Geography Notes for UPSC, Formation of Star UPSC
Table of Contents
1. Star
Stars are the celestial bodies which revolve around the centre of the galaxy.
The stars have their own heat and light, which the stars emit continuously.
Stars are made up of vast clouds of hydrogen gas, helium and dust. Helium gas is the second largest component of the star after Hydrogen (H2).
1. Formation of Star or Stages of Star
Stars pass through a definite evolutionary sequence:
| Nebula | Protostar | Small Star | Red giant | Nova (Produced Residual star and Planetary Nebula) | White dwarf (Residual star have less than equal to 1.44 of solar mass) Black dwarf |
| Protostar | Large Star | Red supergiant | Supernova (Produced Residual star and Planetary Nebula) | Neutron Star (Residual star have greater than 1.44 but less than 3 times of solar mass) Pulsar | |
| Frozen Star (Residual star have greater than equal to 3 times of solar mass) Black Hole |
1 – Nebula
A nebula is a giant cloud of gas (mostly hydrogen and helium) and dust in space.
Nebulae are the birthplaces of stars.
2 – Protostar
First stage of a star formation where nuclear fusion is yet to begin.
A Protostar looks like a star, but its core is not yet hot enough for nuclear fusion to take place.
The luminosity comes exclusively from the heating of the Protostar as it contracts (because of gravity).
Protostars are usually surrounded by dust, which blocks the light that they emit, so they are difficult to observe in the visible spectrum.
3 – T Tauri star
A very young, lightweight star, less than 10 million years old, that is still undergoing gravitational contraction; it represents an intermediate stage between a Protostar and a low-mass main sequence star like the Sun.
4 – Main sequence stars
Main sequence stars are stars that are fusing hydrogen atoms to form helium atoms in their cores.
Nuclear fusion at the core at full swing.
Most of the stars in the universe — about 90 per cent of them — are main sequence stars.
The sun is a main sequence star.
Towards the end of its life, a star like the Sun swells up into a red giant, before losing its outer layers as a planetary nebula and finally shrinking to become a white dwarf.
Red dwarf
The faintest (less than 1/1000th the brightness of the Sun) main sequence stars are called the red dwarfs.
Because of their low luminosity, they are not visible to the naked eye.
They are quite small compared to the sun and have a surface temperature of about 4000 ֯C.
According to some estimates, red dwarfs make up three-quarters of the stars in the Milky Way.
Proxima Centauri, the nearest star to the Sun, is a red dwarf.
5 – Red giant
Red giants have diameters between 10 and 100 times that of the Sun.
They are very bright, although their surface temperature is lower than that of the Sun.
The continued nuclear fusion upset the overall equilibrium of the star and to readjust it 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.
A red giant is formed during the later stages of the evolution as it runs out of hydrogen fuel at its centre.
It still fuses hydrogen into helium in a shell surrounding a hot, dense degenerate helium core.
As the layer surrounding the core contains a bigger volume the fusion of hydrogen to helium around the core releases far more energy and pushes much harder against gravity and expands the volume of the star.
Red giants are hot enough to turn the helium at their core into heavy elements like carbon (this is how elements were formed one after the other).
But most stars are not massive enough to create the pressures and heat necessary to burn heavy elements, so fusion and heat production stops.
Red Supergiant
As the red giant star condenses, it heats up even further, burning the last of its hydrogen and causing the star’s outer layers to expand outward.
At this stage, the star becomes a large red giant. A very large red giant is often called Red Supergiant.
6. Nova and Supernova:
A red giant star phase may end in a Nova/Supernova stage. These are stars whose brightness increases suddenly by ten to twenty magnitudes or more due to a partial or outright explosion in the star. When brightness increases to 20 magnitudes or more, it is called a Supernovae.
Nova
Nova occurs on the surface of a white dwarf in a binary system.
If the two stars of the system are sufficiently near to one another, material (hydrogen) can be pulled from the companion star’s surface onto the white dwarf.
When enough material builds up on the surface of the white dwarf, it triggers a nuclear fusion on a white dwarf which causes a sudden brightening of the star.
Supernova
A supernova is the explosive death of a star and often results in the star obtaining the brightness of 100 million suns for a short time.
The extremely luminous burst of radiation expels much or all of a star’s material at a great velocity, driving a shock wave into the surrounding interstellar medium.
These shock waves trigger condensation in a nebula paving the way for the birth of a new star ― if a star has to be born, a star has to die!
A great proportion of primary cosmic rays comes from supernovae.
7 – Planetary Nebula & Residual star
Planetary Nebula
Planetary nebula is an outer layer of gas and dust (no planets involved!) that are lost when the star changes from a red giant to a white dwarf.
At the end of its lifetime, the sun will swell up into a red giant, expanding out beyond the orbit of Venus. As it burns through its fuel, it will eventually collapse under the influence of gravity.
Residual star
Fusion in a star’s core produces heat and outward pressure, but this pressure is kept in balance by the inward push of gravity generated by a star’s mass.
When the hydrogen used as fuel vanishes, and fusion slows, gravity causes the star to collapse in on itself. This creates a degenerate star/ Residual star.
The matter in this state is called ‘degenerate matter’.
8 – White, Black and Brown dwarf
White dwarf
A white dwarf is a very small, hot star, the second last stage in the life cycle of a star like the Sun.
White dwarfs are the remains of normal stars, whose nuclear energy supplies have been used up.
White dwarf consists of degenerate matter with a very high density due to gravitational effects, i.e., one spoonful has a mass of several tonnes.
Black dwarf
The last stage of stellar evolution is a black dwarf.
A black dwarf is a white dwarf that has sufficiently cooled that it no longer emits significant heat or light.
Because the time required for a white dwarf to reach this state is calculated to be longer than the current age of the universe (13.8 billion years), no black dwarfs are expected to exist in the universe yet.
Brown Dwarfs
Brown dwarfs are objects which are too large to be called planets and too small to be stars.
Brown dwarfs are thought to form in the same way that stars do – from a collapsing cloud of gas and dust.
However, as the cloud collapses, the core is not dense enough to trigger nuclear fusion.
9 – Neutron stars and Pulsar
Neutron stars:
These stars are composed mainly of neutrons and are produced after a supernova, forcing the protons and electrons to combine to produce a neutron star.
Neutron stars are very dense. (mass of three times the Sun can be fit in a sphere of just 20 km in diameter).
If its mass is any greater, its gravity will be so strong that it will shrink further to become a black hole.
Pulsar UPSC:
Pulsars are highly magnetized, rotating neutron stars that emit a beam of electromagnetic radiation.
Pulsars are superdense, rapidly spinning neutron stars left behind when a massive star explodes.
Pulsar is a celestial object that emits regular pulses of radio waves and other electromagnetic radiation at rates of up to one thousand pulses per second.
They are spherical, compact objects that are about the size of a large city but contain more mass than that of the sun.
A pulsar is a rapidly spinning star that is left after a more massive star has exploded. It’s like a lighthouse because it sends out beams of radiation that we can detect as they sweep across Earth, making it appear to “pulse” or blink. Pulsars are relatively small and incredibly dense, with strong magnetic fields.
Cepheids, also called Cepheid Variables, are stars which brighten and dim periodically. This behavior allows them to be used as cosmic yardsticks out to distances of a few tens of millions of light-years.
Black holes UPSC:
Black holes are believed to form from massive stars at the end of their lifetimes.
The gravitational pull in a black hole is so great that nothing can escape from it, not even light.
The density of matter in a black hole cannot be measured (infinite).
Black holes distort the space around them and can suck neighbouring matter into them including stars.
Gravitational lensing:
Light around a massive object, such as a black hole, is bent, causing it to act as a lens for the things that lie behind it.
| Chandrasekhar Limit Chandrasekhar Limit is the maximum mass theoretically possible for a stable white dwarf star. A limit which mandates that no white dwarf (a collapsed, degenerate star) can be more massive than about 1.4 times the mass of the Sun. Any degenerate object more massive must inevitably collapse into a neutron star or black hole. The limit is named after the Nobel laureate Subrahmanyan Chandrasekhar, who first proposed the idea in 1931. He was awarded the Nobel Prize in Physics in 1983 for his work on the physical processes involved in the structure and evolution of stars. White dwarf (Residual star have less than equal to 1.44 of solar mass) – Black dwarf Neutron Star (Residual star have greater than 1.44 but less than 3 times of solar mass) – Pulsar Frozen Star (Residual star have greater than equal to 3 times of solar mass) – Black Hole |
10 – Quasar and Blazar
Quasars
A quasar is an extremely luminous active galactic nucleus (AGN), in which a supermassive black hole with mass ranging from millions to billions of times the mass of the Sun is surrounded by a gaseous accretion disk.
As gas and material in the disk falls towards the black hole, energy is released in the form of electromagnetic radiation, which can be observed across the electromagnetic spectrum, making the quasar shine brightly, sometimes outshining its entire host galaxy.
Quasars, also called quasi-stellar radio sources, are the most energetic and distant members of a class of objects called active galactic nuclei (AGN).
Their spectra contain very broad emission lines, unlike any known from stars, hence the name “quasi-stellar.” Their luminosity can be 100 times greater than that of the Milky Way.
Blazars
Blazars are also active galactic nuclei (AGN), whose jets are aligned with the observer’s line of sight. Some blazars are thought to host binary black holes in them and could be potential targets for future gravitational-wave searches.
The difference between Quasar, radio galaxy and a Blazar is the angle of the stream/jet. If the stream is straight up, it is a radio galaxy and the observer is not in the firing line. If the stream is angled slightly towards the observer, then it is a Quasar and if the stream is angled directly towards the observer, it is a Blazar.
A blazar is a type of quasar, but with a twist – it’s an energetic powerhouse shooting high-energy jets of particles toward Earth. These jets are also powered by a supermassive black hole at the center of a galaxy, but they’re aligned in such a way that the jets point almost directly at us.
Pulsar: A cosmic lighthouse
Quasar: A bright object in the sky
Blazar: An energetic powerhouse
Size: Pulsars are much smaller, with a diameter of around 20 kilometers. Quasars and Blazars are much larger objects, as they are associated with galaxies and their central black holes.
Distance: Pulsars are usually found within our Milky Way galaxy, whereas Quasars and Blazars are much farther away, located in distant galaxies.
Active Galactic Nuclei (AGN)
At the center of most galaxies, there’s a massive black hole with a huge mass accumulating gas, dust, and stellar debris around it.
AGN is formed when the gravitational energy of these materials, being pulled towards the black hole, is converted into light.
A minority of AGN (~15%) emits collimated charged particles called jets travelling at speeds close to the speed of light.
2. Colour of the star
The colour of the star is indicative of its temperature.
Star Colour – Approximate Temperature
Blue – 25,000 K
White – 10,000 K
Yellow – 6000 K
Orange – 4000 K
Red – 3000 K
3. Galaxy
A galaxy is a vast collection of billions of stars, dust and hydrogen gas, and stellar remnants, isolated in space from similar systems. All these heavenly bodies are bound together by their gravitational force.
Galaxies are the major building blocks of the universe.
There are varied sizes of galaxies, but all galaxies contain a large number of stars. The smaller galaxies contain about 1,06,000 stars while the largest galaxy may contain well over 3000 billion stars.
There are innumerable galaxies in the universe and the observable part of the universe contains about 100 billion galaxies.
Galaxies may vary greatly with respect to their size and shape.
Galaxies are labelled according to their shape. Some galaxies are called “spiral”, because they look like giant pinwheels in the sky. The galaxy we live in, the Mandakini, is a spiral galaxy. Some galaxies are called “elliptical”, because they look like flat balls. A galaxy may be called “irregular” if it doesn’t really have a shape.
The centre of each galaxy has a black hole (galactic centre). The galaxy and all the stars in it revolve around the black hole (galactic centre).
It takes about 225 to 250 million years for our star (Sun) to revolve around the centre of our galaxy, Mandakini. This period is called the Cosmic Year. In the beginning, the universe was much smaller as there was less space between the galaxies.
The nearest galaxy to the Milky Way is Andromeda. Andromeda is a spiral galaxy and approximately 2.5 million light-years from the earth. Andromeda is also known as NGC224 and M31.
Our Galaxy (Mandakini)
Mandakini is one of the billions of galaxies existing in the observable universe. Our solar system and our earth belongs to this galaxy.
Mandakini was built about 12 billion years ago. It is a barred spiral shaped galaxy.
The Milky Way is a part of our galaxy Mandakini, seen during the night. The ancient Romans called it the Via Galactica, or “road made of milk”. This is how our galaxy became known as the Milky Way. In India, it is popularly known as Akash Ganga which means ‘white river of light in the sky’.
It is called a spiral galaxy because it has long arms which spin around like a giant pinwheel. Our Sun is a star in one of the arms (middle arm).
| Cluster of Galaxies A cluster of galaxies is a large collection of galaxies in a single gravitational field. Rich clusters of galaxies usually contain at least a dozen large galaxies as massive as the Milky Way, along with hundreds of smaller galaxies. At the centre of large clusters of galaxies there is usually a group of elliptical galaxies called “cD” galaxies. Supercluster of Galaxies Superclusters are the largest collections of massive structures. There are usually many clusters of galaxies in a supercluster, or a single very large cluster at its centre, along with many other groups and collections of galaxies that are collected in the supercluster’s central gravitational field. Superclusters contain many thousands—and sometimes millions—of galaxies. The Milky Way galaxy is located on the outskirts of the Virgo supercluster. Saraswati – In 2017, Discovery of a previously unknown supercluster of galaxies, Saraswati, was reported by a team of Indian scientists. Saraswati, the supercluster of 43 galaxies, is located in the direction of the constellation Pisces. |
4. Constellations
The stars forming a group that has a recognisable shape is called a constellation. Grouping of Stars named for a particular Object, Person or Animal.
We can see only a few bright stars of the constellation with our naked eyes.
The International Astronomical Union recognizes 88 constellations covering the entire northern and southern sky.
The Indian name for constellation is ‘Nakshatras’.
The largest constellation is Hydra and the smallest constellation is Crux, the Southern Cross.
The 12 constellations near the Sun (which have a greater impact on the Earth) are known as the 12 zodiac signs. These 12 zodiac signs are divided into 27 Nakshatra.
Ursa Major
One of the most famous constellations which you can see during summer time in the early part of the night is Ursa Major. It is also known as the Big Dipper, the Great Bear or the Saptarshi.
There are seven prominent stars in this constellation. It appears like a big ladle or a question mark. There are three stars in the handle of the ladle and four in its bowl.
Look at the two stars at the end of Ursa Major. Imagine a straight line passing through these stars as shown in Fig. Extend this imaginary line towards the north direction. (About five times the distance between the two stars.)
This line will lead to a star which is not too bright. This is the Pole star. Observe the Pole star for some time. Note that it does not move at all as other stars drift from east to west.
Ursa Major moves around the Pole star.
Note that the Pole star is not visible from the southern hemisphere. Some of the northern constellations like Ursa Major may also not be visible from some points in the southern hemisphere.
Orion
Orion is another well-known constellation that can be seen during winter in the late evenings. It is one of the most magnificent constellations in the sky. It also has seven or eight bright stars.
Orion is also called the Hunter. The three middle stars represent the belt of the hunter. The four bright stars appear to be arranged in the form of a quadrilateral.
The star Sirius, which is the brightest star in the sky, is located close to Orion. To locate Sirius, imagine a straight line passing through the three middle stars of Orion. Look along this line towards the east. This line will lead you to a very bright star. It is Sirius.
Sirius (Dog Star) is the brightest star in the night sky.
Cassiopeia
Cassiopeiais another prominent constellation in the northern sky. It is visible during winter in the early part of the night. It looks like a distorted letter W or M.
Leo constellation
Leo constellation lies in the northern sky. It is one of the zodiac constellations and one of the largest constellations in the sky. The Solar System
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Formation of Star UPSC
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