The Life Cycle of a Star

Today’s Objective

SWBAT – Analyze the life cycle of a star.

What is a Star?

A star is ball of plasma undergoing nuclear fusion.

Stars give off large amounts of energy.

There are billions of stars throughout the universe.

The closest star to us (besides the Sun) is Alpha Centari A. About 4.22 light years away.

X-ray image of the Sun

A Star is Born….

Stars are formed in a Nebula.

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

Protostars

Dense areas of gas in the nebula become more dense due to gravity.

When dense areas take shape they become protostars.

Protostars are NOT stars, yet.

Protostars

How do protostars become stars? As more gas is added to a protostar, the

pressure in its core increases. The increased pressure causes the gas

molecules to move faster, increasing friction. As temperature increases, heat is generated and

the temperature of the protostars core increases.

A new star!!

Once the core of a protostar is hot enough, nuclear fusion begins/

The protostar ignites and becomes a star.

The bright spot is a new star igniting

Nuclear Fusion

Nuclear Fusion is the process by which two nuclei combine to form a heavier element.

New stars fuse hydrogen to make helium.

Main Sequence Stars

Once the star has ignited, it becomes a main sequence star.

Nuclear Fusion continues Hydrogen fuses into helium. Releases massive amounts of energy.

Stars spend most of their life as a main sequence star.

Why do stars stay as main sequence stars?

• The core of a star is wherethe heat is generated and nuclear fusion takes place. • Radiation zonesand convectionzones 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 Forces• As long as there is a nuclear reaction taking place, the internal forces (nuclear fusion will balance the external forces (pressure and gravity.

• When the hydrogen in a mainsequence star is used up, fusion stops and the forces suddenly become unbalanced. • Mass and gravity cause the remaining gas to collapse on the core.

Red Giant

Collapsing outer layers cause core to heat up. Fusion of helium into carbon begins. Forces regain balance – pressure and gravity =

nuclear fusion 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.

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 Matters

Red Giant

Mass < 3 x sun

White Dwarf

Black Dwarf

Mass > 3x sun

Red Supergiant

Supernova

Neutron Star Black Hole

White Dwarfs

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

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

Planetary nebula around awhite dwarf star.

Red 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, again.

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

Expand up to 1000 times that of the sun

Supernova

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 than whole galaxies for a few years.

Gas ejected from a supernova explosion

Black Holes

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 can’t escape a

black hole, it is hard to tell

what they look like or how they work.