PHYSICS OF STARS & GALAXIES

A star is a celestial body comprising of hot gases that radiates energy derived from thermonuclear reactions in its core. This energy is produced by a reaction known as Nuclear Fusion which involves the combination of two Hydrogen atoms to form a Helium atom. It is this Fusion Reaction that defines a star because, without this reaction, the star is just another massive planet. In other words, if a planet comprising of gases like Hydrogen and Helium are massive enough, then they behave like stars. The huge size of the star is what enables it to sustain a nuclear reaction.

75% of the matter in the Universe is Hydrogen and 23% is Helium; these are the amounts left over from the Big Bang. These elements exist as large stable clouds of cold molecular gas. At some point of time, if a gravitational disturbance, like a supernova or a galaxy collision cause a cloud of gas to collapse, the process of star formation begins. 

It takes 100 of millions of years for a Star to form completely and start a Nuclear Reaction. Stars are categorized based on their masses. The different amounts of mass give a star its properties. As per the laws of Physics, the least massive star possible is about 75 times the mass of Jupiter. In other words, find 74 more Jupiters and mash them together, you’d get a star. The most massive star possible has been an issue of scientific disagreement, but it’s thought to be about 150 times the mass of the Sun. More than that, the star just won’t be able to hold itself together.

When billions of stars, gas and dust are Gravitationally Bound by each other, we have what is called a Galaxy. Our own Galaxy is called the ‘Milky Way’ and our own sun is merely an average star among the millions and millions of stars in the Milky Way. Typically, galaxies range from dwarfs with as few as ten million (10⁷) stars up to giants with one trillion (10¹²) stars, all orbiting a common center of mass.

Galaxies are classified according to their apparent shape. Spiral and elliptic galaxies are the most common ones. It is estimated that there are more than 100 billion (10¹¹) galaxies in the observable universe. A huge number of these galaxies are 1,000 to 100,000 parsecs in diameter and are usually separated by distances on the order of millions of parsecs.

A mysterious entity known as the Dark matter appears to account for around 90% of the mass of most galaxies. So far, Scientists haven’t been able to explain much about the Dark Matter. Astronomers believe that supermassive black holes may exist at the center of many, if not all, galaxies. Our home Galaxy, the Milky Way itself appears to harbor at least one such object within its nucleus.