Type Of Galaxies

Type Of Galaxies

Subject – Science and Tech

Context – In just our observable universe we estimate there are over 2 trillion galaxies!

Concept –

There are four main categories of galaxies: elliptical, spiral, barred spiral, and irregular. These types of galaxies are further divided into subcategories while at the same time other types of galaxies exist based on their size and other unique features.

Spiral Galaxy

• The most common type of galaxy found throughout the universe is the spiral galaxy. Around 77% of the galaxies observed by man are spiral galaxies. A good example of this type is the Andromeda galaxy.
• Around two-thirds of all spiral galaxies have a bar-like structure – thus they are classified as barred spiral galaxies. Our galaxy, the Milky Way is an example of this type of galaxy.
• They have a flat, spinning disk with a central bulge surrounded by spiral arms. The spinning motion reaches speeds of up to hundreds of kilometers/miles per second.
• The bulge located in the center is made up of older, dimmer stars, and is thought to usually contain a supermassive black hole.

Elliptical Galaxies

• Elliptical galaxies have an elongated spherical shape and lack a nucleus or bulge at the center.
• Although there is no nucleus, the galaxy is still brighter in the center and becomes less bright toward the outer edges of the galaxy.
• Their light is dominated by older reddish stars. They appear to also lack spiral arms. The stars, gases and other materials are spread throughout an elliptical galaxy.
• The rarest type of galaxies is the elliptical double-ringed galaxy. PGC 1000714 is an example. Estimates suggest that around 0.1% of galaxies are this type. It is sometimes named the Hoag-type galaxy.
• Elliptical galaxies are usually comprised of very old stars or stars with low mass. They make up around 10-15% of galaxies in the Virgo Supercluster, a supercluster of which we are also part of. They are very dim in comparison with the very bright spiral galaxies.
• The largest galaxies are usually giant elliptical galaxies, containing a trillion or even more stars. They span as much as one million light-years across – 10 times as much as the Milky Way.
• Curious enough, the smallest galaxies are also the elliptical types.

Irregular Galaxies

• Irregular galaxies have no definite shape, though they are in constant motion like all other galaxies. They have a chaotic appearance as they don’t seem to possess a nuclear bulge or traces of spiral arms.
• Some irregular galaxies were once spiral or elliptical galaxies but were deformed by an uneven external gravitational force.
• Irregular galaxies are commonly small, and collectively they make up about a quarter of all the galaxies.
• The oldest and farthest galaxy ever discovered is an irregular type of galaxy called GN-z11. It is 32 billion light-years / 9.8 billion parsecs away from us. It is estimated that the galaxy formed just around 400 million years after the Big Bang.

Lenticular Galaxies

• They appear like cosmic pancakes, fairly flat and featureless in the night sky. These galaxies can be thought of as the “in between” of spiral and elliptical galaxies.
• The majority of star formation has stopped but lenticular galaxies can still have significant amounts of dust in them.