The Milky Way

With the moon below the horizon, a gaze into the space from a place free from the urban pollution reveals a band of light across the dark background of the night sky. That’s the Milky Way, our home galaxy! This bright band of light was first resolved into individual stars by the famous Galileo Galilei with his telescope back in 1610. Until early 1900s, astronomers believed that the Milky Way contained all the stars in the universe. However, observations by Edwin Hubble cleared the air and confirmed the existence of millions of more galaxies in the universe like our own. And of course, our galaxy gets its name because of its appearance in the clear night sky.

The Milky Way is a barred spiral galaxy which was formed around 13.6 billion years ago and is about 100,000 light years across. Like every other entity in the universe, the Milky Way is also in a constant state of motion. If one was to observe from the top, the Milky Way would appear like four giant arms wrapped around a bright central bulge. Our solar system is located on the Orion spur (an extension of Sagittarius arm, or possibly an independent arm). It is reported that our solar system travels at an average speed of 828,000 kmph and with this speed, it would take approximately 230 million years to travel all the way around the Milky Way.  The galactic bulge at the center is the major reason why we can’t see stars (or any other celestial body, for that matter) on the other side.

                                Milky Way overall plan view (Click here for source)

Inside this bulge is present a supermassive black hole (referred to as Sagittarius A*), billions of times the mass of the sun. The black hole may have been smaller in its initial days but the ample supplies of dust and gas have led to its growth into its current gigantic stage. Black holes can’t be seen, but their presence can be detected by the way they bend the paths of the material around it. It is assumed that most galaxies have a black hole at their heart. The galaxy is surrounded by a spherical halo which is made of hot gas, old stars and globular clusters. The halo too spins as rapidly as the galaxy itself and is also detected by its effect on the objects around it.

                                          Milky Way side view (Click here for source)

The Milky Way is a part of a group called the Local Cluster, which consists of 50 other galaxies. It is the most massive galaxy in the local cluster. A rough estimate puts the number of stars in our galaxy at around 200-400 million. Compared to the breadth of the galaxy, the thickness is pretty small (around 1,000 light years). The galaxy contains many ancient stars that were formed after the Big Bang, as the galaxy is almost as old as the universe. It has been formed by consuming smaller galaxies and clusters and is continuing to do so, currently gobbling up Sagittarius Dwarf Spheroidal and the Magellanic Clouds. The Milky Way is predicted to collide with its neighbor, the Andromeda, to form a giant elliptical galaxy in nearly 4 billion years. Andromeda is the largest and brightest in the cluster and contains more ancient stars than the Milky Way.

Illustration depicting the predicted merger of Andromeda and Milky Way as observed from Earth (Click here for source)

Written by:

Divyajyoti Biswal