Thursday, October 29, 2009

The Water Jar

Between the Great Square of Pegasus and the Bandanna of Capricornus lies the rather nondescript constellation of Aquarius the Water Bearer.

Aquarius was traditionally depicted as a man upending an urn and pouring water into the mouth of a fish. A bizarre, inscrutable image, perhaps, but nevertheless, the way the ancients imagined it.

Saving the Aquarius constellation from obscurity are two bright stars and a little asterism (recognizable star pattern). Let’s see if we can spot them.

Aquarius in Johann Bode’s 1801 star atlas
Courtesy of
Linda Hall Library of Science, Engineering and Technology

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 Aquarius the Water Bearer
Star maps created with
Your Sky

2) First, locate the Great Square asterism in Pegasus, high in the sky, just east of the meridian, and the Bandanna asterism of Capricornus, low in the sky, just west of the meridian.

3) Now look between the Great Square and the Bandanna for two bright stars, Sadalsuud and Sadalmelik. Sadalsuud (sah-dull-suh-OOD), from the Arabic for the luckiest of all, is the brightest star in Aquarius. Sadalmelik (sah-dull-MELL-ick), from the Arabic for the luck of the king, is the number two Aquarian. Both stars are yellow supergiants, which is a relatively rare star type.

Don’t confuse them with nearby Enif, the nose of the winged horse Pegusus. Enif is a little brighter than the best and brightest of Aquarius.

4) Just east of Sadalmelik is a little “Y” of four fainter stars. This is the Water Jar asterism, which marks the position of the upended urn in the ancient star picture. If you can’t spot all four, you’ll need to try again at a darker site.

The Water Jar asterism of Aquarius

Of the four stars, only one has a traditional name: Sadachbia. Sadachbia (sah-DUCK-bee-yah), from the Arabic for luck of the tents, is a white star. We call the other three stars Eta, Zeta, and Pi, for their star catalog designations. Zeta, in the center, is the brightest star in the asterism.

Although Sadachbia is not the brightest of the Water Jar quartet, it clearly had enough significance to the ancients to be named and to be considered lucky like Sadalsuud and Sadalmelik. Perhaps whenever the early Arabs thanked their lucky stars, they had Aquarius in mind.

Astronomy Essential: We don’t see celestial objects the way cameras do.

Beginning stargazers are sometimes disappointed by their first telescopic view of a deep-sky object such as a galaxy, nebula, or star cluster. They expect the technicolor, high-resolution panoramas seen in Hubble Space Telescope images, which have become part of our public consciousness.

Instead they get small, fuzzy, black-and-white blobs.

What technology-savvy newcomers don’t realize is that human eyes are limited instruments. Collecting and integrating the faint light from distant objects— light that has traveled many trillions of miles to reach us— pushes our eyes to the limits of their ability. And in low-light situations, we don’t see color well. Compare that to the ability of an ultra-sensitive camera chip to detect extraordinarily faint light and integrate it over a long exposure time. Add to that the ability of specialized image-processing computer software to “stack” multiple images of an object, as well as color-correct the composite to approximate the color signatures of the chemical elements found in each celestial object.

What we can’t accomplish at the telescope eyepiece, we can accomplish at the computer keyboard.

At the end of the day, however, there is no substitute for looking at an astronomical object with your own eyes, collecting on your retinas those faint photons of light that have traveled many years to reach you, and discerning an image of a far-away world in real time, live, under your own power.

Thursday, October 22, 2009

The Little Horse Head

Tucked between Pegasus the Winged Horse and Delphinus the Dolphin is the second smallest constellation in the sky: Equuleus the Foal. Equuleus (ee-KWOO-lee-yuss) is in fact the smallest constellation that can be seen from most latitudes in the Northern Hemisphere.

Like neighboring Pegasus, Equuleus is not a whole horse. Although the entire front half of Pegasus— head, front legs, torso, and wings— soars overhead, poor Equuleus must make do with a disembodied head. Make that an inverted disembodied head. Like Pegasus, Equuleus is upside down in the sky.

First mentioned by the Egyptian astronomer Ptolemy in the second century CE, the origin of the little horse head in the sky is unknown.

Pegasus, Equuleus, and Delphinus in J. Middleton’s 1842 star atlas
Courtesy of
Linda Hall Library of Science, Engineering and Technology

You won’t need a carrot or a sugar cube to coax the stellar foal into view. You will, however, need a dark-sky location and you’ll need to be dark adapted, because his stars are not terribly bright.

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 Equuleus the Foal
Star maps created with
Your Sky

2) First locate the Great Square of Pegasus asterism (star pattern) high in the southeast, and jutting out from it to the west, the right angle asterism that represents the neck and head of Pegasus. The star at the end of the right angle is Enif (EE-niff) which is from the Arabic for nose. Enif marks the snout of Pegasus.

3) Next locate the Dolphin asterism, the prominent star pattern in the little constellation of Delphinus the Dolphin (dell-FINE-uss). The head of Pegasus, punctuated by Enif, points to the Dolphin, which is currently on or near the meridian about an hour after sunset.

4) Now that you have your two celestial landmarks identified, look midway between Enif and the Dolphin— and then slightly south— to locate the four-star quadrilateral asterism that comprises the brightest stars in Equuleus. This asterism is known as the Horse’s Head.

5) The brightest of the four stars is Kitalpha (kitt-AL-fuh), from the Arabic for section of the horse. Kitalpha is a binary star, that is, a system of two stars in orbit around each other. In Kitalpha’s case, the two stars are a yellow giant and a white dwarf. Because of their close proximity from our vantage point, we see their combined light as one star. These two are so close, you won’t even be able to split them in a telescope.

6) The other three stars have no traditional name, so we call them (moving clockwise around the quadrilateral from Kitalpha) Beta, Delta, and Gamma for their star catalog designations. Delta is interesting in that it is also a binary star, with its two components a yellow-white dwarf and a yellow dwarf, the latter quite like our Sun. If you have a telescope, try splitting the two components.

Delta and Gamma mark the snout of the upside-down foal‘s head. He surely must be related to Pegasus. He’s a chip off the old block.

Astronomy Essential: The universe is 13 billion years old.

Currently the best estimate as to the age of our universe is 13 billion years (and change). In other words, 13 billion years have elapsed since the Big Bang.

Astronomers calculate this primarily by: 1) determining the age of the oldest stars, and 2) measuring the rate at which the universe is expanding and then extrapolating that back to the compressed state of the Big Bang.

In turn, identifying and dating the oldest stars is dependent upon what astronomers’ have learned thus far about star mass and about the life cycles of stars.

Measuring the universe’s rate of expansion is dependent upon astronomers’ knowledge of the current density and composition of the universe. In addition, they can peer back in time by observing and mapping the cosmic microwave background radiation, the afterglow of the Big Bang and the oldest light in the universe.

Thursday, October 8, 2009

Get Out Your Bandannas

Note: There will be no October 15 post. Enjoy the October 8 post or browse my older posts. I'll be back with a new post on October 22.

Let’s wrap up our exploration of Capricornus the Sea Goat with a telescopic look at one of my favorite deep-sky objects: the globular cluster Messier 30.

Globular clusters, or “globs” as they are called in the astronomy community, are dense balls of gravitationally bound stars. There are at least 150 known globs scattered about 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 110 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.

Globs are my favorite type of deep-sky object, and Messier 30— or M30 as it is commonly known— is one of my favorites because of its irregular shape. I find the globs that are a bit asymmetric, rather than perfectly spherical, to have the most visual appeal.

Use the star map below to locate M30, just to the left (east) of the Bandanna asterism. If you have a dark sky (Moon-free and light pollution-free), try spotting it with binoculars first. If successful, you’ll see a small fuzzy ball, something like the end of a Q-tip.

Messier 30 in Capricornus
Star maps created with
Your Sky

But honestly, M30 is one of those objects that is best examined in a telescope. Depending on your aperture (diameter of the primary optical element)— which dictates your telescope’s ability to collect light from faint celestial objects— you may begin to resolve the cluster. If so, the stars in the cluster will begin to separate into distinct points of light, and you’ll be able to truly appreciate M30’s endearing quirkiness.

Once you’ve located M30 with a low-power eyepiece (high mm number), switch to a high-power eyepiece (low mm number) and pump up the volume. This object warrants the most magnification you can muster.

Globular cluster M30
Source: NASA/ESA

Honk if you love globs. Or just wave your bandanna.

Astronomy Essential: Human eyes can adapt to the dark.

Seasoned night-sky observers typically go through a ritual called dark adaptation prior to embarking on an evening of stargazing. Dark adapting involves avoiding all white-light sources for 20 to 30 minutes, which prepares their eyes for enhanced seeing in the dark.

In a dark environment, first the human eye responds with pupil dilation. Next, chemical changes occur in our retinas’ receptors: the rods and cones. The rods become super-sensitized to light, thereby enabling us to see in low-light conditions.

As you can imagine, dark adapting— and then avoiding bright light once adapted— results in greater success seeing faint celestial objects through binoculars or telescopes, as well as with the naked eye. Astronomers maintain their dark adaptation by using red-light flashlights, since red light does not interfere with night vision.

Thursday, October 1, 2009

The Sea Goat Revisited

In my previous post I introduced you to Capricornus the Sea Goat and its defining asterism (star pattern), the Bandanna.

This time, let’s take a look at the more prominent stars of the Bandanna— the ones with traditional names. They all lie along the top of the Bandanna. If you need help locating Capricornus, review my previous post.

The top left corner of the Bandanna is marked by Deneb Algedi, from the Arabic for tail of the goat. Deneb Algedi (DENN-ebb uhl-JEDD-ee), the brightest star in Capricornus, is a white star that lies about 39 light years away. A light year is the distance light travels in one Earth year, nearly six trillion miles.

The Bandanna of Capricornus
Star maps created with
Your Sky

Moving right (or west), the next bright star is Nashira (nah-SHEE-rah). Its precise meaning in Arabic is unclear, but it references “luck.” Nashira is also a white star, but it lies nearly 100 light years farther away than Deneb Algedi.

Traveling across the top of the Bandanna toward the upper right corner, we next come to Dabih. Dabih (DAH-bee) is from the Arabic for luck of the slaughterer, a reference to the seasonal sacrifice made by Arabs when Dabih first appeared in the dawn sky before sunrise. Dabih is a binary system, that is, two stars in orbit around one another. The brighter of the pair is an orange giant, and the companion star is a blue-white giant. You should be able to “split” Dabih, that is, separate the two component stars, using binoculars.

The upper right corner of the Bandanna is marked by Algedi, Arabic for goat. Algedi is also two stars, but unlike Dabih, the two components are not gravitationally associated. They simply appear close together by line of sight from Earth. This type of double star is called an optical double. Coincidentally, both components of Algedi are yellow stars, similar in temperature to our Sun.

If you have keen eyesight, you should be able to split Algedi with the naked eye. If this proves difficult, the magnification provided by a pair of binoculars will bring you certain success.

Astronomy Essential: Constellations are three-dimensional.

We experience the night sky as if it’s an upside-down bowl, painted black, with an artistic sprinkle of luminous white dots. In other words, we experience it as a two-dimensional plane.

The connect-the-dot star pictures of the ancients, persisting across the centuries, do nothing to disabuse us of this notion.

We stargazers must, therefore, make a conscious effort to remember that the constellations and asterisms we use to navigate the sky are three-dimensional. Although we perceive a particular star pattern in two dimensions, the stars of that pattern are distributed in three dimensions. Some of the stars are relatively close to us, and some are relatively far from us.

If we could fly out into space and “through” a particular constellation, we would experience this first-hand. Zipping past each star in turn, we would finally see the third dimension— the depth— of space. We would finally understand that constellations and asterisms and any other patterns our brain seeks to make from random star positions are simply the artificial constructs of Earth-bound life forms with active imaginations.