Universe Today

If you’ve got a clear view of the skies, and happen ti live in the southern hemisphere, there’s a relatively obscure constellation you should probably check out. It’s known as Horologium, a region of the sky that is named after an important historic personality, one who is largely responsible for how we measure time.

The constellation of Horologium was one of 14 created by Nicolas Louis de Lacaille to chart southern hemisphere skies. Originally named “Horologium Oscillitorium” to honor Christiaan Huygens – the inventor of the pendulum clock – it was later shortened to its present named when adopted as one of the 88 modern constellations by the IAU.

Horologium spans 249 square degrees of sky and consists of 6 mains stars in the asterism, with 10 Bayer/Flamsteed designated stars. It is bordered by the constellations of Eridanus, Hydrus, Reticulum, Dorado and Caelum. Horologium is visible to all observers at latitudes between +30° and ?90° and is best seen at culmination during the month of December.

Horologium was named to honor Christiaan Huygens, the Dutch mathematician, astronomer and physicist. While traveling in the southern hemisphere and charting the heavens, Nicholas de Lacaille (who loved all things science) found this dim constellation reminded him of Huygen’s newly invented pendulum clock.

Huygens clock incorporated the first harmonic oscillator – increasing the accuracy to within 15 seconds per day. His “horological innovation” so impressed Lacaille that he found the pattern for this invention in the stars.

Horologium is bordered by five different constellations: Eridanus (the Po River), Caelum (the chisel), Reticulum (the reticle), Dorado (the dolphinfish/swordfish), and Hydrus (the male water snake).

Spring driven pendulum clock, designed by Christiaan Huygens (1657) and copy of the Horologium Oscillatorium,  Museum Boerhaave, Leiden. Credit: Flickr/Rob Koopman

The official constellation boundaries are defined by a twenty-two sided polygon. Covering a total of 249 square degrees, Horologium ranks 58th in area out of the 88 modern constellations.

With almost no bright stars to claim, stargazing at Horologium can be a bit tricky. But with binoculars, a telescope, and a chart, there are plenty of opportunities for some picturesque views. Let’s start by taking a look in binoculars with Alpha Horologii – the “a” symbol on our map.

Located about 193 light years from Earth, this very normal K1 orange giant star – quietly fusing its core helium into carbon and oxygen. Nearby is Delta, the “8” symbol. It, too is rather ordinary. Delta is a spectroscopic binary star, located about 175 light years away.

So, with very little in the constellation in the way of stars, what is there to do with a telescope? First of all, there’s NGC 1261 (RA: 03:12:15.3; Dec: -55:13:01). This 8th magnitude globular cluster is very well condensed and is at home in a very picturesque field. Small wonder it made the Caldwell list at number 87. Look for a very bright core region and well resolved chains of stars at the edges of this pretty star cluster.

For larger telescopes, try NGC 1512 (RA 4:03.9 Dec -43:21). At slightly brighter than magnitude 11, this barred spiral galaxy belongs to the Dorado group and is located about 30 million light years away. While you won’t find much details here, NASA’s Galaxy Evolution Explorer show spiral galaxy NGC 1512 sitting slightly northwest of elliptical galaxy NGC 1510.

The two galaxies are currently separated by a mere 68,000 light-years, leading many astronomers to suspect that a close encounter is currently in progress. The overlapping of two tightly wound spiral arm segments makes up the light blue inner ring of NGC 1512. Meanwhile, the galaxy’s outer spiral arm is being distorted by strong gravitational interactions with NGC 1510.

Another challenge? Then try NGC 1433 (RA 3:42.0 Dec -47:13). This magnitude 10 galaxy is an example of a ringed barred spiral. While physically you may only notice a bright nucleus and the soft bar, the stars orbiting the disk of this galaxy shows its internal motions photographically. A small elliptical ring can develop near the nucleus – blue proof of star formation. Always keep a watch, because this galaxy had a supernova event in 1985.

Source: Wikipedia
Chart Courtesy of Your Sky.

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The constellation of Hercules belongs to one of the 48 originals plotted by Ptolemy and has survived time to become one of the 88 modern constellations adopted by the International Astronomical Union. Spanning an impressing 1225 square degrees of sky and containing 22 stars in the asterism, it has 106 Bayer/Flamsteed designated stellar designations. Hercules is bordered by the constellations of Draco, Bootes, Corona Borealis, Serpens Caput, Ophiuchus, Aquila, Sagitta, Vulpecula and Lyra. It is visible to all observers at latitudes between +90° and ?50° and is best seen at culmination during the month of July. There is one annual meteor shower associated with Hercules, the Tau Herculids, which peak on or near June 3. The radiant, or point of origin, is near the Hercules/Corona Borealis border and the meteor shower itself last about a month beginning around two weeks before and lasting about two weeks after the peak date. Most of these meteors are quite faint and at maximum, expect to see no more than 15 per hour average.

The mythology surrounding Hercules is a long and very colorful one. He was considered the greatest of all heroes – both Greek and Roman. The legendary strong man was supposed to be the son of Zeus; immortal, yet forever challenged by Hera by his circumstance of birth. His tasks were many: killing a lion with a hide that could not be punctured, destroying the many headed Hydra, cleaning out nasty stables, fighting birds with knife-like feathers, capturing a bull that breathed fire, taming horses that ate flesh, stealing cattle from monsters, stealing golden apples, fighting dragons, snatching a three-headed dog, loosing the love of his life, accidentally killing his teacher and so much more… It is no wonder that Hercules is so often depicted as kneeling in the sky! Even an immortal would be tired from so much… But at last, Hercules earned his place in the stars and he remains there to this day… The fifth largest constellation in the night sky.

Because the constellation of Hercules has no particularly bright stars, it is sometimes difficult to navigate through with binoculars until you learn a few “key” ingredients. There is a large asterism which is fairly easy to recognize that forms a lopsided box, referred to as the “keystone”. The northeast corner is Pi. The northwest corner is Eta. The southeast corner is Epsilon. The southwest corner is Zeta. Always remember when you look at a star chart that north and south are up and down… But east is to the left and west is to the right! To find the “keystone”, let bright Vega guide you…. just start by looking southwest.

Have you found Pi Herculis, yet? If you’re seeing two stars in your binoculars and you’re not sure which one, Pi is the slightly redder and slightly brighter of the pair. Situated about 370 light years from Earth, Pi Herculis is a cool, red supergiant star that was born about 140 million years ago. Although you can’t see it, Pi also has an orbiting substellar companion about 27 times larger than Jupiter there, too! Now, drop south for Epsilon – another binary star. Chances are good this pair of twin stars are almost identical to each other – about twice the size and mass of our Sun – and orbit each other so closely they nearly touch.

Don’t stop moving south. Our next stop is Gamma Herculis, the “8” shape on our map. Gamma is also a very cool star – one with a dead helium core that’s waiting to become a red giant. In maybe 8 million or so years, it will begin to fuse helium into carbon and become much brighter than it is tonight. If you see a faint companion star, it is only an optical one in binoculars – but Gamma is also a genuine binary star.

Next stop? Further south for Alpha – the “a” shape on our map. Now here is a great star! Named Rasalgethi and located about 380 light years away, here we have one of the finest double stars in the night sky. The primary star is a magnificent red class M supergiant that’s over 475 more luminous than our Sun and whose size would fill up our solar system clear out to the orbit of the asteroid belt. But that’s not all… Aim a telescope at Rasalgethi and you’ll see it has a fifth magnitude companion five seconds of arc away. It is also a binary star – an F2 giant with a close orbiting dwarf star companion. Surrounding this whole system is an envelope of gas expelled from the primary star’s incredible solar winds… Enjoy the unusual red and green hues of this colorful double star! And keep watching… Because Rasalgethi is also an irregular variable star – whose brightness changes from magnitude 2.7 to 4.0 within a period of about three months.

Next up? Return to the “keystone” and the northwest corner for Eta – the “n” shape on our map. Shining away about 50 times brighter than our own Sun at a distance of 112 light years, there is nothing particularly impressive about Eta, except where it leads. Begin moving your optics slowly south towards Zeta and you will encounter the “Great Hercules Cluster” – M13! Easily seen in binoculars, sometimes visible to the unaided eye in a dark sky location and absolutely magnificent in any telescope, Messier 13 is perhaps the most famous of all northern globular clusters. Located about 25,000 light years away and home to more than half a million stars, this 12 billion year old system spans no more than 100 light years across. Also known as NGC 6205, this impressive ball of stars was first discovered by Edmund Halley in 1714 and catalogued by Charles Messier on June 1, 1764. If you aren’t impressed, then take the words of Kurt Vonnegut to heart: “”Every passing hour brings the Solar System forty-three thousand miles closer to Globular Cluster M13 in Hercules — and still there are some misfits who insist that there is no such thing as progress.”

Ready for more? Then take another look at Eta and Pi and form an imaginary triangle on the sky using these two stars as the base. The apex is very near where you will find another amazing globular cluster for binoculars or small telescopes – Messier 92. First discovered by Johann Elert Bode in 1777 and independently rediscovered by Charles Messier on March 18, 1781, M92 is a 16 billion year old beauty – formed back at the Milky Way Galaxy’s beginnings. Hiding in there are 16 variable stars and one rare eclipsing binary. What a treat to have two such bright objects so near to one another!

Ready for an alternative binocular tour of Hercules? Then let’s use what you’ve learned. Start by locating magnificent M13 and move 3 degrees northwest – about a binocular field. What you will find is a splendid loose open cluster of stars known as Dolidze/Dzimselejsvili (DoDz) 5 – and it looks much like a miniature of the constellation Hercules. Just slightly more than 4 degrees to its east and just about a degree south of Eta Herculis is DoDz 6, which contains a perfect diamond pattern and an asterism of brighter stars resembling the constellation of Sagitta. Now we’re going to move across the constellation of Hercules towards Lyra. East of the “keystone” is a tight configuration of three stars – Omicron, Nu, and Xi. About the same distance separating these stars northeast you will find DoDz 9. You’ll see a pretty open cluster of around two dozen mixed magnitude stars. Now look again at the “keystone” and identify Lambda and Delta to the south. About midway between them and slightly southeast you will discover the stellar field of DoDz 8. This last is easy – all you need to do is return to Alpha. Move about 1 degree northwest (Rasalgethi will stay in the field) to discover the star-studded open cluster DoDz 7. These great open clusters are very much off the beaten path and will add a new dimension to binocular and fast-telescope observing!

Would you like a challenge? Then go back to M13 with a large telescope and take a look about 40 arc minutes to the northeast for NGC 6207 (RA 16:43.1 Dec +36:50). At near magnitude 12, this small spiral galaxy isn’t for everyone, but it’s always a smile a bonus when you’re in the area, despite the lack of details. Try NGC 6210 (RA 16:44.5 Dec +23:49), too. This bright planetary nebula is suited for all telescopes and takes magnification very well. Look for a blue/green color in larger telescopes, and adding a nebula filter can sometimes reveal some subtle details of a shell around this one. But be sure to take the filter out if you want to catch the central star!

Sources: Chandra Observatory, SEDS
Chart Courtesy of My Sky.