GalaxygalaxyGalaxy is a tremendous collection of billions of stars, planets, gas and dust bound together by forces of gravity. A diameterdiameterdiameter of a galaxy is of order of thousands light‑years.

The stars observed in galaxies are divided into so‑called young and old stars. They are distinguished on the basis of the content of elements heavier than helium. Old stars contain few metallic elements, while young stars contain a lot of them.

Spiral galaxies are the most common in the Universe. They have both young and old stars. They are given the symbol „S”. Some of the spiral galaxies have in the middle a bar of stars and other objects and are called barred spiral galaxies (denoted by
a symbol “SB”). The further classification of spiral galaxies depends on e.g. the number of its arms.

Elliptical galaxies have elliptical shape and do not have a disk or arms. They contain mostly old stars, and possess little gas or dust. Elliptical galaxies have a symbol “E”.

Irregular galaxies (Irr) have no regular shape. They tend to be smaller objects and to have very hot newer stars mixed in with lots of gas and dust. The examples of irregular galaxies are The Large and Small Magellanic Clouds which orbit around the Milky Way.

Bulge is round structure placed centrally in a galaxygalaxygalaxy. It is made up of old stars, gas and dust. The edges of the bulge are smeared and surrounded by the halo.

In spiral galaxies bulge is surrounded by a flatten disk containing mostly young stars and also gas and dust. These objects are organised in the spiral arms.

The spiral arms in the disk begin in the bulge and extend to the outer edges of a galaxygalaxygalaxy. They are present only in spiral galaxies.

Halo is a space filled with gas, dust, dark matter and clusters of old stars (globular clusters) as well as single old stars. Dark matter are invisible objects that interact gravitationally with other objects, but they do not emit or reflect light, which makes them unobservable.

The white belt seen on a dark night on the sky is called the Milky way.

The Milky Way is a barred spiral galaxyspiral galaxyspiral galaxy formed several billions of years ago and has a diameterdiameterdiameter of about 100000 light‑years. What our eyes see is the densest part of the Milky Way, namely the disk and bulge. Our Galaxy is made of stars, planets, comets, meteors, asteroids and nebula (clouds of dust and gas). All these objects rotate around the centre of the Galaxy. The Solar System is a part of the Milky Way. Its distancedistancedistance from the centre of the Galaxy is about 28000 light‑years. The period of the Sun’s orbit around the centre of Galaxy is 250 million years. Our Solar System lies on a spiral arm called the Orion Arm.

galaktyka

promieniowanie podczerwone

galaktyka spiralna

galaktyka eliptyczna

galaktyka nieregularna

średnica

odległość

rok świetlny