Stellar Evolution (OCR A Level Physics): Flashcards
Exam code: H556
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Define universe.
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Define universe.
The universe is a large collection of billions of galaxies; it is the largest known structure.
Define the observable universe.
The observable universe is the portion of the universe from which electromagnetic radiation has had time to reach Earth since the formation of the universe.
Define a galaxy.
A galaxy is a cluster of billions of stars held together by gravity.
Define a light-year.
A light-year is the distance light travels in one Earth year (1 light-year = 9.5 × 1015 m).
What makes up a solar system?
A star and the gravitationally bound objects that orbit it, such as planets, planetary satellites and comets.
A .......... is a body that orbits a planet, for example the Moon orbiting Earth.
A planetary satellite is a body that orbits a planet, for example the Moon orbiting Earth.
True or False?
As galaxies get further apart, they move away from each other more slowly.
False.
Galaxies are moving away from one another, and the further apart they are, the faster they move apart.
How old is the universe, and what diameter does the observable universe span?
The universe was created 13.8 billion years ago, and the observable universe spans a diameter of 93 billion light-years.
Define a nebula.
A nebula is a giant cloud of hydrogen gas and dust from which all stars form, via gravitational collapse.
Define a protostar.
A protostar is a hot, glowing ball of gas formed by the gravitational collapse of a nebula, before nuclear fusion begins.
What is produced when four hydrogen nuclei fuse into one helium nucleus?
Two gamma-ray photons, two neutrinos and two positrons, along with a massive release of energy.
Define a main sequence star.
A main sequence star is a star in a stable state, where the outward gas pressure and radiation pressure balance the inward gravitational force.
A star is classed as .......... mass if it has a mass more than about 10 times the mass of the Sun.
A star is classed as high-mass if it has a mass more than about 10 times the mass of the Sun.
True or False?
If a star's temperature increases, the outward pressure decreases and the star contracts.
False.
An increase in temperature increases the outward pressure, causing the star to expand.
How can protostars be detected by telescopes?
By observing the infrared radiation they emit.
Define a red giant.
A red giant forms when core fusion stops and the core collapses and heats, causing the outer layers of the star to expand and cool, while shell hydrogen burning continues around the core.
What is shell hydrogen burning?
The fusion of hydrogen nuclei in the shell surrounding the core, once the core's hydrogen has run out.
What causes a low-mass star to form a planetary nebula?
The carbon-oxygen core is not hot enough to fuse heavier elements, so it becomes unstable and collapses, ejecting the outer layers of gas into space.
Define a white dwarf.
A white dwarf is the very hot, dense, solid remnant core of a low-mass star left after the outer layers are ejected; no further fusion reactions take place.
A white dwarf will eventually cool until it no longer emits significant heat or light, at which point it is called a ...........
A white dwarf will eventually cool until it no longer emits significant heat or light, at which point it is called a black dwarf.
True or False?
Core helium burning fuses helium into carbon and oxygen inside the core of a red giant.
True.
Contraction of the core provides temperatures high enough for helium to fuse into carbon and oxygen.
What mass range classifies a star as low-mass?
Between 0.5 and 10 times the mass of the Sun.
Define electron degeneracy pressure.
The outward pressure created when electrons are compressed into the lowest available energy levels within a collapsing core, with nowhere else to go.
Define the Chandrasekhar limit.
The Chandrasekhar limit is the maximum mass of a stable white dwarf star, equal to 1.4 times the mass of the Sun.
Why does a white dwarf no longer undergo nuclear fusion?
The remaining elements (usually carbon and oxygen) cannot fuse further; instead, the white dwarf radiates energy as photons from previous fusion reactions.
If the mass of a collapsing core exceeds the Chandrasekhar limit, .......... pressure can no longer prevent the core from collapsing.
If the mass of a collapsing core exceeds the Chandrasekhar limit, electron degeneracy pressure can no longer prevent the core from collapsing.
True or False?
A stellar core with a mass greater than 1.4 solar masses will form a stable white dwarf.
False.
Only cores with a mass less than 1.4 solar masses form a stable white dwarf; above this, electron degeneracy pressure cannot prevent further collapse.
What happens to protons and electrons when a white dwarf's core exceeds the Chandrasekhar limit?
They combine to form neutrons, leading to the formation of a neutron star.
Define a red supergiant.
The stage after the main sequence in a massive star, where shell and core burning fuse increasingly heavy elements up to iron.
What happens once an iron core forms in a massive star?
It becomes unstable and begins to collapse, as no more fusion reactions can occur.
Define a supernova.
An explosive event where the outer shell of a star is blown out following the collapse of an iron core.
What happens to protons and electrons during the collapse of the core in a supernova?
Gravitational pressure forces them to combine to form neutrons, releasing huge amounts of energy.
The gravitational potential energy transferred during the collapse of the core produces intense ...........
The gravitational potential energy transferred during the collapse of the core produces intense heating.
True or False?
A high-mass star has a mass more than 10 times the mass of the Sun.
True.
What else is created during a Type II supernova, besides a collapsing core?
A shockwave that ejects the outer-layer material into space, and fusion of heavy nuclei with neutrons forms all the known elements beyond iron.
Define a neutron star.
A neutron star is an extremely dense, small star remnant formed when a stellar core with a mass greater than the Chandrasekhar limit collapses.
How does the density of a neutron star compare to a white dwarf and to the Earth?
A neutron star (4 × 1017 kg m-3) is far denser than a white dwarf (1 × 109 kg m-3), which is in turn far denser than the Earth (5 × 103 kg m-3).
Define a pulsar.
A rapidly rotating neutron star (up to 600 times per second) emitting bursts of highly directional electromagnetic radiation.
Define a singularity.
A theoretical point at which matter is compressed to an infinitely small point and the laws of physics, as currently understood, break down.
The boundary at which light and matter cannot escape the gravitational pull of a black hole is called the ...........
The boundary at which light and matter cannot escape the gravitational pull of a black hole is called the event horizon.
True or False?
Light can escape from beyond the event horizon of a black hole if it travels fast enough.
False.
The escape velocity beyond the event horizon is greater than the speed of light, so no light can escape.
What is the diameter of a neutron star with the mass of the Sun?
Only 30 km.
Define main sequence.
The main sequence is the diagonal band on the Hertzsprung-Russell diagram where most stars are found; luminosity increases with surface temperature.
What are the two axes of the Hertzsprung-Russell diagram, and how are they oriented?
Luminosity (relative to the Sun) is on the y-axis, increasing from dim (bottom) to bright (top). Temperature (in kelvin) is on the x-axis, decreasing from hot (left) to cool (right).
Why do red giants and red supergiants appear above the main sequence despite having lower surface temperatures?
They are much larger than main sequence stars, so despite being cooler they have a greater luminosity.
Why are white dwarf stars not very luminous, even though they are hot?
White dwarfs are much smaller than main sequence stars, so despite their high temperature they have low luminosity.
The Hertzsprung-Russell diagram only shows stars in .......... phases, since transitory phases happen quickly relative to a star's lifetime.
The Hertzsprung-Russell diagram only shows stars in stable phases, since transitory phases happen quickly relative to a star's lifetime.
True or False?
Black holes appear on the Hertzsprung-Russell diagram as very dim, cool points.
False.
Black holes are not plotted on the HR diagram at all, since they emit no light.
Which four regions of star types are identified on the Hertzsprung-Russell diagram?
Main sequence, red giants, red supergiants, and white dwarfs.
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