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Sierra NightSky
 
 

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

Current Moon PhaseThe Moon's current phase, courtesy of USNO.


Sierra NightSky for the period starting August 15, 2014 by Jim Kaler

Skylights will next appear on Friday, August 29.

We start our fortnight with the Moon in its waning gibbous phase, which is terminated at third quarter the morning of Sunday, August 17, after Moonrise in North America. It then fades away as a waning crescent as it heads toward new Moon on Monday the 25th. The morning of Friday the 22nd, the slimming crescent will fall between the star Procyon (down and to the right) and the Pollux and Castor pair (up and to the left).

The following morning, that of Saturday the 23rd, the Moon will make a glorious dawn triangle with Jupiter and Venus (Venus the lower and brighter of the two). Then turn around and watch the Moon pop up as a waxing crescent in the early evening sky. Our first glimpse of it will be in western twilight the evening of Tuesday the 26th, though it will be a lot more obvious a day later. The Moon passes apogee, where it is farthest from the Earth, on Sunday the 24th.

We are witness to the rare delight of two pairs of close planets, one in the evening sky the other in the morning. Moving rapidly eastward against the background stars, Mars crossed the border from Virgo into Libra last Sunday the 10th, and finally passes conjunction with much slower moving Saturn on Wednesday the 27th, the red planet four degrees to the south. Look into the southwest as twilight draws to a close. By 10:30 PM Daylight Time they are close to setting.

But if you miss them, all you need do is wait until morning for much brighter Venus and Jupiter. The two planets pass conjunction with each other the night of Sunday the 17th. When they rise, just after the beginning of twilight, they will be less than half a degree apart, Jupiter to the right. The last time they performed like this was in 2012, when they blazed forth together in the evening sky, some taking them for UFOs and invading space aliens. While Venus sinks out of sight into twilight, Jupiter climbs in the other direction, the planets thus quickly separating.

As the Moon moves out of the way, we see the glory of the Milky Way, which is made of the countless stars of the disk of our Galaxy. In mid to late evening the Milky Way plunges down from Cygnus (which turned upside down is the Northern Cross with Deneb at its top), through Aquila and Scutum, and into Scorpius and Sagittarius, finally disappearing below the southern horizon.

It's split into two streams by the Great Rift, a lumpy layer of dark, cold, dusty interstellar clouds in which stars are being born. To the northwest, the Big Dipper in Ursa Major winds around the celestial pole, which is closely marked by Polaris, while in the northeast, the rising "W" of Cassiopeia brings a sign of the coming fall.

STAR OF THE WEEK: HD 140283 (HD 140283 Librae)

And now (once again) for something completely different. Excluding its nuclear-burning core, the Sun is composed of 92 percent hydrogen atoms and 8 percent helium atoms. That's 100 percent.

All the other elements (loosely called "metals") sneak in at a mere 0.15 or so percent, so they don't show up in rounded-off H and He abundances.

Oxygen leads the pack, followed by carbon, neon, nitrogen, and so on down to uranium, which must be there but is too sparse to be detected.

The hydrogen and most of the helium atoms come from the Big Bang, the H primordial, the He from nuclear reactions that took place within the first three minutes of the life of the Universe.

All the other elements (and a bit of helium) were formed in the nuclear cauldrons of massive stars and supernovae that, through their winds and explosions, blew them into interstellar space.

The metal content of the stellar birth clouds therefore increases with time, so that young stars have higher metal abundances than old stars.

The very FIRST stars, however, those formed right after the Big Bang, should have had no metals at all. We can't find them. Presumably, the first stars were all massive and therefore exploded, so there are none left to see.

How far back in metal content, and thus toward the Galaxy's birth, indeed toward that of the Universe, can we go? Not visible to the naked eye, but a clear binocular object, seventh magnitude (7.21) HD 140283 (in the Henry Draper spectral catalogue) is certainly a stop along the way.

Only 190 (give or take under 2) light years away in Libra, HD 140283 is the closest of the really low-metal stars. With a temperature of 5580 Kelvin, the iron abundance relative to hydrogen is 0.0040 that found in the Sun. The oxygen value is higher, but still only 2.1 percent solar.

The star is classified as "sdF3," the "sd" standing for "subdwarf." Subdwarfs were once thought to be dwarfs that are sub-luminous for their classes.

Because of their low metals, however, they are actually hotter and smaller for their luminosities, the low metal abundances giving them classes that are anomalously warm. (All are of class F or cooler. Don't confuse them with similarly named class O and B subdwarfs, which are highly evolved and are on their way to becoming white dwarfs.)

Because they are ancient, subdwarfs such as these all have low masses, below about 80 or 90 percent that of the Sun; those of higher initial mass have long since burned away.

HD 140283, however, once a real subdwarf, is actually a subgiant that is turning into a giant with a dead helium core. From its temperature, luminosity (3.8 times that of the Sun), chemical composition, and theory, HD 140283 seems almost as old as the Universe itself, which dates at 13.8 billion years.

Whatever the final word, the star is certainly among the oldest there are. While close to us now, HD 140283 is a visitor from the ancient Galactic halo, and is just passing through the orderly disk at a speed of 361 kilometers per second relative to the Sun, just a bit more than the true subdwarf Groombridge 1830. Whatever the details, HD 140283 is among the oldest things you can see other than the Universe itself that surrounds you. (Thanks to H. E. Bond et al. in the "Astrophysical Journal Letters," 765:L12, 2013.)

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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