Corvus, the Crow
is related by legend with the constellations of Hydra and Crater,
which lie to the south of Virgo and Leo. The story is told that
Apollo, god of the Sun who rode daily through the sky in his chariot
pulled by four mighty steeds, grew thirsty and sent his liege,
Corvus, to fill his cup, Crater. The crow flew at once to the
fountain, but nearby was a fig tree where the bird stopped to
rest amid the shady leaves. The fruit of the tree was not yet
ripe, and Corvus, always in trouble of some sort, lingered, waiting
for the fruit to ripen. On the way back the bird, suffering from
a guilty conscience, caught a water snake, Hydra, and carried
it in his talons. He then claimed that the reptile had attacked
him and so was responsible for the long delay. But Apollo was
all-knowing and easily saw through the lie. In punishment for
this deception, all three were immobilized and placed among the
stars.
Another story is that
Apollo sent a beautiful silver-white crow to watch over his lover,
Coronis. However, the bird was inattentive to duty and Coronis
was unfaithful.. Apollo became so angry that he turned the crow
black forever. It is not said how Apollo treated Coronis.
Corvus, best observed
in the Spring, is low in the southeast before dawn during November.
It is a small constellation with only a few stars of about magnitude
three. It is easily recognized, however, by the trapezoidal orientation
of those stars.
Most notable of the deep-sky
objects in Corvus is NGC 4038-4039, the Ring-tail Galaxy, or the
Antennae. A representative of the two percent of all known galaxies
called peculiar galaxies, this object is formed by two interlocking
structures. The stars involved in a collision of galaxies do not
themselves collide, for they are separated by immense distances.
However, in such a crash the atoms of the gas and dust clouds
do collide, forming pockets of very dense material from which
new stars are born. Hubble Space Telescope photos prove that the
Antennae is an excellent laboratory for studying the formation
of stars and star clusters since it is the nearest and youngest
example of a pair of colliding galaxies.
NASA’s Hubble
Space Telescope
has uncovered over
1,000 bright, young star clusters bursting to life in a brief,
intense, brilliant “fireworks show” at the heart of
a pair of colliding galaxies.
The image at left,
taken by a ground-based telescope, and the Hubble image at right
provide views of the collision of the two galaxies, the Antennae,
about 63 million years ago. The two long tails in the first image
are luminous matter formed by the force of the collision.
“The sheer number
of these young star clusters is amazing,” says Brad Whitmore
of the Space Telescope Science Institute (STScI), Baltimore, Maryland.
“The discovery will help us put together a chronological
sequence of how colliding galaxies evolve. This will help us address
one of the fundamental questions in astronomy: why some galaxies
are spirals while others are elliptical in shape.”
By probing the Antennae
galaxies (called the Antennae because a pair of long tails of
luminous matter formed by the encounter resembles an insect’s
antennae), Hubble is coming up with a variety of surprises:
The “seeds” for star clusters appear to be giant molecular
clouds (tens to hundreds of light-years across) of cold hydrogen
gas, which are squeezed by surrounding hot gas heated during the
collision and then collapse under their own gravity. Like a string
of firecrackers being ignited by the collision, these reservoirs
of gas light up in a great burst of star formation.
The ages of the resulting clusters provide a clock for estimating
the age of a collision. This offers an unprecedented opportunity
for understanding, step-by-step, the complex sequence of events
which take place during a collision, and possibly even the evolution
of spiral galaxies into elliptical galaxies.
Earlier Hubble pictures show that nearly a third of very distant
galaxies, which existed early in the history of the universe,
appear to be interacting galaxies, like the Antennae. In particular,
the Hubble Deep Field (a “long-exposure” image from
Hubble looking at galaxies far back into time), uncovered a plethora
of odd-shaped, disrupted-looking galaxies. They offer direct visual
evidence that galaxy collisions were more the rule than the exception
in the early days of the universe.
These distant galaxy collisions are too faint and too small to
study in much detail. Astronomers say we are fortunate to have
such a nearby example as the Antennae to study, since collisions
between galaxies are relatively rare today.
In addition to providing a window into how stars and galaxies
formed in the dim past, the Hubble views might also offer a glimpse
of the future fate of Earth’s home galaxy, the Milky Way,
when it either sideswipes or plows head-on into the neighboring
Andromeda galaxy billions of years from now.
Observing other galaxy collisions, the Hubble team discovered
the presence of young star clusters which were very bright and
blue in the case of ongoing collisions, but had faded to become
fainter and redder for the older merger remnants. This allowed
them to place the snapshots of galaxy collisions into a chronological
sequence.