Our place in the Milky Way

The Milky Way is a barred spiral galaxy of a few hundred billion stars. Seen edge-on it has three parts:

• a flat disc, about 100 000 light years across but only about 1000 ly thick, containing the spiral arms — full of gas, dust and young blue stars;
• a central bulge of old stars, with a supermassive black hole (Sagittarius A*) of about 4 million solar masses at its heart;
• a spherical halo containing about 150 globular clusters of very old stars.

The Sun lies in a spiral arm about 26 000 ly from the centre — well out in the suburbs — and takes roughly 225 million years to complete one orbit of the Galaxy.

Why the Milky Way is a band in the sky: we are inside the disc, so looking along it we see huge numbers of stars piled up — but dust blocks our view of the centre in visible light. (Infrared and radio see straight through, which is how the central black hole was found.)

Where are we?

Types of galaxy

Hubble sorted galaxies by their shape, and the classification still stands:

• Spiral (and barred spiral) — a flat disc with a bulge and arms. Plenty of gas and dust, so new stars are still forming in the arms (which look blue). The Milky Way and Andromeda are both barred/spiral.
• Elliptical — a smooth ball or egg with no arms and little gas or dust. Star formation has largely stopped, so the stars are old and red. This class includes both the largest galaxies known and many small dwarfs.
• Irregular — no clear shape, often small and gas-rich, frequently distorted by a gravitational encounter with a bigger galaxy (e.g. the Magellanic Clouds).

Galaxies cluster. The Milky Way belongs to the Local Group (about 80 galaxies), dominated by us and the Andromeda Galaxy (M31), which lies about 2.5 million light years away — and is actually moving towards us.

Dark matter: stars in the outer parts of galaxies orbit far too fast for the visible mass to hold them in. Either gravity is wrong, or most of a galaxy's mass is invisible dark matter. Galaxy rotation curves are the classic evidence.

Redshift and Hubble's law

Take the spectrum of a distant galaxy and the familiar absorption lines are all shifted towards longer (redder) wavelengths. This redshift means the galaxy is moving away from us.

In 1929 Edwin Hubble found something extraordinary: the further away a galaxy is, the faster it is receding, and the relationship is a straight line:

We are not the centre. Space itself is expanding and carrying the galaxies apart, so every observer in every galaxy sees the same law. Think of dots drawn on a balloon: blow it up, and every dot sees the others recede, with the far ones receding fastest.

Your turn — Hubble's law

Your turn — measuring a redshift

Your turn — how far away is it?

What does redshift tell us?

The Big Bang and the evidence for it

Run the expansion backwards and everything was once packed into an unimaginably hot, dense state. About 13.8 billion years ago, space, time, matter and energy began expanding from it — the Big Bang. The rival Steady State theory (an eternal, unchanging universe with matter created continuously) was abandoned because it could not explain the evidence:

• 1. Redshift / Hubble's law — everything is receding, and the further away, the faster.
• 2. The cosmic microwave background (CMB) — a faint microwave glow coming from every direction in the sky, at a temperature of about 2.7 K. It is the light of the hot early universe, stretched by 13.8 billion years of expansion into microwaves. It was predicted by Big Bang theory and found by accident by Penzias and Wilson in 1965 — and the Steady State model has no explanation for it at all.
• 3. The abundance of light elements — the universe is about 25% helium by mass everywhere, far more than stars could have made. Big Bang nucleosynthesis predicts exactly that.

Common trap: the Big Bang was not an explosion into empty space. It happened everywhere at once — space itself expanded. There is no centre to point to, and no edge to fall off.

The best evidence

Sort the cosmos

Tap a statement, then tap where it belongs.

Match the description to the term

Tap a description on the left, then its matching term on the right.

A bigger Hubble constant

Olbers' paradox and the fate of the universe

Olbers' paradox: if the universe were infinite, eternal and unchanging, then every line of sight would eventually hit the surface of a star — and the whole night sky would blaze as brightly as the Sun. It plainly does not. Why?

• The universe has a finite age (13.8 billion years), so light from the most distant regions has not had time to reach us.
• The universe is expanding, so light from very distant objects is redshifted out of the visible band (the most distant light of all now reaches us as the microwave background).

What happens next? Measurements of distant Type Ia supernovae in the late 1990s showed the expansion is not slowing down but accelerating, driven by something unknown called dark energy. Current evidence favours a universe that expands for ever, growing ever colder and darker.

The modern inventory: ordinary matter — everything you have ever seen — is only about 5% of the universe. Roughly 27% is dark matter and about 68% is dark energy. Cosmology's biggest questions are still wide open.

Why is the night sky dark?

The big ideas to know

That is Galaxies & Cosmology covered for Edexcel GCSE Astronomy. Press Finish to see your score.