The Astronomy Book

138

STARS ARE

GIANTS OR

DWARFS

REFINING STAR CLASSIFICATION

IN CONTEXT

KEY ASTRONOMER

Henry Norris Russell

(1877–1957)

BEFORE

1901 Annie Jump Cannon,

working at the Harvard College

Observatory, introduces the

star spectral classes O, B, A, F,

G, K, and M (based on surface

temperature of stars).

1905 Based on analyses of star

spectra, Ejnar Hertzsprung

states that there are two

fundamentally different kinds

of star within some spectral

classes, one of which is far

more luminous.

AFTER

1914 Walter Adams discovers

white dwarf stars—white-hot

but relatively faint.

1933 Danish astronomer

Bengt Strömgren introduces

the term “Hertzsprung–Russell

diagram” to denote a plot

of the absolute magnitudes

of stars against spectral class.

A

round 1912, American Henry

Russell began comparing

stars’ absolute magnitude

(or true brightness) and their color,

or spectral class. Before the early

20th century, no one had figured out

how different star types might be

related in some overall scheme, but

it had long been recognized that

they differ in certain properties,

such as color. While some stars

shine with a pure white light, others

have distinct colors: many have

reddish or bluish hues, while the

sun is yellow. In 1900, German

physicist Max Planck worked out

the precise mathematics to describe

how the mix of wavelengths of light

given off by hot objects, and hence

their color, varies according to their

temperature. Thus, star colors are

related to surface temperature—red

stars have the coolest surfaces, and

blue stars the hottest. By around

1910, stars were considered to fit

into spectral classes related to their

colors and surface temperatures.

The other obvious way in which

stars differ is in their brightness.

Since ancient times, stars have

been classified into brightness

classes. This developed into the

apparent magnitude scale, which

rated stars according to how bright

Stars fall into two

distinct groups when

plotted on a diagram

showing luminosity

and temperature.

However, a few

exceptionally bright stars

do not follow this rule.

These are giant stars.

Stars are either

giants or dwarfs.

Among most stars,

blue stars are brighter

than yellow stars, which are

brighter than orange/red

stars. These are

dwarf stars.