Posted by on Jul 17, 2013 in Constellation Of the Month | 0 comments

Sagittarius the archer was one of the 48 constellations listed by the 2nd century astronomer Ptolemy. It remains one of the 88 modern constellations today. It is the most southerly of the zodiacal constellations and is always very low in the night sky over Yorkshire. Part of it remains permanently below our horizon.

This group of stars is not particularly easy to find. The best method is to locate Antares in Scorpio and then look to the east, about the same distance above the horizon. The brightest stars, which mark Sagittarius, should then be found without a great deal of trouble.

There are some interesting objects in the area, but their low altitude above the horizon prevents their being seen well. However the area is worth sweeping with binoculars. The celebrated ‘Star clouds’ indicate the direction of the centre of our Galaxy.

Star Visual Magnitude Spectrum Absolute Magnitude Distance (Lt Years)
Rukbat 3.96 B8V 0.38 170
Kaus Australis 1.81 B9 -1.1 124
Nunki 2.12 B2 -2.7 300
Ascella 2.61 A2 0.1 140
Kaus Meridionalis 2.71 K2 0.7 84
Kaus Boralis 2.80 K2 1.1 71
Alnasr 2.97 K0 0.1 124
Albaldah 2.89 F2 -0.7 250

 

M 8 (NGC 6523) The Lagoon Nebula.

One of the finest cluster/nebula combinations in the sky. It is an emission nebula. Fairly bright with a magnitude of 3.0 The central area of the Lagoon Nebula is also known as the Hourglass Nebula, so named for it’s distinctive shape. The Hourglass Nebula has it’s shape because of matter propelled by Herschel 36. (The central star in the nebula from which it evolved. It is a very hot blue coloured star. The youngest known main sequence star).

The Lagoon Nebula also features three dark nebulae. The large, scattered open cluster lies at the eastern edge of a large, swirling cloud of nebulosity. The nebula has obvious dark lanes and looping patterns, which in long exposure photography are shown to dense star forming regions. This is a fine sight in almost any telescope, and can be seen by both the naked eye and in binoculars. Long exposure photographs reveal it has a pink hue, common to emission nebulae.

M 20 (NGC 6514) The Trifid Nebula.

Also an emission Nebula. The outside of the Trifid Nebula is a bluish reflection nebula; the interior is pink with two dark bands that divide it into three areas, sometimes called “lobes”. Hydrogen in the nebula is ionised, creating its characteristic colour, by a central triple star, which formed in the intersection of the two dark bands. This often photographed nebula is rather faint in small telescopes, but in moderate size instruments shows a circular patch of light surrounding a double star. This patch of light is more or less equally divided by three intersecting dark lanes meeting near its centre. A fainter region of reflection nebulosity can be seen to the north.

M 17 (NGC 6618) The Omega or Swan Nebula.

This bright nebula looks like a check mark or swan floating in a heavenly pond. It consists of a curving arc of nebulosity connected to a straighter bar shape. The bar portion shows a lot of intricate mottling and streaks. It is considered one of the brightest and most massive star-forming regions of our galaxy. Its local geometry is similar to the Orion Nebula except that it is viewed edge-on rather than face-on.

Photography shows this region to be only a small part of a larger, billowing nebulous region; an open cluster of 35 stars lies embedded in the nebulosity and causes the gases of the nebula to shine due to radiation from these hot, young stars; however the actual number of stars in the nebula is much higher, up to 800. It’s also one of the youngest clusters known, with an age of just 1 million years. The luminous blue variable HD 168607, HD 168607, located in the south-east part of the Omega nebula, is generally assumed to be associated with it; its close neighbour, the blue hyper giant HD 168625, may be too.

The Swan portion of M17, the Omega Nebula in the Sagittarius nebulosity is said to resemble a barber’s pole.

M-22 (NGC 6656)

Despite its relative proximity to us, this metal poor cluster’s light is limited by dust extinction, giving it an apparent magnitude of 5.5 making it one of the finest globular clusters in the sky visible from northern latitudes. It is large, about 15-20′ in diameter, and is rather loosely gathered, allowing us to resolve many individual stars across its face. Some consider this globular to be second only to Omega Centauri in beauty. However, due to its southerly declination, M22 never rises high in the sky and so appears less impressive than other summer sky globulars such as M13 and M5.

M 24 (IC 4715) The Small Sagittarius Star Cloud.

This is a large, disconnected portion of the Milky Way. With an approximate magnitude of 3, it is best viewed in binoculars, which show countless stars and some obvious dark streaks along its length.

NGC 6603

A small very dense star cluster, which some authors claim erroneously to be M-24, is embedded in its north eastern section. It bears no resemblance to Messier’s original description.

NGC 6567

A dim planetary nebula, and Barnard 92, a Bok globule, are also nearby. Bok globules are dark clouds of dense cosmic dust and gas in which star formation sometimes takes place.

NGC 6445

A planetary nebula with an approximate magnitude of 11. A large nebula at over one arcminute in diameter, it appears very close to the globular cluster NGC 6440.

NGC 6522 & NGC 6528

They are small and moderately faint, but lie within the same field of view of a low power eyepiece in a highly populated region of the Milky Way. The longer they are observed, the more background stars become visible, producing a grainy backdrop for these two twin globulars. A great deep sky double!

NGC 6520 & B86

NGC 6520 is a small, rather tight gathering of about twenty stars placed right next to a similarly sized dark nebula, B86.The proximity of the cluster seems to accentuate the darkness of the nebula and make it appear as a hole in the sky, definitely darker than the surrounding region. This dark nebula is commonly known as the Ink Spot because of this.

NGC 6638

This is a dimmer globular at magnitude 9.2, though it is more distant than M71 at a distance of 26,000 light-years. It is a Shapley class VI cluster; the classification means that it has intermediate concentration at its core. It is approximately a degree away from the brighter globulars M22 and M28; NGC 6638 is southeast and southwest of the clusters respectively.

V4641 Sagittari

In 1999 a violent outburst at V4641 Sagittari was thought to have revealed the location of the closest known black hole to Earth, but later investigation increased its estimated distance by a factor of 15. The complex radio source Sagittarius A is also here. Astronomers believe that one of its components, known as Sagittarius A*, is associated with a supermassive black hole at the centre of the Milky Way, with a mass of 2.6 million solar masses.

The Sagittarius Dwarf Elliptical Galaxy is located just outside the Milky Way.

Baade’s Window

This is an area with very little obscuring dust that shows objects closer to the Milky Way’s centre than would normally be visible. NGC 6522, magnitude 8.6, and NGC 6528, magnitude 9.5, are both globular clusters visible through Baade’s Window. 20,000 and 24,000 light-years from Earth, with Shapley classes of VI and V respectively, both are moderately concentrated at their cores. NGC 6528 is closer to the galactic core at an approximate distance of 2,000 light-years.