1. The Universe
Chapter 20 Pg

2. What are Stars?
Stars: huge spheres of very hot gas that emit light and radiation

3. Distances of Stars Stars are located at various distances from Earth
Light-year: distance light travels in one year or 9.5 x 1012 m

4. Stars are formed from clouds of dust and gas, or nebulas, and go through different stages as they age

5. Stars are born in a nebula.
After billions of years, most stars become old and lose their outer layers of gas.
As they begin to die, they may become a planetary nebulaes.

6. Nuclear Fusion Reactions Power Stars
Stars are composed of very hot hydrogen and helium gas that emits light
They are held together by enormous gravitational forces
Stars get their energy from nuclear fusion taking place in the core

7. Hydrogen atoms collide and fuse to form new nuclei called deutrons (one proton and one neutron)
Another proton collides with the deutron to form a helium isotope.
Each time two particles fuse, energy is released

8. Energy moves slowly through the layers of a star
Convection: hot gas moves away from the star’s core
Radiation: energy is transferred to individual atoms

9. Studying Stars
How Giant Telescope Work Video – See Additional Resources Section
Telescopes have allowed astronomers to accurately study stars
Brightness of a star depends on temperature of star

10. Color and Temperature of Stars
Hotter objects have more intensity and shorter wavelengths (blue end of spectrum)
Cooler objects have less intensity and longer wavelengths (red end of spectrum)

11. Life Cycle of Stars Nebula: invisible cloud of gas and dust
Protostar: forms when nebula collapses and begins to spin

12. Main Sequence Star: converts hydrogen into helium in core
Red Giant: large, reddish star late in life, surface is relatively cool while the core is hot

13. White Dwarf: small, dim, very dense star, no longer fuse elements, slowly cool.
Supernova: core collapses because of gravity, rebounds with a shock wave that violently blows the star’s outer layers away from core.

14. Neutron Star: remnant of a supernova, very massive, dense, can be detected as pulsars (spinning neutron stars that emit radio waves)
Black Hole: consists of matter so massive and compressed that nothing can escape its gravitational pull

15. H-R Diagram Pg. 701 of Textbook Shows how stars evolve Compares
Temperature
Absolute magnitude: how bright stars would be if they were the same distance from Earth
Luminosity: total energy output
Pg. 701 of Textbook

16. The Milky Way and Other Galaxies
Section 20.2

17. Galaxies Galaxy: Collection of millions or billions of stars
Young stars are often found near the nebular gas and dust where they were born
Older stars may be found throughout the galaxy or in regions that contain no gas and dust

18. Gravity holds galaxies together
Clouds of gas come together and collapse to form stars
Solar system revolves around the center of the galaxy because of gravity
Takes about 226 millions years to complete one orbit of our galaxy

19. Clusters of Galaxies
Galaxies are grouped together and bound by gravity in clusters
Local Group is a cluster of more than 30 galaxies, including the Milky Way galaxy

20. Spiral Galaxies Have a huge bulge in center
Nucleus is very dense and has many old stars
Spiral arms have gas and dust between the stars (interstellar matter) that provide material for new stars to form

22. Elliptical Galaxies No spiral arms Spherical or egg shaped
Often reddish in color
Contain mostly older stars

24. Irregular Galaxies Lack regular shapes
Do not have well-defined structures
Some contain little interstellar matter, while others have large amounts
More dwarf irregular galaxies than giant galaxies
Often found near larger galaxies and may be oddly shaped due to gravitational pull of nearby galaxies

26. How Galaxies Change Over Time
Scientists do not know what galaxies look like now, but by studying closer galaxies that might be similar to ancient ones, they can slowly piece together the puzzle of how galaxies evolve
When scientists observe distant galaxies, they are looking back in time (light year?)

27. Quasar Quasar: most distant and most radiant objects in space
One explanation: quasars are enormous black holes surrounded by large disks of gas and dust
May be central parts of distant galaxies seen as they were when very young

28. Galaxies Change Over Time
As galaxies consume their gas and dust, they become unable to make stars
As galaxies approach one another, the gravitational attraction changes their shapes

29. Origin of the Universe
Section 20.3

30. What is the Universe?
Universe: consists of all space, matter, and energy that exists – now, in the past, or in the future.
We see the universe now as it was in the past
It takes time for light to travel through space, the farther an object is, the older the light we get from that object
The sun is 8 light-minutes away, we are expressing not only its distance from Earth, but also that we see the sun as it was 8 minutes ago.

31. What Happened at the Beginning?
By studying ancient light from the most distant objects in the universe, and looking at other evidence, scientists have been able to theorize that the universe formed during a cataclysmic event known as the big bang.

32. The Universe is Expanding
Doppler Effect: change in the observed frequency of electromagnetic radiation when an object or observer is moving
Red Shift: Light waves emitted from an object moving away would be stretched out (towards the red end of the spectrum), while objects approaching us have shorter wavelengths (near the blue end of the spectrum)

33. Most galaxies have red shifts and galaxies that are farther away have greater red shifts.
Almost every galaxy is moving away from Earth, therefore, galaxies are also moving away from each other --- the universe is expanding

34. What does this mean?
Expansion implies that the universe was once smaller.
If every galaxy normally moves away from every other galaxy, then as time goes backward, the galaxies move closer together
Long ago, the entire universe might have been contained in an extremely small space

35. Did the universe start with a big bang?
Big Bang Theory: states that the universe began with a gigantic explosion 13 billion to 15 billion years ago
Nothing existed before the big bang
There was no time and no space
The explosion released all of the matter and energy that still exists in the universe today

36. Cosmic Background Radiation
Cosmic Background Radiation: steady but very dim signal from all over the sky in the form of radiation at microwave wavelengths
Supports the big bang theory
Dim remnants of the radiation produced during the big bang.
Cosmic Background Radiation segment starts at 5:00 min on video, watch the whole 9 min video if you want, it’s good regardless.
Big Bang – Cosmic Background Radiation Video
– See Additional Resources Section

37. Big Bang Theory is Constantly Being Tested
As new information emerges, the big bang theory may be refined and revised, or a new theory may become more widely accepted by astronomers
Other theories for the origin of the universe have been proposed over the last 100 years, but the big bang is the one that is best supported by current evidence

38. Future of the Universe Three possible outcomes for the universe:
The universe will keep expanding forever
The expansion will gradually slow down and the universe will approach a limit size
The universe will stop expanding and start to fall back on itself

39. Future of universe depends on mass