There are different methods of measuring the distance to the stars. I'll try to explain a little of each.
1. Annual parallax. With parallax we measure the angle to the star when the earth is one position, and measure it again 6 months later, when the earth
is on the opposite side of the sun. By simple trigonometry we can derive the distance to the star. The angles that we are talking about are extremely
small and very difficult to measure. For example, the closest star, (and thus the star with the largest parallax), Proxima Centauri, has a parallax of
0.7687 ± 0.0003 arcsec. This angle is approximately equivalent to the parallax that you would get by measuring an object 2 centimeters in diameter
located 5.3 kilometers away. Because these measurements are so small, we can only do these measurements for the very closest stars.
In 1989, the satelitte, Hipparcos, was launced that can do these measurements more accurate, and from its measurements the distance to Polaris could
be calculated. The distance was calculated to be 434 light years.
This leads us to the question of how do we know the distance to stars that are further away?
By studying the closest stars, we assume that from what we know of stellar physics, that all stars of the same color, will be equally bright, if we
view them at the same distance. This comes from stellar physics. We can place them in a graph, combining their luminosity(or absolute magnitude) and
color. If we observe a new star, and measure it's color and apparent magnityde accurately, we can calculate how bright that star really is, from the
inverse square law, and then we can calculate it's distance. This works, but only for stars that we can measure accurately.
Here is a picture of the graph, it is called the "Hertzsprung Russel Diagram"
If the star is further away, like in another galaxy, we can't do that anymore, so how do we meausure their distance?
By looking for special stars, and that is where Canopus comes in. It is a Cepheid variable, that is, it's brightness changes in a very fixed way. By
studying a lot of Cepheid stars, astronomers discovered a strong relationship between their luminosity and the frequency of variability. Now, Cepheid
stars are very bright, up to 100 000 times more luminous than our sun, se we can see them standing out from other stars in other galaxies. We can
measure their apparent magnitude, and its period. From that we can calculate how bright it really is (absolute magnitude), and then we can calculate
typical cepheid variable:
Cepheid period vs. luminosity:
There are also different types of Cepheids, so it is not as simple as all that, but it is good enough for this explanation.
With Cepheids we can measure the distance to other galaxies in our local group. By using the Cepheid variables in the local galaxies, and measuring
their red shifts, we can derive a value for Hubble's constant, and by using Hubble's constant, combined with the red shift to a distant galaxy, we can
derive how far that distant galaxy is.
So, the fact that our distance to Polaris might be off, is of no concern to the average person. The distances to distant stars and galaxies have been
redefined many times. Astronomy is a dynamic science. As we learn more, refine instrumentation more, develop more equipment, we learn more. Astronomy
is like any other science, dynamic.
This is just a very short introduction on stellar distance measurements, will post more later, with references.
How far is that star?
edit on 17/12/2012 by Hellhound604 because: Added HR, and cepheid graphs.
edit on 17/12/2012 by Hellhound604 because: (no
edit on 17/12/2012 by Hellhound604 because: (no reason given)
edit on 17/12/2012 by Hellhound604
because: (no reason given)