Science For Everyone: Supernova, Nebula, Neutron Star — Stages In A Star's Life Cycle

Science For Everyone: This week, in ABP Live's science column, we discuss the different stages in the life cycle of a star, and the differences between a supernova, nebula, neutron star and black hole

Science For Everyone: The mass of a star determines the length of its life cycle. The larger the mass of a star, the shorter its life cycle is. A star is formed from the material present inside a nebula, which is a giant, diffuse cloud of dust and gas in space.

Source : NASA

Science For Everyone: Welcome back to "Science For Everyone", ABP Live's weekly science column. Last week, we discussed what El Niño and La Niña are, and how these events impact weather patterns across the globe. This week, we discuss the different stages in the life cycle of a star, and the difference between a supernova, nebula, neutron star and black hole. These astronomical terms are often mistaken for one another because all of them are related to stars. 

In order to understand the difference between a supernova, nebula and a neutron star, it is important to know the stages in the life cycle of a star. 

Stages in the life cycle of a star

The mass of a star determines the length of its life cycle. The larger the mass of a star, the shorter its life cycle is. A star is formed from the material present inside a nebula, which is a giant, diffuse cloud of dust and gas in space. The amount of matter present in a nebula determines the mass of a star. A nebula consists of gases such as hydrogen and helium, which are evenly spread out, but are pulled together by gravity. 

A nebula can be of different types: reflection nebula, emission nebula and planetary nebula. A reflection nebula is one which shines light reflected from nearby stars; an emission nebula is one which shines by emitting energy in the form of light as a result of electrons merging with protons to form hydrogen; and a planetary nebula is the gas and dust remaining after a star similar to the Sun explodes. When ultraviolet light from a nearby star shines on a cloud of hydrogen gas, electrons are emitted. It is these electrons that combine with protons to form hydrogen, as a result of which light is emitted from emission nebulae.

When a dying star explodes, a phenomenon known as a supernova, gas and dust are ejected in all directions, resulting in the formation of a nebula. Nebulae serve as stellar factories because they are rich in star-forming materials, including hydrogen.

As years go by, gravity pulls together the hydrogen gas in the nebula, and as a result, the gas starts to spin. Eventually, the speed with which the gas spins increases, causing it to heat up, and become a protostar. 

The gas can get heated up to a temperature of 15 million degrees, according to NASA. When this stage is reached, nuclear fusion, the process in which small nuclei are combined to form a larger nucleus whose mass is slightly smaller than the sum of the smaller ones, because some amount of energy is lost, occurs in the core of the protostar. Einstein's equivalence principle, which states that energy is equal to mass times the speed of light squared, explains why nuclear fusion results in the formation of a nucleus with a mass less than the combined masses of the smaller nuclei, and the release of energy. Nuclear fusion also occurs in the Sun, and is the source of the star's energy. 

Due to the soaring temperatures and nuclear fusion reactions, the protostar shines brightly, following which it contracts a little. After this, it becomes stable, and turns into a main sequence star. The celestial body remains a main sequence star for millions to billions of years, shining brightly. The Sun is currently a main sequence star.

The gravity which holds the clumps of gas and dust together in a star becomes stronger and stronger as the clumps get bigger and bigger. 

A point is reached when the clumps become so big that the star collapses from its own gravity, resulting in a supernova. It is the material released into interstellar space as a result from this explosion that acts as the material for the formation of other stars. 

The Helix Nebula, located 700 light-years away from Earth, is the closest nebula to our planet. 

Now let us discuss a supernova in detail. The main sequence star glows for millions to billions of years, and during this period, the hydrogen in the core is converted into helium as a result of nuclear fusion. However, when the hydrogen supply in the core of the main sequence star begins to run out, the number of nuclear fusion reactions begins to run out, and the amount of heat produced decreases.

This causes the core to become unstable. As a result, the core contracts. 

Meanwhile, the outer shell of the star, which is still mostly hydrogen, starts to expand, and during this process, it cools and flows red. This stage of the star is called the red giant phase. A red giant is a star that has low surface temperature and a diameter that is large relative to the Sun. A red giant star is red because it is cooler than it was in the main sequence star stage, and is a giant because the outer shell has expanded outward.

What happens after the red giant phase?

Through nuclear fusion, helium in the core of the red giant gets converted into carbon. Up to the red giant phase, all stars evolve in the same way. The mass of a red giant determines what happens next in the star's life cycle. 

Stars are either low-mass, similar to our Sun, or high-mass. In the core of a low-mass star, the helium fuses into carbon, as a result of which the core collapses again. After this, the outer layers of the star are expelled, and from these, a planetary nebula is formed. The collapsed core becomes a white dwarf, and after cooling, is converted into a black dwarf. 

A white dwarf is a star that has exhausted most of all of its nuclear fuel, and has collapsed to a very small size, usually a radius equal to about 0.01 times that of the Sun. However, the mass of a white dwarf star is nearly equal to that of the Sun. Since volume is a function of the cube of radius, and density is inversely proportional to volume, a white dwarf star is one million times denser than the Sun. 

After a white dwarf star has radiated all its energy, a non-radiating ball of gas remains. This is called a black dwarf star. 

The science behind a supernova

A massive star is one whose mass is 10 times or more the mass of the Sun. Both low-mass stars and massive stars are born in the dust and hydrogen gas of nebulae, and are converted into main sequence stars. However, after reaching the red giant phase, a massive star, unlike a low-mass star, explodes into a supernova. 

An extremely bright and powerful explosion of a star is called a supernova. It is the biggest explosion humans have ever seen. 

A supernova is often described as the "last hurrah" of a dying star, which had a mass at least five times that of the Sun, before meeting its end. 

Since massive stars burn large amounts of nuclear fuel at their cores, huge amounts of energy are produced, causing the heat and pressure inside the core to increase to huge extents. It is the nuclear fusion reactions that keep a star from collapsing. The nuclear forces and gravitational act in opposite directions. While nuclear forces create a strong, outward pressure on the star's core, gravity tries to squeeze the star into the smallest and tightest ball possible. Since the outward pressure and heat generated from nuclear reactions resists the inward push of gravity, a star is kept from collapsing. 

However, the pressure inside a massive star drops when it runs out of fuel because the absence of nuclear reactions causes the star to cool off. Therefore, gravity exerts an inward pressure on the star, and causes it to collapse. A star million times the mass of the Earth collapses in 15 seconds. 

The outer part of the star explodes because the collapse of the star happens quickly, and enormous shock waves are produced. 

Another type of supernova is one which occurs when an Earth-sized white dwarf star orbits another star in a binary system, and the two either collide with each other, or the white dwarf star pulls a lot of matter from its nearby star. 

Therefore, a supernova can be the death explosion of a massive star, resulting in a sharp increase in brightness, followed by a gradual fading, or the explosion of a white dwarf which has accumulated enough material from a companion star to achieve a mass equal to the Chandrashekhar limit, which is the maximum mass theoretically possible for a stable white dwarf star. It is impossible for a white dwarf star to be stable if its mass is greater than 1.44 times the mass of the Sun. A dying star with a mass greater than the Chandrashekhar limit either becomes a neutron star or a black hole. 

The supernovae which occur as a result of the death explosion of a massive star are called Type II supernovae, and at peak light output, they can outshine a galaxy. 

According to NASA, the outer layers of the exploding star are blasted out in a radioactive cloud which is expanding, and is visible long after the initial explosion fades from view. The expanding cloud forms the supernova remnant. 

The supernovae formed as a result of the explosion of a white dwarf are called Type Ia supernovae, and have approximately the same intrinsic brightnesses as Type II supernovae. Astronomers use Type Ia supernovae to determine distances. 

The shock waves released as a result of a supernova encounter material in the outer layers of the star, and cause the material to be heated. This results in the formation of new elements and radioactive stars. Most of the common elements are produced as a result of nuclear fusion reactions in the cores of stars, while heavier elements are formed due to unstable conditions of the supernova explosion. 

The supernova remnant, the hot surrounding material, and radioactive isotopes produce X-rays and gamma-rays.

What determines whether a neutron star or black hole is formed?

The fate of the supernova remnant depends on its mass. The supernova remnant can either become a neutron star or a black hole, depending on its mass.

A neutron star is the imploded core of a massive star produced by a supernova explosion, and has a mass about 1.4 to three times that of the Sun, a radius of about eight kilometres, and the density of a neutron. 

All supernovae leave behind a very dense core, and a nebula. Stars more than 10 times the size of the Sun produce black holes. In such cases, the supernova remnant is more than three times the mass of the Sun.

In the case of a black hole, the gravitational forces swallow the supernova remnant. The black hole accretes everything that comes in its vicinity. 

However, when the supernova remnant is 1.4 to three times the mass of the Sun, it gets converted into a neutron star. 

According to the National Radio Astronomy Observatory, a neutron star that is left over after a supernova is a remnant of the massive star which went supernova. Black hole formation can also occur without a supernova explosion, if the star is massive enough. This is because it will collapse directly in less than half a second. 

The collapse of a neutron star can also result in the formation of a black hole if it accretes sufficient material from a nearby companion star, or merges with a companion star. The merging can increase the mass to an extent that the neutron star mass limit is crossed, and a black hole is formed. The mass limit of a neutron star is three solar masses. 

Conversion of a neutron star into a black hole can take more than millions of years, depending on the rate of accretion of material. Once a neutron star crosses the mass limit of three solar masses, it collapses into a black hole in less than a second.