Life cycle of Stars

Sun, the brightest object of our solar system, is the primary energy source of our life, in fact for any living organism. Sun is considered to be the central object of our solar system that holds all the planets and their moons together. The day begins with beautiful sunshine and ends with reddish sky at dusk. Then the night comes and moon shines. This is what a common routine for most people. And according to many religions, God put sun to maintain this daily routine.

But there is science, and astrophysics (science of stars) does not approve this common concept. There are lots of complicated things going on inside and around the sun. This motivates some people to study about this and make good statements of all the dynamics happening in stars miles and miles away from us. Stars have got their own light while planets do not have their own light, this make stars a more comfortable subject to study. Because we can detect light and by spectroscopic analysis we can easily identify molecules and compounds present there. There are some basic questions that come to everyone’s mind; why stars have their own light? How long this light is going to shine? How long a star is going to live? But before answering all these questions, we must have a good understanding of how does a star form. Formation of a star is one of the most interesting topic of astrophysics which gives us a brief idea of everything that is going on in our universe.

Formation of Star:

The universe is very big. There is matter, and some empty spaces spread all over it. Space/distance between any two stars is known as Inter stellar medium (ISM). In very simple words, this is known as stellar neighborhood. This is a huge area between any two stars, and it consist of many layers. In the beginning it was difficult to detect ISM, but after the discovery of radio astronomy, Van Hulst discovered 21cm detection technique for hydrogen present in ISM. This ISM consists of the following regions: HI (neutral hydrogen), HII (ionized hydrogen), Hot corona gas, molecular clouds. Among all the regions, molecular cloud is the main topic of interest, because it is called the birthplace of star. These molecular clouds have densities of more than particles per cubic meter. Clouds are found in the cooler regions of ISM whose temperature is in the range of 10-30 K. Even though these clouds occupy less than 1% ISM space, still it contributes more than 40% of ISM mass. Most molecules in ISM are studied through molecular radio lines. The most extensively studied interstellar molecule is Carbon monoxide (CO), since it has very convenient radio lines. There are many complex as well as organic molecules present in this regions, which combine and eventually form a cloud of sufficient mass.
Gravity plays an important role here. When these cloud’s mass is of the order of solar mass (Jeans Mass), then because of the inward gravitational pull, they contract and starts to collapse. Jeans Mass is the minimum limit of mass that cloud must have to contract. Otherwise there will be no further steps, the star will end up as brown dwarf. Cloud is generally cold (less than 100K) and not very dense, called the dark nebulae. Collapse of the cloud is governed by many factors such as mass, temperature, strength of magnetic field, rotational energy, Jeans instability factors etc. While collapsing at the core, the gravitational energy excites some molecules and releases some thermal energy. This collapsing continues unless the thermal energy at the core becomes half the gravitational energy that is, , which is better known as Virial theorem. These energies cannot escape the core, because while collapsing down the outer layer becomes optically thick. This increases the core temperature to 2000K which dissociates hydrogen molecules. Dissociation of absorbs lots of internal energy for which a second core collapse starts. This stage is known as protostar. After certain time, cloud becomes transparent enough to radiate away some energy, which leads to the increase of luminosity. Once the protostar is contracted down to a radius of about cm after almost 1000 years, its luminosity is about erg/s During this stage the star becomes convective in nature. And after around 1 million years, the star becomes twice that of our sun. Slow contraction in this million period leads to a very high rise in temperature. Which is sufficiently high to initiate thermonuclear reactions at the core. As the protostar prepares to become a main sequence star, it becomes less luminous and the process of contraction continues. After almost 20 millions of years the protostar finally settles down to a ZAMS (Zero Age Main Sequence) star in H-R diagram from where it begins it’s life as a main sequence star.

Now what is H-R diagram or main sequence star?

H-R diagram is mathematically a graph plotted between luminosity on y-axis and temperature on x-axis. Temperature is plotted such that it increases towards left. In the graph we plot hundreds of stars as function of temperature and luminosity. H-R diagram has lots of information. There is a particular line pattern some stars follow on the graph, which is known as main sequence. Main sequence star is the most stable or in equilibrium star in H-R diagram. Our sun is a main sequence star. At the main sequence stage, gravitational collapse ceases and p-p chain reaction starts which produce the energy by thermonuclear reactions.

Mass of initial molecular cloud is very important. If the mass of the contracting cloud very high, then it will take shorter time to reach the main sequence line from protostar, and if its very low, it will take billions of years to reach main sequence line. A graph of this shown below:

Evolutionary tracks of protostar of different masses to main sequence

That is the general process of how a star is formed. After reaching the main sequence, that is the equilibrium line, the process is not completed. How long star is going to stay there?

What happens after the star is formed?
Although the star is in stable equilibrium in main sequence, but their evolution will depend on the availability of fuel supply inside the core. Since, initially most of the energy supply is from hydrogen burning, so, it will be a key factor to understand the depletion of hydrogen supply inside the core. Based on that, the stars from main sequence will undergo further evolution. So, now our sun is in main sequence that means after some billions of years, it will not be there, it will undergo evolution and will become something very unstable. That’s a clear evidence that our sun is not eternal, it will end up as some other structure. Are we safe? Yes, definitely, because that’s a huge amount of years, and probably human race may not be there when it will happen.

Star is formed from ISM, then it evolves, temperature rises, thermonuclear reaction occurs, protostar formed and then main sequence (or stable) star is formed. This cycling process is known as Stellar Evolution. And now we are in a position to discuss the end stages of this stellar evolution. As I have already stated, there are two types of main sequence star, Upper Main Sequence and Lower Main Sequence. Stars with mass greater (>>) than sun, falls in upper main sequence, and stars with mass equal or lower than solar mass falls in lower main sequence.

Both the types have different way of evolution after reaching main sequence.

But we are interested in stars like sun, so we will discuss what happens to such stars after reaching equilibrium point. Such stars spend a time period of years on the main sequence. This totally depend on the availability of fuels (molecules) inside the core. When they are on the main sequence, hydrogen starts converting into helium. As the mass of the helium core increases, core of the star become denser surrounding by a burning hydrogen layer. At this time the star slightly moves above the main sequence line. Luminosity of the star remains same, but surface temperature is reduced at this stage. Energy producing at the core is radiated outwards by convection, which increases the luminosity of the star. At this stage, star moves sharply above the main sequence line. Now the star is very far away, high above the equilibrium point. This stage of the star is known as Red Giant. After reaching the stage, star loses its mass rapidly by stellar wind. Because of this shrinkage, helium burns faster, rising the temperature of the star to K. Electrons inside the core becomes degenerate and core expands to a normal value which decreases the temperature. As a result, the star comes slightly down from its position and moves towards left. It is known as now Variable Star. Since it pulsate out variable energy output at this stage. When Helium and Carbon starts to burn at the core, the star evolves towards higher luminosity but comparatively lower radius. Beyond this point the radiation pressure exceeds the gravitational force of the envelope of hydrogen. This ejects out the envelope in space from the star in the form of planetary nebulae. A compact stellar core is left behind in the process. This compact core is generally formed of degenerate electron gas and hot molecules. No further contraction occurs, energy stored in the core is leaked away into space and the star cools to form White Dwarf. Minimum mass of a stable white dwarf is 1.44 times solar mass. This is known as Chandrasekhar’s mass limit. Beyond this point, no further mass can be supported by degenerate electron pressure. So, for stars like sun, White Dwarf may be considered as the end point of stellar evolution.

However, for upper main sequence star, that is massive stars may end up differently. The white dwarf form by them, if they crosses Chandrasekhar’s limit, then star explodes in the space. And enormous amount of energy is ejected. This explosion is known as Supernova. After supernova explosion, if the mass of the remnant star is of the order of 1.4 times solar times, then remnant is known as Neutron Star. And if remnant is of higher than 2 solar mass, then it ends up as Black Hole in the space. The energy explodes in the supernova again excites some molecules in inter-stellar space. After millions of years, it will again start forming protostar.

So, this is how the life cycle of a star goes on. We can conclude, low mass stars like our sun, will end up their life as White Dwarf, whereas higher mass stars will end up their life either as Neutron star/Black hole or just supernova explosion.

This is what we should know about our only energy source in the solar system. One day our sun will have to go away from us. That is science and that is true. Unfortunately, there will be no God then.

[Note: I have tried to make it as short as possible. So, some intermediate stages of stellar evolution are skipped. I have not included any mathematics so that everyone can read it. Feedback is welcome via email: rupam.istgu@gmail.com]