Section 29.3
Stellar Evolution
Objectives
Determine the effect of mass on a star’s evolution.
Identify the features of massive and regular star
life cycles.
Explain how the universe is affected by the life
cycles of stars.
Section 29.3
Stellar Evolution
The Sun and other stars follow similar life
cycles, leaving the galaxy enriched with
heavy elements.
Review Vocabulary
evolution: a radical change in composition
over a star’s lifetime
Section 29.3
Stellar Evolution
New Vocabulary
nebula
pulsar
protostar
supernova
neutron star
black hole
Section 29.3
Stellar Evolution
Basic Structure of Stars
Mass effects
The more massive a star is, the greater the
gravity pressing inward, and the hotter and
more dense the star must be inside to
balance its own gravity. The temperature
inside a star governs the rate of nuclear
reactions, which in turn determines the star’s
energy output—its luminosity.
Section 29.3
Stellar Evolution
Basic Structure of Stars
Mass effects
The balance between gravity squeezing
inward and outward pressure is maintained by
heat due to nuclear reactions and
compression.
This balance, governed by the
mass of the star, is called
hydrostatic equilibrium, and it
must hold for any stable star.
Section 29.3
Stellar Evolution
Basic Structure of Stars
Fusion
The density and temperature increase
toward the center of a star, where energy is
generated by nuclear fusion.
Section 29.3
Stellar Evolution
Stellar Evolution
Eventually, when its nuclear fuel runs out, a
star’s internal structure and mechanism for
producing pressure must change to counteract
gravity. The changes a star undergoes during
its evolution begin with its formation.
Section 29.3
Stellar Evolution
Stellar Evolution
Star formation
The formation of a star begins with a cloud
of interstellar gas and dust, called a nebula
(plural, nebulae), which collapses on itself
as a result of its own gravity.
As the cloud contracts, its rotation forces it
into a disk shape with a hot, condensed
object at the center, called a protostar.
Section 29.3
Stellar Evolution
Stellar Evolution
Star formation
Friction from gravity continues to increase
the temperature of the protostar, until the
condensed object reaches the ignition
temperature for nuclear reactions and
becomes a new star.
Section 29.3
Stellar Evolution
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Section 29.3
Stellar Evolution
Stellar Evolution
Fusion begins, star gets the on Main
sequence
The first nuclear fusion reaction to ignite in
a protostar is always the conversion of
hydrogen to helium. Once this reaction
begins, the star becomes stable because it
then has sufficient internal heat to produce
the pressure needed to balance gravity.
The object is then truly a star.
Section 29.3
Stellar Evolution
Life Cycles of Stars Like the Sun
It takes about 10 billion years for a star with the
mass of the Sun to convert all of the hydrogen
in its core into helium. Thus, such a star has a
main-sequence lifetime of 10 billion years.
From here, the next step in the life cycle of a
small mass star is to become a red giantmoves out of the main sequence, fusing
helium into carbon.
Section 29.3
Stellar Evolution
Life Cycles of Stars Like the Sun
Red giant
When the hydrogen in a star’s core is gone, it
has a helium center and outer layers made of
hydrogen-dominated gas. Some hydrogen
continues to react in a thin layer at the outer
edge of the helium core. The energy produced
in this layer forces the outer layers of the star
to expand and cool.
Section 29.3
Stellar Evolution
Life Cycles of Stars Like the Sun
Red giant
While a star is a red giant, it loses gas from
its outer layers. Meanwhile, the core of the
star becomes hot enough, at 100 million K,
for helium to react and form carbon. When
the helium in the core is depleted, the star is
left with a core made of carbon.
Section 29.3
Stellar Evolution
Life Cycles of Stars Like the Sun
The final stages
A star with the same mass as the Sun never
becomes hot enough for carbon to fuse, so
its energy production ends. The outer layers
expand again and are expelled by pulsations
that develop in the outer layers. The shell of
gas is called a planetary nebula.
Section 29.3
Stellar Evolution
Life Cycles of Stars Like the Sun
The final stages
In the center of a planetary nebula, the core
of the star becomes exposed as a small, hot
object about the size of Earth. The star is
then a white dwarf made of carbon.
Section 29.3
Stellar Evolution
Life Cycles of Stars Like the Sun
Internal pressure in white dwarfs
A white dwarf is stable despite its lack of
nuclear reactions because it is supported by
the resistance of electrons being squeezed
together. This pressure counteracts gravity
and can support the core as long as the
mass of the remaining core is less than
about 1.4 times the mass of the Sun.
Section 29.3
Stellar Evolution
Life Cycles of Massive Stars
A more massive star begins its life with hydrogen
being converted to helium, but it is much higher
on the main sequence. The star’s lifetime in this
phase is short because the star is very luminous
and uses up its fuel quickly. When the white
dwarf cools and loses its luminosity, it becomes
an undetectable black dwarf.
Section 29.3
Stellar Evolution
Life Cycles of Massive Stars
Supergiant
A massive star undergoes many more reaction
phases and thus produces a rich stew of many
elements in its interior. The star becomes a red
giant several times as it expands following the
end of each reaction stage.
Section 29.3
Stellar Evolution
Life Cycles of Massive Stars
Supergiant
As more shells are formed
by the fusion of different
elements in a massive star,
the star expands to a larger
size and becomes a
supergiant. These stars are
the source of heavier
elements in the universe.
Section 29.3
Stellar Evolution
Life Cycles of Massive Stars
Supernova formation
A star that begins with a mass between about
8 and 20 times the Sun’s mass will end up
with a core that is too massive to be supported
by electron pressure. Once reactions in the
core of the star have created iron, no further
energy-producing reactions can occur, and the
core of the star violently collapses in on itself.
Section 29.3
Stellar Evolution
Life Cycles of Massive Stars
Supernova formation
A neutron star is a collapsed, dense core of a
star that forms quickly while its outer layers are
falling inward. It has a radius of about 10 km
and a mass 1.5 to 3 times that of the Sun, and
it contains mostly neutrons.
Section 29.3
Stellar Evolution
Life Cycles of Massive Stars
Supernova formation
A pulsar is a spinning neutron star that exhibits
a pulsing pattern.
Section 29.3
Stellar Evolution
Life Cycles of Massive Stars
Supernova formation
When the outer layers of a star collapse into
the neutron core, the central mass of neutrons
creates a pressure that causes this mass to
explode outward as a supernova, leaving a
neutron star.
Section 29.3
Stellar Evolution
Life Cycles of Massive Stars
Black holes
A star that begins with more than 20 times
the Sun’s mass will be too massive to form
a neutron star. The resistance of neutrons
to being squeezed is not great enough to
stop the collapse. The core of the star
continues to collapse, compacting matter
into a smaller volume.
Section 29.3
Stellar Evolution
Life Cycles of Massive Stars
Black holes
A black hole is a small, extremely dense
remnant of a star whose gravity is so
immense that not even light can escape its
gravity field.
Evolution of stars- as they get off the main
sequence.( M – is the mass of Sun)
Mass up to 8M
Mass 8-20 M
Mass greater than 20M
1.Red giants
1.Super giants
1.Super giants
2.Planetary nebula
2.Super nova
2.Super nova
3.White dwarf
3.Neutron Star
3.Black hole
(about Earth size, mass
up to1.4M)
(city size, mass 1.5 –
3M)
( point size, very dense ,
even light cannot escape
its gravity)
CH
Study Guide
Key Concepts
Section 29.3 Stellar
Evolution
The Sun and other stars follow
similar life cycles, leaving the galaxy enriched
with heavy elements.
 The mass of a star determines its internal
structure and its other properties.
 Gravity and pressure balance each other
in a star.
CH
Study Guide
Key Concepts
Section 29.3 Stellar
Evolution
 If the temperature in the core of a star
becomes high enough, elements heavier
than hydrogen can fuse together.
 A supernova occurs when the outer layers
of the star bounce off the neutron star core,
and explode outward.
CH
Stars
29.3 Section Questions
The diagram depicts a star that is stable
and will not expand or contract. What is
this balance called?
a. electrostatic equilibrium
b. hydrostatic equilibrium
c. gravitational equilibrium
d. luminosity equilibrium
CH
Stars
29.3 Section Questions
The density of a neutron star is comparable
to that of an atomic nucleus.
a. true
b. false
CH
Stars
29.3 Section Questions
If light cannot escape a black hole, how do
astronomers locate black holes?
Answer: Because light cannot escape, a
black hole is invisible. However, gases
spiraling into a black hole emit X rays.
Astronomers can locate the black hole by
looking for those X-ray emissions.
CH
Stars
Chapter Assessment
Questions
The diagram shows a star with a helium core.
At which stage of its life cycle is this star?
a. main sequence
b. red giant
c. white dwarf
d. helium-carbon
CH
Stars
Chapter Assessment
Questions
Which is the outermost layer of the Sun?
a. corona
b. prominence
c. chromosphere
d. photosphere
CH
Stars
Chapter Assessment
Questions
What is the difference between absolute
magnitude and apparent magnitude?
Possible answer: Apparent magnitude is
how bright a star appears to be from Earth.
Absolute magnitude takes the star’s
distance into account.
CH
Stars
Chapter Assessment
Questions
How is parallax used to determine the distance
from Earth to a star?
CH
Stars
Chapter Assessment
Questions
Answer: As Earth orbits the Sun, nearby
stars appear to shift position in the sky when
compared with more distant stars. The closer
the star, the greater the shift. By measuring
the angle of the change, astronomers can
estimate the distance to the star.
CH
Stars
Chapter Assessment
Questions
What causes the dark bands in a star’s
spectrum?
Answer: The various chemical elements that
make up the star absorb light at specific
wavelengths. This causes dark bands to
appear in the star’s spectrum.
CH
Stars
Standardized Test
Practice
Where does the Sun’s energy come from?
a. radioactive decay
b. X-ray emissions
c. fusion reactions
d. nuclear fission
CH
Stars
Standardized Test
Practice
What do astronomers measure to determine a
star’s motion relative to Earth’s?
a. wavelength shift
b. absolute magnitude
c. angle of parallax
d. apparent magnitude
CH
Stars
Standardized Test
Practice
Which do astronomers use to classify a star?
a. age and size
b. position
c. color and size
d. spectral type
CH
Stars
Standardized Test
Practice
At which part of its life cycle is a Sun-sized star
with a carbon core?
a. protostar
b. main sequence
c. beginning stages
d. final stages
CH
Stars
Standardized Test
Practice
Which property takes a star’s distance
into account?
a. apparent magnitude
b. absorption spectra
c. absolute magnitude
d. emission spectra