Ever since the ancient Greeks, astronomers and scientists
alike have pondered the mysteries of the nebula. What are they composed
of? How large are they? Where are they located with respects to our galaxy?
What are the different classes of nebulae? These questions can now have
answers, thanks to the advances in technology.
Since before the twentieth century, many nebulae
have been discovered. The Orion Nebula
was discovered not to be a star, but instead a nebula, in 1610. Forty-six
years later, the bright inner regions of the nebula were described by Christiaan
Huygans, who found that the center was a compact cluster of inner stars.
So as not to confuse nebulae with comets, Charles Messier composed a catalogue
of 103 nebulous in 1781.
The 20th century has brought many advances in the
study of nebulae. In 1930, Bernhard Schmidt invented an extremely fast
wide angled camera, which allowed the detection of large, faint, nebulae.
Since the 1960’s, photoelectric devices more than 100 times as efficient
as photography has since been produced. Even more efficient than photography
radio telescopes have been produced because they can penetrate the nebulae
unhindered as an optical photograph cannot.
The physical conditions of nebulae are very interesting.
Nebulae are made up of 90% hydrogen, 10% helium, and 0.1% heavy metals.
The nebula is composed of carbon, nitrogen, magnesium, potassium, calcium,
iron, as well as a number of other atoms. Diffuse nebulae, planetary nebulae,
and supernova remnants are made up of mostly ionized atoms, meaning the
atoms have been stripped of a number of their electrons, so the nebula
produces its own light. Only the dark and diffuse nebulae are made up of
mainly neutral atoms and do not produce light, but instead either reflect
or absorb it.
Nebulae differ greatly in size. The Orion
Nebula is the only nebula visible to the naked eye, and is the brightest
of the diffuse nebulae. The largest diffuse nebula is the Gum Nebula ,
which measures 20 in angular diameter. The Helix
Nebula , which measures 20 of arc, is the largest planetary nebula
that we know of. The majority of planetary nebulae are 0.5 of arc. The
best known supernova remnant would be the Crab Nebula which measures 3
of arc.
A supernova remnant is the remnant of a stellar
explosion. When the star is at its height of explosion, it can emit as
much energy in one day as the our sun emits in 3,000,000 years. The star
blasts most of its mass away at thousands of kilometers per second, and
its remnants form clouds of gas that expand away from the site of the explosion.
Planetary nebulae appear as round, symmetrical disks
or rings in space, and often resemble out of focus planets. They have a
very bright surface and are 1000 times as luminous as other nebulae. The
average planetary nebula contains around 100 to 10,000 atoms per cubic
centimeter. The nucleus of the nebula provides ultra-violet light radiation
used for the ionization of the atoms. The closest planetary nebula in our
galaxy is about 300 light years away, but most of them are much too far
away to be absolutely sure. Planetary nebulae can be found near the galactic
center of our galaxy, indicating that they were ‘born’ when the galaxy
was about half formed.
A reflecting nebula reflects the light from nearby
stars. They normally reflect the light from a nearby star with a temperature
of about 20,000K or cooler. Any hotter, and the molecules within the nebula
would be ionized by the star resulting in the nebula producing its own
light and no longer being able to be called a reflecting nebula.
Dark nebulae appear as black patches in the sky,
and always block out the light of other stars behind them. These nebulae
were found because astronomers would notice large black patches in the
sky. They are located in the highly concentrated spiral planes of the galaxy.
The diffuse nebula is the most common of all nebulae.
They can be found in this galaxy as well as other galaxies. Diffuse nebulae
are faint, irregular shapes that produce the radiation they emit. They
are several light years across, and have a density of approximately 10
atoms per cubic centimeter. Although their total mass would be that of
several times our sun, their density distribution is irregular and chaotic.
Like that of the dark nebula, they are located in the spiral arms of our
galaxy.
Over the years, people have found many interesting
facts about nebula, from their composition to their location. Unfortunately,
we will never fully understand them until we can further develop our technology
to uncover the remaining mysteries of the nebula.