WWW.ASTRONOMY.COM 69light that is absorbed can also
provide information about how
much of each element is present.
The more elements an object
contains, the more complicated
its spectrum can become. Other
factors, such as motion, can
affect the positions of spectral
lines, though not the spacing
between the lines from a given
element. Fortunately, computer
modeling allows researchers to
tell many different elements
and compounds apart even in
a crowded spectrum, and to
identify lines that appear
shifted due to motion.
Alison Klesman
Associate Editor
Q: IF ONE WERE TO STAND
ON THE MOON WHEN
EARTH IS ECLIPSING
THE SUN AND LOOK UP,
WHAT WOULD ONE SEE?
Bill Cahill
Long Beach, California
A: Just as the Moon can eclipse
the Sun as seen from Earth,
Earth can eclipse the Sun as
seen from the Moon. These
eclipses may be partial or total.
During the lead-up to a
solar eclipse from the Moon, a
viewer on the Moon would see
the Sun approach the dark disk
of Earth, with the planet’s
nightside facing the Moon.
Earth’s atmosphere would be
lit from behind, creating a red-
dish ring around the planet
that would glow brighter as the
eclipse proceeded. Astronaut
Alan Bean, who experienced
such an eclipse while returning
home on Apollo 12, said in
interviews that the light filter-
ing through Earth’s atmo-
sphere shifted from red into
the full spectrum of colors dur-
ing maximum eclipse. On the
Moon, the landscape would
also grow darker and redder,
as the sunlight of every sunrise
and sunset on Earth filtered
through the atmosphere and
reached the Moon. We see this
reddish color from Earth dur-
ing total lunar eclipses.
During a partial eclipse,
some bright portion of the Sun
would remain uncovered by
Earth. For near-total eclipses,
this would create a diamond
ring effect with the tiny part of
the Sun’s disk as the diamond
and Earth’s atmosphere acting
as the ring. During a total
eclipse, the entire Sun would
vanish, and the outer regions of
the Sun’s corona would become
visible, just as it is during a
total solar eclipse on Earth.
Alison Klesman
Associate EditorQ: WHAT IS THE DIFFERENCE
BETWEEN A GLOBULAR
STAR CLUSTER AND AN
OPEN STAR CLUSTER?
Edward Dugan III
Camas, WashingtonA: “Star cluster” is a generic
way for astronomers to refer
to a group of stars that formed
from the same material and are
gravitationally bound for at
least some period of time.
There are two major types ofstar clusters — globular clusters
and open clusters — and they
are actually quite different.
Globular clusters are old
clusters of stars that have
remained in a gravitationally
bound system. These clusters
are usually roughly spherical
and can contain anywhere
from a few thousand to a
million stars.
Globular clusters are found
in our galaxy’s spherical halo,
so they are distributed above,
below, and level with the plane
of our f lat, disk-shaped Milky
Way. They are typically on the
order of 13 billion years old,
meaning they contain some of
the oldest stars in our galaxy.
As such, astronomers use them
to study our galaxy’s early his-
tory to learn more about what
the Milky Way was like when
these stars formed.
Open clusters are much
younger and smaller than glob-
ular clusters. They are the
recent birthplaces of new stars,
which form out of clouds of
dust and gas, and contain only
hundreds or thousands of stars.
Though the stars in an open
cluster formed together out of
the same material, they do notremain gravitationally bound
over time and spread out,
scattering their stars far and
wide. Because they are not
gravitationally bound, these
clusters can have random and
irregular shapes. They are
located in the plane of our
galaxy, along the spiral arms
where the gas and dust in the
Milky Way resides. Because
open clusters are both younger
and shorter lived, astronomers
use them to study young stars
and the processes of star for-
mation, as well as the dynamics
of how material spreads out in
our galaxy.
Alison Klesman
Associate EditorSend us your
questions
Send your astronomy
questions via email to
[email protected],
or write to Ask Astro,
P. O. Box 1612, Waukesha,
WI 53187. Be sure to tell us
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cannot answer all questions
submitted.The open star cluster NGC 2547 (left) contains young, bright stars roughly 30 million years old that are loosely
gravitationally bound and will spread apart over time. By contrast, the globular cluster 47 Tucanae (NGC 104)
contains millions of stars about 13 billion years old that have been bound in the spherical cluster their entire lives.
NGC 2547: ESO. 47 TUCANAE: ESO/M.-R. CIONI/VISTA MAGELLANIC CLOUD SURVEY. ACKNOWLEDGMENT: CAMBRIDGE ASTRONOMICAL SURVEY UNIT