The Solar System

168 PART 2^ |^ THE STARS

Th e Universe, as has been observed before, is

an unsettlingly big place, a fact which for the

sake of a quiet life most people tend to ignore.

— DOUGLAS ADAMS, THE RESTAURANT AT THE END OF THE UNIVERSE

E

verything has to come from somewhere. Look at
your thumb. Th e atoms in your thumb are billions of
years old. Only a hundred million years ago, they were
inside dinosaurs. Five billion years ago, those atoms were part of a
cloud of gas fl oating in space, and not long before that those atoms
were inside stars. Th e atoms inside your body are old, but the matter
the atoms are made of is even older. Th at matter had its beginning
within minutes of the beginning of the universe 13.7 billion years
ago. If your atoms could tell their stories, you would be amazed to
know where they came from and where they have been.
As you study the solar system, remember the history of the
matter it is made of. Th e iron inside Earth and in your blood, the
oxygen, nitrogen and carbon in Earth’s atmosphere that you
breathe in and out, the calcium in your bones—all exist because
stars have lived and died. You are no more isolated from the rest
of the universe than a raindrop is isolated from the sea.

The Birth of Stars

The stars above seem permanent fixtures of the sky, but

astronomers know that stars are born and stars die. Th eir lives are

long compared with a human life; if you know where to look,

you can see all of the stages of stellar birth, aging, and death

represented in the sky. Astronomers have put those stages into

the proper order and can tell the life story of the stars, a story that

begins in the darkness of interstellar space.

Although space seems empty, it is actually fi lled with thinly

spread gas and dust, the interstellar medium. Th e gas atoms are

mostly hydrogen, a few centimeters apart, and the dust is made

of microscopic grains of heavier atoms such as carbon and iron.

Th e dust makes up only about one percent of the matter between

the stars.

Th e interstellar medium is tenuous in the extreme, yet you

can see clear evidence that it exists. In some places, the inter-

stellar medium is collected into great dark clouds of dusty gas

that obscure the stars beyond. In other cases, a nearby hot star

can ionize the gas and create a glowing cloud (■ Figure P-1).

Astronomers refer to both dark and glowing interstellar clouds

as nebulae (singular, nebula), from the Latin word for cloud

or mist.

P-1

Visual-wavelength image

Horsehead NebulaHorsehead Nebula

Visual-wavelength image

100 ly

Nebula N44Nebula N44

Dusty foreground gas silhouetted

against glowing gas illuminated

by hot, young stars

Roughly 40 young stars are inflating

a bubble of hot gas inside the nebula

from which they formed.

Young star

buried in the

nebula

■ Figure P-1 5 ly

Young stars are found in clouds of gas and dust from which they
have been born. The nebula N 44 is 170,000 ly from Earth in a
nearby galaxy, and the Horsehead Nebula is only about 1500 ly
distant in our own galaxy. Gas in both nebulae is excited to glow
by hot, young stars, and dust is visible as dark, twisted clouds
seen against the bright background gas. (N 44: ESO; Horsehead
Nebula: NOAO and Nigel Sharp)