Emission Nebulae
Emission nebula are clouds of gas in interstellar
space that are emitting light. Their energy comes from nearby
stars or stars that are embedded in the nebula itself. The
light from the stars excites the gas molecules in the cloud,
and they emit more light, usually at specific sets of frequencies
that depend on what the gas is made of. Typical suspects are
oxygen, hydrogen, helium, carbon, and others. Excited emission
is the same process that makes fluorescent light fixtures
glow.
Generally, emission nebulae are actually star-forming
regions. It turns out that a large fraction of the mass of
our galaxy is actually in the form of interstellar gas, so
there is plenty of material available. The cloud of gas is
gravitationally collapsing, and as pockets of local density
reach critical mass, a star is born. Newborn stars, depending
on their mass, burn quite brightly, and their light shines
out into the cloud, making the gas glow. The clouds are rather
large – typically several light-years across. Some "nearby"
emission nebulae cover a patch of sky as big as the moon.
Prime examples are the Great Nebula in Orion (M42 - shown
here), the Lagoon Nebula in Sagittarius (M8), and the Swan
Nebula (M17).
Because emission nebulae emit light at specific
frequencies, it presents the opportunity to use specialized
filters to observe them, increasing contrast and bringing
out structure and detail. We’ll discuss the use of filters
in the Tools section below.
Reflection Nebulae
Reflection nebulae are also clouds, but mainly
of interstellar dust that doesn’t emit light. In this case,
the dust plays the important role – it reflects light from
stars, making them visible. Typically, reflection nebulae
are somewhat fainter and more difficult to see, and the use
of filters is less effective because they are reflecting a
broad spectrum of light, not emitting at specific frequencies.
A prime example of a reflection nebula is the dusty glow surrounding
the Pleiades star cluster in Taurus (shown). The photo is
a time exposure to bring out the nebula. You can see it in
a modest telescope under dark skies, but less distinctly.
Dark Nebulae
Dark nebulae are kinfolk of reflection nebulae,
being clouds of interstellar dust. However, there are no nearby
stars to make them shine. So how do we see them? We only see
them when they lie in the plane of the Milky Way where they
block the light from the dense star clouds behind them. They
look like “holes” in the star cloud, but are actually much
closer to us than the background stars.
You can see large patches of dark nebulae with
the naked eye and binoculars just by looking at the Milky
Way from a very dark sky location. The best clouds
of this sort are in the summer Milky Way, in Cygnus, and in
Sagittarius (shown here). There are also several dark nebulae
that are best viewed in telescopes at low power (with a wide
field of view).
Perhaps the most famous dark nebula is the “Horsehead”
in Orion. It is a dark cloud shaped like a horsehead superimposed
on a background emission nebula.
Planetary Nebulae
The term planetary nebula is somewhat of a misnomer.
Early astronomers observed these curious non-stellar objects
with the limited equipment and knowledge of the era, and dubbed
them planetary nebula because they appeared round(ish), and
were similar in apparent size to planets like Jupiter. Actually,
Planetary Nebula are a shell of glowing gas closely surrounding
a star. They emit light by the same mechanism as emission
nebula, but their formation is entirely different.
As a star nears the end of its life, strange
things begin to happen in its core. It begins life fusing
hydrogen into helium (as our Sun is doing now), and in the
process converts matter into energy, giving the star its power
source. However, over time, it uses up the hydrogen in its
core. When this happens, the force balance in the star’s center
becomes unstable – the force of gravity is no longer balanced
by the radiation pressure from the fusion process. At some
point, gravity wins and the star begins to collapse. This
increases the pressure and temperature in the core dramatically,
until in a flash, the star begins to burn helium, fusing it
into heavier elements. The helium flash results in a sudden
outburst of radiation, causing the star to “burp” off a large
fraction of its outer layers of gas. The outer layers expand
outward rapidly, escaping the star’s gravity. A roughly spherical
shell of expanding gas then continues to grow in diameter,
for millions of years, and is excited into emission by the
central star, now burning hotly. By the time we see them,
the shell of gas is perhaps 1-5 light years across.
Planetary nebula are perhaps some of the strangest
looking and most widely varied objects in the sky. The precise
conditions of their creation give rise to various asymmetries
in their structure. For example, a nice, solitary star that
undergoes a Helium flash might create a nice egg-shell nebula
such as the Ring Nebula in Lyra (M57). It appears as a ring
because when we look along the sides of the shell, the line-of-sight
through the gas is thicker than when we look through the center.
If, however, the star is a double star with
a very close companion, or has a very massive planet in orbit,
the expanding shell of gas is gravitationally distorted, often
resulting in bizarre and wonderful shapes. Some of the most
spectacular Hubble shots are close-ups of these kinds of planetaries.
However, even in modest telescopes, you can clearly see some
of these structures. Prime examples are the Eskimo Nebula,
the Saturn Nebula (named for its dual lobes), The Dumbell
Nebula (M27 – shown here), and the Cat’s Eye.
Because planetaries glow by emission processes,
they are good candidates for the use of filters to enhance
detail and contrast.
Supernova Remnants
When a star has about 5 times the mass of our
Sun, it ends its life abruptly in a massive explosion called
a supernova. These are among the most violent and energetic
events in the Universe. The explosion blows off a large fraction
of the star’s mass into an expanding shell of gas at extremely
high velocities. The gas expands into the interstellar medium
to form interesting nebulae.
One of the most famous supernova remnants is
the Crab Nebula (M1 - shown here) in Taurus. This is the remnant
from a supernova that blew up in 1054 A. D. that was observed
in many parts of the world, with the most detailed observations
coming out of China. Today, we know the nebula in Taurus is
the aftermath of that explosion almost 1000 years ago. Since
then, it has expanded into a bizarre object with various tendrils
and features reminiscent of a crab.
The sky is full of many of these, far older
than the Crab, some being millions of years old. A prime example
of an old (and hence very large) remnant is the Veil Nebula
in Cygnus. The Veil has been expanding for so long that it
covers a region of the sky several degrees across. It has
dissipated so much that all that is left are wispy veils of
nebula that are rather faint. However, because supernova remnants
glow by emission processes, they respond very well to the
use of filters. The Veil in particular is spectacular in a
large telescope using an Oxygen-III filter.
Open Clusters
Open clusters are also known as “galactic clusters”
because they lie in the plane of our Milky Way. These are
groupings of 10 to 200 stars that all formed at about the
same time. Recall that emission nebula are star-forming regions
– open clusters are the result of that process, when most
of the gas has collapsed to form stars, and the remaining
gas has blown away due to the stellar winds of the newborns.
There are hundreds of these in the sky of varying
sizes and densities. The Beehive Cluster in Cancer and the
Pleiades in Taurus are two of the largest and closest clusters,
easily visible with the naked eye and binoculars. One notable
example is the Double Cluster in Perseus (shown here). There
are numerous smaller, fainter, but richly populated clusters
all over the sky.
Globular Clusters
Globular Clusters are one of the most spectacular
types of deep sky objects, especially when viewed in larger
telescopes that can resolve some of the individual stars.
Globulars contain hundreds of thousands of stars, all packed
into a spherical ball only a few light years across. They
reside in a “halo” around our Milky Way, not in the galactic
plane. The details of their origin and formation are still
hotly debated among astrophysicists. They are fairly ancient
objects, coming together shortly after our galaxy was formed.
Some theorize that the center of each globular cluster contains
a black hole that attracts the matter that in turn forms the
stars in the cluster. Globular clusters are observed in similar
halos around other galaxies also.
Prime examples of globulars include the great
Hercules Cluster M13 (shown here), the cluster M22 in Sagittarius,
and Omega Centauri (the largest and closest) visible from
southern latitudes.
Galaxies
Galaxies, galaxies, galaxies. They’re everywhere,
and the deeper we look, the more of them we see. Each galaxy
is akin to our own Milky Way, consisting of literally trillions
of stars each. They come in a variety of shapes and sizes,
and many of them have “active” cores – massive explosions,
cosmic jets, and other little understood phenomena. Galaxies
are typically 40,000 to 150,000 light years across. Our Milky
Way is around 70,000 light years across and is thought to
be rather average. The general consensus in the past few years
is that galaxies each have a super-massive black hole in their
center.
Galaxies tend to be grouped into clusters, and
the clusters are grouped into super-clusters. In between are
vast voids of space. The distances between galaxies are enormous;
our nearest neighbor, Andromeda, is about 2 million
light years away. In spite of these distances, galaxies do
have a tendency to collide because of their mutual gravitational
attraction. There are many examples of collisions in progress,
collisions nearly finished, and collisions about to happen
that we can see with modest telescopes.
Let’s look at some of the major types of galaxies:
Spiral Galaxies: these are what folks
typically think of as a galaxy – a large flattened pinwheel
with spiral arms rotating about a central bulge. Our Milky
Way and Andromeda are both spirals, and are fairly similar.
Spirals come in a variety of sub-classes also:
- Regular spirals typified by
Andromeda, and the Whirlpool (M51 - shown)
- Barred spirals with “bars” coming
out of the central bulge, then swirling into arms
Elliptical Galaxies:these are roughly
spherical or ellipsoidal in shape, with no features such as
spiral arms. It is thought that when spirals collide, the
result (after a billion years or so of settling) is an elliptical.
Lenticular Galaxies:these are disk galaxies
without any obvious structure in their disks. Astronomers
think the lack of structure is because they have used up most
of their interstellar matter and they therefore consist of
old stars only which have found a smooth and even distribution
in the disk. Alternatively, it may be because the galaxy has
not closely encountered any neighbor in the past few hundred
million (or few billion) years.
Irregular Galaxies:these are strangely
shaped, probably due to a recent collision or some incredible
activity in their nucleus that has distorted their shape.
All in all, galaxies are probably the most numerous
type of object in the sky accessible to amateur telescopes.
Galaxy Clusters
Galaxy clusters occur on a variety of distance
scales. Our Milky Way is a member of a small handful of nearby
galaxies called “The Local Group”. The local group is in turn
part of the Virgo cluster, which is in turn part of the Virgo
super cluster. The Virgo cluster is a rich hunting ground
for telescopes (in, you guessed it, the constellation Virgo),
covering about 10x10 degrees, where a 10” telescope can bring
in 20 or 30 galaxies easily. Another cluster is in Coma Berenices,
called the “Coma Cluster”.
In these clusters, you will only see one, or
sometimes 2 or 3 galaxies in a single telescopic field at
a time. More distant clusters are much fainter, but you get
to see 4, 5, or 6 together in a field of view. The Abell galaxy
cluster catalog lists some 100 galaxy clusters, but you’ll
need a serious instrument (16” or larger) to see these wonders.
Tools for the Deep Sky
The tools for deep sky observing are not all
that different than for general astronomy, with a couple of
important differences:
- Light gathering power is critical.
You want the biggest aperture you can afford and transport
- Dark skies are critical. You want to
make the effort to get as far away from light pollution
as possible
- It's important to get above the haze,
so head for the hills.
- Filters deserve some special considerations.
- Finder charts and high-quality star
atlases are critical since most of these objects are far
too faint to see with the naked eye.
Light Buckets
The most commonly used telescope for the deep
sky hunter is the “Big Dob”, a large-aperture Dobsonian telescope,
affectionately referred to as a “light bucket.” An 8” Dob
is a great starter scope for the deep sky enthusiast, and
will bring in many hundreds of objects under good observing
conditions. A 10” or 12.5” is even better, and will really
start resolving globulars, galaxy arms, features of planetary
nebula, and the like. When you get up to the 16” to 25” range,
the views become truly spectacular. Even if you can’t afford
or manage a scope that large, be sure to attend a star party
and get some views through larger instruments. An 18” Dob
is shown at the left.
Filters
Because many of the deep sky objects glow by
emission, narrow-band filters play a large role in accentuating
their features. Additionally, wide-band filters can be very
effective for reducing light-pollution and sky glow, increasing
contrast on globulars, galaxies, and reflection nebula.
Light-Pollution Filters:light
pollution is a pervasive problem, but there are ways to mitigate
its effect on your observing enjoyment. Some communities mandate
Mercury-Sodium vapor streetlights (especially near professional
observatories) because these types of lights emit light at
only one or two discreet wavelengths of light. Thus, it is
easy to manufacture a filter that eliminates only those wavelengths,
and allows the rest of the light to pass through to your retina.
More generally, both wide-band and narrow-band light-pollution
filters are available from major vendors that help substantially
in the general case of a light-polluted metro area.
Nebula Filters:nebula
filters enhance the specific emission lines of emission nebula,
planetaries, and supernova remnants. Most famous is the OIII
(Oxygen-3) filter available from Lumicon. This filter eliminates
almost all the light at other wavelengths other than the Oxygen
emission lines generated by many interstellar nebulae. The
Great Nebula in Orion (M42) and the Veil Nebula in Cygnus
take on an entirely new aspect when viewed through an OIII
filter. Other filters in this category include the H-beta
filter (ideal for the Horsehead nebula), and various other
more general-purpose “Deep Sky” filters that enhance contrast
and bring out faint detail in many objects, including globular
clusters, planetary nebula, and galaxies.
Pointing Devices
Since most of the objects you’ll be locating
are too faint to see, you’ll need some aides to help point
your telescope close to the neighborhood of the object of
interest. While most telescopes come equipped with a finder
scope (typically 6x30 or 8x50), the preferred pointing device
these days is an LED pointer. Some models use a red dot projected
onto a transparent screen, allowing you to aim the scope by
pointing the red dot. Others use a projected reticle (concentric
circles and cross hairs). The now famous “Telerad” was the
first, and still highly popular, device of this sort. Many
finder charts (including printed charts, on-line catalogs,
and PC software) display the stars in the object’s neighborhood
with the reticle pattern drawn in.
Additionally, a pair of binoculars can come
in handy for scouting the stars near the object that you can
use to “star hop” to the object.
Deep Sky Catalogs
The Messier Catalog: In the 1700s and
1800s, a comet hunter named Charles Messier spent night after
night searching the skies for new comets. He kept running
into faint smudges that did not move from night to night,
and so they could not have been comets. For convenience, and
to avoid confusion, he constructed a catalog of these faint
smudges. While he did discover a handful of comets during
his life, he is now famous and best remembered for his catalog
of over 100 deep sky objects visible from the northern hemisphere.
These objects now bear their most-used designation stemming
from the Messier catalog. “M1” is the Crab Nebula, “M42” is
the great Orion nebula, “M31” is the Andromeda galaxy, etc.
Finder cards and books on the Messier objects are available
from many publishers and on the Web, and are highly recommended
if you have a modest telescope and a very dark sky.
The Caldwell Catalog: Additionally, a
new “Caldwell” catalog gathers another 109 objects that are
of similar brightness to the M-objects, but were overlooked
by Messier. These are ideal starting places for the beginning
deep-sky observer. The Caldwell catalog was created by Patrick
Moore, this list doubles the number of "must see"
objects for amateur astronomers.
The NGC Catalog: The New Galactic Catalog,
or “NGC” was compiled by J. L. E. Dreyer in 1888 and replaced
all previous lists and catalogs. It is a superset of the Messiers',
and covers the entire sky. There are approximately 10,000
objects in this catalog, the vast majority of which are accessible
by modest amateur telescopes in dark skies. There are several
observing guides emphasizing the most spectacular of these,
and a high-quality star chart will show thousands of NGC objects.
The IC Catalog: The Supplementary Index
Catalogs was compiled after the NGC list. The first IC contains
1529 objects discovered between 1888 and 1894. The second
IC contains an additional 3856 objects found through 1907.
Arp Galaxies: These are a special subset
of the NGC list. Compiled by Dr. Halton C. Arp. They are a
collection of the more peculiar and irregular galaxies. These
are, generally speaking, quite faint. If you're into the Arp
galaxies, you've probably already made the decision to acquire
a larger aperture telescope – perhaps something in the 12-16"
range. These are, however, within the reach of more modest
telescopes if you can use imaging. These are some very interesting
targets.
The Abell Planetary Catalog: It's particularly
exciting to find a deep sky objects that have only been discovered
in the last half century, because very few people have ever
seen them. To join this exclusive club, check out George Abell's
Catalog of Planetary nebula containing objects discovered
on plates from the Palomar Sky Survey in the mid-1950s.
The Abell Galaxy Cluster Catalog: This
catalog is a homogeneous all-sky catalog of rich galaxy clusters
with populations of 30 or more galaxies. The catalog combines
a northern survey, originally published by George Abell in
1958, and a southern survey begun by Abell and Harold Corwin
in 1975, which (after Abell's untimely 1983 death) was completed
by Corwin and Ronald Olowin in 1987. Most of these clusters
require significant aperture (12.5” minimum) to locate.
Observing Techniques
There are several important considerations and
observing techniques that will help maximize the success of
your deep sky observing efforts.
Dark Sky Site
Not enough emphasis can be given to the benefits
of a truly dark-sky observing site. Light pollution, even
from a modest city of, say, 50,000 people, can create sky
glow 20 miles away, and reduce contrast. The darker the sky,
the more these faint fuzzies will reveal themselves. In addition
to improving the views of deep sky objects, a dark sky is
majestic and beautiful to the naked eye, especially with a
rich Milky Way high in the sky. Spending time with binoculars
in a lounge chair under dark skies is another wonderful reason
to make the trek out there.
The two primary considerations for a good observing
site are darkness and elevation. If you live near mountains,
try to get as much elevation as you can to get above haze
and water vapor (and dress warmly – it can get quite cold
at 7,000 feet, even in August!). Really amazing dark skies
are found a good 50 miles from the nearest city, and have
minimal local lighting. Many campgrounds are good candidates,
if they have open spaces without too many trees.
Dark Adaptation
One of the most important considerations for
seeing detail in dark sky objects is the dark-adaptation of
your own eyes. Visual purple, a chemical responsible for increasing
the acuity of your eyes in low-light conditions, takes 15-30
minutes to develop, but can be eliminated immediately by one
good dose of bright light. That means another 15-30 minutes
of adaptation time. Besides avoiding bright lights, astronomers
use flashlights with deep red filters to help navigate their
surroundings, view start charts, check their mount, change
eyepieces, and so on. Red light does not destroy visual purple
like white light does. Many vendors sell red-light flashlights
for observing, but a simple piece of red cellophane over a
small flashlight works just fine.
Averted Vision
To improve the amount of detail seen, and to
glimpse faint objects at the limit of your telescope’s grasp,
use the technique of “averted vision”. The human retina is
composed of differing sensors called “cones” and “rods”. The
center of your vision, the fovea, is mainly composed of cones
that are most sensitive to bright, colored light. The periphery
of your vision is dominated by rods, which are more sensitive
to low light levels, with less color discrimination. Averted
vision concentrates the light from the eyepiece onto the more
sensitive part of your retina, and results in an ability to
discern fainter objects and greater detail.
Star Hopping
Star hopping is a technique for locating objects
that can’t be seen in a finder scope or binoculars. The idea
is to plan a set of stars, starting with a fairly bright one,
near the object. You first get the initial star in your field
of view, and then move gingerly to a fainter star. Typically,
you estimate the direction and distance from one star to the
next in terms of “clock” directions and field widths. For
example, move in the 2-o’clock direction 3 fields. Continue
this until you get to a known star fairly close to the object.
By sweeping around the neighborhood of the star, you’ll find
your target.
It is important to know the actual field of
view of your eyepiece/telescope combination. To calculate
the actual field, take the apparent field of view of the eyepiece,
and divide it by the magnification. Star hopping, and pointing/locating
in general, is best done with the lowest magnification and
widest field eyepiece you have. Once you locate the object,
you can then increase the magnification if desired. The only
caveat to this rule of thumb is that some planetaries are
fairly small, and at low magnification, will appear star-like.
These are a bit more challenging, and star hopping at higher
magnification will help you find them.
Deep Sky Tour
Let’s take a quick look at some additional deep-sky
objects that are must-see.
| Andromeda Galaxy (M31):The
great Andromeda Galaxy is our nearest full-size galaxy
neighbor. It spans almost 4 degrees of sky (that’s 8 moon
diameters!), but the outer stretches of it are fairly
faint. In dark skies, you can easily see it with the naked
eye – being the farthest object one can see without visual
aid (over 2 million light-years). It is best observed
with a wide-field, low-power eyepiece. It has a bright
central core, and two satellite galaxies. Larger instruments
will clearly show structure in the spiral arms. |
 |
 |
The Ring Nebula (M57):The
Ring is a classic planetary nebula in the constellation
Lyra. This is a good object to experiment with different
kinds of filters to reveal various aspects. The central
star of the Ring is a rather faint magnitude 14 or so,
and is quite difficult to see generally because of the
nebula’s glow. |
| The Helix (NGC 7293):The
Helix is a large, close planetary in the constellation
Aquarius. It has very low surface brightness, making it
challenging to find. A nebula or OIII filter makes a huge
difference on this object. |
 |
 |
The Lagoon Nebula (M8):The
Lagoon in Sagittarius is one of the largest, brightest
diffuse nebula out there. It has beautiful lane structures
and a rich grouping of stars embedded in it. Again, a
nebula filter does wonders on this object. |
Deep Sky Resources
There are thousands of books on astronomy. Below
are some recommendations suitable for folks new to the field
of deep sky gazing. Click on the links to read the reviews
at Amazon:
Links
SEDS Messier Catalog: http://www.seds.org/messier/
(Students for the Exploration and Development of Space)
Deep Sky catalogs, charts, and observing: http://www.utahskies.org/deepsky
Deep Sky Database, observing list generator:
http://www.virtualcolony.com/sac/
Deep Sky Observing Forum at Astronomy.com: http://www.astronomy.com/community/forum/forum.asp?FORUM_ID=3
The Abell Catalog of Planetary Nebulae: http://www.angelfire.com/id/jsredshift/abellpn.htm
CCD Images of Deep Sky Objects: http://www.arizonausa.com/sky/deepsky.htm
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