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  D e s e r t   E x p o s u r e  May 2011



Chamaeleon, the Chameleon

Eighteen degrees below our southern horizon on May evenings, you will find the constellation Chamaeleon, the Chameleon. It is just north of Octans, the home of the South Celestial Pole, and is among the 12 constellations created by Petrus Plancius around 1597. This lizard constellation was first featured in a star atlas in 1603 when Johann Bayer published his Uranometria.

starry dome

Click the image for a bigger map

Chamaeleon is a far southern constellation not visible from the desert southwest. The edge of the Milky Way passes through the constellation. There are three dark clouds in this area, Chamaeleon I, II, and III. One and Two are forming stars, but three seems to be devoid of star formation

As seen from Earth, Chamaeleon is at the edge of the Milky Way, about 60 degrees from the center of our galaxy. The Milky Way's equator is tipped 62 degrees relative to the celestial equator (the projection of the Earth's equator onto the sky), and the Milky Way is 30 degrees wide, so its edge comes within 10 degrees of the South Celestial Pole — and passes through Chamaeleon. Since we are looking nearly along the disc of our galaxy, there are a number of interesting objects in Chamaeleon.

This area of the sky contains a group of dark molecular clouds, occupying most of Chamaeleon and stretching into the constellations Apus, Musca and Carina, called the Chamaeleon complex. The Complex is a large star-forming region, where the dust and gas that make up the clouds slowly collapse into denser knots of gas that eventually become young stars.

These clouds are called "molecular" because they are relatively cold and the atoms have combined much as on Earth to form molecules like cyanogen (CN), methane (CH4), hydrogen (H2), carbon dioxide (CO2) and even water (H2O). These molecules can form here because stars are few and far between in this region. Stars emit energy, much of it in the ultraviolet part of the electromagnetic spectrum, which has more energy than visible light — enough to split molecules back into individual atoms. (Intense ultraviolet light even strips electrons off the atoms, turning the atoms into ions. When the electrons rejoin their ionized atoms, they give off energy as light, which we can see in glowing gas like the Orion or Ring Nebula.) The Chamaeleon Complex contains dark clouds, indicating they are not being bathed in ultraviolet light.

This Complex is divided into three regions. Chamaeleon I is a mere 500 light-years away, one of the closest star-forming regions to Earth, with 200-300 stars. Chamaeleon II lies somewhat behind Chamaeleon I, 580 light-years away, with about 50 stars. Chamaeleon III is even farther away and appears to have no star formation.

The Uhuru X-Ray Observatory, launched in 1970 and in operation until 1973, was the first satellite to survey the sky in the x-ray region of the electromagnetic spectrum. It detected an x-ray source in the Chamaeleon II cloud that astronomers think is connected to a very young T-Tauri star that undergoes powerful flares — much stronger than those on our Sun. If such a flare occurred on our Sun, our electricity and communications grids as well as most computer systems would be destroyed, throwing technology back into the early 1800s!


The Planets for May

Saturn stands alone in our evening sky in May, slowly approaching the bright star Porrima in Virgo as it glides westward among the stars. At midmonth, the Ringed Planet is magnitude 0.6 with a disc 18.7 seconds-of-arc across. The Rings are tipped down 7.5 degrees with the northern face showing and are 42.3 seconds-of-arc across. Saturn is slowly receding from us, having passed opposition last month.

The pre-dawn sky is very busy this month, however, with Venus, Mercury, Jupiter and Mars dancing around. On May 1, Mars, moving eastward, will pass just 0.4 degrees north of Jupiter. Next, Venus and Mercury will both be moving eastward on May 7, when the speedier Venus passes 1.4 degrees north of Mercury, not too far west of Jupiter.

Watch the Skies

(all times MDT)


May 1, 5 a.m. — Mars 0.4° north of Jupiter

May 3, 12:51 a.m. — New Moon

May 6, 6 a.m. — Eta Aquarid meteor shower peaks

May 7, 1 p.m. — Mercury furthest W of Sun (27°)

9 p.m. — Venus 1.4° north of Mercury

May 10, 2:33 p.m. — First Quarter Moon

4 p.m. — Mercury 2° south of Jupiter

May 11, 3 a.m. — Venus 0.6° south of Jupiter

May 17, 5:09 a.m. — Full Moon

May 18, 2 a.m. — Venus 1.4° north of Mercury

May 19, 7 p.m. — Mercury 2° south of Mars

May 22, 9 a.m. — Venus 1.1° south of Mars

May 24, 12:52 p.m. — Last Quarter Moon

Jupiter, Venus and Mercury will almost line up north to south. Mercury and Venus will be moving almost the same speed eastward as they pass Jupiter on May 11. Mercury passes Jupiter first, about 2.0 degrees south; 11 hours later, Venus passes just 0.6 degrees south of Jupiter. Mercury and Venus continue to race eastward, with Venus slightly in the lead.

By May 18, Mercury will have picked up speed, passing Venus only 1.4 degrees to the south, this time not too far west of Mars. May 19 sees Mercury passing 2.0 degrees south of Mars. Two days later, Venus takes its turn, passing 1.1 degrees south of Mars.

Mercury will win the race to the Sun next month, with Venus following a few months later. Jupiter will continue to move up (westward) away from the Sun, but Mars will hold its position relative to the Sun for a little while longer before it too heads west.

At midmonth, Jupiter is moving slowly eastward among the stars of Pisces. It glows at magnitude -2.1 and its disc is 33.9 seconds-of-arc across. Meanwhile, Venus travels eastward from Pisces into Aries; its 90% sunlit disk is 11.0 seconds-of-arc across at magnitude -3.9.

Mercury starts the month in Pisces as well, but it clips the northwest corner of Cetus and then heads into Aries. Moving rapidly eastward, it manages to just make it into Taurus before the month ends. At midmonth, Mercury is at magnitude +0.1, with its disc 55% illuminated and becoming fuller, but shrinking in size at 7.0 seconds-of-arc across. Finally, Mars is moving eastward also, starting the month in Pisces and moving into Aries. Mars is 4.0 seconds-of-arc across and it glows at magnitude 1.3 at midmonth.

By the end of the month, these four planets will have spread themselves across 26 degrees of sky. So enjoy this tight grouping of planets this month 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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