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.