The Life Cycle

From birth to death

What is a star?

The objects that heat and light the planets in a system

A star is a ball of plasma held together by its own gravity

Nuclear reactions occur in stars

(H He)

Energy from the nuclear reactions is released as electromagnetic radiation

Characteristics of Stars

1) DISTANCE

Measured in light-years

The distance which a ray of light would travel in one year

About 6,000,000,000,000 (6 trillion) miles

186,000 miles per second

2) Magnitude (brightness)

A measure of brightness of celestial objects

Smaller values represent brighter objects than larger values

Apparent magnitude

How bright a star appears to be from Earth

Absolute magnitude (luminosity)

How bright a star actually is

3 & 4) Temperature & Color

The color of a star indicates the Temperature of the star

Stars are classified by Temperature

Decreasing Temp. (bright to dim)

http://www.seasky.org/cosmic/sky7a01.html

Hertzsprung-Russell Diagram

Birth of All Stars

In the nebula

Gases swirl and get hot - nuclear

fusion starts = a star is born!!!

Protostar - “earliest” star

Here is a nebula.

……and a protostar is born.

Life SpanDepends on its mass

Less mass = longer life

Small stars can live to 200 billion years

Medium (like our sun) live about 10

billion

Large (15x bigger than the sun) can live 10

million years

Death of Stars

Occurs when they run out of fuel

No more nuclear fusion

Beginning of the end

All main sequence (average) stars

become red giants

This is the beginning of the death

of a star

The inside shrinks and the

outside expands

The core heats up and releases energy.

The star heats, expands, reddens and

becomes a ……

Red Giant

The helium core continues to heat.

The last of the hydrogen gases drift

away.

These drifting gases form a nebula ring around

the star.

The star is dying. It grows fainter and

fainter.

Gravity causes its matter to collapse

inward.

The star is squeezed into a……

White Dwarf

Blue/white core is left behind

Size of Earth - very dense

Glow faintly

A chunk of the white dwarf the size of a sugar cube

They are so packed together…..

When the white dwarf’s energy is

gone,

it becomes a dead star/black dwarf

Would equal the mass of a CAR!

This is a white dwarf with a nebula cloud around it.

Massive Stars

……begin with about 6 times as much mass as our sun.

Like medium sized stars:

They burn hydrogen at first.

They redden as their helium core forms.

But, instead of becoming red giants ….

they become supergiants.

They expand into supergiants.

What happens next?

When the fuel runs out, gravity collapses the core

even more.

The iron core absorbs energy instead of releasing it….

The star breaks apart in a terrific

The entire sky is lit up for weeks.

The temperature can be more than 100 billion degrees.

The iron atoms fuse into uranium.

The uranium and gases explode into space.

A gigantic cloud of gas and dust is left and

may become a new nebula.

New stars can come from the nebula

remains.

A Supernova

This supernova is

surrounded by a nebula ring.

Neutron StarAfter the supernova explosion of a star 6-30 times

the mass of the sun, a neutron star remains.

How big is it?

a chunk the size of a sugar cube

It’s only about 16Km in diameter,

but it has so much mass packed into it,

has the mass of ~ 100

MILLION CARS!!!

Black Hole

Caused by the most massive stars - 40x

more massive than the Sun

Mass is left - gravity is so strong it

compacts

Packing 5 Suns into 15 miles!

The gravity from this black hole

is pulling this matter into itself.

Black holes are like invisible cosmic vacuum cleaners.

They swallow both matter and energy.

How can we find them if we can’t see them?

Matter falling into a black hole

releases large bursts of X-rays.

We can’t, for sure.

We can detect these.