Star Light, Star Bright Going from the Sun to other Stars

Giving a Star a Physical Use Starlight! Distances- use stellar parallax Luminosity- same as sun (careful!) Temperature- same as sun Diameter- use Luminosity and Temperature Mass- save it for later

Distance Stellar Parallax New unit- parsec (pc) 1 pc= 206,265 AU= 3.1x10 16 m “parallax in arc seconds” Distance (pc)=1/parallax (arc sec) Lightyear-distance light travels in one year 1pc=3.3ly

Distance Another Method- Standard Candles Know the brightness a star should have If it appears dimmer, it must be further away Estimate distance based on dimness Often used for extragalactic objects

Parallax Star Sun Earth Parallax Angle 1 AU

Sun’s Neighbors Closest neighbor- Proxima Centauri – 1.3 pc away (4.3 ly) 300,000 times distance between Earth and Sun About 30 stars within 4pc Many are multiple star systems We can measure parallax out to 100pc

Diameter Radius-Luminosity-Temperature relationship Use color to find temperature Use Stefan-Boltzmann Law to find Luminosity L  R 2  T 4 Star w/same T as Sun but Bigger L must be larger in size!

Temperature Remember that Blackbodies will appear different “colors” depending upon Temperature Cool stars – Red Hot stars – Blue

Spectra of a Star Indication of energy emitted at every wavelength of light Tells us many things Composition, temperature, luminosity, velocity, rotation speed are just some

Spectral Classification Detailed spectra of stars allow better classification Lines that are present allow star to be “pigeonholed” Orignal scheme was loosely based on color 4 classes: White, Yellow, Red, Deep Red

History of Spectral Types Edward Pickering at Harvard Hired “computers” Williamina Fleming started with A – Strength of H lines only She classified 10,000 stars Pickering published her work in 1890

History of Spectral Types Annie Jump Cannon developed new scheme Pared Fleming’s number of classes Included subdivisons Classified 400,000 stars in her lifetime Her system is the standard used today

Spectral Classification Pickering (Harvard) assigned letters to original classes (A-M) Annie Jump Cannon rearranged classes based on temperature (Payne’s system) Non-alphabetical OBAFGKM (LT)(RNS)

Families of Stars

Spectral Types O star – Ionized He, weak H lines – T>25,000 K – Electric Blue (peaks in UV) – Example: Stars in Orion’s Belt

Spectral Types B star – Neutral He, moderate H lines – T=25,000 K-11,000K – Blue (peaks in UV) – Example: Rigel

Spectral Types A star Very Strong H lines T=11,000-7,500K Peaks in Violet Example: Sirius, Vega

Spectral Types F star Moderate H lines and Ionized Ca T=7,500-6,000K Blue Example: Polaris, Canopus

Spectral Types G star Weak H lines and Strong Ionized Ca T=6,000-5,000K Yellow Example: Sun, Alpha Centauri

Spectral Types K star Lines of neutral and singly ionized metal, some molecules T=5,000-3,500K Red Example: Arcturus, Aldeberan

Spectral Types M star Strong Molecular Lines T=2,200-3,500K Red (Peaks in IR) Example: Betelgeuse, Proxima Centauri

Spectral Types L star Strong Molecular Lines Includes Water !! T=1,300-2,200 Red (Peaks in IR) Likely a Brown Dwarf

Spectral Types T star Strong Lines of Water and Methane Very Cool! T= K Red (Peaks in IR) Likely a Brown Dwarf

Spectral Types RNS Special classes for “evolved” stars These stars are in old age Puffy atmospheres wash out some lines Others are easier to see

Spectral Types Further divisions 0-9 Based on where temperature is in range Lower the number- hotter the star Sun is a G2 star, cooler than G1 hotter than G3

Why different spectra? Most stars have similar composition Line strength is determined by number of excited electrons What determines this? Temperature differences!

Combination of Tools Spectral Class, Temperature, and Luminosity can be put together Form a very useful tool Hertzsprung-Russell (HR) diagram Relates T, L, D, spectral class of any star! Very important to Astronomers!

HR Diagram Demographic Chart All stars are place on it based on two pieces information Luminosity and Temperature (spectral class) Can provide information about many things

HR Diagram Temperature Increasing Luminosity Increasing Cool, dimHot, dim Hot, bright Cool, bright

HR Diagram Temperature Increasing Luminosity Increasing Main Sequence Red Giants White Dwarfs Red Super Giants

Stellar Populations HR diagram gives information about populations Stars evolve and age Star’s position on HR diagram =info about age Not all stars in sky are same age! Also info about fusion fuel

Main Sequence Most stars Adult star Majority of lifetime spent here Hydrogen fusion Stay in one location on diagram Blue Supergiants to Red Dwarfs Sun is on MS

Red Giants Red Giants x Radius of Sun (R  ) K Red Giants are older than MS stars of same mass No Red Giants within 5pc of Sun 1% of Solar Neighborhood Stopped H-fusion

White Dwarfs Earth-sized (Tiny) Very hot (>6000K) Older than Red Giants No H-fusion 9% of Solar Neighborhood

Luminosity Classes Need more than Spectral Class Example : Both Betelgeuse and Barnard’s Star are M type stars Betelgeuse is 100,000 times more Luminous!

Luminosity Classes Assign LC to distinguish types of stars of same Spectral Class I Supergiants (Ia, Ib) II Luminous Giants III Regular Giants IV Subgiants V Main Sequence Stars

Luminosity Classes Betelgeuse is a M2Ia – Red, Supergiant Barnard’s Star M5V – Red Dwarf, Main Sequence

Distance Again Find distance to ANY star Measure energy received Estimate luminosity from classification Use inverse-square law to find distance Spectroscopic Distance

Stellar Masses Can’t be found from just “size” Two ways to determine Binary Star system Mass-Luminosity Relationship Determines star’s location on MS and ultimately… It’s lifespan!

Binary Star Masses Two stars orbiting a common center 3 types of Binary Stars Visual Binary Spectroscopic Binary Eclipsing Binary

Visual Binary See two stars w/ eye or telescope Example Alcor/Mizar in Big Dipper Widely separated Time of orbit can be observed directly Brighter Star-Primary Fainter Star-Secondary

Spectroscopic Binary Too closer together or too far away to see separate stars Look for Doppler Shift in Spectral Lines Moving toward us –Blue Shift Moving away from us –Red Shift

Spectroscopic Binary Double-line SB – Two stars about same Luminosity – Two sets of lines observed – Each is Doppler Shifted Single-line SB – One star is brighter than other – One set of lines observed – Doppler shifted also

Spectroscopic Binary Animation

Eclipsing Binaries Rarest form Orbital Plane is edge on One star passes in front of other Blocks light (eclipses!) “Star” appears to vary dramatically in brightness Check it out!Check it out! Example: Algol, Sirius AB

Finding Masses Determine the period of orbit Determine distance apart Find the “balance point” of system This is Center of Mass Use this to determine total mass of system Can’t find individual masses unless individual stars can be seen

Single Star Masses Binary techniques don’t work Mass-Luminosity relationship Larger Luminosity – Greater Mass Luminosity  Mass 4 Example A star 2x Mass of Sun (M  ) has a Luminosity 2 4 (16x) the Sun’s (L  )

IMPORTANT! The Mass-Luminosity Relation applies to Main Sequence Stars only! Red Giants and White Dwarfs must use approximations

Mass-Luminosity Relation Range of Masses on MS is not very large 0.1M  -100M  Smaller than this-don’t “turn on” Larger than this –too unstable

Mass-Luminosity Relation Also, tells about lifetimes Big stars have more fuel but… They burn it much, much faster so… They live much shorter lifetimes than smaller stars 1M  - 10 billion years 10M  -20 million years

Mass-Luminosity Relation

Summary Spectral Classes tell about temperature (and color) Luminosity Classes tell about sizes HR diagram VERY IMPORTANT TOOL Luminosity – Radius –Temperature Mass -Luminosity