[Physics Class Notes] on Galaxies Pdf for Exam

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.”

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.

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.