star light, star bright
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Star Light, Star BrightStar Light, Star Bright
Going from the Sun to other Stars
Giving a Star a PhysicalGiving a Star a PhysicalUse Starlight!Distances- use stellar parallaxLuminosity- same as sun (careful!)Temperature- same as sunDiameter- use Luminosity and
TemperatureMass- save it for later
DistanceDistanceStellar ParallaxNew unit- parsec (pc)1 pc= 206,265 AU= 3.1x1016m“parallax in arc seconds”Distance (pc)=1/parallax (arc sec)Lightyear-distance light travels in
one year 1pc=3.3ly
DistanceDistance
Another Method- Standard CandlesKnow the brightness a star should
haveIf it appears dimmer, it must be
further awayEstimate distance based on dimnessOften used for extragalactic objects
ParallaxParallax
Star
Sun
Earth
Parallax Angle
1 AU
Sun’s NeighborsSun’s NeighborsClosest neighbor- Proxima Centauri
– 1.3 pc away (4.3 ly) 300,000 times distance between Earth and Sun
About 30 stars within 4pcMany are multiple star systemsWe can measure parallax out to
100pc
DiameterDiameterRadius-Luminosity-Temperature
relationshipUse color to find temperatureUse Stefan-Boltzmann Law to find
LuminosityL R2 T4
Star w/same T as Sun but Bigger L must be larger in size!
TemperatureTemperatureRemember that Blackbodies will
appear different “colors” depending upon Temperature
Cool stars – RedHot stars – Blue
Spectra of a StarSpectra of a Star
Indication of energy emitted at every wavelength of light
Tells us many thingsComposition, temperature,
luminosity, velocity, rotation speed are just some
Spectral ClassificationSpectral ClassificationDetailed spectra of stars allow
better classificationLines that are present allow star to
be “pigeonholed” Orignal scheme was loosely based
on color4 classes: White, Yellow, Red,
Deep Red
History of Spectral TypesHistory of Spectral TypesEdward Pickering at HarvardHired “computers”Williamina Fleming started with A
– Strength of H lines onlyShe classified 10,000 starsPickering published her work in
1890
History of Spectral TypesHistory of Spectral TypesAnnie Jump Cannon developed new
schemePared Fleming’s number of classesIncluded subdivisonsClassified 400,000 stars in her
lifetimeHer system is the standard used
today
Spectral ClassificationSpectral 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 StarsFamilies of Stars
www.hubblesite.org
Spectral TypesSpectral TypesO star
– Ionized He, weak H lines– T>25,000 K
– Electric Blue (peaks in UV)– Example: Stars in Orion’s Belt
Spectral TypesSpectral TypesB star
– Neutral He, moderate H lines– T=25,000 K-11,000K– Blue (peaks in UV)
– Example: Rigel
Spectral TypesSpectral Types A star
Very Strong H linesT=11,000-7,500KPeaks in Violet
Example: Sirius, Vega
Spectral TypesSpectral Types F star
Moderate H lines and Ionized CaT=7,500-6,000K
BlueExample: Polaris, Canopus
Spectral TypesSpectral Types G star
Weak H lines and Strong Ionized CaT=6,000-5,000K
YellowExample: Sun, Alpha Centauri
Spectral TypesSpectral Types K star
Lines of neutral and singly ionized metal, some molecules
T=5,000-3,500K Red
Example: Arcturus, Aldeberan
Spectral TypesSpectral Types M star
Strong Molecular LinesT=2,200-3,500K
Red (Peaks in IR)Example: Betelgeuse, Proxima Centauri
Spectral TypesSpectral Types
L starStrong Molecular Lines
Includes Water !!T=1,300-2,200
Red (Peaks in IR)Likely a Brown Dwarf
Spectral TypesSpectral Types
T starStrong Lines of Water and Methane
Very Cool!T=900-1300K
Red (Peaks in IR)Likely a Brown Dwarf
Spectral Types Spectral Types
RNSSpecial classes for “evolved” stars
These stars are in old agePuffy atmospheres wash out some
linesOthers are easier to see
Spectral TypesSpectral TypesFurther divisions 0-9Based on where temperature is in
rangeLower the number- hotter the starSun is a G2 star, cooler than G1
hotter than G3
Why different spectra?Why different spectra?
Most stars have similar composition
Line strength is determined by number of excited electrons
What determines this?Temperature differences!
Combination of ToolsCombination of ToolsSpectral Class, Temperature, and
Luminosity can be put togetherForm a very useful toolHertzsprung-Russell (HR) diagramRelates T, L, D , spectral class of
any star!Very important to Astronomers!
HR DiagramHR DiagramDemographic ChartAll stars are place on it based on
two pieces informationLuminosity and Temperature
(spectral class)Can provide information about
many things
HR DiagramHR Diagram
Temperature Increasing
Lum
inosi
ty In
creasi
ng
Cool, dimHot, dim
Hot, bright Cool, bright
HR DiagramHR Diagram
Temperature Increasing
Lum
inosi
ty In
creasi
ng M
ain SequenceRed Giants
White Dwarfs
Red Super Giants
Stellar PopulationsStellar PopulationsHR diagram gives information
about populationsStars evolve and ageStar’s position on HR diagram
=info about ageNot all stars in sky are same age!Also info about fusion fuel
Main SequenceMain Sequence
Most starsAdult starMajority of lifetime spent hereHydrogen fusionStay in one location on diagramBlue Supergiants to Red DwarfsSun is on MS
Red GiantsRed Giants
10-1000x Radius of Sun (R)3000-6000KRed Giants are older than MS stars
of same massNo Red Giants within 5pc of Sun1% of Solar NeighborhoodStopped H-fusion
White DwarfsWhite Dwarfs
Earth-sized (Tiny)Very hot (>6000K)Older than Red GiantsNo H-fusion9% of Solar Neighborhood
Luminosity ClassesLuminosity Classes
Need more than Spectral ClassExample :Both Betelgeuse and Barnard’s Star
are M type starsBetelgeuse is 100,000 times more
Luminous!
Luminosity ClassesLuminosity Classes
Assign LC to distinguish types of stars of same Spectral Class
I Supergiants (Ia, Ib)II Luminous GiantsIII Regular GiantsIV SubgiantsV Main Sequence Stars
Luminosity ClassesLuminosity Classes
Betelgeuse is a M2Ia – Red, Supergiant
Barnard’s Star M5V– Red Dwarf, Main Sequence
Distance AgainDistance Again
Find distance to ANY starMeasure energy receivedEstimate luminosity from
classificationUse inverse-square law to find
distanceSpectroscopic Distance
Stellar MassesStellar Masses
Can’t be found from just “size”Two ways to determine
Binary Star systemMass-Luminosity Relationship
Determines star’s location on MS and ultimately…
It’s lifespan!
Binary Star MassesBinary Star Masses
Two stars orbiting a common center
3 types of Binary StarsVisual Binary
Spectroscopic Binary Eclipsing Binary
Visual BinaryVisual Binary
See two stars w/ eye or telescopeExample Alcor/Mizar in Big DipperWidely separatedTime of orbit can be observed
directlyBrighter Star-PrimaryFainter Star-Secondary
Spectroscopic BinarySpectroscopic Binary
Too closer together or too far away to see separate stars
Look for Doppler Shift in Spectral Lines
Moving toward us –Blue ShiftMoving away from us –Red Shift
Spectroscopic BinarySpectroscopic 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 AnimationSpectroscopic Binary Animation
http://csep10.phys.utk.edu/astr162/lect/binaries/spectroscopic.html
Eclipsing BinariesEclipsing Binaries
Rarest formOrbital Plane is edge onOne star passes in front of otherBlocks light (eclipses!)“Star” appears to vary dramatically
in brightness Check it out!Example: Algol, Sirius AB
Finding MassesFinding Masses
Determine the period of orbitDetermine distance apartFind the “balance point” of systemThis is Center of MassUse this to determine total mass of
systemCan’t find individual masses unless
individual stars can be seen
Single Star MassesSingle Star Masses
Binary techniques don’t workMass-Luminosity relationshipLarger Luminosity – Greater MassLuminosity Mass4
ExampleA star 2x Mass of Sun (M) has a
Luminosity 24 (16x) the Sun’s (L )
IMPORTANT!IMPORTANT!
The Mass-Luminosity Relation applies to Main Sequence Stars only!
Red Giants and White Dwarfs must use approximations
Mass-Luminosity RelationMass-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 RelationMass-Luminosity Relation
Also, tells about lifetimesBig stars have more fuel but…They burn it much, much faster so…They live much shorter lifetimes
than smaller stars1M - 10 billion years
10M-20 million years
Mass-Luminosity RelationMass-Luminosity Relation
http://www.astronomynotes.com/starprop/s13.htm
SummarySummary
Spectral Classes tell about temperature (and color)
Luminosity Classes tell about sizesHR diagram VERY IMPORTANT
TOOLLuminosity – Radius –Temperature Mass -Luminosity
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