Define radiant flux intensity .

The observed intensity of radiation received per unit area at Earth, measured in W m -2

The total power output of a source, measured in watts (W) — a constant for a given star

Light leaving a star spreads uniformly through a spherical shell, so the radiation reaching Earth is spread over an area of ..........

Light leaving a star spreads uniformly through a spherical shell, so the radiation reaching Earth is spread over an area of 4πd 2

Why do more distant stars appear fainter?

Their emitted light is spread over a greater area , so the radiant flux intensity received at Earth is smaller — flux follows an inverse square law with distance

True or False? Doubling the distance to a star halves the radiant flux received

False. Radiant flux follows an inverse square law — doubling the distance spreads the light over four times the area, so the flux falls to one quarter

State the two assumptions of the inverse square law of flux.

The power radiates uniformly through space, and no radiation is absorbed between the star and the Earth

Define stellar parallax .

The apparent shift in position of a nearby star against a background of distant stars, when viewed from different positions of the Earth during its orbit around the Sun

Why are stellar parallax observations taken six months apart?

The Earth has completed half an orbit , maximising how far it has moved from its starting position and giving the largest apparent shift of the nearby star

During stellar parallax the .......... stars appear fixed, while the nearby star appears to shift position

During stellar parallax the distant stars appear fixed, while the nearby star appears to shift position

Up to what distance is the parallax method accurate, and why does it fail beyond this?

Accurate up to about 100 pc — beyond this the parallax angles are too small to measure accurately

What symbol represents an arcsecond?

The double prime " denotes an arcsecond ; the single prime ' denotes an arcminute

Define a standard candle .

An astronomical object with a known luminosity due to a characteristic quality possessed by that class of object

Give two examples of standard candles.

Cepheid variable stars — pulsating stars whose brightness variation has a well-defined link to luminosity Type 1a supernovae — white dwarf explosions whose luminosity is always the same

How is the distance to a standard candle determined?

The radiant flux arriving at Earth is measured; since the luminosity is known , the distance is found using the inverse square law of flux

Define the cosmic distance ladder .

The combination of data from different standard candle methods, each valid over a certain range, used to build up distances from nearby stars to distant galaxies

A direct distance measurement is only possible if the object is .......... to the Earth; otherwise indirect methods must be used

A direct distance measurement is only possible if the object is close enough to the Earth; otherwise indirect methods must be used

True or False? A standard candle works because we measure its luminosity directly from Earth

False. The luminosity is already known from the object's class; astronomers measure the radiant flux at Earth and apply the inverse square law to find distance

What two quantities are plotted on a Hertzsprung-Russell diagram?

Luminosity (relative to the Sun, increasing upwards) on the y-axis against temperature on the x-axis

How is the temperature axis on a Hertzsprung-Russell diagram arranged?

Temperature runs from hot on the left to cool on the right — the reverse of a conventional axis

Define the main sequence .

The diagonal band where most stars are clustered, along which luminosity increases with surface temperature

White dwarf stars sit below and to the left of the main sequence because they are hot but ..........

White dwarf stars sit below and to the left of the main sequence because they are hot but not very luminous

Why do red giants and red supergiants appear above the main sequence?

They show high luminosity at cooler temperatures , which can only be explained by them being much larger than main sequence stars

True or False? Black holes can be located on a Hertzsprung-Russell diagram

False. The HR diagram only shows stars in stable phases ; black holes emit no light and so cannot be plotted

A giant cloud of hydrogen gas and dust from which all stars form

What are the first four stages common to stars of all masses?

Nebula → protostar → nuclear fusion → main sequence star

The inward movement of matter under gravitational attraction, which begins star formation, is called ..........

The inward movement of matter under gravitational attraction, which begins star formation, is called gravitational collapse

What keeps a main sequence star in a stable state?

The inward gravitational force is balanced by the outward gas and radiation pressure , so the forces are in equilibrium

What mass decides whether a star follows the low-mass or high-mass path?

The cut-off is about 1.4 times the mass of the Sun — below this is low-mass , above this is high-mass

What stages follow the main sequence for a low-mass star?

Red giant → planetary nebula → white dwarf

What stages follow the main sequence for a high-mass star?

Red supergiant → supernova → neutron star (or black hole)

True or False? Every high-mass star ends its life as a black hole

False. After a supernova the neutron core forms a neutron star ; only if its mass exceeds about 3 solar masses does it collapse into a black hole

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Astronomy (Edexcel A Level Physics): Flashcards

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FrontInverse Square Law of Flux
Define radiant flux intensity.

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Cards in this collection (34)

Define radiant flux intensity.
The observed intensity of radiation received per unit area at Earth, measured in W m^-2
What is the inverse square law of flux equation?
$F = \frac{L}{4 π d^{2}}$
where F = radiant flux intensity (W m^-2), L = luminosity (W) and d = distance from the star to Earth (m)
Define luminosity.
The total power output of a source, measured in watts (W) — a constant for a given star
Light leaving a star spreads uniformly through a spherical shell, so the radiation reaching Earth is spread over an area of ..........
Light leaving a star spreads uniformly through a spherical shell, so the radiation reaching Earth is spread over an area of 4πd²
Why do more distant stars appear fainter?
Their emitted light is spread over a greater area, so the radiant flux intensity received at Earth is smaller — flux follows an inverse square law with distance
True or False?
Doubling the distance to a star halves the radiant flux received
False.
Radiant flux follows an inverse square law — doubling the distance spreads the light over four times the area, so the flux falls to one quarter
State the two assumptions of the inverse square law of flux.
The power radiates uniformly through space, and no radiation is absorbed between the star and the Earth
Define stellar parallax.
The apparent shift in position of a nearby star against a background of distant stars, when viewed from different positions of the Earth during its orbit around the Sun
Why are stellar parallax observations taken six months apart?
The Earth has completed half an orbit, maximising how far it has moved from its starting position and giving the largest apparent shift of the nearby star
State the parallax equation.
$p = \frac{1}{d}$
where p = parallax in arcseconds (") and d = distance in parsecs (pc)
During stellar parallax the .......... stars appear fixed, while the nearby star appears to shift position
During stellar parallax the distant stars appear fixed, while the nearby star appears to shift position
Up to what distance is the parallax method accurate, and why does it fail beyond this?
Accurate up to about 100 pc — beyond this the parallax angles are too small to measure accurately
What symbol represents an arcsecond?
The double prime " denotes an arcsecond; the single prime ' denotes an arcminute
True or False?
A smaller parallax angle means the star is closer to Earth
False.
Since $p = \frac{1}{d}$ , a smaller parallax angle means the star is further away
Define a standard candle.
An astronomical object with a known luminosity due to a characteristic quality possessed by that class of object
Give two examples of standard candles.
Cepheid variable stars — pulsating stars whose brightness variation has a well-defined link to luminosity
Type 1a supernovae — white dwarf explosions whose luminosity is always the same
How is the distance to a standard candle determined?
The radiant flux arriving at Earth is measured; since the luminosity is known, the distance is found using the inverse square law of flux
Define the cosmic distance ladder.
The combination of data from different standard candle methods, each valid over a certain range, used to build up distances from nearby stars to distant galaxies
A direct distance measurement is only possible if the object is .......... to the Earth; otherwise indirect methods must be used
A direct distance measurement is only possible if the object is close enough to the Earth; otherwise indirect methods must be used
True or False?
A standard candle works because we measure its luminosity directly from Earth
False.
The luminosity is already known from the object's class; astronomers measure the radiant flux at Earth and apply the inverse square law to find distance
What two quantities are plotted on a Hertzsprung-Russell diagram?
Luminosity (relative to the Sun, increasing upwards) on the y-axis against temperature on the x-axis
How is the temperature axis on a Hertzsprung-Russell diagram arranged?
Temperature runs from hot on the left to cool on the right — the reverse of a conventional axis
Define the main sequence.
The diagonal band where most stars are clustered, along which luminosity increases with surface temperature
White dwarf stars sit below and to the left of the main sequence because they are hot but ..........
White dwarf stars sit below and to the left of the main sequence because they are hot but not very luminous
Why do red giants and red supergiants appear above the main sequence?
They show high luminosity at cooler temperatures, which can only be explained by them being much larger than main sequence stars
True or False?
Black holes can be located on a Hertzsprung-Russell diagram
False.
The HR diagram only shows stars in stable phases; black holes emit no light and so cannot be plotted
Define a nebula.
A giant cloud of hydrogen gas and dust from which all stars form
What are the first four stages common to stars of all masses?
Nebula → protostar → nuclear fusion → main sequence star
The inward movement of matter under gravitational attraction, which begins star formation, is called ..........
The inward movement of matter under gravitational attraction, which begins star formation, is called gravitational collapse
What keeps a main sequence star in a stable state?
The inward gravitational force is balanced by the outward gas and radiation pressure, so the forces are in equilibrium
What mass decides whether a star follows the low-mass or high-mass path?
The cut-off is about 1.4 times the mass of the Sun — below this is low-mass, above this is high-mass
What stages follow the main sequence for a low-mass star?
Red giant → planetary nebula → white dwarf
What stages follow the main sequence for a high-mass star?
Red supergiant → supernova → neutron star (or black hole)
True or False?
Every high-mass star ends its life as a black hole
False.
After a supernova the neutron core forms a neutron star; only if its mass exceeds about 3 solar masses does it collapse into a black hole

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