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

Gravity

R1/2 R

a=0

a= GM’/(1/2 R)2 ~ R

a= GM/R2

Astronomers (and what it takes…)

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

The Many Opportunities for Astronomers

1. Academic life2. National or International Research

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

Space Telescope Science Institute

NICMOS (Near Infrared Camera and Multi-Object Spectrometer)

One of the Instruments on the Hubble Space Telescope

The many facets of Astronomers

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

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

The Typical Day of an Astronomer

…but there are also special days

Meeting of the American Astronomical SocietyJune 2002

Some of the important ingredients

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

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

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?

Panorama view of the sky

The Four Basic Parameters of Stars

»Luminosity

»Size

»Mass

»Surface Temperature

Brightness, Distance, and Luminosity

L=4D2 l

luminosity distance

apparent brightnessor flux

l =L/(4D2 )

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

How to measure the surface temperature of a star?

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

2. More accurately, spectroscopically

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.

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.

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:

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.

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 …

I. Visual Binaries

Eclipsing Binaries

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.

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)

The Hertzsprung-Russell Diagram

The Hertzsprung-Russell Diagram

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.

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.

The Hertzsprung-Russell Diagram

The Hertzsprung-Russell Diagram

White Dwarfs

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

The Hertzsprung-Russell Diagram

Size of Star

Mass ofStar

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