Star Formation – Stellar Evolution and Life Cycle of a Star

Have you ever felt the soothing effect when you sleep under the blanket of the night sky all covered with millions of stars? Stars seem to be so fascinating and when one poetically says “We are made of stardust”, it brings a different pride in our eyes.

Interestingly though, this is a fact. Let’s today firstly study the start formation and life cycle of these astonishing bodies and unearth the scientific reasoning behind these poetic lines.

What are Stars?

Stars, from the tiny sparkling dots in the night sky to the huge ball of fire, Sun, are the astronomical objects that consist of a luminous spheroid of plasma held together by its own gravity. Based on the temperature and an atmospheric pressure of a star, its surface brightness and size could be found.

On the basis of their spectra (i.e. emission and absorption), scientists have classified stars into seven types namely O, B, A, F, G, K and M. (There is a famous mnemonic for remembering the order of classification – Oh Boy, An F Grade Kills Me)

HR Diagrams (as shown in the figure) help one to find the life history and evolution pattern of a star. Based on age, distribution and composition of stars in a galaxy, one can trace the history and evolution of the galaxy.

STELLAR EVOLUTION

Stellar evolution is the process by which stars develop over time.

The process of the formation of stars from dust and clouds of the main hydrogen, the formation of protostar followed by a main-sequence star to its death as a white dwarf, nova, supernova, neutron star, or black hole is explained in the underlying paragraphs.

FORMATION OF PROTOSTAR

Stars are formed from a cloud of gas that has a temperature of 20K and typically has hydrogen atoms.

This big cloud keeps condensing till one local region has a high density (1-1000 solar mass) and these smaller pockets of the order of solar mass will contract independently which further condense to form a cluster of primitive stars ‘Protostar’ with a radius of order 10^(15)m.

It collapses freely as till now there is no internal pressure because gravitational energy is used to dissociate first hydrogen molecules into hydrogen atoms and then to ionize hydrogen atoms into electrons and protons.

Around a radius of 10^(11) m, the process of free collapse stops because now charged particles will absorb the energy from photons and get kinetic energy and give pressure, and quasi-virial equilibrium is reached.

The time scale for this free collapse is of the order of 20,000 years and the average temperature of a protostar when rapid collapse under gravity is replaced by slow contraction is 30,000K.

When most of the hydrogen is ionized and as the protostar becomes increasingly opaque to its own radiation, the gravitational energy released is converted into random thermal energy of electrons and ions.

Stars before going into the main sequence go into pre-main-sequence where stars having the mass of the order of sun and less than 10 billion years old slowly blow away.

The surrounding shell of gas and dust with the help of stellar winds and radiation; where the surrounding envelope has cleared. It is called the T-Tauri phase.

The internal pressure rises and the collapse of the protostar is slowed down and the system keeps collapsing till the point nuclear fusion kicks in and after that hydrostatic equilibrium is reached and the star enters the main sequence.

WHY DO THE STARS SHINE?

In a protostar, nuclear fusion occurs in the core. Due to the reaction, the cloud begins to shine brightly. It also contracts a little and becomes stable, thus forming a main-sequence star.

A star remains in this phase for millions of years. The Sun is in this phase presently and will stay in this mature phase for about 10 billion years. The Hydrogen in the core fuels nuclear fusion and keeps on converting to Helium.

When the core runs out of Hydrogen making the heat generation come to a stop, the core becomes unstable and hence contracts. The outer shell of the star (which is mostly Hydrogen) begins to expand.

As it expands, it cools down and glows red. Meanwhile, in the core, Helium further gets converted to Carbon. This phase is known as the Red Giant phase.

(Red because the star seems red on cooling down of the outer layers and Giant because of the expansion it undergoes, basically the outer layers). As the Red Giant star condenses, it heats up, burning the last of its Hydrogen.

This results in further expansion of the outer layers of the star. This very large red giant is known as Red Supergiant. All-stars follow the same path until the Red Giant phase. After this phase, the journey of the star depends on its mass.

WHAT IS PLANETARY NEBULA?

Planetary Nebula is an outer layer of gas and dust that are lost when the star undergoes the conversion from Red Giant star to White Dwarf. No planets are involved in this layer.

WHAT IS A WHITE DWARF?

In low mass stars, the process of ejecting outer layers continues until the core of the star is exposed. This dead yet hot object, of the size of Earth, is referred to as White Dwarf.

A white dwarf does not collapse any further because of the presence of the fast-moving electrons. White dwarfs then gradually cool down and finally disappear in oblivion. This is the fate that awaits the Sun.

WHAT ARE NOVA AND SUPERNOVA?

If a white dwarf is close enough to a companion star or forms part of a binary or multi-star system, its fate is a bit different, as a Nova (meaning ‘new’ in Latin). The white dwarf so formed attracts the Hydrogen from the companion star, building its own outer shell where the nuclear fusion restarts.

This causes the star to shine again for a few days. Because of this shine, there had been illusions that the star is a new star; where it is actually very old. Then the star’s surface explodes and the cycle resumes.

If the white dwarf is big enough (about 1.4 solar mass), it attracts so much mass that it collapses and explodes completely, becoming a Supernova. In Supernova, unlike Nova, the star’s core collapses and then explodes.

In massive stars, nuclear fusion continues even after the formation of Carbon forming Oxygen, Nitrogen, and eventually Iron. When the core contains just Iron, the nuclear fusion stops and the core explodes as it couldn’t support its own mass.

The outer layer initially begins to collapse with the core but then is thrown outwards violently with greater energy. Supernova thus shines very brightly for a few days, outshining the entire galaxy.

Supernova can be triggered in two ways – Type I Supernova or Type II Supernova.

TYPE I SUPERNOVA

Type I supernova occurs in a binary system. In this, there is a sudden re-ignition of nuclear fusion on the surface of a degenerate white dwarf. The core temperature of the white dwarf thus increases igniting carbon fusion. This triggers runaway nuclear fusion completely destroying the star.

DIFFERENCE BETWEEN NOVA AND TYPE I SUPERNOVA

TYPE II SUPERNOVA

Type II Supernova occurs due to the gravitational collapse of the core of a massive star. Take, for example, the supernova of a red giant.

WHAT IS NEUTRON STAR?

If the remnant is 1.4 to 3 solar masses, it forms a Neutron star. Here electrons and protons combine together to form neutrons. They are very dense, just like the nucleus of an atom. Hence, gravitational force at their surface is very high.

WHAT IS BLACK HOLE?

If the core is larger than 3 solar masses, it collapses into a Black Hole. The black hole has such strong gravity that even light cannot escape it. Hence, black holes cannot be seen. They can be detected only because their strong gravitational force attracts outer layers of a companion star into it.

As matter spirals into a black hole, it forms a disc emitting X-rays and gamma rays.

WHAT IS DEGENERATE MATTER?

When the hydrogen in a star runs out, the star begins to collapse on itself. Earlier heat and outward pressure due to nuclear fusion were balanced by inward pressure due to gravity. The star is now known as a degenerate star.

If electrons from their regular shell are pushed closer to the nucleus, the density of the star increases. The matter in this state is known as degenerate matter.

WHAT NEXT?

From the dust and debris leftover by Nova and Supernova, after recycling, new stars and accompanying planetary systems are formed. This is how everything: the planets, the meteors, and we are made of stardust!

WHAT ARE BROWN DWARFS?

Brown Dwarfs are objects which have size somewhere between planets and stars. They are too small to be stars and too large to be planets. They are formed in a similar way as stars, from collapsing clouds of gas and dust.

The core of brown dwarfs is not dense enough to trigger nuclear fusion. Brown dwarfs found to date are part of the binary system. In cosmology, the discovery of brown dwarfs helps to solve the “missing mass” problem.

WHAT IS GALAXY?

Galaxy is a gravitationally bound system of stars, their solar systems, interstellar gas, and dust. There are over 100 billion galaxies in the universe. They are major building blocks of the universe.

Scientists have classified galaxies into two major types – Regular and Irregular galaxies. Regular galaxies are further of two types – Spiral and Elliptical galaxies.

SPIRAL GALAXY

These are the galaxies like the milky way that contains a prominent disc composed of stars, gas, and dust. The disc contains spiral arms, filaments in which stars are continuously being formed. Spiral arms contain bright stars while old stars lie in the centre. These are relatively smaller and less bright.

ELLIPTICAL GALAXY

Also known as the Ellipsoidal Galaxy, they are bigger versions of the spiral galaxies. The entire galaxy consists of random distribution. They evolve from spiral galaxies and hence consist of old stars. The brightest galaxies in the universe are elliptical.

IRREGULAR GALAXY

These are the galaxies that are either not formed completely or are disrupted. The stars in irregular galaxies are generally very old. Irregular galaxies comprise about one-tenth of all galaxies.

Conclusion

Finally, through this article, we learned how a star is formed from mere clouds and dust, forming protostar. Shining brightly for years how the star eventually meets its fate depending on its mass.

Also, we learned about planetary nebulae, white dwarf, nova, supernova, neutron star, and a black hole. Moreover, we learned about degenerate matter and brown dwarfs. We also discussed in significant detail different types of galaxies.