Stars and the Universe

Stars form from collapsing clouds of gas and dust and shine by fusing hydrogen. Sun-like stars swell into red giants and end as white dwarfs; much more massive stars explode as supernovae, leaving neutron stars or black holes. Light from distant galaxies is redshifted: the further away, the faster they recede. The Universe is expanding from a Big Bang about $14$ billion years ago.

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Section 1 of 14

Learning objectives

Describe the Sun and the life cycles of stars of different masses.
Describe galaxies and use the light-year as a unit of distance.
Explain redshift and the evidence for the Big Bang.
Use $v=H_0d$ and estimate the age of the Universe.

Pause and think

The light from a galaxy $10$ million light-years away is reaching your eyes right now. When did it leave the galaxy?

Section 2 of 14

Core idea

Start hereGalaxies running away

Light from distant galaxies is stretched towards red, and the further the galaxy, the more stretched. Every galaxy seems to rush away from every other, as if the whole universe started from one point: the Big Bang.

The big idea

Stars form from collapsing clouds and fuse hydrogen; mass decides their fate. Redshift and Hubble's law $v=H_0d$ show the universe is expanding.

1. Life of a star

Life cycle of a star

Stars like the Sun (top path) end as white dwarfs. Much heavier stars (bottom) explode as supernovae, leaving a neutron star or black hole.

Stellar life cycles

Drawing diagram…

The heaviest elements are made in supernovae.

2. Expansion

Hubble's law: further galaxies recede faster

A straight line through the origin: everything moving apart, as if from one starting point.

H_0\approx2.2\times10^{-18}

s⁻¹, giving an age of about 14 billion years.

Age from Hubble's law

$v=H_0d$
Hubble's law.
$t=\frac dv=\frac1{H_0}$
Time to reach distance $d$ at constant speed.

Worked exampleAge of the universe

$H_0\approx2.2\times10^{-18}$ s⁻¹.

Goal: Age.
Strategy: $1/H_0$ .

$\frac1{2.2\times10^{-18}}=4.5\times10^{17}\text{ s}$
Reciprocal.
$\div3.16\times10^7\approx1.4\times10^{10}\text{ years}$
Seconds to years.

AnswerAbout 14 billion years.

✓ CheckAgrees with other methods ✓.

Common misconceptionRedshift = colour red It's a shift to longer wavelengths, not a red colour.

Summary

Mass decides a star's fate.
Redshift and Hubble's law show expansion.

Redshift: Increase in wavelength from a receding source.

HistoryHubble and Leavitt. Edwin Hubble showed in

1929

that distant galaxies are moving away from us. His distances relied on a method discovered by Henrietta Swan Leavitt in

1912

, who found that certain stars' brightness pulsed at a rate that revealed their true brightness. She was paid

30

cents an hour.

Fun factYou are made of star stuff. Apart from hydrogen, almost every atom in your body, including the carbon in your cells and the iron in your blood, was made inside stars that exploded before the Sun formed. As Carl Sagan put it: "We are made of star stuff."

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