Open
clusters, also called galactic clusters, are loose, irregular bodies
that can contain several thousands of stars. Open clusters are young
star clusters where the stars move together and form a group. They
are concentrated in the disk of a galaxy, which is where interstellar mater
and young stars are found. Very few open clusters are found away
from the galactic plane. Astronomers use open cluster stars to determine
the plane of the Milky Way galaxy (Cambridge, 243-4).
Stars in open clusters are linked weakly by gravitational forces.
Stars escape the pull of gravity about one per hundred thousand years.
From there, stars disperse into the galactic disk. From this information,
astronomers speculate that the life span of open clusters should not exceed
one hundred million years. When viewed though a telescope, open clusters
look like a cluster of small stars.
Globular
clusters are the oldest of stellar clusters and are usually spherical
in shape. They are tightly packed stars in populations of up to several
hundreds of thousands and can be in a volume 300 light years across.
Globular clusters are found in a spherical area of space surrounding galaxies
called the galactic halo, but peak density, roughly 64%, is in the galactic
center. "Each cluster travels in an elongated orbit with a period
of revolution of two hundred million years," (Cambridge, 308). From observations
of globular cluster orbiting the center of the Milky Way, we see the outer
reaches of our galaxy and realize the true size of the Milky Way.
The globular clusters in the Milky Way galaxy, of which there are between
146 and 200, are very old. Their ages are between ten and fifteen
billion years old. They were formed first in the galaxy, and from
this, they have not benefited from the interstellar matter of stellar nucleosynthesis
or supernovae explosions. The observations made of the elements that
make up the globular clusters lend important information about the first
stages of the chemical evolution of our galaxy (308). Globular clusters
have lower amounts of metals, elements heavier than hydrogen or helium,
than any other known celestial objects. Post helium flash stars are
typically the types of stars that are found in globular clusters.
Most of the stars found in globular clusters are low-mass stars, as the
higher mass stars have burned away as red giants. The lower mass
stars are left behind and are usually undergoing core hydrogen fusion.
In the future, these low-mass stars will move into another state of their
lives, the giant state. As core helium fusion occurs and they devour
their fuel, they die. Both M31 in Andromeda and M87 in Virgo have
great numbers of globular clusters (White, 24-8).
Diffuse nebula are "an irregularly shaped cloud of interstellar gas or dust whose spectrum may contain emission lines or absorption lines characteristic of the spectrum on nearby illuminating stars" (Hopkins, 42). These bodies are also known as reflection nebula.
The term planetary nebula refers to an expanding and usually symmetrical
cloud of gas that surrounds certain hot stars. Nearly one thousand
planetary nebulas have been discovered. The name denotes their disk-like
resemblance to planets. Radiation from the centrally located star
excites the gaseous cloud, "causing fluorescence as in bright nebula.
The atoms of the (gas cloud) absorb the ultraviolet radiation from the
star and re-emit it in radio, infrared and visible wavelengths" (Cambridge,
274). The phenomenon is a normal phase in the life of a low-mass
star (between one and five solar masses), occurring between the red giant
and white dwarf phases. The transition phase of the star from red
giant to planetary nebula can be as quick as one thousand years but can
last up to 10,000 years, and the transition phase between planetary nebula
and white dwarf can have an equal transition period.
The clouds that make up a planetary nebula expand at a rate of about
20 kilometers per second. From this information, it is inferred that
an explosion creates a planetary nebula. "The shell of a planetary
nebula contains far more material than the shell thrown off by a nova.
Therefore we can assume that the explosion forming a planetary nebula is
very large and occurs very rarely" (Meadows, 152). To date, no explosion
large enough to produce a planetary nebula has been observed.
The temperature of the central star is hot, between 30,000 and
100,000 Kelvins. The mass of a planetary nebula is approximately
0.10 to 0.20 solar masses. Tenuous gas is 10-20 g/cm3 , and dust
is present.
Supernova remnants are an expanding gas shell of gas from a supernova explosion, consisting of supernova projections and swept-up interstellar gas. "A supernova is a star that temporarily brightens to an absolute magnitude of about -15. A supernova explosion blows off all or most of the star's material at a high velocity, as a result in the final uncontrolled nuclear reactions of stars that reach an unstable state late in their evolution" (Illingsworth, 331). Young supernova remnants are generally optically faint, but emit large quantities of radio and x-ray waves. The Crab Nebula, M1, is an example of a supernova remnant and is especially bright due to a central pulsar. Older supernova remnants appear as rings of bright filaments, which are associated with radio and x-ray wave emission. Compression by an expanding supernova remnant can trigger stellar birth in interstellar clouds and may have initiated the formation of our solar system.
A galaxy is an ensemble of stars of various masses and properties in
a more or less dense interstellar medium, though this medium is virtually
non-existent in elliptical galaxies. Presently, more than a billion
galaxies are observable, though they may possibly be the most under observed
objects in the sky due to light pollution. Galaxies are not randomly
distributed, but form clusters. "Regular clusters of galaxies contain
at least a thousand members. Irregular or open clusters contain no
central nucleus of galaxies, as they appear to be simply a loose collection
of galaxies without the symmetrical shape found in regular clusters.
Open clusters generally contain fewer than galaxies than regular ones"
(Schweighauser). The enormous distances of galaxies from Earth make
it possible to study their evolution. They are receding from one
another with increasing velocities proportional to their distance, from
which one can measure their red shift. The galaxies with high redshift
are the most distant from us and are visible at a younger state of their
evolution.
There are three ways to determine a galaxy's evolutionary progress.
The first is dynamic evolution, involving the way the galaxy is formed
and how its components move and interact. The second is the evolution
of chemical composition. The third is the evolution of luminosity
and color; "an ensemble of stars which are born, evolve, and die (hence
changing luminosity and color), and which enrich the interstellar medium
in chemical elements which they synthesize" (Cambridge 334).
There are four main morphological types of galaxies. These are
know as elliptical, spiral, peculiar, and irregular; in which elliptical
and spiral are the two main types.
Elliptical galaxies are massive and not especially luminous.
They tend to be rather red with low interstellar gas content.
Spiral galaxies are less massive and more luminous. Their interstellar
gas content is 1-10% of the total density of the matter of the disk.
Spirals contain three or four components: a central region or bulge
that looks like a small elliptical galaxy; a disk which is extended and
flattened; spiral arms within the disk, in which the stars and gas are
greatly concentrate; the fourth pertains only to barred spiral galaxies
and that is a "bar", a flattened structure that is between the bulge and
the disk. The outer parts of the disk are blue and the center bulge
is yellow to red in color. The bulge and the disk are surrounded
by a tenuous, extended spherical region, which where globular clusters
are found.
Spiral galaxies are further divided into categories that denote their
outstanding characteristics. SB denotes those galaxies with a central
bar, while S refers to those without a bar. Sa and SBa galaxies have
tightly wound arms and relatively large central bulges. Sc and SBc
galaxies have loose arms and small central bulges. Sb and SBb are
intermediate between the two extremes (Mitton, 359).
10-15% of galaxies do not have a particular form and are thus known
as irregular galaxies. They have low mass, substantial luminosity,
and a large concentration of interstellar gas. Their dominant color
is blue.
Spring is the best time to observe Messier objects, between March 19 and 30 is ideal. A dark sky, low horizon and no moon are quintessential for optimal results. Messier objects are visible only in the Northern Hemisphere.
The Cambridge Atlas of Astronomy; Cambridge University Press, NY. Ny; 1994.
Illingsworth, Vallery; The Facts on File Dictionary of Astronomy, Facts on File, NY, NY; 1979.
Hopkins, Jeanne; Glossary of Astronomy and Astrophysics, University of Chicago Press; 1976.
Kaufmann, William J. III; Discovering the Universe , W.H. Freeman and Co.; 1996.
Kwok, Sun; "A Modern View of Planetary Nebulae", Sky and Telescope; July 1996.
Machholz, Don; " Notes on Messier Marathon", Astronomy ; March; 1980.
Mallas, John H., and Kreimer, Evered ; The Messier Album; Cambridge University Press; 1979.
Malin, David and Murdin, Paul; Colours of the Stars ; Cambridge University Press; 1984.
Meadows, A.J.; Stellar Evolution (second ed), Oxford, NY; Pergamon Press; 1978.
Mitchell, Larry; "The M31 Challenge", Sky and Telescope; November; 1997.
Nisqually Valley Telescope Maker's Workshop, "Just Messier"; 1994.
Parker, Samantha; "The Eagle's Nest", Sky and Telescope; February 1996.
Royal Astronomical Society of Canada, Observer's Handbook 1998; University of Toronto Press Inc.; 1997.
Schweighauser, Charles A.; Astronomy from A to Z: A Dictionary of Celestial Objects and Ideas; Illinois Issues; 1991.
Witkoski, Michael F.; "Observing Galaxies", Astronomy; April, 1980.
White, Raymond E.; "Globular Clusters: Fads and Fallacies", Sky and Telescope; January, 1991.