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Astronomy 110 Announcements: 11.1 Properties of Stars

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Astronomy 110 Announcements: 11.1 Properties of Stars Astronomy 110 Announcements: 11.1 Properties of Stars • 5 min. quiz on stellar properties – start of class • Our Goals for Learning • Reading for tomorrow: pp. 301 – 314, 329 – • How luminous are stars? 330 (change from original schedule) • How hot are stars? • Homework #3 due Friday • How massive are stars? • Remember to hand in any extra-credit from observing They Might Be Giants - Sunshine Luminosity (~Intrinsic brightness): Amount of power a star radiates (energy per second=Watts) Apparent brightness: Amount of starlight that reaches Earth (energy per second per square meter) The brightness of a star depends on both distance and luminosity Thought Question Thought Question These two stars have about the same luminosity -- These two stars have about the same which one appears brighter? luminosity -- which one appears A. Alpha Centauri brighter? B. The Sun A. Alpha Centauri B. The Sun Luminosity passing The relationship between apparent brightness and through each sphere is luminosity depends on distance: the same Luminosity Brightness = 4! (distance)2 Area of sphere: This is the inverse square law for light. 4! (radius)2 We can determine a star’s luminosity if we can measure its distance and apparent brightness: Divide luminosity by area to get brightness Luminosity = 4! (distance)2 x (Brightness) • We observe apparent brightness of stars • To determine the luminosities (total energy output per second), Thought Question we need to know the distance to stars • So how do we determine the distance? How would the apparent brightness of Alpha Centauri change if it were three times farther away? A. It would be only 1/3 as bright B. It would be only 1/6 as bright C. It would be only 1/9 as bright D. It would be three times brighter Parallax: apparent motion of an object relative to the background due to change in viewing positions. Apparent positions of nearest stars shift by about an Parallax is measured by arcsecond comparing as Earth snapshots taken orbits Sun at different times and measuring the shift in angle to star Parallax Most luminous angle stars: depends on 6 distance 10 LSun Least luminous stars: -4 10 LSun (LSun is luminosity of Sun) How hot are the stars? Laws of Thermal Radiation 1) Hotter objects emit more light at all wavelengths Every object emits thermal radiation with a 2) Hotter objects tend to emit light at shorter wavelengths and spectrum that depends on its temperature higher frequencies (bluer) ! Color gives us the star’s temperature Luminosity depends on both the temperature and the size of a star. Hottest stars: 50,000 K •An object of fixed size grows more luminous as its temperature Coolest stars: rises. 3,000 K •An object of fixed temperature grows more luminous as it (Sun’s surface gets bigger. is 5,800 K) (these temps. Refer to surface temp. only) 106 K Level of ionization also reveals a star’s 105 K Ionized temperature Gas 104 K (Plasma) Stars of different temperatures will show 103 K Neutral Gas different absorption lines (due to amount of 102 K Molecules energy available to ionize various elements). 10 K Solid Absorption lines in star’s spectrum tell us ionization level, which tells us its temperature. Remembering Spectral Types (Hottest) O B A F G K M (Coolest) • Oh, Be A Fine Guy, Kiss Me Spectral types are further broken down to sub-classes by numbers from 0 to 9 (hotter to cooler) Lines in a star’s spectrum correspond to a spectral type that reveals its temperature (Hottest) O B A F G K M (Coolest) How massive are stars? Thought Question Which kind of star is hottest? A. M star B. F star C. A star D. K star The orbit of a binary star system depends on strength of gravity Types of Binary Star Systems Visual Binary • Visual Binary • Eclipsing Binary • Spectroscopic Binary We can directly observe the orbital motions of About half of all stars are in binary systems these stars Sometimes we can only detect a “wobble” of one star (can’t see the fainter one). Eclipsing Binary Spectroscopic Binary •Stars orbit in the plane of our line of sight. •We can measure periodic eclipses We determine the orbit by measuring Doppler shifts We measure mass using gravity Need 2 out of 3 observables to Direct mass measurements are measure mass: possible only for stars in binary star systems 1) Orbital Period (p) 4!2 2) Orbital Separation (a or r=radius) p2 = a3 v G (M1 + M2) 3) Orbital Velocity (v) p = period a = average separation r M For circular orbits, v = 2!r / p Isaac Newton Most massive stars: 11.2 Classifying Stars 100 MSun Our Goals for Learning Least massive • How do we classify stars? stars: • Why is a star’s mass its most important property? 0.08 MSun • What is a Hertzsprung–Russell diagram? (MSun is the mass of the Sun) Most of the brightest stars are reddish in color Color and How do we classify stars? luminosity are closely related among the remaining “normal” stars Main-sequence stars Why are some red are fusing stars so much hydrogen into more luminous? helium in their They’re bigger! cores like the Sun Biggest red stars: Luminous main- sequence stars are 1000 Rsun hot (blue) Smallest red stars: Less luminous 0.1 RSun ones are cooler (yellow or red) A star’s full classification includes: How does a star’s mass determine its •spectral type (temperature - OBAFGKM) luminosity? •luminosity class (related to the size of the star): A more massive star needs more internal pressure to be in gravitational equilibrium I - supergiant !Thus, core temperature is higher. II - bright giant III - giant !Higher temps lead to more nuclear reactions ! higher luminosity IV - subgiant V - main sequence Examples: Sun - G2 V Sirius - A1 V Proxima Centauri - M5.5 V Betelgeuse - M2 I.
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