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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

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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


Distance

Distance

  • 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


Distance1

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

Parallax

Earth

1 AU

Star

Parallax Angle

Sun


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


Diameter

Diameter

  • 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!


Temperature

Temperature

  • 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

    OBAFGKM

    (LT)(RNS)


Families of stars

Families of Stars

www.hubblesite.org


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

T=11,000-7,500K

Peaks in Violet

Example: Sirius, Vega


Spectral types3

Spectral Types

F star

Moderate H lines and Ionized Ca

T=7,500-6,000K

Blue

Example: Polaris, Canopus


Spectral types4

Spectral Types

G star

Weak H lines and Strong Ionized Ca

T=6,000-5,000K

Yellow

Example: Sun, Alpha Centauri


Spectral types5

Spectral Types

K star

Lines of neutral and singly ionized metal, some molecules

T=5,000-3,500K

Red

Example: Arcturus, Aldeberan


Spectral types6

Spectral Types

M star

Strong Molecular Lines

T=2,200-3,500K

Red (Peaks in IR)

Example: Betelgeuse, Proxima Centauri


Spectral types7

Spectral Types

L star

Strong Molecular Lines

Includes Water !!

T=1,300-2,200

Red (Peaks in IR)

Likely a Brown Dwarf


Spectral types8

Spectral Types

T star

Strong Lines of Water and Methane

Very Cool!

T=900-1300K

Red (Peaks in IR)

Likely a Brown Dwarf


Spectral types9

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 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

http://csep10.phys.utk.edu/astr162/lect/binaries/spectroscopic.html


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 )


Important

IMPORTANT!

  • 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

http://www.astronomynotes.com/starprop/s13.htm


Summary

Summary

  • Spectral Classes tell about temperature (and color)

  • Luminosity Classes tell about sizes

  • HR diagram VERY IMPORTANT TOOL

  • Luminosity – Radius –Temperature

  • Mass -Luminosity


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