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b¡ STF 1245STF 1255STF 1273STF 1290S Hydrae62 ASTRONOMY • APRIL 2019L
ast December, we
toured the constel-
lation Triangulum.
Small in size, it can
be fully explored
in a single evening, much
like a fisherman can cover
a small pond in one day. In
that December article, I also
promised to explain how you
can explore an expansive
constellation the same way
an angler tackles a large body
of water.
So, is your telescope ready?
Our target, the water snake
Hydra, is a fearsome creature
— both mythologically and
astronomically.
Hydra is the largest of the 88
recognized constellations; with
an area of 1,303 square degrees,
it covers about 10 times as
much sky as Triangulum.
Hydra winds its way across
part or all of eight zones of
right ascension — far too much
territory for a single observing
session. What to do? Here’s a
tactic I learned during my
decades as an avid freshwater
fisherman.
Before heading out to fish, I
look at a map of the lake and
single out a promising area
— perhaps a bay near an inlet.
I concentrate on this one spot,
saving other parts of the lake
for future trips. The same
approach can be used for
covering large constellations,
referring to a star atlas to zero
in on a promising section. For
instance, if I’m in a double star
mood (and when am I not?),
Bruce MacEvoy and WilTirion’s Double Star Atlas iden-
tifies several binaries near a
distinctive group of stars that
forms the head of Hydra. The
area covers only about 20
square degrees, so I’ve essen-
tially turned a vast lake into a
small pond.
Hydra’s head lies on the
boundary with Cancer, so start
by jumping across the border
into Cancer for a peek at the
double star Struve 1245
(STF 1245). This is one of more
than 3,000 pairs cataloged by
the German-Russian astrono-
mer Friedrich Georg Wilhelm
von Struve between 1824 and- According to the online
edition of The Washington
Double Star Catalog (WDS),
its magnitude 6.0 and 7.2
members are separated by 10.1".
Their spectral classes are listed
as F8 and G5, so study this pair
closely to see if you can detect
any colors.
Returning to Hydra, train
your telescope on STF 1255, ½°
east of the 4th-magnitude star
Delta (δ) Hydrae. Fainter than
STF 1245, it’s nevertheless
bright and wide enough (mag-
nitudes 7.3 and 8.6, separation
26") for an easy split in a
small-aperture scope like a
3-inch f/10 reflector. Both are
G-type stars, but the colors
might not be so obvious inOBSERVINGBASICS
BY GLENN CHAPLEWhat’s hiding
in Hydra?
Take a dip in a vast cosmic lake and start exploring
the water snake’s head.such a small instrument.
The next pair will require
a bigger boat, er, telescope. One
of Struve’s doubles (STF 1273)
is better known by its Greek
moniker, Epsilon (ε) Hydrae.
The brighter component, of
magnitude 3.5, is attended by
a magnitude 6.7 companion a
mere 2.9" away. A 5-inch scope
should split this pair, but you’ll
still need an evening of steady
seeing and an eyepiece that
magnifies 150x or more.
If you’re able to split ε Hya,
move on to an even moredifficult challenge, STF 1290.
This magnitude 7.4 and 9.2 duo
is located about 2.5° southeast
of ε Hya. The two stars are
separated by just 2.8". If 150x
doesn’t work with a 5-inch
telescope, try 200x.
Double stars, however, aren’t
the only fish in the Hydra lake.
A variable star, S Hydrae, lies
1.5° south and slightly east of
STF 1290. S Hya is a long-
period variable (LPV). LPVs
are also called Mira-type vari-
ables after the prototype, the
star Mira (Omicron [ο] Ceti) in
the constellation Cetus. Most
are red giants that pulsate insomewhat regular cycles,
changing brightness as they
beat. S Hya cycles from an
average maximum brightness
of magnitude 7.3 to an average
minimum of 13.3 and back
again once every 8.5 months.
The last max was in late
September of last year; the next
should occur sometime in mid-
to late May this year. To moni-
tor the changes, check out S
Hya every seven to 10 days
using a chart, like the one on
the American Association of
Variable Star Observers’ web-
site, that shows the magnitudes
of nearby stars.
Given its astounding size,
there are many other parts of
Lake Hydra that any angler/
astronomer can explore. Next
session, we might just migrate
south to check out the open
cluster M48 and the double
stars STF 1270 (magnitudes 6.9
and 7.5, separation 4.7") and
17 Hydrae (magnitudes 6.7
and 6.9, separation 4.1").
Questions, comments, or
suggestions? Email me at
[email protected].
Next month: What visual
double star has a companion
that circles its main star once
each day? Clear skies!BROWSE THE “OBSERVING BASICS” ARCHIVE AT http://www.Astronomy.com/Chaple.Although Hydra is the largest constellation, if you break the water snake into smaller
parts, it’s much easier — and much more rewarding — to explore. ASTRONOMY: ROEN KELLYGlenn Chaple has been an
avid observer since a friend
showed him Saturn through a
small backyard scope in 1963.Double stars, however, aren’t the only fish
in the Hydra lake.