[/caption]Greetings, fellow SkyWatchers! Are you ready for today’s eclipse? Be sure to follow Ian’s earlier instructions this week and catch the action for yourself! When the Sun is gone at last, then let’s continue through the New Moon weekend with our globular cluster studies and we’ll take a look at some of the summer’s finest for both binoculars and telescopes. If you’re not afraid of the dark, then follow me…
Friday, August 1, 2008 – Mark your calendar! A total solar eclipse occurs today in northern Canada, the Arctic and Asia. Totality will begin at 09:21:07 UT in Canada, with the path crossing Greenland, the Arctic Ocean, Russia, and Mongolia – ending in China at 11:21:28 UT. Maximum occurs at 10:21:08 UT. For those not in the path, a partial eclipse will be visible over northeastern Canada, most of Asia and Europe, and the Middle East, between 08:04:07 UT and 12:38:28 UT. Be sure to consult with online sources such as Mr. Eclipse for accurate locations of the path of totality. And please…NEVER look at the Sun without taking proper precautions. Wishing you clear skies for this event!
(If you aren’t able to view today’s solar eclipse in person, be sure to follow the event by following Ian’s excellent instructions at: Friday’s Total Solar Eclipse Can Be Watched On The Internet.)
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Since tonight is also New Moon, let’s continue our exploration of summer’s globular clusters. These gravitationally bound concentrations of stars contain anywhere from ten thousand to one million members, and attain sizes of up to 200 light-years in diameter. At one time, these fantastic members of our galactic halo were believed to be round nebulae; perhaps the very first to be discovered was M22 in Sagittarius by Abraham Ihle in 1665. This particular globular is easily seen in even small binoculars and can be easily located just slightly more than two degrees northeast of the teapot’s lid, Lambda Sagittarii – Kaus Borealis (RA 18 36 24 Dec -23 54 12).
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!
Saturday, August 2, 2008 – If you’re out tonight at sunset, be sure to watch the horizon in hopes of catching a glimpse of the very beginning of the Moon’s return. Both Regulus and Venus are nearby!
Tonight, let’s return again to look at two globular giants so we might compare roughly equal sizes, but not equal classes. To judge them fairly, you must use the same eyepiece. Start first by re-locating previous study M4. This is a class IX globular cluster. Notice the powder-like qualities. It might be heavily populated, but it is not dense. Now return to another previous study, M13, which is of class V. Most telescopes will achieve at least some resolution and show a distinct core region. It is the level of condensation that creates the different classes. Judging a globular’s concentration is no different from judging magnitudes, and simply takes practice.
Now try your hand at M55 (RA 19 39 59 Dec -30 57 43) along the bottom of the Sagittarius teapot – it’s a class XI. Although it is a full magnitude brighter than the class I cluster M75, can you tell the difference in concentration? For those with GoTo systems, take a quick hop through Ophiuchus and look at the difference between NGC 6356 (class II) and NGC 6426 (class IX). If you want to try one that science can’t even classify? Look no further than M71 in Sagitta (RA 19 53 46 Dec +18 46 42). It’s all a wonderful game and the most fun comes from learning!
Sunday, August 3 – For SkyWatchers tonight, be sure to catch the tender crescent Moon pairing with lovely Saturn just after sunset! Now, let’s return to earlier evening skies as we continue our studies with one of the globulars nearest to the galactic center – M14. Located about 16 degrees (less than a handspan) south of Alpha Ophiuchi (RA 17 37 36 Dec -03 14 45), this 9th magnitude, class VIII cluster can be spotted with larger binoculars, but will only be fully appreciated with the telescope.
When studied spectroscopically, globular clusters are found to be much lower in heavy element abundance than stars such as our own Sun. These earlier generation stars (Population II) began their formation during the birth of our galaxy, making globular clusters the oldest formations an amateur can study. Globulars are distributed in a spherical halo around the galaxy center. In contrast, stars in the disk are mostly much younger, their populations having gone through cycles of starbirth and supernovae, which in turn have enriched the heavy element concentration in nearby star forming clouds. Of course, as you may have guessed, M14 breaks the rules! It contains an unusually high number of variable stars – in excess of 70 – with many of them known to be the W Virginis type. In 1938, a nova appeared in M14, but it went undiscovered until 1964 when Amelia Wehlau of the University of Ontario was surveying the photographic plates taken by Helen Sawyer Hogg. The nova was revealed on eight of these plates taken on consecutive nights and showed itself as a 16th magnitude star – and at its peak was believed to be almost five times brighter than other cluster members. So unlike 80 years earlier with T Scorpii in M80, actual photographic evidence of this event existed. In 1991, the eyes of the Hubble were turned its way, but neither the suspect star nor traces of a nebulous remnant were discovered. But six years later, a rare carbon star was discovered in M14.
To a small telescope, M14 will offer little to no resolution and will appear almost like an elliptical galaxy, lacking any central condensation. Larger scopes will show hints of resolution, with a gradual fading toward the cluster’s slightly oblate edges. A true beauty!
This week’s awesome images are: Total Eclipse – Credit: NASA (Fred Espenak), M22 – Credit: N.A.Sharp, REU program/NOAO/AURA/NSF, M71 – REU program/NOAO/AURA/NSF and M14 – NOAO/AURA/NSF.