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PHYS 183
Tracy Webb

CHAPTER 15: STARS  Hertzsprung-Russell diagram: graph of every single star in the universe based on stellar properties o can place every star on diagram if distance is known o horizontal axis = stellar surface temperature, corresponding to spectral type  temperature decreases left to right o vertical axis = stellar luminosity o provides direct information about stellar radii since luminosity depends on temperature & radius o 4 groups  main sequence, supergiants, giants, white dwarfs o shows us: temperature, colour, luminosity, radius, mass & lifetime o can use this diagram to determine age of the universe  no star should be older than the universe  main sequence stars: prominent streak running from upper left to lower right on an HR diagram, includes the sun; star born with limited supply of core H so remains a H-fusing, main sequence star for limited time o lifetimes depend on mass (determines how much H fuel star contains in core) & luminosity (determines how rapidly star uses H) o low mass stars are cooler & fainter but longer lifetimes  less H to burn so done slower o high mass stars are hotter & brighter but shorter lifetimes  more H at higher temperature  the primary property of a star is mass  luminosity-mass relation: the luminosity of a star is proportional to it’s size & temperature o L = 4 π r T o the more massive a star is, the more luminous it is o a little increase in mass gives a huge increase in energy o as you increase radius of a star, increase luminosity by a factor of 16  luminosity classes: region on the HR diagram in which a star falls; more closely related to size than to luminosity o 4 luminosity classes: supergiants, giants, main sequence & white dwarfs o white dwarfs  very hot but very low luminosity since small radius o giants & supergiants  cold but very high luminosity since large radius  if star runs out of H in core, the core contracts & heats up which puffs up outer layers of star  stellar evolution o start with main sequence star which is burning H in the core o starts to run out of H so new fusion reactions occur creating giants & supergiants o white dwarfs occur when the star is entirely out of fuel  hot  essentially exposed stellar cores  dim  lack an energy source & only radiate leftover heat into space  star cluster: a bound group of stars which formed at the same time out of the same gas cloud o all stars in cluster lie at same distance from earth o all stars in cluster formed at the same time o can use the HR diagram to determine age of a star cluster  globular cluster: densely packed star cluster o mostly found in the halo of the Milky Way o very old & very large clusters  1 million stars o gradually lose stars & grow more compact  velocities of stars increase when passing other stars in cluster & some reach escape velocity o some of the oldest objects in the universe  current estimate between 11-18 billion years (uncertainty range, not age range) but good agreement says universe ~14 billion years old  main sequence turn-off: precise point on the HR diagram that stars diverge from main sequence CHAPTER 16: STAR BIRTH  nebula: a large cloud of gas in the universe  dark patches in the Milky Way (block the light)  interstellar medium: the gas and dust between stars; place where you find a lot of extra energy or energy that is being blocked  molecular clouds: interstellar clouds that are particularly cold & dense that stars are born in o most of the matter in form of molecules o usually have very distinct boundaries/edges o very cold & very dense  interstellar dust: dust made of tiny solid particles of C, Si, Fe & O o larger than molecules & made of heavier elements o also blocks light from stars behind them  interstellar reddening: dust is better at blocking blue light than red light, so detect reddening from Doppler shifts (doesn’t change wavelength of star’s spectral lines), so compare observed colour to colour expected of that spectral type o amount of reddening tells how much dusty gas lies between earth & star o general ring of apparently red stars at edge of molecular cloud o visible light from new star trapped in dense & dusty cloud where it was born  dust grains in gas glow in infrared since they absorb energy from new stars  stars form due to collapses of high density areas in clouds of gas  can only form if gravity can overcome thermal pressure of gas  formation of a star: o turbulent gas cloud has random motions in it causing it to become lumpy o if gravity can overcome pressure in dense regions, collapse forming denser lumps of matter o loses potential energy as it collapses & turns into thermal energy  star heats up & creates pressure o large cloud fragments into many smaller lumps of matter (forms cluster of stars bound together by gravity) o each lump can form one or more new stars  there is a constant war between gravity & pressure o gravity pushes inward, pressure pushes outward o cooling happens when emission lines relieve thermal pressure o thermal energy converted to light & is radiated away o allows cloud to collapse even further  halting c
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