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The Universe
Section 1
Key Ideas
〉 How are stars formed?
〉 How can we learn about stars if they are so far away?
〉 What natural cycles do stars go through?
The Universe
What Are Stars?
〉 How are stars formed?
Section 1
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〉 Stars are formed from clouds of dust and gas, or
nebulas, and go through different stages as they age.
• star: a large celestial body that is composed of gas and
emits light
• light-year: the distance that light travels in one year;
about 9.46 trillion kilometers
The Universe
Section 1
What Are Stars? continued
• Stars are powered by nuclear fusion reactions.
– The core of a star is extremely hot, extremely dense,
and under extreme pressure.
– Nuclear fusion takes place in the core of a star.
– Fusion combines the nuclei of hydrogen atoms into
helium.
– When two particles fuse, energy is released.
The Universe
Section 1
Visual Concept: Nuclear Fusion
The Universe
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What Are Stars? continued
• Energy moves slowly through the layers of a star.
– Energy moves through the layers of a star by
convection and radiation.
– During convection, hot gas moves upward, away
from the star’s center, and cooler gas sinks toward
the center.
– During radiation, atoms absorb energy and transfer it
to other atoms in random directions. Atoms near the
star’s surface radiate energy into space.
The Universe
Structure of the Sun
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The Universe
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Visual Concept: Structure of the Sun
The Universe
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Studying Stars
〉 How can we learn about stars if they are so far away?
〉 The telescope allowed astronomers to study stars in
more detail for the first time.
• Some stars appear brighter than others.
– The brightness of a star depends on the star’s
temperature, size, and distance from Earth.
– The brightest star in the night sky, Sirius, appears so
bright because it is relatively close to Earth.
The Universe
Section 1
Studying Stars, continued
• We learn about stars by studying energy.
– Stars produce a full range of electromagnetic
radiation, from high-energy X-rays to low-energy radio
waves.
– Scientists use optical telescopes to study visible light
and radio telescopes to study radio waves emitted
from astronomical objects.
– Earth’s atmosphere blocks some wavelengths, so
telescopes in space can study a wider range of the
spectrum.
The Universe
Section 1
Studying Stars, continued
• A star’s color is related to its temperature.
– Hotter objects glow with light that has shorter
wavelengths (closer to the blue end of the spectrum).
– Cooler objects glow with light that has longer
wavelengths (closer to the red end of the spectrum).
– Hot stars emit more energy at every wavelength than
cooler stars do.
The Universe
Starlight Intensity Graph
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The Universe
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Studying Stars, continued
• Spectral lines reveal the composition of stars.
– The spectra of most stars have dark lines caused by
gases in the outer layers that absorb light at that
wavelength.
– Each element produces a unique pattern of spectral
lines.
– Astronomers can match the dark lines in starlight to
the known lines of elements found on Earth.
The Universe
Section 1
Visual Concept: Constellation
The Universe
Section 1
The Life Cycle of Stars
〉 What natural cycles do stars go through?
〉 In a way that is similar to other natural cycles, stars are
born, go through various stages of development, and
eventually die.
• The sun formed from a cloud of gas and dust.
– The sun formed about 5 billion years ago.
– The sun was “born” when the process of fusion began
in the core.
The Universe
Section 1
The Life Cycle of Stars, continued
• The sun has a balance of inward and outward forces.
– The fusion reactions in the core of the sun produce an
outward force that balances the inward force due to
gravity.
– Over time, the percentage of the sun’s core that is
helium becomes larger.
– Scientists estimate that the sun can continue nuclear
fusion for another 5 billion years.
The Universe
Section 1
The Life Cycle of Stars, continued
• The sun will become a red giant before it dies.
– As fusion slows, the outer layers of the sun will expand.
– The sun will become a red giant.
• red giant: a large, reddish star late in its life cycle
– When the sun runs out of helium, the outer layers will
expand and eventually leave the sun’s orbit.
– The sun will become a white dwarf.
• white dwarf: a small, hot dim star that is the leftover center of
an old star
The Universe
Section 1
The Life Cycle of Stars, continued
• Supergiant stars explode in supernovas.
– Massive stars evolve faster, develop hotter cores, and
create heavier elements through fusion.
– The formation of an iron core signals the beginning of
a supergiant’s death.
– Eventually the core collapses and then explodes in a
supernova.
• supernova: a gigantic explosion in which a massive star
collapses and throws its outer layers into space, plural
supernovae
The Universe
Section 1
The Life Cycle of Stars, continued
• Some supernovas form neutron stars and black holes.
– If the core that remains after a supernova has a mass
of 1.4 to 3 solar masses, the remnant can become a
neutron star.
– If the leftover core has a mass that is greater than
three solar masses, it will collapse to form a black
hole.
• black hole: an object so massive and dense that not
even light can escape its gravity
The Universe
Section 1
The Life Cycle of Stars, continued
• The H-R diagram shows how stars evolve.
– The vertical line on an H-R diagram indicates
brightness in absolute magnitude.
– The horizontal line on the H-R diagram indicates
temperature.
– Most stars appear in a diagonal line called the main
sequence.
– As stars age and pass through different stages, their
positions on the H-R diagram change.
• The sun is currently a main-sequence star.
The Universe
H-R Diagram
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The Universe
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Visual Concept: Types of Stars
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