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Download Time From the Perspective of a Particle Physicist
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The Nature of Stars • Measure properties of Stars Distance Mass Absolute Brightness Surface Temperature Radius • Find that some are related Large Mass Large Brightness • Determine model of stellar formation and life cycle PHYS 162 1 Stellar Sizes • For a few close, big stars seen in a telescope as non-point objects • Measure angular size; if know distance then get size of star Example: Betelgeuse 300 times larger radius than the Sun • If further away but a binary star, get size of stars when they eclipse each other length of time one star passes in front or behind each other PHYS 162 2 Binary Star Orbits - Eclipses PHYS 162 3 Stellar Sizes PHYS 162 4 Mass vs Luminosity always on these plots it is the Absolute Luminosity of the star High mass High brightness PHYS 162 5 Surface Temperature of Stars • peak wavelength tells temperature wavelength(max) = A/Temp • OR measure relative intensity at a few wavelengths like RED GREEN Easy to do BLUE PHYS 162 6 HST image. “add” together images taken with different color filters PHYS 162 7 Spectral Classes Light passing through star’s photosphere gives dark line absorption spectrum. Tells: • What atoms are present • Motion of the star by the Doppler shift of the absorption lines • temperature of the photosphere: relative intensity of different absorption lines amounts of different molecules and ions PHYS 162 8 Spectral Classes PHYS 162 9 Spectral classes originally ordered A,B,C,D… based on the amount of hydrogen absorption in the visible: • Now order by surface temperature Spectral Class Temperature O oh hottest B be A a Don’t need to F fine know G girl/guy K kiss M me coolest PHYS 162 10 HertzprungRussell Diagram Plot Luminosity versus surface temperature PHYS 162 11 HertzprungRussell Diagram Stars with larger sizes are brighter then a smaller star with the same surface temperature PHYS 162 12 Temperature vs Luminosity vs Radius of Stars Energy emitted by surface of star due to EM radiation is Energy/area = sT4. Examples • Two stars. Same temperature and radius same Luminosity • Two stars. Same temperature. Radius(B) = 2xRadius(A). So surface area(B)= 4xsurface area(A) Luminosity(B)= 4xLum(A) Radius = 1 radius = 2 PHYS 162 13 Temperature vs Luminosity vs Radius of Stars Energy emitted by surface of star due to EM radiation is Energy/area = sT4. Examples • Two stars. Same radius. Temperature(B) = 2xTemp(A). (Energy/Area)B = 24(Energy/Area)A or (Energy/Area)B = 16x(Energy/Area)B Luminosity(B) = 16xLum(A) Temp = 6000 Temp = 12,000 PHYS 162 14 Hertzprung-Russell Diagram • Most stars are on a “line” called the MAIN SEQUENCE with hot surface temp large radius medium temp medium radius cool surface temp small radius • Stars with cool surface temperature but very large radius: RED GIANTS • Stars with hot surface temperature but very small radius: WHITE DWARVES PHYS 162 15 PHYS 162 16 Spectroscopic Parallax • If we use well-understood close stars to determine the overall brightness scale of a specific class of star, then measuring the spectrum can be used to give the distance for stars > 500 LY away 1. Determine Surface Temperature + spectral class of star 2. Determine where on HR diagram should go 3. Read off absolute luminosity from HR diagram 4. Measure apparent luminosity and calculate distance • works best if many close-by stars PHYS 162 17 Stars: Birth, Life and Death • Stars are formed from interstellar material which is compressed by gravity • spend >90% of their lives burning Hydrogen into Helium and on main sequence • how they “die” depends on mass large stars blow up Supernovas • understand stars’ lifecycles by studying their properties and also groups of stars PHYS 162 18 Nebula Historic term for any extended patch of light • • • • • • • galaxy comets star clusters supernova remnants material ejected from Red Giants gas clouds dust clouds PHYS 162 19 Stars: Birth, Life and Death • Stars are formed from interstellar material which is compressed by gravity • spend >90% of their lives burning Hydrogen into Helium • how they “die” depends on mass large stars Supernovas neutron stars/black holes stars like our Sun end up as white dwarves PHYS 162 20 Interstellar Medium • • • • Interstellar space is filled with Gas (mostly H and He) Dust (silicates, ices) and molecules (even complicated like sugars, alcohol and amino acids) usually cold (100O K or -300O F) usually almost perfect vacuum with 1 atom/cm3 (1 g water = 1023 atoms) Local concentrations: compressed by gravity and form stars. Called Giant Molecular Clouds as molecules have been observed. Need about 1,000,000 times the mass of the Sun in 100 LY volume to initiate star formation PHYS 162 21 Emission Nebula • • • • If gas cloud heated up by being near stars, will emit light and spectrum tells: chemical composition temperature density velocity (by Doppler shift) PHYS 162 22 Dust Clouds If dense gas and dust (very small particles) between stars and us see as dark image Horsehead nebula • IR can often see through • regions were new stars are being formed PHYS 162 23 Star Forming Region Eagle nebula PHYS 162 24 Star Formation STEPS 1. Collapsing Gas Cloud 2. Protostar: hot ball but no fusion 3. Star: nuclear fusion but not final equilibrium 4. Main Sequence Star: final equilibrium with excess gas blown away PHYS 162 25 Star Formation gas cloud protostar Star PHYS 162 equilibrium 26 Gravity and Star Formation • • • • • gravity causes the material (gas and dust) in a cloud to be attracted to each other compresses into smaller volume increases temperature and density If the temperature at the center becomes large enough (5 million degrees) then H to He fusion can occur: Star is born Many stars formed from same cloud PHYS 162 27 Star Clusters stars are usually near other stars - CLUSTER • formed at the same time • similar chemical composition • about the same distance from us Can classify by appearance and use to: • study stellar lifetimes • measure distances (spectroscopic parallax) PHYS 162 28 Open Star Clusters can see individual stars by eye or modest telescope • usually some bright, hot stars • 100-1000 stars in region of about 50 LY with few LY separating stars • have significant amount of heavy elements like Carbon and Oxygen Understood as group of recently formed stars PHYS 162 29 Open Star Clusters - Pleiades “Seven Sisters” being chased by Orion the hunter (Greek) Subaru cluster (Japan) PHYS 162 Subaru Telescope30Hawaii Globular Star Clusters “fuzzy cotton ball” by eye or modest telescope • usually dim red stars • dense with 100,000 stars in 50-300 LY region with less than LY separating stars • no heavy elements. Just Hydrogen and Helium • often outside plane of galaxy Understood as group of old stars formed in early history of the galaxy PHYS 162 31