Page 59 - BBC Sky at Night - September 2017 UK

THE SKY GUIDE SEPTEMBER 59

COMETS AND ASTEROIDS

56 Melete – a minor planet that offers us the chance to do some serious science

56 Melete occults a star in Orion this Another occultation is
month, but be warned, it’s a quick predicted to occur during the
event lasting only a minute TAURUS morning of 24 September,
r 1 56 Melete when 56 Melete will occult the
mag. +11.0 TYC 1313-01143-1,
d
1 Sep 21 Sep which is in the northern part of
GEMINI r 2
Orion, near the Hunter’s ‘club’.
The occultation track passes
11 Sep
56 Melete across the south of the Republic
of Ireland, much of Wales and
21 Sep TYC 1313-01143-1 central England. The event is
26 Sep
predicted to occur between
h
i Meissa 04:05-04:06 BST (03:05-03:06 UT)
on 24 September. At that time,
j
56 Melete will be mag. +13.8
Bellatrix a and cause the star to dim by
ORION
2.9 magnitudes.
As asteroids go, 56 Melete is
Minor planet 56 Melete is a 0.15 magnitudes caused by its be seen from Earth – is finite. pretty faint, remaining at mag.
main belt asteroid discovered rotation. The repeat of this Within the track, accurate +13.8 all month long. Despite
160 years ago on 9 September curve, and hence the asteroid’s timings can help derive the this, imagers should be able to
1857. It takes 4.18 years to orbit rotation period, is 18.15 hours. profile width of the asteroid. pick it up as it passes from the
the Sun, its elliptical orbit Occasionally asteroids If enough observers record southeast corner of Taurus into
bringing it in as close as 1.97 appear to pass in front of stars, timings within the track path, the northern part of Orion. If
AU to our star and out as far as and 56 Melete was observed doing it is possible to add all of their you are located within the
3.21 AU. It’s a dark object, 129km so in 1997 and 2002. Such events occultation results together occultation track, this is a great
across, its light curve showing a are important because the and built an approximate opportunity to do some quite
small variation in brightness of occultation track – where it can profile of the asteroid’s shape. important science.

STAR OF

THE MONTH NGC 404
ANDROMEDA
Mirach – the haunted q
Mirach
semi-regular varaible M31
Mirach (Beta (`) Andromedae) is halfway i
along Andromeda’s body, between Alpheratz
(Alpha (_) Andromedae) and Almach
(Gamma (a) Andromedae). The name +
Mirach probably results from a corruption / Alpheratz _
of mi’zar, meaning girdle or waist-cloth. Almach `
Mirach and Alpheratz have the same a
average brightness of mag. +2.06, but look Mirach b
markedly different because Mirach is a ¡
red giant and Alpheratz a hot blue star.
Mirach is 197 lightyears away and 1,900 ! Mirach and NGC 404 appear close as a line of sight effect; the galaxy is 51,000 times farther
times more luminous than the Sun. Its radius
is estimated at around 0.4 AU, or 85 times Like many stars, Mirach is not alone and has Through the eyepiece Mirach has an
larger than the Sun, giving it a circumference a 14th-magnitude main-sequence companion additional deep sky companion in the form
equivalent to the orbit of Mercury. As the that is 60,000 times dimmer than Mirach itself. of the isolated dwarf lenticular galaxy, NGC
star is approaching the end of its life its core Their orbits bring them physically to within 40 404. The apparent separation between star
probably consists of helium or carbon. As is times the Sun-Pluto distance of one another. and galaxy is just 7 arcminutes giving NGC
common with old red giants, Mirach’s light is From Mirach the companion would appear as 404 the appearance of a lens flare caused
inconstant, and it’s classed as a semi-regular bright as Venus appears from Earth. Mirach by Mirach. Known informally as Mirach’s
variable. The changes in brightness are small, would appear 120 times brighter than the full Ghost, one of the biggest hurdles to seeing it
ranging between +2.01 to +2.10. Moon from the companion. is the brightness of Mirach itself.

skyatnightmagazine.com 2017