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Looking at the Night Sky (cont.)
The electromagnetic spectrum is a
continuous range of wavelengths.
Different parts of the electromagnetic
spectrum have different wavelengths and
different energies. You can see only a small
part of the energy in these wavelengths.
An astronomical unit is the average
distance between Earth and the Sun,
about 150 million km.
Measuring Distances (cont.)
• A light-year is the distance light travels
in 1 year.
• One light-year is about 10 trillion km.
Measuring Brightness
The apparent magnitude of an object is
a measure of how bright it appears from
Earth.
Measuring Brightness (cont.)
• Luminosity is the true brightness of an
object.
• The luminosity of a star, measured on an
absolute magnitude scale, depends on
the star’s temperature and size, not its
distance from Earth.
The Sun and Other Stars
• nuclear fusion
• photosphere
• star
• chromosphere
• radiative zone
• corona
• convection
zone
• HertzsprungRussell
diagram
How Stars Shine
• A star is a large ball of gas held together
by gravity with a core so hot that nuclear
fusion occurs.
• Nuclear fusion occurs when the nuclei
of several atoms combine into one larger
nucleus.
How Stars Shine (cont.)
• Nuclear fusion releases a large amount
of energy.
• A star shines because when energy
leaves a star’s core, it travels throughout
the star and radiates into space.
Composition and Structure of Stars
Spectra of the Sun and other stars provide
information about stellar composition.
stellar
Science Use anything related
to stars
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Composition and Structure of Stars
(cont.)
• There are three
interior layers of
a typical star.
• When first formed,
all stars fuse
hydrogen into
helium in their
cores.
Composition and Structure of Stars
(cont.)
• The radiative zone
is a shell of cooler
hydrogen around a
star’s core.
• In the convection
zone, hot gases
move toward the
surface as cooler
gases move down
into the interior.
Composition and Structure of Stars
(cont.)
Beyond the convection zone are
the three layers of a star’s atmosphere—
the photosphere, the chromosphere, and
the corona.
Composition and Structure of Stars
(cont.)
The photosphere is
the apparent surface
of a star, where light
energy radiates into
space.
Composition and Structure of Stars
(cont.)
• The chromosphere
is the orange-red
layer above the
photosphere.
• The corona is the
wide, outermost
layer of a star’s
atmosphere.
Changing Features of the Sun:
Sunspots
• Cooler regions of
magnetic activity
• Seem to move as
the Sun rotates
• Number varies on
an 11-year cycle
Digital Vision/PunchStock
Changing Features of the Sun:
Coronal Mass Ejections (CMEs)
• Huge gas bubbles
ejected from the
corona
• Larger than flares
• May reach Earth
• Can cause radio
blackouts
NASA
Changing Features of the Sun:
Prominences and Flares
• Prominences—
clouds and jets of
gases forming loops
into the corona
• Flares—sudden
increases in
brightness, often
near sunspots or
prominences
SOHO Consortium, ESA, NASA
Changing Features of the Sun:
The Solar Wind
• Caused by
charged particles
streaming away
from the Sun
• Extends to the
edge of the solar
system
• Causes auroras
CORBIS
Groups of Stars
• Most stars exist in star systems bound
by gravity.
• Many stars exist in large groupings
called clusters.
• Stars in a cluster all formed at about the
same time and are the same distance
from Earth.
Classifying Stars
• Scientists classify stars according to
their spectra.
• Though there are exceptions, color in
most stars is related to mass.
Blue-white stars tend to have the most
mass, followed by white stars, yellow stars,
orange stars, and red stars.
The Hertzsprung-Russell diagram
is a graph that plots luminosity against
temperature of stars.
The y-axis of the H-R diagram displays
increasing luminosity and the x-axis
displays decreasing temperature.
Most stars exist along the main sequence.
The mass of a main-sequence star
determines both its temperature and its
luminosity
Evolution of Stars
• nebula
• white dwarf
• supernova
• neutron star
• black hole
Life Cycle of a Star
• Stars form deep inside clouds of gas
and dust.
• A cloud of gas and dust is a nebula.
nebula
from Latin nebula, means
“mist” or “little cloud”
Life Cycle of a Star (cont.)
Star-forming nebulae are cold, dense,
and dark.
Photo courtesy of NASA/Corbis
Life Cycle of a Star (cont.)
• Gravity causes the densest parts to
collapse, forming regions called
protostars.
• Over many thousands of years, the
energy produced by protostars heats
the gas and dust surrounding them.
Life Cycle of a Star (cont.)
• A star becomes a main-sequence star
as soon as it begins to fuse hydrogen
into helium.
• Low-mass stars stay on the main
sequence for billions of years, and
high-mass stars are there for only a few
million years.
Life Cycle of a Star (cont.)
• When a star’s hydrogen supply is
nearly gone, the star leaves the main
sequence and begins the next stage
of its life cycle.
• All stars form in the same way, but
stars die in different ways, depending
on their masses.
• Massive stars eventually become red
supergiants.
End of a Star
• After helium in the cores of lower-mass
stars is gone, the stars cast off their
gases exposing their cores.
• The core eventually becomes a white
dwarf, a hot, dense, slowly cooling
sphere of carbon.
• This is what is expected to happen to the
Sun.
The Sun will remain on the main
sequence for 5 billion more years.
When the Sun becomes a red giant for the
second time, it will probably absorb Earth and
push Mars and Jupiter outward.
When the Sun becomes a white dwarf, the
solar system will be a cold, dark place.
End of a Star (cont.)
• Very massive stars can explode in a
supernova, which destroys the star.
• Iron in the core does not fuse and the
core collapses quickly under the force of
gravity.
• The normal space within atoms is
eliminated, leaving a dense core of
neutrons, or a neutron star.
End of a Star (cont.)
• For the most massive stars, atomic
forces holding neutrons together are not
strong enough to overcome so much
mass in such a small volume. Gravity is
too strong, and the matter crushes into a
black hole.
• A black hole is an object whose gravity
is so great that no light can escape.