The Caldwell Objects

60 & 61
W HILE SWEEPING WITH HIS POWERFUL RE-
flector through Corvus, the Crow, on February 7,
1785, William Herschel chanced upon a nebula
unlike any other he had seen. Of it he wrote:
"Large opening with a branch, or two nebulae
very faintly joined." What an utterly simple yet
profound comment. It's as if Herschel were
describing two butterflies dangling from a branch
in some romantic embrace. How innocent he
seems to us today, when we look at his records —
we who live in a time of
astrophysical sophistication.
In his day
Herschel had no way of
fathoming what he had
discovered. He could
only describe what he
saw and ponder its
significance in the light
of other faint and diffuse
objects he had seen (no
pun intended). Imagine
his
response had he suddenly learned the incredible
truth: that he had stumbled upon two
extragalactic cannibals, two savage galaxies
ripping each other apart — each disemboweling
billions of stars from the other and flinging them
across several hundred million light-years of
space, where they continue to shine like
luminous entrails. Such is the case with NGC
4038 (Caldwell 60) and NGC 4039 (Caldwell 61),
the finest and most popular pair of interacting
galaxies in the heavens.
NGC 4038 and NGC 4039 are also the
brightest of the five galaxies plotted in the second
edition of Sky Atlas 2000.0 near the star 31
Crateris (which lies within the modern borders
for Corvus). To find the galaxy pair, look 3¾°
southwest of magnitude-2.6 Gamma (γ) Corvi
and about ¾° north and ever so slightly
240
east of 31 Crateris. In photographs the galaxies
look like two tadpoles bonded together at their
eastern ends. Two long "tails" extend from the
bridge connecting the tadpoles, one curving to
the north, then east, the other to the southeast,
then southwest. Computer simulations show that
such filaments form during close encounters
between spiral galaxies. When two spirals collide,
their constituent stars do not smash together like
cars; the distances between the stars in each
galaxy are too great for that to
happen. But a battle of
tidal forces does ensue,
and ultimately it
disrupts each galactic
system. Furthermore,
while gravity yanks
each galaxy a-part, gas
clouds collide,
triggering star formation.
Then, as the galaxies'
respective nuclei
move apart from one
another, tides pull billions of stars from each
spiral's disk. As a result, the galaxies lose mass
and, with it, some of the gravitational pull that
once held each of them together. The orphaned
stars flee unimpeded into space, forming two
taffylike streamers. Yet the battle has not ended.
Like two warriors gathering strength, the
galaxies separate, then orbit one another, until
their mutual gravities force them back into battle
once again. Today we see NGC 4038 and NGC
4039 in such a second round of engagement. Not
until some time in the dim and distant future,
perhaps a billion years from now, will the war
end, with the two galaxies merging into a single
chaotic stellar swarm.
A report in the Astrophysical Journal for April
1, 1997, confirms that massive star formation has
been triggered by this extragalactic
Deep-Sky Companions: The Caldwell Objects