People have studied stars since time immemorial, and they have been important to many civilisations around the world. They have inspired many a tale of divine legends, mythology and traditions, but more practically, constellations have also marked the passage of seasons and have been used to define calendars. Early civilisations also relied on stars for navigation across land and sea.
Throughout history, stars have baffled the minds of many academic minds. Through innovations and breakthroughs in astronomical technology, we can now calculate all their physical properties, composition, age, and even how they are formed.
Life Cycle of Stars
Planets and stars have their own life cycle, which ranges from a few million years up to trillions of years. During their life cycle, they undergo changes in their composition, relative position and properties, and we have enough information to determine the stage of life that stars are in.
Birth of Stars
Stars are formed when matter gathers into a higher density to make molecular clouds. These clouds mostly contain hydrogen, with about 25% helium and various other gasses. They are around 1,000 to 10 million times the size of our Sun, and the formation may take hundreds of thousands, if not millions of years, to fully form and go into the Main Sequence.
Different Groups and Phases of Stars
There are lots of different types of stars based on size, physical attributes and luminosity. The 5 main groups of stars define the life cycle of stars as stars evolve from one phase to the next.
Main Sequence Stars
When the clumps of dust and molecular clouds gain mass, they start to spin and generate heat. After heating up to millions of degrees, nuclear fusion occurs, producing two protons to form Hydrogen atoms nuclei and one helium nucleus.
Releasing energy, the heat creates pressure that pushes against gravity, thus creating a Main Sequence star. This is the longest period in a star’s life, and around 90% of the stars in the universe are in this phase. They can range from one-tenth to 200 times the size of our Sun.
Red Giants
During nuclear fusion, a smaller Main Sequence star can run out of oxygen in its core. This forces the star to collapse under its own gravity, and the fusion will begin to convert helium into carbon, ejecting the outer layers. As helium fusion begins moving outwards, it causes the star to expand and form a red giant. These stars are unstable, sometimes expanding and even ejecting portions of their atmosphere.
Our Sun is around 5 billion years old, and it is estimated that in another 5 billion years, it will become a Red Giant. When our Sun becomes a red giant, it will have swollen to 200 times its size today, engulfing Mercury, Venus and possibly Earth.
White Dwarfs
After going through the Red Giant phase, it is predicted that our Sun will become a White Dwarf. This is the phase after the red giant has lost all of its atmosphere, and only the core remains intact. White dwarfs are about the size of Earth, but they have a tremendous density.
They do not produce any more heat, so the matter gradually cools down, and they emit a white-blue or red light. They cannot be seen without the use of instruments and telescopes.
Neutron Stars
On the subject of density, Neutron stars pack more mass than the Sun into a tiny area. Neutron stars are the collapsed cores of massive supergiant stars and result from the explosion of a massive star.
The core is far more compressed than white dwarfs. Extremely dense, they are also very small and rich in elements that are heavier than hydrogen and helium. They may evolve further through collisions, but they do not generate any more heat.
Red Dwarfs
These are small main sequence stars that are far smaller than our Sun. They do not generate nearly as much heat as the Sun and look more orange than red. They steadily burn through their entire supply of hydrogen without changing their internal structures.
Red dwarfs can, therefore, live extremely long. The stars are too faint to see with your eyes, but with a commercial telescope, it may be possible to see some red dwarfs.
Brown Dwarfs
While they are put in the same bracket as stars, brown dwarfs are technically not stars. They are just massive planets that can generate a little infrared light. But they do not emit visible light, and they do not have enough mass to make fusion like a main sequence star can.
They may also have disks of gas and dust, but you cannot see them without the use of advanced instruments.
Death of Stars
Eventually, all stars run out of hydrogen to convert into helium. As the star expands, fusion will then convert helium into carbon, and the star will completely blow out its surface layers. A planetary nebula will form, and the white dwarf will form. The next step is when the star’s core collapses, and the nuclei will rebound, creating a massive shock wave.
This explosion is called a supernova, and the core will ultimately survive to become either a neutron star or a black hole. The material cast out into space can travel over to other molecular clouds and, in time, go on to form new stars.
Key Takeaways About the Life Cycle of Stars
There is a remarkable continuity in the life of a star, as after a supernova, it can go on to create a new generation of stars. Thus, the universe will always grow, and galaxies will continue to thrive.