Luminosity is the total power output of radiation emitted by a star, measured in watts (W).

Define radiant flux intensity .

Radiant flux intensity is the radiant power transmitted normally through a surface per unit area, measured on Earth.

The inverse square law of flux assumes that no radiation is .......... between the star and the Earth.

The inverse square law of flux assumes that no radiation is absorbed between the star and the Earth.

True or False? A star's luminosity decreases as it moves further away from Earth.

False. Luminosity is a fixed, intrinsic property of the star; it is the radiant flux intensity (observed brightness) that decreases with distance, not the luminosity.

Define a 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.

Give two examples of standard candles used in astronomy.

Cepheid variable stars Type 1a supernovae

Define a Cepheid variable star.

A Cepheid variable is a type of pulsating star whose brightness increases and decreases over a set time period, with this period related to its luminosity.

What is a type 1a supernova, and why is it useful as a standard candle?

A type 1a supernova is an explosion involving a white dwarf. Its peak luminosity always reaches the same value, so it can be used to estimate distance.

Outline how the distance to a standard candle is determined.

Its radiant flux intensity is measured from Earth. Since its luminosity is known (as it is a standard candle), the distance is calculated using the inverse square law of flux.

Why are type 1a supernovae more suitable than Cepheid variable stars for measuring distances to the most distant galaxies?

Type 1a supernovae have a much greater luminosity , so their radiant flux intensity is much greater and they are more likely to be observed at very large distances.

A .......... distance measurement, such as using parallax, is only possible if an object is close enough to Earth; more distant objects require indirect methods such as standard candles.

A direct distance measurement, such as using parallax, is only possible if an object is close enough to Earth; more distant objects require indirect methods such as standard candles.

True or False? Standard candles all have the same radiant flux intensity as observed from Earth.

False. Standard candles have a known luminosity ; their observed radiant flux intensity varies with distance, which is what allows distance to be calculated.

A LevelPhysicsCambridge (CIE)Flashcards25. Astronomy & CosmologyStandard Candles

Standard Candles (Cambridge (CIE) A Level Physics): Flashcards

Exam code: 9702

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FrontLuminosity & Radiant Flux
Define luminosity.
FrontLuminosity & Radiant Flux
State the inverse square law of flux equation, and define each symbol.
BackLuminosity & Radiant 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 (14)

Define luminosity.
Luminosity is the total power output of radiation emitted by a star, measured in watts (W).
Define radiant flux intensity.
Radiant flux intensity is the radiant power transmitted normally through a surface per unit area, measured on Earth.
State the inverse square law of flux equation, and define each symbol.
$F = \frac{L}{4 π d^{2}}$
- F = radiant flux intensity (W m^-2)
- L = luminosity of the source (W)
- d = distance between the star and Earth (m)
Explain, in terms of surface area, why radiant flux intensity decreases with distance from a star according to an inverse square law.
Radiation spreads out uniformly over the surface of an expanding sphere of area $4 π d^{2}$ , so the same total power (luminosity) is spread over an increasingly large area as distance d increases.
The inverse square law of flux assumes that no radiation is .......... between the star and the Earth.
The inverse square law of flux assumes that no radiation is absorbed between the star and the Earth.
True or False?
A star's luminosity decreases as it moves further away from Earth.
False.
Luminosity is a fixed, intrinsic property of the star; it is the radiant flux intensity (observed brightness) that decreases with distance, not the luminosity.
Define a 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.
Give two examples of standard candles used in astronomy.
- Cepheid variable stars
- Type 1a supernovae
Define a Cepheid variable star.
A Cepheid variable is a type of pulsating star whose brightness increases and decreases over a set time period, with this period related to its luminosity.
What is a type 1a supernova, and why is it useful as a standard candle?
A type 1a supernova is an explosion involving a white dwarf. Its peak luminosity always reaches the same value, so it can be used to estimate distance.
Outline how the distance to a standard candle is determined.
Its radiant flux intensity is measured from Earth. Since its luminosity is known (as it is a standard candle), the distance is calculated using the inverse square law of flux.
Why are type 1a supernovae more suitable than Cepheid variable stars for measuring distances to the most distant galaxies?
Type 1a supernovae have a much greater luminosity, so their radiant flux intensity is much greater and they are more likely to be observed at very large distances.
A .......... distance measurement, such as using parallax, is only possible if an object is close enough to Earth; more distant objects require indirect methods such as standard candles.
A direct distance measurement, such as using parallax, is only possible if an object is close enough to Earth; more distant objects require indirect methods such as standard candles.
True or False?
Standard candles all have the same radiant flux intensity as observed from Earth.
False.
Standard candles have a known luminosity; their observed radiant flux intensity varies with distance, which is what allows distance to be calculated.

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