What's the most important thing to know about a star? Its brightness (apparent magnitude) might top the list, but right behind would be its spectral type. Without its spectral type a star is a meaningless dot. Add a few letters and numbers like "G2V" or "B5IV-Vshnne" and the star suddenly gains personality and character. To those who can read its meaning, the spectral code tells at a glance just what kind of object the star really is: its color, size, and luminosity compared to the Sun and stars of all other types; its peculiarities, its history, and its future.

The modern spectral classification system is so successful that it has hardly been changed since 1943. It is based on just two physical properties that imprint themselves on the spectrum of a star's light: the star's surface temperature and atmospheric pressure. These reveal an abundance of information that paints the star's portrait and tells its life story.

The temperature sets the star's color and determines its surface brightness: how much light comes from each square meter of its surface. The atmospheric pressure depends on the star's surface gravity and therefore, roughly, on its size — telling whether it is a giant, dwarf, or something in between. The size and surface brightness in turn yield the star's luminosity (its total light output, or absolute magnitude) and often its evolutionary status (young, middle-aged, or nearing death). The luminosity (when compared to the star's apparent brightness in our sky) also gives a good idea of the star's distance. Appended to the basic spectral type may be letters for chemical peculiarities, an extended atmosphere, unusual surface activity, fast rotation, or other special characteristics.

Every starwatcher needs to have a feel for spectral types. Here are the most important things to know.

Dissecting Starlight

Our tale began in 1802, when the English experimenter William Wollaston passed a beam of sunlight through a thin slit and then through a prism. The slit provided a sharp, high-resolution view of the familiar rainbow spectrum, with no color overlapping another. When seen this way, Wollaston noticed, the Sun's spectrum was marked by many narrow, black lines of various intensities. These dark lines stayed at exactly the same places in the colorful band from day to day and year to year. They were later measured and cataloged by Josef von Fraunhofer, for whom they are still called "Fraunhofer lines."

Similar spectral lines showed up in laboratory experiments. Using a slit and prism, physicists discovered that when a solid, liquid, or dense gas is heated to glow, it emits a smooth spectrum of light with no lines: a continuum. A rarefied hot gas, on the other hand, glows only in certain colors, or wavelengths: bright, narrow emission lines instead of a rainbow band. If a cooler sample of the same gas is placed in front of a glowing object emitting a continuum, dark absorption lines appear at the wavelengths where the emission lines would be if the gas were hot.

By 1859 the situation was clear: we see the Sun's hot, relatively dense surface through a cooler solar atmosphere that imposes the dark lines.

Here, scientists realized, was a way to bring the Sun down into the laboratory. Every element, every chemical compound, shows its own set of spectral lines. They are as unique as fingerprints. They reveal not only which atoms and molecules are present but also many other physical conditions, starting with temperature. And when astronomers put the same slit-and-prism device (a spectroscope) on a telescope, they could even see spectral lines in the light of stars.

It was the 19th century's greatest astronomical breakthrough. Philosophers had cited the makeup of stars as an example of something that was beyond all possible human knowing. Now finding the composition of the Sun and stars was just a matter of comparing spectral lines seen in a telescope with those seen in a laboratory. This wasn't always simple, but it gave birth to modern astrophysics — the treatment of stars as physical objects to be studied and understood, rather than as mere points of light on the sky to be measured.