Stellar Evolution (Edexcel IGCSE Physics (Modular): Unit 2): Flashcards
Exam code: 4XPH1
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Define red star.
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Define red star.
A red star is the coolest type of star, with a surface temperature of around 3000 K.
Define blue star.
A blue star is the hottest type of star, with a surface temperature of around 30 000 K.
What determines a star's colour?
A star's colour depends on its surface temperature.
Warm objects emit ______ radiation, while extremely hot objects also emit ______ light.
Warm objects emit infrared radiation, while extremely hot objects also emit visible light.
What happens to a star's colour and surface temperature as it evolves into a red giant?
It becomes redder, as it expands and cools.
True or False?
A white dwarf becomes redder as it cools and expands.
False.
A white dwarf becomes whiter as it contracts and heats up.
Define nebula.
A nebula is a giant interstellar cloud of gas and dust from which stars form.
Define protostar.
A protostar is a hot ball of gas formed when gravity pulls the particles of a nebula closer together.
What happens to the density and temperature of a protostar as it forms, and why?
Both increase, because gravity pulls the particles closer together, causing more frequent collisions.
What condition must be met for a protostar to become a main sequence star?
The protostar must become hot enough for nuclear fusion reactions to begin in its core.
During the main sequence, a star is in ______ and said to be ______.
During the main sequence, a star is in equilibrium and said to be stable.
State, in order, the life cycle stages of a star similar in size to the Sun after the main sequence.
Main sequence → red giant → white dwarf (→ black dwarf).
Why does a main sequence star similar in size to the Sun become a red giant?
Hydrogen fuel runs low, so core fusion dies down. The star begins fusing helium, causing the outer layers to expand and the surface to cool, appearing red.
True or False?
A white dwarf forms when a star's helium fusion reactions finish and the star collapses.
True.
The white dwarf then cools over time, emitting less energy, eventually becoming a black dwarf once it no longer emits light.
State, in order, the life cycle stages of a high-mass star after the main sequence.
Red supergiant → supernova → neutron star or black hole.
What key difference in fusion products distinguishes a high-mass star from a lower-mass star at the end of its life?
A lower-mass star fuses helium into carbon; a higher-mass star fuses helium into much heavier elements, such as iron.
Define supernova.
A supernova is the gigantic explosion caused by the sudden collapse of the core of a red supergiant.
What happens to the outer remnants of a star during a supernova, and what can they form?
They are ejected into space, forming new clouds of dust and gas (nebulae), which may go on to form new planetary systems.
Define black hole.
A black hole is an extremely dense object with a gravitational pull so strong that not even light can escape it.
A high-mass star will stay on the main sequence for a ______ time than a lower-mass star before becoming a red supergiant.
A high-mass star will stay on the main sequence for a shorter time than a lower-mass star before becoming a red supergiant.
How can a neutron star go on to become a black hole?
In the most massive stars, the neutron star continues to collapse under the force of gravity until it forms a black hole.
True or False?
The heaviest elements are formed during a supernova.
True.
These heaviest elements are ejected into space along with the outer remnants of the star.
Define luminosity.
Luminosity is the total amount of light energy emitted by a star; it is a measure of a star's brightness or power output.
Define apparent magnitude.
Apparent magnitude is the perceived brightness of a star as seen from Earth.
What two factors determine a star's apparent magnitude?
The star's luminosity and its distance from Earth.
True or False?
On the apparent magnitude scale, the brighter a star is, the higher its magnitude value.
False.
The scale runs backwards: the brighter the star, the lower the magnitude; the dimmer the star, the higher the magnitude.
Define absolute magnitude.
Absolute magnitude is a measure of how bright a star would appear if all stars were placed the same distance away from Earth.
Why do astronomers use absolute magnitude rather than comparing stars' apparent brightness directly?
Because a bright star that is far away can look the same as a dim star that is nearby, so direct brightness comparisons are unreliable.
The standard distance used for absolute magnitude is 10 parsecs, or ______ light-years.
The standard distance used for absolute magnitude is 10 parsecs, or 32.6 light-years.
What is plotted on the x-axis and y-axis of a Hertzsprung-Russell (HR) diagram?
Luminosity on the y-axis and surface temperature on the x-axis.
True or False?
On a Hertzsprung-Russell diagram, the temperature axis increases from left to right.
False.
The temperature axis is plotted backwards, from hottest on the left to coolest on the right.
Define the main sequence (on the H-R diagram).
The main sequence is the band of stars running diagonally from top left to bottom right on the H-R diagram, where most stars are found.
Where are white dwarfs found on the H-R diagram, relative to the main sequence?
Below the main sequence (and slightly to the left) — they are hot but dim.
Where are red giants and red supergiants found on the H-R diagram, relative to the main sequence?
Above the main sequence on the right-hand side, with red supergiants directly above the red giants.
The ______, brightest stars are red supergiants, while the ______, dimmest stars are white dwarfs.
The coolest, brightest stars are red supergiants, while the hottest, dimmest stars are white dwarfs.
What type of stars are the coolest, dimmest stars on the H-R diagram?
Red dwarfs — the smallest main sequence stars.
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