Stellar Evolution: All about Birth and Death of Stars
Stellar evolution is a complicated process. All stars go through the continuous change and their life cycle is made of immature stage, mature stage and final changes towards end of their lives.
Stars pass through a definite evolutionary sequence, which can be broadly divided into three parts viz. Pre-main sequence, Main sequence and Post-main sequence stages. A star which is currently in its main mature period of its life cycle is called Main Sequence Star. Main sequence stars convert hydrogen into helium and are in an equilibrium state. The stars which are not yet in the main sequence are called pre-main sequence or infant stars. The stars which have already lived their main sequence life are called post-main sequence or elderly stars.
Pre-main sequence phases
Protostar
Birth of any star begins with the gravitational collapse of a giant molecular cloud (nebula) spread across hundreds of light years. When it collapses, the molecular cloud breaks into smaller fragments releasing gravitational potential energy as heat. Its temperature and pressure increases and one of the fragments condenses into a rotating sphere of superhot gas to be known as Protostar. A Protostar is a highly condensed cloud of gases, mainly hydrogen and helium. It continues to grow by accretion of gas and dust from the molecular cloud. However, its further development depends upon its mass. If it is of low mass, it would turn into a brown dwarf, while if it is high mass, it would further evolve into main sequence star.
Brown Dwarf
A brown dwarf develops from a low mass protostar in which absence of required temperature and pressure leads to no nuclear fusion chain reaction. However, they still have more mass than any of the planets in solar system. They are dim, emit very less visible light and can be found only by using infrared telecopy. Brown dwarfs have been identified lately only after development of infrared telecopy. Now it is assumed that there are so many brown dwarfs out there that they can outnumber all other different types of stars in galaxy. However, still, red dwarfs are considered to be te largest number of stars in galaxy.
Main sequence star
If the protostar is massive and its core temperature is worth starting a proton-proton chain reaction; it would onset its journey to become a main sequence star. Its mass decides which path it would further take. If it is of low mass, it would turn into a red dwarf; if it is intermediate mass, it would turn into a red giant. Both of them would burn for 6-12 trillion years and would end their life as post mains sequence white dwarf. However, if it is a higher mass, it would turn into a red giant or blue giant.
Red Dwarf
A red dwarf as discussed above, is a low-mass, main-sequence star, which is hot and massive than brown dwarfs and is capable to sustain proton-proton chain reaction; but is cool and less massive then other stars such as red giants, blue giants etc. The temperature of their photosphere is around 3,000°K. They are small and faint than other stars and have no radiative zone between their core and the convection zone. It is thought that red dwarf stars are in largest population in galaxy among all kinds of stars.
Red Giant
A red giant is also a main sequence star but has a higher mass than red dwarfs. The red dwarfs convert hydrogen into helium via nuclear fusion and over its life, the outward pressure of fusion is balanced against the inward pressure of gravity. However, once the hydrogen is finished off, the fusion would stop and gravity would take lead. This would upset the overall equilibrium of the star and to re-adjust it. During this readjustment, the star’s outer region expands while the core shrinks. Due to the large expansion of the outer shell, the star becomes very big, and its colour changes- to red. However, its core would compress and get tighter and smaller. This contraction increases the temperature at core and reaches at levels where Helium fuses with Carbon and turns into a white dwarf in post main sequence life.
We note that since energy in red giants is spread across large area, they have surface temperature cooler (around 2200-3200°C) and thus are little over half as hot as Sun. They shine in the red part of the spectrum and thus are called red giants.
Similarities and differences between Red Dwarfs and Red Giants
Similarities
- Both are main sequence stars
- Both end their lives as white dwarfs
Differences
- While red dwarf is of low mass, red giant is of intermediate mass
- While red dwarfs sustain hydrogen fusion, red giants go further and result in helium fusion reaction
- While red dwarf has little light emanating from it, red giants are little more radiative.
Yellow Dwarf
A yellow or G-dwarf star has a surface temperature of 5300-6000K and converts hydrogen to helium in its core by nuclear fusion. Sun, Alpha Centauri A etc. are some of the yellow dwarfs. The term yellow is a misnomer because yellow is white {for example Sun would appear white if there was no atmosphere}.
Sun’s future as a red giant
Scientists believe that sun, currently a yellow / white dwarf, would deplete its hydrogen in next 5-6 billion years and once that happens – it will start to expand. At its largest size, its photosphere would engulf Venus, mercury and possibly earth. However, this is only a hypothesis. It is argued that when sun loses its hydrogen, it would cause earth and other planets to farther away due to lesser gravity.
Blue Giant
A blue giant big and blue. Such stars are usually high-mass stars on the main sequence. Blue giants live for only a million years or so, glowing a million times brighter than the Sun before they blow apart in titanic supernova explosions.
Post-main sequence stars
A giant star phase ends in white dwarfs, nova or super nova depending upon mass and some other factors. S. Chandrasekhar had proposed that only stars that have a certain mass limit would end their life as white dwarfs. He proposed that a star with mass above about 1.4 solar masses would collapse beyond the white dwarf stage and turn into something far denser and more compact. This upper mass limit is today called the Chandrasekhar limit.
Nova and Supernova
These are stars whose brightness increase suddenly by ten to twenty times or more due to a partial or outright explosion in the star. When brightness increases to 20 magnitudes or more, it is called a Supernova.
If the mass of the star is above Chandrasekhar limit, a tremendous explosion occurs at its core giving rise to a supernova. When that happens, it takes only a fraction of a second for the stellar core to collapse into a dense ball about ten miles across. The temperature and pressure becomes almost immeasurably hot and high.
There are two general types of supernovae.
- A Type I supernova is the result of an existing, older white dwarf that gains enough mass to exceed the Chandrasekhar limit, causing a runaway collapse.
- A Type II supernova is produced by a single highmass star whose gravity is so strong that its own weight causes the stellar core to reach a mass beyond the Chandrasekhar limit.
Neutron Star
A supernovae explosion in a star bigger than Sun but not more than twice as big, may leave behind an extremely dense, residual ‘core of that star, reaching a density of 1014gms/cm3, known as Neutron Star. This serves as matter’s last line of defense against gravity. In order to stay internally supported as an object and not be crushed into a singularity, the neutrons in the object press up against one another in a state known as neutron degeneracy. This state, which resembles the conditions within an atomic nucleus, is the densest known form of matter in the universe.
A neutron star is about as dense as a neutron itself. In other words, it has the density of an object more massive than the Sun, yet it is only about ten miles across. Its density is such that a single teaspoon of neutron star material would weigh about five billion tons!
Pulsar
When a neutron star spins incredibly fast, it forms magnetic field billions of times stronger than Earth’s field. This magnetic field interacts with nearby electrically charged matter and can result in a great deal of energy being radiated into space, a process called synchrotron radiation.
The slightest unevenness or surface feature on the neutron star can cause a significant “blip” or “pulse” in the radiation being emitted. Each time the neutron star spins around once, a pulse of radiation comes out. Such an object is called a pulsar.
As of now, more than 1,000 pulsars have been found throughout our galaxy. Perhaps the best known one is the Crab Nebula pulsar. It is at the center of the Crab Nebula and is a remnant from a supernova that was first observed in 1054 AD.
It pulses once every 33 milliseconds, which shows that the body with the mass of the Sun is spinning more than 30 times per second!
X-Ray Star
An X-ray star emits a great deal of X-ray radiation. X-ray stars may emit thousands of times more X rays than visible light radiation. X-ray stars are almost always binary star systems or multiple star systems. The interaction between the two or more stars in the systems—one of which is usually a compact object like a white dwarf, neutron star, or black hole—is what causes the strong X-ray emission.
Binary Star
A binary star is a pair of stars that are so close together in the sky that they appear to be closely associated with one another.
Some binary stars, called apparent binaries, are merely close together because of our point of view from Earth; they have nothing to do with one another physically.
Black Holes
A black hole is an object With such strong gravitational field that even light cannot escape from its surface. Black holes are formed from neutron stars after the Supernova explosions of big stars.
Factors that influence, how stars will evolve
The most important factor that affects evolution of star is its initial mass. On this basis, the stars can be of very low mass (0.01 solar mass), low mass (0.1 solar mass),
intermediate mass (about 1 solar mass), high mass (about 10 solar masses), and very high mass (up to about 100 solar masses). By this definition, Sun is intermediate mass star. The more is the mass of the sun, more it spends time in its main sequence and the hotter it is during its main sequence.
