The Caldwell Objects

and the 100-inch telescopes atop Mount Wilson.
But the situation changed when the 200-inch
reflector on Palomar Mountain began operation,
for it soon revealed a thin equatorial plane,
indicative of a disk, in NGC 3115. Based on the
new evidence, NGC 3115's classification was
changed, hovering somewhere between a
transitional E7-S0 and a pure SO. As with NGC
4697 (Caldwell 52), changing technology changed
a galaxy's classification. Of course, this is how
science evolves.
Today there is no question that NGC 3115 is a
pure lenticular (S0) galaxy with a very bright
nucleus and modest disk inclined 24° from edge
on. Redshift measurements show the galaxy
receding at 700 km per second. If R. Brent Tully's
distance of 22 million light-years is correct, NGC
3115 has a true linear diameter of nearly 40,000
light-years and a luminosity of about 7 billion
Suns. The Hubble Space Telescope has confirmed
the existence of a supermassive black hole in the
core of NGC 3115 — one with the mass of 2
billion Suns. That's twice the mass estimated by
the University of Michigan team that discovered
the black hole in 1992 using ground-based
telescopes. The confirmation was detailed in the
March 10, 1996, issue of Astrophysical Journal
Letters, and it marked the third time the Hubble
Space Telescope had documented the existence
of a supermassive one in a galaxy core.
Speaking of black holes, how can you find
NGC 3115 in the dreaded Sextans void?
Actually, the situation is not that bad. You just
have to forget about Sextans and concentrate on
the brighter stars of Hydra. Those whose telescopes
have setting circles should start with 2ndmagnitude
Alpha (α) Hydrae, or Alphard.
Center Alphard in your field of view; move your
telescope 9° due east, then 1° due north; and
look in the eyepiece. You should see NGC 3115
about ½° due west of a pair of stars (with
53
magnitudes of 6.6 and 7.5, respectively) oriented
northeast-southwest. Do not confuse this pair of
stars with another, brighter pair, 17 and 18
Sextantis, 1° to the southeast. A simpler, shorter
route for star-hoppers starts from 3.6-magnitude
Lambda (λ) Hydrae. The stars 17 and 18 Sextantis
lie only 4° due north of
Lambda. So owners of a Telrad sighting device
simply have to place Lambda Hydrae on the
southern end of the device's luminous outer circle;
17 and 18 Sextantis will be on the northern
end of the same circle. You can hop 1° northwest
to the fainter pair of stars and then to NGC 3115.
Houston also preferred this approach. "Not only
is [Lambda Hydrae] an easy naked-eye star," he
said, "but a distinct asterism formed by several
surrounding stars will offer positive
identification even in small finders."
Under clear, dark skies NGC 3115 is easily
visible in 7x35 binoculars, and the view only gets
better with a telescope. At 23x in the Genesis the
9th-magnitude galaxy immediately reveals its 7'long
spindle, which is oriented northeast to
southwest. Look for a bright central condensation
and a tapered halo. Webb called NGC 3115
"[v]ery distinct, with much
The Caldwell Objects 213