AQA · A-Level · Physics

    Course version: 7408 · Version from Sept 2016

    Use of type 1a supernovae as standard candles to determine distances.…

    Full specification wording

    Supernovae, neutron stars and black holes (A-level only): Use of type 1a supernovae as standard candles to determine distances. Controversy concerning accelerating Universe and dark energy.

    Type 1a supernovae occur when a white dwarf in a binary system accretes matter and exceeds the Chandrasekhar limit, triggering a thermonuclear explosion.

    Checked against the AQA 7408 specification (opens in a new tab) on .

    • Point 4 of 6
    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Type 1a supernovae occur when a white dwarf in a binary system accretes matter and exceeds the Chandrasekhar limit, triggering a thermonuclear explosion. Because the trigger mass is nearly the same, their peak luminosity is nearly standard, so they act as standard candles. Astronomers measure apparent brightness b and use b = L / (4πd²) to find distance d, or use the distance modulus m − M = 5 log₁₀(d / 10 pc). Comparing distances with redshifts shows that distant supernovae are fainter than expected in a decelerating universe, implying accelerated expansion. This is attributed to dark energy, a controversial component making up about 68% of the energy density of the Universe.

    What earns the marks

    • Explain that type 1a supernovae have a consistent peak luminosity because they explode at a similar trigger mass, the Chandrasekhar limit.
    • Use the inverse-square law b = L / (4πd²) to relate apparent brightness to distance for a standard candle.
    • Apply the distance modulus m − M = 5 log₁₀(d / 10 pc) to determine distance from apparent and absolute magnitudes.
    • Describe how comparing supernova distances with their redshifts tests the expansion history of the Universe.
    • Explain that observed dimming of distant supernovae indicates accelerating expansion, attributed to dark energy.
    • Discuss the controversy: dark energy is inferred but not directly understood, and its nature remains an open problem in physics.

    Common mistakes and how to put them right

    • Treating type 1a supernovae as perfectly identical in peak luminosity.

      Put it right: Describe them as standardisable candles, since corrections for light-curve shape and colour reduce scatter.

    • Using the inverse-square law with luminosity in solar units and distance in parsecs without converting.

      Put it right: Use SI units, converting parsecs to metres, or use the distance modulus consistently.

    • Claiming that dark energy has been directly detected.

      Put it right: State that it is inferred from supernova data and other observations, and its nature is not yet known.

    Exam tips

    • State clearly that the standard candle assumption relies on a known absolute magnitude M before using the distance modulus.
    • Show the rearrangement of b = L / (4πd²) to d = √(L / (4πb)) when calculating distance.
    • When discussing dark energy, distinguish between observational evidence and theoretical interpretation.

    Try the lesson

    The start of the Supernovae, neutron stars and black holes (A-level only): Use of type 1a supernovae as standard candles to determine distances. Controversy concerning accelerating Universe and dark energy. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Type 1a supernovae are used as standard candles in astronomy. What property makes them suitable for determining distances to distant galaxies?

    Listen

    Type 1a supernovae occur when a white dwarf in a binary system accretes matter and exceeds the Chandrasekhar limit, triggering a thermonuclear explosion. Because the trigger mass is nearly the same, their peak luminosity is nearly standard, so they act as standard candles. Astronomers measure apparent brightness b and use b = L / (4πd²) to find distance d, or use the distance modulus m − M = 5 log₁₀(d / 10 pc). Comparing distances with redshifts shows that distant supernovae are fainter than expected in a decelerating universe, implying accelerated expansion. This is attributed to dark energy, a controversial component making up about 68% of the energy density of the Universe. A strong answer earns marks for these points. Explain that type 1a supernovae have a consistent peak luminosity because they explode at a similar trigger mass, the Chandrasekhar limit. Use the inverse-square law b = L / (4πd²) to relate apparent brightness to distance for a standard candle. Apply the distance modulus m − M = 5 log₁₀(d / 10 pc) to determine distance from apparent and absolute magnitudes. Describe how comparing supernova distances with their redshifts tests the expansion history of the Universe. A common slip: Treating type 1a supernovae as perfectly identical in peak luminosity. Instead, describe them as standardisable candles, since corrections for light-curve shape and colour reduce scatter. Exam tip: State clearly that the standard candle assumption relies on a known absolute magnitude M before using the distance modulus.

    See it

    A top-down flowchart. The first box says a white dwarf in a binary system accretes matter, leading to a box saying it exceeds the Chandrasekhar limit. This leads to a thermonuclear explosion: type 1a supernova, then to nearly standard peak luminosity: standard candle. From there, measure apparent brightness b, then find distance d using b = L / (4πd²), then compare distances with redshifts. This leads to distant supernovae fainter than expected in a decelerating Universe, which implies accelerating expansion, attributed to dark energy, and finally a box noting the controversy: inferred but not directly understood.
    It shows how type 1a supernovae act as standard candles and how comparing their distances with redshifts leads to accelerating expansion and the controversial idea of dark energy.

    Key words

    Say what each word means before you turn the card.

    standard candles

    Try answering aloud or in your head before turning the card.

    dark energy

    Try answering aloud or in your head before turning the card.

    distance modulus

    Try answering aloud or in your head before turning the card.

    See every definition
    standard candles
    Type 1a supernovae have a nearly standard peak luminosity because they explode at a similar trigger mass, the Chandrasekhar limit, so they act as standard candles.
    dark energy
    A controversial component making up about 68% of the energy density of the Universe, to which accelerated expansion is attributed.
    distance modulus
    The relation m − M = 5 log₁₀(d / 10 pc), used to determine distance from apparent and absolute magnitudes.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The first box says a white dwarf in a binary system accretes matter, leading to a box saying it exceeds the Chandrasekhar limit. This leads to a thermonuclear explosion: type 1a supernova, then to nearly standard peak luminosity: standard candle. From there, measure apparent brightness b, then find distance d using b = L / (4πd²), then compare distances with redshifts. This leads to distant supernovae fainter than expected in a decelerating Universe, which implies accelerating expansion, attributed to dark energy, and finally a box noting the controversy: inferred but not directly understood.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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