The Life Cycle of a StarThe Life Cycle of a Star

Mr. Anderson and Mrs. Gucciardo

What is a Star?What is a Star?

A star is ball of plasma undergoing nuclear fusion.

A star is almost entirely made up of Hydrogen and some Helium

Stars give off large amounts of energy in the form of electromagnetic radiation.

X-ray image of the Sun

A Star is Born….A Star is Born….

Stars are formed in a Nebula.

A Nebula is a very large cloud of gas and dust in space.

NebulaNebula Dense areas of gas in the

nebula become more dense due to gravity.

Soon the dense areas of gas take on a definite shape and are called protostars.

Sometimes these dense areas block starlight from shining through, they look like dark spots

The Nebula to the left is called the horsehead nebula, it can be found in Orion

ProtostarsProtostars

As more mass (gas) is added to a protostar, the pressure in its core increases.

The increased pressure causes the gas molecules to heat up and move faster, each collision contains more energy (kinetic theory).

As mass continues to get pulled into the protostar, temperature and pressure continue to rise, the gravitational field gets stronger.

ProtostarsProtostars This process

takes a really long time. Many stars are still being born 13 billion years after the “Big Bang”

A new star!!A new star!!

Once the core of a protostar reaches 27,000,000o F, nuclear fusion begins and the protostar is no more.

The protostar is now a star.

The bright spot is a new star igniting

NuclearNuclear FusionFusion

Nuclear Fusion is the process by which 4 hydrogen atoms combine to form a helium atom.

New stars initially will fuse hydrogen nuclei together to form helium.

Animation

NuclearNuclear FusionFusion

Nuclear Fusion produces a tremendous amount of energy from a pretty small package

The thermonuclear bomb pictured above is “old” 1960s technology and yielded about the explosive power of 680,000,000 pounds of TNT

Main Sequence StarsMain Sequence Stars Once the star has ignited, it becomes a main

sequence star. Main Sequence stars fuse hydrogen to form

helium, releasing enormous amounts of energy.

It takes about 10 billion years for a star like our Sun to consume all of its hydrogen.

Smaller stars last longer, big stars burn bright and then die

Balancing ActBalancing Act

The core of a star is wherethe heat is generated. The radiative and conductive zones move energy out from the center of the star.

The incredible weight ofof all the gas and gravitytry to collapse the star on its core.

Unbalanced ForcesUnbalanced Forces

As long as there is a nuclear reaction taking place, the internal forces will balance the external gravity.

When the hydrogen in a mainsequence star is consumed, fusion stops and the forces suddenly become unbalanced. Mass andGravity causes the remaining gasto collapse on the core.

Unbalanced ForcesUnbalanced Forces

It actually takes the light thousands of years to get from the center to the outside where it can race outward

The center of the sun is opaque.The light moves through the core of sun only slightly faster than it would through rock

Red GiantRed Giant

Collapsing outer layers cause core to heat up. fusion of helium into carbon begins. Forces regain balance. Outer shell expands from 1 to at least 40 million

miles across. ( 10 to 100 times larger than the Sun) Red Giants last for about 100 million years (this is

a really short time in the life of a star).

Unbalanced Forces (again)Unbalanced Forces (again)

When the Red Giant has fused all of the helium into carbon, the forces acting on the star are again unbalanced.

The massive outer layers of the star again rush into the core and rebound, generating staggering amounts of energy.

What happens next depends on how much mass the star has.

Mass MattersMass Matters

Red Giant

Mass < 3 x sun

White Dwarf

Black Dwarf

Mass > 3x sun

Red Supergiant

Supernova

Neutron Star Black Hole

Life Cycle of a star

White DwarfsWhite Dwarfs

The pressure exerted on the core by the outer layers does not produce enough energy to start carbon fusion.

The core is now very dense and very hot. (A tablespoon full would weigh 5 tons!)

The stars outer layers drift away and become a planetary nebula.

A white dwarf is about 8,000 miles in diameter.

After 35,000 years, the core begins to cool.

Planetary nebula around awhite dwarf star.

Black DwarfsBlack Dwarfs

As the white dwarf cools, the light it gives off will fade through the visible light spectrum, blue to red to back (no light).

A black dwarf will continue to generate gravity and low energy transmissions (radio waves).

Red SupergiantsRed Supergiants

If the mass of a star is 3 times that of our sun or greater, then the Red Giant will become a Red Supergiant.

When a massive Red Giant fuses all of the helium into carbon, fusion stops and the outer layers collapse on the core.

This time, there is enough mass to get the core hot enough to start the fusion of carbon into iron.

Red SupergiantsRed Supergiants

Once Helium fusion begins, the star will expand to be between 10 and 1000 times larger than our sun. ( Out to the orbit of Uranus )

Betelgeuse is Orions right shoulder

SupernovaSupernova

When a Supergiant fuses all of the Carbon into Iron, there is no more fuel left to consume.

The Core of the supergiant will then collapse in less than a second, causing a massive explosion called a supernova.

In a supernova, a massive shockwave is produced that blows away the outer layers of the star.

Supernova shine brighter then whole galaxies for a few years.

Gas ejected from a supernova explosion

SupernovaSupernova This supernova is in

the crab nebula, when it exploded in 1054 AD it was so bright it could be seen for 23 days during the day!

During the night it could be seen for 2 years. Now it is a pulsar

Supernova in crab nebula

SupernovaSupernova Supernovas can

also form when binary stars (two stars revolving around each other) get too close and one sucks mass from the other until BOOM!

Gas ejected from a suprnova explosion

Neutron StarNeutron Star Sometimes the core will survive the

supernova. If the core collapses with just the

right force, and has a solar mass less than 3 Suns it becomes a Neutron star.

In a neutron star the electrons in the atoms get forced into the nucleus.

Neutron stars are extremely dense, according to wikipedia, a cube of a neutron star would have approximately the same mass as every human on Earth combined

6 miles in diameter

Black HolesBlack Holes

If the mass of the surviving core is greater than 3 solar masses, then a black hole forms.

A black hole is a core so dense and massive that it will generate so much gravity that not even light can escape it.Since light cant escape a

black hole, it is hard to tellwhat they look like or howthey work.