Wing IV · the domeRoom VI of VI
Inv. no. IV.6 · the plates
The colour of a star
Spread a star's light through a prism and you get a rainbow crossed by dark lines. Which lines, and how dark, tells you how hot the star's surface is, almost at a glance. Astronomers sort stars into seven letters by it, hottest to coolest: O B A F G K M. The Sun is a G. Hot stars are blue-white and show mostly helium and hydrogen. Cool ones are orange-red, and most of their lines come from metals and molecules. They are made of much the same stuff; the temperature decides which lines show up.
A star and its lines
Fig. I. The disc in the colour the eye would see, the black-body glow above the visible rainbow, and the spectrum as a plate shows it: the darker a line, the stronger.
- Class
- Like
- The glow peaks at
- Strongest lines
Each dark line is light taken out by atoms in the star's outer layers: an atom swallows a photon of exactly the energy that lifts one of its electrons to a higher step. Slide the temperature and watch the families take turns. Hydrogen's lines are strongest around 10,000 kelvin, in the A stars like Vega and Sirius. Hotter, and the hydrogen loses its electron altogether and the lines fade, while helium's appear. Cooler, and hydrogen's electrons sit too low to absorb visible light; then calcium, iron and sodium take over, and below about 3,800 kelvin molecules such as titanium oxide survive and saw broad bands into the red light.
Shift the same lines a little red or blue and the spectroscope reads how the star is moving; the wobble that gives away a planet is read the same way.
Sorting the plates
Fig. II. Thirteen spectra on glass, a magnifier running along them. Sorted by hydrogen, the plates are a muddle. Sorted by temperature, each family of lines takes its turn.
- Top to bottom
At Harvard in the 1880s and 1890s the spectra came in on glass plates by the thousand, and the first sorting went by the strength of the hydrogen lines: A for the strongest, then B, C and on down the alphabet. Press by hydrogen to see why that went nowhere: the colours jump about and the other lines come and go at random.
Annie Jump Cannon kept seven of the letters and put them in the order of their colours and lines, and with it, as it later turned out, of temperature: O B A F G K M, each split into ten steps from 0 to 9. Press by temperature and the plates fall into a smooth sequence. A quarter of a century later Cecilia Payne showed why it works: the stars are made of nearly the same stuff, mostly hydrogen and helium; only their heat differs.
Who worked it out
Where it already matters
A quarter of a million stars, by eye. Annie Jump Cannon classified the spectra of some 225,000 stars for the Henry Draper Catalogue, published from 1918 to 1924, and many more after it, at a pace of several a minute. The letters she settled on are still the ones in use.
Made of hydrogen, 1925. In her doctoral thesis Cecilia Payne worked out how much of each element the stars' lines really point to, allowing for how hot the gas is. Her answer was that stars are mostly hydrogen and helium. Nobody expected that, and she was talked into calling the result probably spurious. It was right.
G2V. In 1943 William Morgan and Philip Keenan added a Roman numeral for how big and bright the star is, from I for supergiants to V for ordinary stars burning hydrogen. The Sun is a G2V; Betelgeuse an M1–M2 Ia, a supergiant. Plotting the letters against brightness, as Ejnar Hertzsprung and Henry Norris Russell did around 1911–1913, gives the HR diagram, the map in the life of a star.
Past M. Since 1999 the coolest objects have letters of their own: L, then T, and Y, added in 2011. Most of them are brown dwarfs, too small to burn hydrogen; the coolest Y dwarfs are only a few hundred degrees warm.
Oh Be A Fine Girl Kiss Me. The order has been learned by generations of students with a mnemonic, these days often with Guy for Girl.