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Supernovas occur with stars that have masses more than five times that of our sun. As a result of their huge gravitational fields, they develop extremely high pressures in their cores. This allows them to continue to generate energy by fusion even when their hydrogen supply has been depleted (remember that stars fuse hydrogen to produce helium plus huge amounts of energy). The fusion of helium produces even heavier elements and vast amounts of energy. These fusion reactions continue until elements as heavy as iron are formed. It is thought that fusion reactions involving iron result in the catastrophic collapse of the star's core. The outer layers of the massive star are violently blown off in a supernova explosion.
For several months, supernovas may shine 10 billion times more brightly than a normal star. Supernovas are rare, occurring only about once per century in a galaxy the size of the Milky Way.
Black holes are regions of space that exert incredibly powerful gravitational fields.
As a result, they act like gigantic cosmic vacuum cleaners that can suck in anything
that comes within their grasp including comets, planets, and even clouds of gas.
Not even light can escape the intense gravitational pull of a black hole. As a result,
they remain black and invisible.
When an object is pulled into a black hole it is crushed to infinite density and disappears forever.
Black holes are formed by the collapse of massive stars. These huge stars are
able to maintain their normal size during most of their life time because the heat generated
from thermonuclear reactions expands the star outwards and offsets the huge gravitational
pull. When the star expends its nuclear fuel, however, the star collapses under its
huge gravitational pull and a black hole is formed. To put the magnitude of
these forces into perspective, a
star the size of our sun, 864,950 miles ( 1,391,704 kilometers) in diameter, would have to shrink to less
than 1.9 miles ( 3.06 kilometers) to become a black hole!
Black holes often seem to be part of binary systems, where the black hole and a visible
star revolve around one another. The black hole, with its massive gravitational
field, pulls gas from the visible star into itself. When this happens the gas emits
strong X-rays which astronomers can detect.
A neutron star is the core of a
star that remains after a supernova explosion. Neutron stars emit intense X-rays. A neutron star has a mass about equal to the sun, but has a diameter of
only about 10 miles (16 kilometers). Neutron stars have a density estimated
to be about 100 million tons per cubic inch (5.5 million metric tons per cubic centimeter). At this
high pressure electrons and protons combine to form the neutrons of which the star is composed.
The temperature of the core is as high as 10 billion degrees F and the surface temperature can be 18 million degrees F.
Pulsar is an abbreviation for "pulsating radio star." This name was given for the obvious reason that they emit extremely regular pulses of radio waves. Some pulsars, however, emit visible light, X rays, or gamma rays.
Pulsars are thought to be rapidly spinning neutron stars that are extremely dense (a neutron star is the core of a star that remains after a supernova explosion). Most pulsars have a diameter of 6 miles (10 kilometers) or less.
A pulsar behaves much like a lighthouse beacon when it emits electromagnetic radiation (radio waves, visible light, X-rays, or gamma rays). Scientists believe that charged particles (protons and electrons) released from the surface enter an intense magnetic field that surrounds the star and rotates along with it.
The particles give
off electromagnet radiation as they are accelerated to near the speed of light. Hence the pulsar swings
an electromagnetic radiation beam around the star much like a beacon. Scientists are able to detect this pulsating radiation.
The interval between pulses ranges from about 4 seconds, for the slowest, to about
1.55 milliseconds for the fastest (the Millisecond Pulsar). It has been estimated that the Millisecond Pulsar
is spinning at 642 times per second! Scientists believe that this is near the limit of rotation, because
at faster speeds the pulsar would rip apart as a result of centrifugal force at the equator.
Quasars, or quasi-stellar radio sources, are believed to be
the highly luminous cores, or nuclei, of distant galaxies. The most baffling property
of quasars is the vast amount of energy that they produce. Many are
are up to a thousand times as bright as an average galaxy.
Calculations have indicated that many quasars are receding at greater than 90%
of the speed of light and that their distance from the Earth is enormous. Some are estimated to
be 14 billion light-years away. This incredible distance makes them the most distant objects
known and they probably mark the horizon of the known universe.
Quasars are estimated to be only one or two light-years in
diameter. This is surprising because they emit 10 to
1,000 times more electromagnetic radiation than a normal galaxy which
can have a diameter of greater than 100,000 light years.
Galaxies are enormous cluster of stars. Small galaxies may contain fewer than a million stars while very large galaxies may contain more than one trillion stars. Galaxies can be up to 100,000 light-years in diameter.
Galaxies come in four general shapes: 1) elliptical, 2)spiral, 3) barred spiral and 4) irregular. Elliptical galaxies show very little structure and are flat to spherical in shape. Spiral galaxies are flat and generally have a bright nucleus, and arms that come out of the nucleus and form a giant pinwheel shape. Barred spiral galaxies are also flat and have arms that extend sideways in a short straight line before curving into a spiral shape. Irregular galaxies are usually small and have no particular shape.
The Milky Way is our Galaxy. Some astronomers estimate that the Milky Way has more than 100 billion stars. The Milky Way is a member of a group of about 20 galaxies know as the Local Group. Farther out, there are estimated to be billions of other galaxies (sometimes called the extragalactic nebulae).
The stars that occur in the Milky Way are classified as Population I stars or Population II stars. Population I stars occur primarily in the spiral arms of the galaxy and include stars ranging in age from a few hundred thousand to 10 billion years old. Population I stars are rich in heavy elements (elements heavier than helium). Population II stars occur primarily in the area around the galactic nucleus. Population II stars are approximately 12 to 15 billion years old and are very deficient in heavy elements. These stars are the oldest members of our galaxy.
Now for some speculation. The earth and the life that occupies the planet are composed of numerous elements that are heavier than helium. It is indeed an interesting thought that the atoms that make up our bodies were forged in the core of distant stars and distributed throughout the galaxy by massive supernova explosions. Since Population I stars are rich in heavy elements, it is these stars that have the highest probability of harboring planets and life. Hence, if other life is to be found in the Milky Way, the greatest probability would be in the spiral arms and not in the galactic core.
Comets are characterized by a brilliant nucleus surrounded by a
fuzzy coma of light. From the coma extends a less luminous tail.
The nucleus, or core, of a comet has a composition resembling a large, dirty snowball.
They are thought to consist of approximately 25 percent dust and/or chunks of rock and about 75 percent ice. The ice consists primarily of frozen water,
with smaller amounts of methane, ammonia, and carbon
dioxide radicals.
So far approximately 850 comets have been recorded. The nucleus of comets range in size from about 0.3 mile (0.5 kilometer) in diameter up to 42 miles (88 kilometers) in diameter. The tail, however, can be up to
to 93 million miles (150 million kilometers) long. The tail of a comet always faces away from the sun
as a result of the solar wind.
When a comet approaches the sun its ice
begins to sublime (i.e. it passes from a solid to a gas form).
The sublimed gas also contains dust particles. The
gas and dust spread out around the nucleus to form a large, thin atmosphere
that is called the coma. Sunlight causes the gas atoms in the coma to glow. The
amount of gasses present in the coma determine how bright the comet will be.
Astronomers refer to comets as short-period comets or long-period comets. Short-period comets have orbital periods of less than 200 years and are usually members of the inner solar system. Long-period comets, on the other hand, can have periods on the order of millions of years and may have orbits that extend halfway to the nearest stars.
Stars are gigantic globes of incandescent gases (primarily hydrogen). They are powered by thermonuclear fusion which is the process of combining two light elements to form a heavier element plus large amounts of energy.
The brightness of a star depends upon its size and temperature. Astronomers refer to the brightness of a star as its magnitude. Magnitude is defined in two ways, the apparent magnitude and the absolute magnitude. Apparent magnitude refers to how bright the star appears from Earth . Absolute magnitude is the value its apparent magnitude would be if the star were a distance of 10 parsecs (a parsec is equal to 3.26 light years) from Earth. Stars can range in size from one tenth the size of our sun to more than 100 times as large.
The most common variety of stars include 1) giants and supergiants, 2) main sequence stars, and 3) white dwarfs. Giants and supergiants are generally yellow or red in color. These are very large stars that radiate huge amounts of energy but are generally cooler than our sun. Main sequence stars make up the majority of stars that we observe. These stars tend to radiate energy that is proportional to their surface temperature. Hence, hot main sequence stars emit much more energy that cooler main sequence stars. White dwarf stars are very small in size. They are white in color and they emit more energy per unit area of surface than our sun. However, the TOTAL energy output from a white dwarf is much lower than our sun as a result of their small size.