Thursday, July 30, 2009

Gobs of Globs

Note: There will be no August 6 post so that I may prepare for a She Is An Astronomer outreach event. Enjoy the July 30 post or browse my older posts. I'll be back with a new post on August 13.

Last week we looked at a bright open cluster in the summer constellation Ophiuchus the Snake Handler. Now let’s look at a different type of star cluster of which Ophiuchus contains a plethora: the globular cluster.

Globular clusters, or “globs” as they are commonly known, are dense balls of gravitationally bound stars. There are at least 150 known globs in our home galaxy, the Milky Way. Globs contain tens of thousands to millions of stars.

A number of the brighter globs that can be spotted from the Northern Hemisphere are found in the Messier catalog (MESS-ee-yay). This is the catalog of deep-sky objects compiled by the famed 18th century French astronomer Charles Messier, who observed from the rooftops of Paris. The Messier catalog contains some of the finest binocular and telescope objects in the night sky and is widely used by amateur astronomers as an observing list.

With seven each, Ophiuchus (oh-fee-YOO-kuss) and Sagittarius contain more Messier globular clusters than any other constellation.

If you’re not sure how to locate Ophiuchus, read last week’s post. The map below shows the locations of the Messier globs that call the Snake Handler “home.” Try spotting them with binoculars from a dark site, orienting from the Coffin asterism in Ophiuchus. Most should be visible as small fuzzy balls in modest-sized binoculars like 7x35s or 7x50s. The globs M9 and M107 will be tough to impossible, so don‘t bother with those. Use the binocular targeting tip I’ve discussed before.

The Messier catalog globular clusters in Ophiuchus
Star maps created with
Your Sky

Now try observing them all with a telescope, if you have one. Depending on how much aperture (diameter of your primary lens or mirror) you have, you’ll either see a larger, brighter fuzzy ball or you’ll begin to resolve the cluster, that is, separate the stars into distinct points of light. Once resolved, you’ll see why I call globular clusters “galactic sugar piles.”

One theory among astronomers is that globs are the cores of ancient galaxies that collided with the Milky Way. All the outer stellar material was ripped from the galaxies and shredded by our home galaxy's powerful gravitational force. What remains are the densely star-packed galactic cores, studding the Milky Way like trophies on a hunter’s wall.

It’s a jungle out there.

Astronomy Essential: There is gravity in space.

There is gravity everywhere, since it's produced by all objects with mass in the universe. Its force, however, decreases with distance.

The “weightlessness” or “Zero G” (zero gravity) that an astronaut experiences in space is caused by the balancing act between the centrifugal force on his body and the gravitational force on his body.

In other words, Earth’s gravity— although a little weaker at that distance— is still pulling him towards the planet. However, the centrifugal force of his orbiting spacecraft propels him outward, away from the center of rotation (Earth), thus canceling out the effect of the gravity.

Friday, July 24, 2009

Caroline and the Cluster

If you ask an amateur astronomer to name a famous female astronomer, the first one who leaps to mind will most likely be Caroline Herschel (1750 to 1848). Sister to the renowned astronomer and Uranus discoverer William Herschel, she was an accomplished observer in her own right.

Although born in Germany, William and Caroline emigrated to England, where William became the royal astronomer to King George.

Caroline’s discoveries included eight comets; in fact she was the first woman to discover a comet. She also discovered 14 deep-sky objects, and she catalogued many stars and nebulas. The diminutive Caroline’s primary instrument was a modest-sized reflecting telescope, around four inches in diameter.

Let’s take a look at the brightest deep-sky object she discovered: the open cluster IC 4665 in Ophiuchus the Snake Handler. It is appropriate that we search for it in July, as she discovered it on July 31, 1783.

1) About an hour after your local sunset time, 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.

Looking south to Scorpius and Ophiuchus
Star maps created with
Your Sky

2) Locate the distinctive curve of the scorpion’s body, just above the southern horizon. Ophiuchus (oh-fee-YOO-kuss) is a large constellation that essentially rides on the back of the scorpion. So look north of the scorpion for a large asterism (star pattern) known as the Coffin, which comprises the brightest stars in Ophiuchus. It’s known as the Coffin because it resembles an old-fashioned casket with a pointed head. We’ll take a closer look at the Coffin stars in a future post.

The Coffin asterism of Ophiuchus with open cluster IC 4665

3) Point your binoculars or small-aperture telescope just above the star Cebalrai (SEH-buhl-rye) near the top of the Coffin. You don’t want to use a large-aperture (large-diameter) telescope or a lot of magnification for an object like this with a large apparent size (the amount of sky it covers from our perspective on Earth).

Can you spot the open cluster IC 4665? An open cluster is a loose collection of stars that formed around the same time in the same nebula (gas and dust cloud). You might want to think of an open cluster as a sort of family group. IC 4665 has around 30 members in its family.

Open cluster IC 4665
Image credit: Digital Sky Survey

Hats off and a tip of the teapot to Caroline Herschel, astronomer.

Astronomy Essential: Shooting stars are meteors.

The phenomenon we call a “shooting star” is actually a bit of space rock or dust called a meteoroid burning up as it enters Earth’s atmosphere. The streak of light produced as the meteoroid is incinerated is called a meteor. Most meteoroids are very small: the size of grains of sand or grains of rice. It is the high entry velocity of the particles that produces the lovely light show.

Occasionally, larger meteoroids will survive their high-speed encounter with Earth’s atmosphere and hit the ground; these rocks are known as meteorites.

Thursday, July 16, 2009


Monday, July 20, 2009 marks the 40th anniversary of the first Moon landing. It’s hard to believe it’s been only 40 years since astronauts Neil Armstrong and Buzz Aldrin left their historic footprints on the Moon. It seems like eons ago, although I’m not sure why. Perhaps it’s because so much has transpired since then in the arena of space exploration.

Source: NASA

Humankind has launched numerous space exploration missions since 1969, taking our technology, if not our bodies, to the edge of our solar system. Add to that the prolific output of the Hubble, the Chandra, and the Spitzer space telescopes: spectacular, surreal images of deep-space objects in the Milky Way, along with those beyond our home galaxy, billions of light years away.

Sometimes it feels as if I’m comfortably seated on the observing deck of a swift spacecraft, while brilliant scientists and engineers sweat it out in the boiler room in order to incrementally extend my view out the picture window.

Of course, my little conceit stems from our common, virtual experience of space. We are for the most part a species of armchair travelers, exploring the universe from the comfort of terra firma. Just a few brave souls suit up to ride rockets to the International Space Station and back. Only six times have humans landed on the Moon. Only six times have any of us looked back at Earth while standing on another world.

In 40 years, yes, our vision has penetrated far into the universe. But our footprints have not.

The Sea of Tranquility provided the site for the historic Apollo 11 moon landing in July 1969. You can easily spot this large dark feature, a lava-filled crater, with the naked eye.
Give it a try when the Moon is waxing (growing).

Astronomy Essential: A galaxy is a large system of stars and planets, bound by gravity and rotating around a dense core.

The word galaxy (GA-leck-see) derives from the Greek word for milk. Galaxies are typically categorized by their shape and/or their orientation to Earth. Some of the shape-based categories are spiral (like our home galaxy, the Milky Way), elliptical, lenticular, and irregular. Common orientation categories are face-on and edge-on.

There are an estimated 200 billion galaxies in the universe.

Thursday, July 9, 2009

The Riddle of the Snake

Fans of astronomy trivia may know the answer to this riddle: what’s the only constellation that is both one and two?

Answer: Serpens the Snake. The celestial snake imagined by the ancients struggles in the unyielding grasp of Ophiuchus the Snake Handler, winding between his legs. When 20th century astronomers were laying out the boundaries of the modern constellations, they kept the ancient figure of Ophiuchus (oh-fee-YOO-kuss) intact and divided Serpens into two parts: the head, on the Snake Handler’s left, and the tail, on the Snake Handler’s right. Thus the intertwined figures of old begat the only modern constellation with two disconnected parts.

Ophiuchus and Serpens in John Flamsteed’s 1729 star atlas
Courtesy of
Linda Hall Library of Science, Engineering and Technology

Modern constellation boundaries of Ophiuchus and Serpens (in green)
Star maps created with
Your Sky

Let’s aim for the head.

1) About an hour after your local sunset time, 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.

Looking south

2) Just under the horseshoe-shaped asterism (star pattern) of the constellation Corona Borealis (which is located on or near the meridian) and east of the bright star Arcturus, look for a small triangle of stars. This is the head of the snake. If your vision is keen and your skies dark, you may also spot the two fainter stars above the triangle. Add them to the other three and you’ll have an “X” that marks the spot where the snake’s head lies.

3) Due south of the snake’s head is a star that’s brighter than any of the “X” stars. This is Unukalhai (uh-NOO-kuh-lye), an orange giant and the brightest star in Serpens. Unukalhai is from the Arabic for serpent’s neck, the star’s location in the snaky figure of antiquity.

4) This section of Serpens containing the head and the neck is known as Serpens Caput, to distinguish it from Serpens Cauda, the section east of Ophiuchus. Serpens Caput (SIRR-penz KAH-put) is Latin for snake head, while Serpens Cauda (SIRR-penz COW-duh) is Latin for snake tail.

Serpens Cauda is a tail for another day.

Astronomy Essential: A nebula is a cloud of gas and dust in space.

The word nebula (NEBB-yoo-luh) is from the Latin word for cloud. In general, there are four types of nebulas or nebulae (NEBB-yoo-lee): diffuse nebulas, planetary nebulas, supernova remnants, and dark nebulas.

Diffuse nebulas are stellar nurseries, gas and dust clouds where stars are born. Planetary nebulas are the remains of average-sized stars (like our Sun) that have died. Supernova remnants are the remains of massive stars that died in cataclysmic explosions. Dark nebulas are gas and dust clouds where the dust is so thick it completely obscures the light from the stars behind it.

Thursday, July 2, 2009

A Pair of Ragged Claws

One of the signature sights and true delights of summer stargazing is the curvy constellation Scorpius. But preceding the scorpion in the sky are its long-lost appendages: the Claws. It was the ancient Romans who reportedly hacked them off in order to make a twelfth zodiac sign, Libra the Scales.

Scorpius and Libra in Alexander Jamieson’s star atlas
Courtesy of
Linda Hall Library of Science, Engineering and Technology

Let’s reunite the celestial arachnid with its pincers.

1) About an hour after your local sunset time, 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.

Looking south to Libra
Star maps created with
Your Sky

2) Just to the right (west) of the arc of stars that marks the scorpion's head is a quadrilateral of stars— two bright and two dim. The brighter ones mark the tips of the Claws. The bright one closest to the southern horizon is the star Zubenelgenubi (zoo-BENN-uhl-jenn-NOO-bee). Its name is from the Arabic for southern claw. It’s actually a double star that may be “split” with the naked eye, that is, you should be able to see the stars as two separate points, if you have relatively good eyesight and you’re in a dark location that’s relatively free of light pollution.

"The Claws" in Libra

3) The slightly brighter star above Zubenelgenubi is the blue-white dwarf star Zubeneschamali (zoo-BENN-esh-uh-MAH-lee), from the Arabic for northern claw. The luminous Zubeneschamali is about 130 times brighter than our Sun.

4) Of the two dimmer stars in the quadrilateral, only the southern one has a traditional name: Zubenhakrabi. Zubenhakrabi (zoo-BENN-hock-RAH-bee) is from the Arabic for scorpion’s claw. It’s a red giant star.

We call the northern star Gamma, for its star catalog designation. Gamma is an orange star.

It's interesting to note that Libra is the only constellation of the zodiac that represents an inanimate object. It may also be the only constellation whose brightest stars present a devilishly difficult trio of tongue twisters.

Astronomy Essential: Gravity has shaped our universe.

Gravity is one of the four fundamental forces known in the universe. Although the weakest of the four, it has shaped the large-scale structure of our universe by initiating star and galaxy formation.

Gravity pulls objects together. Every object produces a gravitational force that attracts all other objects. The more mass (quantity of matter) an object has, the stronger its gravitational force.

The gravity that causes a gas cloud in space to compress until it becomes a star is the same force that keeps us attached to the surface of planet Earth. The gravity that causes stars to congregate into massive collections called galaxies is the same force that keeps Earth in orbit around the Sun.