Greetings, fellow SkyWatchers! As the week begins, keep an eye out for members of the Draconid meteor shower as we begin a in-depth look at globular cluster – M15. There will be plenty to explore, so get out your optics and head for the stars, because…
Here’s what’s up!
Monday, October 9 – Tonight is the peak of the Draconid meteor shower with its radiant near the westering constellation Hercules. This particular shower can be quite impressive when comet Giacobini-Zinner passes near Earth. During that time, the fall rate jumps to 200 per hour and has even reached 1000! Comet Giacobini-Zinner achieved perihelion on July 2, 2005. Because tonight’s fast rising Moon will greatly interfere with these faint meteors, we can still keep watch – but first let’s practice a little lunacy.
Through binoculars, look along the northeast shore of Mare Serenitatis for the bright ring of Posidonius. Now look at Mare Crisium and get a “feel” for its size. A little more than one Crisium’s length west of Posidonius you’ll meet Aristotle and Eudoxus. Drop a similar length south and you’ll find tiny, brilliant crater LinnÃ© exposed on the expanse of Mare Serenitatis. So what’s so cool about this little white dot? With only binoculars you are resolving a crater one mile wide, found within a seven mile wide bright patch of ejecta from a quarter million miles away!
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Tonight we again visit the M15 globular and learn more about the scale of the Universe – circa 1900. On a decent night, a modest telescope will resolve about a dozen 13th magnitude stars outside M15’s core region. Most of these stars are red giants with absolute magnitudes of -2. Such stars appear 15 magnitudes fainter than they would be if they were at an astronomically standardized distance. Based on this 15 magnitude loss in intensity, we should be able to figure out how far away M15 is, but this is circular reasoning. In the early 1900s, astronomers didn’t know that the brightest stars in M15 were absolute magnitude -2. They first needed to know how far away the globular was to make sense of that. Here’s where the H-R diagram helps out. The most massive and swollen red giants (those nearing the end of their lives such as Betelgeuse and Antares) can be as luminous as absolute magnitude -6, but you can’t assume that the brightest red giants in a globular cluster are as bright as Antares and Betelgeuse. Why? Because we later discovered that all stars in a globular cluster entered the main sequence about the same time – some 12 billion years ago. Meanwhile, the very brightest ones – the Denebs – are no longer around. They exited the main sequence, became red giants and exploded a long, time ago, and possibly in a dwarf galaxy far, far away!
Tuesday, October 10 – Today in 1846, William Lassell was busy at the eyepiece and made an extraordinary new discovery – Neptune’s brightest moon Triton. At magnitude 13.5, we might expect to see Triton using a scope of moderate aperture; however Triton gets no further than 17 arc-seconds from the planet’s 7.9 magnitude globe. To make a run at Triton, you can use techniques similar to locating Pluto. Through a 10″ or larger scope, locate Neptune – a little more than 1 degree northwest of Iota Capricorni. At high power, make a field sketch of Neptune and neighboring faint stars. Be sure to come back again and again. (You may also wish to visit the Association of Lunar and Planetary Observers – ALPO – on the Internet for more information on planetary and satellite locations throughout the solar system.)
Thanks to the late-rising Moon… M15 awaits!
With the advent of the H-R diagram, all kinds of stars could be plotted for color-temperature and luminosity. With a large enough sample, you could begin to make solid guesses about how bright those 13th magnitude red giants in M15 really were. All you needed was a large sample of stars known to be at the same distance. Of course, you can pretty much assume that all stars in a globular cluster are at the same distance – even if you don’t exactly know what that distance is. Something peculiar was noticed about all globular clusters investigated using this approach. Their red giants were more plentiful and much dimmer than you might expect. They weren’t nearly as massive as Antares or Betelgeuse – and it also implied they were very old stars of lower mass than the Denebs of the galaxy…
When revisiting M15 (or neighboring M2 a fist width due south) you will observe something incredibly old. As these clusters aged, many of their brightest and finest died out a very, very, long time ago. Today, as a result of that great age, stars only several times more massive than our Sun are swelling up to become red giants. In the case of M15, something equally remarkable is occurring – the cluster’s core is collapsing in on itself. The cause is not thousands of huge perfectly-formed monoliths, but perhaps those high-mass red giants now taking the form of powerful black holes…
Once the Moon climbs up from the east, and the lunar terminator has not advanced too far at your location, have a look at the southeast shoreline of Mare Crisium for Agarum Promontorium. Look at how boldly it progresses northward across the dark plain before it disappears beneath the once molten lava. There were times in the past when great lunar observers noted a mist-like appearance in this area – a transient phenomenon observers should make note of and report whenever seen.
Wednesday, October 11 – With early dark tonight we have more time to spend contemplating the mystery of globular clusters.
Class IV M15 lies within a large flat triangle of three 7th magnitude stars. On a good night, two or three dozen outlying members can be resolved through a modest telescope. It also reveals a very compact and brilliant, blue core – a core far more concentrated in appearance than that of neighboring Class II M2 to the south, which may very well be the most uniformly dense cluster visible in the night sky. Hundreds of stars seem to hint at resolution. Double the aperture and they all come out to play!
If most stars in globulars are reddening with age, we wonder why so many appear blue? The bluest stars in the sky – such as Vega – all have higher surface temperatures – certainly twice our own Sun. Such stars are three or four times more massive and don’t live nearly as long due to their higher consumption rate of hydrogen. If Sol were at the distance of M15 it would appear as a star of the 19th magnitude. Such stars are not the ones waiting to become red giants. These “solar type” stars hardly contribute to the light of the cluster at all. Blue stars like Vega and Fomalhaut would begin around the 15th magnitude at M15’s distance. These stars are within the H-R diagram’s main sequence and provide the bulk of the cluster’s “blue” light.
Thursday, October 12 – Today in 1892, astronomy great E. E. Barnard puzzled over an early photographic plate. In doing so, he became the first astronomer to discover a comet – 1892 V – using photography! But it wasn’t Barnard’s first comet. His career began in 1881 and 1882 with three comet discoveries through a 5″ equatorial refractor purchased at great personal expense five years earlier. While a student of mathematics at Vanderbilt University, Barnard discovered eight comets at the eyepiece of the University’s 6″ refractor. It was at this telescope in 1884 that he discovered the 1.7 million light-year distant dwarf galaxy NGC 6822, located in northeastern Sagittarius. Tonight, before it heads too far southwest, head about a fist width west of Beta Capricorni and discover it for yourself!
Friday, October 13 – Today marks the founding of the British Interplanetary Society in 1933. “From imagination to reality,” the BIS is the world’s longest established organization devoted solely to supporting and promoting the exploration of space and astronautics.
Tonight we’ll do them proud by having a speculative look at distant Pluto – the only member of the solar system yet to see a “flyby” from an Earth-sourced probe. The “unexplored planetoid” now lies a little less than one degree south-southeast of Xi Ophiuchi. Consult with star charts available on the Internet to determine which little 14th magnitude “star” is our solar system’s 9th planet. (Or use the sketch method described elsewhere in this book to follow its motion.)
Saturday, October 14 – With comfortable early evening dark skies, let’s head off again to M15 and M2 – the last in a series of bright, beautiful globular clusters to be seen after skydark until spring.
Astronomers now know there are more than 150 globular clusters associated with the Milky Way Galaxy. These globulars take up positions throughout the galaxy with most orbiting the galactic core outside the galactic plane. We also know that some 400 globulars are associated with our next-door neighbor in space – the Andromeda Galaxy. If only we knew precisely how far away such intergalactic beacons are, we could truly get a sense of the scale of the Universe…
Unraveling the mystery of globular cluster distances took more than simply analyzing the break off point on the H-R diagram to show at what point bright semi-massive blue stars start to become red giants. Using the H-R diagram – in conjunction with other methods – eventually allowed astronomers to deduce a certain globular cluster’s distance. The globular was discovered by Jean-Dominique Maraldi in 1746, then added to Messier’s list as M15 in 1764. We now know it is located 33,600 light-years away.
Sunday, October 15 – Today in 1963 saw the first detection of an interstellar molecule. This discovery was made by a team of scientists headed by Sander Weinreb using the MIT Millstone Hill 84-foot dish. Using new correlation receiver technology, hydroxyl molecules were found in the interstellar medium (ISM) based on absorption bands associated with light coming from supernova remnant Cas A. By the dawn of the new millennium, nearly 200 different interstellar molecules had been identified and many are considered organic in nature…
Tonight, let’s see what’s up there in the region of Cas A using visible light. The nearest bright star to Cas A is Beta Cassiopeiae – the bright star westward of the “W.” To locate the region of Cas A, go about three finger-widths due west of Beta and follow the subtle curve of three 5th magnitude stars. Cas A lies less than one degree south-southwest of the second star in the sequence of three. This star is a complex 5th magnitude multiple star system associated with variable star AR Cas.
Through binoculars, two stars of the AR system are easily resolved – the 4.9 magnitude primary is seen to be led across the sky by a 7.1 magnitude secondary (component C) which is a very tight double itself. Its 8.9 magnitude partner is resolvable in mid-sized scopes. Large aperture scopes may also be able to distinguish a 9.3 magnitude, second (B) component from the primary. Smaller scopes are back in the running again when attempting three 11th magnitude stars – none of which are close to the primary. Intermediate scopes can also hope to pick out a 12.9 magnitude H component northwest of C. 8.9 magnitude F also has a 9.1 magnitude near twin to the east-northeast. If you can see them all you should probably wrap an observatory building around your telescope – if one isn’t there already!
If you like to follow brightness changes in variables – AR Cas is not a good choice. This eclipsing type variable only fluctuates by a tenth of a magnitude over a period of 6 earth days.
May all your journeys be at light speed… ~Tammy Plotner with Jeff Barbour.