All 88
constellations are covered in this section. Each page will contain
information about the constellation in general and a selection of
interesting deep sky objects. Underneath the name of
each constellation will be text that tells what the constellation is
supposed to be. To the right, there will be two icons. One will tell you
whether it is in the north or south celestial hemisphere and the other
icon will tell you the season that the constellation is visible in. Some
of the constellations in the northern celestial hemisphere can be seen
from southern countries and also the other way around. Also seasons are
reversed in the southern hemisphere of Earth, so when it is winter in
the north, it will be summer in the south.
There will be descriptions of some of the deep sky objects
in the constellations. Next to each deep sky object that is described
will be an icon that represents either an eye, pair of binoculars or a
telescope. This is the minimum equipment required to see the object. A
better instrument than the one shown in the icon will show the object in
more detail.
The ordering of the deep sky objects on each page is from
popular to obscure with bright well known objects at the top and faint
obscure objects at the bottom. Also some deep sky objects have an
informal popular name such as NGC 2024 in Orion also being referred to
as the Flame Nebula. Popular names are given to some objects because
they resemble certain objects or they were discovered by famous
individuals. An object isn't necessarily well known if it has a popular
name such as the obscure nebula DWB-111 also having the name of the
Propeller Nebula. Some objects might not resemble the object referred to
in their popular name in images as they only have the resemblance
visually through a telescope as photographs can show the fainter parts.
Also some deep sky objects have more than one popular name, for example
the emission nebula known as M17 is also called the Swan Nebula and
Omega Nebula.
Most pages will have some images illustrating both popular
and obscure objects and every single image is true colour. The colours
seen in the images will look a lot more dim with the eye through a
telescope. There are many reasons for this such as receptors in the eyes
shutting down at night. The primary reason is the vast distances the
light from deep sky objects has to travel to reach the eyepiece. Another
reason is that eyes are more sensitive to green light, most deep sky
objects that show any colour through a telescope are green or blue
planetary nebulae. Even with huge professional observatories, virtually
no colour can be seen. Colours of star clusters can also be seen, this
is more apparent with star clusters that have different coloured stars
such as the Jewel Box Cluster. This is because the colours seem enhanced
by a contrast effect. The colours of stars will look different to
everybody, what looks like a red star to one person might look orange
coloured to another.
Colour is not the only thing that is barely visible through
a telescope, many details and structures of galaxies and nebulae seen
in photographs are not seen through a telescope. The reason they are
visible in photographs is the more efficient light gathering
capabilities of high resolution cameras, they are vastly more sensitive
than our eyes. Another reason is that most images are the result of
being exposed for many hours over multiple nights. Most images have
exposure times of tens of hours as the longer the exposure time is, the
more brighter, colourful and detailed an object will appear.
In the photography of nebulae, details can be accentuated
or new details revealed if they are imaged with specialised filters. The
most common one of these is an 'hydrogen alpha' filter as it
specifically isolates the light emitted by hydrogen molecules. This
works best with emission and planetary nebulae as well as certain
reflection nebulae. Another one is 'OIII' and isolates light from oxygen
atoms, this can be used when imaging planetary nebulae and supernova
remnants. When certain planetary nebulae are imaged with OIII or Ha
filters, a halo can be seen surrounding them. Examples include the Ring
Nebula and the Dumbbell Nebula. These filters aren't specifically for
astrophotography, they are also used by visual observers to observe deep
sky objects and to aid in finding them although a 'H-beta' filter
rather than Ha is used for observing emission nebulae.
Many images of nebulae taken with professional
observatories and the Hubble Space Telescope are usually false colour.
The reason is to uncover parts that aren't visible normally as well as
revealing the distribution of gases. These images are taken with Ha,
OIII and SII filters and are also commercailly available to amateur
astronomers. Even though they are false colour, they are still
considered to be in the optical wavelength. None of the images on the
constellation pages are false colour but some of them link to one as
they are quite dramatic and beautiful.
Images can be taken in different wavelengths such as
infrared and ultraviolet. These make a dramatic difference in revealing
details. For example infrared images can reveal star clusters in star
forming regions that are invisible to the human eye. This can apply to
objects other than nebulae, an image of a star taken in 2007 with an
ultraviolet observatory revealed a huge pluming tail, this would also be
invisible optically. Images of galaxies taken in different wavelengths
can reveal more information than true colour optical images such as how
active a galaxy is and massive radio jets that aren't visible optically
such as the ones in the peculiar galaxy, Centaurus A.
Deep sky objects have been catalogued following the invention of the telescope. The
most well known catalogues are the Messier and the New General
Catalogue. This can also be abbreviated to ‘NGC’ and Messier is commonly
referred to as ‘M’. There are other catalogues and these are referred
to by their abbreviations. On the constellation pages, when there is a
description of an object belonging to a catalogue that hasn't been
previously mentioned, an explanation of the catalogue and what the
abbreviation stands for is provided.
Some catalogues are dedicated specifically to a certain
type of object, such as the Sharpless catalogue of nebulae and the Abell
galaxy cluster catalogue. There are very few that encapsulate various
different types but include the aforementioned NGC and Messier catalogue
that are the staples of telescopic observations as well as less common
ones such as the 'IC', the Index Catalogue, which is a subset of the New
General Catalogue.
Many objects tend to be in more than one catalogue. For
example the Pinwheel Galaxy in Ursa Major is commonly referred to as
M101 but is also known as NGC 5427. It also happens to be the 26th
entry in the Arp atlas of peculiar galaxies. Another example is the
famous Orion Nebula, M42 is the instantly recognisable designation but
it is also NGC 1976 as well as Sharpless 281. |