Reading Starlight
The Sun (§3.1) is one star among hundreds of billions in our galaxy, and they are far from identical. The most obvious difference is colour. Some stars burn blue-white, others a warm yellow, others a dull red — and colour is not decoration. As we saw with light and the spectrum (§0.4), a glowing object's colour reveals its temperature: hotter surfaces glow blue, cooler ones glow red, exactly as an iron bar runs from red to white as it heats.
Astronomers grade that colour ladder with seven letters — O, B, A, F, G, K, M — the spectral classes, running from the hottest blue stars to the coolest red ones. An O star can reach 30,000 degrees Celsius at its surface (a 3 followed by four zeros); an M red dwarf simmers at only about 3,000. Generations of students have remembered the order with the line "Oh, Be A Fine Girl/Guy, Kiss Me." Our own Sun is a middle-of-the-road G star at about 5,800 degrees — yellow, ordinary, unremarkable.
Colour is only half the story; stars also differ enormously in brightness, or luminosity — the total light a star pours out, which we measure in Suns (the Sun's output is defined as 1). Plot every star with temperature across the bottom and luminosity up the side and a remarkable pattern appears: the stars are not scattered at random but gather into a few families. This is the Hertzsprung–Russell diagram, the single most important chart in all of stellar astronomy. Most stars — the Sun included — sit along a diagonal band called the main sequence, the long, stable prime of life during which a star fuses hydrogen in its core. Off the band sit two oddball families: the giants and supergiants in the upper right (cool-surfaced but so vast they outshine everything), and the white dwarfs in the lower left (scorching-hot but Earth-sized, so they barely glow). Step through the marked stars below with the arrow keys and watch each one's class, colour and home region light up.
Charts like this one used to be drawn from a few thousand stars. Today they are drawn from nearly two billion. The European Space Agency's Gaia mission — the telescope of this chapter — measured the positions, distances, colours and brightnesses of about 1.8 billion stars, yielding the finest, most crowded Hertzsprung–Russell diagrams ever made and turning a hand-sketched idea from 1911 into a precise census of the Milky Way.
Mass Is Destiny
Why do stars come in such different kinds at all? The answer is a single number fixed at birth: a star's mass — how much gas it gathered when it formed. Mass alone sets a star's temperature, its colour, its brightness, how long it lives, and how it dies. Everything else follows.
The rule is steep, and at first it seems backwards. A heavy star has more fuel — yet it burns out far faster, because greater weight crushes its core harder and makes it fuse that fuel at a furious rate. A faint red dwarf of about one-tenth the Sun's mass sips its fuel so slowly that it can shine steadily for trillions of years — longer than the present age of the Universe. The Sun, at one solar mass, will last about ten billion years. A massive O star of twenty Suns or more blazes thousands of times brighter, races through its fuel, and dies in only a few million years — a cosmic eyeblink. Drag the slider below to set a star's birth mass and watch its lifetime, colour and final fate change.
That birth weight also decides the ending. A modest star like the Sun fades quietly into a white dwarf; a heavier one explodes as a supernova and leaves behind a neutron star or a black hole. How a star travels along this path — and what it scatters back into space on the way — is the story of §3.3, and what survives the end is the subject of §3.4.
Seven letters of colour, one diagram of families, and a single number — mass at birth — that writes a star's whole biography in advance. Next we follow that biography unfolding: how a star is born, lives, and changes.