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The Nature of Stars

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.

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The Nature of Stars

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  1. 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

  2. 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

  3. Binary Star Orbits - Eclipses PHYS 162

  4. Stellar Sizes PHYS 162

  5. Mass vs Luminosity always on these plots it is the Absolute Luminosity of the star High mass  High brightness PHYS 162

  6. Surface Temperature of Stars • peak wavelength tells temperature wavelength(max) = A/Temp • OR measure relative intensity at a few wavelengths like RED GREEN BLUE  Easy to do PHYS 162

  7. HST image. “add” together images taken with different color filters PHYS 162

  8. 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

  9. Spectral Classes PHYS 162

  10. 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 F fine G girl/guy K kiss M me coolest Don’t need to know PHYS 162

  11. Hertzprung-Russell Diagram Plot Luminosity versus surface temperature PHYS 162

  12. Hertzprung-Russell Diagram Stars with larger sizes are brighter then a smaller star with the same surface temperature 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 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

  14. 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

  15. 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

  16. PHYS 162

  17. 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

  18. 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

  19. 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

  20. 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

  21. 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

  22. 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

  23. 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

  24. Star Forming Region Eagle nebula PHYS 162

  25. 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

  26. Star Formation gas cloud protostar Star equilibrium PHYS 162

  27. 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

  28. 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

  29. 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

  30. Open Star Clusters - Pleiades “Seven Sisters” being chased by Orion the hunter (Greek) Subaru cluster (Japan) Subaru Telescope Hawaii PHYS 162

  31. 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

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