New Chelyabinsk Results Yield Surprises (Continued.....) (November 7, 2013)
by Kelly Beatty, skyandtelescope.com
New analyses suggest that an asteroidal fragment's collision with Earth on February 15, 2013, might not be the once-per-century event that researchers thought. Instead, these potent wallops might occur more frequently and with more destructive power than previously thought.
Researchers have also pulled together estimates of the impact's energy, gauged as the kinetic-energy equivalent of exploding TNT. A research team led by Peter Brown (University of Western Ontario), visible-light emission alone implies a blast of at least 470,000 tons (470 kilotons) of TNT. But the powerful seismic shock yields a rather uncertain "best estimate" of 430 kilotons, while sensors on military satellites suggest 530 kilotons. Finally, records from the CTBTO's infrasound network imply a somewhat higher yield of 600 kilotons.
Uncertainties aside, the Chelyabinsk blast represents the most energetic impact on Earth since the iconic blast over the Tunguska region of Siberia in 1908. The rocky object had a diameter close to 62 feet (19 m) and a mass of roughly 1,200 metric tons (nearly twice the initial estimate). The ground-level damage might have been much greater, researchers conclude, save for two fortunate circumstances. First, the body came in at a very shallow angle, just 17° from horizontal. As it broke up, the resulting shock wave expanded with a cylindrical shape, rather than from a single explosive point, which tended to spread the shock energy over a wider, less concentrated area. Had the flight path been more vertical, as was the case with the Tunguska blast, the damage would likely have been far more extensive.
Second, recovered fragments show that the incoming object was a single cohesive body — but only barely so, according to analysis by the "Chelyabinsk Airburst Consortium," led by Olga Popova (Institute for Dynamics of Geospheres, Russian Academy of Sciences).
Uncertainties aside, the Chelyabinsk blast represents the most energetic impact on Earth since the iconic blast over the Tunguska region of Siberia in 1908. The rocky object had a diameter close to 62 feet (19 m) and a mass of roughly 1,200 metric tons (nearly twice the initial estimate). The ground-level damage might have been much greater, researchers conclude, save for two fortunate circumstances. First, the body came in at a very shallow angle, just 17° from horizontal. As it broke up, the resulting shock wave expanded with a cylindrical shape, rather than from a single explosive point, which tended to spread the shock energy over a wider, less concentrated area. Had the flight path been more vertical, as was the case with the Tunguska blast, the damage would likely have been far more extensive.
Second, recovered fragments show that the incoming object was a single cohesive body — but only barely so, according to analysis by the "Chelyabinsk Airburst Consortium," led by Olga Popova (Institute for Dynamics of Geospheres, Russian Academy of Sciences).
This fragment from the Chelyabinsk impactor, about 4cm across, shows crisscrossing fractures that are filled with shock-melted material.
Olga Popova & others
The teams reports that the Chelyabinsk parent body must have endured an abrupt thermal or collisional event 4.45 billion years ago, 115 million years after the solar system formed, that left its interior crisscrossed by a network of fractures filled with metal-rich glass. These preexisting fractures, explains consortium member Peter Jenniskens (SETI Institute), "left the body weaker, and it broke apart along those veins."
But researchers are hardly feeling complacent about all this. For example, Brown and his team went on to compile a worldwide catalog of all such airbursts over the past two decades. (Chelyabinsk might have been the most powerful, but it wasn't the only one recorded.) They find that the impacts from objects a few tens of meters across must be occurring 7 to 10 times more frequent than estimates based only on telescopic surveys, though there's a lot of uncertainty because the events are so sparse. Still, Brown's census suggests that a Chelyabinsk-type blast should happen not just once per century on average, as had been thought, but instead every few decades.
Another concern is researchers didn't believe objects in this mass range would disintegrate so low in the atmosphere. Now the thinking is that these blasts are driven deeper down by their own momentum, an idea first put forward six years ago by Mark Boslough (Sandia National Laboratories) to explain Tunguska's 800 square miles of devastation. Moreover, Boslough points out, impacts appear to be "more damaging than nuclear explosions of the same yield" because up to half the energy in a nuclear blast escapes as radiation rather than as shock and heat.
But researchers are hardly feeling complacent about all this. For example, Brown and his team went on to compile a worldwide catalog of all such airbursts over the past two decades. (Chelyabinsk might have been the most powerful, but it wasn't the only one recorded.) They find that the impacts from objects a few tens of meters across must be occurring 7 to 10 times more frequent than estimates based only on telescopic surveys, though there's a lot of uncertainty because the events are so sparse. Still, Brown's census suggests that a Chelyabinsk-type blast should happen not just once per century on average, as had been thought, but instead every few decades.
Another concern is researchers didn't believe objects in this mass range would disintegrate so low in the atmosphere. Now the thinking is that these blasts are driven deeper down by their own momentum, an idea first put forward six years ago by Mark Boslough (Sandia National Laboratories) to explain Tunguska's 800 square miles of devastation. Moreover, Boslough points out, impacts appear to be "more damaging than nuclear explosions of the same yield" because up to half the energy in a nuclear blast escapes as radiation rather than as shock and heat.
It's a set of results both exciting and sobering. Millions of Earth-threatening objects with masses comparable to Chelyabinsk's have yet to be discovered. To make matters worse, February's rogue impactor approached from Earth's sunlit side, making it impossible to detect beforehand.
So what's the "action plan" to defend Earth from objects once considered too small to do any real damage? Ideas are being kicked around. A team at the University of Hawaii hopes to operate a Asteroid Terrestrial-impact Last Alert System (ATLAS) by 2015. The privately funded B612 Foundation has proposed its Sentinel spacecraft, which would scan the infrared sky from a location well inside Earth's orbit. Conceptually similar is NEOCam, first proposed in 2005.
Meanwhile, NASA managers have launched a "Grand Challenge" to bring innovative concepts to light, and the United Nations is trying to establish an "International Asteroid Warning Group".
These are useful first steps toward a more comprehensive global strategy. But some researchers wonder whether February's eye-opener, combined with the realization that relatively small asteroids can do serious damage, should spur a more rapid buildup of detection and defense systems. One has to wonder how different the response would have been had that errant space rock come in over Chicago, instead of Chelyabinsk.
So what's the "action plan" to defend Earth from objects once considered too small to do any real damage? Ideas are being kicked around. A team at the University of Hawaii hopes to operate a Asteroid Terrestrial-impact Last Alert System (ATLAS) by 2015. The privately funded B612 Foundation has proposed its Sentinel spacecraft, which would scan the infrared sky from a location well inside Earth's orbit. Conceptually similar is NEOCam, first proposed in 2005.
Meanwhile, NASA managers have launched a "Grand Challenge" to bring innovative concepts to light, and the United Nations is trying to establish an "International Asteroid Warning Group".
These are useful first steps toward a more comprehensive global strategy. But some researchers wonder whether February's eye-opener, combined with the realization that relatively small asteroids can do serious damage, should spur a more rapid buildup of detection and defense systems. One has to wonder how different the response would have been had that errant space rock come in over Chicago, instead of Chelyabinsk.
Catching Summer's Best Deep-Sky Object's before They're Gone!
(August 26, 2013)
As the summer night sky starts to fade and autumn constellations take the stage, there are still some cosmic objects that may call out to skywatchers
equipped with a small telescope, binoculars or their own two eyes.
Compiling a "Top Five" list of summer deep sky objects is, of course, very subjective, but here is one observer's list of "Don't Miss" summer sky objects to try and catch before they're gone. They are listed in ascending order but each is special in its own way:
1) The Coathanger - A Binocular Treat!
equipped with a small telescope, binoculars or their own two eyes.
Compiling a "Top Five" list of summer deep sky objects is, of course, very subjective, but here is one observer's list of "Don't Miss" summer sky objects to try and catch before they're gone. They are listed in ascending order but each is special in its own way:
1) The Coathanger - A Binocular Treat!
Thumb through most astronomy books or skywatching guides and you'll find most of the attention focused on the most brilliant and splashy star patterns such as Orion, the Hunter, Scorpius, the Scorpion or (for southern observers), the region around Crux, the Southern Cross. However, there are smaller, fainter star patterns called 'asterisms' that are interesting in their own right, usually because they look like more common objects of modern life. They usually don't get much attention though, because they are harder to spot. One pattern I always look for, is Brocchi's Cluster, nicknamed the "Coathanger", for its remarkable similarity to that everyday device. It can be seen with the naked eye from dark sky locations but is most easily picked up in binoculars even from fairly light-polluted skies. Be aware, though, that it is fairly tiny in comparison to a constellation of stars and it does appear upside-down to viewers in the northern hemisphere. This sky map shows where to look for it.
Although some have classified this grouping of stars as a "cluster", it has been shown that their motions through space are not related and that they are a just a random alignment of stars as seen from our position in the Milky Way. They are certainly worth looking for!
Although some have classified this grouping of stars as a "cluster", it has been shown that their motions through space are not related and that they are a just a random alignment of stars as seen from our position in the Milky Way. They are certainly worth looking for!
The Coathanger is located just less than halfway between two of the brightest stars in the summer sky, Vega above and Altair to the south.
2) The Dumbbell Nebula (M27) - An Amazing Planetary Nebula
Sighted in wide-field binoculars or a telescope's viewfinder, the constellation Sagitta (the Arrow) helps us locate the beautiful Dumbbell nebula (M27) about 1/3 of the way from the tip of the arrow to the bright star Alberio in the nose of the constellation Cygnus. This compact glowing cloud is actually the expelled outer layers of a dead red giant star whose only remains is a glowing white dwarf core that makes the cloud fluoresce with its copious emissions of UV energy. Fom earth's position in space, we can view the cloud at low power through a small telescope as a glowing bubble encompassing two hazy patches of light; it assumes more of a dumbbell appearance at higher power.
The name "Dumbbell" was, in fact, derived from the description by the Rev. T.W. Webb (1807-1885) of two hazy masses in contact. And while you're scanning in this area of the sky, be sure to also look for the Arrow (Sagitta) and Job's Coffin, a lozenge-shape pattern of four stars that represents Delphinus, the Dolphin. Scanning in this region may even reveal the odd open cluster of stars as well!
The name "Dumbbell" was, in fact, derived from the description by the Rev. T.W. Webb (1807-1885) of two hazy masses in contact. And while you're scanning in this area of the sky, be sure to also look for the Arrow (Sagitta) and Job's Coffin, a lozenge-shape pattern of four stars that represents Delphinus, the Dolphin. Scanning in this region may even reveal the odd open cluster of stars as well!
3) The Ring Nebula (Messier 57) - Bright Planetary Nebula
The little constellation of Lyra is supposed to represent Apollo's harp. Six fainter stars form a little geometric pattern of a parallelogram attached at its northern corner to an equal-sided triangle. Vega gleams as the brightest star in the summer sky over at the western part of the triangle. But tucked in this region is also the acclaimed Ring nebula.
The little constellation of Lyra is supposed to represent Apollo's harp. Six fainter stars form a little geometric pattern of a parallelogram attached at its northern corner to an equal-sided triangle. Vega gleams as the brightest star in the summer sky over at the western part of the triangle. But tucked in this region is also the acclaimed Ring nebula.
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The two lowest stars in the parallelogram are Beta and Gamma Lyrae. Beta is sometimes also known as Sheliak. Between these two stars, but a trifle nearer to Gamma, is where you will find the Ring nebula - the tenuous expelled shells of a dying star's atmosphere blown away during its final stages of helium burning.
The sky map below shows the location of the Ring nebula in the Lyra constellation. The nebula shines at magnitude +8.8, and thus is far too faint to be seen with the unaided eye. Any good pair of binoculars will locate it, though it will look almost star-like in appearance because of its small apparent diameter. The ring shape might just begin to become evident to most eyes in small telescopes using a magnification of 100-power, although at least a 6-inch telescope is recommended to see the ring clearly. With larger instruments and higher magnifications, the ring appears distinctly as a tiny ghostly doughnut. 4) The Lagoon Nebula (Messier 8) - A Giant Stellar Nursery The Lagoon Nebula, also known as M8 or Messier 8, is a large gas cloud within the Milky Way Galaxy, barely visible to the human eye under good conditions. It appears a few degrees above and to the right of the Teapot of Sagittarius. Visually about three times the size of the full moon, the Lagoon Nebula is the largest and brightest of a number of nebulosities in and around Sagittarius. As just a very faint patch to the unaided eye, the nebula takes on an oblong shape in binoculars. A brighter nucleus (the so-called “hour glass”) is visible on one side, separated by a dark rift, from an open star cluster on the other side. Unlike the published timed-exposure photographs, to the unaided eye the faint nebulosity appears grayish with little if any hint of color. |
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The Lagoon Nebula is widely visible throughout the populated areas of North America. However, due to its location in the sky (-24 degrees declination), observers farther south see it higher in the sky, which is better for observing. A line drawn from Phi Sag through Lambda Sag and onward approximately as far as those two stars are apart will lead the general area of M8.
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M8 is about 5000 light years away, and roughly 130 light years across in the longer dimension. Composed primarily of hydrogen, much of it ionized (heated or energized) by radiation from the nearby superstar Herschel 36, M8 is known as an emission nebula. As such it also is a star-forming region, sometimes called a “stellar nursery.” There is an open star cluster, NGC 6530, of young, hot, blue stars probably only a few million years old. In addition to these young stars, there are also many dark “Bok” globules of condensing gas and dust on their way to becoming “protostars” and ultimately full-fledged stars like those already formed nearby.
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Observing Herschel's Garnet Star...(continued)
Colored variable stars appear redder when near their minima. The good news is, we’re currently near a maximum in brightness for Mu Cephei. In the case of Herschel’s Garnet Star, it appears to fade from orange to red as it cycles. An optical illusion, known as the Purkinje effect, can often cause an observer to overestimate the brightness of a crimson-colored star. The longer you stare at it, the brighter it appears. Throwing the star out of focus slightly can cause this apparent change in brightness to vanish.
Burnham’s Celestial Handbook notes that Mu Cephei “Appears a deep orange-red, but on occasion takes on a peculiar purple tint.” Can you see this spurious effect? We think it’s interesting that Herschel chose the word “garnet” to describe this star, as the gemstone can vary from shades of red to orange to even green!
Stars like Mu Cephei are also great targets, as they hold up well under light-polluted suburban skies. Herschel’s Garnet Star is also the reddest of the non-carbon stars.
And that leads us to the “wow factor” of what you’re truly seeing. Mu Cephei is a class M2Ia Supergiant star, about 2,400 light years distant (estimates still vary widely). It shines with a luminosity of 600,000 times that of our own Sun, making it one of the brightest known stars in the Milky Way. Move Mu Cephei to the standard absolute magnitude distance of 32.6 light years, and it would shine at magnitude -7.6, 16x times brighter than Venus at its best.
Mu Cephei is also 19 times more massive than our Sun, and has a radius of 1,650 times larger than Sol. Place Mu Cephei in our solar system, and it would extend out to the orbit of the planet Saturn. This makes Mu Cephei one of the largest stars known in terms of volume, with only a handful of larger stars known.
The surface temperature of Mu Cephei is also relatively cool among stars, if you consider 3,690 Kelvin to be cool. Astronomers also detected water vapor in the spectrum of the star as early as 1964, another rarity. Massive stars such as Mu Cephei are destined to “live fast, die young” with life spans measured in the millions of years instead of the sedate billions of year life span of stars like our own Sun. Mu Cephei will one day grace our skies with a fine supernova, an event that could be on its way tomorrow night or millions of years from now.
All food for thought as you track down this fine scarlet delight in Cepheus, just as Sir William Herschel did centuries ago.
Burnham’s Celestial Handbook notes that Mu Cephei “Appears a deep orange-red, but on occasion takes on a peculiar purple tint.” Can you see this spurious effect? We think it’s interesting that Herschel chose the word “garnet” to describe this star, as the gemstone can vary from shades of red to orange to even green!
Stars like Mu Cephei are also great targets, as they hold up well under light-polluted suburban skies. Herschel’s Garnet Star is also the reddest of the non-carbon stars.
And that leads us to the “wow factor” of what you’re truly seeing. Mu Cephei is a class M2Ia Supergiant star, about 2,400 light years distant (estimates still vary widely). It shines with a luminosity of 600,000 times that of our own Sun, making it one of the brightest known stars in the Milky Way. Move Mu Cephei to the standard absolute magnitude distance of 32.6 light years, and it would shine at magnitude -7.6, 16x times brighter than Venus at its best.
Mu Cephei is also 19 times more massive than our Sun, and has a radius of 1,650 times larger than Sol. Place Mu Cephei in our solar system, and it would extend out to the orbit of the planet Saturn. This makes Mu Cephei one of the largest stars known in terms of volume, with only a handful of larger stars known.
The surface temperature of Mu Cephei is also relatively cool among stars, if you consider 3,690 Kelvin to be cool. Astronomers also detected water vapor in the spectrum of the star as early as 1964, another rarity. Massive stars such as Mu Cephei are destined to “live fast, die young” with life spans measured in the millions of years instead of the sedate billions of year life span of stars like our own Sun. Mu Cephei will one day grace our skies with a fine supernova, an event that could be on its way tomorrow night or millions of years from now.
All food for thought as you track down this fine scarlet delight in Cepheus, just as Sir William Herschel did centuries ago.
A comparison of the size of mu Cephei to the size of the orbits of some of the planets in the solar system (Saturn's orbit is the largest shown) and to some of the other largest stars near us in the Milky Way galaxy. The sun is too small to be shown here.
Strewnfield Map of Chelyabinsk Meteorites
Priming for Summer's Globular Clusters ...continued...
It doesn't require much aperture to start to recognise M3's true nature, as even a 80-mm 'scope at low power shows it as grainy and apertures of 100mm and above will start to resolve the cluster, making it a harder proposition in this respect than its principle rivals in the northern sky, M13 and M5. M3 is rated as class V1 in the 12-point scale of degree of condensation (with I being very dense and compact to XII being extremely diffuse), making it slightly more diffuse than its aforementioned rivals, both rated V. Its apparent size through the eyepiece depends on aperture, appearing around seven arcminutes in a 100mm increasing to 15 arcminutes in large amateur 'scopes. Spectacular images are possible through even modest 'scopes with deep LRGB CCD images revealing M3's breathtakingly beautiful form extending to almost 20 arcminutes.
Despite being in a relatively barren part of the sky M3 is quite easy to find; just look over half way between brilliant Arcturus (alpha Boötis) and Cor Caroli (alpha CVn), a little closer to the former. At the moment M3 culminates due south just after 11pm BST at a very healthy 65 degrees or so and there's a five-hour observing window until the morning twilight intervenes around 4am.
Messier 5
Scanning the sky with binoculars in the direction of Serpens, close to the border with Virgo, you may come across a hazy spot, like an out of focus star. This is the big, bright and beautiful globular cluster Messier 5 (NGC 5904), so good in fact that the great observer, Edward Emerson Barnard thought it much more beautiful than M13.
It doesn't require much aperture to start to recognise M3's true nature, as even a 80-mm 'scope at low power shows it as grainy and apertures of 100mm and above will start to resolve the cluster, making it a harder proposition in this respect than its principle rivals in the northern sky, M13 and M5. M3 is rated as class V1 in the 12-point scale of degree of condensation (with I being very dense and compact to XII being extremely diffuse), making it slightly more diffuse than its aforementioned rivals, both rated V. Its apparent size through the eyepiece depends on aperture, appearing around seven arcminutes in a 100mm increasing to 15 arcminutes in large amateur 'scopes. Spectacular images are possible through even modest 'scopes with deep LRGB CCD images revealing M3's breathtakingly beautiful form extending to almost 20 arcminutes.
Despite being in a relatively barren part of the sky M3 is quite easy to find; just look over half way between brilliant Arcturus (alpha Boötis) and Cor Caroli (alpha CVn), a little closer to the former. At the moment M3 culminates due south just after 11pm BST at a very healthy 65 degrees or so and there's a five-hour observing window until the morning twilight intervenes around 4am.
Messier 5
Scanning the sky with binoculars in the direction of Serpens, close to the border with Virgo, you may come across a hazy spot, like an out of focus star. This is the big, bright and beautiful globular cluster Messier 5 (NGC 5904), so good in fact that the great observer, Edward Emerson Barnard thought it much more beautiful than M13.
Amazing spring/summer globular Messier #5 in Bootes. Image by Bob Franke.
Gottfried Kirch in Berlin first recorded M5 in May 1702, Charles Messier noted it in 1764 but William Herschel was the first to resolve it in 1791. It is similar in many categories to M3 and M13; it is the equal to M13 in brightness, shining at mag. +5.7 and has only a slightly inferior apparent size of 20 arcminutes as opposed to M13's 21 arcminutes. This gives it an actual size of 150 light-years across at its distance of 26,620 light years; M13 is slightly closer and larger with M3 beating both of at 190 light years in size at its more remote distance of 34,170 light years. M5 could contain as many as half a million stars and weighs in at 800,000 solar masses. M5's age is a point of contention; cited previously as one of the oldest, studies in 1997 by Raul Jimenez and Paolo Padoan reported a youthful ten billion years, which if correct would make it one of the youngest.
M5 can be found with the naked eye by eagle-eyed observers at dark sites some 25 degrees south-east of Arcturus and eight degrees west of alpha Serpentis. Its impact is somewhat diminished by its low northerly declination and this is where M3 and M13 score heavily. On the other hand, it does mean M5 is visible from both hemispheres. Small scopes reveal a distinctly elliptical shape with a bright core and resolution of the outlying stars. Moving up to 'scopes in the 150-200-mm class gives magnificent views, with resolution more or less down to the core at moderate magnifications. M5 can be observed as soon as it gets dark and is at its best at 12.45am BST when it's 40 degrees up. Pick up a copy of the May issue of Astronomy Now for an in-depth, exhaustive observing, sketching and imaging guide to M5.
Messier 13
Unquestionably the finest globular cluster in the Northern Hemisphere, M13 (NGC 6205) is many observers' first experience of a globular cluster and from then on are hooked. M13 is only eclipsed by the great southern globulars Omega Centauri and 47 Tucanae, with M22 in Sagittarius providing very stiff competition. M13 is very easy to find on the west side of the Keystone asterism of Hercules, lying a third of the way down from eta to zeta Herculis and it's just within naked-eye range too from the darkest of sites, glowing at an integrated magnitude +5.7.
M5 can be found with the naked eye by eagle-eyed observers at dark sites some 25 degrees south-east of Arcturus and eight degrees west of alpha Serpentis. Its impact is somewhat diminished by its low northerly declination and this is where M3 and M13 score heavily. On the other hand, it does mean M5 is visible from both hemispheres. Small scopes reveal a distinctly elliptical shape with a bright core and resolution of the outlying stars. Moving up to 'scopes in the 150-200-mm class gives magnificent views, with resolution more or less down to the core at moderate magnifications. M5 can be observed as soon as it gets dark and is at its best at 12.45am BST when it's 40 degrees up. Pick up a copy of the May issue of Astronomy Now for an in-depth, exhaustive observing, sketching and imaging guide to M5.
Messier 13
Unquestionably the finest globular cluster in the Northern Hemisphere, M13 (NGC 6205) is many observers' first experience of a globular cluster and from then on are hooked. M13 is only eclipsed by the great southern globulars Omega Centauri and 47 Tucanae, with M22 in Sagittarius providing very stiff competition. M13 is very easy to find on the west side of the Keystone asterism of Hercules, lying a third of the way down from eta to zeta Herculis and it's just within naked-eye range too from the darkest of sites, glowing at an integrated magnitude +5.7.
The glorious monster globular cluster of the northern hemisphere, Messier #13. Image credit: Marco Burali, Tiziano Capecchi, Marco Mancini (Osservatorio MTM).
Binoculars will snare M13 easily, flanked by two seventh-magnitude stars, but only show an extended fuzzy spot with a brighter core. Small telescopes of good quality in the 80-100-mm class should show an apparent diameter of eight to ten arcminutes and start to resolve some of the outlying stars in this giant ball of suns. Upgrading to a 150-mm and employing magnification of 200x quite simply gives stunning views.
M13 was discovered by Edmond Halley in 1714 and Messier added it to his list of comet-like nebulous objects in 1764. The indefatigable and brilliant William Herschel was the first to recognise its true nature 20 years later. M13 has an eccentric, 500 million year orbit around the galactic centre and it can be as remote as 80,000 light years from us but at present it lies much closer at 26,000 light years. M13 is one of the larger clusters with a physical diameter of 160 light years, equating to an apparent diameter on the celestial sphere of 21 arcminutes. Astronomers believe M13 contains no more than one million stars with a total mass of 600,000 solar masses.
Messier 92
M92 (NGC 6341) is a very good globular cluster residing in Hercules but is often overshadowed by the great globular cluster M13, only ten degrees to its south-west. M92 is smaller and fainter (mag. +6.5 and 14') than M13 (+5.7 and 21') because it is physically smaller and further away. M92 weighs in at 400,000 solar masses, crammed into a 110 light year diameter lying 27,000 light years away.
M13 was discovered by Edmond Halley in 1714 and Messier added it to his list of comet-like nebulous objects in 1764. The indefatigable and brilliant William Herschel was the first to recognise its true nature 20 years later. M13 has an eccentric, 500 million year orbit around the galactic centre and it can be as remote as 80,000 light years from us but at present it lies much closer at 26,000 light years. M13 is one of the larger clusters with a physical diameter of 160 light years, equating to an apparent diameter on the celestial sphere of 21 arcminutes. Astronomers believe M13 contains no more than one million stars with a total mass of 600,000 solar masses.
Messier 92
M92 (NGC 6341) is a very good globular cluster residing in Hercules but is often overshadowed by the great globular cluster M13, only ten degrees to its south-west. M92 is smaller and fainter (mag. +6.5 and 14') than M13 (+5.7 and 21') because it is physically smaller and further away. M92 weighs in at 400,000 solar masses, crammed into a 110 light year diameter lying 27,000 light years away.
Sometimes known as the "Rodney Dangerfield" cluster because it just gets no respect, Messier #92 is beautiful in its own right. Image Credit: Daniel Bramich (ING) and Nik Szymanek.
So M92 is easy to locate in northern Hercules, its high northerly declination another plus point in its ease of detection. But what is the view like through the eyepiece? Amateurs like to resolve individual stars in globulars and try to resolve stars all the way to the core of the cluster if possible. The globulars with a higher degree of condensation are harder to resolve and M92's high rating, which is an advantage in finding it, becomes a hindrance when trying to resolve it. Apertures in the region of 75-100-mm will start to resolve the outer regions of M92, with the compact, nebulous core hinting at resolution. But it will probably require a 250-300-mm aperture to fully resolve M92 right to the core.
Try switching the view between M13 and M92 and see how you rate them? Vary the magnification and take plenty of time observing each cluster. M92 appears asymmetric even in scopes as small as 80-mm, with the central condensation offset to the north-east and there are no obvious star chains, unlike those in M5 and M13.
Both M92 and M13 are superbly placed on May evenings, basically observable all night, culminating in the early hours. M92 is circumpolar from the UK (never sets).
Messier 10 & 12
Ophiuchus is home to no less than seven Messier globular cluster with the M10 (NGC6254) and M12 (NGC6218) pair (only slightly over three degrees apart) being the most accessible to Northern Hemisphere observer and happen to be two of the finest anywhere in the heavens. M10 is slightly superior to its neighbour in all categories, shining at mag. +6.6 and extending out to almost 20 arcminutes in apparent diameter.
Try switching the view between M13 and M92 and see how you rate them? Vary the magnification and take plenty of time observing each cluster. M92 appears asymmetric even in scopes as small as 80-mm, with the central condensation offset to the north-east and there are no obvious star chains, unlike those in M5 and M13.
Both M92 and M13 are superbly placed on May evenings, basically observable all night, culminating in the early hours. M92 is circumpolar from the UK (never sets).
Messier 10 & 12
Ophiuchus is home to no less than seven Messier globular cluster with the M10 (NGC6254) and M12 (NGC6218) pair (only slightly over three degrees apart) being the most accessible to Northern Hemisphere observer and happen to be two of the finest anywhere in the heavens. M10 is slightly superior to its neighbour in all categories, shining at mag. +6.6 and extending out to almost 20 arcminutes in apparent diameter.
M10 Globular cluster in Ophiuchus.
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M12, another beauty with long star chains twisting away from the core. Image by Dick Miller.
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It is visible in binoculars but the view will be so much better in a small telescope and even an 80-mm 'scope at magnification x100 will start to resolve individual stars. M10 is of medium compactness and concentration, classed as VII, and it will only take a 150-mm 'scope to fully resolve stars right to the core. M10 is quite a large globular with a physical diameter of 140 light years but in common with M107 (also in Ophiuchus) is it somewhat average, consisting of about 250,000 solar masses.
M12 is a noticeably looser globular even through small apertures (IX classification) and if you have a 100-150-mm 'scope then you should be able to fully resolve this cluster to its core if the seeing permits a high enough power. M12 is smaller and fainter than M10; although there's not much in it in brightness terms, a mere two tenths at +6.8, in size it loses out by five arcminutes despite lying 4,000 light years closer than its neighbour at 20,760 light years. This is down to its true physical size of about 85 light years, much smaller than M10. By virtue of being south of the celestial equator (albeit marginally) both M10 and M12 don't pull well clear of the southeastern horizon until midnight and culminate around 2am, when they are both comfortably over 30 degrees above the southern horizon.
There are many other globular clusters within reach of small to moderate-sized telescopes this summer, but give these beauties a try first, even under city lights. You're not likely going to be disappointed by their appearance!
M12 is a noticeably looser globular even through small apertures (IX classification) and if you have a 100-150-mm 'scope then you should be able to fully resolve this cluster to its core if the seeing permits a high enough power. M12 is smaller and fainter than M10; although there's not much in it in brightness terms, a mere two tenths at +6.8, in size it loses out by five arcminutes despite lying 4,000 light years closer than its neighbour at 20,760 light years. This is down to its true physical size of about 85 light years, much smaller than M10. By virtue of being south of the celestial equator (albeit marginally) both M10 and M12 don't pull well clear of the southeastern horizon until midnight and culminate around 2am, when they are both comfortably over 30 degrees above the southern horizon.
There are many other globular clusters within reach of small to moderate-sized telescopes this summer, but give these beauties a try first, even under city lights. You're not likely going to be disappointed by their appearance!