Unit 1:  Our place in the Universe                                          1   2   3   4  

3. The components of the Universe

3.1. Galaxies

Galaxies are enormous accumulations of matter in the form of interstellar dust, nebulae and stars, some of which have planetary systems.

All these components keep together and revolve around the galactic centre (black hole) due to the gravity.

In galaxies, the space among stars is not empty. It contains a mixture of gases (helium and hydrogen mainly) and interstellar dust.

It is supposed that in the observable universe (a sphere of 13.700 light-years in diameter which surround us) about 100.000 million galaxies exist (10¹¹)

Edwin Hubble was the first to made a classification of galaxies. According to their shape, galaxies can be:

• Elliptical. They are elipse shaped and are little structured. They are made up of old stars and have little interstellar matter. They can be really huge and their origin seems to be the collision and fusion of other galaxies. They are called with the letter E and a number from 0 to 7 (it indicates the excentricity) The E0 type is practically spherical, while the E7 one is very elongated.

• Lenticular. They are called as S0. They are disc-shaped and are similar to spiral galaxies but without arms.

• Spiral. They are rotating discs of stars and interstellar matter. In them it is possible to distinguish a bulky central part  formed by old stars, and a variable number of arms formed by young stars. Letter S is used joined to other letter (a, b, or c) which indicates if the arms are near the nucleus (Sa) or separated from it (Sc).

• Barred Spiral (SB).They are similar to spiral galaxies but is possible see a bar which conects the nucleus with the arms. They are classified as SBa (long central bar) y SBc (short central bar).

Besides these types there is another kind of galaxies, the irregular galaxies (I) without defined shape and caotic structure. They are formed by young stars and a lot of interstellar gas ans dust. They seem to be the result of star explosions and formation of other stars from the remains.

3.1. Answer these questions:

       a. Which are the components of a galaxy?

       b. What criterion is used to classify galaxies?

       c. What is the difference between a spiral galaxy and a barred spiral galaxy?