Galaxies and Hierarchic Structures in the Universe Revision Notes

In these revision notes for Galaxies and Hierarchic Structures in the Universe, we cover the following key points:

• What are galaxies? How are they organized?
• What are the main features of our galaxy?
• How many types of galaxies are there? What are the main features of each of them?
• How is the Universe is organized?
• What is cosmic radiation? What does it cause?
• What is dark matter? Can we see it?

Galaxies and Hierarchic Structures in the Universe Revision Notes

A galaxy is a huge collection of gas, dust, and billions of stars and their solar systems held together by gravity. The Milky Way is the galaxy our Solar System belong to. It has about 200 billion stars.

The Milky Way has a disc shape with a spherical bulge at its centre, called the bulb. The diameter of the Milky Way is 100 000 light years while the bulb thickness is 3000-4000 light years. However, the thickness of the bulb around the centre of galaxy is about 10000 light years. The Milky Way is a spiral galaxy.

We can identify three important parts of our galaxy containing a large number of stars each: disc, bulb and halo. All these three parts contain groups of stars and lonely stars that revolve around the centre of galaxy.

The Sun is 28 000 light years (or about 2/3) away from the centre of the Milky Way galaxy. It completes one revolution about the centre of galaxy every 200 million years. We are able to see different regions of the Milky Way due to rotation of the Earth around the Sun.

Galaxies are classified in three major categories determined by their shape: spiral, elliptical and irregular. Spiral galaxies are similar to the Milky Way, only the dimensions vary. Elliptic galaxies have 3-dimensional elliptic shapes (like eggs). They are common in the universe but are less visible because the brightness they produce is not very high. Irregular galaxies represents collections of stars without any specific shape.

Stars may exist alone (such as the case of our Sun), in pairs, in groups of three, four, or even tens, hundreds and thousands. When the number of stars in a group is large, they are known as clusters. Clusters may be of two types: open and spherical clusters. The dimensions of open clusters reach tens of thousands of light years. As for spherical clusters, they exist mainly in galaxy halos and contain stars ranging from a few hundred thousand to millions in number. In spherical clusters, stars are much closer to each other than in open clusters.

All groups of stars share a common feature: they revolve altogether around the centre of their galaxy.

Galaxies themselves are organized in even larger structures based on the mutual attraction effect caused by gravitational forces. These structures are known as groups of galaxies. For example, the Milky Way galaxy is part of a group that includes 31 galaxies known as the Local Group.

Groups of galaxies on the other hand, are not distributed uniformly in space; they tend to gather in supergroups leaving enormous empty spaces between them.

Cosmic radiation is an important component of the Universe. It does not belong to any of star structures mentioned above but it fills uniformly the entire Universe. Cosmic radiation is isotropic (equal in all directions), and it has the same features of a black body at temperature 2.73 K. For this reason, the temperature of sky is taken as 2.73 K.

Dark matter is, potentially, a kind of invisible matter that fills the entire universe. The only indicator that dark matter exists is the mathematical difference resulting in the total gravitational force existing in the universe and the theoretical gravitational force if the existence of this dark matter is not considered. It is probable that the amount of dark matter in the universe is 10-100 times greater than that of the actually detectable matter.