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.Determination 3.Do only what you like to do 4.Flexibility

The Many Opportunities for Astronomers 1.Academic life 2.National or International Research Centers/Observatories 3.Industry 4.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.Instrumentalist 4.Laboratory Scientist

The Typical Day of an Astronomer

…but there are also special days Meeting of the American Astronomical Society June 2002

Some of the important ingredients 1.Collaborations (with peers) 2.Interactions (with younger/future peers) 3.Living above your Universe Island

Astronomers and Family 1.Perfectly compatible carriers 2.Most of the astronomers I know have families to which they contribute their half share 3.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=4  D 2 l luminositydistance apparent brightness or flux l =L/(4  D 2 )

There is a Big Range of Stellar Luminosities Out there! StarLuminosity (in units of solar) Sun1 Proxima Centauri 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. R Stellar Radii vary in size from ~1500xR Sun for a large Red Giant to 0.008xR Sun for a White Dwarf.

Temperature, Luminosity, and Size – pulling them all together Stefan-Boltzmann Law Luminosity Stellar radius Surface temperature L=4 π R 2 σ T 4 A star’s luminosity, surface temperature, and size are all related by the Stefan-Boltzmann Law:

Measurements of Star Properties Apparent brightness Distance Luminosity Temperature Radius Direct measurent Parallax Distance + apparent brightness ( L=4  D 2 l) Spectral type (or color) Luminosity + temperature (L=4  R 2  T 4 ) 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 Stars 1)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 Main Sequence - all main sequence stars fuse H into He in their cores - this is the defining characteristic of a main sequence star.

The Hertzsprung-Russell Diagram Red Giants - Red Giant stars are very large, cool and quite bright. Ex. Betelgeuse is 100,000 times more luminous than the Sun but is only 3,500K on the surface. It’s radius is 1,000 times that of the Sun.

The Hertzsprung-Russell Diagram

White Dwarfs - White Dwarfs are hot but since they are so small, they are not very luminous.

The Hertzsprung-Russell Diagram Size of Star Mass of Star

Mass-Luminosity relation Most stars appear on the Main Sequence, where stars appear to obey a Mass-Luminosity relation: L  M 3.5 For example, if the mass of a star is doubled, its luminosity increases by a factor ~ 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