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Galaxies and the Milky Way

Updated on October 3, 2019
Mtanzil profile image

Astro science research enthusiast loves to read about astronomy and space exploration. In love with gaining and sharing knowledge.

Galaxies

Man must rise above the EARTH—to the top of the atmosphere and beyond—for only thus will he fully understand the world in which he lives.” |source


What is a galaxy?

Galaxy is a collection of stars, stellar remnants, interstellar gas, dust, and dark matter which are gravitationally bound together to form a system. The word galaxy has been derived from the Greek word galaxias which means "Milky". Galaxies vary in size ranging from Dwarf to Giants. Dwarf galaxies contain a few hundred million stars whereas the giant galaxies may contain as many as hundred trillion stars which orbit around the galaxy's center of mass. Recent research suggests that there are about 2 trillion or more galaxies in the observable universe.

Galaxies across space

How are galaxies categorized?

Galaxies are categorized according to their visual morphology. Some galaxies are spiral-shaped. They have curved arms that make it look like a pinwheel. Other galaxies are smooth and oval-shaped. They’re called elliptical galaxies. And there are also galaxies that aren’t spirals or ovals. They have irregular shapes and look like blobs. The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical, up to E7, which is highly elongated.

1. Elliptical galaxies

Elliptical galaxies are shaped like ellipses (stretched circles). They are divided into eight types: E0-E7 depending on how elliptical they are. E0 ellipticals are nearly circular, while E7s are very stretched out. Elliptical galaxies are made up of mostly old stars and do not have much gas and dust. There is very little new star formation in these galaxies. Elliptical galaxies also come in many sizes. The largest galaxies we see are ellipticals, but, elliptical galaxies can also be small. About 60% of all galaxies are ellipticals.


2. Spiral galaxies

Spiral galaxies are the most common type in the universe. Spirals are large rotating disks of stars and nebulae, surrounded by a shell of dark matter. The central bright region at the core of a galaxy is called the “galactic bulge”. Many spirals have a halo of stars and star clusters arrayed above and below the disk. Spirals that have large, bright bars of stars and material cutting across their central sections are called “barred spirals”. A large majority of galaxies have these bars, and astronomers study them to understand what function they play within the galaxy. In addition to bars, many spirals may also contain supermassive black holes in their cores. Subgroups of spirals are defined by the characteristics of their bulges, spiral arms, and how tightly bound those arms are.

3. Irregular galaxies

Irregular galaxies are as their name suggests: irregular in shape. The best example of an irregular that can be seen from Earth is the small magnetic cloud. Irregulars usually do not have enough structure to characterize them as spirals or ellipticals. They may show some bar structure, they may have active regions of star formation, and some smaller ones are listed as “dwarf irregulars”, very similar to the very earliest galaxies that formed about 13.5 billion years ago. Irregulars are characterized by their structures (or lack of them).

The Milky way!

Milky Way Galaxy is a large spiral system consisting of several hundred billion stars one of which is the sun. It is an irregular luminous band of stars and gas clouds that stretches across the sky as seen from Earth. The term Milky Way is a translation of Latin word via lactea and the Greek word galaxías kýklos, which means “Milky circle”.

Facts about Milky Way!

  1. The Milky Way is a barred spiral galaxy with a diameter between 150,000 and 200,000 light-years.

  2. It is estimated to contain 100–400 billion stars and more than 100 billion planets.

  3. The Solar System is located at a radius of 26,490 (± 100) light-years from the Galactic Center, on the inner edge of the Orion Arm.

  4. The Milky Way is the second-largest galaxy in the Local Group, with its stellar disk approximately 100,000 ly (30 kpc) in diameter and, on average, approximately 1,000 ly (0.3 kpc) thick.

  5. The Milky Way is approximately 1.5 trillion times the mass of the Sun.

  6. The Milky Way has grown by merging with other galaxies through time. It is currently acquiring stars from a very small galaxy called the Sagittarius Dwarf Spheroidal, as well as gobbling up material from the Magellanic Clouds.

  7. The Milky Way moves through space at a velocity of about 552 kilometers per second (343 miles per second) with respect to the Cosmic Microwave Background radiation.

  8. The Milky Way’s central core contains a supermassive black hole. It is commonly referred to as Sagittarius A*. It contains the mass of about 4.3 million Suns.

  9. The stars, gas, and dust of the Milky Way all orbit the center at a rate of about 220 kilometers per second.

  10. Our galaxy will collide with Andromeda Galaxy in about 5 billion years. Some astronomers refer to our two galaxy as a binary system of giant spirals.


Structure of the Milky Way.

The Milky Way can be broken into four structural components:

  1. Bulge.

  2. Disk.

  3. Stellar ("normal") halo.

  4. Non-baryonic ("dark matter") halo.

1. Bulge

In astronomy, a bulge is a tightly packed group of stars within a larger formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies. Bulges were historically thought to be elliptical galaxies that happened to have a disk of stars around them, but high-resolution images using the Hubble space telescope have revealed that many bulges lie at the heart of a spiral galaxy.

2. Disk

The thin disk is the defining component of disk galaxies in general and spiral galaxies in particular. It contains stars, star clusters, gas and dust which are confined to the galaxy’s plane of rotation. The thin disks of spiral galaxies contain a lot of gas and dust, and are therefore an active site for ongoing star formation, especially in the spiral arms. For this reason, stars in the thin disk tend to be relatively young (average age around 6 billion years), although individual ages range from 0 to 10 billion years.

3. Stellar

Far above the plane of the disk, we find hundreds of globular clusters. The stars in globular clusters are very old. The stellar halo extends much farther from the center of our galaxy than the visible disk. It's hard to see in the Milky Way, so consider the halo of the Andromeda Galaxy, another spiral galaxy.

4. Dark matter Halo.

We also believe that our galaxy is surrounded by an enormous spherical cloud of dark matter, material which is not composed of ordinary protons, electrons, and neutrons, but of some other sort of elementary particles. As its name indicates, this material doesn't emit much light at least, not as much light per kilogram as stars do.


Cosmology brings us face to face with the deepest mysteries, questions that were once treated only in religion and myth.” |source

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

Comments

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    • Mtanzil profile imageAUTHOR

      Tanzeela mushtaq 

      2 days ago from banglore

      @jannat Hossain, indeed it is!

    • Jannat Hossain profile image

      Jannat Hossain 

      3 days ago from Bangladesh

      Astonishing, isn't it?!

    • Mtanzil profile imageAUTHOR

      Tanzeela mushtaq 

      10 days ago from banglore

      thank you @haris

    • profile image

      haris 

      10 days ago

      good article

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