The Astronomy Book

126

additional dimension, creating
the Yerkes classification system,
otherwise called the MKK system
after William Morgan, Philip
Keenan, and Edith Kellman, the
astronomers based at the Yerkes
Observatory in Wisconsin who
formulated it. This system denotes
luminosity with Roman numerals,
although a few letters are also used.
The advantage of the MKK
system is that it gives a star a size as
well as a temperature, so that stars
can be described in colloquial terms
such as white dwarf, red giant, or
blue supergiant. The main sequence
stars, including the sun, are all small
enough to be called dwarfs. The sun
is a G2V star, which indicates that it
is a yellow dwarf with a surface
temperature of about 5,800 K.

Classes and characteristics
The hottest class of star, O-types
have a surface temperature in excess
of 30,000 K. Most of the radiation
these stars emit is in the ultraviolet
part of the spectrum and appear
blue when viewed in visible light.
O stars are mainly giants, typically
20 times as massive as the sun and
10 times as wide. Only 0.00003
percent of main sequence stars

are this hot. O-type stars burn their

fuel very quickly and release huge

amounts of energy. As a result, they

have a short life expectancy, which

is measured in tens of millions

of years, compared to billions for

cooler stars. Members of this class

have weak lines of hydrogen in

their spectra, and strong evidence

of ionized helium, which is present

because of the high temperature.

With a surface temperature of

between 10,000 and 30,000 K, B-type

stars are brighter in visible light than

O-types, despite being cooler. This

is because more of the radiation is

emitted as visible light, making them

“blue-white.” Again, B-type dwarfs

are rare, making up less than 0.1

percent of main sequence stars.

When they do occur, they are

perhaps 15 times more massive

than the sun. B-type stars have

non-ionized helium in their spectra

and more evidence of hydrogen.

Because they live for only a short

time, B-type stars are found in

molecular clouds or star-forming

regions, since they have not had

time to move far from the location

in which they formed. About

THE CHARACTERISTICS OF STARS

twice as large as the sun, main

sequence A-type stars have a

surface temperature of between

7,500 and 10,000 K. They have

strong hydrogen lines in their

spectra and emit a wide spectrum

of visible light, which makes them

look white (with a blueish tinge).

As a result, they are some of the

most easily seen stars in the night

sky, and include Vega (in Lyra),

Gamma Ursae Majoris (in the Big

Dipper), and Deneb (in Cygnus).

However, only 0.625 percent of main

sequence stars are A-type stars.

Cooling stars

As dwarf stars cool, the hydrogen

in their spectra becomes less

intense. They also exhibit more

absorption lines due to metals. (To

an astronomer, everything heavier

than helium is a metal.) This is

not because their composition is

different from that of hotter stars

but because the gas near the surface

is cooler. In hotter stars, the atoms

are too ionized to create absorption

lines. F-type stars have a surface

temperature of between 6,000 and

7,500 K. Called yellow-white dwarfs,

The prism has revealed to us

something of the nature of

the heavenly bodies, and the

photographic plate has made

a permanent record of the

condition of the sky.

Williamina Fleming

RELATIVE STRENGTH

DECREASING TEMPERATURE

O B A F G K M

The strengths of the absorption lines of different elements

vary according to the surface temperature of the star. Lines of

heavier elements are more prominent in the spectra of cooler stars.

Neutral

helium Hydrogen

Neutral

metals

Ionized

helium

Ionized

metals Molecules