Yassou folks. Welcome to another post of The Scientific Revelation, the star genesis and stelliferous era, in which we will discuss star formation and the stelliferous era. In the previous posts of after the bang of the Big Bang, the fundamental forces of nature, inflation, and matter formation we have focused on the big bang, early stages of the universe, fundamental forces and particle, how inflation had shaped the universe, the formation of matter and protogalaxies. After the recombination event universe plunged into dark ages where for millions of years no visible light was present. The first-generation stars were very large and made up dominantly of hydrogen. Those stars were very massive and to support the massive gravity they had used their fuel very fast. They burst open in the supernova and gave birth to the second generation of stars.
The importance of stellar physics
The study of stars is very important from an astronomical point of view. Their composition is a sign of the era they belong to. This knowledge helps us in recognizing the type of stars. Our sun is a population 1 star and contains 5–6% of heavy matter. The composition of stars and its size also defines the planetary system for example very massive stars have massive radiation and gravity due to which an Earth-like planet is not possible.
Read this post on stars by Hubble space telescope.
Stars are formed inside interstellar clouds that are widespread in the universe. Because of many reasons like supernova and Steller winds these normally evenly spread galactic clouds form clots/clumps. With the passage of time these clumps evolve into protostars and eventually bright stars. So whenever you look up in the sky try to cherish these bright objects that take millions of years to form. Not to mention life is possible due to heavy matter that was forged inside the core of a star.
The stars are landmarks of the universe.
Before there was light
Before the stelliferous era universe was plunged into the darkness. After the recombination period in which the universe had seen light for the first time, it again became dark. When electrons and nuclei combined together the subatomic soup melted away and the universe was transparent for the first time. Photons were able to travel long distances and the universe was lit for few seconds before plunging into darkness.
Our recent technology can not pinpoint exactly when the cosmic dark ages ended. Although our best estimate is that it lasted between 300 million to 1 billion years.
The Dark ages
During the dark ages, the universe entered a crucial period that is known as Reionization. After the recombination stage(at the end of this phase CMBR was produced) most of the universe was filled with dark matter. The percentage of baryonic/normal matter was very less. The dark matter had made a halo-like structure due to their gravity. This gravity of dark matter had swept the normal matter under its influence and hasten the process of galaxy formation. Some regions had more density of atoms and others have very less. The overdense regions collapse under their gravity and underdense regions keep on expanding. The atoms in dense regions became fuels for our first stars.
Research had shown that this formation had released ultraviolet radiation which in turn had started reionization. To actually find when cosmic dark ages had ended is a challenge for scientists. The intergalactic medium has the property to absorb the ultraviolet light so it is a challenge to find when the cosmic dark ages ended AKA when the first galaxies formed. Read more about this in the posts, matter formation, and inflation of The scientific Revelation.
The dark matter detection
The dark matter was detected when scientists have found out that stars at the arms of galaxies are rotating at the same speed as near the center. If we go with the simple gravitational calculation the stars near the center should rotate faster and those near the arms should rotate slower. Their similar rotation had made speculation for an unseen matter which is now known as dark matter. Their existing can also explain gravitational lensing in which the cloud of dark matter bend/distorts the light from a distant object.
The stelliferous era is the star dominated time period of the universe that is also our current era. As a species and civilization, the energy of the star is a life-supporting system. In the vast coldness of the cosmos, it would have been not possible for life to sprout on earth we didn’t have the sun. Stars give us energy and they are furnace where metals forms. After the cosmic era when the first stars had lit the sky to right now, we are still progressing in stelliferous and will continue to progress for trillions of years. We can say that the stelliferous is the brightest time period of the universe. It took us quite a while to discover and understand what stars are made of, their types, and their distribution in the cosmos. We are still in the progress of unlocking more about them.
The star formation
Stars are formed out of very dense molecular clouds. They are predominately made up of hydrogen and a little bit of helium. Their temperature is very less and they are found in molecular forms. The less temperature helps gravity in the accumulation of the matter. These star regions are very dense so it is difficult for visible light to move across them. They are opaque to visible light and we can detect them using star telescope and infrared.
From clumps to the protostar
A gas clump separates from a bigger cloud and then it collapses under its own gravity. These cores normally carry 10 to 50 solar masses in them. It keeps on contracting and squeezing under. The nearby lose gas matter keeps on getting attracted/falling into the core (which is the densest part of a cloud). They then form a protostar and this whole process of star formation takes up to millions of years(10 million years). Proto star is just a stage before star because it’s ready to do nuclear fusion but not started yet.
Angular momentum, nuclear fusion, and IR
The falling gas produces kinetic energy because of heat and temperature as well as pressure increase inside the core of the star. Then with the ceasing temperature, the protostar produces IR energy. When the gas clump separates from the galactic cloud it is irregular and without any shape but under the effect of angular momentum it turns into spherical with a rotating disc. The infall of matter increases the size of the protostar and this in fall of matter/the nearby matter that falls inside the dense proto star stops when nucleosynthesis starts(at that same time it also produces strong ejection of matter from the poles).
Note- The molecular clouds are transparent enough that it lets light pass. But when clumps form it becomes opaque to light. Another point is that in the start a protostar has very little mass in comparison to its final stage.
The simplest and easy to understand post on star formation by Arizona education.
The main sequence stars
Let’s talk about the main sequence stars and their type. If we talk about our sun so it is a medium star. It had taken 50 million years for our sun to mature. By maturity I mean, since the time when our sun as a clump had separated from interstellar gas to the time when the first nucleosynthesis happened. Sun will be able to shine for another 5 billion years then it will run out of fuel. The fuel for stars comes from the nucleosynthesis when hydrogen turns into helium. This process produces pressure that provides an outward push. The gravity pushes inward which is balanced by pressure from the core’s activity. The more the mass the higher the inward push of gravity.
The lowest main sequence stars are red dwarfs that are very dim and have approx. 10% of the solar mass. It only has a temperature between 3000 to 4000 k. These are one of the most widespread stars in the universe.
On the other hand of the spectrum, we have massive stars also called hypergiant with 100 or more solar masses and surface temperatures of more than 30,000 k. They emit an immense amount of energy but have a few million years of life span. These stars used to be very common at the start of the stelliferous era but now only a few are there per galaxy.
Steps of star formation
- Out of the molecular cloud many gas clumps form. These clumps have irregular shapes and due to angular momentum, they form a spherical shape. With the rotation motion, they form into a sphere. This initial star had very little mass and it is not very densely concentrated.
- The nearby matter(fragmented gas clouds/bits and material) keeps on falling/ getting attracted to the gas clump(future protostar). Most of the attracted matter increases the star’s mass but some also give rise to planets or can turn into the comet, asteroid.
- As the matter keeps on falling the mass of protostar keeps on increasing. Temperature and pressure keep on increasing and the temperature of its core reaches the point of IR radiation. In this, we can detect the protostar in spite of it being invisible.
- The birth of the main sequence star is quite an event. Protostar spins rapidly, to begin with, but after the collapsing of the star, this formation spins even faster increasing the temperature even more. There is a simple physics behind it. The more molecular clouds get contracted the denser and smaller they get and the faster they spin. A mature star is denser and smaller than a protostar.
- When nucleosynthesis starts in the star it stops collecting the nearby matter and excess matter excrete out by the jets of matter from both poles of the star.
T-Tauri star phase
As I have mentioned in the previous flowchart that after the start of nucleosynthesis when hydrogen started using as fuel and converting into helium. The extra matter that was stored/collected by the star get ejected through the jets of matter from both of the poles. It can be seen by radio telescope and called the T- Tauri phase. The telltale sign of this phase is jets of matter from both of the poles.
Steps of formation:
T- Tauri stars usually have a large circumstellar disc(most of the time two; nearby one feeds the protostar and outer one give rise to planets.) During the formation of planets, this disc dissipates and dissolves. T Tauri can lose up to half of their mass in jet ejection. They can evolve into a red giant or supergiant.T- Tauri begins its life as a very compact star. As years pass star loses its hydrogen fuel. As we know the stars only shines when nuclear fission starts inside them. With the start of nucleosynthesis, hydrogen gas starts to fuse into helium, and then helium will fuse into the heavier matter. As the star fuel start to diminish star increases in size and gets dimmer. A protostar is evolved from the cluster of molecular/galactic clouds into a cluster of T-Tauri stars that are hot and produce Stellar winds.
Sometimes we see clusters of young stars with other clusters of young stars coexisting in the nearby vicinity. They formed due to supernova. Supernova produces very strong Steller waves during the explosion. It is so powerful that it makes nearby interstellar clouds to compact and form more stars. It is one of the prime reasons for star production. Because of supernova explosions when the interstellar medium gets disturbed it produces a high number of stars.
Any star has three distinctive characters, vigorous activities like eruptions, strong Stellar winds, and matter jets in the starting stages of star formation.
A brown dwarf is definitely not a star but it is an important chapter of star formation. It is a fascinating structure in the universe between a gas planet(size of Jupiter) and a star. A brown dwarf typically carries 0.08 solar mass. It is a very low mass. Because of low mass gravitational push never reaches the limit where it can create enough pressure. When there is not enough pressure so the temperature never increases to start the thermonuclear process.
Due to insufficient temperature and pressure hydrogen never synthesis into helium and no light produces. For gravitational push to act star should have at least 75 Jupiter masses. So brown dwarf due to insufficient mass never turns into a star. And when it doesn’t reach a certain mass it doesn’t cross a specific temperature. The temperature for thermonuclear should at least be 10,000,000 kelvin. A brown dwarf has red color. Its outer layer heats up a bit only to resembles a red dwarf for few millions of years but it again fades away.
A pseudo star with no light
The reason that why a brown dwarf never produces light is because its core becomes degenerate even before nuclear fusion can start. When the degeneration of the core started the infalling of gas stopped and the pressure does not increase if the pressure doesn’t increase temperature doesn’t increase and no nuclear fusion happened. Brown dwarf does produce infrared radiation(due to infalling gaseous material in the star core) when potential energy turns into kinetic energy. In astronomical distances, it is very difficult to distinguish between a red dwarf, a planet, and a brown dwarf. A red dwarf is of the lowest level of a true star. It produces its own energy by nuclear fusion. A planet and brown dwarf are very similar in size and they probably have the same atmospheric constituents so it is very different to distinguish among themselves.
The first stars
In this period universe was hot and filled with hydrogen gas. The temperature was still very high and things were still moving fast. To make the first stars the density of hydrogen had to increase. 10 million years after Recombination/CMBR universe was still hot and things need to cool down. The universe had neutral hydrogen and millions of years had passed before it turned into molecular hydrogen. This molecular hydrogen became the building blocks of the first stars.
The first-generation stars formed 1 hundred million years after the big bang. They were hot, short-lived, and massive. They had no heavy metals inside their core. Our sun is a population third star with 3–5% of the massive element in it. They were 100 to 300 times bigger than our sun and brightness was blinding very high than today’s stars. They were very bright consequently their fuels burned faster. Not to mention those starts were heavy, very heavy and to mention the gravity fuels burned faster. It was a double tap for the star’s life. At the end of their life, they burst open by the supernovae explosion. They were the place/factories where heavier elements were born.
Population 1 and 2 stars
Our sun and its coexisting stars come under population 1 star and found at the spiral arms of the galaxies. There is a distinct quality of these stars that they have matter content produced from the supernovae of the previous stars. They have circular to elliptical orbit and younger pop. 1 population stars are forms in the galactic arms. On the other hand older ones/population 2 have formed near the galactic centers.
On the other hand population, 2 stars are found in the globular clusters and nucleus of galaxies. They are old from more than 10 billion years to 13 billion years. Their masses are less than or equal to 0.8 solar masses. Their orbits are random means no order and highly elliptical. These stars are nutrient-poor and cold with mostly hydrogen plus helium inside.
Death of a star
Every star has a different fate. The more massive the star the faster it will die off. The more massive stars the faster they will use their energy. Normally the star has two forces working on it. Firstly, the inner pressure/push of nuclear fusion. The process of nucleosynthesis produces the bright blinding light of a star. Secondly, we have a gravitational force from outside. The more mass a star has the higher the gravitational force it will feel. To balance a high gravity star has to produce more energy means a higher rate of nucleosynthesis. This is the reason why more massive stars have a lower lifespan.
The transition into a red giant
After extinguishing its energy star move towards the stage of the red giant. When all the hydrogen in its core finished up nuclear reaction stops. Because there is no energy to balance gravitational force the core begins to collapse in itself. It’s temperature increases even more. Even though there is no fuel inside the core, stars do contain hydrogen in the outer shells so fusion continues outside. This increases the diameter of the star. The hot inner core also pushes the star outwards. This leads to the expansion of the star turning into a red giant.
After the stage of the red giant comes the death of a star. If the star is sufficiently massive nuclear fusion can upgrade up to the synthesis of iron. Though the question arises that how other heavier matter forms? At the time of supernova explosion (it is undoubtedly one of the most massive and fascinating even in the universe) the energy of core and star rise higher creating conditions for the formation of heavier metals. There are types of the end for the stars like a neutron star or black hole or white dwarf.
Stars equivalent to our sun becomes a white dwarf. After the death of the core out shells expand until they totally dissipate in the space and only the core remains visible. Stars bound to become white dwarfs have mass up to the limit of 1.4 solar masses. The pressure from fast-moving electrons keeps the core intact. The more massive the star would be in defined limit the denser its core will be. And this denser core means its diameter will be smaller.
Scientists have observed an interesting phenomenon in white dwarfs. If a white dwarf forms in a binary/multiple star system and has more mass, it attracts the hydrogen from nearby stars and attaches it to oneself. These massive white dwarfs sometimes collect so much hydrogen that they can explode into novae.
Massive main sequence stars can die through supernovae explosions. A supernova is different from a nova. In nova(which happens on the white dwarf) surface of the star explodes but in a supernova, the core of a massive star collapses and explodes. The massive stars reach a higher level of nuclear fusion synthesizing iron. After the formation of iron, the star core can’t synthesize heavier elements because it has no hydrogen left. When there is no fuel and nucleosynthesis no energy produces. Star can not support its massive mass anymore and it collapses.
The core that is normally thousands of miles shrinks up to as little as 100 miles and energy increases up to 100 billion degrees. With the collapse of the core, the outer layer also shrinks initially but then it expands outward violently and explodes with a boom. Supernova is so violent that it can be brightly visible for weeks. The massive shock waves released creates disruptions in otherwise uniform interstellar gas hastening the process of star formation. The elements, that are heavier than iron forms at the spike of energy during a supernova when atoms are boomed out of the star.
The aftermath of a supernova explosion can be a black hole or neutron star. We know that if a star’s mass is up to 1.4 solar masses it will become a white dwarf. And if a star mass is in the range of 1.4 to 3 solar mass it will become a neutron star. The process is the same as the white dwarf. The star can not handle its gravity as there is no fuel to exert pressure and heat. The fast-moving electron would have been somehow balancing the weight of the core but because the core is even heavier than their range the electrons and protons combine to make neutrons forming neutron stars. Neutron stars are extremely dense. They are so dense that if they are in a multiple star system they can suck the other star’s outer layers.
Neutron stars have a very strong magnetic field and it causes the acceleration of neutrons at the poles. Because of it, a strong beam of radiation leaves past the poles and it can be observed from earth too.
When the core of a dying star is more than 3 solar masses heavy it becomes a black hole. Black holes are very strong gravitational objects that suck everything in their vicinity. Light also can not escape the gravity of a black hole that’s why we can not detect a black hole directly. Although we can indirectly detect a black hole by its gravitational activity. When a black hole come closer to a star it started sucking the outer layers of the star. This creates a rotating, heated disc of sucked material around the black hole. The immense heating of material produces the x and gamma rays that are visible from far away. We also know at the center of spiral galaxies massive black holes lie. Milky way also has a massive black hole Sagittarius A* at its center.
During supernova explosions, dust and debris get mixed with nearby interstellar clouds that in the future will become part of new stars. The heavier matter produced in supernovae will be part of a rocky planet like our earth.
The future of the cosmos
There are many theories that had predicted the end of our universe. We are currently living in the stelliferous era after this will come the degenerate era, the era of black holes. Our universe can end in a big crunch or big rip. Maybe the universe will start contracting instead of expanding or maybe it will continue to grow apart until even atoms will be ripped apart.
refer to these posts for a detailed understanding of star formation.
- one of the best posts I have read on star formation and many related topics of the cosmos.
- Post on The stars by NASA. Very informative and understandable.
- A very detailed description of stars formation and evolution by britannica.com.
- video description of star formation by Launchpad Astronomy.
- A lecture on The Formation of Stars and Planets.
There are many speculations about the evolution of the universe. We shall read more about them and other ideas in the next posts. So, stay tuned and do the revelation.
As one star sinks behind the horizon, another comes into the picture.