186 PART 2^ |^ THE STARS
What Are We? Children of the Milky WayHang on tight. The sun, with Earth in its
clutch, is ripping along at about 220 km/sec
(that’s 490,000 mph) as it orbits the center of
the Milky Way Galaxy. We live on a wildly
moving ball of rock in a large galaxy that
some people call our home galaxy, but the
Milky Way is more than just our home. Perhaps
“parent galaxy” would be a better name.
Except for hydrogen atoms, which have
survived unchanged since the universebegan, you and Earth are made of atoms
heavier than helium. There is no helium in
your body, but there are plenty of atoms
such as calcium and iron. There is calcium
in your bones and iron in your blood. All of
those atoms and more were cooked up
inside stars or in their supernova deaths.
Stars are born when clouds of gas are
compressed and gravity pulls the gas
together. That process has given birth togenerations of stars, and each generation
has produced elements heavier than helium
and spread them back into space. The
abundance of these atoms has grown slowly
in the galaxy. About 4.6 billion years ago a
cloud of gas enriched in those heavy atoms
contracted and produced the sun, Earth,
and you. Your atoms have been cooked up
by the Milky Way Galaxy—your parent
galaxy.186 PART^2 | THE STARSSummary
▶ (^) The interstellar medium (p. 168), the gas and dust between the
stars, can be seen as nebulae (p. 168). Small, dense, dark clouds are
called Bok globules (p. 170). Stars are born when clouds of gas and
dust contract to form protostars (p. 169).
▶ (^) Protostars are cool and hidden inside clouds of gas and dust, so they
are most easily observed in the infrared.
▶ (^) Planets form in the disks of gas and dust around protostars.
▶ (^) Stars that generate energy by hydrogen fusion, including the sun, are
called main-sequence stars (p. 169).
▶ (^) The lowest-mass stars, the red dwarfs, (p. 172) can survive for many
billions of years.
▶ (^) The sun is a medium-mass star and can live for about 10 billion years
before it expands to become a giant (p. 172), expels its outer layers
to form a planetary nebula (p. 172), and collapses to form a white
dwarf (p. 172).
▶ (^) The most massive stars live only a few million years and expand to
become supergiants (p. 173). Giant and supergiant stars exhale gas
back into the interstellar medium. The most massive stars die in explo-
sions called supernovae (p. 173) that can form elements heavier than
iron and blow them plus atoms of lighter elements made earlier in the
star’s life back into the interstellar medium. Supernova explosions can
leave behind neutron stars (p. 173) or black holes (p. 173) as stel-
lar remnants.
▶ (^) Stars cook hydrogen atoms into atoms of heavier elements and return
those atoms to the interstellar medium when the star dies.
▶ (^) The Milky Way (p. 176), the hazy band of light across the night sky,
is our galaxy, the Milky Way Galaxy (p. 176), seen from the inside. It
contains about 100 billion stars and is about 80,000 ly in diameter. We
live about two-thirds of the way from the center to the edge.
▶ (^) Billions of galaxies fi ll the sky, and some are elliptical (p. 178), some
spiral (p. 178), and some irregular (p. 179). Some spiral galaxies,
including the Milky Way Galaxy, contain a bar-shaped nucleus and are
called barred-spiral galaxies (p. 178).
▶ (^) Collisions between galaxies are common and can trigger star formation.
▶ (^) The redshifts of the galaxies show that the universe is expanding, and
imagining running the expansion backward in time leads to inference
of the high-temperature, high-density beginning of the universe called
the big bang (p. 181).
▶ (^) The cosmic microwave background radiation (p. 182) is light
released from matter soon after the big bang that has been redshifted
by the expansion of the universe into the infrared and radio parts of
the spectrum. It is direct evidence that the big bang really happened.
Because Earth is part of the big bang, the radiation is detectable in
every direction in the sky.
▶ (^) The energy of the big bang produced protons and electrons (hydrogen
gas), and some of that hydrogen fused into helium during the fi rst few
minutes, but no heavier elements could form.
▶ (^) Gravity drew the gas into great clouds that formed galaxies, and
stars fused hydrogen into heavier elements. Matter passes through a
gas–star–gas cycle and is enriched in elements heavier than helium.
A number of generations of stars have produced the chemical elements
in Earth and in you.