Category: Astronomy

[/caption]

The zodiacal constellation of Sagittarius resides on the ecliptic plane and was one of the original 48 constellations charted by Ptolemy to be later adopted as a modern constellation by the IAU. It spans 867 square degrees of sky and ranks 15th in constellation size. It has 7 primary stars in its main asterism and 68 Bayer Flamsteed designation stars within its confines. Sagittarius is bordered by the constellations of Aquila, Scutum, Serpens Cauda, Ophiuchus, Scorpius. Corona Australis, Telescopium, Indus, Microscopium and Capricornus. It is visible to all observers located at latitudes between +55° and ?90° and is best seen at culmination during the month of August.

The easily recogniged “tea pot” shape of Sagittarius was well known in mythology as being represented by the half-man, half-horse – the Centaur. According to some legends, he was the offspring of of Philyra and Saturn. Named Chiron, he turned himself into a horse to hide from his jealous wife and was eventually immortalized in the stars. He is often depicted as an archer as well, with his arrow pointed directly at the red heart of the Scorpion – Antares. Sagittarius may represent the son of Pan, who invented archery and was sent to entertain the Muses who threw a laurel wreath at his feet. No matter what identity you choose, one thing is for certain – there’s no mistaking the presence of the nearby Sagittarius arm of the Milky Way!

(Since the constellation of Sagittarius is simply slopping over with deep sky objects, creating a small, workable chart here would be very confusing. For this reason, I have only chosen a few of my favorite objects to highlight and I hope you enjoy them, too!)

Let’s begin our binocular tour of Sagittarius with its alpha star – the “a” symbol on our map. Located far south in the constellation, Alpha Sagittarii is far from being the brightest of its stars and goes by the traditional name of Rukbat – the “knee of the Archer”. It’s nothing special. Just a typical blue, class AB dwarf star located about 170 light years from Earth, but it often gets ignored because of its position. Have a look at Beta while you’re there, too. It’s the “B” symbol on our map. That’s right! It’s a visual double star and its name is Arkab – the “hamstring”. Now, power up in a telescope. Arkab Prior is the westernmost and it truly is a binary star accompanied by a 7th magnitude dwarf star and seperated by about 28 arcseconds. It’s located about 378 light years from Earth. Now, hop east for Arkab Posterior. It is a spectral type F2 giant star, but much closer at 137 light years in distance.

Now turn your attention towards Epsilon Sagittarii – the backwards “3” symbol on our chart. Kaus Australis is actually the brightest star in the bottom righthand corner of the teapot and the brightest of all the stars in Sagittarius and the 36th brightest in the night sky. Hanging out in space some 134 light years from our solar system, this A-class giant star is much hotter than most of its main sequence peers and spinning over 70 times faster on its axis than our Sun. This rapid movement has caused a shell to form around the star, dimming its brightness… But not nearly as dim as its 14th magnitude companion! That’s right… Epsilon is a binary star. The disparate companion is well seperated at 32 arc seconds, but will require a larger telescope to pick away from its bright companion!

Ready for more? Then have a look at Gamma – the “Y” symbol on our map. Alnasl, the “arrowhead” is two star systems that share the same name. If you have sharp eyes, you can even split this visual double star without aid! However, take a look in the telescope… Gamma-1 Sagittarii is a Cepheid 1500 light year distant variable star in disguise. It drops by almost a full stellar magnitude in just a little under 8 days! Got a big telescope? Then take a closer look, because Gamma-1 also shows evidence of being a close binary star, as well has having two more distant 13th magnitude companions, W Sagittarii B, and C separated by 33 and 48 arcseconds respectively. How about Gamma-2? It’s just a regular type-K giant star – but it’s only 96 light years from Earth!

Located just slightly more than a fingerwidth above Gamma Sagittarii and 5500 light-years away, NGC 6520 (RA 18 03 24 Dec -27 53 00) is a galactic star cluster which formed millions of years ago. Its blue stars are far younger than our own Sun, and may very well have formed from what you don’t see nearby – a dark, molecular cloud. Filled with dust, Barnard 86 literally blocks the starlight coming from our galaxy’s own halo area in the direction of the core. To get a good idea of just how much light is blocked by B 86, take a look at the star SAO 180161 on the edge. Behind this obscuration lies the densest part of our Milky Way! This one is so dark that it’s often referred to as the “Ink Spot.” While both NGC 6520 and B 86 are about the same distance away, they don’t reside in the hub of our galaxy, but in the Sagittarius Spiral Arm. Seen in binoculars as a small area of compression, and delightfully resolved in a telescope, you’ll find this cluster is on the Herschel “400” list and many others as well.

Are you ready for a whirlwind tour of the Messier Catalog objects with binoculars or a small telescope? Then let’s start at the top with the “Nike Swoosh” of M17.
Easily viewed in binoculars of any size and outstanding in every telescope, the 5000 light-year distant Omega Nebula was discovered by Philippe Loys de Chéseaux in 1745-46 and later (1764) cataloged by Messier as object 17 (RA 18 20 26 Dec -16 10 36). This beautiful emission nebula is the product of hot gases excited by the radiation of newly born stars. As part of a vast region of interstellar matter, many of its embedded stars don’t show up in photographs, but reveal themselves beautifully to the eye at the telescope. As you look at its unique shape, you realize many of these areas are obscured by dark dust, and this same dust is often illuminated by the stars themselves. Often known as “The Swan,” M17 will appear as a huge, glowing check mark or ghostly “2” in the sky – but power up if you use a larger telescope and look for a long, bright streak across its northern edge with extensions to both the east and north. While the illuminating stars are truly hidden, you will see many glittering points in the structure itself and at least 35 of them are true members of this region, which spans up to 40 light-years and could form up to 800 solar masses. It is awesome…

Keeping moving south and you will see a very small collection of stars known as M18, and a bit more south will bring up a huge cloud of stars called M24. This patch of Milky Way “stuff” will show a wonderful open cluster – NGC 6603 – to average telescopes and some great Barnard darks to larger ones. M24 is often referred to as the “Small Sagittarius Star Cloud”. This vast region is easily seen unaided from a dark sky site and is a stellar profusion in binoculars. Telescopes will find an enclosed galactic cluster – NGC 6603 – on its northern border. For those of you who prefer a challenge, look for Barnard Dark Nebula, B92, just above the central portion.

Now we’re going to shift to the southeast just a touch and pick up the M25 open cluster. M25 is a scattered galactic cluster that contains a cephid variable – U Sagittarii. This one is a quick change artist, going from magnitude 6.3 to 7.1 in less than seven days. Keep an eye on it over the next few weeks by comparing it to the other cluster members. Variable stars are fun! Head due west about a fist’s width to capture the next open cluster – M23. From there, we are dropping south again and M21 will be your reward. Head back for your scope and remember your area, because the M20 “Triffid Nebula” is just a shade to the southwest. Small scopes will pick up on the little glowing ball, but anything from about 4″ up can see those dark dust lanes that make this nebula so special. The “Trifid” nebula appears initially as two widely spaced stars – one of which is a low power double – each caught in its own faint lobe of nebulosity. Keen eyed observers will find that the double star – HN 40 – is actually a superb triple star system of striking colors! The 7.6 magnitude primary appears blue. Southwest is a reddish 10.7 magnitude secondary while a third companion of magnitude 8.7 is northwest of the primary.

Described as “trifid” by William Herschel in 1784, this tri-lobed pattern of faint luminosity broken by a dark nebula – Barnard 85 – is associated with the southern triple. This region is more brightly illuminated due to the presence of the star cluster and is suffused with a brighter, redder reflection nebula of hydrogen gas. The northern part of the Trifid (surrounding the solitary star) is fainter and bluer. It shines by excitation and is composed primarily of doubly ionized oxygen gas. The entire area lies roughly 5000 light-years away. What makes M20 the “Trifid” nebula, are the series of dark, dissecting dust lanes meeting at the nebula’s east and west edges, while the southernmost dust lane ends in the brightest portion of the nebula. With much larger scopes, M20 shows differences in concentration in each of the lobes along with other embedded stars. It requires a dark night, but the Trifid is worth the hunt. On excellent nights of seeing, larger scopes will show the Trifid much as it appears in black and white photographs!

You can go back to the binoculars again, because the M8 “Lagoon Nebula” is south again and very easy to see. Easily located about three finger-widths above the tip of the teapot’s spout (Al Nasl), M8 is one of Sagittarius’ premier objects. This combination of emission/reflection and dark nebula only gets better as you add an open cluster. Spanning a half a degree of sky, this study is loaded with features. One of the most prominent is a curving dark channel dividing the area nearly in half. On its leading (western) side you will note two bright stars. The southernmost of this pair (9 Sagittarii) is thought to be the illuminating source of the nebula. On the trailing (eastern) side, is brightly scattered cluster NGC 6530 containing 18 erratically changing variables known as “flare stars.” For large scopes, and those with filters, look for small patches of dark nebulae called “globules.” These are thought to be “protostar” regions – areas where new stars undergo rapid formation. Return again to 9 Sagittarii and look carefully at a concentrated portion of the nebula west-southwest. This is known as the “Hourglass” and is a source of strong radio emission.

This particular star hop is very fun. If you have children who would like to see some of these riches, point out the primary stars and show them how it looks like a dot-to-dot “tea kettle.” From the kettle’s “spout” pours the “steam” of the Milky Way. If you start there, all you will need to do is follow the “steam” trail up the sky and you can see the majority of these with ease.

At the top of the “tea kettle” is Lambda. This is our marker for two easy binocular objects. The small M28 globular cluster is quite easily found just a breath to the north/northwest. The larger, brighter and quite wonderful globular cluster M22 is also very easily found to Lambda’s northeast. Ranking third amidst the 151 known globular clusters in total light, M22 is probably the nearest of these incredible systems to our Earth, with an approximate distance of 9,600 light-years. It is also one of the nearest globulars to the galactic plane. Since it resides less than a degree from the ecliptic, it often shares the same eyepiece field with a planet. At magnitude 6, the class VII M22 will begin to show individual stars to even modest instruments and will burst into stunning resolution for larger aperture. About a degree west-northwest, mid-sized telescopes and larger binoculars will capture the smaller 8th magnitude NGC 6642 (RA 18 31 54 Dec -23 28 34). At class V, this particular globular will show more concentration toward the core region than M22. Enjoy them both!

Now we’re roaming into “binocular possible” but better with the telescope objects. The southeastern corner of the “tea kettle” is Zeta, and we’re going to hop across the bottom to the west. Starting at Zeta, slide southwest to capture globular cluster M54. Keep heading another three degrees southwest and you will see the fuzzy ball of M70. Just around two degrees more to the west is another globular that looks like M70’s twin. The small globular M55 is out there in “No Man’s Land” about a fist’s width away east/south east of Zeta .

Ready for a big telescope challenge? Then try your hand at one the sky’s most curious galaxies – NGC 6822. This study is a telescopic challenge even for skilled observers. Set your sights roughly 2 degrees northeast of easy double 54 Sagittarii, and have a look at this distant dwarf galaxy bound to our own Milky Way by invisible gravitational attraction…

Named after its discoverer (E. E. Barnard – 1884), “Barnard’s Galaxy” is a not-so-nearby member of our local galaxy group. Discovered with a 6″ refractor, this 1.7 million light-year distant galaxy is not easily found, but can be seen with very dark sky conditions and at the lowest possible power. Due to large apparent size, and overall faintness (magnitude 9), low power is essential in larger telescopes to give a better sense of the galaxy’s frontier. Observers using large scopes will see faint regions of glowing gas (HII regions) and unresolved concentrations of bright stars. To distinguish them, try a nebula filter to enhance the HII and downplay the star fields. Barnard’s Galaxy appears like a very faint open cluster overlaid with a sheen of nebulosity, but the practiced eye using the above technique will clearly see that the “shine” behind the stars is extragalactic in nature.

Now look less than a degree north-northwest to turn up pale blue-green NGC 6818 – the “Little Gem” planetary. Easily found in any size scope, this bright and condensed nebula reveals its annular nature in larger scopes but hints at it in scopes as small as 6″. Use a super wide field long-focus eyepiece to frame them both!

Be sure to get a good star chart and enjoy the constellation of Sagittarius to its fullest potential – there’s lots more out there!

Sources:
SEDS
Chandra Observatory
Wikipedia
Chart Courtesy of Your Sky.

[/caption]

Sagitta belongs to the original 48 constellations charted by Ptolemy and it remains one of the 88 modern constellations defined by the International Astronomical Union. Located north of the ecliptic plane, it spans 80 square degrees of sky, ranking 86th in constellation size. Sagitta contains 4 main stars in its asterism and has 19 Bayer Flamsteed designated stars within its confines. It is bordered by the constellations of Vulpecula, Hercules, Aquila and Delphinus. Sagitta is visible to all observers located at latitudes between +90° and ?70° and is best seen at culmination during the month of August.

In ancient history, the constellation of Sagitta was once known as Sham – a name applied today to its Alpha star. It was the Romans who named it Sagitta, the “Arrow”. In ancient Greek myth, it depicted the weapon which Hercules used to kill the eagle Aquila – or perhaps the Stymphalian birds. Perhaps it is Cupid’s Arrow, or the one which the Centaur shot at Chiron… No matter which tale you choose, it’s unmistakeable arrow shape was clear to all cultures, including the Persians, Hebrews, Greeks and Romans. Only Johann Bayer seemed to have trouble with it… For it is one of those constellation in which he named the bright stars out of order!

Let’s begin our binocular tour of Sagitta with Alpha – the “a” symbol on our chart. While Sham isn’t the brightest star in the constellation, this yellow bright giant star’s name really does mean “arrow”. Located about about 475 light years from Earth, it has a stellar luminosity 340 times that of the Sun and is about 20 times larger. Sham falls inside the “Hertzsprung Gap,” a perimeter of stellar temperature and luminosity that few stars fit inside. From its point in stellar evolution, it should have stopped fusing hydrogen to helium and began to brighten. However, that’s not the case. For some reason, Sham’s surface shows an abundance of nitrogen – a state which could only occur from interior helium fusion. A Cepheid variable star in the making? Perhaps!

Take a look at Beta in binoculars – the “B” symbol on our map. It’s G-type yellow star like our own Sun. Beta Sagittae is a giant star and, like Sham, is only about 467 light years away from our solar system. Delta, in the center of the arrow, is a spectroscopic binary star. It consists of a class M giant star and a quiet little hydrogen fusing dwarf star. Both are happy at a distance of about 448 light years from here and both happily separated from each other by a little less than 9 AU. Don’t forget red giant star, Gamma, either! The “Y” star on our chart might be 275 light years away, but it shines 640 times brighter than our Sun! It, too, is highly evolved…. Surrounded by a shell and well on its way to becoming a Mira-type variable star and eventually a white dwarf star about the size of the Earth.

For large binoculars and small telescopes, set your sights towards Messier 71 (RA 19 : 53.8 Dec +18 : 47). At around 8th magnitude, this loosely structured globular cluster is a challenge for smaller optics, but a wonderful study. It was originally discovered by Philippe Loys de Chéseaux in 1746 and included by Charles Messier in his Messier catalog of comet-like objects in 1780. Residing about 12,000 light years away and spanning about 27 light years across, there has long been a debate about this star cluster’s proper designation… globular cluster or concentrated galactic star cluster? Thanks to modern photometry, astronomers have detected a short “horizontal branch” in the H-R diagram of M71, which is characteristic of a globular cluster. Its low metallicity content has now been recognized as that of a “youthful” globular cluster and its lack of RR Lyrae variables places it at an age of between 9 and 10 billion years old.

For two challenging large telescope studies, let’s try your hand with planetary nebulae. The first is NGC 6879 (RA 20 : 10.5 Dec +16 : 55). At an apparent magnitude of 13, this challenging study will require high magnification and careful alignment to pick out from the stellar field. However, don’t be discouraged, because the nebula itself is rather bright and conspicuous as a “hairy star”. Just as challenging is NGC 6886 (RA 20 : 12.7 Dec +19 : 59). While the central star is a magnitude brighter at 12, you’re going to need at least an 8″ telescope to detect this one. It has an unusual chemical composition which an OIII filter helps to reveal.

Sources:
Wikipedia
Chandra Observatory
SEDS
Chart Courtesy of Your Sky.

[/caption]

Located south of the ecliptic plane, the small, faint constellation of Reticulum was first named Rhombus by astronomical clock creator – Isaac Habrecht. It was later renamed Reticulum by Nicolas Louis de Lacaille in 1763. It spans 114 square degrees of sky, ranking 82nd in constellation size, has 4 main stars in its primary asterism and contains 11 Bayer Flamsteed designated stars within its confines. Reticulum is bordered by the constellations of Horologium, Dorado and Hydrus. It is visible to all observers located at latitudes between +23° and ?90° and is best seen at culmination during the month of January.

Since Reticulum is a “new” constellation, there is no mythology associated with it – only the story of how its name came to be. Originally named Rhombus by Isaac Habrecht, it was a name the fit the star pattern, considering a rhombus is a basic diamond pattern. Habrecht and his brother were talented Germany clock makers and one of their specialities was in fashioning astronomical clocks. As a matter of fact, they built the second astronomical clock in Strasbourg between 1571 and 1574. It was designed by mathematician Christian Herlin, and as well as the Habrecht brothers, had astronomer and musician David Wolckenstein to assist. This fantasy clock had a staircase, huge amounts of artwork, musical embellishments, but was best known for its complexity as an astronomical device. It had a calendar dial, the astrolabe, the indicators for planets and eclipses… and a celestial globe. When Lacaille made his sojourn to the Cape of Good Hope, his intent wasn’t to usurp Habrecht’s place in astronomical history – but to unify astronomical catalogs. In an attempt to honor instruments of science and his telescope for which he used to chart the southern skies, Lacaille named this trapezoidal collection of stars Reticulum, the Latin derivative for the reticule crosshairs on his spyglass which enabled him to accurately pinpoint star positions. The name Reticulum stuck and was later adopted as one of the 88 modern constellations by the International Astronomical Union.

Let’s begin our binocular tour of Reticulum with its brightest star – Alpha – the “a” symbol on our chart. Alpha Reticuli is a yellow G class giant star which is about 163 light years away from Earth. It shines about 237 times brighter than our Sun and is about 21 times larger. It will eventually end its life quietly as a white dwarf star. But, take out your telescope and have a closer look! You’ll find out that Alpha is also a binary star with a very disparate 12th magnitude companion star nearby. While the star hasn’t moved in the last 150 years, the pair does display the same proper motion.

Keep binoculars handy and hop west for Zeti Reticuli. This binary star system located about 39 light years away from our own solar system. The pair of twin suns are very much like our own in temperature and mass – yellow dwarf stars – but it’s there the similarities end. At one time, astronomers believed the Zeta pairing to be old galactic halo Population II subdwarf stars, but recent research indicates the may belong to the younger galactic disk population. This makes the twin Zetas far older than our Sun – in the neighborhood of 8 billion years old. And they aren’t moving along alone! The pair belongs to the Zeta Herculis Moving Star Group. Both stars share similar proper motions and distances – and despite being so widely spaced, they are a true binary star with an orbital period of an estimated million years!

While viewing Zeta, keep in mind all the legends behind this particular pair. In 1961, alien abductees – Barney and Betty Hill – were “taken” by citizens who imparted information to Betty that their home star was the Zeta system. After a map was constructed by an amateur astronomer and eventually debunked by Carl Sagan, then later sensationalized by Bob Lazar, the Zeta “planet” theory eventually went into hibernation for fear of media attention. On September 20, 1996 a tentative discovery of a “hot Jupiter” in the Zeta system was discovered and quickly retracted as being “pulsations” from the star… and while conditions are possible for Earth-like planets to exists around these twins suns, low solar metallicity makes their presence unlikely.

Before you give up planetary hopes, hop to Epsilon Reticuli – the backwards “3” symbol on our map. Now here’s a binary system located approximately 59 light-years away that really does have a confirmed planet! The primary star is an orange subgiant star, while the secondary star is a white dwarf star. As of 2000, an extrasolar planet has been confirmed to be orbiting the primary star in the system! It is roughly the size of Jupiter and it orbits around the star every 418 days. What’s more, there could possibly be an Earth-like trojan accompanying it!

For binoculars, keep a watch on R Reticuli – a Mira-type variable star. While it takes 278.3 days for it to go through it’s changes, they are very dramatic. You’ll find this incredible star begins by shining at respectable magnitude of 6.5 only to virtually turn telescopic at magnitude 14 during its minima. Now that’s variable!

For a small telescope and big binocular challenge, try your hand at NGC 1313 (RA 3 : 18.3 Dec -66 : 30). At magnitude 9 and more than 8 arc minutes in size, this starburst galaxy is often referred to as the “Topsy Turvy” because of its unusual supershell spiral galaxy structure. Located about 15 million light years away, large telescopes will pick out strange features, like spiral arms which are lopsided and its rotational axis is not at the center of the nuclear bar.

Now, have a go at NGC 1559 (RA 4 : 17.6 Dec -62 : 47). Although this barred spiral galaxy is over a magnitude fainter and about half the size of the last, you’ll still find it quite bright and rich in the telescope. Although it was originally thought to be a member of the Dorado Galaxy Group, this Seyfert Galaxy is all alone in space. NGC 1559 has massive spiral arms and strong star formation. It contains a small bar which is oriented nearly east-west and spans 40?. Its bar and galactic disc are the source of very strong radio emissions!

Our last galaxy is NGC 1543 (RA 4 : 12.8 Dec -57 : 44). Also about magnitude 10.5 and about 4 arc minutes in size, Dunlop 100 really is considered part of the Dorado Galaxy Group. Look for a very bright nucleus in this spiral galaxy, with a faint east to west extension!

Chart Courtesy of Your Sky.