2. OUR GALAXY

Talking book 

Galaxies are the places in the Universe where stars, nebulae and planets are grouped together. The Earth is one of millions of celestial bodies found in our galaxy, the Milky Way.

2.1. The Milky Way

Talking book 

The name of our galaxy comes from its whitish appearance. We can often see it on a cloudless night in areas without light pollution. It is also known El Camino de Santiago.

The Milky Way is composed of millions of stars and has four arms. The Solar System is located in the Orion Arm. The galaxy has three main parts: the nucleus, the disk and the halo.

1. Halo: this surrounds the core and the disk and contains isolated stars.
2. Disk: this is formed by four arms containing the youngest stars.
3. Nucleus o bulge: this is the centre of the galaxy and it is formed by the oldest stars.

2.2. The night sky: the constellations

Talking book 

All the stars we see in the night sky belong to the Milky Way. They are the part of the Milky Way that is visible from Earth.
Each star is a different distance from Earth, but with the naked eye we can't perceive the depth and they all appear to be the same distance away. As a result, stars appear to be close to each other, but in reality, they are far away from each other. Due to this perception, the stars in the Milky Way appear to be grouped in different imaginary shapes. These are called constellations. Not all constellations are visible from any place on Earth or at all times of the year. From the Northern Hemisphere we can only observe 37 of the 88 constellations.

2.3. Stars

Talking book 

A nuclear reaction inside stars called nuclear fusion occurs. This results in the release of a lot of energy in the form of light and heat.

Stars are distinguished by their colour, size and brightness.

• The colour of a star is defined by its surface temperature. 
  Weblink 3: Cosmic classifier 
  Key concepts

  • The Milky Way is a spiral Galaxy that belongs to a galaxy cluster called the Local Group. 
  • The Milky Way has a nucleus of old stars, a disk with four spiral arms and a halo.

  • The constellations are groups of stars seen from the Earth that form imaginary shapes.
  • Stars are large spheres of gas that emit light and heat. 

   

As a star consumes the hydrogen it contains, it changes colour, until the hydrogen is used up and the star dies.

• The size of a star is measured relative to the Sun. Stars that are bigger are called giants; stars that are the same size are medium-sized and stars that are smaller than the Sun are called dwarf stars.

• The brightness of a star varies according to how far away it is, the quantity of energy it releases and its size.

Stars form groups called star clusters. These clusters can be very dense (globular clusters); or less dense, formed by disperse stars (open clusters). Some stars attract different bodies and form a planetary system. This is the case of our Solar System.