The Life cycle of Stars- A complete Guide

Updated: Nov 20, 2020

"We are a way for the universe to know itself. Some part of our being knows this is where we came from. We long to return. And we can, because the cosmos is also within us. We are made of star-stuff," - Carl Sagan.

Carl Sagan uttered above quote in his show "Cosmos" aired on PBS. Why did he say we are star-stuff? What did he mean? Did life on this planet jump out of stars and live on Earth one day? Of course not! All organic matter containing carbon formed in stars and got here through explosions called Nova.

SuperNova is a stage in the life cycle of stars where massive ones explode billion times brighter than our Sun. We know how stars form from the giant molecular clouds called Nebula and how they sustain themselves through fission reaction. In this article, we deal with the life cycle of stars and all the stages involved.

Mass of the stars determines their fate, larger the star shorter the lifespan. We measure the mass of stars, black holes, nebulae etc. using the unit Solar mass. One solar mass is approximately equal to the mass of our sun.

On average,

  • In Massive stars, that are many times larger than our Sun, the fusion reaction takes place at a faster rate. They exhaust their fuel faster and die early.

  • Small-sized stars burn slowly. The fusion reaction takes place at a much slower rate.

For Example, Stars about 40 solar masses live for a million years, while they estimate the smaller stars with almost one third the mass of our sun to live for 560 billion years (Our universe itself is 13.2 billion years old). Mind-blowing, Right?! Now let's jump right in.

How are Protostars formed?

The gas cloud collapses towards the centre under gravity and builds a low mass protostar. A pancake looking disc of gas and dust surrounds the young star. It makes protostars hard to observe. Imagine a sunny side up egg, the egg white surrounds the Yolk like a disk, in the same manner, the protostar is in the centre and the gas cloud surrounds it.

The gas, dust and the other fragments in the disk continue to rain into it. The protostar enters pre-main sequence stage after all it dispenses the material in the disk.

What are the Main-Sequence Stars?

A main-sequence star is any star that fuses hydrogen to form helium in its core. Almost 90% of stars in the Universe including our sun, is a main-sequence star. Main-sequence stars attain stability, the energy released by the fission reaction keeps the star collapsing under its gravitational pull towards the core.

For example, imagine blowing up a balloon while your friend is trying to crush it down. Nothing happens to the balloon when the air pressure you blow is equal to the compressing pressure, the same way the gravity wants to make the star collapse on itself while the nuclear fusion pushes the energy outwards. Main sequence stars attain stability between these two opposite forces.

Red Giant Stage:

At some point, in larger stars of the Main-sequence stage fuses all the hydrogen in its core, it leaves them with only helium. The core exhausts all the hydrogen fuel and leaves the star with only Helium. As it exhausts hydrogen in the core, the fusion reaction spreads outwards causing the star to expand 400 times its size. It swallows the nearby planets. Eventually, the hydrogen in the shell gets exhausted leaving the star with low temperatures and a faint reddish glow. This is the Red Giant phase and it can span around for a few thousand to 1 billion years.

What happens to Low-mass stars?

In case of Stars with mass up to 3 solar masses, the expansion during the Red giant phase continues till the star sheds its outer shells and the white core gets exposed. The core now exposed forms a White Dwarf.

The outer shell shed by the star forms a planetary nebula. The Planetary Nebula is completely unrelated to planets or exoplanets but it is a layer of ionised gas that surrounds a dying star.

White Dwarfs are roughly the size of Earth. Pressure from fast-moving electrons in the core keeps the star from collapsing any further. White dwarfs glow faintly with low energy and eventually cool down forming a Black Dwarf. Black Dwarves are dead stars.

The following infographic explains the life of low-mass stars with pictures:

What Happens to High-mass stars?

Stars with core mass up to 8 solar masses die in a dramatic supernova that can shine billion times brighter than the sun. Note that in planetary Nebula only the shell of the star erupts but in a supernova, the entire star collapses including the core. Supernova can outshine entire galaxies space and can last for days to weeks. A rich array of subatomic particles and elements produced during these titanic explosions enrich the planets and star systems within their range. In our case, these supreme explosions made our planet optimal for survival. Dying stars gave us life that's the poetic nature of the Universe we live in!

Massive stars that larger than 8 solar masses the core survives at the white dwarf stage and doesn't die down to form black dwarfs but the core continues to collapse. The inward pull of the core is so intense even the protons and electrons in the core combines to form neutrons. When all the protons and electrons in the star have combined, it becomes a Neutron star. Neutron stars are extremely dense as they have very high mass packed into a minuscule volume. They have powerful magnetic fields that speed up atomic particles present near its poles and this produces powerful beams called Pulsars which we can observe from the Earth.

Stars with surviving core mass greater than 3 solar masses collapse to form a black hole. Black holes are infinitely dense objects with an infinite mass that can create a tear in the fabric of space-time. Nothing escapes their gravitational pull, not even light! The following mind map compares the life of Low-mass stars with high-mass stars:

The image describes the stages in the life cycle of stars
The Life cycle of stars ( Image credits: NASA and the Night Sky Network)

New star formation:

The Nova and Super Nova that spread through the cosmos in the interstellar space form huge molecular clouds. These clouds act as stellar nurseries for a new generation of stars to form.

161 views0 comments

Recent Posts

See All