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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

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Star light star bright

Star Light, Star Bright

Going from the Sun to other Stars

Giving a star a physical
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


  • Stellar Parallax

  • New 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


  • 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



1 AU


Parallax Angle


Sun s neighbors
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


  • Radius-Luminosity-Temperature relationship

  • Use color to find temperature

  • Use Stefan-Boltzmann Law to find Luminosity

  • L  R2  T4

  • Star w/same T as Sun but Bigger L must be larger in size!


  • Remember that Blackbodies will appear different “colors” depending upon Temperature

  • Cool stars – Red

  • Hot stars – Blue

Spectra of a star
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
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
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 types1
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 classification1
Spectral Classification

  • Pickering (Harvard) assigned letters to original classes (A-M)

  • Annie Jump Cannon rearranged classes based on temperature (Payne’s system)

  • Non-alphabetical



Families of stars
Families of Stars

Spectral types
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 types1
Spectral Types

B star

  • Neutral He, moderate H lines

  • T=25,000 K-11,000K

  • Blue (peaks in UV)

  • Example: Rigel

Spectral types2
Spectral Types

A star

Very Strong H lines


Peaks in Violet

Example: Sirius, Vega

Spectral types3
Spectral Types

F star

Moderate H lines and Ionized Ca



Example: Polaris, Canopus

Spectral types4
Spectral Types

G star

Weak H lines and Strong Ionized Ca



Example: Sun, Alpha Centauri

Spectral types5
Spectral Types

K star

Lines of neutral and singly ionized metal, some molecules



Example: Arcturus, Aldeberan

Spectral types6
Spectral Types

M star

Strong Molecular Lines


Red (Peaks in IR)

Example: Betelgeuse, Proxima Centauri

Spectral types7
Spectral Types

L star

Strong Molecular Lines

Includes Water !!


Red (Peaks in IR)

Likely a Brown Dwarf

Spectral types8
Spectral Types

T star

Strong Lines of Water and Methane

Very Cool!


Red (Peaks in IR)

Likely a Brown Dwarf

Spectral types9
Spectral Types


Special classes for “evolved” stars

These stars are in old age

Puffy atmospheres wash out some lines

Others are easier to see

Spectral types10
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
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
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
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 diagram1
HR Diagram

Hot, bright

Cool, bright

Luminosity Increasing

Hot, dim

Cool, dim

Temperature Increasing

Hr diagram2
HR Diagram

Red Super Giants

Red Giants

Main Sequence

Luminosity Increasing

White Dwarfs

Temperature Increasing

Stellar populations
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
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

  • 10-1000x Radius of Sun (R)

  • 3000-6000K

  • 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
White Dwarfs

  • Earth-sized (Tiny)

  • Very hot (>6000K)

  • Older than Red Giants

  • No H-fusion

  • 9% of Solar Neighborhood

Luminosity classes
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 classes1
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 classes2
Luminosity Classes

  • Betelgeuse is a M2Ia

    • Red, Supergiant

  • Barnard’s Star M5V

    • Red Dwarf, Main Sequence

Distance again
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
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
Binary Star Masses

  • Two stars orbiting a common center

  • 3 types of Binary Stars

  • Visual Binary

  • Spectroscopic Binary

  • Eclipsing Binary

Visual 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
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 binary1
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
Spectroscopic Binary Animation

Eclipsing binaries
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!

  • Example: Algol, Sirius AB

Finding masses
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
Single Star Masses

  • Binary techniques don’t work

  • Mass-Luminosity relationship

  • Larger Luminosity – Greater Mass

  • Luminosity  Mass4

  • Example

  • A star 2x Mass of Sun (M) has a Luminosity 24 (16x) the Sun’s (L )


  • The Mass-Luminosity Relation applies to Main Sequence Stars only!

  • Red Giants and White Dwarfs must use approximations

Mass luminosity relation
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 relation1
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 relation2
Mass-Luminosity Relation


  • Spectral Classes tell about temperature (and color)

  • Luminosity Classes tell about sizes


  • Luminosity – Radius –Temperature

  • Mass -Luminosity