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

2. HW #6 Due this Wednesday You will make a quiz covering material up to April 8th You need to have answers for each question 70 points –Use a similar style to my previous quizzes Cumulative You can’t use (or be too similar to) previous questions on quizzes You can’t use questions used by other students (no collaboration with other students in making your quiz) I will use the “best” quiz as your next quiz

3. If you are unhappy with any of your grades: You can write a 10 page paper on an astronomical subject to replace it 12 point font Times New Roman font Double space No figures or plots Due by May 1st

5. Star expands quickly Becomes more luminous because the star becomes bigger Because the star is expanding, the energy coming from the interior becomes more spread out The surface becomes cooler

7. Helium Fusion Helium can fuse into Carbon at temperatures above 100 million kelvin

9. Degenerate gas Core compresses But can only compress so far since electrons can not occupy the same space if they have the same energy This is called a degenerate gas

10. Stars less than 0.5 solar mass can’t compress any more Stars from 0.5-2 solar masses get so compressed that gravitational contraction and radiation from the fusion shell releases energy that is absorbed by some of the electrons The core can then compress more Helium fusion can occur

11. Variable Star Star whose luminosity changes over time

13. Lightcurves

14. Lightcurves plot brightness of a star versus time Period is the interval of time where a pattern of brightness repeats

15. Different Types of Variable Stars RR Lyrae Variables – Mass comparable to Sun but ~40 times more luminous Mira Variables – Red cool supergiants that have periods of many months to years Cepheid Variables –Yellow supergiants that are more massive than the Sun and ~10,000 times more luminous T-Tauri Irregular Variables – Pre-main sequence stars that appear to brighten as material falls sporadically onto the forming star

17. Giant stars pulsate because their atmospheres trap their radiated energy Heats their outer layers that expand The expanded gas cools and pressure drops Gravity then recompresses the gas

18. Cepheid Variables Obey a period-luminosity relation The more luminous a Cepheid is, the slower it pulsates

19. Why is there a relationship? Larger Cepheid Variables have weaker surface gravities than the smaller Cepheid Variables Gravity pulls Larger Cepheid Variables inward slower than smaller Cepheid Variables So Larger ones pulsate slower

21. Cepheids as Standard Candles Can be used as standard candle So luminous that you can determine distances to galaxies that are millions of light years distant You determine the pulsation period You can then determine its luminosity You can measure its brightness Then determine distance from inverse-square law B = L/4  d 2

22. http://www.youtube.com/watch?v=iyisAjHdhas

23. Any Questions?