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

4. Our place in the Universe

4.1. Our galaxy: the Milky Way

The Milky Way is a barred spiral galaxy (SBa type).

It is formed by between 100,000 and 300,000 millions of stars. It is 100,000 light years in diameter. Its age is estimated in about 13,000 millions of years and its mass is about a million of millions (10¹²) of solar masses.

It is formed by different parts:

- Centre. It is occupied by a supermassive black hole around which the rest

  of the elements rotate.

- Bulge. It is formed by several millions of old stars very close together.

- Disc. It is where the young stars, nebulae, and cosmic dust al located.

  It is flat-shaped and is formed by four main arms (Perseus, Sagittarius,

  Centaurus and Cygnus) and other secondary four ones. In one of these

  secondary arms (Orion), the Solar System is.

- Halo. It is spherical-shaped and involves the other parts. It contains

  globular clusters (formed by very old stars) and isolated stars. 

Video: Guide of our galaxy (ESA)

The Milky Way is part of a group formed by 54 galaxies and called Local Group.

It is divided into two groups:

- 20 galaxies gravitationaly joined to the Milky Way and much smaller the it.

- 15 small galaxies that surround the galaxy M31 (Andromeda). Andromeda is 250,000 light-years in diameter. It is more than the doble of our galaxy although its mass is no bigger than 4.10¹¹ solar masses. It is at 2.5 million light years from us.

In addition, there is an elevated number of small galaxies do not related to neither of them.

The Local Group is in the border of a bigger structure, the Virgo Cluster, formed by between 1,300 and 2,000 galaxies and that is located in the centre of another bigger structure, the Virgo Supercluster.

4.1. Answer the questions:

 a.  Why is almost impossible to know how many stars are really in the Milky Way?

 b.  What galaxies form part of the Local Group?

 c.  Which parts can be distinguished in the Milky Way?

4.2. To measure distances in the space we need bigger units than those used

        in the Earth.

a.   Which unit is used to measure distances within the Solar System?

      Which one will we use to measure distances between stars?

      Indicate the equivalence of both in kilometres.

b.   Andromeda galaxy is 2.5 million light-years from the Earth. How much time would it take us to arrive there if we could travel at the speed of light?

c.   If our spacecraft could only reach 40,000 Km/h, how long does the journey be?  

      Express the result in years.