Sky and Telescope - October 2016 - 49

Learn how to use detailed star charts to ﬁnd the very faintest
things your telescope can show: skyandtelescope.com/charts.

Alan MacRobert

The Two Top Miras
Chi Cygni high overhead, and Mira
rising up late at night, are the two brightest Mira-type stars in the sky: aging red
giants that slowly pulse and fade through
very large brightness swings with
periods of many months. This class is
also known as the long-period variables,
dating from back when no other kinds
of variable stars with long periods were
known. This fall you can watch these two
leaders of the Mira menagerie progress
through opposite behaviors, on opposite
sides of the sky.
Chi Cygni starts out bright. The
American Association of Variable Star
Observers predicts that it will peak in
mid-September, then fade for the next 7
or 8 months. Chi Cyg tends to alternate
between bright and faint maxima. It
reached 4th magnitude last time, so this
time it will probably top out at 5th or 6th.
But who knows?
Mira, on the other hand, should bottom
out in late October around magnitude 9.4.
That's brighter than the maxima of many
variables that amateurs follow. A magnitude-9.2 orange star lies just 2 arcminutes
east of it, as shown on the bottom chart,
oﬀering a convenient comparison. Mira
variables tend to brighten faster than they
fade; the AAVSO predicts that Mira itself
should climb to 3.4 by late February, making it easy to the naked eye.
On these charts, comparison-star
magnitudes are given to the nearest tenth
with the decimal point omitted. At the
Top: Chi (χ) Cygni, high overhead these
evenings, is the brightest star of spectral type
S. It should peak at 4th to 6th magnitude in
September, then start fading. The black rectangle shows the area of the enlargement. (For
deeper comparison-star charts, go to aavso.org
and at "Star Name Here," type Chi Cyg.) Right:
Mira is in good view in the east by about 1 or
2 a.m. in September, 11 p.m. or midnight in
October, 9 or 10 p.m. in November, and so on.
In late October, you may catch it matching the
9.2-magnitude star 2′ east of it. This winter, it
should become easy to spot naked-eye.

eyepiece, ﬁnd the comparison star that
looks just a little brighter, and the one
just a little fainter, than the variable itself.
Interpolate the variable's magnitude
carefully between them. Then spend
some time examining whether this ﬁrst
impression is really the best. Visual variable-star work is rather exacting; strive for
the greatest accuracy you can eke out.
On what date will falling Chi and rising Mira cross in brightness?

g
f
e
k2
k1

_

CYGNUS

a

CYGNUS

d

73

56

d

39
71

68

b

Deneb

+35$
64

χ

50

47

60

χ

62
75

60

Albireo

`

VULPECULA
55
+30$

47

VULPECULA

_

`
Albireo
31

a

_
28

36

a
38

b

41

57

61

_Psc

58

54

63

R

0°

M77
69

55

54

60

CETUS
58
68

3h 00m

2h 30m

75

64 92 Mira
73
88
86
80
71
67

57
73

2h 00m
-5°

Sk yandTelescope.com October 2016

49
http://www.skyandtelescope.com/charts http://www.aavso.org http://www.SkyandTelescope.com
Sky and Telescope - October 2016

Table of Contents for the Digital Edition of Sky and Telescope - October 2016
