1. Survey of the Universe Tom Burbine tburbine@mtholyoke.edu

2. Quiz #4 This Wednesday Covers up to last Wednesday Cumulative You can bring in one 8 ½ by 11 inch piece of paper with anything written on it

3. Life of a Star A star-forming interstellar cloud is called a molecular cloud because low temperatures allow Hydrogen to form Hydrogen molecules (H 2 ) Temperatures like 10-50 K Contain Hydrogen and Helium with particles of silicates, carbon, and iron coated with irces

4. Region is approximately 50 light years across

5. Condensing Molecular clouds tends to be lumpy These lumps tend to condense into stars That is why stars tend to be found in clusters

6. Protostar The dense cloud fragment gets hotter as it contracts The cloud becomes denser and radiation cannot escape The thermal pressure and gas temperature start to rise and rise The dense cloud fragment becomes a protostar

8. When does a protostar become a star When the core temperatures reaches 10 million K, hydrogen fusion can start occurring

9. http://www.youtube.com/watch?v=7YhRW9kLghY

10. T-Tauri Stars Pre-main-sequence stars that vary erratically in brightness

12. Because nuclear reactions have not yet begun in the protostar’s core, this luminosity is due entirely to the release of gravitational energy as the protostar continues to shrink and material from the surrounding fragment

13. http://www.youtube.com/watch?v=W13ZYepDB vohttp://www.youtube.com/watch?v=W13ZYepDB vo http://www.youtube.com/watch?v=QFklLMB_Z OI&feature=relatedhttp://www.youtube.com/watch?v=QFklLMB_Z OI&feature=related

14. Brown Dwarfs Failed stars Not enough mass for fusion Minimum mass of gas need for fusion is 0.08 solar masses (80 times the mass of Jupiter)

15. Main Sequence Is not an evolutionary track –Stars do not evolve on it Stars stop on the main sequence and spend most of their lives on it

16. Hydrostatic Equilbrium

17. Fusion reaction in high mass -stars

18. Main Sequence Lifetimes The more massive a star on the main sequence, the shorter its lifetime More massive stars do contain more hydrogen than smaller stars However, the more massive stars have higher luminosities so they are using up their fuel at a much quicker rate than smaller stars

19. Main Sequence Lifetime (t) t = M/L x 10 10 years M is in Solar masses and L is in Solar Luminosities For example, Sirius has a mass of 2 solar masses and a luminosity of 20 solar luminosities t = (2/20) x 10 10 years = 0.1 x 10 10 years

21. Ages Universe is thought to be about 14 billion years old So less massive stars have lifetimes longer than the age of the universe More massive stars have ages much younger So stars must be continually forming

23. Blue stars have formed recently since they have young main sequence lifetimes

24. Core Uses up hydrogen When the core uses up its hydrogen Gravity causes the core to shrink The star leaves the main sequence

28. Any Questions?