Twinkle, Twinkle, Little Star ...

How I Wonder What You Are ...

• Stars have

• Different colors– Which indicate different temperatures

• Red stars- cooler

• White/blue stars- hotter

• The hotter a star is, the faster it burns its life away.

Stellar NurserySpace is filled with the stuff to make stars.

New Stars are not quiet !

Expulsion of gas from a young binary star system

Fusion– Inside a star, the density and temperature

increase toward the center, where energy is generated by nuclear fusion.

– Fusion reactions involving elements other than hydrogen can produce heavier elements, but few heavier than iron.

– The energy produced according to the equation E = mc2 stabilizes a star by producing the pressure needed to counteract gravity.

A Balancing Act

• Energy released from nuclear fusion counter-acts inward force of gravity.

• E = MC2

Throughout its life, these two forces determine the stages of a star’s life.

Hydrostatic equilibrium

Star Life Cycle

• What happens during a star’s life cycle depends on its mass.

• Higher mass stars live shorter lives!– It takes about 10 billion years for a star

with the mass of the Sun to convert all of the hydrogen in its core into helium.

– When the hydrogen in its core is gone, a star has a helium center and outer layers made of hydrogen-dominated gas.

The Beginning of the End: Red Giants

– Some hydrogen continues to react in a thin layer at the outer edge of the helium core. This forces the outer layers of the star to expand and cool and the star becomes a red giant.

– As the star cools it begins to contract and heat again, eventually starting fusion all over.

– When the helium in the core is all used up, the star is left with a core made of carbon.

The end for solar type stars

Planetary Nebulae

After Helium exhausted, outer layers of star expelled

White dwarfs

• At center of Planetary Nebula lies a • White Dwarf.

• Size of the Earth with Mass of the Sun “A ton per teaspoon”

• Inward force of gravity balanced by repulsive force of electrons.

Fate of high mass stars

• After Helium exhausted, core collapses again until it becomes hot enough to fuse Carbon into Magnesium or Oxygen.

– 12C + 12C --> 24Mg

OR 12C + 4H --> 16O

• Through a combination of processes, successively heavier elements are formed and burned.

The End of the Line for Massive Stars

• Massive stars burn a succession of elements.

• Iron is the most stable element and cannot be fused further.– Instead of

releasing energy, it uses energy.

Supernova !

Supernova Remnants: SN1987A

a b

c d

a) Optical - Feb 2000• Illuminating material

ejected from the star thousands of years before the SN

b) Radio - Sep 1999c) X-ray - Oct 1999d) X-ray - Jan 2000• The shock wave from

the SN heating the gas

Elements from Supernovae

All X-ray Energies Silicon

Calcium Iron

What’s Left After the Supernova

• Neutron Star (If mass of core < 5 x Solar)• Under collapse, protons and electrons

combine to form neutrons.• 10 Km across

• Black Hole (If mass of core > 5 x Solar)• Not even compacted neutrons can

support weight of very massive stars.

A whole new life: X-ray binariesIn close binary systems, material flows from normal star toNeutron Star or Black Hole. X-rays emitted from disk of gas around Neutron Star/Black Hole.

Reprise: the Life Cycle

Sun-like Stars Massive Stars