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


Sierra NightSky for the period starting January 16, 2015 by Jim Kaler

Back to fortnights for a bit: thanks for your patience. The next skylights will appear January 30, 2015.

We start with the Moon late in its waning crescent phase, which ends at new Moon on the morning of Tuesday, January 20. The morning of Saturday the 17th, the rising crescent will appear well to the left of Antares in Scorpius and down and to the left of Saturn. From new Moon we look to evening western twilight to see the waxing crescent climb toward first quarter, which takes place on the night of Monday the 26th around the time of moonset in North America. It then enters the waxing gibbous phase as it plows toward full Moon on February 3.

The night of Wednesday the 21st the Moon passes well north of Venus, then the next night north of Mars. After the quarter, the Moon invades Taurus. The night of Wednesday the 28th, find it to the west of the Hyades and Aldebaran and to the south of the Pleiades. The following evening it will switch to the other side of the bright orange star. The Moon passes perigee, where it is closest to Earth, on Wednesday the 21st.

In the west, Venus is getting ever higher, and as our fortnight opens sets just after the end of evening twilight. It will make a glorious sight this spring and early summer. Early in our extended period, Venus hovers near fainter Mercury, the little planet quickly disappearing then passing inferior conjunction with the Sun on Friday the 30th.

The show really belongs to Jupiter, which rises (still in western Leo to the west of Regulus) about as Venus sets and twilight ends, and is with us all night, crossing the meridian high to the south shortly past midnight. Not long after (about 3:30 AM at the start of our session, 2:30 AM at the end) we get to see Jupiter's brother planet Saturn loft itself over the southeastern horizon just off the tip of the head of Scorpius and nicely above Antares.

Back in the west, Mars, now deep in Aquarius approaching Pisces, reliably sets at 8 PM. On Monday the 19th, the red planet will pass a mere two-tenths of a degree south of much fainter Neptune, making for a fine telescopic event.

With good fortune, if it's bright enough, you might get to see Comet Lovejoy as it climbs west of Taurus and the Pleiades then southwest of Perseus, between it and Triangulum (the Triangle). Lovejoy is clearly a drop-in from the distant Oort Cloud of icy comets that were ejected by the giant planets during the formation of the solar system. As such it is pretty unpredictable. If you can't see it, try scanning around with binoculars.

Perseus and Auriga stand high nearly overhead in mid evening, the two making a nifty triangle with Taurus to the south, and Orion even farther below, the winter skies filling with brilliant stars. Fainter, Eridanus, the River, winds to the southwest of Orion then for northerners below the horizon where it ends in beautiful bluish Achernar.

STAR OF THE WEEK: PHI PER (Phi Persei)

An amazing star, Phi Persei is an extreme system that inspires more than a bit of awe. Not part of any of Perseus's usual outlines, mid-fourth magnitude (4.07), Phi Per is just barely over the border with Andromeda and about midway between the classical figures of Perseus and Cassiopeia, so it's easy to miss, and that's too bad. In the Milky Way at a distance of 718 light years (give or take 31), Phi Per is dimmed by two-thirds of a magnitude (54 percent) by interstellar dust.

Were the line of sight clear, it would shine at magnitude three (3.40) and might even have a proper name. It's a hot class B (B2) hydrogen-fusing dwarf, an emission-line star with a surrounding radiating disk in the mold of Gamma Cassiopeia and Zeta Tauri, indeed a "shell star" in which the disk is presented more or less edge on.

Consistently, Phi Per is spinning madly, with an equatorial velocity of at least 430 kilometers per second, making it one of the fastest-rotating stars known. All "Be" stars are fast rotators, though just how the rotation factors into the creation of the surrounding disk is not very well understood. Like almost all Be stars, Phi Per is variable, though only slightly, wobbling between magnitudes 4.03 and 4.11.

Of vastly more interest, Phi Persei is double with a strange hot companion that takes 126.7 days to orbit its brighter master. From the 25,400 Kelvin temperature found for Phi Per proper (the dwarf, Phi A), needed to estimate the high amount of ultraviolet radiation, and the distance, we get a luminosity of 16,100 times that of the Sun, which leads to a radius of 6.6 times solar and a rotation period of under 0.77 days. The disk's radius is roughly 10 times as large. Luminosity and temperature plus theory yield a mass of 12 Suns, which may be a bit high because of contamination of the visible dwarf's light by the odd companion, but not by much.

With a temperature estimated at 53,000 Kelvin, the companion is a "subdwarf O" star. Cool subdwarfs are low-metal stars of the Galaxy's halo. Very different, hot subdwarfs are stars that have lost their outer envelopes and are in some stage of development toward becoming white dwarfs. Phi Per B, now the lighter star, was once the mightier, which means it was slated to die first. In the process of evolution, it has lost most of its mass, a lot of it to Phi Per A, which is NOW the more massive, the two switching the dominant role.

Algol, Beta Persei, shows much the same kind of action, though it's in an earlier stage. The disk around Phi A may then be the result of accretion from what is now the secondary (Phi B). Mass accretion toward the side of Phi A, the class B dwarf, then spun it up to its present state of ultra-rapid rotation. Phi A now seems to be shoveling some of its mass back to the subdwarf, Phi B, whence it creates at least an illusion of another disk, each star having one.

Study with the Hubble Space Telescope suggests an orbital radius for the subdwarf of 1.1 Astronomical Units and respective masses for Phi A and B of 9.3 and 1.14 Suns, making the Be star too luminous for its mass (and too hot as well), no surprise given the intense activity and mass transfer going on. With such a high degree of mass loss and illuminating ultraviolet radiation, one might expect an extended planetary nebula (which has nothing to do with planets) to surround the system. Perhaps the evolution has not gotten that far. We don't know.

The star, however, is a guide to a genuine and well-known planetary nebula, Messier 76, which lies just under a degree to the north-northwest. (Thanks to: D. R. Gies et al. in the Astrophysical Journal, 493: 440, 1998; S. Stefl, W. Hummel, and Th. Ravinius in Astronomy and Astrophysics, 358: 208, 2000; and W. Hummel and S. Stefl in Astronomy and Astrophysics, 368: 471, 2001.)

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

 

 
 
 
 
 
 
 

   

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