The Solar System

PERSPECTIVE: ORIGINS 169

Th ese nebulae not only adorn the sky, they also mark the
birthplace of stars. As the cold gases of a nebula grow denser,
gravity can pull parts of it together to form warmer, denser bod-
ies that eventually become protostars—objects destined to
become stars.
Astronomers can’t see these protostars easily because they
are hidden deep inside the dusty gas clouds from which they
form, but they are easily detected at infrared wavelengths. As
the more massive protostars become hot, luminous stars, the
light and gas fl owing away from the newborn stars blow the
nebula away to reveal the new stars (■ Figure P-2). In this way,
a single gas cloud can give birth to a cluster of stars. Our sun
was probably born in such a nebula about 5 billion years
ago.
If you look at the constellation Orion, you can see one of
these regions with your unaided eye. Th e patch of haze in Orion’s
sword is the Great Nebula in Orion. Read Star Formation
in the Orion Nebula on pages 170–171 and notice four
points and one new term:

Th e nebula you see is only a small part of a vast, dusty cloud.

You see the nebula because the stars born within it have ion-

ized the gas and driven it outward, breaking out of the

cloud. In some cases, the turbulence has produced small,

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dense clouds of gas and dust called Bok globules that may be

in the process of forming stars.

Also notice that a single very hot star (classifi ed O6) is

almost entirely responsible for producing the ultraviolet

photons that ionize the gas and make the nebula glow.

Infrared observations reveal clear evidence of active star for-

mation deeper in the cloud just to the northwest of the

Trapezium, behind the visible nebula.

Finally, notice that many stars visible in the Orion Nebula

are surrounded by disks of gas and dust. Planets form in

such disks, but the disks do not last long and are clear evi-

dence that the stars are very young.

As protostars contract, something happens that is quite

important to planetwalkers like you—planets form. You will

see in the next chapter how our solar system formed from the

disk of gas and dust that orbited the protostar that became

our sun.

You could identify the birth of a star as the moment when it

begins fusing hydrogen into helium. Th at nuclear fusion reaction

releases energy, supports the star, and stops its contraction. Stars

that generate energy at their cores by hydrogen fusion are called

main-sequence stars.

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3

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a Visual-wavelength image

b Visual + infrared-wavelength image

These massive stars were

triggered into formation by

compression from the

formation of earlier stars out

of the image to the left.

New stars are forming in

these dense clouds

because of compression

from the stars to the left.

If you lived inside this monster cluster,

Earth’s sky would contain hundreds of

stars brighter than the full moon.

■ Figure P-2

(a) An earlier generation of massive stars out of the frame to the left
ejected high-speed gas that compressed nearby gas clouds and triggered
the formation of young, extremely massive stars in the bright region to
the left. Those stars are now triggering the birth of a third generation of
stars. (NASA, ESA, The Hubble Heritage Team, AURA/STScI) (b) Compression can
trigger the formation of very massive star clusters. The super star cluster
Westerlund 1 contains roughly half a million stars, some extremely mas-
sive. It must have formed no more than 5 million years ago. (ESO)