Star Formation (Stellar Evolution or Life Cycle of A Star)

• Outlined below are the steps involved in a star’s evolution, from its formation in a nebula, to its death as a white dwarf or a neutron star.
• Nebula: a cloud of gas (mostly hydrogen and helium) and dust in space. Nebulae are the birthplaces of stars.
• Protostar: an early stage of a star formation where nuclear fusion is yet to begin.
• T Tauri Star: a young star still undergoing gravitational contraction; it represents an intermediate stage between a Protostar & a low-mass main sequence star.
• Main Sequence Star: E.g., Sun – full of life (nuclear fusion at the core is in full swing).
• Red Giant (in case of a small star) and Red Supergiant (in case of a large star).
• Planetary Nebula (in case of a small star) and Supernova (in case of a large star).
• White dwarf (in case of a small star) and Neutron Star or Black Hole (in case of a large star).

Protostar

• A Protostar looks like a star, but its core is not yet hot enough for nuclear fusion. 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.

Main sequence stars

• Main sequence stars fuse hydrogen atoms to form helium in their cores. Most of the stars in the universe, about 90 per cent of them including the sun, are main sequence stars.
• Towards the end of its life, stars like the sun swells up into a red giant, before losing their outer layers as a planetary nebula and finally shrinking to become a white dwarf.

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 the Sun’s.
• 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. An enormous red giant is often called Red Supergiant.

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 beyond the orbit of Venus. As it burns through its fuel, it will eventually collapse under gravity. The outer layers will be ejected in a shell of gas (planetary nebula) that will last a few tens of thousands of years before spreading into the vastness of space.

White Dwarf

• A white dwarf is a very small, hot star, whose nuclear energy supplies have been used up. It consists of degenerate matter with a very high density due to gravitational effects, i.e., one spoonful has a mass of several tonnes. It is the last stage in the life cycle of a star like the Sun.

Black dwarf

• The last stage of stellar evolution is a black dwarf. A black dwarf is a white dwarf that has sufficiently cooled and 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.

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. A great proportion of primary cosmic rays comes from supernovae.
• The extremely bright burst of radiation expels much of the 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 must be born, a star has to die!
• Supernovae can be triggered in one of two ways:

1. Type I supernova or Type Ia supernova (read as one-a) and
2. Type II supernova.

Type I supernova or Type Ia supernova (read as one-a)

• Type I supernova occurs when there is a sudden re-ignition of nuclear fusion on the surface of a degenerate white dwarf in a binary system. A degenerate white dwarf may accumulate sufficient material from a companion star to raise its core temperature, ignite carbon fusion, and trigger runaway nuclear fusion, completely disrupting the star.

The Importance of Type Ia Supernovae

• All Type Ia supernovae are thought to have nearly the same maximum brightness when they explode. Such consistency allows them to be used as beacons to measure the rate of expansion of the universe. The weaker the light, the farther away the star is (cosmological redshift).

The Difference Between Nova and Type I Supernova

Type II supernova

• Type II supernova is a supernova that occurs by the gravitational collapse of the core of a massive star (mostly made of iron). E.g., Supernova of a red supergiant.

Neutron stars

• Neutron stars are composed mainly of neutrons and are produced after a supernova, forcing the protons and electrons to combine to produce a neutron star.
• A neutron star is very dense (a mass of three times the Sun can be fit in a sphere of just 20km in diameter). If its mass is any greater, its gravity will be so strong that it will shrink further to become a black hole.

• Chandrasekhar Limit: it is the maximum mass at which a star near the end of its life cycle can become a white dwarf and above which the star will collapse to form a neutron star or black hole.

Black holes

• Black holes are believed to form from massive stars at the end of their lifetimes. The density of matter in a black hole cannot be measured (infinite!). The gravitational pull is so great that nothing can escape from it, not even light.
• Black holes distort the space around them and can suck neighbouring matter into them including stars.

Constellations

• The stars forming a group that has a recognisable shape is called a constellation.
• A few famous constellations are Great Bear (the Big Dipper or Saptarshi or Ursa Major), Orion (hunter), Cassiopeia & Leo Major.
• Ursa Major moves around the Pole Star. In fact, all the stars appear to revolve around the Pole Star.
• The northern constellations like Ursa Major may also not be visible from some points in the southern hemisphere.
• Orion can be seen during winter in the late evenings. The star Sirius, 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 Sirius.
• Cassiopeia is 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/M.