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HERMANUS ASTRONOMY CENTRE
OBSERVATION SECTION
July and August 2015
THE BIG FIVE IN SAGITTARIUS
(AND ONE TINY ONE)
INTRODUCTION
In accordance with ASSA's wishes for an Observation Section in each Centre, HAC will in future publish a
"Big Five" newsletter listing at least a number of deep sky objects (DSOs) that are well worthwhile
observing. (Note: The "Big Five" concept is based on Auke Slotegraaf's "Big Five" proposal for the Southern
Sky).
SELECTED CONSTELLATION
The selected constellation for July and August 2015 is SAGITTARIUS. In the evening sky, at about one hour
after sunset, this constellation is rising in the east and is at a convenient altitude for observation (> 30º).
STARFIELD IMAGE
To assist observers, a starfield image for SAGITTARIUS (Sgr) is attached to the covering email. It is
recommended that you print this typed list and have the starfield image displayed on your computer screen
when you read this list.
The image was taken on 18 August 2014 from the observatory in Fisherhaven and is annotated to assist in
the orientation of our observers. Slightly to the left of the centre of the image the well-known asterism The
Tea-pot in Sgr has been outlined by connecting the lines. In the bottom right corner of the image is the
stinger and tail of the constellation SCORPIUS (Sco). Somewhere between the Tea-pot and the tail of Sco is
the centre of our galaxy, the Milky Way.
The overwhelming feature of the image is the Milky Way itself. However, the Milky Way has already been
mentioned by Auke in his initial "Big Five" letter, as such we will ignore it for the present (this somewhat
similar to ignoring the proverbial elephant in the room!).
THE BIG FIVE IN SAGITTARIUS
In order of magnitude from bright to less so, the following objects have been selected for observation:
Number1. #1 is the Sagittarius Star Cloud, also known as Delle Cautiche and numbered by Messier as M24.
To describe this cloud, the following extract from freestarcharts.com is inserted below:
M24 also known as the Sagittarius Star Cloud is a large naked eye expanse of stars, clusters, nebulosity and other objects
located in Sagittarius. At apparent magnitude +4.6 and covering 1.5 degrees of sky. It's visible to the naked eye as a large
detached part of the Milky Way. The object is a fantastic sight in binoculars or small telescopes; it's claimed that M24 has the
densest concentration of individual stars visible using binoculars, around 1,000 stars within a single field of view. The
Sagittarius Star Cloud is not a "true" deep sky object but results from a chance alignment between the Earth and the centre of
our galaxy. We would expect this region to be packed with interstellar dust, however by chance we are looking through a
"tunnel" in the interstellar dust, revealing many thousands of distant stars, clusters and nebulae that would otherwise be
obscured.
The coordinates for M24 are RA 18h 18m 48s and Dec S 18º 33' 00". Its magnitude is 3.1 and its
dimensions 95 x 35 arc'.
Observers wishing to use a finder chart to find M24, are invited to visit the following website:
http://freestarcharts.com/20-guides/messier/315-messier-24-m24-sagittarius-star-cloud
Number 2. #2 on our list is the open cluster known as M25. It is visible on the starfield image somewhat to
the left of #1. It is an open cluster of magnitude 4.6 and dimension 26 arc minutes. It is clearly
marked on the same finder chart as for M24. The co-ordinates for M25 are RA 18h 31m 42s and Dec
S 10º 07' 00". Its magnitude is 4.6 and its diameter 26 arc'.
Number 3. #3 on the list is the bright nebula numbered M8 and titled the Lagune Nebula. The extract from
Wikipedia reads as follows:
Compiled by Johan Retief
Draft
18 July 2015
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The Lagoon Nebula ) is a giant interstellar cloud in the constellation Sagittarius. It is classified as an emission nebula. The
Lagoon Nebula is estimated to be between 4,000-6,000 light years from the Earth. In the sky of Earth, it spans 90' by 40',
which translates to an actual dimension of 110 by 50 light years. Like many nebulas, it appears pink in time-exposure colour
photos but is grey to the eye peering through binoculars or a telescope, human vision having poor colour sensitivity at low light
levels.
It is clearly marked on the same finder chart as for M24. The co-ordinates for M8 are RA 18h 3m 12s
and Dec S 24º 23' 00". Its magnitude is 5.0 and its dimensions 45 x 30 arc'.
Number 4. #4 is the globular cluster numbered M22 and titled the Sagittarius Cluster. M22 is one of the
closer globular clusters to Earth and numbers in excess of 70,000 stars. It is clearly marked on the
same finder chart as for M24. The co-ordinates for M25 are RA 18h 36m 24s and Dec S 23º 54' 12".
Its magnitude is 5.2 and its diameter 32 arc'.
Number 5. #5 is the bright nebula numbered M20 and titled the Trifid Nebula. Its face also looks like the
face of a black and white kitten and it is also known as the pussy-cat nebula. The extract from
Wikipedia reads as follows:
Its name means 'divided into three lobes'. The object is an unusual combination of an open cluster of stars; an emission
nebula a reflection nebula and a dark nebula (the apparent 'gaps' within the emission nebula cause the trifurcated
appearance).
It is clearly marked on the same finder chart as for M24. The co-ordinates for M20 are RA 18h 02m
23s and Dec S 23º 01' 48". Its magnitude is 6.3 and its diameter 20 arc'.
Bonus: A tiny dark nebula. #6 is a tiny dark nebula numbered B86 and titled the Ink Spot Nebula. It is also
known as Herschel's Hole in the Heavens. Its location is indicated by a dark circle in the starfield
image somewhat above and to the eft of the tip of the Tea-pot's spout. The co-ordinates for B86 are
RA 18h 02m 58s and Dec S 27º 52' 06". Its diameter 4 arc'.
Enjoy!
The following members are available should you have any questions
Pierre de Villiers
028 314 0830
082 854 2277
Deon Krige
028 314 1045
084 528 0947
Johan Retief
028 315 1132
082 807 6793
Email address for the Centre
Hermanus.astronomy@gmail.com
Compiled by Johan Retief
Draft
18 July 2015