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Chapter 26
26. 1 The Sun
What is the sun?
 Without the sun’s radiation, Earth would be
dark, cold, and lifeless.
70% hydrogen, 28% helium
A. Energy from the sun
Gives off energy in the form of electromagnetic
radiation
2. The sun’s energy is produced in its central region
by the fusion of hydrogen nuclei into helium
nuclei.
3. Been a stable energy source for billions of years.
4. The sun remains stable because the inward pull of
gravity balances outward push of thermal pressure
from nuclear fusion.
5. The sun will remain stable as long as there is a
steady energy source
1.
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5 billion more years
B. The sun’s interior
1. Consists of:
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Core, radiation zone, and convection zone
2. Core – central region where fusion occurs
3. Radiation zone- dense region of compressed
gas, just outside the core, can take 100,000
years for energy to pass through it.
4. Convection zone- outer layer of the sun’s
interior, Energy is transferred by convection
currents.
C. The Sun’s Atmosphere
1. Has 3 layers:
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Photosphere, Chromospheres, and Corona
2. Photosphere- innermost, visible surface,
temp. 5800K, bubbly appearance
3. Chromospheres- 10,000K, middle layer, only
see during total solar eclipse
The Sun’s Atmosphere Cont.
4. Corona- outer layer, can only see during a
solar eclipse, very hot = 1 million K.
a. Thins into solar winds- electrically charged
particles flow into the solar system
D. Features of the sun’s
atmosphere
1. Sunspots- cooler areas on the sun’s surface
that look darker. Gives off less energy.
2. Prominences- near sunspots, huge loops of
gas that can be up to 10,000 km above the
sun’s surface
3. Solar Flare- sudden release of energy
Section 26.2: Stars
1. Star- large glowing ball of gas in space, gets
its energy from nuclear fusion.
2. Light year- distance light travels in a
vacuum in a year = 9.5 trillion km.
A. Properties of Stars
1. Classified by:
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Color
Size
Brightness
Composition
2. Color and Temperature
a. Hot is blue
b. Cold is red
3. Brightness- does not always tell the distance of
the star.
a. Apparent brightness- brightness of a star
as it appears from Earth
b. Absolute brightness- how bright a star
really is.
4. Size and Mass
a. No way to directly find this out, it is an
estimate
b. Use gravity to determine
5. Composition
a. Most have a chemical make-up similar to
the sun
b. H and He make-up 96-99% of the mass
B. H-R Diagram
Graph of the surface temp., or color and absolute
brightness of sample stars
2. Used to estimate the sizes of the stars and their
distances, and to infer how stars change over time
3. Main sequence- diagonal line on the diagram
where 90% of stars are found
4. Supergiants- very bright, very large stars
5. Giants- large and bright
6. White Dwarf- small, dense remains of low-medium
mass stars, dimmer than the main sequence stars.
1.
26.3 Life Cycles of Stars
1. A star is formed when a contracting cloud of
gas and dust becomes so dense and hot that
nuclear fusion begins
2. A star’s mass determines the star’s place on
the main sequence and how long it will stay
there.
3. A dwindling supply in a star’s core ultimately
leads to the star’s death as a white dwarf,
neutron star or black hole.
26.4 Groups of Stars
Star Systems
1. A group of 2 or more stars held together by
gravity
2. Astronomers have determined that more
than half of all stars are members of star
systems
Galaxies
 A huge group of
individual stars, star
systems, star clusters,
dust, and gas bound
together by gravity
Galaxies
Astronomers classify them into 4 main types:
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Spiral, barred-spiral, elliptical, irregular
Types of Galaxies
1. Spirals- has a bulge in the center and arms
that extend outward like a pinwheel
2. Barred-Spiral- has a bar of stars running
through the middle with arms extending off
the bar.
3. Elliptical- spherical or oval with no spiral
arms
4. Irregular- very disorganized appearance,
lots of dust and gas
The Milky Way Galaxy!!!
 Has 200- 400 billion stars and a diameter of
over 100,000 light years.
 Our solar system is in one of the spiral arms
2/3 of the way from the center.
 Takes the sun 220 million years to orbit
around the center of the galaxy.
 Evidence shows there is a massive black hole
at the center of our galaxy.
The Doppler Effect
1. When the wavelength and frequency of
sound or light change because the waves
are still moving
2. When stars move away/towards the Earth
we use the Doppler Effect to look at the
wavelengths.
3. To see this we use the visible light spectrum
(ROY G BIV)
4. If the wavelengths are moving towards the
blue end (shorter wavelengths) the star or
galaxy is moving towards Earth.
5. If it moves towards the red end of the light
spectrum (longer wavelengths) the star is
moving away from the Earth. This is called the
RED SHIFT.
Hubble’s Law (1920)
 Hubble- 1920’s astronomers found a red shift
on other galaxies. This means they are
moving away from Earth.
 Galaxies farther away from Earth moved
away faster than closer galaxies.
 HUBBLE’s LAW- the speed at which a galaxy
moves away is proportional to it’s distance
 The observed red shift in
the spectra of galaxies
shows that the universe is
expanding.
The Big Bang Theory
 Astronomers theorize that the universe came
into being at a single moment, in an event
called the big bang.
 Big Bang Theory (not the tv show!)- states
that the universe began in an instant, billions
of years ago, in an enormous explosion.
After the Explosion
 The universe expanded and cooled quickly
 Gravity started pulling elements together to
form atoms
Dust clouds
stars.
Evidence for the Theory
 The existence of cosmic microwave
background radiation and the red shift in the
spectra of distant galaxies strongly support
the big bang theory.
Age of the universe
 Use microwave background radiation to
measure how fast the universe is expanding.
 They can now estimate that the universe is
13.7 billion years old.
DARK MATTER
 matter that does not give off radiation
 It cannot be seen directly, but we can detect
it by observing its gravitational force on
visible matter.
 They do not know all the details about dark
matter.