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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 March 27, 2015 by Jim Kaler

Skylights' current two-week period is the result of travel and medical issues. I hope to go back to weekly reports. Thanks for your patience.

The next skylights will appear April 10, 2015.

The current fortnight is the opposite of the previous one, at least with events involving the Moon. This version of Skylights is again bracketed by the lunar quarters, but reversed. Now we begin with first quarter, which is passed on Friday, March 27, while the third quarter takes place the night of Saturday April 11, just after our period ends. Then it includes lunar apogee, with the Moon farthest from Earth, rather than perigee (closest), on Wednesday, April 1.

And now it's full Moon that is encountered, the morning of Saturday the 4th. Prior to that, the Moon waxes in its gibbous phase, while after it's in the waning gibbous.

Since solar and lunar eclipses more or less come in pairs, this full Moon suffers a total eclipse. Best visibility falls in the middle of the Pacific Ocean. Western US and Canada, though, get to see totality, while in eastern North America, the Moon sets while it is entering full Earth shadow. Europe is shut out. The Moon enters full Earth shadow at 3:15 AM Pacific Daylight Time, hits totality at 4:54 AM. Since the Moon just skims the northern edge of full shadow, central eclipse takes place just minutes later, at 5:00 AM. Total eclipse ends at 5:06 AM, and the show is over at 6:45 AM (ignoring the penumbral stages). Add one hour for MDT, two for CDT, three for EDT.

Our two-week period also features a pair of planetary passages. The night of Sunday the 29th, the Moon will start to slide south of Jupiter, then will appear northwest of Saturn the morning of Wednesday the 8th, to the northeast of the planet the following morning.

To the west in the evening we see the glory of Venus, which does not set until more than an hour past the end of twilight (two by the end of our period), somewhat after Jupiter crosses the meridian to the south. The giant planet is then with us until somewhat before dawn.

Still to the west of Leo and Regulus, Jupiter ceases retrograde motion on Wednesday the 8th. By the end of our fortnight Saturn, which has just begun retrograde, is rising an hour before midnight Daylight Time northwest of Antares in Scorpius.

In lesser news, Uranus passes conjunction with the Sun on Monday the 6th, while Mercury goes through superior conjunction with the Sun four days later. Mars sets as twilight ends.

Orion's two hunting dogs, Canis Major with bright Sirius and Canis Minor with Procyon, bound toward the west, both to the south of Gemini. The constellation's two bright stars, Castor and Pollux, point downward to the roundish head of Hydra, the Water Serpent, which winds far to the southeast.

STAR OF THE WEEK: XI-1 CMA (Xi-1 Canis Majoris)

In a brilliant constellation like Canis Major, in the Milky Way with not one but two first magnitude stars (Sirius, actually minus first mag, and Adhara at the end of the list), you run out of names fast, and to keep going within the Greek alphabet you have to stretch things a bit, hence the use of attached numbers for stars close together.

Usually listed west to east, the members of such combinations rarely have anything to do with each other. In the central part of the Big Dog lies fourth magnitude (4.33) Xi-1 Canis Majoris.

A hot blue class B (B0.5) subgiant (but see below), it's on the edge of rare class O and as such is fated to explode. At a distance of 1382 light years (give or take 117), Xi-1 is three times as far as Xi-2, the latter a class A0 giant just under a degree to the northeast of Xi-1 but 440 light years away. Were there any significant interstellar matter in the area, Xi-1, at a temperature of 27,720 Kelvin, is hot enough to produce a diffuse nebula.

Even along the whole line of sight there is not very much, as interstellar dust dims Xi-1 by only 0.13 magnitudes. Allowing for a lot of ultraviolet radiation, the star shines with the light of 35,000 Suns, its radius 8.1 times solar.

For its class, Xi-1 is a slow rotator, the projected equatorial rotation velocity a mere 27 kilometers per second, giving it a rotation period that could be as long as 15.2 days. The star could of course be rotating with its rotation pole more or less pointed toward us, giving a false impression of its spin. We don't know.

But it's enough to produce a magnetic field some 600 times that of Earth, if indeed rotation is involved.

Luminosity and temperature combined with the theory of stellar structure and evolution reveal a mass of 15 times that of the Sun, well over the limit beyond which a stellar core fuses to iron and then collapses, the resulting energy blowing the rest of the star up as a supernova. But it has a ways to go.

Not a more advanced subgiant as given by its spectral class, Xi-1 is still a dwarf that is perhaps three-fourths or so the way through its hydrogen-fusing lifetime of 12 million years. Such dichotomies in class are common among the B stars and mean little.

Like many stars of its kind, X-1 is a subtle "Beta Cephei" variable. Such stars commonly oscillate in brightness by a few hundredths of a magnitude with multiple periods of under a day. Xi-1 CMa is a rather unusual monoperiodic variable that jitters by just 0.03 magnitudes (well under that detectable by eye alone) over a period of 5.03 hours. Xi-1 CMa is accompanied by two "companions."

The motion of 14th magnitude Xi-1 B relative to the B0.5 dwarf 27 seconds of arc away shows "B" to be just a line-of-sight coincidence. A similar 14th magnitude neighbor 28 seconds of arc away but in a different direction presents a better case, but it too is probably coincidental. If not, it would be at least 12,000 Astronomical Units away and take 300,000 years to make a full circuit of the primary star. Not even a member of an association of other hot stars, it seems likely that X-1 CMa will have to go through its remaining evolution and its eventual destruction all alone.

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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