Chapter: 11th 12th std standard Class Physics sciense Higher secondary school College Notes
Milky Way galaxy
Milky Way galaxy
Milky Way looks like a stream of
milk across the sky. Some of the important features are given below.
(i) Shape and size
Milky Way is thick at the centre
and thin at the edges. The diameter of the disc is 105 light years.
The thickness of the Milky Way varies from 5000 light years at the centre to
1000 light years at the position of the Sun and to < 500 light years at the edges. The Sun is at a distance of about
27000 light years from the galactic centre.
(ii) Interstellar matter
The interstellar space in the
Milky Way is filled with dust and gases called inter stellar matter. It is
found that about 90% of the matter is in the form of hydrogen.
(iii) Clusters
Groups of stars held by mutual
gravitational force in the galaxy are called star clusters. A star cluster
moves as a whole in the galaxy. A group of 100 to 1000 stars is called galactic
cluster. A group of about 10000 stars is called globular cluster.
(iv) Rotation
The galaxy is rotating about an
axis passing through its centre. All the stars in the Milky Way revolve around
the centre and complete one revolution in about 300 million years. The Sun, one
of the many stars revolves around the centre with a velocity of 250 km/s and
its period of revolution is about 220 million years.
(v) Mass
The mass of the Milky Way is
estimated to be 3 × 1041 kg.
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