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Topic 6 · 6.2

Stars and the Universe

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Describe the Sun as a medium-sized star of hydrogen and helium radiating mostly infrared, visible light and ultraviolet; know stars are powered by fusion EXTENDED.
  • State that galaxies contain billions of stars, that the Sun is in the Milky Way, and that distances are measured in light-years.
  • Describe the life cycle of a star EXTENDED.
  • Describe redshift and how it supports the Big Bang Theory; know the diameter of the Milky Way.
  • Describe the CMBR; use \( H_0 = \dfrac{v}{d} \) and \( \dfrac{1}{H_0} \) as the age of the Universe EXTENDED.

📚The physics

The Sun and the galaxies

The Sun is a star of medium size, consisting mostly of hydrogen and helium; it radiates most of its energy in the infrared, visible and ultraviolet regions. EXTENDED Stars are powered by nuclear reactions; in stable stars these are the fusion of hydrogen into helium.

  • Galaxies are each made up of many billions of stars. The Sun is a star in the Milky Way.
  • The other stars in the Milky Way are much further away from the Earth than the Sun is.
  • A light-year is the distance travelled in (the vacuum of) space by light in one year: about 9.5 × 1015 m.
  • The Milky Way is one of many billions of galaxies in the Universe; its diameter is about 100 000 light-years.

The life cycle of a star EXTENDED

A flow chart. An interstellar cloud of gas and dust collapses under gravity into a protostar, which becomes a stable star when the inward pull of gravity is balanced by the outward force from its hot core. When it runs out of hydrogen in its core, a less massive star becomes a red giant, then a planetary nebula with a white dwarf at its centre; a more massive star becomes a red supergiant, then explodes as a supernova, leaving a neutron star or a black hole and a nebula from which new stars and planets can form.
EXTENDED The mass of the star decides its path.
  1. EXTENDED A star forms from an interstellar cloud of gas and dust containing hydrogen.
  2. A protostar is the cloud collapsing and heating up because of its internal gravitational attraction.
  3. It becomes a stable star when the inward force of gravity is balanced by an outward force due to the high temperature at its centre.
  4. All stars eventually run out of hydrogen for their nuclear reactions.
  5. Most stars expand into red giants; more massive stars into red supergiants, once most of the core’s hydrogen is helium.
  6. A red giant forms a planetary nebula with a white dwarf at its centre.
  7. A red supergiant explodes as a supernova, forming a nebula containing hydrogen and new heavier elements, leaving a neutron star or a black hole.
  8. The nebula from a supernova may form new stars with orbiting planets.

The expanding Universe

Two spectra: from a laboratory source, dark absorption lines at certain wavelengths; from a distant galaxy, the same pattern of lines shifted towards the red, longer-wavelength end.
Redshift: an increase in the observed wavelength of light from receding galaxies.

Light from distant galaxies is redshifted compared with light emitted on the Earth. Redshift is evidence that the Universe is expanding and supports the Big Bang Theory. EXTENDED Microwave radiation of a specific frequency is observed at all points in space: the cosmic microwave background radiation (CMBR). It was produced shortly after the Universe was formed and has been stretched into the microwave region as the Universe expanded.

A graph of speed of recession, in thousands of kilometres per second, against distance in millions of light-years for ten galaxies. The points lie close to a straight line through the origin: speed is proportional to distance.
EXTENDED The gradient is the Hubble constant.
\[ H_0 = \frac{v}{d} \qquad \text{age of the Universe} \approx \frac{d}{v} = \frac{1}{H_0} \]

EXTENDED The speed v of a galaxy is found from the redshift of its starlight; its distance d from the brightness of a supernova in it. The current estimate of H0 is 2.2 × 10−18 per second. \( 1/H_0 \) estimates the age of the Universe, evidence that all the matter in the Universe was once at a single point.

✏️Worked example

EXTENDED (Modelled on 0625/42 June 2026 Q11(c).) (a) Define a light-year. [1] (b) A galaxy is 2.5 × 108 light-years away. Taking H0 = 2.2 × 10−18 /s and 1 light-year = 9.46 × 1015 m, calculate its speed of recession in m/s. [3] (c) Estimate the age of the Universe in years. [2]

(a) The distance travelled by light in (the vacuum of) space in one year.

(b) d = 2.5 × 108 × 9.46 × 1015 = 2.37 × 1024 m; v = H0d = 2.2 × 10−18 × 2.37 × 1024 = 5.2 × 106 m/s.

(c) 1/H0 = 4.5 × 1017 s; ÷ (3.15 × 107 s per year) = 1.4 × 1010 years (about 14 billion years).

Check it. 5.2 × 106 m/s is under 2% of the speed of light — sensible for a galaxy “only” 250 million light-years away.
Leaving d in light-years. H0 is per second, so d must be in metres to give v in m/s.

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Multiple choice.) The star Sirius is 8.6 light-years from the Earth. What is this distance in kilometres? (1 light-year = 9.5 × 1012 km.) A: 1.1 × 1012 km. B: 8.2 × 1013 km. C: 9.5 × 1013 km. D: 8.2 × 1016 km.
B. 8.6 × 9.5 × 1012 = 8.2 × 1013 km.
2. (Theory.) Describe what is meant by redshift, and state what it shows about the Universe. [3]
An increase in the observed wavelength of light from a receding star or galaxy (the spectrum lines move towards the red end). It shows that distant galaxies are moving away, so the Universe is expanding, supporting the Big Bang Theory.
3. (Theory.) State the approximate diameter of the Milky Way. [1]
About 100 000 light-years.
4. (Theory.) EXTENDED Explain why a stable star does not collapse or expand. [2]
The inward force of gravitational attraction is balanced by the outward force due to the high temperature (and pressure) at the centre of the star.
5. (Theory.) EXTENDED Describe what happens to a star much more massive than the Sun at the end of its life. [3]
When most of the hydrogen in its core has become helium, it expands into a red supergiant, which explodes as a supernova; this leaves a neutron star or a black hole, and a nebula containing hydrogen and heavier elements.
6. (Theory.) EXTENDED What is the CMBR, and why is it now in the microwave region? [2]
Cosmic microwave background radiation: microwave radiation of a specific frequency observed from all directions in space. It was produced shortly after the Universe formed; the expansion of the Universe has stretched its wavelength into the microwave region.
7. (Theory.) EXTENDED State how the distance to a far galaxy can be found, and how its speed is found. [2]
Distance: from the brightness of a supernova in that galaxy. Speed: from the change in wavelength (redshift) of its starlight.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • ESA — Planck and the cosmic microwave background
  • NASA — “Star Life Cycle” and the Hubble constant