You're watching FreeSchool!
Space: it's vast and full of secrets waiting
to be discovered. Of all the incredible objects
scientists have found, out among the stars,
some of the most spectacular are nebulae.
Although most nebulae are completely invisible
to the naked eye, modern telescopes are able
to give us close-up views of some of the most
beautiful objects in the galaxy.
A nebula, named for the Greek word for 'cloud,'
is in fact an interstellar cloud of dust and
gas. Like the clouds in the sky, nebulae come
in interesting shapes, like flowers, insects,
animals, or people. This is how many of them
get their names.
There are four main types of nebulae.
First, there are star-forming regions, also
known as stellar nurseries. In these huge
areas of hydrogen gas, gravity pulls the materials
together into larger clumps, which in turn
attract more material. More and more gas squeezes
together harder and harder, making it grow
hotter and hotter. When it becomes hot enough,
it will be a new star. Whatever is left over
may become planets, circling them.
An example of a star-forming region and one
of the most famous nebulae in the sky is the
Orion Nebula. The Orion nebula is the most
active area of star formation in our galaxy.
Although it is visible to the unaided eye,
it is only a vague glow. Through a telescope,
the extent of its wonder becomes visible.
The Orion Nebula is 15 light years across,
and is made mostly of hydrogen. When you look
at this nebula, you are looking at stars being
born.
This nebula gets its glow from something called
the Trapezium, a cluster of four stars, each
thousands of times brighter than our sun.
Their energy has cleared a space in the gases
around them.
You can see that other stars are being born
inside the nebula, too. These ovals are blankets
of dust surrounding newborn stars. The rings
inside are probably new planetary systems
being formed around these stars. 1300 light
years away, the Orion Nebula is only a little
glow in Orion's sword.
Second, there are planetary nebulae. This
kind of nebula doesn't really have anything
to do with planets, but when early astronomers
saw them through their telescopes, they looked
so round and bright they though they must
be planets. Planetary nebulae are formed by
the slow death of ordinary stars.
This beautiful object is known as the butterfly
nebula, and it is an example of a planetary
nebula. Although it looks peaceful, and like
a delicate butterfly from afar, it is actually
made of raging clouds of gas, heated to more
than 36,000 degrees fahrenheit. For comparison,
that is more than three times hotter than
the surface of the sun! The gas is also in
motion - moving at more than 600,000 miles
an hour. At that speed, you could go from
the earth to the moon in just 24 minutes!
The third kind of nebulae are called supernova
remnants. Sometimes when a star dies it doesn't
die slowly, it dies with a bang! A supernova
is a huge explosion that spreads the material
from the star across a huge expanse of space.
The leftovers form a nebula, which may over
time collect and form new stars.
The crab nebula is an example of a supernova
remnant. This nebula is especially exciting
because we know when the star that created
it exploded! Nearly 1,000 years ago, Chinese
astronomers noticed a bright new star in the
sky. It stayed there for almost two years,
and then slowly vanished. Modern astronomers
studying the Crab Nebula realized that the
bright, temporary star decribed in the ancient
record was an excellent match for the supernova
that would have created the Crab Nebula. Knowing
when the star went nova allowed scientists
to learn a great deal about the way this type
of nebula develops.
The fourth kind of nebulae are dark nebulae.
A dark nebula is a cloud of gas and dust that
is only visible because of bright nebulae
and stars behind it. Like a shadow or a silhouette,
they take their shape from the light we do
not see.
One of the most famous dark nebulae is the
Horsehead Nebula. It is easy to see where
this nebula got its name! The Horsehead Nebula
is made of thick dust clouds, so dense that
they block the light of stars behind them.
Just because the cloud is dark, does not mean
that it is cold. Looking at the nebula through
an infrared camera that allows us to look
at the heat it gives off, we can see that
the Horsehead Nebula is very hot, indeed.
There are even bright spots visible, which
show where new baby stars are forming.
No matter what type they are, nebulae are
among the most dazzling celestial objects to
be found in the sky. They are a window into
the life cycle of the universe. In the end,
the same raw materials that form stars are
left behind when stars die, to form in turn
new stars - and the cycle begins again.
I hope you enjoyed learning about nebulae
today. Goodbye till next time!
