D e s e r t E x p o s u r e
June 2008
Constellation of the Month: Centaurus, the Centaur
Sitting right on our southern horizon in June, the constellation of Centaurus, the Centaur, appears to be galloping eastward, even as the daily motion of the stars carries it westward. It is so far south that the fabled half-man, half-horse has his legs always hidden below our southern horizon. The two "front hooves" lost from view are two of its brightest stars, Hadar and Rigel Kentaurus — the latter also known as Alpha Centauri, the closest multiple star to our solar system.
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Click the image for a larger map
Centaurus grazes along our southern horizon on June evenings. Two of its brightest stars are below our southern horizon, Alpha Centauri and Hadar. The only named star in Centaurus visible from our latitude is Menkent. Omega Centauri is located just under the "e" in Centaurus. |
Centaurus is one of Ptolemy's original 48 constellations, but its exact mythological basis is not certain. The most common identification is with the wise centaur Chiron, who was a tutor to both Jason (of the Argonauts) and Hercules and who is credited with inventing the constellations. In a battle with other centaurs, Hercules inadvertently injured Chiron with an arrow tipped with the poison blood of the Hydra. An immortal, Chiron could not die, but the poison kept the wound from healing and caused him great pain. As Chiron's suffering increased, he desired to die, but could not.
Meanwhile, the Titan Prometheus was chained to a rock, enduring unending torture, for stealing fire from Mount Olympus as a gift for humans. Prometheus' only escape was for an immortal to take his place and give up immortality. Chiron agreed; when the centaur died Zeus placed him in the heavens.
Centaurus is the ninth-largest constellation in the sky. Among its interesting objects is the Alpha Centauri star system. While Alpha Centauri is the brightest in that multiple-star system, the red dwarf Proxima Centauri actually is the closest star to our Sun. Proxima's name was suggested in 1915 by Robert Innes, then director of the Union Observatory in Johannesburg, South Africa. He determined that this star was traveling with the same motion as Alpha Centauri, so it had to be near our Sun, and suggested the name "Proxima," meaning nearby. Two years later, Dutch astronomer J. Voute determined that Proxima was indeed only 4.22 light-years away.
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Omega Centauri skimming
the southern horizon. This moonlit photo was made from Chaco Canyon using
a 5-inch telescope. Photo courtesy David Cortner. |
Another interesting object is Omega Centauri, which 2,000 years ago Ptolemy put in his catalog as a 3.7-magnitude star. Actually, however, Omega is a giant globular cluster that appears to the naked eye as a star. Its true nature was first identified in the 1830s by English astronomer John Herschel. The brightest and largest of this type of cluster in our sky, Omega Centauri is 18,300 light-years away and about 97 light-years across. It is about 12 billion years old.
This globular cluster orbits our galaxy, just like the rest of the local globular clusters (other galaxies have their own cloud of globular clusters surrounding them). What makes Omega Centauri the biggest? A black hole, some 40,000 times the mass of our Sun, at the center of Omega Centauri keeps this vast collection of stars together. Another interesting feature is that unlike other globulars, there are several generations of stars in Omega, just like in a galaxy. These facts have given rise to the theory that Omega Centauri is actually the core of a dwarf galaxy that was once several hundred times its current size, but was swallowed up by our Milky Way galaxy. The outer regions of Omega Centauri diffused into our galaxy, leaving the inner core, held together by the black hole, orbiting our galaxy as a globular cluster.
The Planets for June 2008
Cancer is the starting point for Mars this month. The Red Planet soon moves into Leo and will be just west of Regulus by month's end. Mars continues to fade, being magnitude 1.6 at mid-month. At that time, its disc will be a paltry 4.7 seconds-of-arc across and 93 percent sunlit. Mars will be a little over a third of the way up in the western sky at dusk, setting around 11:45 p.m.
Saturn is also in Leo, just a little east of Regulus. It is moving slowly eastward, away from Regulus, and sets around 12:15 a.m. At mid-month, the Ringed Planet is magnitude 0.6, and its disc is 17.2 seconds-of-arc across. The rings are 38.9 seconds-of-arc across, tilted up 9.1 degrees with the southern face showing. The rings continue to close and early next year they will be invisible.
The King of the Gods is moving westward, back toward the handle of the Teapot in Sagittarius. Rising at 10 p.m., Jupiter glows brilliantly at magnitude -2.7 as it travels from the east-southeast across our southern sky to the southwest each night. At mid-month Jupiter's disc is 46.3 seconds-of-arc across, slowly increasing as it heads toward opposition next month.
Watch the Skies
(all times MDT) June 3, 1:23 p.m. — New Moon June 7, 9 a.m. — Mercury in inferior conjunction with the Sun ; 8 p.m. — Mars 1.1 degrees north of the Moon June 8, 9 p.m. — Regulus 1.4 degrees north of the Moon June 10, 9:04 a.m. — First Quarter Moon June 18, 11:30 a.m. — Full Moon June 20, 6 p.m. — Summer Solstice; summer begins June 23, 3 a.m. — Moon goes in front of Neptune June 26, 6:10 a.m. — Last Quarter Moon |
After being in the evening sky all of May, Mercury is down near the Sun as June begins. It passes through inferior conjunction with the Sun on June 7. Toward the end of the month, Mercury will appear in the morning sky, but it will be very hard to see. At the end of the month it will be 33 percent illuminated and becoming increasingly full, magnitude 0.6. Its disc will be 8.3 seconds-of-arc across and shrinking. The Messenger of the Gods will be easier to see in July.
Summer begins in the northern hemisphere on June 20 at exactly 6 p.m., when the Sun reaches as far north of the equator as it gets this year. That day will be the longest day of the year and that night the shortest. From then on, the Sun will turn south and the days will start getting shorter.
Venus will disappear behind the Sun on June 9. This is the midway point between the 2004 Transit of Venus across the Sun, which was not visible here, and the 2012 Transit that will be visible here. The 2012 Transit of Venus will be the last one until 2117. We will be talking more about it in a few years, so in the meantime, "keep watching the sky"! k