L=4 R 2 SBT 4. Galaxies (AS 7007) The physics of stars. A closer look at the physics of stars

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1 Galaxies (AS 7007) Autumn 2007 Teacher: Göran Östlin Assistant: Jens Melinder Lecture 3, contents: - Stellar physics basics - Impact of stellar evolution on galaxies - Properties of local galaxies A closer look at the physics of stars Galaxies are made up of stars (among other things) Stars make the light that make us see galaxies We need to understand the physics of stars and their evolution in order to undertstand galaxies Sun is the reference: R, L, M Dwarfs (e.g. Sun) and Giants: differ in R, L and M Dwarfs = main sequence stars (hydrogen burning) Giants = evolved inflated stars The physics of stars Hydrogen (H,X), Helium (He,Y), Metals (N>3,Z) Hydrostatic equilibrium: gravity balances pressure If central Temp sufficiently high fusion reactions (H He) will take place. This is determined by mass. If M < 0.08 M then no fusion => Brown dwarf Upper mass limit, approx 100 M Luminosity of black body L=4 R 2 SBT 4 Stellar spectra are black bodies modified by absorption in stellar atmospheres Defines effective temperature: T eff Stars born collectively from gravitationally unstable molecular clouds. Low and high mass stars together, their relative proportions set by the IMF 1

2 Stellar evolution Any star spend most of its life on the so called main sequence fusing hydrogen to helium. 0.7% goes to E=mc 2 L M α, α = 2.2 to 5 on main sequence Massive stars run out of fuel faster! (metallicity effect) Luminosity Herzsprung-Russel / Colour-Magnitude Diagram HR diagram / CMD After hydrogen is exhausted the core of the star contracts and its outer parts expand => red luminous giant If the temperature gets high enough in the core, then He Can be fused into Oxygen, etc. This depends on the mass Of the star, but there is a limit AGB stars (pulsating) give WD, massive stars give SN blue Temperature red Iso-chrones / Stellar populations Metal-rich Stellar evolution time scale is set by Mass Z = metallicity (mass fraction of elements heavier than He) Positions of stars of given age RGB more sensitive to metallicity than age (star climbs RGB until degenerate He core ignites which occur at same He mass standard candle & metallicity indicator) Not to be confused with redshift: z Metal-poor 2

3 End points of stellar evolution: Planetary nebulae white dwarf Nucleosynthesis in massive stars Succesive burning requires higher temperature, hence less extended burning zones Fe is most stable element no further burning possible End points of stellar evolution: (core collapse) supernova neutron star or black hole SN1987A A supernova may outshine an entire galaxy 3

4 Star formation Hydrostatic equilibrium: Stars are born in dusty molecular clouds Dust allows the gas to cool Sound cross time Free fall time Collapse if t S = R /c S t ff =1/ G" t ff < t S M Jeans = c 3 S /(G 3 / 2 " 1/ 2 ) Jeans mass for typical molecular clouds solar masses Contraction leads to decreasing M jeans and fragmentation Star formation is collective => populations Unveiling the IMF in the solar neighbourhood is more complex due to the presence of many stellar generations IMF of the Pleiades young star cluster Star clusters = single stellar populations 4

5 R136 cluster of young stars in 30 Dor (LMC) Young massive stars in R136 (Massey & Hunter) R136 is the central star cluster in the 30 Dor starburst complex IMF: Salpeter (1955): dn " M #$ dm $ = 2.35 Galaxy spectra: composite of all stars present, weighted by their luminosity Massey & Hunter Work on R136 confirmedsalpeter Slope for young masssive stars + gas (cold, warm, hot) + dust (BB) Massive stars are rare but blue and luminous, die young, produce and return lots of metals Low mass stars are numerous red and faint, live long but don t return much - Mass sink 5

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7 Stars and their impact on galaxies: Summary Mass - luminosity colour relation Mass vs lifetime Stellar death and ISM enrichment IMF and stellar populations Photometry: the magnitude system Galaxy spectra: a time integral of IMF, stellar evolution and brithrate The local galaxy population Isolated galaxies (cf field galaxies) Groups Clusters Superclusters Stars: main generator of electro-magnetic radiation (reradiated by dust and ionised gas) 7

8 Galaxy head on colission Peculiar galaxies often result from interaction/merging HI Cen A E0 pec and Radio Galaxy E0 galaxy with dust lane due to eaten disk galaxy Antlia dsph Very low surface brightness makes the galaxy hard to see at all Radio lobes due to central AGN 8

9 M87 another peculiar giant Elliptical, here due to a jet Ground based HST Detail from Stephans quintet - a compact group where galaxies tug and tear each other M82 ionised gas (red) outflows along minor axis due to SN feedback 9