"Let your soul stand cool and composed before a million universes" Walt Whitman

The Moon's current phase, courtesy of USNO.

Sierra NightSky for the period starting Friday, May 25, 2012 - by Jim Kaler

Skylights now resumes its normal weekly schedule.

Our Moon begins our week in a fat crescent phase as it approaches its first quarter on Monday, May 28. North Americans will see it rising in the afternoon daytime sky. In the following days, it will then grow into the waxing gibbous phase, full Moon not achieved until rather well into next week, when it will undergo an eclipse seen throughout most of the Pacific basin and western and central US and Canada.

On the morning of Tuesday the 29th, the Moon will pass seven degrees to the south of Mars (the wide angle due largely to lunar orbital tilt). On the previous evening (Monday the 28th), the lunar disk will be a bit to the southwest of the planet. Then on the night of Thursday the 31st, we will find the Moon gliding about the same angle to the south of Saturn. Since Saturn is north (and a bit east) of Spica, the three (going north, the Moon, Spica, Saturn) will lie in a nice row, the Moon rather close to the star, making it a bit difficult to see.

Venus, which has been gloriously with us during the whole of 2012 (and for some time before), is now effectively gone, at the beginning of our week already setting in late twilight. It is preparing to sweep across the face of the Sun in a rare transit that for North Americans will start late in the afternoon on Tuesday, June 5, at 5:05 PM CDT, and for the contiguous US and most of Canada will run until sunset. Optical aid is needed, and since the Sun is so bright, so is a professionally-made filter. Projection of the image works well too.

This week, it is Mercury's turn, as the planet goes through superior conjunction with the Sun on Sunday the 27th, the planet in back of the Sun, the event, such as it is, quit invisible. It will thereafter pop up in the early evening sky. The night then belongs to Mars and Saturn, the two nicely visible at the same time. The western one of the two, Mars is now moving easterly to the south of the eastern hindquarters of classical Leo well to the southeast of Regulus heading for Virgo (and in mid-August for Saturn). Well into the western sky at dusk, the red planet now sets around 2 AM. Transiting the meridian in late twilight, Saturn follows Mars to the horizon by about an hour and a half. Regulus, Mars, Spica and Saturn make for a fine evening's viewing.

If you are far enough south (meaning here mostly Hawaii), this is the season for Crux, the Southern Cross. However, northerners can still admire some of the bright stars of Centaurus (the Centaur) that more or less wrap around the Cross to the north. Just get a clear horizon and look well to the south of Spica past the dim tail of Hydra, the Water Serpent.

STAR OF THE WEEK: LAMBDA CEN (Lambda Centauri)

Crux, the Southern Cross, was in the seventeenth century carved from the feet of Centaurus, the Centaur, the two constellations thus bearing an intimate relation with each other. Starting to the southeast (with Alpha and Beta Centauri), Centaurus wraps around the Cross to the north and then a bit to west, where we find the luminous mid-third magnitude (3.13) star Lambda Cen, a class B (B9) giant (some say bright giant) 420 light years (give or take 18) away. It's a part of the "Lower Centaurus Crux" association of O and B (and lesser) stars that are something of a family with members born more or less at the same time, the system now expanding and dissipating into space.

"LCC" lies at an average distance of 385 light years, Lambda Cen thus being a bit on the back side of it. The star is dimmed by about six percent by interstellar dust, a surprisingly small amount given its position in the Milky Way. Separated from Lambda proper ("Aa") by under a second of arc lies Ab, which in infrared light shines at seventh magnitude. The assumption that it is the same in visual light (perhaps a bit rash) drops Aa to magnitude 3.17. Several temperature measures are all over the place, averaging 10,170 Kelvin. Lambda itself (Aa) shines with the light of 955 Suns, which leads to a radius of 10 times solar.

Measures of projected equatorial rotation speed are odd, an early citation giving zero, two later ones averaging 185 kilometers per second, which is probably the more correct. If so, the star completes a rotation in under 2.7 days. It's fast enough (supporting the rapid rotation) that the chemical abundances are not weird, as they are in so many stars of its class.

That said, the star does appear to be metal-rich with an iron abundance as much as two to three times normal, with silicon and carbon a bit down. As are so many stars within its association, Lambda is on the massive side, carrying about 4.5 times the mass of the Sun. Not yet a true giant, the star is now in the process of becoming one with a dead helium core, its age about 125 million years. The companion may be a mid-class A star that carries around double the solar mass.

With a mean orbital size of at least 90 Astronomical Units, from Kepler's Laws Ab must take more than 335 years to make a full orbit. Around 16 seconds of arc distant we find 11.5 magnitude Lambda B, which if real is a solar-type star separated from the inner pair by at least 2000 AU in an orbit that must take at least 34,000 years to complete. From the outer star, the stars of the inner double would appear two or three degrees apart, their orbit making quite a sight, were there anyone there to see it. X-ray radiation may be associated with the lesser companion. Lambda Cen is associated with a small nebula, but it's probably just a line-of-sight coincidence.

Do you have a favorite star or one you would like to see highlighted on the Star of the Week? Send a suggestion to Jim Kaler. 

Sierra NightSky thanks to Jim Kaler.
Check out his site for more