Transcript What is a Star? - Lisle CUSD 202

The Life Cycle of a Star
Mr. Anderson and Mrs. Gucciardo
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….
 Stars
are formed in a
Nebula.
 A Nebula is a very
large cloud of gas and
dust in space.
Nebula
 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
Protostars
 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.
Protostars
 This
process
takes a really
long time. Many
stars are still
being born 13
billion years
after the “Big
Bang”
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
Nuclear Fusion
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

Nuclear Fusion
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 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 Act
The core of a star is where
the heat is generated. The
radiative and conductive
zones move energy out
from the center of the star.
The incredible weight of
of all the gas and gravity
try to collapse the star on
its core.
Unbalanced Forces
As long as there is a nuclear
reaction taking place, the
internal forces will balance the
external gravity.
When the hydrogen in a main
sequence star is consumed, fusion
stops and the forces suddenly
become unbalanced. Mass and
Gravity causes the remaining gas
to collapse on the core.
Unbalanced 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 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)
 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
Mass > 3x sun
White Dwarf
Red Supergiant
Black Dwarf
Supernova
Neutron Star
Black Hole
Life Cycle of a star
White Dwarfs

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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 a
white dwarf star.
Black 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 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 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
Supernova
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Gas ejected from a supernova explosion
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.
Supernova
 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
Supernova
 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 Star
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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 Holes


Since light cant escape a
black hole, it is hard to tell
what they look like or how
they work.
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.