Click the icon to view Nebulae of the Messier Catalog
The icon shows the Horsehead Nebula (Barnard 33),
a dark nebula superimposed on an emission nebula (IC 434).
Diffuse nebulae are clouds of interstellar matter, namely thin but widespread
agglomerations of gas and dust. If they are large and massive enough they are
frequently places of star formation, thus generating big associations or
clusters of stars. Some of the young stars are often very massive and so hot
that their high energy radiation can excite the gas of the nebula (mostly
hydrogen) to shine; such nebula is called emission nebula. If the stars
are not hot enough, their light is reflected by the dust and can be seen as
white or bluish reflection nebula. As most diffuse emission nebulae also
contain dust, they typically have a reflection nebula component also.
When a star like our sun has used up all its central nuclear fuel, it finally
ejects a significant portion of its mass in a gaseous shell which is then
visible in the light emitted due to high-energy excitation by its extremely hot
central star, which previously was the core of the stellar progenitor (thus,
planetary nebulae are a special kind of emission nebulae). These nebulae
quickly expand and fade while their matter is spread in the interstellar
Stars which are considerably more massive than our Sun, and have at least about
3 solar masses left after their giant state, can most probably not evolve
quitely into an end state as a white dwarf, but when coming to age, explode in
a most violent detonation which flashes up at a luminosity of up to 10 billion
times that of the sun, called supernova (of type II) and ejecting the very
greatest part of the stellar matter in a violently expanding shell.
Alternatively, infalling matter on a white dwarf star can cause it to explode
as a supernova of type I.
The nebulous ejecta of supernovae of either type are called supernova remnants.
The only supernova remnant in Messier's catalog is the first object, the
Crab Nebula M1, the remnant of a type II supernova.
Although none of them is in Messier's catalog, some of these objects are
conspicuous. Unlike the others, the bright nebulae, these dust clouds are only
visible by the absorption of light from objects behind them. They are
distinguished from diffuse nebula mainly because they happen to be not
illuminated by embedded or nearby stars.
The term "Nebula" has varied in the history of astronomy. In pre-telescopic
times it was used to distinguish objects which look non-stellar from the
pointlike stars. Most "nebulae" known at that time have been shown to be
open star clusters. The term "Nebula" was thus used
for what we now call "Deepsky Object".
In early telescopic times, the nature of these objects was still widely
unknown. With open clusters resolved, still all other deepsky objects were
summarized as "Nebulae". Only the use of large telescopes, the discovery of
spectroscopy and the invention of photography in the second half of the 19th
century made it possible to distinguish "real" nebulae - i.e., gas and dust
clouds - with certainty from objects made up of stars
(globular clusters and
Now that the nature of nebulae as interstellar masses of gas and dust is known,
there are still several classification schemes. The first is based on
spectroscopy and the light which is seen from the nebulae:
This scheme tells nothing on the nature of the nebulae.
- Emission Nebulae: Emit light because the atoms in their gases are
excited by high energy radiation of stars involved. They show emission line
- Reflection Nebulae: Reflect light of nearby stars by their dust
particles. Therefore, their spectra are the same as those of the stars,
typically continuous spectra.
- Absorption Nebulae or Dark Nebulae: Absorb light: Their gas
component can be seen as absorption spectra in the light of background stars,
their dust component by absorbing and reddening background light.
A more modern scheme distinguishes star-forming or pre-stellar
nebulae (basically diffuse and dark nebulae) from post-stellar
nebulae (basically planetary nebulae and supernova remnants).
The first of these classes typically includes clouds of interstellar matter
of a mass of several 100 or several 1,000 stars, while the latter is related
to one specific star in advanced state of evolution, at or just beyond the end
of its nuclear life.
There are a number of variations and special classes of nebulae such as the
Herbig-Haro Nebulae (related to stars in the process of formation, and
emit jets of gaseous material, thus often found near large diffuse nebulae with
star formation) and
Wolf-Rayet Nebulae (related to hot Wolf-Rayet stars, stars of some age
that have ejected matter they now cause to shine).
On cosmic timescales, all these types of nebulae, in particular the bright
nebulae, undergo rapid changes and have only comparatively short lifetimes,
so that those we observe are all rather young objects. Planetary nebulae and
supernova remnants usually have only a few thousands of years before they fade
and spread their matter into the this interstellar matter of their environment,
while star forming H II regions shine bright for the few 100,000 or million
years they are brightened by the very hot massive O and/or B stars that formed
within them. The giant molecular clouds have a somewhat longer life of some
10s of millions of years, while they form new stars and star clusters.
One should keep in mind that all Messier nebulae are members of our
Milky Way Galaxy (together with many others).
Other galaxies contain nebulae, too, which can be
detected with considerably sensitive instruments within the images of these
- Webb Society Deep-Sky Observer's Handbook
Vol. 2: Planetary and Gaseous Nebulae, by Kenneth Glyn Jones (ed.).
Enslow Publishers 1978/1979.
- James B. Kaler, Cosmic Clouds, Scientific American Library,
W.H. Freeman, 1997 (German edition: Kosmische Wolken, 1998)
Last Modification: November 17, 2005