D  e  s  e  r  t     E  x  p  o  s  u  r  e     March 2005


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Constellation of the Month: Hydra, the Water Snake

Our April constellation is the largest in the sky, Hydra, the Water Snake. Hydra is also known as a dragon-serpent or a sea monster.

Hydra did not make it into the sky because of any great deed or epic adventure, but because he was used as an excuse. Zeus had a pet raven with him at Olympus. One day he sent the raven to earth to get a cup of spring water. The raven took a cup and flew down to a nice spring. Nearby, some figs were starting to ripen, so Zeus' raven decided to wait for the fruit. Eating the figs when they were ripe delayed the raven a few more days.

While the brightest star in Hydra, Alphard, is high in the south at 9:45 p.m., you have to wait until almost midnight to see all of Hydra spread across our southern sky. Leo is above Alphard and Jupiter is near Spica, with Hydra below both. (Click the image for a larger map.)

Realizing that Zeus wouldn't be happy with his slowness, the raven cast around for an excuse. Spying a water snake, he grabbed it with his claws and, with the filled cup in his mouth, flew back to Olympus.

The raven told Zeus a story about the water snake delaying him. Zeus saw through this fabrication and threw the cup, the raven and the water snake into the sky, where they became Crater (The Cup), Corvus (The Raven or Crow) and Hydra (The Water Snake).

Hydra has also been associated with the Lernaean Hydra, a nine-headed water snake that Hercules defeated as one of his Twelve Labors. The Hydra's blood was used to tip Hercules' arrows, which became instantly fatal from even a small scratch, and which eventually killed Hercules.

Winding its way across the southern part of our spring sky, Hydra is a whopping 1,303 square degrees. With all this real estate in our sky, you'd think there would be many bright stars in this constellation, but all are rather faint. The brightest, Alphard, is but magnitude 1.98. Alphard is a giant star, so big that if it were in our Solar System, it would stretch halfway to the orbit of Mercury. Imagine seeing a sun in the sky 40 times larger than our own!

Another difference would be color: Alphard is a reddish star, making it look like an eternal sunset. Star color is a measure of the temperature on its surface. Just as a heated metal glows dull red, then red, yellow and eventually white hot, so too does the surface of a star.

If you separate the light from a star into its individual colors with a prism, you can see bright lines that are the signature of various elements (in gaseous form) on the star's surface. When astronomers first started recording the spectrum, as these stellar rainbows are called, they were not sure what all the bright lines meant and how they related to the structure of the star and its evolution.

In 1872, astronomer Henry Draper made the first photograph of a star's spectrum. He realized what a powerful tool a spectrum was and started a project to catalog the spectra of all the stars in the sky. Draper died before he could finish the project, but his widow donated a large sum of money to Harvard College Observatory to continue the project. Harvard physicist Edward Charles Pickering took on the project in 1886. He invented a way of taking the spectra of all the stars in the field of the telescope at once.

Pickering hired a group of "Lady Computers" to analyze the spectra that were coming in batches from the telescope. One woman, Annie Jump Cannon, who analyzed 350,000 spectra, came up with a classification system. Spectra had originally labeled by letters of the alphabet, starting with "A." Cannon regrouped them into seven categories, from the hottest to coolest—O, B, A, F, G, K, M—and subdivided the categories by adding a number. So our Sun is as a class G2 star, while Alphard is a K3, cooler and redder than our sun.

The Planets for April 2005

Saturn remains in Gemini, hanging high in the western sky after sunset. It continues to fade slightly, at magnitude -0.4, and to shrink (18.2 seconds of arc across at midmonth). The rings have passed their maximum tilt, and are starting to close at 23.8 degrees, with the southern face showing, and are 41.2 seconds of arc across.

Jupiter is the bright "star" in the east-southeast as evening begins. It will be in Virgo all month, shining at magnitude -2.5. Jupiter reaches its closest point to Earth ("opposition") this year on April 3, when it will be as big as it gets through a telescope, 44.2 seconds of arc across. The Moon will be near it on April 22.

Moving from Capricorn into Aquarius, Mars continues in the morning sky, rising about 3:45 a.m. Still too small for any significant observing, Mars is 6.3 seconds of arc across and magnitude +0.8.

After appearing in the evening last month, Mercury will be in the morning sky through most of April. Mercury will best be visible later in the month, but will not be as high in the sky as it was during March. It's highest on April 26, when it will be magnitude 0.5.

The Sun will be grazed by the Moon late in the afternoon of April 8 in what for the desert southwest will be a very partial solar eclipse. The eclipse will begin at 3:20 p.m. and reach maximum at 4:02 p.m., when almost seven percent of the Sun will be covered. The eclipse ends at 4:42 p.m. The central line of this total-annular eclipse runs mostly through the southern Pacific Ocean, but also crosses Panama and Colombia, and ends in Venezuela. Be sure not to look at the Sun without proper protection. For further information on safe eclipse viewing, see sunearth.gsfc.nasa.gov/eclipse/SEhelp/safety.html online. Enjoy the warmer weather and "keep watching the sky"!

An amateur astronomer for more than 35 years, Bert Stevens is
co-director of Desert Moon Observatory in Las Cruces.

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