Thursday, May 28, 2009

The Pond

Last week, we looked at one of my favorite spring star patterns, Three Leaps of the Gazelle. We learned that ancient Arabic star lore holds that the gazelle leapt from a spot in the sky known as the “Pond.” We can view that spot with the naked eye, as it is the well-known Coma Star Cluster in the constellation Coma Berenices (KOH-mah bare-uh-NIGH-seez).

Coma Berenices means Berenice’s Hair. Queen Berenice was a real person, a monarch of ancient Egypt. The Greek legend associated with Her Highness relates that she pledged her hair to the gods if they would keep her husband safe in battle. When he returned in one piece, she cut off her locks as promised and placed them in a temple. By the following day, they had disappeared. The court astronomer— apparently as politically astute as he may have been scientifically minded— determined that they had ascended to the heavens and could be seen as the spangled patch near Leo the Lion’s tail.

Let’s comb the sky for the queen’s tresses.

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.

Star map created with Your Sky

2) Tilt your head back and look at the zenith, the point in the sky that’s directly overhead. Near the zenith, or a bit below it toward the southern horizon, you should spot a fuzzy patch of stars. It will be slightly above and slightly east of Leo the Lion’s tail. This is the Coma Star Cluster (also known by its catalog designation Melotte 111)— what the Greeks saw as a bejeweled head of hair.

This star cluster is the most prominent naked-eye feature of Coma Berenices, and it’s the best way to tell when you’ve navigated into that particular constellation.

3) If you have binoculars or a small, wide-field telescope like the Starblast Astro, examine the cluster a bit more closely. Use your lowest power eyepiece (highest mm number) in your telescope. This is a really big object, so you don’t want to put much magnification on it.

Coma Star Cluster
© T. Credner & S. Kohle,

The Coma Star Cluster is an open cluster, a collection of stars that formed around the same time in the same nebula, or cloud of gas and dust. This cluster is believed to have around 40 member stars. You may want to think of them as a sort of family group.

Coma Galaxy Cluster
Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

Coma Berenices doesn’t really have any other notable star patterns. Its other crowning glory, the Coma Galaxy Cluster, lies hidden to the unaided eye. The dense cluster contains thousands of galaxies, some of which can be observed by an amateur with a large-aperture reflecting telescope.

Astronomy Essential: The planets orbit the Sun at different speeds.

As the planets of our solar system orbit the Sun, they travel at different speeds. The closer the planet’s orbit is to the Sun, the greater its speed. The farther the planet’s orbit from the Sun, the slower its speed. Therefore Mercury, the nearest planet to the Sun, travels at an average orbital speed of 29.7 miles per second, whereas Neptune, the most distant planet, travels at a much slower average orbital speed of 3.3 miles per second.

The acceleration of planets that are closer to the Sun occurs because the closer the planet is, the greater the gravitational force the Sun exerts on it.

The “average orbital speed” is given here because, during its orbit, each planet speeds up when it is nearer the Sun and travels more slowly when it is far from the Sun. This variation in distance from the Sun occurs because the planets travel in elliptical orbits, not perfectly circular ones.