Luminosity is the total power radiated by a star, measured in watts (W).

Define radiant flux intensity .

Radiant flux intensity , F , is the power received per unit area from a star observed at Earth, measured in W m -2 .

State two assumptions made when using the inverse square law of flux.

The power from the star radiates uniformly through space No radiation is absorbed between the star and Earth

The radiant flux from a star follows an .......... law with distance from the star.

The radiant flux from a star follows an inverse square law with distance from the star.

True or False? If the distance to a star doubles, the radiant flux intensity received on Earth halves.

False. Flux follows an inverse square law, so doubling the distance spreads the light over four times the area, meaning the flux intensity falls to a quarter , not a half.

Define stellar parallax .

Stellar parallax is the apparent shifting in position of a nearby star against a background of distant stars, when viewed from different positions of the Earth during its orbit about the Sun.

Why are the two observations used to measure stellar parallax taken six months apart?

Six months is half an orbit, so the Earth is at its most different position , giving the largest possible baseline and therefore the largest, most measurable parallax angle.

Why do distant background stars appear not to move during a stellar parallax measurement?

They are so far away that their apparent shift against the Earth's changing viewpoint is negligible , so they act as a fixed background against which the nearby star's movement is measured.

The equation p = 1/ d is only accurate for distances up to .......... pc.

The equation p = 1/ d is only accurate for distances up to 100 pc.

True or False? The equation p = 1/ d can be used accurately to find the distance to any star, however far away.

False. Beyond about 100 pc , the parallax angle is so small that it becomes too difficult to measure accurately.

Define standard candle .

A standard candle is an astronomical object which has a known luminosity, due to a characteristic quality possessed by that class of object.

Define cosmic distance ladder .

The cosmic distance ladder is the combination of different distance-measurement methods, each valid over a certain range, collated to build up the scale of the universe.

Give two examples of standard candles.

Cepheid variable stars Type 1a supernovae

A .......... variable star is a pulsating star whose brightness changes over a set period, a variation that is well related to its luminosity.

A Cepheid variable star is a pulsating star whose brightness changes over a set period, a variation that is well related to its luminosity.

True or False? A single standard candle method can be used to measure the distance to any object in the universe, however far away.

False. Each standard candle method is only reliable over a certain range of distances, so several methods must be combined into the cosmic distance ladder to cover the full scale of the universe.

Define the main sequence .

The main sequence is the band on the Hertzsprung-Russell diagram where most stars are found, and where luminosity increases with surface temperature.

Describe the axes of the Hertzsprung-Russell diagram.

x-axis: temperature (K), decreasing from hot (left) to cool (right) y-axis: luminosity relative to the Sun, increasing from dim (bottom) to bright (top)

Why do red giants appear above the main sequence despite having a cooler surface temperature?

A cooler star can only have a higher luminosity if it has a much larger surface area , so red giants must be much larger than main sequence stars.

Why do white dwarfs appear below and to the left of the main sequence?

White dwarfs are hot but not very luminous, so for a given temperature they must be much smaller than main sequence stars.

The Hertzsprung-Russell diagram only shows stars in .......... phases of their life cycle.

The Hertzsprung-Russell diagram only shows stars in stable phases of their life cycle.

True or False? Black holes can be plotted on the Hertzsprung-Russell diagram.

False. Black holes emit no light, so they cannot be observed and therefore cannot be plotted.

A nebula is a giant cloud of hydrogen gas and dust from which stars form, via gravitational collapse.

A protostar is the stage of star formation where gravitational collapse causes the gas and dust to heat up and glow, detectable by telescopes observing infrared radiation .

What is the mass cut-off between a low-mass star and a high-mass star?

A mass of about 1.4 times the mass of the Sun (1.4 M Sun ).

State the stages in the life cycle of a low-mass star, from the main sequence to its death.

Main sequence → red giant → planetary nebula → white dwarf

State the stages in the life cycle of a high-mass star, from the main sequence to its death.

Main sequence → red supergiant → supernova → neutron star or black hole

What determines whether the remnant of a supernova becomes a neutron star or a black hole?

If the remnant neutron core's mass is greater than 3 times the solar mass , the core collapses further to form a black hole ; otherwise it remains a neutron star .

A main sequence star is in a stable state where the inward gravitational force is balanced by outward .......... forces.

A main sequence star is in a stable state where the inward gravitational force is balanced by outward pressure forces.

True or False? All stars, regardless of their initial mass, eventually become a white dwarf.

False. Only low-mass stars (less than 1.4 M Sun ) become white dwarfs; high-mass stars end their life cycle as a neutron star or black hole .

International A Level (IAL)PhysicsEdexcelFlashcards5. Thermodynamics, Radiation, Oscillations & CosmologyAstronomy

Astronomy (Edexcel International A Level (IAL) Physics): Flashcards

Exam code: YPH11

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FrontInverse Square Law of Flux
Define luminosity.
FrontInverse Square Law of Flux
State the inverse square law of flux, defining each term.
BackInverse Square Law of Flux
$F = \frac{L}{4 π d^{2}}$
F = radiant flux intensity (W m^-2)
L = luminosity of the source (W)
d = distance between the star and Earth (m)

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Cards in this collection (32)

Define luminosity.
Luminosity is the total power radiated by a star, measured in watts (W).
Define radiant flux intensity.
Radiant flux intensity, F, is the power received per unit area from a star observed at Earth, measured in W m^-2.
State the inverse square law of flux, defining each term.
$F = \frac{L}{4 π d^{2}}$
- F = radiant flux intensity (W m^-2)
- L = luminosity of the source (W)
- d = distance between the star and Earth (m)
State two assumptions made when using the inverse square law of flux.
- The power from the star radiates uniformly through space
- No radiation is absorbed between the star and Earth
The radiant flux from a star follows an .......... law with distance from the star.
The radiant flux from a star follows an inverse square law with distance from the star.
True or False?
If the distance to a star doubles, the radiant flux intensity received on Earth halves.
False.
Flux follows an inverse square law, so doubling the distance spreads the light over four times the area, meaning the flux intensity falls to a quarter, not a half.
Define stellar parallax.
Stellar parallax is the apparent shifting in position of a nearby star against a background of distant stars, when viewed from different positions of the Earth during its orbit about the Sun.
Why are the two observations used to measure stellar parallax taken six months apart?
Six months is half an orbit, so the Earth is at its most different position, giving the largest possible baseline and therefore the largest, most measurable parallax angle.
Why do distant background stars appear not to move during a stellar parallax measurement?
They are so far away that their apparent shift against the Earth's changing viewpoint is negligible, so they act as a fixed background against which the nearby star's movement is measured.
State the equation relating parallax angle to distance in parsecs.
$p = \frac{1}{d}$
- p = parallax angle (")
- d = distance to the star (pc)
The equation p = 1/d is only accurate for distances up to .......... pc.
The equation p = 1/d is only accurate for distances up to 100 pc.
True or False?
The equation p = 1/d can be used accurately to find the distance to any star, however far away.
False.
Beyond about 100 pc, the parallax angle is so small that it becomes too difficult to measure accurately.
Define standard candle.
A standard candle is an astronomical object which has a known luminosity, due to a characteristic quality possessed by that class of object.
Define cosmic distance ladder.
The cosmic distance ladder is the combination of different distance-measurement methods, each valid over a certain range, collated to build up the scale of the universe.
Give two examples of standard candles.
- Cepheid variable stars
- Type 1a supernovae
Explain how the distance to a standard candle is determined.
The radiant flux intensity, F, received at Earth is measured. Since the luminosity, L, of a standard candle is already known, the distance d can be calculated using the inverse square law of flux, $F = \frac{L}{4 π d^{2}}$ .
A .......... variable star is a pulsating star whose brightness changes over a set period, a variation that is well related to its luminosity.
A Cepheid variable star is a pulsating star whose brightness changes over a set period, a variation that is well related to its luminosity.
True or False?
A single standard candle method can be used to measure the distance to any object in the universe, however far away.
False.
Each standard candle method is only reliable over a certain range of distances, so several methods must be combined into the cosmic distance ladder to cover the full scale of the universe.
Define the main sequence.
The main sequence is the band on the Hertzsprung-Russell diagram where most stars are found, and where luminosity increases with surface temperature.
Describe the axes of the Hertzsprung-Russell diagram.
- x-axis: temperature (K), decreasing from hot (left) to cool (right)
- y-axis: luminosity relative to the Sun, increasing from dim (bottom) to bright (top)
Why do red giants appear above the main sequence despite having a cooler surface temperature?
A cooler star can only have a higher luminosity if it has a much larger surface area, so red giants must be much larger than main sequence stars.
Why do white dwarfs appear below and to the left of the main sequence?
White dwarfs are hot but not very luminous, so for a given temperature they must be much smaller than main sequence stars.
The Hertzsprung-Russell diagram only shows stars in .......... phases of their life cycle.
The Hertzsprung-Russell diagram only shows stars in stable phases of their life cycle.
True or False?
Black holes can be plotted on the Hertzsprung-Russell diagram.
False.
Black holes emit no light, so they cannot be observed and therefore cannot be plotted.
Define nebula.
A nebula is a giant cloud of hydrogen gas and dust from which stars form, via gravitational collapse.
Define protostar.
A protostar is the stage of star formation where gravitational collapse causes the gas and dust to heat up and glow, detectable by telescopes observing infrared radiation.
What is the mass cut-off between a low-mass star and a high-mass star?
A mass of about 1.4 times the mass of the Sun (1.4 M_Sun).
State the stages in the life cycle of a low-mass star, from the main sequence to its death.
Main sequence → red giant → planetary nebula → white dwarf
State the stages in the life cycle of a high-mass star, from the main sequence to its death.
Main sequence → red supergiant → supernova → neutron star or black hole
What determines whether the remnant of a supernova becomes a neutron star or a black hole?
If the remnant neutron core's mass is greater than 3 times the solar mass, the core collapses further to form a black hole; otherwise it remains a neutron star.
A main sequence star is in a stable state where the inward gravitational force is balanced by outward .......... forces.
A main sequence star is in a stable state where the inward gravitational force is balanced by outward pressure forces.
True or False?
All stars, regardless of their initial mass, eventually become a white dwarf.
False.
Only low-mass stars (less than 1.4 M_Sun) become white dwarfs; high-mass stars end their life cycle as a neutron star or black hole.

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