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