T HE S UN AS A S TAR Susan Cartwright University of Sheffield

W HAT DO WE KNOW ABOUT THE S UN ? 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

T HE S UN ’ S COMPOSITION AND TEMPERATURE The spectrum of the Sun tells us both its composition and its temperature

H OW THE S UN GENERATES ENERGY Fusion of hydrogen to helium How does this work? He atom weighs slightly less than 4 H atoms (0.7%) E = mc 2 How do we know? Only mechanism that lasts long enough! We detect neutrinos

T HE S UN AS A S TAR Joe Morris,

H OW TYPICAL IS THE S UN ? 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

T HE S UN ’ S LIFE AND DEATH : Y OUTH 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

T HE S UN ’ S LIFE AND DEATH : Y OUTH

T HE S UN ’ S LIFE AND DEATH : P RIME 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??

T HE S UN ’ S LIFE AND DEATH : O LD A GE 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

T HE S UN ’ S LIFE AND DEATH : O LD A GE

T HE S UN ’ S LIFE AND DEATH : D EATH 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

T HE S UN ’ S LIFE AND DEATH : D EATH

T HE 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

T HE L IFE AND D EATH OF S TARS Simulations from /ddr/stellar/beginner.html

P LANETARY S YSTEMS

O THER S TARS WITH P LANETS 276 planets detected to date!

C ONCLUSIONS 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

G LIESE 581 Probably like Venus Possibly habitable!