Reach for the Stars

The Highway of the Stars, the Milky Way

In the broader sense of the term, our home is the Milky Way, the star system to which the Sun also belongs. The shape of the Milky Way, a spiral galaxy, resembles a disc. Its diameter spans 100–200 thousand light-years, while the ‘hump’ in the middle is about 10 thousand light-years across (1 light-year = 9,464,616 million km), and its mass is roughly 1,500 billion times that of the Sun. The Solar System is located 27,200 light-years from the centre of the galaxy and has an orbital period of 250 million years.

The centre of our galaxy

The rotational centre of our galaxy can be observed from Earth in the Sagittarius constellation. It is a dense, 10 thousand
light-year thick region with a diameter of 20 thousand light-years, and a mass
10 billion times that of the Sun.
It is made up of stars and interstellar matter.

Disc

The flat disc of the Milky Way is
100–200 thousand light-years across and 100 light-years thick. It is made up of stars, star clusters and interstellar dust and gases.

Halo

Our Milky Way is surrounded by a roughly spherical halo, consisting of an outwardly rarifying mass
of stars, gases and dark matter. 90 % of this mass
is concentrated within a sphere 100 thousand
light-years in diameter.

• The Solar System is located approximately 27,200 light-years from the centre of our galaxy, in the Orion Arm.

• The centre of the Galaxy is a supermassive black hole. Its mass is more than 4 million times larger than the mass of the Sun. The gravitational force of black holes is so strong that nothing, not even light, can escape from them, hence their name.

The Milky Way is made up of hundreds of billions of stars, star clusters, planets, interstellar dust and gas, and dark matter. There is a supermassive black hole at its centre, surrounded by massive stars.

Spiral arms: Viewed from above, the Milky Way looks like a whirlpool. The stars
are arranged in spiral arms, curving outwards from the centre. Our Solar System is situated in the Orion Arm, one of the minor spiral arms.

Stars: There are around 200–400 billion stars in the Milky Way.

Star clusters: Groups of gravitationally bound stars that can be separated into
two main types, globular clusters and open clusters.

Nebula: A cloud of interstellar dust, gas and plasma.

Interstellar gas: Gas contained in interstellar matter, consisting mainly of hydrogen and helium.

Interstellar dust: Dust particles ranging from 0.01 and 0.1 μm that account for approx. 1% of the Milky Way’s mass, consisting of carbon, silicate, ice, methane, etc.