The galaxies
Here on the left, the southern
sky is represented for October 15, at 10pm. In front of you stands the great Pegasus square. From the upper left corner the Andromeda constellation departs, whose stars are marked in yellow. If the night is clear and there are no disturbing nearby lights, it is possible to see a flock of diffuse light in the position marked in red. This object is called the Andromeda Nebula. It is object no. 31 of the Messier catalog, so it is also known as M31. Currently the distance to Andromeda is estimated as slightly more than 2 million light years. Notice also that a little higher, the disk of our own Galaxy can be seen edge on.
A long exposure picture like this shows that M31 is a rather complicated structure made by stars. In order to find the distance to M31 it is not possible to use the parallax method. You have indeed seen that this method allows reliable measurements only for the distances of stars relatively close to the Sun. One can then use some kind of standard candle. As you know, this term indicates an object whose luminosity is known and that is easily recognizable. In 1924 the astronomer Edwin Hubble discovered that the Andromeda galaxy contains variable stars of the Cepheid type. The luminosity of these stars varies periodically, going from a minimum to a maximum, and this behavior repeats itself in a certain period of time. It is believed that these variations are due to the fact that the star is pulsating. Even before 1924 it had been discovered that the more luminous a Cepheid is, the longer is its period. This period-luminosity relation is very important. Indeed it allows us to establish the intrinsic luminosity of a star by determining its period, that is with a rather simple operation. Using this relation Hubble found the luminosity of the variables contained in M31, and comparing it with their apparent luminosities he could determine the distance of the Andromeda nebula. This distance resulted much larger than that of the farthest objects then known. Hence Hubble concluded that M31 is an isolated system of stars, separated from our Galaxy. Actually several spiral nebulae had been known for a long time, and the Hubble discovery made clear that the stars are not distributed homogeneously in the Universe, but they gather in systems like M31. It was also known that the nebulae have very different apparent dimensions. Assuming that they have, more or less, the same intrinsic dimension, this observation means that they are at larger or smaller distances from us. M31 has the largest apparent dimension, so one can think that it is the closest one. And for this reason Hubble chose it for his studies. After the realization of larger and larger telescopes, and the systematic use of photography, it has been possible in the past to make sky atlases containing fainter and fainter objects. Thousands of very far stellar systems were then discovered, and it has been learned that the spiral objects are just a fraction of the existing ones. Of course, one of the first tasks that were carried out, was the classification of the galaxies. In order to understand what galaxies are and how they formed, we cannot limit to a classification. For each astronomical object that you have seen, you tried to determine its basic physical properties, and this is what you can do now. Of course, galaxies are objects much more complex than the Solar System or a star cluster. So it is also harder to reach some conclusions, and the uncertainties are anyhow large. Some estimates are however enough in order to have a good picture. Now you can have the answer to the following questions. The enormous distances of the galaxies have one consequence: do you remember that the light does not arrive immediately after being emitted, but it takes a time proportional to the distance of the source? The galaxies that we see are observed how they were million or billion years ago. For example the picture of M31 shows it how it was 2 million years ago.As every other astronomical object that you have seen, even a galaxy emits radiation of all frequencies. Even outside the visible spectrum, the emission of the so called "normal" galaxies is not much different than the sum of that of its single stars. Besides this emission one observes ultraviolet and radio emission from the star formations areas. In the infrared the emission coming from the dust is important and in the radio wavelengths it is possible to reveal the radiation emitted by the neutral, cold hydrogen. However, besides the common systems, there are also active galaxies, where, instead, large amounts of energy are released by mechanisms different from the stellar ones, in particular from the nucleus of these galaxies. This video shows you how the M81 galaxy appears, by looking at it with "eyes" sensitive to several bands of the spectrum. How does the relative importance of the emission of the arms and the bulge change?
You can now watch some videos showing better some of the properties of the galaxies that you have seen. Firstly you will see the variation of light for a Cepheid variable. Then a simulation will show how the galaxies could have been formed some ten billion years ago. Finally, you will able to see how a quasar could appear from a close view, according to the current ideas.
In Nature it often happens that the astronomical objects tend to group together, like the planets in planetary systems, and the stars in clusters and galaxies. Even galaxies follow this trend. Go then forward to the following page, whose password is "alcor". Clustering and motions of the galaxies
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