| Canes Venatici
| Canis Major
| Canis Minor
| Coma Berenices
| Corona Australis
| Corona Borealis
| Leo Minor
| Piscis Austrinus
| Triangulum Australe
| Ursa Major
| Ursa Minor
If the stars should appear one night in a thousand years, how would man believe and adore, and preserve for many generations the remembrance of the city of God?
- Ralph Waldo Emerson
|Area:||867 sq deg|
|Co-ordinates1:||Right Ascension 19h, Declination −25°|
On a deserted beach long ago, a young man looked up at more stars than he could possibly count. He was looking at the centre of our galaxy, the Milky Way, which lies in the constellation Sagittarius.
Douglas Adams started the first book in the Hitchhiker series with the words
Far out in the uncharted backwaters of the unfashionable end of the Western arm of the galaxy lies a small unregarded yellow sun.
- Douglas Adams (Intro) The Hitchhiker's Guide to the Galaxy
He was of course referring to our own sun, and while we may quibble with the designations uncharted or unfashionable, no one is arguing about the words 'far out'. Those who read science fiction
might note that the opening quote from Emerson was also used to begin a story by Isaac Asimov called Nightfall
. The story was about a civilisation living in the middle of a globular cluster much closer to the heart of the galaxy. That civilisation also did not have street lights, and so when it got dark it got really dark there were an unimaginable numbers of stars. Those numbers really exist on Earth, but a person living in a densely-populated city such as London, Tokyo, or Los Angeles may never see them. Even those with a clear dark sky cannot see things like the Pistol Star without using an infra-red telescope, because of all the dust and dark matter between our arm of the galaxy and the central core.
The constellation Sagittarius appears in the summer sky east of Scorpius between 20° and 40° south of the celestial equator. Many star charts will show an asterism here called 'The Teapot'. Scientific study of the sky in Sagittarius indicates that a resident black hole lurks at the heart of the Milky Way, with a mass calculated at greater than 2.6 million times that of our sun.
Sagittarius depicts a centaur holding a bow and arrow. Many sources say Ptolemy patterned both this constellation and the more southerly Centaurus after the same figure in mythology: the centaur named Chiron2. Most centaurs were a surly lot, but not Chiron. Educated by the Greek gods, he was wise and kind. Among his students were Achilles, Jason, Hercules, and Asclepius3, the latter two having their own constellations. According to some versions the Archer has his arrows poised to shoot the scorpion in case he decides to sting anyone.
In the table below you will find the first column contains letters such as alpha or beta. The scientific star names are from the lower case Greek alphabet. Where two stars share the same Greek letter they have been given a suffix such as gamma-2. Sagittarius is one of those constellations where alpha is not the designation for the brightest star (epsilon is actually the brightest). Some stars have proper names as well, and where no such names were found HR designations have been used.
|υ||Upsilon ||6832||+4.61||1,672||B2 (A1)|
|ν1 ||Nu-1 ||Ain Al Rami||+4.83||1,850||K1|
|ψ||Psi ||7039||+4.85||330||K0 (A8)|
|ν2 ||Nu-2 ||7120||+4.9||270||K1|
|ξ2 ||Xi-2 ||Nergal||+5.08||372||G8|
|ξ1 ||Xi-1 ||7150||+5.1||2,350||K0|
|χ||Chi ||7362||+5.43; 5.03||220||A5|
|κ1 ||Kappa-1 ||7779||+5.59||244||A0|
|κ2 ||Kappa-2 ||7787||+5.64||371||A5|
|λ||Lambda ||Kaus Borealis||+5.64||77||K1|
The Pistol Star
Until the advent of orbiting telescopes this star was unknown. It has been calculated that if the space between that solar system and ours was clear it would be a fourth magnitude star and appear in the chart above. However because of the intervening matter it is only visible in infra-red or X-ray.
Located in our galaxy's central bulge, originally this star was one of the most massive in existence (200 times the size of our sun). It is still a giant and in the course of its life has thrown out an emission nebula over four light years in diameter.
Nebulae, Star Clusters and Galaxies
The NGC catalogue was compiled by John Louis Emil Dreyer (Director of the Armagh Observatory, 1882 - 1916). The M numbers are from The Messier Catalogue, by Charles Messier. These Messier Objects were compiled during the 1700s and predate the NGC Catalogue.
|M8||Lagoon nebula ||N||+6.0||5200|
|M17||Omega nebula ||E||+7.0||greater than 5,000|
|M18|| Star cluster||O||+7.5||4,000|
|M20||Trifid nebula ||E/R||+9.0||less than 9,000|
|M22|| Star cluster||G||+5.1||10,400|
|M24 ||Star cloud||--||--||10,000 to 16,000|
|M54 || Star cluster||G||+7.6||87,000|
|M55 || Star cluster||G||+6.3||17,300|
|M69 || Star cluster||G||+7.9||29,700|
|M70 || Star cluster||G||+7.6||29,300|
|M75|| Star cluster||G ||+8.5||67,500|
Legend: N=Nebulosa4, E=Emission, R=Reflection, P=Planetary, G=Globular, O=Open, ElG=Elliptical galaxy, IrG=Irregular galaxy
A Stellar Mystery
The stars in M24 are not gravitationally bound together - they just happen to lie in the same direction. Astronomers sometimes bring up the idea that the sky should be lit up everywhere, but it is not. So why is the sky dark, if there are such a large number of stars out there? Some say that interstellar dust is the reason, but their arguments are hardly convincing. Others blame dark matter, that unknown substance that makes up a sizable percentage of the mass of the Universe. Whichever answer is correct, it is visibly apparent that M24 has less of it than most places and so we see many stars ranging from 10,000 - 16,000 light years away, clearly which would otherwise be somewhat dimmed.
Over 2,000 years ago the Greek philosophers Aristotle and Epicurus argued about whether there were other Earth-like planets orbiting distant suns. Today scientists are coming up with ways to actually detect such planets, and the techniques are being so fine-tuned that planets throughout our galaxy can be discovered and studied.
There have been many extrasolar planets found in the constellation Sagittarius, two of which orbit the star HD 169830. The size of the extrasolar planets use as reference the mass of Jupiter, our Solar System's largest planet. This is known by astronomers as the Jovian scale.
Extrasolar Planets Table
|Star name or
|year discovered||Planet mass
|HD 169830||HD 169830 b||2000||2.88||0.81||225.6|
|HD 169830||HD 169830 c||2003||4.04||3.6||2,102|
|HD 179949||HD 179949 b||2000||0.95||0.045||3.092|
|HD 190647||HD 190647 b||2007||1.9||2.07||1,038|
|OGLE TR-10||OGLE TR-10 b||2004||0.63||0.41||3.1|
|HD 187085||HD 187085 b||2006||0.75||2.05||986|
|HD 171238||HD 171238 b||2009||2.6||2.54||1,523|
|HD 181720||HD 181720 b||2009||0.37||N/A||956|
|HD 164604||HD 164604 b||2010||2.7||1.13||606.4|
|HD 180902||HD 180902 b||2010||1.6||1.39||479|
|HD 181342||HD 181342 b||2010||3.3||1.78||663|