Question 1

What is the difference between a red giant and a red supergiant?

Accepted Answer

A red giant forms from a low-mass star (like the Sun) after it exhausts hydrogen in its core. It is relatively small and will eventually become a white dwarf. A red supergiant forms from a high-mass star (more than 8 times the Sun's mass) and is much larger and more luminous. It ends in a supernova, leaving a neutron star or black hole.

Question 2

How does the Doppler effect cause redshift?

Accepted Answer

The Doppler effect causes a change in the observed wavelength of light when a source is moving relative to an observer. If a galaxy is moving away from Earth, the light waves are stretched, shifting them to longer (redder) wavelengths. This is called redshift. The greater the redshift, the faster the galaxy is moving away.

Question 3

What is the cosmic microwave background radiation?

Accepted Answer

The cosmic microwave background (CMB) is faint electromagnetic radiation that fills the universe. It is the leftover heat from the Big Bang, now cooled to about 2.7 Kelvin. The CMB is uniform in all directions, providing strong evidence that the universe began in a hot, dense state and has been expanding ever since.

Question 4

Why do planets stay in orbit around the Sun?

Accepted Answer

Planets stay in orbit because of the gravitational force between the planet and the Sun. This force provides the centripetal force needed to keep the planet moving in a circular path. The planet's forward motion (inertia) tries to take it in a straight line, but gravity pulls it toward the Sun, resulting in a stable orbit.

Question 5

What happens when a star runs out of hydrogen?

Accepted Answer

When a star runs out of hydrogen in its core, nuclear fusion stops, and the core contracts under gravity. This heats the core and causes the outer layers to expand, turning the star into a red giant (low-mass) or red supergiant (high-mass). For low-mass stars, helium fusion begins briefly, then the star sheds its outer layers to form a planetary nebula, leaving a white dwarf. High-mass stars continue fusion of heavier elements until iron, then collapse and explode as a supernova.

Question 6

How do we know the universe is expanding?

Accepted Answer

We know the universe is expanding because of two main pieces of evidence: redshift and the cosmic microwave background. Redshift shows that distant galaxies are moving away from us, and the farther they are, the faster they recede (Hubble's law). The CMB is the cooled remnant of the Big Bang, and its uniformity supports the idea that the universe expanded from a single point.

Topic 7 – Astronomy

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How to Revise Topic 7 – Astronomy — Edexcel GCSE Physics

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