Parallax and the parsec

As the Earth orbits the Sun, a nearby star appears to shift slightly against the far more distant background stars. Half of that total annual shift is the star's parallax angle, p, measured in arcseconds.

The catch: parallax angles are tiny (always under 1 arcsecond) and get smaller with distance, so ground-based parallax only works out to a few hundred parsecs. Space missions (Hipparcos, then Gaia) do far better because they are above the blurring atmosphere.

Your turn — how far is the star?

Your turn — into light years

Apparent and absolute magnitude

A star can look faint either because it really is feeble, or because it is very far away. So astronomers use two magnitudes:

• Apparent magnitude (m) — how bright the star looks from Earth.
• Absolute magnitude (M) — how bright it would look at a standard distance of 10 parsecs. This is a fair measure of the star's true output (its luminosity).

Example: Sirius looks the brightest star in our sky (m = −1.4) only because it is close (2.6 pc). Its absolute magnitude is +1.4. Rigel looks fainter (m = +0.1) but is around 250 pc away — its absolute magnitude is about −7. That is 8.4 magnitudes brighter than Sirius, making it thousands of times more luminous.

Remember the direction: both scales run backwards — the smaller (more negative) the number, the brighter the star. The Sun's apparent magnitude is −26.7; its absolute magnitude is only +4.8.

Your turn — absolute magnitude

A fair comparison

Colour, temperature and the HR diagram

A star's colour tells you its surface temperature: blue stars are the hottest, red stars the coolest. Splitting starlight with a spectroscope also reveals dark absorption lines, which show which elements are present. Stars are sorted into spectral classes:

Plot luminosity (up) against temperature (increasing to the left — a quirk of history) and you get the Hertzsprung-Russell diagram. Stars are not scattered randomly:

• Main sequence — a diagonal band from hot and luminous (top left) to cool and dim (bottom right). About 90% of stars, including the Sun, sit here, fusing hydrogen into helium.
• Red giants and supergiants — top right: cool but hugely luminous, because they are enormous.
• White dwarfs — bottom left: hot but very dim, because they are tiny (Earth-sized).

Mass rules everything. A star's mass fixes where it sits on the main sequence and how long it lives. A massive O star blazes through its hydrogen in a few million years; a small red dwarf will last for hundreds of billions.

Read the HR diagram

The life cycle of a star

All stars begin the same way: a cloud of gas and dust — a nebula — collapses under gravity, heats up, and forms a protostar. When the core reaches about 15 million K, fusion of hydrogen into helium begins, the outward pressure balances gravity, and the star joins the main sequence. What happens next depends entirely on mass.

• Low mass (like the Sun): when core hydrogen runs out, the core contracts and the outer layers swell → a red giant. Helium fuses to carbon. Eventually the outer layers drift off as a planetary nebula, leaving the hot, dense core behind — a white dwarf, which slowly cools forever.
• High mass: the star swells into a red supergiant and fuses ever heavier elements, up to iron. Iron cannot release energy by fusing, so the core collapses in under a second and rebounds in a titanic explosion — a supernova. It leaves behind a neutron star, or, if massive enough, a black hole — an object so dense that not even light can escape.

Why you are made of stardust: everything heavier than helium was forged inside stars, and elements heavier than iron are made in supernovae. The carbon in you and the iron in your blood were made in stars that died before the Sun was born.

How will the Sun end?

Whose life story?

Tap a stage, then tap the kind of star it belongs to.

Match the description to the term

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

Live fast, die young

Cepheid variables — measuring the far distances

Parallax runs out of accuracy after a few hundred parsecs. To reach other galaxies astronomers use a standard candle — an object whose true luminosity is known, so that comparing it with how bright it looks gives the distance.

• Cepheid variables are giant stars that pulsate, swelling and shrinking so that their brightness rises and falls in a very regular cycle of days to weeks.
• Henrietta Leavitt discovered the period-luminosity relationship: the longer the pulsation period, the more luminous the star. So timing the period gives its absolute magnitude free of charge.
• Compare that with its apparent magnitude, and you get the distance — which is precisely how Hubble proved that the "spiral nebulae" were entire galaxies far beyond the Milky Way.

Type Ia supernovae are even better standard candles: they all explode at almost the same mass and so reach almost the same peak luminosity, and they are bright enough to be seen billions of light years away.

Why a Cepheid is useful

The big ideas to know

That is Stars covered for Edexcel GCSE Astronomy. Press Finish to see your score.