Trocha historie VIII
Redakce, Neděle, 13. Září 2009
Pod názvem Trocha historie bychom chtěli postupně uveřejnit ukázky z nejstarších čísel Instantního Bílého trpaslíka (doplňku zpravodaje Bílý trpaslík společnosti Amatérská prohlídka oblohy), na který navazují tyto stránky.
Ukázky jsou exkurzí do druhé poloviny 90. let na dění v Amatérské prohlídce oblohy, v astronomii a historickým pohledem na oblohu v těchto letech.
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* I N S T A N T N I B I L Y T R P A S L I K c. 008 *
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Comet Hyakutake is spectacular I.
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Had my first views of Hyakutake from Lowell Observatory's Anderson Mesa
"dark sky" site tonight (Mar 21.3-21.4 UT). The sky had just gone from being
photometric, and a few thin cirrus were discernable. The naked-eye mag. limit
was still V ~7.2 (typical for the site and my eyes); sky brightness up a bit
from scattered starlight by the aerosols: I'll estimate muV ~21.5 per square
arcsec at the zenith (normal is 21.8).
The comet is simply stunning! The tail goes right to the zodiacal band
where it merges with the gegenschein, passing between gamma and delta Vir---
check those charts, boys and girls, that's 30 degrees! The glow in this area
was quite remarkable and took some time to sort out. The gegenschein was a
big blob, now well east of the hind legs of Leo toward gamma Vir, and the
zodiacal band appeared to extend in two directions from it toward the east.
The southern branch is the zodiacal band heading toward Scorpius; the northern
one leads right to the head of the comet. Notably, the tail this far out was
_fainter_ than the zodiacal band, so obviously is something that will require
"true-dark" skies to pick up.
I can confirm the descriptions by Rusty Lederman and Dave Knisely that
there is a nice spur on the northern side of the main tail; I made it about
3 degrees long in 7x35mm and 10x50mm binoculars. The main tail itself seemed
to have a brightness gradient laterally across it, giving at first glance the
impression that it is bifurcated, too. There also seemed to be some structure
on the south side of the main tail within a few degrees of the coma, but
several stars in this region may have caused this. The binoculars showed the
narrow main tail to about 15 degrees or a bit more, becoming very difficult to
discern beyond that. I also looked for but did not see anything unexpected on
the sunward side of the coma.
Without glasses (MyopiaVision [tm] version -8.5 diopters), I made the
coma to be about 0.6 to 0.8 mag. fainter than Spica (V=1.0), thus mv ~1.7 for
the comet. Since the distance change each night is now fairly small, this
presumably accounts for the slowdown in night-to-night brightness increase.
As an aside, I "calibrate my eyeball" for delta-mags from pairs of stars
(either naked-eye or telescopic) with reliable photometry; e.g. Castor and
Pollux have delta-m of 0.45 mag., gamma and 40 Leonis about 2.5, etc. A good
exercise for learning to see small differences in brightness is trying to
order the Big Dipper stars by brightness and then comparing with photoelectric
photometry in a list such as the RASC Handbook, Sky Catalogue 2000, or the
Yale "Bright Star Catalogue".
Again with glasses, the core of the coma appeared substellar (i.e. not
quite stellar), and seemed comparable with direct vision to stars as bright
as 109 Vir or zeta Boo (both around V=3.7) or possibly brighter. This was
fiendishly difficult to do, mainly from the difficulty of really getting a
zero-angle direct-vision look (as compared to very slightly averted, which at
least my eyes seem to prefer). Anyway, the point is this impression is
subject to large errors.
I think it will be interesting to watch the nuclear condensation (the one
that's just a few arcseconds across, not the naked eye one of the previous
paragraph) as many of us have described in the last few days. At closest
approach the "image scale" will be about 75 km/arcsec, so we're getting into
the county-sized region of the comet's core. Is there anyone with one of
those 25+ inchers out there who can give us some verbiage from high power
views?
The comet was moving at about 17'.5 per hour tonight, so the motion was
easy to pick up in a few minutes with binoculars. It was interesting watching
stars drift through the field on successive frames of the CCD camera on our
1.1-m telescope, which was being autoguided on the comet.
Finally, the comet group at Lowell are busy studying this object. We'll
probably have some images somewhere on the Lowell Web page in a few days,
which I'll send a note out about if/when.
Brian Skiff (bas@lowell.edu)
Comet Hyakutake is spectacular I.
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Not quite clear this morning (Mar 22.4 UT) observing from the catwalk at
the Lowell Observatory 1.1-m telescope on Anderson Mesa as a weak trough
receded. Cloudy until about 1am/8h UT. By 2am/9h UT, stars beyond mag. 7
were visible naked-eye near the zenith about 20 degrees from the head of the
comet. My check stars are HD 113865 (V=6.5), HD 112734 (V=7.0), HD 112887
(V=7.2), and HD 113493 (V=7.3), all in Coma around the triangle of bright
stars that encloses the Coma galaxy cluster.
With the unaided eye I could detect the tail out past the zodiacal band
and gegenschein south of the ecliptic, for a total length near 40 degrees.
Specifically, the tail has rotated around to the southwest quite dramatically
since last night, and was pointed through gamma Virginis. (The head of the
comet is more than 10 degrees farther north than last night, but the tail at
gamma Vir is actually slightly farther south!) The gegenschein is centered
presently near eta Vir, which is the first naked-eye star west of gamma Vir.
The zodiacal band extends along the Regulus-Spica line and southeast to alpha
Librae. I saw an additional "nebula" past gamma Vir, reaching roughly to the
lip of the cup of Crater, i.e. between theta and eta Crt, at about 11h50m and
and -12deg. I played around with convincing myself of this by viewing with
the telescope dome blocking the comet east of gamma Vir, with my head
upside-down, and by sweeping my head left-&-right fairly rapidly---a technique
that helps show large, subtle structures in the sky during twilight.
The southern extension of the tail below the ecliptic persisted with
sidereal motion for about an hour, so this was not a cloud. Thus I'm pretty
convinced it's there. Observers should be careful to isolate and identify the
various night-sky stuff in order to differentiate them from faint extensions
of the tail. As last night, the tail this far from the comet is fainter than
the zodiacal band, so the latter should be identifiable separately from the
comet.
The "easy" part of the tail is about 20 degrees long.
With 7x35mm and 10x50mm binoculars, the tail structure in the first ~5
degrees has changed markedly since last night. The "spur" on the north side
has now elongated and broadened, nearly merging with the long main tail. The
tail generally is stronger (higher surface brightness) than last night. The
inner coma is now strongly elongated, noticeable even with the 7x35s---it was
circular last night. CCD frames being taken with the 1.1-m telescope showed
that this change extends right to the nuclear condensation. High-magnification
visual observations with significant aperture are desired!
Total magnitude: without glasses (-8.5 diopters out of focus, so all
stars look like 1.5-deg comets!) it was very similar to Spica (V=1.0) or
slightly fainter. I'll be conservative tonight and call the comet mv = 1.2.
A delta-mag estimate against Arcturus (V=0.0) gave a value of 0.8, however.
Definitely brighter than any of the Big Dipper stars.
Let's see, did any superlatives creep in?
Brian Skiff (bas@lowell.edu)
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