Sky and Telescope - July 2016 - 51

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The Nova Ophiuchi Watch

SERPENS
η

ξ
24

OPHIUCHUS
6356

-18°
78

M9
86

6342

-20°

-22°

6325
6401

-24°
33

17h 30m

θ
17h 20m

17h 10m

71
78

-18° 30'

-19°
87

Aldebaran Occultation
97

109

86
118

-19° 30'

17h 34m

When Nova Ophiuchi 1998 erupted to 10th magnitude in June of
that year, no record existed of any previous outburst at its location. But
its behavior and spectrum hinted that it might be a recurrent nova -
so Ashley Pagnotta, Bradley Schaefer, and others searched its location
on the century-spanning collections of sky-patrol plates at Harvard
College Observatory and Sonneberg Observatory in Germany. They
found that indeed, the star had erupted previously: in June 1900.
They then analyzed the "discovery eﬃciency" for its eruptions,
based on the sky-patrol plate coverage. They wrote, "We deduce a
recurrence time for V2487 Oph [as it's now known] of approximately 18
years, which implies that the next eruption is expected around 2016."
This prediction is a long shot, but diligent amateurs can be good at
catching long shots.
V2487 Ophiuchi normally simmers uneasily at 17th magnitude. If it
rises to 10th again some night, you could catch it visually with a 4-inch
scope. It's in the legs of Ophiuchus, east of Antares.
Use the charts at left to get in the habit of taking a peek. Five
globular clusters on the wide-ﬁeld chart at top invite you if the nova
is missing (and see Sue French's Deep-Sky Wonders, page 54). M9 is
magnitude 7.7; the rest are brighter than magnitude 10.6.
Also: could an unknown outburst be lurking on one of your old
images of this much-photographed area? Go check. Don't be misled by
the 11.8-magnitude star 1.3 arcminutes west of the nova's position.
Someday V2487 Ophiuchi should dazzle the Earth far more
brightly. As in all classical and recurrent novae, a white dwarf here
is accreting hydrogen from a close companion star. Periodically, the
dwarf's fresh new surface material erupts in a runaway hydrogenfusion reaction. The white dwarf in V2487 Ophiuchi is unusually massive: 1.35 ± 0.01 solar masses. That's close to the Chandrasekhar Limit
of about 1.4 solar masses, at which a white dwarf starts to collapse in
on itself and explodes completely as a Type Ia supernova.
When that day comes, V2487 Oph should shine down on the world
at 1st magnitude or brighter. At the white dwarf's present estimated
rate of mass accretion, expect it to happen in about 4 million years.

17h 32m

17h 30m

Ophiuchus stands in the south over Scorpius on July evenings,
as shown on our all-sky constellation map on pages 42-43.
There you'll ﬁnd 2nd-magnitude Eta (η) Ophiuchi right at the
map's crease. Starting from Eta, use the charts here to star-hop
to the repeating nova's precise location. The 11.8-magnitude star
on the closeup chart lies 76″ west of the point to watch.

On the morning of July 29th, the bright limb of the waning crescent Moon
will occult 1st-magnitude Aldebaran for the southern and eastern US. The
event happens in darkness for Texas and surrounding areas, in twilight
for much of the Midwest and South, and after sunrise farther northeast.
Some times: at Austin, disappearance 4:39 a.m. CDT, reappearance 5:19
a.m. CDT; Atlanta, d. 5:48 a.m., r. 6:40 a.m. EDT; Miami, d. 5:33 a.m., r.
6:43 a.m. EDT; Pittsburgh, d. 6:11 a.m., r. 6:44 a.m. EDT; Washington DC,
d. 6:05 a.m., r. 6:53 a.m. EDT; Boston, d: 6:20 a.m. EDT, r. 7:03 a.m. EDT.
The Moon will also occult the star-pair Theta1 and Theta2 Tauri, magnitudes 3.4 and 3.8, about three hours earlier (in darkness with the Moon
much lower) for parts of the East and Northeast.
For all three occultations, ﬁnd timetables for many cities at lunar
-occultations.com/iota/bstar/bstar.htm. ✦

Sk yandTelescope.com July 2016

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Sky and Telescope - July 2016

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