The Sun as a Star

1. The Sun as a Star Susan Cartwright University of Sheffield

2. What do we know about the Sun? • Its size and its distance from us • Its luminosity (energy generated per second) • Its (surface) chemical composition and temperature • How it generates energy • Its (approximate) age • from radioactive dating of solar system material • Its history and future • from our understanding of stars in general

3. The Sun’s composition and temperature • The spectrum of the Sun tells us both its composition and its temperature

4. How the Sun generates energy • Fusion of hydrogen to helium • How does this work? • He atom weighs slightly less than 4 H atoms (0.7%) • E = mc2 • How do we know? • Only mechanismthat lasts long enough! • We detect neutrinos

5. Joe Morris, http://joemorris.mystarband.net/ The Sun as a Star

6. How typical is the Sun? • Compare Sun with nearby stars • It is on the main sequence • It is brighter and more massive than average • but fainter and less massive than most bright naked-eye stars • It is not a binary • this is not rare, but probably more than half of all stars are binaries • It has planets • this is probably very common

7. The Sun’s life and death: Youth • The Sun was born from a giant cloud of cool gas • this contracted under gravity • as it contracted it heated up • eventually the core reached 10 million degrees and hydrogen fusion began Most of following images taken from http://outreach.atnf.csiro.au/education/senior/astrophysics

8. The Sun’s life and death: Youth

9. The Sun’s life and death: Prime • The Sun’s main sequence lifetime is 10 billion years • Less massive stars live longer • More massive stars live less long • The Sun gets slightly brighter as it evolves on the main sequence • “faint young Sun problem” • Why didn’t early Earth freeze solid??

10. The Sun’s life and death: Old Age • When the Sun has used up its core hydrogen, it will become a Red Giant • Red giants still use hydrogen as fuel, but outside the (pure helium) core • The helium core will get larger and hotter • Eventually it will reach 100 million degrees and helium fusion will begin • this is much less efficient and will not last as long

11. The Sun’s life and death: Old Age

12. The Sun’s life and death: Death • As helium fusion continues, the Sun will shed its outer envelope • the hot inner core will be revealed as a White Dwarf • about 0.6 times the Sun’s current mass, but only the size of Neptune

13. The Sun’s life and death: Death

14. The lives of other stars • Stars up to about 8 times the Sun’s mass live and die like the Sun • only more massive ones do it faster • Stars more than 8 times the Sun’s mass can fuse heavier elements • they are responsible for making most of the Periodic Table! • But they die young in a spectacular explosion called a Supernova

15. The Life and Death of Stars Simulations from http://rainman.astro.uiuc.edu/ddr/stellar/beginner.html

16. Planetary Systems

17. Other Stars with Planets • 276 planets detected to date!

18. Conclusions • The Sun is a star • a little brighter than most • a bit more massive than most • not a binary • Studying the Sun can tell us about stars • it is far closer than any other star and can be studied in much more detail • Studying other stars can tell us about the Sun • range of ages, masses and chemical composition available for study • planets are common – range of very different planetary systems can be studied

19. Gliese 581 Probably like Venus Possibly habitable!