* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Astronomy 110 Announcements: 11.1 Properties of Stars
Dialogue Concerning the Two Chief World Systems wikipedia , lookup
Constellation wikipedia , lookup
Aries (constellation) wikipedia , lookup
Star of Bethlehem wikipedia , lookup
International Ultraviolet Explorer wikipedia , lookup
Canis Minor wikipedia , lookup
Dyson sphere wikipedia , lookup
Auriga (constellation) wikipedia , lookup
Corona Australis wikipedia , lookup
Corona Borealis wikipedia , lookup
Cassiopeia (constellation) wikipedia , lookup
Cygnus (constellation) wikipedia , lookup
Type II supernova wikipedia , lookup
Future of an expanding universe wikipedia , lookup
Star catalogue wikipedia , lookup
H II region wikipedia , lookup
Observational astronomy wikipedia , lookup
Canis Major wikipedia , lookup
Perseus (constellation) wikipedia , lookup
Aquarius (constellation) wikipedia , lookup
Timeline of astronomy wikipedia , lookup
Cosmic distance ladder wikipedia , lookup
Stellar classification wikipedia , lookup
Malmquist bias wikipedia , lookup
Stellar kinematics wikipedia , lookup
Stellar evolution wikipedia , lookup
Astronomy 110 Announcements: • 5 min. quiz on stellar properties – start of class • Reading for tomorrow: pp. 301 – 314, 329 – 330 (change from original schedule) • Homework #3 due Friday • Remember to hand in any extra-credit from observing 11.1 Properties of Stars • Our Goals for Learning • How luminous are stars? • How hot are stars? • How massive are stars? 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 The brightness of a star depends on both distance and luminosity (energy per second per square meter) Thought Question Thought Question These two stars have about the same luminosity -which one appears brighter? A. Alpha Centauri B. The Sun These two stars have about the same luminosity -- which one appears brighter? A. Alpha Centauri B. The Sun Luminosity passing through each sphere is the same The relationship between apparent brightness and luminosity depends on distance: Brightness = Area of sphere: 4! (radius)2 Divide luminosity by area to get brightness Luminosity 4! (distance)2 This is the inverse square law for light. We can determine a star’s luminosity if we can measure its distance and apparent brightness: Luminosity = 4! (distance)2 x (Brightness) Thought Question • We observe apparent brightness of stars • To determine the luminosities (total energy output per second), 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. B. C. D. It would be only 1/3 as bright It would be only 1/6 as bright It would be only 1/9 as bright It would be three times brighter Parallax: apparent motion of an object relative to the background due to change in viewing positions. Parallax is measured by comparing snapshots taken at different times and measuring the shift in angle to star Apparent positions of nearest stars shift by about an arcsecond as Earth orbits Sun Most luminous stars: Parallax angle depends on distance 106 LSun Least luminous stars: 10-4 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 spectrum that depends on its temperature 2) Hotter objects tend to emit light at shorter wavelengths and higher frequencies (bluer) ! Color gives us the star’s temperature Hottest stars: 50,000 K Luminosity depends on both the temperature and the size of a star. •An object of fixed size grows more luminous as its temperature rises. Coolest stars: 3,000 K •An object of fixed temperature grows more luminous as it gets bigger. (Sun’s surface is 5,800 K) (these temps. Refer to surface temp. only) 106 K 105 K 104 K Absorption lines in star’s spectrum tell us ionization level, which tells us its temperature. Ionized Gas (Plasma) 103 K Neutral Gas 102 K Molecules 10 K Solid Level of ionization also reveals a star’s temperature Stars of different temperatures will show different absorption lines (due to amount of energy available to ionize various elements). 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. B. C. D. M star F star A star 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 About half of all stars are in binary systems We can directly observe the orbital motions of these stars Sometimes we can only detect a “wobble” of one star (can’t see the fainter one). Eclipsing Binary •Stars orbit in the plane of our line of sight. •We can measure periodic eclipses Spectroscopic Binary We determine the orbit by measuring Doppler shifts We measure mass using gravity Direct mass measurements are possible only for stars in binary star systems p2 = 4!2 G (M1 + M2) p = period a3 a = average separation Need 2 out of 3 observables to measure mass: 1) Orbital Period (p) 2) Orbital Separation (a or r=radius) 3) Orbital Velocity (v) For circular orbits, v = 2!r / p v r Isaac Newton Most massive stars: 100 MSun Least massive stars: 0.08 MSun (MSun is the mass of the Sun) 11.2 Classifying Stars Our Goals for Learning • How do we classify stars? • Why is a star’s mass its most important property? • What is a Hertzsprung–Russell diagram? M Most of the brightest stars are reddish in color How do we classify stars? Main-sequence stars are fusing hydrogen into helium in their cores like the Sun Color and luminosity are closely related among the remaining “normal” stars Why are some red stars so much more luminous? They’re bigger! Biggest red stars: Luminous mainsequence stars are hot (blue) Less luminous ones are cooler (yellow or red) 1000 Rsun Smallest red stars: 0.1 RSun A star’s full classification includes: •spectral type (temperature - OBAFGKM) •luminosity class (related to the size of the star): I - supergiant II - bright giant III - giant IV - subgiant V - main sequence Examples: Sun - G2 V Sirius - A1 V Proxima Centauri - M5.5 V Betelgeuse - M2 I How does a star’s mass determine its luminosity? A more massive star needs more internal pressure to be in gravitational equilibrium !Thus, core temperature is higher. !Higher temps lead to more nuclear reactions ! higher luminosity