D e s e r t E x p o s u r e
August 2010
Constellation of the Month: Corona Australis, the Southern Crown
Many of the constellations in the southern part of our sky were created when the European explorers went south of the equator, beginning in the Renaissance, and first saw the stars of the southern sky. Going south, our constellation of the month, Corona Australis, or the Southern Cross, is the last of the ancient constellations before we enter the realm of modern constellations with Telescopium being just south of it. Corona Australis has Sagittarius just to its north and was originally thought to represent the crown worn by the Centaur Sagittarius. In ancient times, this constellation was sometimes referred to as "Corona Sagittarii." But in Greek star maps, this constellation is shown more as a wreath than a crown. Corona Australis was in Ptolemy's list of the 44 ancient constellations, the Almagest.
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Click the image for a larger map
Corona Australis sits in our southern sky just below Sagittarius. This little circlet of stars was once part of Sagittarius, but was finally separated out by the time the IAU created the official 88 constellations. This little constellation has the closest neutron star and the globular cluster NGC 6541. The constellations to the north of Corona Australis have ancient mythological names and the ones south have the names of modern scientific instruments, like Telescopium. |
A different myth is that Corona Australis is said to be the crown Dionysus tossed into the sky after he freed his mother, Semele, from Hades. But the same myth has been associated with the constellation Corona Borealis (the Northern Crown) as well.
Even though this is an ancient constellation, the stars are all rather faint, with the brightest being Beta Coronae Australis, an orange giant shining at magnitude +4.09. The star that Johann Bayer thought was the brightest when he assigned his Greek alphabet designations, Alpha Coronae Australis, is a slightly fainter +4.11. It is the constellation's only named star (Alphekka Meridiana).
Another star in this constellation, which can only be seen by the largest telescopes, is RX J1856.5-3754. The star number is derived from its location in the sky as measured in Right Ascension (18 hours 56.5 minutes) and Declination (-37 degrees 54 minutes). This star is a very faint magnitude +25.6, but is bright in the X-ray portion of the spectrum. Astronomers believe this is a neutron star that is only 400 light years away from us, making it the closest neutron star to Earth. What makes it even more interesting is that this neutron star is traveling alone in space, not as part of a binary star system like most neutron stars we can detect. Its closeness makes it possible for us to study it more closely.
This star was first found by the Roentgensatellit (ROSAT), a joint German, US and British orbiting X-ray observatory, in 1992. Additional observations have been made by the Hubble Space Telescope and the Chandra X-Ray Observatory. Combining these observations, astronomers have deduced that this neutron star is just 18 miles in diameter. Packed into this 18-mile sphere is 1.4 times the mass of our Sun, something that could not happen if it were composed of normal matter.
RX J1856.5-3754 is actually composed of "degenerate" matter. During a supernova explosion, the electrons and protons in the core of the progenitor star are crushed into each other, forming neutrons that join the already existing neutrons in the core. This mass of neutrons, no longer held apart by electromagnetic force, is packed tightly together s by the star's intense gravity; the resulting material is so dense that a sugar-cube size lump would weigh 1.4 trillion pounds.
The temperature on the surface of this neutron star is about 1.3 million degrees Fahrenheit, compared to the surface of our Sun at a paltry 10,800 degrees. When the light from this star is analyzed, the dark and bright lines that usually appear in the spectrum of a star cannot be found. Additional analysis indicates that there is a very strong magnetic field coming from the neutron star that blurs the normally visible lines so much that they cannot be seen.
RX J1856.5-3754 is the brightest and closest of the Magnificent Seven, a group of nearby neutron stars. They were all discovered by the ROSAT orbiting X-ray observatory.
The Planets for August 2010
Mercury finishes the apparition it started last month in our evening sky. Mercury will actually spend the month in Leo, except for two days when it clips the northeast corner of Sextans on August 5-6. It will be low in the west as it starts to get dark, barely nine degrees up on August 1. By midmonth, it will have headed back toward the Sun and be lost in the evening twilight. On August 1, it will be 55% illuminated and moving toward a crescent phase. It is 7.0 seconds-of-arc across and growing larger, glowing at magnitude 0.2. Mercury sets around 9:15 p.m.
Saturn is also in the west as twilight ends and sets around 9:45 p.m. The Ringed Planet moves slowly eastward among the stars of western Virgo, and its disc is 16.1 seconds-of-arc across at midmonth. The Rings are 36.4 seconds-of-arc across, tipped down 2.5 degrees with the northern face showing. Saturn shines at magnitude 1.1. Mars passes two degrees south of Saturn on August 1, and Venus passes three degrees south of the Ringed Planet on the evening of August 9.
Venus spends the month in Virgo, setting just before 10 p.m. At midmonth it is well up in the west as it gets dark, shining at magnitude -4.3. The disc is 50% illuminated and 23.3 seconds-of-arc across. Venus will catch up to Mars on August 23, passing two degrees to its south. Venus will be very near the first-magnitude star Spica on the last day of the month. You can observe these three as they perform their graceful dance in our evening sky. Watch them each evening and note where they are relative to each other.
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The God of War also spends the month in Virgo, 20 degrees up in the west-southwest as it gets dark and setting around 10 p.m. Mars' disc is 4.5 seconds-of-arc across and continues to fade at magnitude 1.5. Mars will pass Spica early next month.
Jupiter rises around 9:40 p.m. on the eastern horizon, and is visible the rest of the night. The second-brightest planet in our sky is magnitude -2.8 at midmonth, its disc 47.6 seconds-of-arc across. The King of the Planets is moving slowly westward among the stars of central Pisces.
The annual Perseid meteor shower occurs this month just after New Moon, so this will be a relatively favorable year to view these meteors, with only a small crescent Moon confined to the evening sky. These meteors are tiny particles from the comet P/109 Swift-Tuttle, "burning up" as they hit our upper atmosphere at a speed of 40 miles per second. The bad news is that the peak of the shower occurs in the evening for us, so the best time for viewing these meteors is in the hour before morning twilight begins on the mornings of August 12 and 13. The best way to view them is to sit in a folding lounge chair facing northeast. So stay warm on those cool August mornings 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.