Space

How does a star form?

A nebula (gas + dust) collapses under gravity, heats up, and nuclear fusion starts (hydrogen → helium). The star becomes stable.

What are key features of stars?

Brightness, temperature/colour (blue = hottest, red = coolest), size, mass, composition (mostly hydrogen and helium).

What is solar mass, and how does it compare stars?

1 solar mass (M☉) = mass of our Sun. Small stars < 0.5 M☉; medium stars 0.5–8 M☉ (like the Sun); large stars > 8 M☉. More mass = hotter, brighter, shorter-lived.

What is a galaxy?

A huge collection of stars, gas, dust, and dark matter held together by gravity. Our galaxy is the Milky Way.

What are major features of galaxies?

Stars, nebulae, dark matter, supermassive black holes at the centre, and spiral arms (in spiral galaxies).

What are the three types of galaxies?

Spiral (Milky Way), Elliptical (M87), Irregular (Large Magellanic Cloud).

What is a light-year, and how is it used?

The distance light travels in one year (≈ 9.5 trillion km). Within galaxies: light-years. Between galaxies: millions/billions of light-years.

What are the major features of the solar system?

The Sun (99.8% of mass), 8 planets (4 rocky, 4 gas giants), dwarf planets, moons, asteroid belt, Kuiper Belt, Oort Cloud, comets.

What units are used for planet sizes and distances?

Sizes: kilometres (Earth ≈ 12,742 km; Jupiter ≈ 139,820 km). Distances: AU (1 AU = Earth–Sun distance ≈ 150 million km). Mercury ≈ 0.39 AU; Neptune ≈ 30 AU.

How did the solar system form?

A solar nebula collapsed under gravity into a spinning disk. The centre formed the Sun. Dust clumped (accretion) into planets. Rocky planets formed close; gas giants further out.

What is an optical telescope?

Collects visible light with lenses/mirrors. Used to observe planets, stars, galaxies, nebulae.

What is a radio telescope?

Collects radio waves with large dishes. Works day/night, through clouds. Used to find pulsars, quasars, CMB.

What is a space telescope?

Sits above Earth's atmosphere. Gives sharper images and sees wavelengths blocked from the ground (UV, infrared, X-rays). E.g. Hubble, James Webb.

What are the advantages of a space telescope?

No atmospheric distortion, no light pollution, and can see wavelengths blocked by the atmosphere.

What have space telescopes discovered?

Hubble: universe's expansion rate, deep-field galaxies, exoplanets. James Webb: earliest galaxies, exoplanet atmospheres, star formation.

What is the Big Bang theory?

13.8 billion years ago, all matter/energy was in a hot, dense point. It expanded (space itself expanded), cooled, and formed atoms, stars, and galaxies. Still expanding today.

What is the CMB, and why does it support the Big Bang?

A faint glow of microwave radiation filling space — leftover heat from the Big Bang. Discovered 1965; confirms the universe was once hot and dense.

How does galaxy redshift support the Big Bang?

Galaxies are moving away, and further ones move faster — the universe is expanding, so it was once smaller and denser.

How does the abundance of light elements support the Big Bang?

The universe's hydrogen: helium ratio (about 75:25) matches what the Big Bang should have produced. Stars alone can't explain it.

What evidence was used to calculate the age of the universe?

Running the expansion rate backwards (Hubble's Law) → ~13.8 billion years. The CMB confirms this. Oldest stars (~13 billion years) match.

What are the three pieces of evidence for the Big Bang?

(1) CMB radiation, (2) galaxy redshift (expansion), (3) hydrogen:helium ratio.

What are the three galaxy types? Give examples.

Spiral (Milky Way), Elliptical (M87), Irregular (Large Magellanic Cloud).