How do astronomers measure distances between the planets of the Solar System, between the stars in our Galaxy, or between the galaxies? Determine the distance between the two space objects of your choice
It has to be changed slightly if we take General Relativity and the expansion of the universe into account, but these changes are irrelevant for all stars in our galaxy and in other nearby galaxies.
Using this law, if we know the brightness of a star in a fixed reference distance, and compare this to the measured brightness, we can say how far away the star is.
The standard reference distance used in astronomy is 10 parsec; the brightness a star would have in this distance from us is called the absolute brightness of the star. Its actual brightness, which we can observe on earth, is called the apparent brightness.
When we determine the distance of an object like a cluster or a galaxy using the H-R Diagram, we have to measure the brightness of lots of stars, including faint ones, in order to get a reliable diagram.
Unfortunately, faint stars can't be seen if the observed object is too far away (because of the inverse-square law), hence one has to resort to other methods:
We use bright objects for which the absolute brightness is known. By measuring the apparent brightness and comparing it to the absolute brightness, one again gets the distance
The only remaining problem is that we have to know the absolute brightness of the observed object. Fortunately, several types of objects exist for which one can determine their absolute brightness without knowing their distances. Such objects are called standard candles
If we closely examine the spectra of far galaxies, we notice that the well-known spectral lines of the elements don't appear at the usual places, but are shifted: they all appear at larger wavelengths than usual.
Because "larger wavelength" corresponds to "redder", this shift is called the red shift. Furthermore, the relative shift is the same for all spectral lines. This red shift was first noticed in 1926 by Hubble, who was also the first to identify individual stars in other galaxies.
An important observation was made by Hubble in 1929 after observing the red shifts of several galaxies: the red shift (and therefore the speed with which they fly away from us, in the Doppler interpretation) is directly proportional to the distances of the galaxies to us! This means that one can write the simple equation
v = H × d, *H = 500 kilometer/second/million parsec. H = 70 kilometer/second/million parsec,