Sky & Telescope - September 2023 - 27

TINY VARIABLE M31-V1's coordinates are
RA 00h 41m 27.3s, Dec. +41° 10′ 10.4″. If you
center the small arrow-shape asterism in your
field of view (12′ × 12′ at left), you should be
able to spot Hubble's variable star (arrowed).
(officially cataloged as M31 V0619).
AAVSO members observed the star for
about six months beforehand in order
to generate multiple light curves over
the star's period of 31.4 days. This was
so Hubble scientists would know when
to aim the telescope at the star - they
wanted to catch it at or near maximum
and minimum brightness, and detailed
studies of the star's phase didn't exist
(even if basic parameters were known).
HST observed the target four times between December 2010
and January 2011. M31-V1's light curve displayed the typical
profile of a Cepheid: a relatively slow decline in brightness for
most of the cycle (25 days for M31-V1), followed by a more
rapid recovery in six to seven days.
I was curious about M31-V1, and in my correspondence
with Matthew Templeton, then science director at the
AAVSO, I learned that all observations thus far had been
imaging. So I decided to observe it visually. The star's magnitude
range - 18.5 to 19.8 - is accessible in my 32-inch reflector
from my home in Minnesota. But the autumn weather
was poor for several years. Then, the late evening of September
23, 2019, was magnificent. There was no dew or wind,
and I had until just past midnight before a waning crescent
Moon would enter the sky. I rapidly found the field using
Megastar charts and a red plate from the National Geographic
Society - Palomar Observatory Sky Survey.
The Cepheid sits within an arrowhead-shape asterism of
GLORIOUS ANDROMEDA M31, more familiarly known as the
Andromeda Galaxy, is a naked-eye object under dark skies and
an easy target even in small instruments. But you'll need a really
big telescope, such as the author's 32-inch, to identify Hubble's
variable star.
observations in the Astrophysical Journal. That same year, 1929,
Hubble recognized the relationship between galaxies' distances
and their speeds of recession - the farther a galaxy lies
from us, the faster it's receding. The universe was expanding!
These two revelations must have offered the young astronomer
a profound sense of discovery and accomplishment.
Homing in on M31-V1
In a tribute to the legendary man and his observations of
this star, in late 2010 NASA planned to point the Hubble
Space Telescope (HST) at M31's famous Cepheid, M31-V1
seven stars 1.5′ across, with the tip pointing west. The three
westernmost stars of that group form an equilateral triangle
20″ on a side, with M31-V1 its southeastern member. The
arrow's tip star to the Cepheid's west-northwest is magnitude
18.0, just a bit brighter than our target star's brightest magnitude.
I hadn't calculated where in its cycle the star would
be that night, so I didn't know what its magnitude should
be, but I could tell right away that it was quite a bit fainter
than the tip star. As chance had it, the Cepheid was on day
23 of its 31.4-day cycle, just before the minimum magnitude
of 19.8. I observed the area for about 75 minutes, employing
a number of eyepieces that gave magnifications from 363×
to 650×, in seeing that was 7/10 and transparency 7-8/10.
Finally, with a 6-mm Zeiss Abbe Ortho at 521×, I confirmed
M31-V1 just before midnight. Being sure to see it several
times, I rejoiced in a journey retracing the history - and
appreciating the importance - of this singular star.
¢ DAVE TOSTESON enjoys when history and cosmology combine
with atmospheric clarity to recreate " Eureka! " moments of
observation.
sk yand tele scope .o r g * SEPTEMBER 2023 27
WIDE VIEW OF M31: MASIL IMAGING
TEAM; ZOOM: POSS-II / STSCI / CALTECH /
PALOMAR OBSERVATORY
http://skyandtelescope.org
Sky & Telescope - September 2023

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