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The Life Cycles of Stars The Life Cycles of Stars and our Sun and our Sun

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Page 1: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

The Life Cycles of StarsThe Life Cycles of Starsand our Sunand our Sun

Page 2: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

Gravity

R1/2 R

a=0

a= GM’/(1/2 R)2 ~ R

a= GM/R2

Page 3: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

Astronomers (and what it takes…)

1. Passion!2. Determination3. Do only what you like to do4. Flexibility

Page 4: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

The Many Opportunities for Astronomers

1. Academic life2. National or International Research

Centers/Observatories3. Industry4. Education (e.g. Planetariums, Museums, etc.)

Page 5: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

Space Telescope Science Institute

Page 6: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

NICMOS (Near Infrared Camera and Multi-Object Spectrometer)

One of the Instruments on the Hubble Space Telescope

Page 7: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

The many facets of Astronomers

1. Observational (telescopes, ground and space; data)

2. Theoretical (numerical, analytical)3. Instrumentalist4. Laboratory Scientist

Page 8: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

The Typical Day of an Astronomer

Page 9: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

…but there are also special days

Meeting of the American Astronomical SocietyJune 2002

Page 10: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

Some of the important ingredients

1. Collaborations (with peers)2. Interactions (with younger/future peers)3. Living above your Universe Island

Page 11: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

Astronomers and Family1. Perfectly compatible carriers2. Most of the astronomers I know have families to

which they contribute their half share3. Many places are supportive of `dual career’ needs

Page 12: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

Stars

“The stars are distant and unobtrusive, but bright and enduring as our fairest and most memorable experiences.”

Henry David Thoreau (1849)

Are Stars similar to our Sun?How far away are they?Where did they come from?What do they do?Do they live forever?

Page 13: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

Panorama view of the sky

Page 14: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2
Page 15: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2
Page 16: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

The Four Basic Parameters of Stars

»Luminosity

»Size

»Mass

»Surface Temperature

Page 17: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

Brightness, Distance, and Luminosity

L=4D2 l

luminosity distance

apparent brightnessor flux

l =L/(4D2 )

Page 18: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

There is a Big Range of Stellar Luminosities Out there!

Star Luminosity (in units of solar)

Sun 1

Proxima Centauri 0.0006

Rigel (Orion) 70,000

Deneb (Cygnus) 170,000

Page 19: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

How to measure the surface temperature of a star?

1. Overall spectral shape (the peak of the blackbody continuous spectrum)

2. More accurately, spectroscopically

Page 20: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

Spectral Types

The sun has a spectral type: G2

For historical reasons, astronomers classify the temperatures of stars on a scale defined by spectral types, called O B A F G K M, ranging from the hottest (type O) to the coolest (type M) stars.

Page 21: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

Stellar Size

• Stars are very spherical so we characterize a star’s size by its radius.

RStellar Radii vary in sizefrom ~1500xRSun for a large Red Giant to 0.008xRSun for a WhiteDwarf.

Page 22: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

Temperature, Luminosity, and Size – pulling them all together

Stefan-Boltzmann Law

Luminosity Stellarradius

Surfacetemperature

L=4πR2 σT4

A star’s luminosity, surface temperature, and size are all related by the Stefan-Boltzmann Law:

Page 23: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

Measurements of Star Properties

Apparent brightness DistanceLuminosity

TemperatureRadius

Direct measurentParallaxDistance + apparent brightness( L=4D2 l)Spectral type (or color)Luminosity + temperature(L=4R2 T4)

Luminosity and temperature are the two independent intrinsic parameters of stars.

Page 24: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

How do you weigh a star?

• Mass is the single most important property in how a star’s life and death will proceed.

• We can “weigh” stars that are in binary systems (two stars orbiting each other). Fortunately, most stars fall into this category.

• Most stars in binary systems have a mass that is very similar to its companion …

Page 25: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

I. Visual Binaries

Page 26: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

Eclipsing Binaries

Page 27: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

In Review• There are four principal characteristics of a

star:– Luminosity– Surface Temperature– Size– Mass

How may we classify stars?We can take a census of stars and see what’s out there.

Page 28: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

Classification of Stars1) Collect information on

a large sample of stars.

2) Measure their luminosities(need the distance!)

3) Measure their surface temperatures(need their spectra)

Page 29: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

The Hertzsprung-Russell Diagram

Page 30: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

The Hertzsprung-Russell Diagram

Page 31: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

The Hertzsprung-Russell Diagram

The Main Sequence

- all main sequencestars fuse H into Hein their cores

- this is the definingcharacteristic of a main sequence star.

Page 32: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

The Hertzsprung-Russell Diagram

Red Giants

- Red Giant starsare very large, cooland quite bright.

Ex. Betelgeuse is100,000 times moreluminous than the Sunbut is only 3,500K onthe surface. It’s radiusis 1,000 times that of theSun.

Page 33: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

The Hertzsprung-Russell Diagram

Page 34: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

The Hertzsprung-Russell Diagram

White Dwarfs

- White Dwarfsare hot but sincethey are so small,they are not veryluminous.

Page 35: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

The Hertzsprung-Russell Diagram

Size of Star

Mass ofStar

Page 36: The Life Cycles of Stars and our Sun. Gravity R 1/2 R a=0 a= GM’/(1/2 R) 2 ~ R a= GM/R 2

Mass-Luminosity relation

•Most stars appear on the Main Sequence, where stars appear to obey a Mass-Luminosity relation:

L M3.5

•For example, if the mass of a star is doubled, its luminosity increases by a factor 23.5 ~ 11. •Thus, stars like Sirius that are about twice as massive as the Sun are about 11 times as luminous. •The more massive a Main Sequence star is, the hotter (bluer), and more luminous.•The Main Sequence is a mass sequence


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