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Topic 6.2 · Unit 6

Stars and the Universe

The Sun as a star, how stars form, live and end, and how redshift and the cosmic microwave background radiation are evidence for an expanding Universe that began with the Big Bang.

In this topic

  1. 6.2.1The Sun as a star
  2. 6.2.2Stars
  3. 6.2.3The Universe

Key points

6.2.1 The Sun as a star

  • The Sun is a star of medium size.
  • The Sun is made mostly of hydrogen and helium.
  • The Sun gives out most of its energy as infrared, visible light and ultraviolet radiation. These are regions of the electromagnetic spectrum.
  • Stars are powered by nuclear reactions that release energy. Extended
  • In stable stars, the nuclear reaction is the fusion of hydrogen into helium: hydrogen nuclei join to make helium nuclei (see 5.1). Extended

6.2.2 Stars

  • A galaxy is a very large group of stars. Each galaxy is made of many billions of stars.
  • The Sun is a star in the galaxy called the Milky Way.
  • The other stars in the Milky Way are much further away from the Earth than the Sun is.
  • Distances in space are so large that we measure them in light-years.
  • One light-year is the distance that light travels through (the vacuum of) space in one year. A light-year is a distance, not a time.
  • One light-year is equal to 9.5 × 1015 m. Extended
  • The life cycle of a star Extended
    1. A star forms from an interstellar cloud of gas and dust. The cloud contains hydrogen.
    2. The cloud’s own gravitational attraction pulls it inwards. As it collapses, its temperature increases. This collapsing, heating cloud is a protostar.
    3. Two forces now act on the star. Gravity pulls its material inwards. The very high temperature at the centre causes a force outwards. When these two forces are balanced, the star stops collapsing. It is now a stable star.
    4. Hydrogen is the fuel for the star’s nuclear reactions. Sooner or later, every star uses up its hydrogen fuel.
    5. When most of the hydrogen in the centre has been converted to helium, the star expands. Most stars become red giants. More massive stars become red supergiants.
    6. Less massive star: the red giant forms a planetary nebula (a shell of gas drifting away) with a white dwarf star at its centre.
    7. More massive star: the red supergiant blows apart in a huge explosion called a supernova. The material thrown out makes a nebula (a cloud of gas and dust). There is hydrogen in this nebula, and also new heavier elements. At the centre, a neutron star or a black hole remains.
    8. The nebula from a supernova may form new stars with orbiting planets.
  • Summary of the two paths: Extended
    • less massive: cloud → protostar → stable star → red giant → planetary nebula + white dwarf
    • more massive: cloud → protostar → stable star → red supergiant → supernova → nebula + neutron star or black hole

6.2.3 The Universe

  • The Universe is made of many billions of galaxies. The Milky Way is one of them.
  • The diameter of the Milky Way is about 100 000 light-years.
  • Redshift is an increase in the observed wavelength of electromagnetic radiation from stars and galaxies that are moving away from us (receding).
  • Light from distant galaxies is redshifted when we compare it with light given out on the Earth. The wavelengths we observe are longer.
  • This redshift shows that distant galaxies are moving away from us. So the Universe is expanding.
  • Redshift is evidence for the Big Bang Theory. This theory says that the Universe began from a single point and has been expanding ever since.
  • Cosmic microwave background radiation (CMBR) is microwave radiation of a particular frequency. It is observed at all points in space around us. Extended
  • The CMBR was produced shortly after the Universe formed. As the Universe expanded, the radiation was stretched into the microwave region of the electromagnetic spectrum. Extended
  • The speed v at which a galaxy is moving away from the Earth can be found from the change in wavelength of its starlight (its redshift). A bigger change in wavelength means a higher speed. Extended
  • The distance d of a far galaxy can be found from the brightness of a supernova in that galaxy. Extended
  • Extra detail: some supernovae always give out about the same amount of light. So the dimmer such a supernova looks from the Earth, the further away its galaxy is.
  • The Hubble constant: H0 = v / d. It is a galaxy’s speed away from the Earth divided by its distance from the Earth. Extended
  • The current estimate of H0 is 2.2 × 10−18 per second. Extended
  • Because v = H0 × d, galaxies that are further away are moving away faster. Extended
  • Assume a galaxy has always moved away from us at the same speed v. Then d / v is the time it has taken to reach its distance d. Extended
  • The speed may not have been constant, so d / v is only an estimate. Extended
  • d / v = 1 / H0, which is the same for every galaxy. So 1 / H0 gives an estimate of the age of the Universe. Extended
  • This is evidence that all the matter in the Universe was once at a single point. Extended

Model

The life cycle of a star: switch between the low mass and high mass star and look for the stage where the two paths split, after the stable star runs out of hydrogen in its centre.

3D modelThe life cycle of a star
The life cycle of a starOpen full screen: The life cycle of a star

Equations

  • One light-yearExtended

    1 light-year = 9.5 × 1015 m

    one light-year = the distance light travels in a vacuum in one year

  • Hubble constantExtended

    H0 = v / d

    H0 = Hubble constant (s−1), current estimate 2.2 × 10−18 per second; v = speed at which the galaxy is moving away from the Earth (m/s); d = distance of the galaxy from the Earth (m)

  • Age of the UniverseExtended

    d / v = 1 / H0

    d / v = estimate of the age of the Universe (s); H0 = Hubble constant (s−1)

One light-year Extended

Where does 9.5 × 1015 m come from? Then: a nebula is 3.8 × 1018 m from the Earth. How far is this in light-years?

  • One year = 365 × 24 × 3600 s = 31 536 000 s = 3.15 × 107 s
  • One light-year = speed of light × one year = 3.0 × 108 m/s × 31 536 000 s = 9.46 × 1015 m
  • To 2 s.f., one light-year = 9.5 × 1015 m
  • Distance in light-years = 3.8 × 1018 ÷ 9.5 × 1015
  • Distance = 400 light-years

Hubble constant Extended

A galaxy is 3.0 × 1024 m from the Earth. It is moving away at 6.6 × 106 m/s. Find H0.

  • Given: v = 6.6 × 106 m/s, d = 3.0 × 1024 m
  • H0 = v / d
  • H0 = 6.6 × 106 ÷ 3.0 × 1024
  • H0 = 2.2 × 10−18 s−1 (per second)

A second galaxy is moving away at 1.1 × 107 m/s. Use H0 = 2.2 × 10−18 s−1 to find its distance.

  • Rearrange: d = v / H0
  • d = 1.1 × 107 ÷ 2.2 × 10−18
  • d = 5.0 × 1024 m

Age of the Universe Extended

Use H0 = 2.2 × 10−18 s−1 to estimate the age of the Universe in seconds and in years.

  • Assume the galaxies have always moved at constant speed.
  • d / v = 1 / H0 = 1 ÷ 2.2 × 10−18
  • Estimated age = 4.5 × 1017 s (2 s.f.)
  • In years: 4.5 × 1017 ÷ 3.15 × 107 = 1.4 × 1010 years (about 14 billion years). This is an estimate.
  • Check with the second galaxy above: d / v = 5.0 × 1024 ÷ 1.1 × 107 = 4.5 × 1017 s. This is the same estimate.

Common mistakes

  • Students write that a light-year is a unit of time. / The mark scheme wants: a light-year is a distance, the distance light travels in one year.
  • Students write that the Sun gives out most of its energy as visible light only. / The mark scheme wants all three regions: infrared, visible light and ultraviolet.
  • Students write that a star is stable because gravity has stopped. / The mark scheme wants a balance: the inward force of gravity is balanced by the outward force due to the high temperature in the centre. Extended
  • Students write that every star ends as a supernova or that a red giant becomes a black hole. / The mark scheme wants the two paths: less massive star → red giant → planetary nebula + white dwarf; more massive star → red supergiant → supernova → neutron star or black hole. Extended
  • Students write that redshift means the light turns red. / The mark scheme wants: an increase in the observed wavelength of the radiation from a receding galaxy.
  • Students write that the CMBR was produced as microwaves. / The mark scheme wants: it was produced shortly after the Universe formed, and its wavelength was stretched into the microwave region as the Universe expanded. Extended

Exam tips

  • State the facts in this topic exactly: the Sun is a medium-sized star; it is mostly hydrogen and helium; the Milky Way is about 100 000 light-years across.
  • State these values exactly: 1 light-year = 9.5 × 1015 m; H0 = 2.2 × 10−18 per second. Extended
  • Describe the life cycle as a chain of named stages with arrows. Say which path is for a less massive star and which is for a more massive star. Extended
  • Explain how redshift supports the Big Bang Theory in steps: observed wavelength is longer → galaxies are moving away → the Universe is expanding → it was smaller in the past and began at a single point.
  • Explain the CMBR in two steps: produced shortly after the Universe formed; stretched into microwaves as the Universe expanded. Extended
  • Calculate: for distances in light-years, multiply by 9.5 × 1015 to get metres, or divide metres by 9.5 × 1015 to get light-years. Check that powers of ten are typed correctly into your calculator. Extended
  • A typical 1-mark answer: “One light-year is the distance light travels in space in one year.”
  • A typical 2-mark answer to “What makes a protostar become a stable star?”: “Nuclear fusion starts in the centre (1). The inward force of gravity is balanced by the outward force due to the high temperature (1).” Extended
  • A typical 3-mark answer to “How is H0 found?”: “Measure the redshift of the galaxy’s starlight to find its speed v (1). Use the brightness of a supernova in the galaxy to find its distance d (1). Divide v by d (1).” Extended