Astronomy News for the two-week period starting Friday, December 147, 2007

We start, as always, with the ever-popular Moon, which begins the week well along in its waxing crescent phase. Passing first quarter on Monday the 17th, it then enters the waxing gibbous as it heads toward full next week. On Sunday the 16th, the Moon will pass north of Uranus, which still hangs out in eastern Aquarius to the southwest of the Circlet of Pisces.

Four planets plan big events. At the bottom of the list is Pluto, which passes conjunction with the Sun on Thursday the 20th, making it REALLY invisible. Next up is Mercury, which also passes conjunction with the Sun, on Monday the 17th. Given its position inside the Earth's orbit, it (like Venus ) can go through two kinds of conjunctions, one when it is between us and the Sun, an "inferior conjunction", the other (like this one) when it is on the other side of the Sun, a "superior conjunction." Saturn then comes forward by entering retrograde, or westerly, motion against the stars on Thursday the 20th, as the Earth prepares to swing between it and the Sun. Watch for the ringed planet's rising in southern Leo around 10:30 PM.

The week, indeed the rest of the year, however, really belongs to Mars, which in this orbital round makes its closest approach to the Earth on Tuesday the 18th, when it is 0.589 Astronomical Units (88.2 million kilometers, 54.8 million miles) away. It is also then at its brightest, just a hair brighter than the brightest star, Sirius. Mars reaches opposition with the Sun on December 24th. That closest approach does not occur at opposition is the result of Mars's rather eccentric orbit. Near opposition, the red planet now rises in southwestern Gemini just after sunset and crosses the meridian high to the south about half an hour after midnight.

All of this activity is watched by far-brighter Venus, who still brilliantly glorifies the southeastern morning sky, rising just before 4 AM. And by Comet Holmes. Still in Perseus, it remains a nice binocular object.

It's more than past time to note the rising of Orion, who lofts himself above the eastern horizon as twilight draws to a close. Look for his three star belt and the two first magnitude supergiants, Betelgeuse, to the northeast of the belt, and Rigel (actually zeroth magnitude) to the southwest.

STAR OF THE WEEK: 25 ORI (25 Orionis)

Like Cassiopeia and other famed prominent constellations, Orion has so many bright stars that several fainter ones of considerable interest are neglected or ignored. Such is the case for 25 Orionis. While sometimes referred to as Psi-1 Ori, it's better known by Flamsteed's number 25. It's not just interesting in its own right, but through its surroundings as well. Fifth magnitude (4.95), at a distance of 1110 light years, 25 Ori is dimmed a quarter magnitude by interstellar dust. One of the hotter blue hydrogen-fusing dwarfs, this class B (B1) star glows with a temperature (that is not all that well determined) of nearly 25,000 Kelvin. Allowance for a lot of ultraviolet light yields a luminosity 10,500 times that of the Sun, a radius of 6 times solar, and a mass of 10.5 times that of the Sun, just over the limit at which it should explode as a supernova. Not just your ordinary hot B star, the rapid minimum equatorial rotation of 310 kilometers per second (giving a rotation period of just under a day) has through a still-mysterious mechanism turned "25" into a "B-emission" ("Be") star, one that also radiates from a circumstellar disk. Moreover, it is listed as a "shell star," one whose disk appears more or less on edge and thus thick, making the rotation parameters actual figures, not just limits. Like all class B dwarfs, the star is young, under 10 million years old. While brilliant and notable, Be stars are so bright that we see a lot of them. 25 Ori's special claim to notoriety is the flock of very young "pre-dwarfs" that surround it, lower mass stars that are still in the late stages of their formation, enough of them that the group is named after the star as the "25 Orionis association." Orion is filled with, indeed made of, such stellar "associations," groups of young stars that were born about the same time, but that are gravitationally unbound and expanding from a common center. They are usually recognized by their luminous hot stars, hence are called "OB associations" even though they contain many lower mass stars. They often come in hierarchies. The 25 Ori group is a subassociation that seems to belong to the Orion OB1a association (stars up and to the right of the Belt), which in turn is included Orion OB1 association, which includes other subassociations that contain the Belt, Sword, and the Orion Nebula and its stars, as well as the wonderful multiple, Sigma Orionis.

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. 

More Astronomy info.

Weekly SkyLights thanks to Jim Kaler.
Check out his site for more