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The Universe
Section 1
Section 1: The Life and Death of Stars
Preview
• Key Ideas
• Bellringer
• What Are Stars?
• Studying Stars
• The Life Cycle of Stars
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
Section 1
Bellringer
Based on what you have learned, as well as previous knowledge,
answer the following questions.
1. People have studied the stars for centuries. The ancient Greeks
named groups of stars, called constellations.
List at least five constellations.
2. Explain why scientists collect information from electromagnetic
waves, such as visible light, microwaves, and X rays, to study
stars.
3. Stars do not all look the same. Some are brighter than others,
and many have different colors.
Write a paragraph explaining the possible reasons for the
differences.
The Universe
Section 1
What Are Stars?
〉 How are stars formed?
〉 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
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The Universe
Section 1
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
Section 1
The Universe
Section 1
Visual Concept: Structure of the Sun
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The Universe
Section 1
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
Section 1
The Universe
Section 1
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
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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
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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
Section 1
The Universe
Section 1
Visual Concept: Types of Stars
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