Chapter 15 (last chapter of the year!!!)- Stars, Galaxies, and the Universe
CUE WORDS or QUESTIONS
What are constellations?
NOTES WRITTEN
Constellations are groups of stars that make imaginary patterns

How are stars different from
each other?
Different constellations can only be seen at certain times of the year,
when they aren’t blocked out by the sun
Stars are huge spheres of glowing gas produced by nuclear fusion and are
classified by color, temperature, size, composition, and brightness
1. Color- stars are different colors depending on their temperatures
2. Temperature- the hottest stars burn blue, while the coolest stars burn
red
3. Size- stars are different temperatures colors depending on their size
(larger stars are hotter, smaller stars are cooler)
4. Composition- scientists determine the gases in star by using a
spectrograph- an instrument that separates light into wavelengths
and shows which part of the wavelength are absorbed and which are
emitted
5. Brightness- the amount of light stars give off (depends of size and
temperature)
How is distance measured in
space?
Light-year- unit used to measure distances between stars and galaxies

1 light-year = 9.5 million million km (the distance light travels in one
year)

Proxima Centauri is our the closest star to Earth at
39,900,000,000,000 km, or 271,000 AU, or 4.2 light-years away
What is the H-R diagram?
Hertzsprung- Russel diagram (H-R diagram)- graph that classifies stars
and shows how they change over time
What is the life cycle of a
star?
All stars are born from nebulas but their ‘death’ depends on their size
1. Nebula- a cloud of dust and gas
2. Protostar- contracting cloud of dust and gas
3. Main sequence star- typical star, like our sun, that makes energy via
Hydrogen fusion


SUMMARY
Cool stars are dim
Hot stars are bright
What is the fate of our sun?
What is the fate of high mass
stars?
What is a galaxy?
What are the types of
galaxies?
Death of low to medium mass stars (like our sun):
1. Red giant- large bright star with a low surface temperature
2. Planetary nebula- glowing cloud of gas
3. White dwarf- core of a star after planetary nebula (hot but not bright)
Death of high mass stars (bigger than our sun):
1. Super red giant- very large bright star with a low surface temperature
2. Supernovas- sudden explosion caused by the star not making energy
3. Neutron star- smaller and more dense than white dwarfs
4. Pulsars- spinning neutron star
5. Black holes-region in space with an intense gravity (no light escapes)

only the largest stars become black holes

scientists believe that at the center of every galaxy is a black hole
Galaxies- groups of billions of stars
Types of galaxies:
1. Irregular galaxy- look irregular (no shape), has young stars
and lots of dust and gas
2. Spiral galaxy- look like a pinwheel, has young stars & dust and gas
 Our galaxy (the Milky Way galaxy) is a spiral galaxy
 The Milky Way is 100,000 light years across
3. Elliptical galaxy- look round or oval, has old stars & no dust and gas
How big is the universe?
What is the Big Bang theory?
What is the solar nebula
theory?
What is the fate of the universe?
SUMMARY
Universe- all of space and everything in it
 The universe is 1010 light years across
Big Bang theory- the universe was at one time a small speck and there was
a big explosion about 14 billion years ago
Evidence of big bang:
1. The galaxies are all moving away from each other
2. There is background radiation coming from every direction in space
3. Based on speed of galaxies scientists can estimate how long it has
been expanding for
Solar nebular theory- ~5 billion years ago our star formed from a nebula
The universe will continue to expand forever
(the end)