1. Heirarchy of the Universe
Middle School Science(summer 2009)‏

2. The Heirarchy
Stars
Star Clusters
Galaxies
Galactic Clusters

3. Sun/Stars
Most common star we are familiar with is our sun, but the sun is only 1 of more than 300 billion stars in our galaxy(The Milky Way).
Take this into account, that the Milky Way is only 1 of an estimated 100 billion galaxies in the Universe
So the sun is only 1 of approximately 30,000,000,000,000,000,000,000 stars in the known universe(estimated)‏

4. Sun/Stars So the sun is quite unremarkable:
-it is of average size and temperature compared to most stars
-it falls into the classification of being a main sequence star(virtually all stars fall into this classification)‏

5. Sun/Stars
Because Main Sequence stars are so common and our sun happens to be one it seems important to know what will happen to the sun as it ages.
This leads us to our big goal of the day. By the end of the period you should be able to describe the life cycle of main sequence stars from their formation to their eventual death.
Why is this important?
Our Sun is a main sequence star, so let's continue!

6. Main Sequence Stars
Main Sequence Stars like our Sun have 4 main phases in their lifecycle and each will be described in detail today. The phases in order are as follows:
1)Protostar
2)Yellow dwarf
3)Red Giant
4)Nebula/Supernova

7. Main Sequence Stars Protostar
--1st Phase in the life cycle(Before a star forms)‏
-loosely packed molecular cloud/gases
-Stars don't just form from nothing, they usually form using recycled gases from stars that have burned out
-no fusion reactions(fuel being burned) so it's relatively cool

8. Main Sequence Stars Yellow Dwarf -2nd phase(lasts 10 billion years)‏
-small to medium mass stars
-our sun is in this category
-surface temperature approximately 10,000 degrees F)‏
-fuel/energy/heat comes from a nuclear fusion reaction; 2 hydrogen atoms fusing into 1 helium atom
-enough hydrogen for about 10 billion years of fuel

9. Main Sequence Stars Red Giant
-occurs at the end of a stars life cycle(after all the hydrogen in the core has fused into helium)‏
-nuclear fusion now occurs at the outer shell of the star
-the star expands to a size much larger than our sun(up to times larger in diameter
-much cooler than our sun(under 10,000 degrees F) on the surface
-This will happen to our sun in roughly 5 billion years. The Sun will engulf the inner planets and could possibly engulf the earth

10. Fate # 1=Nebula(smaller to medium sized stars)‏
-shed their outer surface layers. These layers can be recycled in new star formation.
-the remaining core is very hot/bright and light from it reflects off the ejected outer layers(this is what created the beautiful image to the right)‏
-the core is now called a white dwarf (mass of the sun, size of the earth)=very dense
-the core will continue to cool over hundreds of trillions of years before becoming a black dwarf(emits no more light)-because no more hydrogen fuel remains for fusion reactions.
Most stars(including the sun) meet this fate(97% of all stars)‏

11. Fate #2=Supernova(larger stars)‏
-larger star exhausts all of it's hydrogen fuel(nuclear fusion)‏
-gravitationally collapse into a neutron star. Then stellar(star) material is violently ejected at velocities near the speed of light(explosion)‏
-a black hole or neutron star is left behind
-the ejected material may be used for new stars/planets
-These happen about once every 50 years in the Milky way(very rare) but because they are so violent they can destroy nearby star systems

12. Star Clusters
-groups of stars held together by each others gravitational pull. Same concepts that keep planets in orbit
may contain anywhere from 100 to 100,000 stars

13. Galaxies
-consists of stars, interstellar medium(gas/dust), stellar remnants, and dark matter.
-all components are gravitationally bound
-Estimated 100,000,000,000(100 billion) galaxies in the observable universe
-10 million – 1 trillion stars in each galaxy
-galaxy types(spiral, barred spiral, elliptical)‏

14. Spiral Galaxy consists of: -flattened disk
-stars forming(usually a 2 armed) spiral
-central concentration of stars(bulge)‏

15. Barred Spiral Galaxy consists of: -flattened disk
-stars forming(usually a 2 armed) spiral
-central concentration of stars(bulge)‏
-as the name implies there is a bar shaped concentration of stars in the galaxy

16. Elliptical Galaxy Ellipsoidal to spherical in shape
-very little interstellar matter(means very little new star formation)‏
-most of the stars are very old

17. Lenticular Galaxy also known as disc galaxies
-very little interstellar matter(very little new star formation)‏