"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, February 3, 2012 - by Jim Kaler

Skylights now resumes its normal weekly schedule; thanks for your patience.

We start the week with the Moon in its waxing gibbous phase as it heads towards full on Tuesday, February 7, just before Moonrise in North America, allowing one to see the Snow Moon, the Wolf Moon, in all its rising glory. Our companion will then fade in the waning gibbous during the rest of the week. As it plies its near-monthly path, the Moon will take a bead on Leo and Mars. The night of Tuesday the 7th finds the Moon to the west of the star Regulus, while the following night sees it to the southeast of the star and south of Leo's classical figure. Then look the night of Thursday the 9th to find the red planet smack in between the fat gibbous and Leo's "tail star," Denebola.

Venus is strikingly beautiful in the early evening sky. Dominating the west, the brilliant planet does not set until after 8:30 PM, almost two hours past the end of evening twilight.

It's hard to turn one's head away a bit to the south to admire number two in the planetary sky, Jupiter, which stays with us until shortly before midnight.

Then shortly before Venus sets, Mars serves to replace it, rising in the east to the south of western Leo, as noted above. One, two, three bright evening planets, and now number four, Saturn, which rise nearly as Jupiter sets, just before midnight.

The morning sky keeps Mars and Saturn, the former transiting the meridian to the south about 2:30 AM, Saturn following about 5 AM, just before dawn begins to light the eastern sky. Saturn makes additional news, as on Wednesday the 8th, it begins retrograde motion, to the west against the stars, allowing it to close in a bit on Spica in Virgo, the star lying to the southwest of the ringed planet. In lesser news, on Thursday the 9th, Venus passes just three-tenths of a degree to the north of ranus, one planet bright enough to cast shadows, the other barely visible to the naked eye under the best of circumstances.

In the invisible news department, Mercury goes through superior conjunction with the Sun (on the other side of it) on Tuesday the 7th.

Look nearly overhead in the early evening to see Perseus, the rescuer of Andromeda. Its luminary, Mirfak (the Alpha star), ranks number 35 in brightness, while the Beta star (Algol, the Demon Star), a fine example of an "eclipsing double" (two stars in orbit, in which one each gets in front of the other), winks at us every 2.9 days. To the east find Auriga with bright Capella, ranking sixth and the most northerly of the first magnitude stars.

STAR OF THE WEEK: 40 Per (40 Persei)

Many are the treasures of Perseus, the Hero who slew the Sea Monster in the rescue of Andromeda. Among them are collections of massive stars, many of which are fainter than those that have garnered proper or Greek letter names. One of them is fifth magnitude (4.97) 40 Persei, a hot class B (B0.5) hydrogen- fusing dwarf known best by its Flamsteed number.

The star's other popular name, "o" Persei (lower case Roman "oh", the Roman letters used by Bayer after he ran out of Greek ones) probably should not be used, as the letter is prone to confusion with Greek "omicron." It's a special issue with 40 Per, as Omicron Persei (Atik), a brighter and somewhat similar star, lies just 1.7 degrees to the south-southeast of 40 Per. As a result, 40 Persei sometimes gets the Greek letter name, and is mistakenly called "Omicron Per," which it isn't.

It is, however, one magnificent star that is dimmed by almost a full magnitude by intervening interstellar dust. Were the pathway clear of the obscuring tiny dust grains (made largely of silicates and carbon), 40 Per would shine at magnitude 4.14 and be more a part of its parent constellation.

The obscuration carries a side benefit, however, as (especially given the simplicity of a class B spectrum) the starlight provides a fine background against which to study the composition of and motions within interstellar gas, which always goes along with the dust (indeed, the mass of interstellar gas being some 100 times that found in the dust).

With a high surface temperature of 28,700 Kelvin, 40 Per's radiation lies mostly in the ultraviolet. If it were all stuffed into the visual spectral domain, and if we could get rid of the intervening dust, we'd see 40 Per as first magnitude! Using the star's distance of 1055 light years (give or take 72), we find a great luminosity of 23,600 times that of the Sun, from which is calculated a radius of 6.2 solar.

Measurements of projected equatorial rotation speeds are all over the place. Adopting 30 kilometers per second yields a rotation period under 10 days, relatively long for the class. The theory of stellar structure and evolution then tells of a mass 14 times that of the Sun. Confirmed as a dwarf, 40 Per is about halfway through its hydrogen-fusing lifetime.

The star is clearly above the limit at which stars explode. After it completes its helium core, 40 Persei will go on to become a grand red supergiant that will "burn" its core helium through other products such as neon, magnesium, and sulfur to iron. The iron cannot fuse to anything, leading to total core collapse and a magnificent supernova.

Forty Per is listed with two "companions." Its tenth magnitude optical "mate," 26 seconds of arc away, is moving much too fast and is clearly just a line of sight coincidence. A spectroscopically-detected neighbor does not seem to have been confirmed. The star is probably a loner. But not quite, as 40 Per is a member of the large, expanding, gravitationally unbound Perseus OB2 association of hot stars that includes a far more famed star, third magnitude Zeta Persei, and possibly (its membership disputed) nearby Omicron Persei as well, thus completing the circle.

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