How distance of stars is determined?

Question

As far a I understand, In solar system usual triangulation is used, in long distances, differences of light strength compared with usual type of star - candlelights. But how they come out with so precise results, if they can just relatively tell - this is less bright for this type of star, so its farther away.

Answer 1

The idea is to use geometry to measure the distance. One builds a triangle. For the base of the triangle, use the diameter of the orbit of the Earth around the Sun. It's about 93,000,000 miles to the Sun, so the diameter is twice that, or 186,000,000 miles. Look at a star just before sunrise, and note the precise angle location. Then wait nearly six months, and just after sunset, find that same star and note the precise angle. (Really measure the angle relative to stars near it, which one hopes are much farther away, and so aren't moving so much.) This yields a base length and two angles of a triangle, it's a simple matter to figure out how far it is to the the vertex at the end, where the star is. The phenomenon is called parallax. The difference in the angles measured is referred to as the parallax angle.  

Answer 2

for close stars, triangulation works.  The triangulation points used are the earth on opposite sides of the sun, so it takes 6 months to triangulate.  For farther stars, the standard candle method that you mentioned works well.  For distant galaxies, the measure of red-shift is the best method.  When possible, 2 or 3 methods are used to verify and improve results.

Generally, results aren't very precise.  For example, if they say a galaxy is 4.2 billion light years away, that's a precision of 100 million light years, which is not very precise (generally, I think, they can resolve the distances to galaxies to better precisions than this though).  Or if they say a star is 50,000 lightyears away, that's probably to a precision of 2-4 thousand light years.  Not very precise.  But generally, good enough for the applications the information is used for.

Answer 3

For nearer stars, the same type of triangulation is used, but using the diameter of the earth's orbit as the baseline. Even for the nearest stars, the amount of parallax is quite small though: less than a second of arc. However, the Hipparchos satellite measured the parallaxes for stars up to about 1000 light years distance.

For stars that are farther than that, it turns out to be easier if the stars are part of a cluster. It is then possible to use a modification of parallax to find the distance to the cluster. The technique uses the fact that parallel lines appear to converge at a distance. If you watch the cluster for long enough to determine that 'convergence point', it is possible to find the distance to the cluster.

For still farther stars, we simply compare to stars that are similar based on composition, temperature, etc to find the absolute brightness. Then, by measuring the observed brightness, we can figure out how for the star is. But this technique requires the two previous ones to get enough comparison stars. This method works for most stars in our galaxy. In particular, it allows us to find the brightness and distances of certain variable stars called Cepheid variables. It turns out that these stars can be seen in other galaxies and this gives a way to determine the distances to nearby galaxies.

The distance to a galaxy can also be found if  a certain type of supernova (a type IA) can be found in that galaxy. It turns out that all such supernova are the same brightness. This allows us to find distances to very distant galaxies.

Answer 4

The distance to nearby stars is determined by the parallax method (check http://en.wikipedia.org/wiki/Parallax)

Once you know the distance to a star and how bright it looks from the Earth (this is called apparent magnitude), you can calculate it's intrinsic brightness (or absolute magnitude).  By compiling the data for hundreds of nearby stars,  astronomers determined that there is a close correlation between the absolute magnitude of most stars and their surface temperature (and color, and spectral type).  They graphed the results in what is called the H-R diagram (http://en.wikipedia.org/wiki/H-R_Diagram...  Bright stars have high surface temperatures, while cooler stars are dimmer.

The same correlation between star's magnitude and color can then be used to estimate the distance to distance stars, that are too far away for the parallax method.  You just have to measure the apparent magnitude and color of a star (which can be done easily with a telescope), and then use the H-R diagram to find out what the absolute magnitude is.  With the absolute magnitude taken for the H-R diagram and apparent magnitude measured with a telescope, you can estimate the distance.

This method works well for main sequence stars.

Astronomers also found that certain variable stars (called cepheids) bear close relation between their brightness and the period of their changes in the luminance.  Hence, cepheids can be used to measure the distance to nearby galaxies.  What you have to do is to find a cepheid, measure its period and apparent magnitude, and then use this data to estimate the absolute magnitude and distance.

Also, certain type of supernovae are very consisten when it comes to their brightness.  These can be used to measure distance to galaxies.

How was this achieved?  Via thousands and thousands of observations, and comprarison of results obtained via different methods, until a good correlation between one method and another was found.

Answer 5

Distance of stars is determined by light years. One light year same with 9,5 trilion kilometres. 1 mile equal with 1.6 kilometres. I try my best to give a best answer. Please give me 10 points so I can reached my next level. Your kindness will be appreciated.

Answer 6

Here's something I wrote about the "standard candles" for h2g2:

In 1908, the American astronomer Henrietta Swan Leavitt (1868 - 1921), working at the Harvard College Observatory, did a detailed study of photographs of the Small Magellanic Cloud which had been taken at Arequipa, Peru using the 10-inch 'Metcalf' refractor telescope. She identified 25 stars of the type known as Cepheid variables, named after the prototype star delta Cephei. These are stars whose brightness varies in a regular fashion over a few days. They get sharply brighter, then fade slowly away, only to repeat the pattern exactly after a few days. It is now thought that the variation in brightness is caused by an increase in the volume of the star, which collapses after a few days back to its original size.

Some stars are bright because they are close to us, while others are bright because they are actually very luminous but are far away. Astronomers talk about the apparent magnitude, which is how bright the star appears from Earth, and the absolute magnitude, which is how bright the star really is when you get close to it. We can easily measure the apparent magnitude, but normally to calculate the absolute magnitude we need to know the distance of the star. Leavitt's work turned this on its head.

Leavitt measured the period of variation of each of the Cepheid variables. By assuming that the Cloud is very far away, she was able to treat all the stars as being the same distance from us (in the same way that Dublin and London can both be considered the same distance from, say, Malaysia). This meant that she could work out the relative values of their absolute magnitudes. If one of these stars appeared to be twice as bright as another to us observing from thousands of light years away, then its absolute magnitude was actually twice as bright. She discovered that the brighter the star in absolute terms, the longer was the period of its variability, and the two values were connected by a mathematical relationship. Over the next four years she refined her theories and published her findings in 1912.

These results led to a method of calculating the distance of such stars: knowing the period of variation of a star, the absolute magnitude can be calculated. Relating this to the apparent (observed) magnitude allows the distance of the star from us to be calculated. This method is one of the most useful for measuring the distances of remote clusters or galaxies - just find a Cepheid variable within the group, and you can calculate how far away it is. The Cepheid variable method is now the standard 'yardstick' for measuring distances within the Local Group of galaxies. For galaxies further away, it is usually impossible to identify individual stars, making this method impractical.

Answer 7

BY USING LIGHTYEARS AS A MEANS OF MEASUREMENT DISTANCE b/w STARS R DETERMINED.

Answer 8

Triangulation for nearby stars works very accurately.  Think about parallax.  Then think about where the earth is in January vs. July.  186 million miles (2 AU) difference of position.  Just like the way a person with two eyes can estimate  the distance of something nearby with their depth perception, seeing it from two different angles, or measuring the distance (a much better explanation of parallax) of a rock on the other side of the canyon by measuring where it appears from point A, walking to point B, and knowing the geometric angle of your triangle, you can measure that rock without pulling out a tape measure and trying to jump across that chasm...  

For stars in distant galaxies, distance is determined from these "candle-lights" you described.  Edwin Hubble's work in the 1920's while searching for a Cepheid Variable (a type of star we know has a very specific luminosity) he was able to determine that the Andromeda "nebula" could not be in the Milky Way, because of the apparent brightness of one of these stars.  It was only shortly after that when we realized our universe was much larger than just our galaxy.

Most extreme distances still use this method.  So really, the answer to your question was already in your question!