Table of Contents
Very few questions are asked in prelims [time to benefit ratio is too low] from this section. You can ignore these concepts if you found them too scientific.
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Star Formation-Stellar Evolution-Life Cycle Of A Star
- Outlined below are the many steps involved in a stars evolution, from its formation in a nebula, to its death as a white dwarf or neutron star.
- T Tauri Star
- Main Sequence Star
- Red Giant
- White dwarf, Neutron Star or Black Hole
- A nebula is a cloud of gas (hydrogen) and dust in space.
- Nebulae are the birthplaces of stars.
- Nebulae consist mostly hydrogen and helium gas.
- A Protostar looks like a star but its core is not yet hot enough for fusion (fusion of 2 hydrogen atoms into a helium atom with the liberation of huge amount of energy) to take place.
- The luminosity comes exclusively from the heating of the Protostar as it contracts.
- Protostars are usually surrounded by dust, which blocks the light that they emit, so they are difficult to observe in the visible spectrum.
T Tauri star
- A very young, lightweight star, less than 10 million years old, that it still undergoing gravitational contraction; it represents an intermediate stage between a protostar and a low-mass main sequence star like the Sun.
Main sequence stars
- Main sequence stars are stars that are fusing hydrogen atoms to form helium atoms in their cores.
- Most of the stars in the universe — about 90 percent of them — are main sequence stars.
- The sun is a main sequence star.
- The faintest stars are the red dwarfs, less than one-thousandth the brightness of the Sun.
- 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.
- This is a large bright star with a cool surface.
- It is formed during the later stages of the evolution as it runs out of hydrogen fuel at its centre.
- Red giants have diameter’s between 10 and 100 times that of the Sun.
- They are very bright, although their surface temperature is lower than that of the Sun
- Very large stars (red giants) are often called Super Giants.
- The most common red giants are stars nearing the end but are still fusing hydrogen into helium in a shell surrounding a degenerate helium core.
- As the star condenses and 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.
- Red giants are hot enough to turn the helium at their core, which was made by fusing hydrogen, into heavy elements like carbon. But most stars are not massive enough to create the pressures and heat necessary to burn heavy elements, so fusion and heat production stop.
- A cloud of Gas and Dust. [No Planets Involved].
- Planetary Nebula are the outer layers of a star 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. The outer layers will be ejected in a shell of gas that will last a few tens of thousands of years before spreading into the vastness of space.
- This 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.
- A great proportion of primary cosmic rays comes from supernovae.
- Supernovae can be triggered in one of two ways
- by the sudden re-ignition of nuclear fusion in a degenerate star; or
- by the gravitational collapse of the core of a massive star.
- Nova: In the first case, a degenerate white dwarf may accumulate sufficient material from a companion to raise its core temperature, ignite carbon fusion, and trigger runaway nuclear fusion, completely disrupting the star.
- In the second case, the core of a massive star may undergo sudden gravitational collapse, releasing gravitational potential energy that can create a supernova explosion.
- Nuclear explosion on a white dwarf, which causes a sudden brightening of the star.
- Novae are thought to occur 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 can be pulled from the companion star’s surface onto the white dwarf.
- A nova is caused by the accretion of hydrogen onto the surface of the star, commencing a runaway fusion reaction.
- This is very small, hot star, the last stage in the life cycle of a star like the Sun.
- White dwarfs are the shrunken remains of normal stars, whose nuclear energy supplies have been used up.
- White dwarf consist of degenerate matter with a very high density due to gravitational effects, i.e. one spoonful has a mass of several tonnes.
- 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.
- Great densities are only possible when electrons are displaced from their regular shells and pushed closer to the nucleus, allowing atoms to take up less space. The matter in this state is called ‘degenerate matter’.
- 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
- At the moment, they are strictly theoretical.
- A main sequence star that lacks the mass necessary to explode in a supernova will become a white dwarf, a ‘dead’ star that has burned through all of its hydrogen and helium fuel. But the white dwarf remains hot for some time, much like a stove burner still emits heat even when it has been turned off.
- After enough time has passed, all of the leftover heat will have radiated away. No longer emitting heat or light, the white dwarf will become a black dwarf
- However, the black dwarf would still retain its mass, allowing scientists to detect the effects produced by its gravitational field.
- 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, it does not form an object which is dense enough at its core to trigger nuclear fusion.
- Brown dwarfs were only a theoretical concept until they were first discovered in 1995.
- 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.
- Typical stars having a mass of three times the Sun but a diameter of only 20 km.
- If its mass is any greater, its gravity will be so strong that it will shrink further to become a black hole.
- 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.
- Black holes distort the space around them, and can often suck neighboring matter into them including stars.
1. Which of the following sequences below correctly describes the evolution of the Sun from young to old?
A) White dwarf, red giant, main-sequence, protostar
B) Red giant, main-sequence, white dwarf, protostar
C) Protostar, red giant, main-sequence, white dwarf
D) Protostar, main-sequence, white dwarf, red giant
E) Protostar, main-sequence, red giant, white dwarf
2. A planetary nebula is
A) another term for the disk of gas around a young star.
B) the cloud from which protostars form.
C) a shell of gas ejected from a star late in its life.
D) what is left when a white dwarf star explodes as a supernova.
E) the remnants of the explosion created by the collapse of the iron core in a massive star.
3. Stars like the Sun probably do not form iron cores during their evolution because
A) all the iron is ejected when they become planetary nebulas.
B) their cores never get hot enough for them to make iron by nucleosynthesis.
C) the iron they make by nucleosynthesis is all fused into uranium.
D) their strong magnetic fields keep their iron in their atmospheres.
4. As a star like the Sun evolves into a red giant, its core
A) expands and cools.
B) contracts and heats.
C) expands and heats.
D) turns into iron.
3) B ==> Only bigger stars can form iron cores.
4) B ==> At Red Giant Stage the star expands whereas the core contracts due to accumulation of heavier elements.