An Astronomical Introduction: Galaxies

Galaxies are vast groups of stars held together by a pulling force called gravity. Inside a galaxy the region in between stars called interstellar, can be empty or full with gaseous dust clouds like nebula, rocks planets, ice elements, etc.

Found Something Interesting? This is our galaxy, we call it MILKY WAY.

People used to think that our own galaxy, the Milky Way, was the whole universe. So, the universe is often called ‘the Galaxy’.

Later, the famous astronomer Edwin Hubble proved that swirly star clouds called spiral nebulae (later named as Spiral galaxies) were not inside our galaxy, they are separated and very far away from ours. He also showed that the farest galaxies are moving farther with time.

Suddenly, we had found the universe is a lot bigger. And, it is expending.

With increasing distance, our knowledge fades, and fades rapidly. Eventually, we reach the dim boundary—the utmost limits of our telescopes. 

Edwin Hubble

(1889-1953)

Types of Galaxies

Even though there are many type of classifications for galaxies but most popular classification is based on by their shapes and they are three types. 

Spiral and barred spiral galaxy

A barred spiral (SB) galaxy is a spiral galaxy (S) with a central bar-shaped structure composed of stars.

Irregular Galaxy

Irregular galaxy (Irr) doesn’t fit into spiral and elliptical type of galaxies. Some of deformed spiral or elliptical became irregular galaxy also.

Elliptical and Lenticular Galaxy

A lenticular galaxy ( S0) is a type of galaxy intermediate between an elliptical (E) and a spiral galaxy (S).

A diagram called "Hubble Tuning Fork Diagram" can show the galactic classification.

Other Well-known Types of Galaxies

The  constellation of Ursa Major (The Great Bear) is home to Messier 101,  the Pinwheel Galaxy. One of the biggest and brightest spiral galaxies in  the night sky, Messier 101 is also the subject of one of Hubble's most  famous images (heic0602). Like the Milky Way, Messier 101 is not alone, with smaller dwarf galaxies in its neighbourhood. NGC  5477, one of these dwarf galaxies in the Messier 101 group, is the  subject of this image from the NASA/ESA Hubble Space Telescope. Without  obvious structure, but with visible signs of ongoing starbirth, NGC 5477  looks much like an archetypal dwarf irregular galaxy. The bright  nebulae that extend across much of the galaxy are clouds of glowing  hydrogen gas in which new stars are forming. These glow pinkish red in  real life, although the selection of green and infrared filters through  which this image was taken makes them appear almost white. The  observations were taken as part of a project to measure accurate  distances to a range of galaxies within about 30 million light-years  from Earth, by studying the brightness of red giant stars. In  addition to NGC 5477, the image includes numerous galaxies in the  background, including some that are visible right through NGC 5477. This  serves as a reminder that galaxies, far from being solid, opaque  objects, are actually largely made up of the empty space between their  stars. This  image is a combination of exposures taken through green and infrared  filters using Hubble's Advanced Camera for Surveys. The field of view is  approximately 3.3 by 3.3 arcminutes. 

Dwarfs Galaxy

NGC 5477, small size

NGC 5128 Active Galaxy

Active Galaxy

NGC-5128,Active Galactic Nucleus (AGN)

The Antennae Galaxies, a pair of peculiar interacting galaxies

Peculiar Galaxy

The Antennae Galaxies, peculiar interacting galaxies

NGC 7742 Seyfert galaxy

Seyfert Galaxy

NGC 7742, bright core

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