GALAXIES, THE COSMIC ISLANDS

Spiral galaxies

Characterized by a nucleus of stars, prevalently old (over 10 billion years) from which "spiral arms" branch off, formed by young stars (few tens of million years, up to one billion years), gas and dust.

NGC 300 spiral galaxy, of the Sculptor group, at a distance of approximately 10 million light years. The blue colour of the arms indicates the presence of young and brilliant stars. The reddish colour of the nucleus indicates the presence of old stars. (JPEG, 211 K) (ESO)

 

NGC 1365 spiral galaxy. Its nucleus is crossed by a bar of stars, at the extremity of which two spiral branches, made of stars, gas and dust, depart. Its distance is 50 million light years. This type of spiral galaxies is called "Barred Spiral". (JPEG, 305 K) (ESO)

Elliptic and lenticular galaxies 

They are formed by a nucleus of old stars (over 10 billion years) crossed by a disc of younger stars with dust and low quantities of gas.

The M104 "Sombrero" galaxy is one of the most spectacular objects in the southern sky. It is 44 million light years far from us. The dark face is formed by dust that absorbs the light emitted by the nucleus of the galaxy. (JPEG, 85 K) (ESO)

Irregular galaxies

They lack any kind of symmetry. They are constituted by young stars, gas, and dust.

The Large Magellanic Cloud. It is a satellite galaxy of the Milky Way, 170,000 light years far. (JPEG, 397 K) (ESO)

CLUSTERS OF GALAXIES

The galaxies tend to gather in clusters constituted by hundreds or thousands of galaxies.
The nearest cluster of galaxies is that of the Virgin, about 50 million light years far, formed by approximately 1,000 galaxies.
The clusters of galaxies, tend to gather in larger structures, called "superclusters". Their shape is that of a "pancake" quite thin (only 10 million light years) and with diameters of 150 million light years.

This cluster of galaxies is 5 billion light years far. In the picture it is possible to count over 200 galaxies. (JPEG, 76 K) (NASA-STScI)

THE MILKY WAY, OUR GALAXY

The Milky Way is the galaxy in which we are living. It contains approximately 100 billion stars and is 15 billion years old.

The Solar System is situated on one of the spiral arms of the Milky Way, our galaxy, which in turn is part of a cluster of about thirty galaxies, called Local Group. (JPEG, 130 K) (Drawing: Michelangelo Miani)

 

The Milky Way is constituted by a dense nucleus of stars, as old as the galaxy, around which the other stars orbit. The spiral arms depart from the centre of the galaxy, forming a disc with a diameter of 100,000 light years. (JPEG, 154 K)

The spiral arms contains the younger stars, from a few million to a few billion years old. The spiral arms also contain gas and dust, besides the stars.
The Sun is situated at the periphery of the galaxy, approximately 28,000 light years from the centre and requires approximately 250,000,000 years to complete an orbit around the centre of the Milky Way.
The Milky Way, together with the Large Magellanic Cloud, the Andromeda galaxy and about fourty other smaller galaxies constitutes the so-called Local Group, a mini-cluster of galaxies which, gravitationally attracted by the near cluster of the Virgin, is "falling" on it at the speed of 220 km per second.

The Andromeda galaxy. The largest galaxy of the Local Group. It is 2,400,000 light years far from the Milky Way. (JPEG, 406 K) (D.Berry, STScI)

 

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