a Star?

How does one know, by looking at it, if a star is a red giant or a red dwarf?

How do we know the distance to Delta Cephei?

It is said that Delta Cephei is a "yellow-white class F (F5) supergiant" but how do we know it's a supergiant?

Supposing there are frozen sodium ions in between Delta Cephei and the Earth, wouldn't that slow the light down on it's way to the Earth?

 

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Well you can tell what kind of star you are looking at by examining the spectrum of light it gives off. Depending on what stage of it's lifecycle the star is in it will be burning mostly hydrogen, helium etc, which can be seen in the spectrum. Basically you can tell what it is made of. Once you know these things about the star you can figure out how bright such a star should be and by comparing this luminosity to that observed in the sky you can figure out how far away it is.
Details about particular stars or star types I do not know, but I'm sure someone else does.

 

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REPLY: OUTER SPACE is quite empty. What little is there is mostly hydrogen. Out of pure curiousity: why did you pick sodium ions as the medium for altering the speed of light ? traveler

 

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What do you mean? Let's take Delta Cephei for example. What can you tell me about it's spectrum of light?

Don't stars burn hydrogen and helium at all stages of their life according to the standard model?

How can you tell what a star is made of?

I don't know them that's why I'm asking.

I'm still confused.

 

It doesn't look empty to anyone that I know.

Just look at all the superclusters of galaxies, nebulae, gas, and dust, and you'll see that it's not empty.

Since it's the simplest atom that is what one would expect.

Because it's a scientific fact that they do.

 

The matter density of the universe is about 10^-30 g/cm^3. That is pretty empty.

Obviously there are dense regions/objects but the interstellar medium is very dilute. And if you are seeing a star then the chances are there are not dense objects in the way.

 

How do you measure the matter density of the universe from the Earth?

Apparently people require spacesuits because space is filled with charged particles and flying ions of all kinds.

So if I see a quasar in the galactic nucleus of NGC 7319, does that mean there is no black hole there but just empty space?

 

Spacesuits are required because there's no air and it's a zero-pressure environment.

 

OK but there are still ions in space.

bnl.gov/discover/Fall_03/NSRL_2.asp

 

Cosmological models are very sensitive to the density of the universe and any sensible model will have a density orders of magnitude lower than that required to significantly affect the passage of most light rays.

Ignoring dark energy we can estimate the density required to give us an approximately flat universe which is what we observe. This is the critical density which is about 10^-26 kg/m3. In fact taking into account dark energy the density of the universe is about 1/3 of this. And there are too few stars/nebulae to account for even that hence a reason for dark matter to be hypothesized.

 

If you look at the spectrum of a star, you will see bright lines called "emission lines" These lines are due the elements heated at the surface of the star. By comparing the brightness of these lines for different elements we can find the temperature/color of the Star. By looking at how broad these lines are we can tell how large the star is( Dwarf or Giant).

By parallax measurements. We note the position of the star against background stars when the Earth is at different points of its orbit. By measuring how far the star shifts and a little geometry, we can calculate it's distance. We the help of the Hubble, we have been able to do this with an accuracy of 4%.

Answered above; by measuring the broadness of its spectral lines.

The only reason I can see for your mentioning Sodium is that it is used to make Bose-Einstein condensates, which have been used to slow light significantly.

The problem is that such condensates cannot be found in open space. They they need to be at temps that are almost 0°K. Space is permeated with a microwave radiation of 3°K, which while low, is still to hot for BECs to exist in.

Another reason that we know that there is nothing significant between us and Delta Cephei is that we see it sharp and clear. Any substance that slows light also scatters it. Besides that, this scattering and slowing of light is wavelength dependent. Meaning different frequencies of light would be slowed different amounts. This would be particularly notable with Delta Cephei as it is a variable star which changes brightness over a set period. If the light from it was slowed on its way here, the spectrum would be out of phase; the red light from the star would dim and brighten at different times than the blue light.

The fact that Delta Cephie is a variable star is important in another way too. It is the model for the Cephied stars. These are important because there is a fixed relationship between their brightness and period of variability. This allows them to be used as "standard candles" for determining distances to objects such as galaxies.

 

I follow you up to here.

Can you say more? Is a red quasar cold? What is the temperature of a red quasar vs. the temperature of a blue quasar?

This makes absolutely no sense to me whatsoever. Can you please explain this further?

The parallax of the nearest star is less than .7 seconds of arc.

Interesting and informative. Thanks.

This is what I am trying to understand. How is it possible to know the distance of a variable star simply by looking at it? It's brightness tells you nothing about distance and it's period tells you nothing about distance.

 

Quasars are entirely different animals from stars. Quasars are black holes at the center of galaxies absorbing matter.
For stars, it goes like this:

Red < 3700K
Orange 3700-5200K
Yellow 5200-6000K
Yellow-White 6000-7500K
White 7500-10000K
Blue-White 10000-30000K
Blue >30000K

Here's an image of a spectrum showing the Hydrogen lines:

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Broadening of the lines means that the individual lines appear thicker. This broadening effect is sensitive to the density of the gas at the surface of the star. Giant stars have less dense surfaces, so we can use this effect to determine the size of the star. So by looking at the spectrum lines brightness we can tell the temperature of the star and by looking at how thick these lines are we can tell whether it is a dwarf or giant.

So? The Hipparcos satellite can measure parallaxes for stars up to 1600 ly away. Delta Cephei is well within that range.

By comparing its brightness/period to cepheid stars that we already know the distance to by other methods such as parallax. The brightness/period law was discovered by studying cepheids in one of the Magellanic clouds. The size of the cloud compared to its size means that, for all intents and purposes, the stars in them all are the same distance from Earth. Then it was just a matter of comparing visual magnitude to period.

We use the same type of system to determine the distance to main sequence stars. There is a relationship between color/spectral class and luminosity. We know the distance to a large number of stars from their parallax, and by using these stars as standards, we can determine the distances to further stars by comparing their spectrum to their visual magnitude. So if a star has the same spectrum as a known distance star, but is 1/9 as bright, we know that it is 3 times further away.

 

Each atom on the periodic table of elements will produce a unique absorption/emission pattern in a spectrograph and the quantity of any element will determine the intensity of this pattern. Young and large stars burn hot, we see this in the intensity of the emission lines. Old stars have a higher abundance of heavy elements than younger stars because the fusion process has had more time to produce heavier elements than hydrogen and helium. The frequency and boldness of the absorption lines will tell you what stage of the fusion process that star is in. A large star will burn through its H and He supply faster then a small star but the larger star can produce a higher quantity of heavier elements than an older dwarf star. A young large star will have a bolder H and He absorption line than a young dwarf star simply because the larger star contains a higher quantity of H and He.

Well, I know of three ways to find the distance to any object. The first is parallax measurements but this seems to loose its accuracy as we try to measure between galaxies. The second is luminosity. This plays off the fact that a 100 watt light bulb appears to dim as you move further away from it. This ratio has been quantified by observing specific objects such as a supernova signature that will only occur in a specific class of star. The third way is to measure the cosmological red-shift of a distant light source. This is observed as a change in the position of an elements absorption frequency relative to its distance. This relies on our understanding of how light behaves as the space around it stretches during its long journey across the universe. So to answer your question, distance is found through a combination of all three measurements, each supporting the shortcomings of the others.

 

To add to Janus' excellent post, here is a reference regarding the parallax measurement of Delta Cephei via the Hubble:
http://www.iop.org/EJ/article/1538-3881/124/3/1695/202044.text.html

Exactly. Those "standard candles" provide a means to measure the distance to remote objects so long as scientists have a way to calibrate those standard candles. That requires measuring the distance to a Cepheid variable by some other means, such as parallax.

 

Well measuring a stars distance away from something is usually done in light years, for example, aplha centuri os about almost 1 quadrillion miles away or 4 light years, so the light year term is used for distances that great.

 

Measuring the distance to stars is a multi-phase process.

• First measure the distance to various nearby stars using parallex methods, which requires knowledge of elementary geometry & Trigonometry.
  
  
• Then notice that for stars with certain spectral characteristics the intrinsic brightness can be estimated. This is done using stars with distances known via parallex measurments.
  
  
• Then measure apparent brightness of similar stars (see above) too distant for accurate parallex methods. From apparent brightness & known intrinsic brightness, you can calculate distance. This method works for distances too far for parallex methods, but does not work for very distant stars (It is difficult to resolve individual stars in distant galaxies).
  
  
• Next: Notice special stars like Nova or variable stars whose distance can be determined due to being in a nearby galaxy with stars whose distance is known due to above methods. These measurments result in knowing that Nova with certain characteristics have a known intrinsic brightness. Similarly, it can be determined that for certain types of variable stars, intrinsic brightness is related to the period of the variation.
  
  
• Now apparent brightnes of nova & variable stars can be used to determine the distance to stars too distant for the above methods.
  
  
• All of the above is used to determine the relationship between cosmological red shift & distance. Then red shift can be used to estimate the distance of stars too distant for above methods.

The above provides the general idea. The actual details are more complex.

 

Well, here is added tip on observation.

A object at 250 light years may have a error of as much 77.4 degrees from its true positon. The rate of error from true position increases with distance. Example Are closest neighboring star Alpha Centauri resting at a distance of 4.3 light years would have a estimated error range of 93.67 miles from ture postion when viewed from earth, a star at 250 lightyears would have a error of 77.4 degrees or 5346.46 miles from true postion.


At the distance of the moon the rate of error is 1.45003 millimeters

At a distance of Pluto when at its farthest the rate of error is 88.03 feet


Simple formula for Lightyears is; 21.385858425 miles per lightyear, equals the position error range. for 10 lightyears the distance from true position would be 213.858 miles or 3 degrees.

I don't know if this rate of error is included in star catalogues, so you will have to check the details. Really i do not think that newer catalogues such as Hipparcos and Tyco consider the rate of error from earths surface, but older catalogues may.

DwayneD.L.Rabon

 

Dwayne, stop posting nonsense.

Consider yourself warned.

 

Well DH

What does you consider nonsense. A warning and the character you exspress defines your own limits.

DwayneD.L.Rabon