Star date: 12:21:98
There is a First for Everything
Many, and perhaps most professional amateur astronomers begin their
affair with astronomy when they are children. At this age, when wonder
of childhood looks to the stars, and magnificence is seen.
It seems that everyone, or nearly everyone, sometime in their life,
looks to the stars and is filled with a sense of awe.
Perhaps a first telescope is bought for a child, or even for an adult.
What kind of objects can be seen? Where does one begin?
First, we understand that there are many different kinds of objects
in the night sky. There are the Moon, the planets, nebulae and galaxies
to gaze upon. Also there are globular and open clusters, comets (occasionally),
and several other objects.
The Moon is almost always the first object which beginning amateur astronomers
gaze upon. It is big (as seen from Earth), bright, and is among the easiest
objects to find with a telescope. It's rich surface is full of detail,
from the Southern Highlands (a mountain range near the "bottom"
of the Moon) to its vast maria (plains of basalt which are the dark patches
which make up features in the "Man in the Moon").
Next are the planets. Venus, Jupiter, and Saturn are all very bright,
and stand out from the so called "background stars". Mars is
fairly bright, and can be seen shining with a bright red color which reminds
some people of the color of blood. This is why it was named after the Roman
god of war. Mercury takes some effort to see, due to the fact that it is
always seen close to the Sun. This is due to the simple fact that Mercury
orbits so close to the Sun. Uranus and Neptune are invisible to the naked
eye, but are an easy find with any good pair of binoculars or a telescope.
Just the thought of seeing objects unknown to the human race until a few
hundred years is always a wonder to me.
People who have the fortune to be enjoying astronomy during the passing
of a bright comet can explore the rich detail of a comets structure. From
the subtle hues which make up the coloration of the comet as a whole, to
the intricate structure of the head, comets are a rare delight.
All of these objects, however, are in our solar system. They are all
within six billion kilometers (about 4 billion miles) of Earth. this may
seem like a tremendous distance, but in the scales of astronomy, these
are very small distances. Where do you look when you first wish to view
beyond the Suns family of planets?
One of the most beautiful classes of object to view are nebulae.
There are two types of these.
One of these is created at the beginning of a stars life, when a star
first forms from a cloud a gas and dust. A slowly collapsing cloud of hydrogen
and helium (and sometimes heavier elements) starts to slowly heat due to
gravity. All the time, the object is rotating, and forming a disk around
it due to centrifugal force. When the star gets hot enough for nuclear
fusion to begin, the star becomes a huge, continually exploding hydrogen
bomb. The star then lives most of its life balanced between the nuclear
forces pushing it outwards, and gravitation trying to collapse it inwards.
It all seems simple enough. The real trick here is to get the collapse
started. It may be triggered by a nearby nova or supernova, or perhaps
by a passing object.
There is also another possibility. Some astronomers believe there are
pressure waves passing through the disk of our galaxy. This would be much
like a sound wave, passing back and forth across our galaxy. This is how
planetary nebula form into new stars and solar systems.
The first planetary nebula most people see can be easily found this
week, looking like the middle "star" in the sword of Orion. This
is M42, the famous Orion Nebula.
But what about the other type of cluster, the globular clusters?
Globular clusters can be used to tell us the age of galaxies. They are
found orbiting in a "cloud" around galaxies, and there is no
mechanism currently known which will produce much new star formation in
most members of this class of objects. Therefore, the age of the stars
inside the cluster is also the age of the cluster. Since each galaxy can
be associated with several globular clusters, we can assume that the globular
clusters were formed along with their parent galaxy, and are therefore
the same age. Thus, if we can find the age of stars in globular clusters
surrounding a galaxy, they will be the same age as that galaxy.
It turns out that we can, in fact, tell the age of stars. There are
different classes of stars, not surprisingly called spectral classes. These
are, from the most massive to the least, O, B, A, F, G, K, M and the newly
discovered L class. This sequence of letters is remembered by astronomy
students throughout the English speaking world with the phrase "Oh,
Be A Fine Girl (or Guy), Kiss Me". The team which discovered the new
L class suggested amending this to "Oh, Be A Fine Girl (or Guy), Kiss
My Lips".
If you plot the luminosity of stars on the vertical side of a graph
(sometimes called the Y, or Sine axis), and the spectral class of stars
on the horizontal (also known as the X, or cosine axis) of a graph, you
produce what is known as a Hertzsprung/Russell diagram. It is often called
an H/R diagram. Most stars in that diagram fall along one line, called
the Main Sequence. It is only when a star dies that it is said to have
"gone off the main sequence". Since the lifetime of a star is
determined by it's mass, if we know the mass of a star which has just turned
off the main sequence, we can determine it's age.
This is one of the most important graphs in astronomy, and one of the
reasons for this is because it allows us to tell the age of galaxies.
Happy holidays, clear skies, and good viewing.
Back
To Starwatch Index
This site operated
by tmcGraphics, East Swanzey, NH 03446