History of Galaxies | Classification of Galaxies | Satellite Galaxies | Galaxies Part-2

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Our own Milky Way galaxy is home for billions of stars, planets, Nebulae and more. We are located in suburbs at a distance of 26,000 light away from the galactic center.

History of Galaxies:

At the beginning of 20th century, astronomers are trying to find all of these. In 1920, astronomers have only two ideas about the Universe. One is the Milky way and the other is spiral nebulae.

American scientist Harlow Shapely argued that, everything we see was in the Milky Way. He thought that the spiral nebulae seen to be rotating was in the Milky Way. Another American astronomer Heber Doust Curtis argued that, the spiral nebulae was like our Milky Way and Milky Way is one of them. He thought, if galaxies were as big as Shapely said then they should be large and impossibly far away.

After few years, observations unlocked the mystery about the Universe. American astronomers Edwin P. Hubble and Milton L. Humason made observations on the great spiral nebula M31 in Andromeda, using the largest telescope at that time. They found several number of stars pulsating in the M31, called Cepheid Variables. They rapidly change their size and temperature, leading the brightness to increase or decrease suddenly.

Andromeda_galaxy_Into_the_dark_space
Image credit: NASA/JPL-Caltech

Cepheid Variables helps in finding distances. From observations, they found the distance to M31 as astonishing 0.9 million light years, which is clearly outside the Milky Way. Finally, they concluded that the Great Andromeda Nebula was actually the Andromeda galaxy. At that moment, our view towards the Universe had been changed. Every galaxy consists of billions of stars, planets and huge amounts of gas and dust. Galaxies have different sizes and variety of shapes.

Classification of Galaxies:

Galaxies are mainly classified based upon their shapes as,

1. Spiral galaxies

2. Elliptical galaxies

3. Peculiar galaxies

4. Irregular galaxies

Types_of_galaxies_Into_the_dark_space
Image credit: By Antonio Ciccolella - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=23121824

Spiral Galaxies:

We live in one of the spiral galaxies. Spiral galaxies are flat rotating disks of stars, gas and dust. They consist of central bulge of older redder stars and a huge extended halo of older stars. These galaxies mostly have stars in the huge curved arc like structures called spirals, in which most of the stars, nebulae are present.

They also come with different designs, some have patchy arms called flocculent spirals. They contain huge nebulae glowing brightly and huge dust clouds obstructing the light from the centre. Some spiral galaxies look flat due to their angle towards us. They form disc like shapes (flat) due to the spin of the galaxy.

Elliptical Galaxies:

Elliptical galaxies are shaped as Elliptical. They don't have spiral arms but are swollen. Some of the elliptical galaxies are small called as Dwarf Ellipticals. They contain lot of older stars and don't have enough gas and dust to form new ones. We don't know much about their formation, but we have a thought that they might have been formed due to galactic collisions.

Galactic collisions means the entire galaxies will collide to form new massive and larger galaxy. Galactic collisions takes place for millions of years due to their huge size. In early stages of collision, due to tidal effects, the stars near to the galaxies pulled towards strongly and stars which are far away will get stretched outwards forming long curved arc like structure.

Gas clouds also merge together to form stars at furious rates. Collisions of galaxies may glow up them, due to the collisions of the gas clouds in them. When galaxies collide they merge and form a newer elliptical type galaxy.

Sometimes large galaxies consume the smaller ones. In fact, our own Milky Way galaxy is in the process of consuming some other galaxies like Terzan 5 and other dwarf galaxies. These galaxies were torn apart by Milky Way.

Peculiar Galaxies:

Peculiar type of galaxies are the remnants of galactic collisions. They have uneven shapes. Sometimes smaller galaxies pass right through the larger one forming the ring type of galaxies like Cartwheel galaxy, Hoag's Object etc., But we don't know much about them.

Irregular Galaxies:

Irregular galaxies are small and shapeless. These are also the victims of the galactic collisions. Smaller galaxies don't have perfect shapes due to the effect of low gravity along the galaxy.

Satellite Galaxies:

Many galaxies have satellite galaxies. Our own Milky Way galaxy has dozens of such type of satellite galaxies. In which, two biggest satellite galaxies are Large and Small Magellanic clouds, which are visible from the southern hemisphere. Both Large and Small Magellanic clouds are irregular in shape.

Milky_Way_satellite_galaxies_Into_the_dark_space
Image credit: By !Original: AndrewRTVector: Slashme - Richard Powell originally from http://www.atlasoftheuniverse.com, CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=10672565

Most of the large clouds form stars in them like The Tarantula Nebula which is producing many stars in it. It is in the process of forming globular cluster. There are other types of galaxies too, such as active galaxies. Some of the galaxies are found in groups called clusters.

After every discovery in the Universe at long distances, our eye towards the Universe is increasing and may increase in future. At first we thought our Earth is our Universe, later it changed to solar system, later to Milky Way and now we are sticked to Observable Universe.

Many more to come about galaxies, stay tuned.

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