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Ch. 29 Sec. 3. Stellar Evolution. Cygnus supernova. 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. I. Basic Structure of Stars. A. Mass effects.

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ch 29 sec 3

Ch. 29 Sec. 3

Stellar Evolution

Cygnus supernova


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

i basic structure of stars
I. Basic Structure of Stars

A. Mass effects

  • Larger mass
  • a. greater the gravity
  • b. hotter
  • c. more dense
  • 2. Temperature controls rate of nuclear reactions
  • 3. Nuclear rate determines luminosity

B. Hydrostatic equilibrium

a. Balance between gravity and outward pressure

b. Must hold for any stable star

Fig. 29.17 page 847


C. Fusion

When 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.

ii stellar evolution
II. Stellar Evolution

A. Star formation

  • Nebula (plural, nebulae) collapses
  • a. Gravity/pressure becomes unbalanced
  • b. Core density changes
  • Protostar
  • a. Cloud contracts
  • b. Rotation forces it into a disk shape
  • c. Hot, condensed object at the center

B. Fusion begins

1. Friction increases temperature of protostar

2. Conversion of hydrogen to helium

a. Core density increases

b. Temperature


c. Luminosity


3. Star becomes stable (gravity=pressure)

iii life cycles of stars like the sun
III. 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 giant.


A. Red giant

  • Helium center
  • Hydrogen fuses in core’s outer shell
  • a. Energy forces the outer layers to expand and cool
  • b. Luminosity increases
  • 3. Temperature decreases
  • 4. Loses gas from its outer layers
  • 5. Helium reacts and forms carbon in core

B. The final stages

1. Energy production ends

a. Not enough heat for carbon fusion

2. The outer layers expand again

a. Expelled by pulsations in the outer layers

3. The shell of gas is called a planetary nebula


4. Center of a planetary nebula

a. Core of the star

b. Small, hot object

c. Size of Earth

d. White dwarf made of carbon


C. Internal pressure in white dwarfs

  • Stable
  • Gradually cools
  • a. Loses luminosity
  • b. Black dwarf
iv life cycles of massive stars
IV. 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.


A. 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.


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.


B. Supernova formation

  • 8 to 20 times the Sun’s mass
  • Reactions in core have created iron
  • No further energy-producing reactions can occur
  • Core of the star violently collapses

5. Neutron star - collapsed, dense core of a star

a. Radius of about 10 km

b. Mass 1.5 to 3 times that of the Sun

c. Contains mostly neutrons


6. Pulsar - spinning neutron star

a. Exhibits a pulsing pattern


7. Supernova

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.


C. Black holes

  • 20 Solar masses
  • 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

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.

Black holes emit x-rays, allowing astronomers to identify their locations.


If light cannot escape a black hole, how do astronomers locate black holes?

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.


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.

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.