A Stars Lifespan
depends on its
Mass
Massive Stars live shorter
lives. Low mass stars live
longest.
1.Gravity contracts the
Hydrogen gas
2. Gas Spins
3. Gas Heats
4. Protostar Stage
5. Fusion begins in the
clouds core
6. Cloud glows brightly
7. Main Sequence Star
Gravity pulls the
densest pockets of
hydrogen gas
inward
The Gas
spins
faster, and
heats up.
Hydrogen collects in the
center of the swirling
disk .
The cloud begins to
shine brightly, a
young star is born in
the cloud
Sun Like Star – Long Lifetime
The protostar is now a
stable main sequence star .
Gravity pulls in – Pressure
pushes out
Star is in balance
Neither shrinks or expands
Yellow shining mass
The Sun is a Main Sequence Star
It fuses hydrogen
gas into helium
Lifetime: 10 billion
years.
Near the end hydrogen fuel is
depleted and the
star begins to die.
Our Sun is considered to be an
ordinary star with a spectral
classification of G2 V, a yellow
dwarf main sequence star.
Sun Like Stars how do they do it?
• In the star’s core protons collide and stick
together with a strong nuclear bond.
• A chain reaction occurs, 4 protons weld
together to make 2 protons & 2 neutrons.
• Hydrogen converts to Helium through nuclear
fusion.
• Every second the Sun through thermonuclear
reaction converts 600 million tons of hydrogen
into Helium within its core and emits a tiny
fraction of energy E=MC2,
• the radiation escapes into space bathing the
star’s surroundings in heat and light.
• This is what warms our solar system
As the Sun ages,
Eventually, the
Supply of hydrogen
in the core ends, and
a shell of hydrogen
surrounds the helium
core.
The Sun’s core
becomes unstable
The helium core
contracts and gets
hotter.
Red Giant star seen
from a planet
The Sun’s
hydrogen shell
expands
The Sun is
now a Red
Giant
Hydrogen in
the shell
around the
core continues
to burn
Its core temp
continues to
increase
Red Giant Phase
• Now the Helium
core contracts
• When the Hydrogen
shell ignites:
• The shell continues
to push outward
• Sun becomes
enormous
• It goes from
• 1 million to 100
million miles in size
• Helium ignites, it starts
to fuse into Carbon and
Oxygen. The core
collapses.
• The outer layers are
expelled.
• It becomes a brilliant
cool variable star for
thousands of years like
Betelgeuse in Orion.
Actual photograph of
Betelgeuse
Eventually all of the
hydrogen gas in the outer
shell of the Red Giant is
blown away by stellar winds
to form a ring around the
core. This ring is called a
planetary nebula. The core
is now a hot white dwarf star.
A white dwarf star is left
in the center of the
dying red giant star,
surrounded by the red
giant’s expanded
atmosphere
• A White dwarf star is a
dense stable star about
the size of the Earth
weighing three tons per
cubic centimeter.
• It radiates its left-over
heat for billions of years.
• When its heat is all
dispersed, it will be a
cold, dark black dwarf essentially a dead star
When massive stars ( At
least 5 times larger than
the Sun) reach the red
giant phase,
their core temperature
increases because
carbon is formed from
the fusion of helium.
Gravity pulls carbon
atoms together.
The core temp goes
higher forming oxygen,
then nitrogen, and
eventually iron.
•
The core becomes iron, fusion
stops. No energy.
•
Iron is the most stable element
and requires the most energy
of any element to fuse.
•
So, the core heats to 100
billion degrees, the sudden
lose of energy causes the core
to collapse
•
•
The iron atoms in the core are
crushed.
•
The core becomes rigid.
•
In falling layers of the star
strike the core,
•
then recoil in a Shockwave.
•
The shockwave hits the
surface and the star explodes.
If the core of a massive
star collapses when it is
1.5 to 3 times as
massive as our Sun’s
core. It ends up as a
neutron star. The
protons and electrons are
squeezed together by gravity,
leaving a residue of neutrons,
creating a neutron star.
Neutron stars (right) are about ten miles in
diameter. Spin very rapidly (one revolution takes
mere seconds!). Neutron stars are fascinating
because they are the densest objects known
except for black holes. A teaspoon of neutron star
material weighs 100 million tons.
Massive Stars (8 times or
more larger than the Sun.
Core remains massive after
the supernova.
Fusion is stopped.
Nothing supports the core.
The core is swallowed by its
gravity.
It becomes a black hole
Black holes are detected by Xrays given off matter that falls
into the black hole.
If Black Holes are Black,
How do We See Them ?
Optical
• Material swirls around
central black hole.
• Gas near black hole
heats up to UV and X-ray
temperatures.
• This heats surrounding
gas, which glows in the
optical.
Ultraviolet
Seeing Matter Disappear
• Hubble observed pulses of UV
light emitted by material as it fell
into a black hole.
• Pulses arise from material
orbiting around intense gravity of
the black hole.
• Light pulses, lasting 0.2 s, are
red-shifted from X-ray to UV, as
they fall into gravity of the black
hole.
Radio Jets from Black Holes
Many black holes emit jets.
 Material in jet moving at 0.9c.
 Jet likely composed of electrons and positrons.
Magnetic fields surrounding black hole expel
material and form the jet.
 Interaction of jet material with magnetic field
gives rise to Radio emission.
M87 - An Elliptical Galaxy
With a curious feature
Radio shows the origin of the Jet
X-ray: Jets
Cen A is known to be a peculiar
galaxy with strong radio emission.
Optical image of Cen A
But it is also a strong X-ray
emitter, and has an X-ray jet.
Chandra image of Cen A
Black Holes