The nebulae are regions of rarefied gas and dust, with a diffused appearance,
present almost everywhere in our galaxy and in others. Years ago, the name
indicated all the distant luminous sources with a diffused appearance.
Later, it became clear that many of them were actually external galaxies,
like for example the Andromeda nebula.
In general, there is a distinction between dark and bright
nebulae: the latter are gas clouds illuminated by a star. They originated
in many ways, for example during the explosion of a supernova,
or when a planetary nebula expels
the outer layers of gas: in such case, a very hot stars remains in the
centre of the nebula, emitting ultraviolet radiation; the radiation excites
the gas of the nebula and causes the emission of light from it. In the
case of the remains of a supernova, the gas has has a speed of 1,000-2,000
Km/s and heats up in the collision with the interstellar gas, emitting
radiation. Moreover, the electrons of such gas are accelerated by the magnetic
field and emit radio waves, through a mechanism known as synchrotron.
Then there are reflection nebulae, that contain grains of dust that
diffuse the light of the nearby stars.
The dark nebulae are gas clouds containing dust and with no stars to illuminate them; they do not emit light and obscure any object at the back.
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The NGC 604 nebula in the Triangle constellation. NGC 604 is near the M33 galaxy, and is a large region of stellar formation: it is almost 1,500 light years long and there are over 200 hot and massive stars. They heat up the gas and cause the observed luminescence. (HST) |
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The Eta Carinae nebula consists of two huge fluctuating clouds of gas and dust. This picture has been reprocessed using various techniques and has one of higher resolutions: even though the nebula is over 8,000 light years far, it is possible to distinguish details of "barely" 15 billion kilometres, comparable to our Solar System. (HST) |
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N132D, the remains of a supernova in the Large Magellan Cloud, the
larger of the two nearest galaxies. The complex structure of the gas is
due to the collision of the hot material expelled by the supernova at a
high speed, (over 2,000 kilometres per second), with the cold interstellar
gas. Such impact produces a shock front that emits radiation. The blue-green
streaks correspond to gas regions rich in Oxygen, emitted from the centre
of the star.
(HST) |
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The nebula called "Cygnus Loop" covers a region of the sky that
is six times the diameter of the full moon. It is what remains after the
explosion of a supernova, 15,000 years ago.
(HST) |
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The NGC 7027 planetary nebula, situated 3,000 light years far from
the Cygnus constellation, shows new details of the death of a star.
(HST) |
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The NGC 3324 dark nebula, 9,000 light years far from us.
(Courtesy of Bill Arnett) |
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The Helix Nebula, in the Aquarius constellation. It is the nearest
planetary nebula. The comet-like formations that you can see are probably
due to collisions between the hot gas emitted by the star and the clouds
of colder gas of the surrounding environment.
(HST) |
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NGC 6523, the Lagoon Nebula, was named after the dark band of dust
that separates the two bright regions. It is 4,500 light years far from
us, in the Sagittarius constellation.
(HST) |
The Trifid Nebula in the Sagittarius constellation. (SEDS)
M27, the nebula with the shape of a handle,
in the Vulpecula constellation. (SEDS)
