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Milkyway Galaxy – Definition and Composition

Grade 6
Aug 22, 2022

Milkyway Galaxy

 Key Concepts

  • Galaxy
  • Milky way galaxy


Clusters of stars held together by gravity is called a galaxy. Astronomers think that our galaxy contains more than 200 billion stars and that the universe may have around 100 billion galaxies. Stars move around the center of their galaxy. Galaxies vary in size, structure, and age. Based on their shapes they are divided into three groups—elliptical, irregular, and spiral

A spiral galaxy looks like a vortex. Some of the spiral galaxies are barred galaxies. These galaxies have a ‘bar’ of stars, gas, and dust all through their center. The spiral arms appear from this bar. 

The shape of an elliptical galaxy is like a football and has no spiral arms and no dust or very little dust. 

An irregular galaxy has no particular shape; they are irregular in shape. The irregular shape may be due to the collision of the galaxy with other galaxies.   

Types of galaxies 


biospherePlanets and Orbits

We all know that gravity is the force of attraction among all objects. Gravity causes the things to fall onto the floor, and this force keeps planets in orbit. The amount of gravitational force between the Sun and a planet depends on its mass. The higher the mass of an object, greater is the attraction between them. Distance is also one of the factors that affect gravitation. The higher the distance between the planets and the Sun, the lower will be the pull of gravity. 

 Force of gravity between sun and planets 

The planets revolve in orbit due to inertia. Inertia is the tendency of a moving thing to remain in motion. Gravity will pull objects, i.e., planets, towards the Sun, because of the huge mass of the Sun. This does not take place because the planets have inertia, and they are moving. Inertia causes the planets to move in straight lines. The balance between the force of gravity and the inertia of the planets keeps the planets moving in their orbits around the sun. The movement of planets is in a straight line due to inertia, but gravity pulls the planets and makes their path in a curved shape into orbits around the sun. 

Milkyway Galaxy: 

Milky Way is a spiral galaxy where stars are grouped around a core. All the stars in the Milkyway and Sun orbit the core. If the distance between the star and its core is less, then the star orbits faster. Many spiral arms come out of the core. Our solar system is located on one of the spiral arms of the Milkyway galaxy. The arm consists of gas and dust. The center of the Milkyway cannot be seen because there is a lot of dust between the core and us. But from Earth we can see many stars. 

Milkyway galaxy 


What is Big bang? 

When light passes through the drop of water, it separates into various color bands because white light is a combination of rainbow colors. This band of various colors is called the spectrum. The heated gases of stars produce light, and when this light passes through an outer atmosphere of a star, some light is absorbed by the outer atmosphere of the star. When scientists noticed a spectrum of this starlight, they saw the absorbed light had fallen from the spectrum and formed dark lines called absorption lines. When you look at a spectrum from a galaxy, you can see that the absorption line pattern does not exist at the same point in the spectrum. The position of the pattern looks shifted because the galaxies are moving away from each other, and the space between them has increased. 

When the absorption lines of the spectrum shift towards the end of the blue color of the spectrum, it is called a blueshift. This means the galaxy is moving in our direction. If the absorption lines are shifted towards the red zone of the spectrum, then it is called a redshift; this means that the galaxy is moving away from us. The absorption lines of almost all galaxies are red shifted. As there is an expansion in space, the absorption lines show an expansion redshift. There is no center point to this enlargement. Suppose if we consider ourselves to be in the center of the galaxy, then we can see that the other galaxies are moving away from each other. 



 Redshift and blue shift 

Astronomers think that in the past, galaxies must have been close to each other. In the beginning, the universe was very small, hot, and dense, but after that, the universe expanded very quickly. This expansion of the universe is called the big bang, and it sent matter in all directions. 

As per Big Bang theory, the universe is enlarging, and its density and temperature are decreasing. Galaxies and stars are formed by the collection of matter into clumps due to gravity. 

Formation of the solar system: 

Scientists think that after many years of the big bang, dust and gas present in Milkyway collected into a nebula that is huge and large enough to rotate. A shock wave from a supernova shot at the nebula, and due to the wave, clusters of dust and gas were formed. Gravity causes these clumps of dust and gas to contract. As the nebula contract, it starts rotating. At the center of the cloud, gravity increases, and most clumps get pulled towards the center of the cloud. These clumps joined to form a protostar, and the leftover clumps became protoplanets, also called young planets. The center part of the cloud became more of dust and gas, and also became hot and huge. The high temperature causes the star to become a main-sequence star, the sun with planets around it. 


Formation of the solar system 


  • Clusters of stars held together by gravity is called a galaxy.
  • Galaxies are divided into three groups based on their shapes.
  • They are elliptical, irregular, and spiral. The amount of gravitational force between the Sun and a planet depends on the mass of the latter.
  • Inertia is the tendency of a moving thing to remain in motion.
  • Milkyway is a spiral galaxy where stars are grouped around a core.
  • Galaxies and stars are formed by the collection of matter into clumps due to gravity.


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