The Life-Cycle of a StarThe Life-Cycle of a Star

The Nebular Model

• A nebula is just a cloud of interstellar dust and gas.• Nebulae are sometimes referred to as “baby

factories for stars”.• Stars are formed from the dust and gas from a

nebula.• The nebula is important because it is dense enough

to form stars – most other locations in the universe are not that dense.

• are believed to be formed by exploding stars or left over from the beginning of the universe.

The Bubble Nebula

The Ghosthead NebulaDo you see the ghost?

The Eagle Nebula

The Beginning

• Our Solar System began as a Nebula.

The Beginning

• Our Solar System began as a Nebula.

• Something, a large star passing by maybe, started the nebula spinning in a counterclockwise direction.

The Beginning

• As this loose mass of gas and dust spun, it began to flatten out, kind of like pizza dough.

The Beginning

• As this loose mass of gas and dust spun, it began to flatten out, kind of like pizza dough.

The Beginning

• As this loose mass of gas and dust spun, it began to flatten out, kind of like pizza dough.

• But as it flattens it begins to form a bulge in the center.(I wonder what will form here?)

The Beginning

• The Sun will form in the bulge and the planets will form in the accretion disk.

• 99% of all the mass of the Nebula ends up in the bulge.

Sun

Accretion Disk

The Formation of the Sun

• Our Sun forms in the bulge of the nebula.

• As the gasses and dust became more compact, they began to attract each other towards the center. This is called gravitational contraction.

The Formation of the Sun• As the particles got closer

together they sped up - this increased their kinetic energy (energy of motion).

• Since this increase in kinetic energy also means a increase in temperature, the bulge is getting hotter.

• It is also getting denser.

• At this point what will become our local star, the Sun, is just a protostar.

The Formation of the Sun

• Ultimately the temperature and density reach critical values for nuclear fusion to occur.

• At this point our protostar has become a star. We are so proud!!!

The Formation of the Sun• In nuclear fusion, two hydrogen

atoms are given enough energy to come together and form a helium atom.

• This releases more energy.

• The energy released in this process is what powers the sun. Some of it causes more hydrogen to fuse into helium, and the rest works its way into space.

• Let’s watch.

H H

Fusion of Hydrogen in the Sun

H H

Fusion of Hydrogen in the Sun

H H

Fusion of Hydrogen in the Sun

H H

Fusion of Hydrogen in the Sun

H H

Fusion of Hydrogen in the Sun

H H

Fusion of Hydrogen in the Sun

H H

Fusion of Hydrogen in the Sun

H H

Fusion of Hydrogen in the Sun

HH

Fusion of Hydrogen in the Sun

Fusion of Hydrogen in the Sun

He

The fusion reaction in the core of a star doesn’t stop at Helium.

Helium (Atomic # = 2) can fuse with Hydrogen (Atomic # = 1) to form Lithium (Atomic # = 3).

Two Helium (Atomic # = 2) atoms can fuse to form Beryllium (Atomic # = 4).

In stars the size of the Sun (medium to small) this process continues up to Carbon (Atomic # = 6). Larger stars can provide more energy so this process continues up to Iron

(Atomic # = 26).

This is another view of the Eagle Nebula. At the top of each pillar you can see stars being

born.

This is a view of another Nebula, Abaurigae. At the center you can see a star being born.

This is a view of another Nebula. In the upper right you can see a star being born.

This is a view of the Orion Nebula. The bright spot is a star which has formed. The dark ring is an

accretion disk where planets may form.

Fusion & GravityFusion exerts an external Pressure outward on the star

and gravity pulls inward. The two opposing forces balance each other out. This determines the size of the

star

FusionGravity

Fusion & GravityFusion exerts an external Pressure outward on the star

and gravity pulls inward. The two opposing forces balance each other out. This determines the size of the

star

FusionGravity

Fusion & GravityFusion exerts an external Pressure outward on the star

and gravity pulls inward. The two opposing forces balance each other out. This determines the size of the

star

FusionGravity

Fusion & GravityFusion exerts an external Pressure outward on the star

and gravity pulls inward. The two opposing forces balance each other out. This determines the size of the

star

FusionGravity

Fusion & GravityFusion exerts an external Pressure outward on the star

and gravity pulls inward. The two opposing forces balance each other out. This determines the size of the

star

FusionGravity

Fusion & GravityFusion exerts an external Pressure outward on the star

and gravity pulls inward. The two opposing forces balance each other out. This determines the size of the

star

FusionGravity

Fusion & GravityFusion exerts an external Pressure outward on the star

and gravity pulls inward. The two opposing forces balance each other out. This determines the size of the

star

FusionGravity

Gravity & Fusion

• Gravity & Fusion determine the size of a star.

• Gravity pulls the star inward.

• Fusion pushes the star outward.

• The size of the star is determined by the equilibrium between these two forces.

Size and Color of a StarThe size of a star is determined by the tug-o-war between gravitational contraction and the outward pressure of the fusion reaction.

FusionGravity

Size and Color of a StarThe size of a star is determined by the tug-o-war between gravitational contraction and the outward pressure of the fusion reaction.

But the speed of the fusion reaction determines the color of the star.

FusionGravity

Size and Color of a StarThe size of a star is determined by the tug-o-war between gravitational contraction and the outward pressure of the fusion reaction.

But the speed of the fusion reaction determines the color of the star.

If the fusion reaction is slow, the star is small, cool (3200 K) and red.

FusionGravity

Size and Color of a StarThe size of a star is determined by the tug-o-war between gravitational contraction and the outward pressure of the fusion reaction.

But the speed of the fusion reaction determines the color of the star.

If the fusion reaction is a little faster, the star is bigger, warmer (5800 K) and yellow-orange.

FusionGravity

Size and Color of a StarThe size of a star is determined by the tug-o-war between gravitational contraction and the outward pressure of the fusion reaction.

But the speed of the fusion reaction determines the color of the star.

If the fusion reaction is even faster, the star is bigger, hot (45,000 K) and blue.

FusionGravity

Size and Color of a StarIronically, the bigger the star, the shorter its lifespan.

This is because the fusion reaction is running so fast in large stars that the available fuel is used up very quickly.

A blue star lasts around 800,000 years.

Our Sun (Yellow) 10 billion years.

A red star about 2,000 billion years.

FusionGravity

The Death of Our SunIn about 5 billion years our sun will use up all its Hydrogen and the

fusion reaction will stop. At this point gravity is the only force in the sun and the sun will begin to collapse.

The Death of Our SunBut as the sun begins to Collapse it will get hotter, just like when it

first formed.

The Death of Our SunAnd Hotter !!!

The Death of Our SunUntil fusion begins again and …..…..

The Death of Our SunThe sun expands …..

The Death of Our SunInto a …..

The Death of Our Sun

Until …..a red giant

enveloping and ultimately incinerating the inner 3

planets. This will include us.

Ouch !!!!

BetelgeuseBetelgeuse is a red supergiant located in the constellation of

Orion. Betelgeuse is Orion’s right shoulder.

Betelgeuse will go supernova within the next 10,000 years.

When it does we will see it for two weeks during the day and it

will cast shadows at night.

If you are lucky you may get to see this spectacular event!!!

The Death of Our Sun

Finally the Sun will use up its remaining fuel creating elements up through

Carbon. Bigger, hotter stars can actually create elements through Iron.

The Fusion Reaction Stops at Carbon (Iron)

The Death of Our SunAt this point the outer layer of the sun will be

blown into space along with the elements formed in its core and the heavier elements formed during the “explosion”. These elements will be used to

form other stars and planets.

The atoms that formed you probably came from the death of an earlier star!!!!!

What remains of the Sun will collapse into a white dwarf- a cooler but denser burnt out ember of the

sun

White Dwarf H1504

Eventually this white dwarf will

cool and no longer emit light. At this point it

will no longer be visible.

This is the fate of our Sun.

If the sun were slightly larger it would become a neutron star.

A neutron star is much denser than a white dwarf.

1 teaspoon of matter from a neutron star would weigh as much as a mountain.

If the sun were 3 times as large it would never stop collapsing on itself and become a

black hole.