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# Star Light, Star Bright - PowerPoint PPT Presentation

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

Going from the Sun to other Stars

• 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

Earth

1 AU

Star

Parallax Angle

Sun

• 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

• 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

• Indication of energy emitted at every wavelength of light

• Tells us many things

• Composition, temperature, luminosity, velocity, rotation speed are just some

• 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

• 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

• 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

• 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)

www.hubblesite.org

O star

• Ionized He, weak H lines

• T>25,000 K

• Electric Blue (peaks in UV)

• Example: Stars in Orion’s Belt

B star

• Neutral He, moderate H lines

• T=25,000 K-11,000K

• Blue (peaks in UV)

• Example: Rigel

A star

Very Strong H lines

T=11,000-7,500K

Peaks in Violet

Example: Sirius, Vega

F star

Moderate H lines and Ionized Ca

T=7,500-6,000K

Blue

Example: Polaris, Canopus

G star

Weak H lines and Strong Ionized Ca

T=6,000-5,000K

Yellow

Example: Sun, Alpha Centauri

K star

Lines of neutral and singly ionized metal, some molecules

T=5,000-3,500K

Red

Example: Arcturus, Aldeberan

M star

Strong Molecular Lines

T=2,200-3,500K

Red (Peaks in IR)

Example: Betelgeuse, Proxima Centauri

L star

Strong Molecular Lines

Includes Water !!

T=1,300-2,200

Red (Peaks in IR)

Likely a Brown Dwarf

T star

Strong Lines of Water and Methane

Very Cool!

T=900-1300K

Red (Peaks in IR)

Likely a Brown Dwarf

RNS

Special classes for “evolved” stars

These stars are in old age

Puffy atmospheres wash out some lines

Others are easier to see

• 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

• Most stars have similar composition

• Line strength is determined by number of excited electrons

• What determines this?

• Temperature differences!

• 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!

• Demographic Chart

• All stars are place on it based on two pieces information

• Luminosity and Temperature (spectral class)

• Can provide information about many things

Hot, bright

Cool, bright

Luminosity Increasing

Hot, dim

Cool, dim

Temperature Increasing

Red Super Giants

Red Giants

Main Sequence

Luminosity Increasing

White Dwarfs

Temperature Increasing

• 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

• Most stars

• Majority of lifetime spent here

• Hydrogen fusion

• Stay in one location on diagram

• Blue Supergiants to Red Dwarfs

• Sun is on MS

• 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

• Earth-sized (Tiny)

• Very hot (>6000K)

• Older than Red Giants

• No H-fusion

• 9% of Solar Neighborhood

• Need more than Spectral Class

• Example :

Both Betelgeuse and Barnard’s Star are M type stars

Betelgeuse is 100,000 times more Luminous!

• 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

• Betelgeuse is a M2Ia

• Red, Supergiant

• Barnard’s Star M5V

• Red Dwarf, Main Sequence

• Find distance to ANY star

• Estimate luminosity from classification

• Use inverse-square law to find distance

• Spectroscopic Distance

• 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!

• Two stars orbiting a common center

• 3 types of Binary Stars

• Visual Binary

• Spectroscopic Binary

• Eclipsing 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

• 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

• 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

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

• 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

• 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

• 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

• Range of Masses on MS is not very large

• 0.1M -100M

• Smaller than this-don’t “turn on”

• Larger than this –too unstable

• 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

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

• Spectral Classes tell about temperature (and color)

• Luminosity Classes tell about sizes

• HR diagram VERY IMPORTANT TOOL