Brief Astro Review

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Astronomy & Astrophysics
Michael Reid

AST201 15.2 Patterns Among Stars • Color = surface temperature (red: cooler/blue: hotter) • Luminosity is dependent on surface temperature, and surface area • If two stars have the same surface temperature, the larger will be more luminous • H-R Diagrams: - luminosity & spectral type are all that’s required to plot a star on H-R - surface temperature: x-axis (relates to spectral type) -temperature decreases from left to right - luminosity: y-axis (in units of the sun’s luminosity (sun) - each tick on the y-axis is 10 times that of the one before it - stellar radii are plotted diagonally - stellar masses decrease downward along the main sequence - stars cluster in 4 major groups: main sequence: -most stars fall along here (including the sun) - runs from the upper left corner to the lower right on the H-R supergiants: - upper right (very large, and bright) giants: - smaller in radius that supergiants - lower luminosity - still much larger and brighter than main sequence stars of same spectral type white dwarfs: - lower left - small in radius - white (due to their high temperature) • Luminosity Classes: - the region on the H-R in which a star falls - for the “in-between” stars (i.e. IV: subgaints, bigger than main sequence stars, but smaller than giants) I – supergiants II – bright giants III – giants IV – subgiants V – main-sequence stars * white dwarfs are classified as “wd” as they fall out of this system • Spectral Type: - designated by OBAFGKM (O: hottest & bluest/M: coldest & reddest) • Main Sequence: - All stars on the main sequence are fusing hydrogen to helium in their cores - m.s star’s mass determines its other properties because it sets the point at which the energy produced by fusion in the core equals the radiative energy output from its surface, which keeps gravity stable • Low mass stars are more common than high mass ones • Mass is the most important attribute of a hydrogen burning star • Only main sequence stars have a direct relationship between mass, temperature, and luminosity • Main sequence stars are born with a limited supply of core hydrogen, and therefore can only remain as main sequence stars until it runs out (main sequence lifetime) • Massive stars have shorter lives because they fuse their hydrogen into helium so fast • Their lifetime depends on mass (how much hydrogen they have) and luminosity (how much they use to make them brighter) • Giants and Supergiants: - bright red stars - upper right hand of the H-R - larger in radius than the sun (because they are cooler, but more luminous) - stars nearing the ends of their lives - they have already burned the hydrogen in their central cores - gravity threatens to crush them, so they emit fusion energy at high rates (this explains their high luminosity) - rarer than main sequence stars • White Dwarfs: - exposed stellar cores left behind by giants and supergiants - very hot - dim because they lack energy source - typically not larger than earth, but a mass similar to the sun’s (dense) • Pulsating variable star: - some stars can’t achieve a balance between the energy in the core and that being radiated from the surface - other stars have opaque surfaces that don’t allow energy to escape, so pressure builds up inside it, and pushes the outer layers out so the star expands (making it more luminous) - this makes the outer layers transparent enough to let out enough energy for the star to shrink (decreasing the star’s luminosity) and then the process repeats itself - this makes the star look like it’s blinking (they typically peak every 50 days) - they are found in the instability strip on the H-R diagram (between main sequence and the red giants) 15.3 Star Clusters • All stars within a cluster are formed relatively around the same time • There are 2 types of clusters: open clusters: - modestly sized - always found in the galactic disk - made up of younger stars - contain up to several thousand stars - are about 30 light-years across globular clusters: - found in the halo of
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