Sky & Telescope - September 2020 - 60

SEPTEMBER 2020 OBSERVING
Going Deep

Not One but Two Stars
The very hot white dwarf (120,000 kelvin) at the center of the Ring is responsible for ionizing the surrounding nebula.
The star was at the top of my must-see
observing list for more than 20 years. It
wasn't until ﬁrst light with my 20-inch
f/5 Obsession reﬂector on September 14,
1991, from my suburban backyard that I
ﬁnally saw it. M57 was my ﬁrst deep-sky
object with this scope, too:
I worked up to 507× (!) before I ﬁnally
caught a series of ﬂeeting glimpses -
this is a faint star! Under a dark sky it
should be easier, but I don't think it will
be easy because the surrounding nebula
in the center of the Ring will be brighter
too. I spent an hour on this; just to
show how tough it is.

I've since learned that much smaller
scopes can show the central star. In
fact, the German astronomer Friedrich
von Hahn reported in 1800 that he had
discovered it several years prior with a
12-inch reﬂector. Many observers with
60

SEPTEMBER 2 02 0 * SK Y & TELESCOPE

comparably sized instruments since then
have also spotted it. I could have seen it
with my 12.5-inch scope some years earlier if only I'd used more magniﬁcation.
A combination of good seeing and
high magniﬁcation is essential for spotting the white dwarf. At magnitude 15.7,
it's a tiny spark that looks even fainter
due to the Ring's surrounding brightness. Reduced contrast with the subtle
nebulosity in the nebula's center doesn't
help either. This star is worthy of its
tough reputation. That said, on the very
best nights, the white dwarf's light-blue
color can be seen in large scopes.
Northwest of the white dwarf a magnitude-16.2 star that's unrelated to M57

peeps through the Ring's dark center. I
ﬁgured this star should be visible nearly
as often as the central star - it's only
half a magnitude fainter - but it took
me another 15 years to see the darn
thing. My notes from August 23, 2006,
when observing with the 28-inch read:
The second star is ﬁnally, deﬁnitely
seen! Wahoo! Averted vision is needed,
but with the real central star so easy
with direct vision (and a deﬁnite tinge
of blue) and the seeing quite excellent,
this had to be the night. Best view was
at 1422× and 1138× - but 1422× was
the most impressive. Chuck [Dethloff,
cofounder of the Oregon Star Party]
saw it too so we're both excited. How
cool is this!!

In short, this is a much more difﬁcult
catch than the central star. Good luck.

Central star

Halo

The other star

Main ring
Dark center
Nebulous bands

p DISSECTING THE RING The large circle shows the visible extent of the inner halo, which has
a diameter of 1.5 light-years. The main ring - the most visible part of the Ring Nebula - is about
1 light-year across its major axis and 0.7 light-year along its minor axis.

HOWA RD BA NICH (2)

During the summer of 2019 I
saw just a single, wide band with my
28-inch scope. Although the outer edges
of this band were distinct, no matter
the magniﬁcation, ﬁlter or no ﬁlter,
I couldn't see a dark lane dividing it
in two. The wide band's edges and the
bright inner borders of the Ring form
two dark gaps that highlight the band.
Although I saw it unﬁltered, a UHCstyle ﬁlter boosted contrast.
Whether one band or two, they're a
feature of the Ring's polar axis bubble.
Imagine a lozenge-shaped bubble of
delicate nebulosity, girded in its middle
by the main Ring, with its major axis
more or less pointed in our direction.
The banded features are on both the
approaching and receding surfaces of
the bubble, so they only appear to be
inside the dark center because of our
perspective. I love this stuff!

t THE OTHER STAR This eyepiece sketch
shows that the planet-size white dwarf is in the
exact center of the dark oval and the other star
is just to its northwest. The latest Gaia DR2
data release indicates that the second star is
nearly four times farther away than the central
star - and pins down their magnitudes.

Sky & Telescope - September 2020

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