The Life Cycles of Stars

RVCC Planetarium - Last updated 7/23/03

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Stars have

• Different colors Which indicate different temperatures

• Different sizes

• Different masses

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

Stellar Evolution

Stars begin their lives as clouds of gas and dust called Nebulas

Clouds Contain:•70% H•28% He•2% HeavierElements

Stars start from Nebula Clouds

Collapse to Protostar

• Nebula compresses

• Gravitational pull attracts more material.

• Temperature and pressure increases

• Nebula shrinks and begins to spin

• Spinning nebula begins to flatten into a disk

….A protostar is formed

Protostars are forming in Orion

Protostars are forming in the Orion Nebula

•Pressure builds inside the protostar over millions of years creating heat

•Temp increases to 10 million°C

•Nuclear fusion begins….A star is born!!

•Main Sequence stage of a star begins

•It is the longest stage in life of a star

Nuclear Fusion !

• At 15 million degrees Celsius in the center of the star, Hydrogen Atoms fuses into Helium.

• It takes 4 Hydrogen atoms to make one Helium atom

• Sun has 1056 H atoms to burn which would last 8 billions year!

The Beginning of the End: Red Giants

After Hydrogen is exhausted in core:

• Core collapses, releasing energy to the outer layers

• Outer layers expand

The Core of a Red Giant

•Giants are 10 times bigger than the sun

•Supergiants are 100 times bigger than the sun

Expansion of the outer Layers

Betelgeuse – A Red Giant Star

Outer Layers are Expelled …

… And a Planetary Nebula forms

Planetary Nebulae

After Helium exhausted, outer layers of star expelled

Some Planetary Nebulae are Round

As the dead core of the star cools, the nebula continues to expand, and dissipates into the surroundings.

… and Bipolar

Some look like an Hourglass …

.. or a Cat’s Eye …

… or even look like an Eskimo!

White Dwarfs

At Center of a Planetary Nebula …

… sits a White Dwarfs

White dwarfs shine for billions of years before they cool completely.

The Hubble Space Telescope has detected white dwarf stars (circled) in globular clusters:

White Dwarfs are Small and Heavy

Size of the Earth with the Mass of the Sun

“A ton per teaspoon”

Nova is a white dwarf star that suddenly increases in brightness by several magnitudes. It fades very slowly.

Fate of High Mass Stars

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

• 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.

A Massive Star Explodes

A Supernova is an exploding massive star

Supernova 1987a

What’s Left After the Supernova

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

combine to form neutrons.

Black Hole (If mass of core > 5)• The force of contraction crushes the

dense core of the star• The gravity of a black hole is so great

that not even light can escape from it.

Supernova interaction

This compression starts the collapse of gas and dust to form new stars.

Supernovae compress gas and dust which lie between the stars.

Which brings us back to …

Sun-like Stars Massive Stars

Stellar Recycling

Nebula

Protostar

Low mass main sequence star

Red Giant

Planetary Nebula

White DwarfNova

High mass main sequence star

Supergiant

Supernova

Neutron Star

Black Hole