Stars: Stars and their Properties Astronomy 110 Class 10 WHEN I heard the learn d astronomer; When the proofs, the figures, were ranged in columns before me; When I was shown the charts and the diagrams, to add, divide, and measure them; When I, sitting, heard the astronomer, where he lectured with much applause in the lecture-room, How soon, unaccountable, I became tired and sick; Till rising and gliding out, I wander doff by myself, In the mystical moist night-air, and from time to time, Look d up in perfect silence at the stars. Walt Whitman Leaves of Grass Basic Properties Temperature Distance Luminosity Size Velocity Mass Age 1
Stellar Colors Not all stars same color Some red Some yellow Some blue Temperature Stars near Black Body Hotter is shorter Hotter is brighter Cooler is longer Cooler is fainter Spectral Type 2
Spectral Type Originally alphabetic by Hydrogen strength O B A F G K M O stars hottest ~50,000 K M stars coolest ~3,000 K Sun is G2 Spectral type independent of Distance Why Different Lines Lines depend on excitation more than abundance 30,000 K no lines 10,000 K strong hydrogen 5,000 K 3,000 K weaker hydrogen molecules Distance All stars too far for direct measurements Triangulation (parallax) used for nearest Parallax 3
1 Arc Second Arc seconds used to measure small angles Sometimes denoted π 1 arc-second is 1/3600 of a degree Dime at 2 miles Parsec Astronomical distances 3.26 Light Years Parallax Arc Second D = 1/p Limitations of Parallax Nearest stars only 300 light years Hipparcos satellite catalog 118,000 Future surveys will go out ¼ of galaxy 4
Luminosity Apparent brightness How bright star seems to us Intrinsic Luminosity Power emitted by star You have to know distance Inverse Square Law Light spread out as distance increases Stars get dimmer as they get farther Solar Luminosity Sun s Luminosity is 3.8 x 10 26 Watts Know as 1 L O For star 1. measure apparent brightness 2. determine distance 3. use inverse square law to find luminosity 5
Hertzsprung Russell Diagram Graph of Temperature versus Luminosity Or Spectral Type versus Magnitude First plotted early 20 th century by Henry Norris Russell Ejnar Hertzsprung Hertzsprung Russell Diagram Size of the Stars Virtually all stars are points of light We can t measure size directly 6
Size and Temperature Luminosity Depends on Temperature Diameter Bigger == Brighter Hotter == Brighter Main Sequence Most stars are Main Sequence stars Sun is a Main Sequence star Radius 696,000 km 110 x Earth radii Main Sequence Mass 0.1-60 Solar Mass Radius 0.1 10 Solar Radii Luminosity 0.0001 10,000 Solar Lums. Massive stars die young 7
Red Giants White Dwarfs Size Summary Stars have many sizes.01 100 times sun Our sun is typical Giants are bigger Supergiants are huge 8
Velocity of Star Motion side to side proper motion Motion to and fro Doppler effect Proper Motion Stars position change Nearest move most Need to know distance to get velocity Doppler Shift Moving objects shift spectrum Away red shift Towards blue shift 9
Groupings Sun is isolated unusual Binary stars about 50% Clusters common Optical Doubles Chance association Different distances e.g. Mizar Visual Binary Physically associated Orbit around each other About Center of Mass 10
Eclipsing Binary Close together Orbit in line of sight Get in the way of each other Light Curve What Do We Learn from Binaries? Mass From Kepler s Third Law (as per Newton) Mass of Sun 1.989 x 10 30 kg Stars range 0.1 60 M sun Radius sometimes Mass Luminosity Relation Massive stars are much Brighter L == M4 Massive stars die younger 11
Clusters Larger groupings Stars with same Distance Age Composition Different Masses Open Clusters No regular shape Many hot stars Hundreds of stars In plane of Galaxy Pleiades Seven Sisters Open Cluster Reflection nebula 12
Open Cluster in another Galaxy Globular Clusters Concentrated in Center No hot stars 10,000 + stars Found off the plane Globular Cluster M13 In Hercules 13
How do you find Age of Cluster? Scenario All stars start out at the same time All stars start out on the main sequence The evolve into Red Giants and White Dwarfs As they run out of nuclear fuel HR Diagram Stellar Evolution Hot Stars die young 50,000 K O stars about 1 million years 20,000 K B stars about 100 million years 10,000 K A stars about 1 Billion years 6,000 K F stars about 10 Billion years 14
Pleiades Globular Cluster 15