What’s That Bright Star in the East?
No, that’s not the Christmas star come early.  It’s Jupiter rising big, bright, and lovely just south of the great square of Pegasus.  The planet is much higher for northern observers than it has been for several years, and it lies close to the celestial equator so it’s well placed for observers all over the world.  The planet reaches opposition on September 21 and presents a stunning disk that spans some 49 arc-seconds.  Which means in a telescope at 40x, Jupiter has an apparent diameter as large as the full Moon appears to the unaided eye (if I did the math right…)

In binoculars, you’ll see the four brightest moons of Jupiter.  In a telescope, you’ll see the moons too, along with dark belts and bright zones across the planet’s disk.  And though the southern Equatorial Belt is still missing (see image above), it may return at any time!  So keep a lookout… when the belt reappears, it will do so suddenly.  More on this later in the month…

And a bonus… the planet Uranus hangs tight with Jupiter all month.  On September 17-18, the two planets are less than a degree apart, so you can nearly fit the two planets into the same field of view of a low-to-moderate power eyepiece.  Uranus looks star-like at low mag.  But pump up the power to 100x or more, and you’ll see the planet’s tiny pale-grey disk.

Jupiter and Uranus south of Pegasus on September 15 (click to enlarge)

What’s That Bright Star in the West??
While Jupiter puts on a good show for telescopic observers, no optics are required to see the star Spica and the planets Mars and dazzling Venus in the western sky.  They all move about low on the horizon after sunset this month.  On September 4-5, all three are within 4 degrees of each other.  And on the 11th, the crescent Moon joins the show.

If you’re keen to see Saturn and Mercury this month, then I have bad news.  Both are lost in the Sun’s glare.

What’s That Bright Star in the North??
After midnight, northern-hemisphere observers may see a madly-twinkling star in the northeastern sky.  That is the star Capella in the constellation Auriga.  It will take its place in the northern winter sky by year’s end.

What’s That Bright Star in the South??
Northern-hemisphere observers will also notice a single bright star in the south.  That’s Fomalhaut in the constellation Piscis Austrinus.  In 2008, this star was the first to reveal to the Hubble Space Telescope an extra-solar planet at visible wavelengths.

In the southern hemisphere, Fomalhaut is nearly overhead.

Where’s the Moon?
Moon phases this month…
Last Quarter: September 1, 6:22 UT
New Moon: September 8, 11:30 UT
First Quarter: September 15, 6:50 UT
Full Moon: September 23, 10:17 UT

September 18 also marks the first ever International Observe the Moon Night. Events are planned all over the world.  Check here to see if there’s anything going on near you…

And if you miss this event, don’t worry.  We have a something new in the works at One-Minute Astronomer to help you learn your way around the Earth’s nearest celestial neighbor.  Stay tuned in the coming weeks to learn more…

Equinox
The September Equinox occurs at 4:52 a.m. Universal Time on September 23.  Welcome to spring in the southern hemisphere, and autumn (sigh) in the north.  Summer was nice while it lasted…

That’s it for this month.  If you have questions, or want to see anything in particular in the pages of One-Minute Astronomer, please let us know.

In dark sky, IC4665 is just visible to the unaided eye roughly 1 degree NE of Celebrai.  If you’ve got a little light pollution, you’ll need binoculars to spot it.  The cluster is spread out over a full degree, more than twice the diameter of the full Moon, so it looks fainter than its integrated magnitude of 4.7

In binoculars, you’ll see perhaps a dozen stars; a small telescope at 25-35x shows a few dozen blue-white stars.  At an age of 35 million years, this little cluster is young compared to robust grand-dad clusters like the Beehive (M44) which looks similar but is more than 600 million years old.  The youth of IC4665 means few if any stars have evolved into red giants or supergiants.  So the color of its stars is fairly uniform.

Since the cluster doesn’t need much optics to resolve, you won’t be surprised to know it was discovered long ago… around 1745 by Philippe Loys de Cheseaux, a quarter century before Messier compiled his famed list.  Caroline Herschel saw it too.

Three star clusters near Taurus Poniatowski in the constellation Ophiuchus (click to enlarge)

If southwest is “up” in your field of view when you look at IC4665, look carefully at the inner stars. They form the pattern of the word “HI”, like a big friendly cosmic greeting.  While not obvious at first, it’s a little unnerving when the pattern finally jumps out at you!   This image (from AllTheSky.com) shows the “HI” reversed: move your mouse over the image to reveal the annotated circle around the cluster.  Within the yellow circle, the “I” is on the lower left, and the “H” is on the upper right.

IC 4665 is a little unusual because it’s about 15 degrees above the plane of the Milky Way.  Most young open star clusters are along the plane of our galaxy.  The cluster is about 1,100 light years away and some 20 light-years across

If it’s so darned pretty and easy to see, why isn’t IC4665 better known?  Well, it doesn’t have a very catchy name, I’m sure you’ll agree.  And it’s a little too faint to be easily seen with the unaided eye.  It’s also a little too spread out to see easily in a telescope.  But never mind.  Try to find this cluster for yourself just above the little bull in Ophiuchus.  It presents a lovely spray of stars in a modest pair of binoculars for observers north or south, city or country.

A bonus: in this same area, you’ll find two more dispersed open clusters IC4756 and NGC 6633.  Steve O’Meara calls these two clusters Tweedledee and Tweedle-dum, since they are chubby and fat with stars and seemingly match quite closely.  These clusters were also known by de Cheseaux and Caroline Herschel, though neither had the wide-field instruments to enjoy their splendour.

This little star pattern (or asterism) is called Taurus Poniatowski, or Poniatowski’s Bull.  It was named in 1777 by Abbe Poszobut after King Stanislaus Poniatowsk of Poland.  For a time, this little group was considered a constellation,  as in the old star map above, though it’s now part of the large constellation Ophiuchus.  While “Taurus P” didn’t make the cut as a modern constellation, the name of this star group remains.

And it’s a pretty little group.  In binoculars, the background is flecked with fainter 9-th and 10th- magnitude stars that straggle off the western edge of the Milky Way.  The V-shaped head of the bull consists of three stars: 67, 68, and 70 Ophiuchi.  The two stars at the back end of this little beast are gamma and beta Ophiuchi (Cebalrai).  Long-time readers will remember meeting 70 Oph a couple of years ago… it’s a fine yellow-orange double star and one of the few you can see revolve during a human lifetime.  In binoculars, it makes a nice contrast with the blue-white 67 Oph.

Map of Taurus Poniatowski in the constellation Ophiuchus (click to enlarge)

This asterism can be seen in both hemispheres, though it will appear upside down in the southern hemisphere.

While it’s off the Milky Way, Taurus Poniatowski holds a few sparse open star clusters and one dazzler, IC4665, just one degree northeast of beta Ophiuchi.  We’ll meet this star cluster and a few others next week.  But for now, that’s it!

Here’s the image taken on May 6 of this year, when MESSENGER was 183 million kilometers from the Earth.  The two bright specks in the lower left are the Earth and Moon.  Pretty cool, yes?

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

MESSENGER acquired this image as it was searching for “vulcanoids”, which are small rocky objects which may lurk between Mercury and the Sun.  None have been found, but the craft is well positioned to look for these faint objects when it passes close to the Sun.

That’s it for today.  If you can, get out and do a little stargazing this weekend.  It will make you feel good.

The Northern Coalsack extends over a 6ºx5º span of sky between the stars Deneb, Sadir, and Gienah in northeastern Cygnus. There are many dark nebula in the area. But the Northern Coalsack marks the beginning of the Great Rift of the Milky Way, a huge dark cloud of gas and dust stretching from Cygnus into Sagittarius.  This cloud, which gives the northern Milky Way a mottled appearance, contains some 1 million solar masses of material.

Here’s a map to help you find it.

The Northern Coalsack in the constellation Cygnus

And here’s a spectacular image of the Great Rift in the northern Milky Way.  Make sure to move your mouse over the image to get find out which feature is which.

While not nearly as famous as the southern Coalsack, this northern incarnation has been known since antiquity.  The ancient Greeks, who loved a good story, tell of the legendary Phaeton who took the reins of the sun-god’s chariot and lost control during a joyride across the sky.  Zeus struck Phaeton down with a lightning bolt, and he fell into the river Eridanus.  But his brother, Cygnus, gathered Phaeton’s remains and buried them properly.  Zeus rewarded Cygnus’ devotion by placing him in the sky as a Swan.  Phaeton’s reckless ride across the sky is marked by the darkness of the Great Rift which ends (or begins) at the Northern Coalsack.

The Northern Coalsack is easy to spot if you have dark sky.  You’ll need no optics, though binoculars give you a closeup view of a few foreground stars and the attenuated light from background stars.  Don’t bother with a telescope… it will show you only a tiny fraction of the cloud.  Many believe the Northern Coalsack is not nearly as dark at the southern version in Crux.  But accurate measurement shows each is similar in darkness.  The southern Coalsack simply appears darker by contrast because it appears in a brighter part of the Milky Way.

So go out and see the Northern Coalsack.  While it may seem odd for a stargazer to look at absences of stars, you may acquire a fondness for observing dark nebula.  As astronomy writer Garrett Serviss said nearly a century ago, “Infinity seems to acquire a new meaning in the presence of these black openings in the sky”.

The Cat’s Eye, cataloged as NGC 6543, is the only planetary nebula in the long constellation Draco which winds between the Big and Little Dippers.  The nebula is located about halfway between the stars delta (δ) and zeta (ζ) Draconis.  At 30-40x, you’ll just be able to discern the nebula from the surrounding stars.  It looks slightly fuzzy, with perhaps a greenish color.  More magnification (100x or more) will show you its true nature.

The Cat’s Eye Nebula (NGC 6543), between the stars Altais (δ Draconis) and ζ Draconis.

In the 18th and 19th centuries, astronomers thought planetary nebula were patches of unresolved stars.  But the English amateur astronomer William Huggins turned a spectroscope to the Cat’s Eye and discovered its light was completely different from any star.  He attributed the strange spectrum to an undiscovered element he called “nebulium”.  Decades later, spectroscopists determined  “nebulium” was really a form of ionized oxygen that exists only in the rarified vacuum of space.  This type of oxygen ion is called OIII (“Oh-Three”).  You can now get OIII filters to attach to the eyepiece of your telescope to pass the light from this ion and increase the contrast against the background sky of the Cat’s Eye and other nebulae.

While the Cat’s Eye is modest in a backyard scope, it is dazzling in long-exposure photographs.  A splendid image from the Hubble Telescope made this nebula famous nearly two decades ago.  Here’s an updated image from the HST…

And the most amazing thing about the Cat’s Eye?  Nearly all its wispy structure, which is caused by the intermittent gasps of a dying star like our sun, is quite new… less than a thousand years old.  So unlike most astronomical events, planetary nebulae like the Cat’s Eye Nebula happen quickly and briefly.  Our own star will experience the same quick and final fate in some 5 billion years.

The Cat’s Eye is for northern observers only.  But we never forget our southern subscribers.  For you, look for the False Comet tangled in the tail of the great scorpion Scorpius.  To get the best view, you don’t even need a telescope…

Sorry to say, that’s not going to happen.  Here’s why you got the email, and what you will really see this month…

The email got started in 2003. In August of that year, Mars came within 56 million km of Earth, the closest approach in 60,000 years. It was an impressive event. But even then, when the planet was a breathtaking magnitude -2.9, Mars measured about 25 arc-seconds across. That’s about 75x smaller than the full moon.

As it turns out, the original email about the close approach of Mars correctly mentioned this fact. It said:

“The encounter will culminate on August 27th when Mars comes to within 34,649,589 miles (55,763,108 km) of Earth and will be (next to the moon) the brightest object in the night sky. It will attain a magnitude of -2.9 and will appear 25.11 arc seconds wide. At a modest 75-power magnification Mars will look as large as the full moon to the naked eye.”

This was true… in a telescope at 75x, Mars would subtend an angle of about 1/2 degree, the same as the full moon.

I suspect when the email was forwarded, eventually the words “At a modest 75-power magnification…” were left out. That’s when the confusion began. And on some versions of the email, the year in which the event took place was left out, which is why the email seems to get most traffic just before August.

To be fair, I think the email going around today isn’t really a hoax, but a misunderstanding. But if it appears in your inbox, remember, its claims are completely bogus, although they was founded in fact back in 2003.

Right now, Mars is about 190 million kilometers away and appears 4x smaller than it was in August 2003.  Though it’s a puny 7 arc-seconds across, the planet puts on a nice show in the western sky after sunset for naked-eye observers. You can see it between Leo and Virgo just after the sun goes down.  Here’s an image of what you’ll see this week…

Mars, Venus, and Saturn in the western evening sky in early August 2010.  The planets will appear slightly higher above the horizon in the southern hemisphere (click to enlarge).

Because Mars is so far away, you won’t see much surface detail, even in a powerful telescope.  At a magnification of 260x, it will appear as large as the full moon appears to the unaided eye.  But you won’t see much.

The best planet for viewing this month is Jupiter, visible in the southeast sky after 11 p.m. local time. It’s the brightest object in that part of the sky (except for the moon).

Messier 6

M6 is among the brightest of dozens of open star clusters that fleck the Milky Way in Scorpius and Sagittarius.  Look for it about 4 degrees north of the bright star Shaula in the Scorpion’s tail (see map below).  M6 is a respectable 4th magnitude, though its light spreads over an area as large as the full Moon.  You’ll see the cluster without optics in dark sky; a full Moon or city lights make it harder to see, especially if the cluster is near the horizon.

A map of the tail of Scorpius showing M6 (the Butterfly Cluster) and M7 (click to enlarge).

M6 is often called the Butterfly Cluster, and a glance through a small telescope reveals why.  At 40-50x, the cluster has 3 bright stars running through the center (the body of the butterfly), with two irregular loops of stars on either side (the wings).  A little imagination reveals the butterfly’s “antennae” to the northeast.  Experiment with different eyepieces to get the best view.

The cluster lies some 1,600 ly away from us towards the galactic center.  It holds a little more than 300 stars, though you’ll see just a few dozen in binoculars, and perhaps 100 stars in a 6-inch scope.  It’s a young cluster… about 100 million year old… so it contains mostly blue stars, with one orange-giant star (BM Scorpii) in the northeast corner.

Here’s an image of M6… lovely, yes?

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Discover how to take great astro-photos with your digital camera.  Capture images of the crescent moon and planets at sunset, or the star clouds of Sagittarius rising over the trees above the southeastern horizon  No specialized knowledge required.  Click here to learn more…

* * * * *

Messier 7

Look 3.5 degrees southeast of M6 to find the cluster M7 set in one of the richest sections of the Milky Way (see image at top).  Though they’re closeby in the sky, the two clusters are not physically associated.  M7 is closer, just 780 light years away.  To the unaided eye, in the words of Stephen J. O’Meara, the spray of light from M7 looks “like the eruption of distant fireworks.”

M7 appears larger and brighter than M6, though both were known in antiquity.  But M7 is the southernmost Messier object, so it’s rarely seen well at northern latitudes, and it presents a real challenge for observers in northern Europe.  The view of this cluster from the southern hemisphere, however, where it’s high overhead this time of year, is jaw-dropping.

The cluster spans more than a full degree of sky, twice the size of the full Moon.  So stick with binoculars or a low-power eyepiece.  At 30x, the center of M7 looks square, or to some, cross-shaped.  Since M7 is twice as old as M6, some of its 80 stars have begun to evolve off the main sequence and turn orange-red.  The hottest and most massive blue stars have, presumably, burned out long ago.

Make sure you include M6 and M7 on your observing list this month and next.  They prove the point that, like diamonds, no two open star clusters are alike, yet all are beautiful in their own way.

Perseid Meteor Shower

For casual stargazers, the highlight of the month is the fine Perseid meteor shower.  This annual event peaks on the nights of August 11-12, when the Earth passes through a stream of particles left over from Comet Swift-Tuttle.  In dark sky, you can see many dozens of meteors each hour, especially after midnight as the Earth turns into the stream.   And this year, the sky will be dark since the Moon will set long before midnight.

You can learn more about the history and science of the Perseids here…

Planets at Sunset

Mars, Saturn, and brilliant Venus continue their square dance in the western sky at sunset.  Never far from each other, or from the brighter stars of Leo and Virgo, these three planets show you the dynamic solar system in action.  They gather to within 5 degrees of each other on August 8 (see below), then spread out a bit by August 12 when they’re joined by a slender crescent Moon.  This will be a great opportunity to take a simple snapshot of the Moon and three planets.  Here’s how you do it…

Venus, Mars, and Saturn just after sunset on August 8, 2010.

Jupiter

The next two months will give you the best view this year of the “Lord Planet” Jupiter.  Rising in Pisces about 9:30 p.m. local time by mid month, the planet brightens and grows bigger as it approaches opposition next month.  It shines at a stunning magnitude -2.9, far brighter than any star.  You can’t miss it.  The diameter of the planet’s disk grows to a fat 49″, so it will be a fine sight in a telescope.  If you can, observe the planet when it’s highest in the sky to avoid the turbulence of our atmosphere.

Even a small scope will show you the four largest moons of Jupiter, and the dark belts and white zones running across the planet, especially the thick belt just north of the equator.  While usually easily visible, the south equatorial belt “disappeared” a couple of months ago– a rare occurrence– and no one knows when it will become visible again.

For recent images of Jupiter (including the missing south equatorial belt), check out Chris Go’s website here.

The Milky Way

You can’t beat lying back on an August night under dark sky to examine the stunning celestial  sights along the Milky Way.  Wherever you are in the world, this is a great month to look at star clouds, dark nebulae, and naked-eye clusters along the “backbone of the night”.  One knot of stars is so dense and bright, Messier included it in his catalog as M24 (see the thumbnail image at the top of the page).  It’s just north and east of the lid of the “teapot” shape of Sagittarius, and south the the lovely Swan Nebula.   We’ll give you a few more ideas of things to look for in the coming issues.

The M24 star cloud (square cross-hairs, top), just below the Omega (or Swan Nebula) in Sagittarius

Like all dark nebulae, the Coalsack is a region of cold gas and dust that obscures background starlight.  It is more remarkable than most because it stands out against the bright background of the Milky Way, and covers a patch of sky some 7×5 degrees… so large it barely fits into the field of view of most binoculars.

The dark nebula called the “Coalsack” in the constellation Crux

The dark expanse of the Coalsack impressed preshistoric and renaissance stargazers.  Aboriginal stargazers have known about the Coalsack for at least 40,000 years.  Bark drawings found on Australia’s Groote Eylandt depicts the Coalsack as a fish speared by two brothers who are represented by the two brightest stars of Crux.  Polynesians called the Coalsack “Te Paniwi a Taewa”, the Black Fish.  Vespucci called it the “Black Canopus”.

While it looks like a patch of unoccupied space, the Coalsack is the quite the opposite.  It’s made of thousands of solar masses of gas molecules and cold dust grains which scatter starlight out of our field of view, much like dust in our atmosphere dims the setting Sun (see inset).  This nebula spans about 50 light years at its widest.  But it’s not a single, simple blob.  By studying light extinction of dim backgrounds stars, astronomers conclude the Coalsack is two overlapping dust clouds 610 and 790 light years away.  And it’s not completely dark.  Ground and space-based observation show that dust grains of the nebula reflect a modest amount of blue and ultraviolet light from nearby stars.

In good sky, the Coalsack presents a grand sight for stargazers without optics.  But it’s worth exploring with binoculars, and even with a telescope at low power.  Look for structure among the blackness.  Keen-eyed observers see a rib-like structure in the Coalsack, especially in the area nearest Acrux.

Look also for the small, dim star cluster NGC 4609 about 2 degrees east-northeast of Acrux.  It lies eight times farther away than the Coalsack, yet its young stars pack enough punch to remain visible through this interstellar cloud.