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ASTR 200 : Lecture 15 Ensemble Properties of Stars 1 Midterm exam • • • • • In lecture, October 28 Will contain short answer questions and problems Formula sheet will be provided No calculators Will cover lectures and associated textbook readings listed on the course web site up to and including Friday Oct 21 • A previous year's midterm will be posted, but it is a different format and course topic order 2 Absence • I will be away Monday-Thursday inclusive at the Division of Planetary Sciences meeting • Monday and Wednesday's lecture will be given by Professor Hickson (last year's ASTR 200 instructor) • HW4 will not be due until FRIDAY Oct 21, 4 PM • TA office hours Wed+Thurs as normal; Prof Gladman's Thursday office hour will be moved to Friday Oct 21, 1:30-2:30 3 We can measure stellar T and L ● In 1910, Ejnar Hertzsrung and Henry Norris Russell both independently decided to look at the dependence of luminosity on stellar surface temperature. ● Note, in practice that getting luminosity is tricky, and they used 'spectral class' as a proxy for temperature (see ASTR 205) ● 4 The discovered that this was NOT a uniform diagram. ● Stars of a given temperature are not at all luminosities ● Given L = 4 R2T4 , one would think a star of fixed T could have any R, and thus any L, but no.... The HR diagram 100x Sun Luminosity Sun 0.01 Sun ~20,000 K 5 ~3,000 K Temperature The HR diagram 100x Sun Luminosity Sun M ain Se qu en ce 0.01 Sun ~20,000 K 6 ~3,000 K Temperature Hipparcos stars Relatively close stars, so rare very luminous stars are sparse ● Temperature (top scale) increases to the left! ● The main sequence is the most populated because stars spend most of their life there ● The most luminous main sequence stars have ~3000 solar luminosities ● 7 Mass-Luminosity relation • Text gives: 8 L~M3.9 for stars >0.7 solar masses L~M2.6 for stars <0.7 solar masses Mass-Luminosity relation • Text gives: 9 L~M3.9 for stars >0.7 solar masses L~M2.6 for stars <0.7 solar masses So 10x So larger stars burn much brighter • We haven't discussed the energy source yet • But, since the energy source is inside the star, it is reasonable to assume that the amount of energy E for the star's life is proportional to the mass • For large stars, L∝ M 4 is the rate energy is used • So the total lifetime t must go as M −3 t∝ 4∝M M • The Main Sequence lifetime of the Sun is ~10 Gyr so a 10 solar mass star will only live ~10 Myr (!) 10 Along the main sequence: ● Masses: ● 60 to 0.05 solar masses ●Lifetimes: ● ~Myr to a >trillion yr ● ● ● ● ● 11 Off the main sequence: Radii vary a lot Super giants ~1000 times Sun White Dwarfs ~0.01 time Sun Post main-sequence evolution Most stars will leave the main sequence and evolve to become giants for a while ● Driven by changes in energy production in core ● 12 The previous HR diagrams were for 'all stars nearby', which have a variety of ages • But star clusters exist, where all the stars are thought to have been formed in a short interval of time M11 open cluster 13 HR diagram for M11 star cluster Luminosity 14 Temperature Luminosity 15 This is the positions of a set of stars of a large mass range, all 200 Myr after start of collapse of their interstellar clouds, based on equations of stellar structure (next week) Temperature Cluster+stars Was formed ~200 Myr ago Luminosity 16 Temperature ? Cluster+stars Was formed ~200 Myr ago Luminosity 17 Temperature Gas Cloud → Main Sequence → ??? • So, a large cloud (1000s to ~million solar masses) gets cold enough that many cores collapse into stars, giving a cluster • Each star clears gas disk away, but the cluster as a whole also blows out all the remaining interstellar gas, shutting down star formation • The stars settle onto the main sequence for their `hydrogen burning phase' • When they finish, they leave and become giants, and then – Explode in supernovae (outshine a galaxy!) • leaving neutron stars or black holes behind – Turn into planetary nebulae, ejecting their outer layers • leaving a white dwarf 18 Post main-sequence evolution This is, thus, actually the `evolutionary track' on the HR diagram of a solarmass star ● 19