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GALAXIES
• Galaxies fall into (roughly) two different structural types.
SPIRAL galaxies consist of a nucleus, a disk, a halo, and
spiral arms. Interstellar material such as gas and dust are
found in and near the spiral arm structure in the disk of
the galaxy. The spiral arms are sites of star formation,
which typically contain bright stars and OB associations,
which make the spiral arm structure easily discernible. Our
own Galaxy is a spiral. Spirals that are face-on towards us
appear like giant pinwheels.
• The other types of galaxies are ELLIPTICALS. As their name
suggests, they have elliptical or spheroidal like shapes.
The majority of their stars are old (K and M giants), and
they have much less dust and gas than the spirals.
– How are stars born?
• When a heavy body passes near or through the nebula,
its gravity causes swirls and ripples. It is no different in
a nebula when a star passes by. The "piles" of matter
continue to group together in the nebula until they are
gigantic clumps of dust and gas.
• At this stage, the star is called a protostar.
As the clumps of gas and dust become larger, gravity
squeezes them tighter, causing pressure and heat
to build.
Then, when the pressure in the center, the core, reaches a
temperature of 18,000,000 degrees F (10,000,255 K),
hydrogen fusion is initiated.
• Now, the protostar has become a star. It shines with its
own light. Its solar wind quickly pushes away the rest of
the dust and gas in its vicinity (solar wind is explained
in the comets section).
SUPERNOVA
• Main Sequence Star Death
• After about ten billion years, a main sequence star has
used up most of its hydrogen. The hydrogen core begins
to contract, and the outer layers begin to expand. At
that point, helium fusion begins. The star is now called a
red giant. Life expectancy from here on is about one
hundred million years.
• After 100,000,000 years, the red giant tries to fuse
its carbon into iron, but it does not possess enough
outer pressure to do so.
• The outer layers of the star drift off into space, and
become what is called a planetary nebula. Its core
collapses into a white dwarf star.
• Supergiant Star Death
• After about fifteen million years, a supergiant's
core fuses into iron. Then, when it attempts to fuse
the iron into heavier elements, it explodes in what is
called a supernova. A supernova explosion is
brighter than its parent galaxy.
• One supernova explosion on A.D. July 4,1054, was so
bright that it could be seen in broad daylight for
twenty-three days. The nebula it created is called
the Crab Nebula.
• A supernova creates a nebula surrounding the now
dead star. In the center, it leaves behind either a
neutron star, a pulsar, or a blackhole.
NEBULAE
• Nebula is a vast cloud of dust and gas which stars are born.
• Emission Nebulae
• Emission Nebulae are the most colorful. They shine internally
from young stars still in their stellar nursery. A large telescope
(8+ inches) will reveal most of the colors. To see all of the colors
a long-exposure photograph is required.
– Reflection Nebulae
• Reflection nebulae are nebulae which reflect stars' light. The
Pleiades are a good example; the star Merope's light is reflected
by a blue, wispy nebula near it.
– Dark Nebulae
• All nebulae in the truest sense are dark nebulae. They produce
no light of their own; they either reflect light from stars, or stars
illuminate them from their interior.
• However, the only nebulae that are classified under "dark" are
the ones that are dark. They have no stars near or within them
with which to illuminate themselves. They are only seen when
they omit light from background stars.
– Inplanetary Nebulae
• Planetary nebulae are the nebula which are
created when a main sequence star grows into a
red giant and casts off its outer layers. Some,
though, are remnants from the Big Bang.
• They came to be named "planetary" nebulae when
nineteenth-century astronomers saw that they
looked like the recently discovered Neptune and
Uranus. The name has stuck ever since.
– Supernova Remnants
• These nebulae are the creations of ancient
supernovas. The most famous example is the Crab
Nebula, created by a supernova on July 4, 1054.
BLACKHOLE
• A black hole, in theory, is a type of dead star.
After the supernova, if the remaining core is
three solar masses (three times the sun),
gravity will cause the core to collapse into a
neutron star or a pulsar. If the core is nine or
more solar masses, gravity causes it to
collapse into an infinitely dense point, which
makes such a large gravitational well that not
even light can escape. Thus, since nothing
ever comes out of it, it is called a hole. Since
you can't see it because light can't escape, it
is called a black hole.
PULSAR
– A pulsar is the same thing as a neutron star. The
only difference is that a pulsar emits two very highenergy beams of radiation from its magnetic poles.
Its magnetic field is one trillion times that of the
Earth's.
– A pulsar spins very rapidly. Most spin about once
every second, but the fastest one has been clocked
at 642 rotations per second.
– The anatomy of a pulsar is very simple. The outside
is a solid crust formed of neutrons. Inside is a liquid
neutron "soup." In the very center is a solid core of
neutrons.
NEUTRON STAR
– If a star has between 1.4 and 9 solar masses, it will become a
neutron star.
– A neutron star is a star made entirely of neutrons, as the name
suggests. After a star goes supernova, the remaining core
collapses. Gravity shrinks and condenses it into a sphere about
the size of Manhattan (fifteen mile diameter) in a few seconds.
However, to do this it has to break the barrier that keeps the
electrons and protons separated. Because of the mass of the
star, gravity can overcome this force. When that happens, the
electrons and protons cancel each other out. All that is left are
the neutrons.
– This process creates such a dense star, that if you could
somehow take a pinhead of material from it, it would weigh as
much as a super tanker.
• White Dwarfs
DRAWF
• A white dwarf is what remains of a dead main
sequence star. A white dwarf has the
approximate size of the Earth (7,500 mile
diameter). Its maximum possible size is 1.4
solar masses.
• It you were able to take a enough material
from a white dwarf to fill a matchbox, it would
have the weight of an elephant.
• A white dwarf is made of carbon. It does not
produce a fusion reaction. Therefore, it no
longer produces heat. As it floats through
space, it loses its heat over the course of