galaxies and the universe ch 25 section 3. what are the objects in the sky? galaxies! groups of...
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Galaxies and the UniverseCh 25
Section 3
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What are the objects in the sky?
GALAXIES!Groups of stars, dust, and gases held together
by gravity
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Classification of GalaxiesBy shape & age of stars:
As # increases, the shape becomes more elliptical.
As the lowercase letters advance through the alphabet, the arms becomes more tightly wound.
ellipse
spiral bar
spiral
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1. Spiral (disc-like) – has spiral arms
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2. Elliptical – nearly circular shape
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3. Irregular - no distinct shape
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Milky Way Galaxy• Spiral shape• 100 billion+ stars• Looks “milky” flat disk we view from inside & see stars in EVERY direction• Solar system completes one orbit in galaxy every 200 million years
• Made of:– Galactic disk: younger stars– Nuclear bulge: center; highest density of stars older
stars– Halo: around bulge & disk; thin gas; 90% of mass is
here
What does this mean about the formation of our galaxy?
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What’s at the center?
A galactic black hole• Found by motion of stars close to Sagittarius A
(near center)• Probably formed early in universe history
– Gas clouds and stars probably collided, formed a massive object, and then collapsed
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Where Are We?• The Orion arm
(about 3/4 of the way down)• How fast is our solar
system moving?– 220 km/s
• How long does it take our solar system to make one orbit around the Milky Way?– About 240 million years– about 20 times so far
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Milky Way
We Are Here
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Big Bang Theory• Theory that the universe began as a point and
has been expanding ever since– Thought to have begun as an infinitesimally small,
hot, and dense “singularity”.– About 14 (13.7) billion years ago
• Singularities are zones which defy our current understanding of physics– pressure is thought to be so intense that finite matter
is actually squished into infinite density
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Radiation Domination• After big bang
– Universe was hot & made of energy (radiation)
– Expansion & cooling sub-atomic particles formed (protons, neutrons, etc.)
– H+ nuclei started to form (still too hot for atom formation)
• Big bang theory constantly being tested– As new discoveries emerge– Scientists come up with
hypotheses & confirm over time using laws of science & infer what happened
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Expansion of the UniverseUniverse has two opposing forces:1. Momentum of outward expansion2. Gravity pushing inward to slow expansion
Momentum Expansion
Gravity Gravity
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• What happens depends on which of the forces is stronger
• Creates a period of expansion followed by cooling that continues to happen now
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Expanding Universe: Evidence1. Doppler Effect
– Red shifts of galaxies• Edwin Hubble (1929): galaxies shifting away light
waves stretched• Hubble’s Law:
– Objects moving away from Earth universe is expanding– Galaxies are retreating from Earth @ a speed that is
proportional to their distance» Greater red shifts= faster speeds
– Imagine dots on a balloon being blown up
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What do you notice about the galaxies?
Scientists have been able to track the distances between galaxies and how far away they are moving
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Expanding Universe Evidence2. Cosmic Background Radiation
• Discovered in 1965 (background noise in
radio antenna)• Shorter wavelengths
(when hot) that became longer (when cooled)
Weak radiation left over from the early, hotstages of the Big Bang expansion
• Today are microwaves (1 mm) in radio portion of electromagnetic spectrum
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Outcome of UniverseThree possibilities:1. Open
– expansion will never stop (density insufficient for gravity)
2. Closed– expansion stops and
begins to contract (density high enough gravity pulls mass in)
3. *Flat
– expansion slows to a halt, but does not contract
*widely accepted theory by scientists
Outcome depends on density of universe (unknown).
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Closed Flat Open
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Another View of the Universe’s Outcome
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Solar System vs. Galaxy vs. Universe?
• Solar System: Consists of the Sun, and everything bound to it by gravity. This includes the 8 planets and their moons, the asteroids, the dwarf planets, all the Kuiper belt objects, the meteoroids, comets and interplanetary dust.
• Galaxy: large system of stars held together by mutual gravitation and isolated from similar systems by vast regions of space. The Milky Way measures about 100,000 light-years across, and is thought to contain 200 billion stars.
• Universe: the totality of known or supposed objects and phenomena throughout space; the cosmos; macrocosm.
So to sum it up:• We live on planet Earth which is part of our local Solar System.
• Our Solar System includes the Sun and everything that orbits the Sun.• Our Sun, is just one Star in the Milky Way Galaxy.
• The Milky Way Galaxy is just one Galaxy in the Universe.