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Birth and Death of Stars
• A ball of gas that gives off a tremendous
amount of electromagnetic energy.
• Stars form in nebula.
• The energy of stars comes from nuclear
• Stars vary in color and size.
• Astronomers learn about stars by looking
at their spectrum from the light they emit.
Nuclear Fusion
• The process by which nuclei of small atoms
combine to form new, more massive nuclei.
• The process releases energy.
Composition of Stars:
Star Spectrum
• Different elements absorb different
wavelengths of light. The spectrum of a
star lets us know what elements are in the
• The most abundant elements in stars are
Hydrogen and Helium.
Temperatures of Stars
• The surface temperature of a star is indicated by
the star’s color.
Surface Temp (0C)
> 30,000
10 Lacartae
10,000 – 30,000
Rigel, Spica
7,500 – 10,000
Vega, Sirius
Yellow – White
6,000 – 7,500
Canopus, Procyon
5,000 – 6,000
Sun, Capella
3,500 – 5,000
Arcturus, Aldebaran
< 3,500
Betelgeuse, Antares
Motion of Stars
• Stars appear to move in a circular motion
around Polaris (North Star) because of Earth’s
Motion of Stars
• Stars appear to shift slightly to the west each
night because of Earth’s revolution.
Distances to Stars
• Astronomers use parallax to determine a
star’s distance.
• Close stars appear to shift relative to
farther stars as the Earth goes around the
• The closer the star the greater the shift.
Stellar Brightness
• Apparent magnitude is
the brightness of a star as
it appears from Earth.
This depends on the
amount of light emitted by
the star and how close it
is to the Earth. The lower
the number the greater
the apparent magnitude.
• Absolute magnitude is
how bright the star would
be if all stars were the
same distance from Earth
(32.6 light years).
Sirius (brightest -1.46
Faintest star
with unaided
Classifying Stars
• By plotting stars by their surface
temperature against their luminosity gives
us a consistent pattern called the
Hertzsprung – Russell (H-R) diagram .
• Astronomers use the H-R diagram to
describe the life cycles of stars.
• Most stars that are plotted fall within a
band that runs through the middle of the
H-R called the “main sequence”.
H – R Diagram
A large cloud of gas and dust in
interstellar space; a region in
space where stars are born
Eagle Nebula
Orion Nebula
Horsehead Nebula
Life cycle of Stars: Protostar
• Hydrogen gas in the nebula is pulled
inward by gravity and starts to spin. As the
gas spins faster, it heats up and becomes
a protostar. Eventually the temperature
reaches 10,000,000 oC and nuclear fusion
occurs in the cloud's core. The cloud
begins to glow brightly, contracts a little,
and becomes stable in size.
Stellar Equilibrium
• Stars become stable
in size once the
inward force of
gravity is balanced
by the outward
pressure from
nuclear fusion and
radiation from the
• A star's life cycle is determined by
its mass. The larger its mass, the
shorter its life cycle.
Life Cycle of Stars – Main
• The longest stage of the star is the main
sequence stage.
• Hydrogen continues to fuse into Helium and
produce energy.
• Stars with the mass of about the sun’s mass will
stay on the main sequence for about 10 billion
• Stars more massive than our sun may only stay
on the main sequence for 10 million years.
Life Cycle of Low Mass Star
• When almost all of the hydrogen within a
star’s core has fused to helium, the star
contracts under it’s own gravity. The core
becomes hotter and transfers energy to
the outer shell of hydrogen which allows
fusion to continue. This on-going fusion
radiates energy outward and the outer
shell expands and cools creating a red
Life Cycle of Low Mass Star
• The red giant will stop fusing in the core
once the helium atoms have fused to
carbon and oxygen.
• The star’s outer gases will drift away and
be heated by the remaining core creating
a planetary nebula.
Planetary Nebula “Ant Nebula”
Planetary Nebula “Cat’s Eye”
Planetary Nebula “Eskimo Nebula”
Planetary Nebula “Ring Nebula”
Planetary Nebula
“Hourglass Nebula”
Life Cycle of Low Mass Star
• As the planetary nebula disperses, gravity
causes the remaining matter in the star to
collapse inward. A hot, extremely dense
core of matter is formed called a white
• When the white dwarf no longer gives off
light it becomes a black dwarf.
• Some white dwarfs are part of a binary
star system.
• If a white dwarf revolves around a red
giant, it may capture gases from the red
giant which creates pressure. The
pressure is released in large explosions
called nova.
Nova “Cygni”
• Sometimes a white dwarf in a binary
system collects so much mass on its
surface that gravity overwhelms the
outward pressure. Pressure builds up until
the star blows up. This is a thousand times
more violent than a nova. This is called a
Supernova “1987A”
Life Cycle of High Mass Star
• When almost all of the hydrogen within a
massive star’s core has fused to helium,
the star contracts under it’s own gravity.
The core becomes hotter and transfers
energy to the outer shell of hydrogen
which allows fusion to continue. This ongoing fusion radiates energy outward and
the outer shell expands and cools creating
a red supergiant.
Red Supergiant: Betelgeuse
Life Cycle of High Mass Star
• After the supergiant stage, massive stars
contract with a gravitational force much
greater than low mass stars. The high
pressures and temperatures that result
causes nuclear fusion to begin again. This
time the core fuses into heavier elements
such as oxygen, magnesium, or silicon.
Fusion continues until the core is made of
iron. The star then builds up pressure until
it explodes in a supernova.
Life Cycle of High Mass Star
• After the supernova, the core contracts
into a very small but incredibly dense ball
of neutrons called a neutron star. These
rotate very rapidly. Some neutron stars
emit a beam of radio waves as they rotate.
These are called pulsars.
Pulsar in Crab Nebula
Life Cycle of High Mass Star
• Some massive stars produce leftovers too
massive to become stable neutron stars. If
the remaining core is > 3 times the mass
of the sun, the star may contract further
under its greater gravity. The force of
contraction crushes the dense core and
creates an object so massive and dense
that even light cannot escape its gravity.
These are called black holes.
Finding Black Holes