Stars

Life cycle of starhttp://www.youtube.com/watch?v=H8Jz6FU5D1A

http://aspire.cosmic-ray.org/labs/star_life/starlife_proto.html

1. Nebula = cold, dark cloud of dust and gas [mostly H]

gravity pulls particles, to clump together

Accretion: accumulating more mass

Clumps increase in size [accretion]pressure and temperature increases [due to KE

of particles]

2. Protostar when forces are in equilibrium : gravity collapses particles together, high temperature/pressure

pushes particles apart.

At equilibrium Gas pressure = gravity

If protostar too small, becomes brown dwarf, never a real star.

4. STAR If enough mass, critical temperature protostar core begins

to fuse H into He :star is born.

The star is stable while the gravitation

pulling the star together is balanced

by the internal pressure pushing it apart.

During lifetime, stars fuse Hydrogen into Helium

and then fuse helium into Carbon.

Massive stars can fuse carbon into heavier elements.

3. Main sequence

90% of stars spend majority of life on the Main Sequence

On the main sequence, hydrogen is fused into helium

H is fused into He..

Main sequence is a grouping of stars by size, color, luminosity

We compare other stars to the brightness [luminosity ] of the Sun:

Temperature determines the color of a star

Temperature Color

30,000 - 60,000 K Blue stars

10,000 - 30,000 K Blue-white stars

7,500 - 10,000 K White stars

6,000 - 7,500 K Yellow-white stars

5,000 - 6,000 K Yellow stars (like the Sun)

3,500 - 5,000K Yellow-orange stars

< 3,500 K Red stars

Mass-luminosity relationship for Main Sequence Stars

L = Lo[M/Mo]3.5

Lo = luminosity of SunMo = mass of Sun

The Herzsprung-Russell Diagram H-R diagram

shows relationship of luminosity, temperature, size

[ H-R diagram]

Note the scales are log scales, not linear.

Mo is the mass of the Sun

Low mass stars: 0.5Mo

medium mass stars: 0.5Mo – 3Mo

massive stars: >3 Mo

Larger, hotter stars burn out very fast - in a few million years.

Smaller, cooler stars burn slowly for billions of years

The Sun is a medium yellow star.

The surface temperature is about 6000K

The Sun has a life expectancy of about 10 billion years

Try the interactive star lab: http://aspire.cosmic-ray.org/labs/star_life/starlife_sequence.html

Wh

After becoming red giantlow mass stars [mass of Sun] shrink

into white dwarfs.

Radius is close to radius of Earth.

Medium mass stars [ 1.4 to 3 Mo] expand into Red Giants , collapse

and then explode into a supernova

Finally becomes a neutron star

[about the size of Manhattan]

Massive stars collapse and begin to fuse carbon.They explode as supernova and the core is

crushed into a black hole .

After all H is fused, star leaves the main sequence.

Pressure increases, stars expand enormously and cool.

Small stars expands and sheds outer layers

Medium stars expand into red giants

After about 100,000 years the protostar has drawn in enough dust and gas to become a

star.

When core temperature reaches 10 million degrees, H begins to fuse into He,

releasing energy [heat]

Most stars are Main Sequence Stars

When core H has all fused to He,

About 1 million Earth’s fit in the Sun

http://www.youtube.com/watch?v=g4iD-9GSW-0

Sun compared to largest star

http://www.youtube.com/watch?v=Bcz4vGvoxQA

HR diagramshttp://www.youtube.com/watch?v=Kqe6F-Qf9Tk

Black hole swallows a starhttp://www.youtube.com/watch?v=O3Z5AS3TTS4