Thursday, February 26, 2009

Messier’s Lagniappe

As a young child growing up in Louisiana, I couldn’t help but assimilate a few Cajun words into my lifelong vocabulary. One of my favorites is lagniappe. A lagniappe (LANN-yapp) is a little something extra for free. Consider the thirteenth donut in a baker’s dozen, and you’ve got the idea.

Everybody likes something for free. And for stargazers, a freebie in the sky is simply irresistible.

In the 18th century, the French astronomer Charles Messier (MESS-ee-yay) produced a catalog of over 100 celestial objects that might be mistaken for comets. He was a dedicated comet hunter and didn’t want to waste time looking at fuzzy things that weren’t comets. The resulting Messier catalog has become a staple of amateur astronomy and is typically the first observing list new telescope owners work their way through.

Charles Messier

The catalog object Messier 46, also known as M46, lies in the constellation Puppis the (ship’s) Stern. Puppis (PUH-piss) lies east and south of the well-known winter constellation Canis Major (KAY-niss) the Big Dog, prominent now in the southern sky. M46 is a visually pleasing star cluster— with a lagniappe.

For this exercise, I’d recommend a reflector telescope with at least six inches of aperture (mirror diameter). Let’s go collect our cosmic freebie, shall we?

1) Wait at least one hour after sunset to begin observing, so your sky is good and dark.

2) Face south. If you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your right shoulder to the west, and you’ll be facing approximately south.

Star maps created with Your Sky

3) First locate the Hourglass asterism (recognizable star pattern) of the constellation Orion the Hunter, on or near the meridian. It hangs halfway to two-thirds of the way up from the southern horizon toward the zenith (the point directly above your head). Now locate Orion’s Belt, a diagonal line of three evenly-spaced stars cinching the middle of the hourglass.

4) Now let’s locate Canis Major. The stellar landmark that helps us find the Big Dog is blue-white Sirius, the brightest star in the night sky.

Find Sirius by drawing an imaginary line through the three stars of Orion’s Belt, moving from upper right to lower left, and continuing on until you come to a very bright star. This is spectacular Sirius.

Because Sirius (pronounced SEER-ee-us) follows Orion across the sky, ancient peoples called it the Dog, and it has come to be known as the Dog Star. In ancient star lore, Canis Major was considered one of Orion’s hunting dogs. Sirius marks the location of the dog’s snout.

5) East or left of Sirius, about double the distance between Sirius and the star Mirzam (which is just to its right, or west), lie two star clusters, M46 and M47. Both are open clusters. An open cluster is a loose collection of stars that formed around the same time in the same nebula (cloud of gas and dust).

Point your scope at the spot shown on the map above. Of the two clusters, M46 is the one farthest east. Both clusters will be around the same size in your field of view, but M47 is a bit brighter. However, M46 is our target, and you’ll know you’re looking at the correct cluster when you spot what looks like a fuzzy donut or smoke ring in it. This is the planetary nebula NGC2438— and our lagniappe.

A planetary nebula is the nebula formed when an average-sized star dies and ejects a gaseous shell of stellar material into surrounding space. Despite their name, planetary nebulas have nothing to do with planets; some of them simply resembled planets when viewed in earlier telescopes.

In the case of NGC2438, our dying star does not belong to the M46 cluster. Although we say it is “in” M46, the nebula is actually a foreground object, lying a little more than halfway to the cluster. At several billion years old, the disintegrating star is much older than the relative youngsters of the cluster. The stars of M46 are estimated to be 300 million years old.

M46 with planetary nebula in foreground

Although the American French word “lagniappe” didn’t come into use until the mid 19th century— nearly 30 years after Messier’s death— I like to call the planetary nebula in M46 “Messier’s Lagniappe.” I think that would have made the good Frenchman smile.

Astronomy Essential: There’s a black hole at the center of our galaxy.

It’s a supermassive black hole, to boot, estimated at four million times the size of our Sun. A black hole is an extremely dense object with gravity so strong that not even light can escape it.

Our Milky Way galaxy is not, at its heart, unique. Black holes are thought to reside at the center of most galaxies. Astronomers think that black holes may be a critical component in galaxy formation, providing the mass needed to create an environment where stars can form.

You’ll never see our galaxy’s black hole, because its extreme gravity traps the light that might illuminate it. Astronomers have to watch the behavior of stars near it to infer its existence. And at 27,000 light years away, you’re not likely to make it there in your spaceship. Just one light year is nearly six trillion miles!

Thursday, February 19, 2009

Evening Star

Hanging high in the western sky at sunset is a brilliant star-like object that incites rubbernecking and double-takes. This marvel is the planet Venus.

The convention of referring to planet Venus as both the “Evening Star” and the “Morning Star” began long ago. The ancients, confused by what they observed at different times of the year, thought Venus was two separate bodies— two “wandering stars,” as planets were originally known. Whether we see Venus in the evening after sunset or in the morning before sunrise depends on where it is in its orbit around the Sun— to the “left” (east) or the "right" (west) of the Sun from our Earth-bound perspective.

In its current evening apparition, Venus will be visible in the western sky at sunset until nearly the end of March, after which it will become the “Morning Star,” visible in the eastern sky before sunrise.

Venus imaged in violet light (NASA)

Venus is the second planet from the Sun, orbiting between Mercury (closest to the Sun) and Earth (third rock). Venus is only slightly smaller than Earth. Like Earth, Venus is a terrestrial planet, that is, it's composed primarily of rock and metal. And that’s pretty much where the similarity ends.

Thick clouds of sulfuric acid blanket Venus, and its atmosphere is predominantly carbon dioxide. Its barren landscape is arid and covered with old lava flows. Air pressure at the surface is extremely high, comparable to the pressure we would experience ocean diving at 3,000 feet. This dense atmosphere coupled with the cloud blanket raise the planet’s surface temperature high enough to melt lead. An unsuspecting interplanetary tourist going ashore at Venus would be asphyxiated, crushed, and cooked in one fell swoop.

Lava flows on Venus (NASA)

But from a safe distance, Venus is an ornament for our sky, beautiful and stunningly bright. Why is Venus so darn bright? For one thing, it's the closest planet to Earth, so its apparent size (the amount of sky it covers from our perspective on Earth) is significant. For another thing, its thick cloud cover is extremely reflective, so most of the sunlight it receives bounces back.

Right now, Venus has a surprise for an observer with a telescope. Venus goes through phases, just like the Moon, and currently it’s exhibiting a lovely crescent phase. If you don’t have a telescope, beg a view from someone who does. You can also check with your local astronomy club, planetarium, or science center for scheduled observing opportunities.

Venus in crescent phase (NASA)

Venus in a crescent phase is much brighter than when the planet’s disk is more fully illuminated. This seems counter-intuitive, doesn’t it? We’re accustomed to, for example, a Full Moon being brighter than a crescent Moon. However, Venus in crescent phase is far closer to Earth than when it’s more illuminated, that is, when it’s heading behind the Sun from our perspective or re-emerging from behind the Sun. This proximity accounts for its increased brightness as a crescent.

At a recent public star party, a man took a good long look at Venus through my telescope while I explained that Venus goes through phases just like the Moon and that it was currently sporting a crescent. Finally, he looked up with a sheepish grin and said, “I thought it was the Moon."

Apparently, Venus can fool some of the people some of the time.

Astronomy Essential: Stars are nuclear reactors.

It’s true. What look like dainty little pinpricks of light sprinkled across the evening sky are in truth colossal glowing spheres. Blazing furnaces of unimaginably hot gas. Churning urns (to borrow from James Taylor) of burning funk.

A star begins as an unassuming clump of material— primarily the simple element hydrogen— inside a nebula, an immense cloud of gas and dust. Gravity causes the clump to contract until it’s a spinning sphere. Further contraction increases internal pressure and temperature until nuclear fusion ignites in the core in a complex series of energetic reactions that convert hydrogen into helium. As a by-product of these reactions, energy is emitted from the sphere in the form of light and heat, and a star is born.

Nuclear fusion is the engine that makes a star go.

Thursday, February 12, 2009

Castor and Pollux

Although the names of the two brightest stars in the constellation of Gemini the Twins date from the mythologies of classical Greece and Rome, the luminaries we know as Castor and Pollux were known by many cultures throughout antiquity and were almost always paired. They’ve been known as the Twins, the Brothers, the Sons, the Boy and Girl, the Two Horsemen, the Elder and Younger Gods, the Two Figures, the Two Infants, the Two Angels, the Two Gazelles, the Two Kid Goats, and even the Two Sprouting Plants. It’s not hard to see why.

First, the two stars are of similar apparent magnitude, that is, how bright a star appears to us on Earth. Second, they are close together in the sky, approximately four finger-widths apart, if you hold your hand at arm’s length against the sky, and measure across the knuckles. Third, there are no other bright stars right near them, so they stand out. Procyon (PRO-see-ahn), the nearest star that’s brighter than the twin stars, is about two fist-widths away, in the constellation Canis Minor the Little Dog.

Gemini the Twins in Alexander Jamieson’s 1822 star atlas
Courtesy of
Linda Hall Library of Science, Engineering and Technology

For the ancient seafaring peoples of the Mediterranean region, the appearance of Gemini in the sky signaled the approach of fairer weather. The Twins were considered to be the friends and protectors of navigation, and they were known as the Sailor’s Stars. When I see them high in the sky after sunset, I know that spring is on its way.

Let’s set a course for the bright lights of Gemini.

1) About an hour after sunset, face south. If you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your right shoulder to the west, and you’ll be facing approximately south.

Star map created with Your Sky

2) First locate the Hourglass asterism (star pattern) of Orion the Hunter, just east of the meridian. Above it, to the north, find the Pentagon asterism of Auriga. Midway between the Hourglass and the Pentagon, but over to the east, are the Gemini Twins. Pollux is on the left (farthest south), and Castor is on the right (farthest north).

3) Pollux (PAH-lucks) is the brighter of the two. An orange giant star ten times the diameter of our Sun, Pollux is of particular importance because of the planet discovered in orbit around it.

4) Castor (KASS-turr), in contrast to colorful Pollux, is a white star. But bland it is not. Look at Castor through a telescope, and you’ll see that it is actually two stars. The two stars, Castor A and Castor B, are in orbit around each other. Naked eye, we see their combined light as one star. But we’re not finished. There’s a Castor C too! You won’t see it, but Castor C is in orbit around the orbiting pair of A and B.

Wait, there’s more! Castor A, Castor B, and Castor C each have their own companion star. They are, in effect, each an orbiting pair of stars. That’s right, what looks like one star in Gemini is really six stars.

Forget the “Sailor’s Star.” I propose we call Castor the “Sextuplet Star.”

Astronomy Essential: Starlight takes years to reach your eyes.

The stars in the night sky lie far beyond the distant edges of our solar system. Even the one closest to Earth, Proxima Centauri (PROCK-simm-uh senn-TAH-ree), lies over four light years away.

Distances in outer space are vast, which is why we express them in terms of light years. A light year is the distance light travels in one Earth year, nearly six trillion miles. And as far as we know, nothing in the universe travels faster than light.

So, if you look at Proxima Centauri (you'll need to be in the Southern Hemisphere, and you’ll need a telescope), the light reaching your eyes left the star about four years ago. In other words, you are seeing the star as it existed four years ago. You might say that stargazing is a form of time travel.

The nearest naked-eye star visible to Northern Hemisphere observers (not counting the Sun) is Sirius, which is also the brightest star in the night sky, as seen from anywhere on Earth. Sirius is easily seen in the winter sky, southeast of Orion. When you look at Sirius, consider this: the light reaching your eyes left the star about eight and a half years ago. What were you doing, eight and a half years ago?

Thursday, February 5, 2009

Kemble's Cascade

In the otherwise unremarkable minor constellation of Camelopardalis the Giraffe (kah-MELL-oh-PARR-duh-liss), there is a sort of “scenic overlook” complete with a waterfall. This waterfall is an asterism (star pattern) known as Kemble’s Cascade. It is naked-eye visible only to those with keen eyesight and a dark observing site. Even so, the naked-eye view is a mere smudge compared to the stunning view through binoculars.

Grab your binoculars, and let’s go stretch our necks.

Camelopardalis in Johannes Hevelius’s 1690 star atlas
Courtesy of
Linda Hall Library of Science, Engineering and Technology

1) About an hour after sunset, face north. If you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your left shoulder to the west, and you’ll be facing approximately north.

Star maps created with Your Sky

2) To find the grazing grounds of the shy giraffe, first find the Lazy W asterism in Cassiopeia. Looking north, it should be easy to spot it, just west of the meridian. The Lazy W will be oriented upside down, so it will look more like a big “M.” Draw an imaginary line between the two end stars of the “W” or “M,” and extend it toward the southeast the same distance again. This will bring you very close to Kemble’s Cascade, in the heart of the Giraffe.

3) Another way to home in on the celestial waterfall is to locate the “backbone” of the constellation Perseus, the chain of stars known as the Segment. If you draw an imaginary line between the middle star of the Cassiopeia “M” and the star to its right, or southeast, the next bright group of stars you come to is the Segment. Locate the brightest star in the curved string of six naked-eye stars; this is Mirfak (MURR-fahk), the brightest star in Perseus.

On your way to the Segment from the Lazy W, you crossed a little fuzzy patch in the sky. This is the famous Double Cluster, a pair of star clusters whose combined light is visible to the naked eye. Spot it? If not, you’ll probably need a darker location. If you can see it, draw an imaginary equilateral triangle between Mirfak, the Double Cluster, and a point to the southeast. Now you’re within penny-tossing distance of the waterfall.

By the by, when you’re finished goggling at Kemble’s Cascade, you’ll want to return to the Double Cluster. It’s another superb binocular object.

4) Now that you know where to look, using either method, fix your eyes on the spot where you think the Cascade lies. Don’t take your eyes off that point as you slowly bring the binoculars up to your eyes. You’re going to want to, but don’t do it. If you look AT the binoculars, you will lose your focus on the target in the sky. Keep your eyes glued to your sky target until the bino eyepieces touch your face. Your target should be in the field of view or very close by. This takes a little practice, because the impulse is to glue your eyes to the binoculars first and then sweep around until you (maybe) find the object. You’ll be amazed at how efficient your targeting becomes when you master this little trick.

If you don’t see a long, lazy chain of stars (about 20-plus) in your field of view, make small and ever-widening circles with the binoculars until you spot it.

Here’s a nice image of Kemble’s Cascade that appeared on the Astronomy Picture of the Day website.

5) The sparkling cascade of starlight seems to go on forever, doesn't it? Sweep back and forth along it and look for subtle color differences. You should see some yellowish stars scattered among the white ones.

The stars of Kemble’s Cascade are unrelated. It’s just a happy accident that a nice collection of stars— distant from each other— arranged themselves in space for our stargazing pleasure.

The lovely asterism was discovered by— and named after— the late Father Lucian Kemble, a Franciscan friar and avid amateur astronomer from Alberta, Canada. He discovered it using modest binoculars: 7x35s. So, we have no excuses, eh?

Astronomy Essential: Our solar system has eight planets.

In orbit order from the Sun, the eight planets of our solar system are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Pluto was once considered the ninth planet in our solar system. However, in 2006, the International Astronomical Union (IAU), the recognized world authority for assigning designations to celestial bodies, demoted Pluto to dwarf planet status. This set off a firestorm of controversy and debate that continues today.

The definition of a dwarf planet— as set forth by the IAU— differs from that of a planet in that a dwarf planet has not cleared the neighborhood around its orbit of debris and small solar system bodies such as asteroids. For Pluto, its environment was its undoing. Its orbit skirts the edge of the Kuiper Belt (KY-purr), a region beyond Neptune’s orbit that is filled with tens of thousands of small orbiting bodies.