If you walk away from the city lights on a clear night without a full moon, you will be able to see thousands of points that form dozens of constellations . All these stars are balls of gas and plasma similar to our own Sun, and yet they are so far away that to the naked eye they are nothing more than faint points of light . Some of those spots shine brighter than others, but they are all similar. So much so that we could perfectly well think that they are all located at the same distance and that the only difference is in their size . This thought ancient civilizations, such as the Greek. According to its philosophers and thinkers, the stars were all contained in a sphere that surrounded our planet and rotated every day.
We now know that in addition to having very different sizes, each star is located at a different distance from each other . And we know this because we have been able to measure the distance to millions and millions of stars, galaxies and other astronomical objects. This measurement can be done by different methods, which take advantage of the laws of physics.
Parallax
The first time that the parallax of a star was used to measure its distance was in 1838 , by the German astronomer Friedrich Bessel . This method takes advantage of a very simple and well-known effect and takes it to atomic scales. We can understand it with an everyday experience. Place the index finger of either hand in front of your eyes, with your arm fully extended . Place it in front of a painting, television or a landscape, so that it partially covers them. If you now close only one of your eyes, with the other you will see that your finger covers a portion of the image in front of you . Without moving your finger, open the closed eye and close the open one . You will see that the portion covered by your finger of the far image has changed . Your finger seems to have moved, although all you have done is change your eye.
Well, this apparent movement of your finger is what is known as parallax. If we exchange the outstretched finger for a star and the alternately open eyes for two telescopic observations made at opposite points in the Earth’s orbit, we obtain a method for accurately obtaining the distance to nearby stars. With this method, however, we can only measure the distance to the closest stars , up to distances of a few thousand light years.
variable stars
For more distant stars we need to resort to other methods, which in turn take advantage of the results of the parallax measurement. In astronomy there is the concept of “ standard candles” which are types of stars that, due to their internal physical mechanism, we know to have a well-defined luminosity , so that we can calculate their distance depending on how the measured luminosity differs from the expected one. A good example is the Cepheid stars. These are variable stars, with periodic changes in their luminosity. What is most interesting is that the period of these changes is directly related to its luminosity , as observed by Henrietta Leavitt in 1908 . So if we can measure the distance to a small number of these stars with their parallax, then we can use the relationship between oscillation period and luminosity to get the distance to even more distant Cepheids, for which the parallax would be tiny. With these stars , the distance to some nearby galaxies was measured for the first time , a century ago.
Colour
We have also observed, after studying thousands of nearby stars, that there is a clear relationship between the color of a star and its luminosity. This relationship is captured in the Hertzsprung-Russell diagram .
The color of a star is relatively easy to measure , with a spectrometer, with which we can obtain its theoretical luminosity and compare it with that observed directly through our telescopes . This method, however, is not perfect, as it requires making assumptions about the size of the star studied. As you can see in the diagram above, a red star can have different luminosities, depending on whether it is a red dwarf like Proxima Centauri or a red giant like Betelgeuse. However, using other methods we can often rule out the least likely sizes and narrow the distance accurately .
red shift
You probably know that the universe is expanding . This means that distant galaxies are moving away from ours. Like little dots on an inflating balloon, all galaxies are moving away from each other (unless they are gravitationally connected). Also, the further away a particular galaxy is, the faster it will be moving away from the Milky Way , because more intergalactic space will be expanding between them. This relationship is so obvious that we can use the speed at which a galaxy is receding to establish its distance. We can do this by measuring the red shift (consequence of the Doppler effect) suffered by the frequencies of the light emitted by said galaxy. This method of course will only serve us to measure the distance to incredibly distant galaxies or clusters of galaxies, beyond our Local Group and our cosmic neighborhood.
