What is a Galaxy?
Mother Earth, our home planet, is a part of the solar system which consists of 9 planets (including Pluto). The solar system where we live is a part of the Milky Way Galaxy.
A galaxy is a system of millions of stars consisting of gas and dust which are bound by each other through gravitational force. Our Milky Way Galaxy has a supermassive black hole and its centre is known as “Sagittarius A.”
[Image will be Uploaded Soon]
During night time, we can see the stars of the Milky Way Galaxy if we look up in the sky. We can also have a choice to view the diary band of the Milky Way, when it is too dark at night and far from citylights.
Besides our Milky Way galaxy, there are millions and millions of galaxies in the known universe. The Hubble Space Telescope discovered more than 10,000 galaxies by viewing a small part of space continuously for 12 days.
What is the Milky Way?
Our Sun is a star, which is the central part of our solar system. The Sun along with the planets is part of the Milky Way galaxy. A galaxy is a system of stars, dust and gas which are bound by gravitational force. Galaxies are found in various sizes, shapes and colors.
The Milky Way is our home galaxy. The Milky Way is spirally shaped having length of about 1,00,000 light-years, and width of 10,000 light-years. It contains billions of stars which include our sun. You can get a glimpse of the band of stars in the Milky Way from a dark place at night. Our solar system is located somewhere between the centre and the edge of the Milky Way.
[Image will be Uploaded Soon]
The band of the Milky Way is usually seen from dark sky areas. Scientists or space lovers use the Atacama Large Millimeter/sub-millimeter Arra (ALMA) antenna to see it.
What Galaxy is Earth in?
The Milky Way: It is the home galaxy of our planet. The spiral arm of the Milky Way is made up of giant clouds of gas and dust with star clusters. Some parts of the galaxy are also made of dark matter. Our galaxy consists of four spiral arms which emerge from the central bar.
What Type of Galaxy is the Milky Way?
Barred spiral galaxy- The shape of the Milky Way galaxy is of barred spiral type. The stars that we see at night are part of it. The galaxy appears as a milky band of light in the sky, that's why it is called the Milky Way Galaxy.
A bright band of light stretches across the sky in the night sky. It can be seen from naked eyes when the sky is very dark. This band is the disk of stars that forms the structure of the Milky Way. It depicts that the Milky Way is flat.
What is the Milky Way Made of?
The Milky Way galaxy is made up of approximately 90% dark matter and about 10% “luminous matter.” This large amount of dark matter initiates an invisible halo which causes the Milky Way to spin.
What are the Types of Galaxies?
There are basically four main types of galaxies:
Elliptical
Irregular
Spiral
Barred spiral
These main categories of galaxies are further divided into subcategories. Some other types of galaxies also exist based on their size and other features.
● The most common type of galaxy found in the universe is the spiral-shaped galaxy. Almost 77% of the galaxies discovered till date are spiral shaped galaxies. For example, the Andromeda galaxy is a spiral shaped galaxy.
● A majority of spiral galaxies have a bar-like structure called a barred-spiral galaxy. Around two-thirds of spiral galaxies are barred shaped. Our Milky Way galaxy is a barred shaped galaxy.
● An elliptical doubled-ringed galaxy is the rarest type of galaxy. The galaxy PGC 1000714 is such a type of galaxy. According to scientists around 0.1% of galaxies are of this type. It is also called the Hoag-type galaxy.
● Irregular galaxies are also found, which are usually smaller in size. Around a quarter of known galaxies are irregular in shape. These galaxies don’t have a distinct shape and give a chaotic appearance.
● Elliptical galaxies are also found which are usually composed of old stars having a low mass. They normally occur in Virgo superclusters. Around 10 - 15% of known galaxies are of this type. The starlight from these galaxies is very dim as compared to spiral galaxies.
[Image will be Uploaded Soon]
FAQs on Galaxies
1. What is a galaxy in simple terms?
A galaxy is a massive, gravitationally bound system containing stars, stellar remnants, interstellar gas, dust, and dark matter. Think of it as a vast cosmic city of stars, gas, and dust. Our own Solar System is located within one such galaxy, the Milky Way.
2. What are the four main types of galaxies astronomers have identified?
Astronomers classify galaxies based on their visual shape. The four primary types are:
- Spiral Galaxies: These feature a central bulge, a flat rotating disk, and distinct spiral arms. Our Milky Way is a prime example.
- Elliptical Galaxies: These are shaped like a flattened sphere or an oval and are primarily composed of older stars with very little gas and dust.
- Lenticular Galaxies: These are considered an intermediate type between spiral and elliptical, possessing a central bulge and a disk but lacking spiral arms.
- Irregular Galaxies: These galaxies have no defined or regular shape, often resulting from gravitational interactions or collisions with other galaxies.
3. What is the role of dark matter in a galaxy?
While galaxies are filled with visible stars, gas, and dust, a huge portion of their mass comes from dark matter. This is an invisible substance that does not emit or reflect light. Its immense gravitational pull is believed to be the 'glue' that holds the galaxy together and prevents the rapidly rotating stars on the outer edges from flying off into space.
4. How do galaxies move in the universe?
Galaxies exhibit two main types of motion. Firstly, they rotate around their own centre of mass. Secondly, they move through space. Due to the expansion of the universe, most galaxies are moving away from each other. However, galaxies within a gravitationally bound cluster can move towards each other, overriding the expansion on a local scale.
5. How many galaxies are estimated to exist in the observable universe?
Based on deep-sky observations from instruments like the Hubble Space Telescope, scientists estimate there are between 100 billion and 200 billion galaxies in the observable universe. This number is an approximation, as many more are likely too distant, small, or faint to be detected with our current technology.
6. What is the fundamental difference between a galaxy and a solar system?
The key difference is scale. A solar system consists of a single star (or a binary star system) and all the objects gravitationally bound to it, such as planets, moons, and asteroids. A galaxy is a far more massive structure containing billions of individual solar systems, all bound together by gravity on a cosmic scale. Our solar system is just one of billions inside the Milky Way galaxy.
7. Why is the centre of the Milky Way galaxy so bright and active?
The centre of our galaxy is incredibly bright because it has a very high concentration of stars. At its very core lies a supermassive black hole named Sagittarius A*. This black hole's immense gravity pulls in surrounding gas and dust, forming a hot, glowing accretion disk. This process releases enormous amounts of energy, making the galactic centre a highly dynamic and luminous region.
8. If the universe is expanding, why is the Andromeda galaxy on a collision course with our Milky Way?
This happens because over relatively 'short' cosmic distances, gravity is stronger than the force of cosmic expansion. The Milky Way and Andromeda are the two largest members of a gravitationally bound cluster called the Local Group. Their mutual gravity is powerful enough to overcome the universe's expansion, pulling them towards an eventual merger that is predicted to occur in about 4.5 billion years.
