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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 May 22, 2015 by Jim Kaler

The next skylights will appear June 5, 2015.

We begin the new fortnight with the Moon shining as a fat waxing crescent not far from first quarter, the phase passed on Monday, May 25, about the time of its daylight rise. It then brightens through waxing gibbous to full Moon on Tuesday, June 2, with the Moon out of sight. That evening, the Moon will rise a bit past the full phase as it begins fading through the waning gibbous phase. The night of Saturday the 23rd the Moon will pass five degrees south of Jupiter, the two making a fine sight. By the following night the Moon will appear south of the line connecting Jupiter and Leo's Regulus. Then look to see the nearly full Moon to the northwest of Saturn the night of the 30th, to the northeast of it the following night. Our companion passes apogee, where it is farthest from Earth, 5.5 percent more distant than average, on Tuesday the 26th, just a day after first quarter.

As twilight deepens, Venus remains brilliant in the west, the planet not setting until after 11:30 PM Daylight Time. As the month changes, Venus appears to the left of Castor and Pollux in Gemini. Our session ends with Venus just a day shy of its greatest eastern elongation, 45 degrees east of the Sun, when it will appear half illuminated quite like a half-Moon when it is at the first or third quarter of its orbit.

The phases of Venus constitute one of the classic Galilean proofs that the planets go around the Sun, not the Earth. About a week before, on Saturday the 30th, Mercury passes inferior conjunction with the Sun as it goes from being an evening to a morning planet.

That leaves us with Jupiter and Saturn.

In western evening skies, Jupiter, maintaining its position to the west of the Sickle of Leo, now sets around local midnight. With Venus more or less fixed, the giant planet draws ever closer to it as the two brightest of planets prepare for a close passage in late June/early July (formal conjunction taking place July 1).

About as Jupiter sets, Saturn rises north of Antares in Scorpius. The telescope shows Saturn's rings to be "open," tilted as much toward the observer as possible, which makes the planet especially bright.

Following the curve of the Big Dipper's handle to the south leads you to bright Arcturus, then on to Spica in Virgo. To the right of Spica, is a distorted box that makes the ancient constellation of Corvus, the Crow or Raven. Further to the west is the dim figure of Crater, the Cup, both of them riding the back of Hydra, the Water Serpent. As the front bowl stars of the Dipper point to Polaris, so the top two stars of Corvus point back, to the east, toward Spica, completing the loop.


Among the dimmest of constellations, ancient Crater (the Cup) balances on the back of Hydra (the Water Serpent) to the west of Corvus (the Crow), which does the same, the three once considered a sort of "triplet." Though Crater's outlining stars are only fourth and fifth magnitude, under a dark sky it quite stands out.

To the ancient Greeks it was Zeus's Goblet. Oddly the brightest star is Delta Crateris, while the Alpha designation went to fainter number two, which is in a virtual tie with Gamma but has the constellation's only proper name (Alkes).

At nearly fifth magnitude (4.48), Beta Crt is actually number four, which shows once again that Bayer had something else also in mind (usually position) when he Greek-lettered the stars, Beta falling at the figure's base. A spectral class A (A2) "giant," Beta Crt is 340 light years away, give or take 24. The temperature of 8954 Kelvin, consistent with class, is of minor consequence since most of the light falls in the visual spectrum with only a small correction for ultraviolet. That along with distance yields a luminosity of 147 Suns and a radius of 5.1 times solar.

The projected equatorial rotation velocity of 54 kilometers per second is low enough to expect separation of elements, resulting in a "metallic" star like Sirius. But the chemical composition is solar, which suggests atmospheric stirring from a faster spin, so that the rotation pole must be rather tipped toward us.

Theory gives a mass of 2.9 to 3.0 times that of the Sun and shows the star to be not so much a giant as a subgiant that has just quit core hydrogen fusion after a lifetime of 350 million years.

What makes Beta Crt really special is its 13th magnitude (13.4) hot (34,000 Kelvin or so) white dwarf companion, which puts the star back in league with Sirius.

There is an apparent dearth of such binaries, so astronomers are avidly looking for them. Hints of a companion (from spectral Doppler shifts) go back to 1928, but the actual discovery from the white dwarf's strong ultraviolet radiation is only recent.

White dwarfs, with masses that range from about half to 1.4 Suns, are the carbon/oxygen cores of stars initially born with masses from 0.8 to about 10 times solar, most of a star's mass being lost to space during late evolution. There are two kinds, "DA" wd's like Sirius B and Beta Crt B that have thin, pure hydrogen skins, and DB's with helium envelopes.

Since higher mass stars have shorter lives, Beta Crt B must have evolved first and be the product of a star born with more than 3 Suns, yielding a white dwarf mass of 0.70 solar. But the mass, estimated at just at just 0.43 Suns, is far too low, suggesting strong ancient mass transfer to Beta Crt A, which then would have a birth mass under 3 Suns, confusing the issue.

On the other hand, "B" seems too far away from "A" for any such action to have taken place.

Factoring in an estimate of ultraviolet radiation, the white dwarf's luminosity is still some three-quarters that of the Sun. Given the temperature, its radius is just under three Earths, which is actually rather large (Sirius B is 0.92 the size of Earth), but consistent, as the smaller a white dwarf's mass, the larger it is thanks to a lowered pull of gravity.

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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