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Sierra NightSky

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

Sierra NightSky for the period starting February 27, 2015 by Jim Kaler

The next skylights will appear March 13, 2015.

The two week period spans the full Moon, our bright neighbor more or less blotting out the stars. We begin with the Moon in the waxing gibbous phase just past first quarter; full phase, with the Moon opposite the Sun, is hit on Thursday, March 5, around noon in North America. It thus rises a bit shy of full the evening of Wednesday the 4th, a bit after that Thursday night. After fading through the waning gibbous phase, it passes third quarter on Friday the 13th, roughly around noon and about the time of moonset. We'll have a fine view of it that morning. The Moon passes apogee, where it is farthest from Earth, just ten hours before full, its slight extra distance (just over five percent above average) notably weakening the usual full Moon high tides at the coasts.

The night of Monday the 2nd, the waxing gibbous visits Jupiter, gliding around five degrees south of the giant planet, and then the following evening more of less tucks itself between Jupiter and fainter Regulus in Leo. Just before its third quarter, on the morning of Thursday the 12th, look for the Moon just above and to the left of the other giant planet, Saturn, both set immediately to the northeast of the three-star head of Scorpius and above the first magnitude red supergiant at the Scorpion's heart, Antares.

Venus dominates the western evening skies. There is no mistaking it. Having passed its conjunction with Mars on February 21, Venus is climbing upward above the much dimmer red planet, which reliably still sets at 8 PM, while Venus-set lingers past the end of twilight by a good hour. The brilliant planet will just keep getting better and better, not disappearing back into dusk until mid-summer.

Toward the east find also-bright Jupiter, which transits the meridian to the south in late evening well before midnight, the planet near the Leo-Cancer border to the west of Regulus, as noted above. Shortly after midnight Saturn rises. For fans of the distant planet Uranus, Venus passes just a tenth of a degree north of it on Wednesday the 4th, while Mars does the same on Wednesday the 11th, though with three times the angular separation.

The eye seems to catch long strings of stars. Draco, the Dragon, winds between the Dippers (the Big One climbing the early evening northeastern sky), while Hydra, the Water Serpent (the longest of all constellations), slithers south of Leo and Virgo. Streaming off into evening twilight is Eridanus, the River, which ends far south of the celestial equator in brilliant Achernar.

STAR OF THE WEEK: 54 ERI (54 Eridani).

Immensely long, Eridanus, the River, winds from just northwest of Rigel, the seventh brightest star of the sky, deep into the southern hemisphere, where it ends at Achernar, number nine, both stars of magnitude zero. Not all of Eridanus's more modest stars are on the winding stream, however. On the far southern bank lies 53 Eri, a cool class K fourth magnitude (3.87) giant 110 light years away, and farther south our star, 54 Eri, an even cooler also fourth magnitude (4.37) class M (M4) giant 368 light years distant (give or take 25). 54 Eri (a Flamsteed number) is also a semi-regular variable that goes by the alternative name DM Eridani, the star wandering slightly between magnitudes 4.28 and 4.36 (unnoticeable to the naked eye) over a rough 30-day period.

It presents something of a mystery. In 1877 and on two occasions in the 1920s it was observed as double, the companion seen to be almost as bright as 54 Eri proper. On the other hand, lots of other astronomers never saw it, even in modern times using Hipparcos data and interferometry. So (from Bill Hartkopf, US Naval Observatory) "the duplicity of 54 Eri is definitely not definite (or certainly not certain) ... if it is double it has either closed to under 0.03 seconds of arc, or the magnitude difference is much larger, or some combination of the two...my feeling is that it's single - perhaps they were seeing some reflection due to the brightness."

More telling, the spectrum shows no trace of another star, just that of 54 Eri itself, the class M giant. So single it is, 54 Eri quite vividly revealing the problems astronomers can be faced with. The temperature is as ill-defined as the duplicity. Three values range between 3050 and 3660 K, averaging 3385 Kelvin. Whatever the true temperature, a lot of the star's radiation falls in the infrared. Including that, the distance gives a luminosity 2430 times that of the Sun and a radius of 143 solar radii (0.67 Astronomical Units), which makes the star almost as big as the orbit of Venus.

Like most of 54 Eri's parameters, the mass is not well-defined either, but may fall around double that of the Sun. No surprise, the evolutionary status is also unclear. Possibly the star is in the early phase of brightening and growing for the second time, now with a dead carbon/oxygen core, but it could also be in one of the stages of its first brightening with a dead or burning helium core.

Whatever the case, the star's outer envelope will eventually be expelled, and the core will be left to cool forever as a white dwarf. Both 53 and 54 Eri are moving fairly quickly relative to the Sun, 53 at (oddly) 53 kilometers per second, 54 at 63 km/s, respectively away and towards us at more than three and four times normal, the two obviously having nothing to do with each other except for their contiguity in the Flamsteed catalogue. (Thanks to Bill Hartkopf for detailed commentary.)

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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