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Chapter 8:The Family of Stars We already know how to determine a star’s

• surface temperature

• chemical composition

• motion

Next, we will learn how we can determine its

• distance• luminosity• radius• mass

How do we know how far away the stars are?

Measuring the Distance to Stars

The best method formeasuringdistances of nearbystars is called_________.

This involvesobserving a starfrom two different_______.

The apparentshift in positionof a _____________ againstmore distantbackgroundobjects, due tothe changinglocation of theobserver

Parallax on Earth Parallax on Earth

Parallax happens becausethe observer changes herlocation

Distance betweenobservations = ________

2

Baseline = ______

The Distance to the Stars• We observe a star twice, on

opposite sides of Earth’s ____

• In 6 months Earth has moved_____

• A closer star will appear tomove more with respect tobackground stars

Distance vs. Parallax

• Big distance (object is far away) => _____ parallax• Small distance (object is close) => _____ parallax

Parallax

• Most stars have a very small parallax angle: p

• p is usually measured in ___ ________

• Distances to stars are measured in either: light years, or parsecs. (1 pc = 3.26 LY)

• parsec = PARallax of one arcSEC

Parallax and Distance The Parsec Lecture-Tutorial:Pages 35-37

• Work with a partner or two• Read directions and answer all questions carefully.

Take time to understand it now!• Come to a consensus answer you all agree on before

moving on to the next question.• If you get stuck, ask another group for help.• If you get really stuck, raise your hand and I will

come around.

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How Powerful Are the Stars?

• Some stars are more powerful than others

• “Power” is energy output per _______. (Example: 100 Watts = 100 joules per second)

• Astronomers measure the power, or brightness of stars in2 ways: luminosity and ___________.

Magnitudes• ________ Magnitude: how bright the star

appears• ________ Magnitude: how bright the star

actually is– Sun: apparent -26, absolute 4.8– Alpha Centauri: apparent -0.01, absolute 4.3

Luminosity:

Amount of power astar _______

Apparentbrightness:Amount of starlightthat reaches ____

Luminositypassing througheach sphere is_________

Area of sphere: 4π (radius)2

Luminosity doesnot decrease withdistance, butapparentbrightness does

Apparent brightness obeys an_______-______ relation

Example: Say two stars (A and B) have thesame luminosity. Star B is four times closer toEarth than Star A. How does the apparentbrightness of Star B compare to that of Star A?

Luminosities of Different Stars• Faint M Star: 0.0001 Lsun• G Star 1.0 Lsun• A Star (Vega) : 50 Lsun• Giant star: 100 Lsun• Most luminous star known (Pistol Star): 10 Million

LSun !

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What about size?• How do we know how big a star is?• What is the range of sizes for stars?

Characterize by _____

Bigger than ______

Smaller than _____… Miniscule compared to ___________:

Sun (1 pixel)

Which star would appear mostred in color?

Which star has the highesttemperature?

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Which star is putting off thelargest total amount of energy?

Luminosity, Temperature, andRadius

Imagine two hot plates of the same size (butthe right one is hotter):

Which will cook a pot of spaghetti faster?

Luminosity, Temperature, andRadius

Imagine two hot plates of the sametemperature (but the right one is larger):

Which will cook a pot of spaghetti faster?

Luminosity, Temperature, andRadius

Imagine two hot plates with different sizesand different temperatures:

Which will cook a pot of spaghetti faster?

Luminosity, Temperature, andSize Lecture Tutorial: Page 53• Work with a partner or two• Read directions and answer all questions carefully.

Take time to understand it now!• Come to a consensus answer you all agree on before

moving on to the next question.• If you get stuck, ask another group for help.• If you get really stuck, raise your hand and I will

come around.

Organizing the properties of stars:The H-R Diagram

_________ versus __________

Lum

inos

ity

Temperature

Spectral type: O B A F G K M

Hertzsprung-Russell Diagram

orA

bsol

ute

mag

.

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Use of H-R Diagrams• H-R Diagrams have

been called theastronomers “__________ ____”

• They quickly tell usthe type of star

• They can also revealthe ____, ___ &________ of the star.

Stars of thesame radiusfollow diagonallines

The HertzsprungRussell (HR)Diagram

Most stars arefound along the____ ________

H-R Diagram:Masses of Stars Masses in units

of solar masses

Mass

More massive stars onthe Main Sequence

appear to be ______.

Using the H-RDiagram we can readoff the _________ ofstars of differentmass.

Masses ofStars Masses in units

of solar masses

Low m

asses

High masses

Mass

_______ Stars on theMain Sequence are

more _______

An O type star on theMain Sequence has20x the Sun’s mass.

An M type star on theMain Sequence hasonly 1/10th of theSun’s mass.

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H-R Diagram Lecture Tutorial:Pg. 109-110

• Work with a partner or two• Read directions and answer all questions carefully.

Take time to understand it now!• Come to a consensus answer you all agree on

before moving on to the next question.• If you get stuck, ask another group for help.• If you get really stuck, raise your hand and I will come

around.

Which star looks brighter fromEarth?

62Y

14X

AbsoluteMagnitude

ApparentMagnitudeStar

Which star is more luminous?

62Y

14X

AbsoluteMagnitude

ApparentMagnitudeStar

Which star is closer to Earth?

62Y

14X

AbsoluteMagnitude

ApparentMagnitudeStar

Which star lives the longest?

• Stars which are fusing hydrogen live on the____ ________ in the H-R diagram.(Note: The “____ ________” isn’t a place in space!)

• Stars live until they burn up a portion of theirhydrogen fuel.

• Then they “evolve”, or age, and finally die.

• The stars which live the longest are the oneswhich ______ ____.

Stars or Cars?

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“Burning” Hydrogen Fuel

• O Type Stars are the most luminous (most energy)

• The more quickly Hydrogen is used up, the _____ thelife of the star.

• O Type stars live for “only” 1 million years• G Type stars, like the Sun live for 5 billion years• M Type stars can live for 100 billion years!Because they burn fuel ____ ______.

Mass determines thefate of the star:whether or not it willexplode.

Mass -> luminosity ->lifetime

But how do wemeasure a star’sMass?

Masses ofStars

Binary Stars• Most stars are in multi-star

systems• Binaries, triplets,

quadruplets, etc…– Sirius– Polaris– Alcor and Mizar in Big

Dipper

The Sun, all by itself, is nottypical

Binary Stars

• If bound by ______, both stars orbitaround the center of mass

• Center of mass is always _____ to themore _______ star

m M

Center of Mass

How do we find the mass ofstars?

Recall Kepler’s 3rd Law: _________

We can use Kepler’s Third Law to measure the masses of binarystars.

Newton improved Kepler’s law using the Law of gravity.

MA + MB =aAU

3____Py

2

(MA and MB are the ________ of the stars in units of solarmasses)

Low m

asses

High masses

Mass

Mass_______ as

you go up theMain

SequenceSo doesluminosity!

_____ _____ and______ do notobey the Mass-Luminosityrelation

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The Mass-Luminosity Relationon the ____ _________

L ~ M3.5

When we plot Massand Luminosity, we findthat more massivestars are much _____luminous.

Luminosity depends onmass to the 3.5 power

Mass-Luminosity Relation• Example: If star A is three times as massive as star B, how

much more luminous is it?

L ~ M3.5

LA/LB = (MA/MB)3.5

LA/LB =

LA/LB =

LA/LB =

H-R DiagramsHRDiagrams

A Census of the StarsFaint, red dwarfs(low mass) are

the most________ stars.

Giants andsupergiantsare _______

____.

Bright, hot, bluemain-sequence

stars (high-mass) are ____

____.