Page 114 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

DEEP SKY

NEBULAE

NEBULAE COME IN A PLETHORA OF SHAPES AND
SIZES, AND ARE AMONG THE MOST DIVERSE AND
BEAUTIFUL DEEP-SKY OBJECTS YOU CAN SEE

Before the dawn of the
telescope, only exceptionally
bright objects were visible to
those looking skywards. They
only hinted at the wide variety
of different objects lurking in
the depths of our Universe: the
so-called deep-sky objects.
In the old days they all
The Horsehead Nebula
went by the Latin name of
is a great example of
nebulae, or ‘little mists’ in
a dark nebula
English, simply because they
all looked like small, light,
fuzzy, foggy patches in the many lightyears across. This Nebulae are divided into
otherwise black night. Once doesn’t make nebulae any less different classifi cations,
telescopes became more interesting though, because and every one of them lets us
powerful, it became clear that they still include many of know what is going on in the
T.A.RECTOR (NOAO/AURA/NSF) AND HUBBLE HERITAGE TEAM (STSCI/AURA/NASA), N. SMITH (UNIVERSITY OF CALIFORNIA, BERKELEY) AND
not all nebulae were the same. the most visually stunning cloud or how it was formed. from the newborn stars created
Astronomers became aware things in deep space. Their These include general emission, in the nebula causes the cloud
that there were objects as varied colours can be revealed by a refl ection and dark nebulae, of gas and dust to glow, and
as galaxies, globular clusters good camera attached to a as well as planetary and this lets us see it. The nebula
and supernova remnants. telescope. They’re also the supernova remnants. Each has itself is actually giving out
NOAO/AURA/NSF, NASA/ESA/J. HESTER AND A. LOLL (ARIZONA STATE UNIVERSITY), PAUL WOOTTON, WILL GATER
The term ‘nebula’ became result and cause of some of a story to tell. light, hence its name.
narrower, being used to describe the most exciting activity in Emission nebula, like
deep-sky objects made up of the Universe: star death the Orion Nebula, are the LIT BY THE STARS
huge clouds of dust and gas and star birth. birthplace of stars. Radiation Refl ection nebulae, like the
one around the Pleiades star
cluster, are only visible because
THE STELLAR NURSERY there are some stars nearby
that light up the gas and dust,
just as the Sun lights up a cloud
in an otherwise blue sky.
Dark nebulae, such as the
Horsehead Nebula, are dark
dust clouds. They are only
visible because they are in front
of a bright nebula or fi eld of
stars. We effectively see a
silhouette of the cloud, but
1 Nebulae are where stars are 2 Gravity causes clumps of the 3 If the temperature in the clump no detail in it.
created. One idea of how it all nebula to pull together. The reaches 10 million °C, the nuclear You might think that
starts is that a shock wave from pressure at the centre of the furnace that powers stars ignites. planetary nebulae, such as the
a nearby supernova explosion clumps builds and the temperature Over tens of millions of years it
compresses the cloud. Once the rises dramatically. If there is settles into normal life and joins Ring Nebula, have something
density of the gas passes a critical enough gas to fuel the process, what’s called the main sequence, to do with planets, but you’d
point, gravity takes over. the region can become a protostar. like our own Sun. be wrong. They get their name
114 skyatnightmagazine.com 2012