"Let your soul stand cool and composed before a million universes" Walt Whitman
The Moon's current phase, courtesy of USNO.
Sierra NightSky for the period starting July 18, 2014 by Jim Kaler
The Moon passes through its third quarter the night of Friday, July 18, shortly before Moonrise in North America, and then spends the rest of the week thinning out in the waning crescent phase, new Moon not reached until Saturday the 26th.
Early risers that have a clear eastern horizon can see the thin crescent just to the right of Venus the morning of Thursday the 24th.
The two giant planets make the biggest splash. Jupiter, which has been invisible in the solar glare for some time now, finally passes conjunction with the Sun on Thursday the 24th, and thus officially becomes a morning planet. Its rising, though, will not clear dawn until the middle of August.
Earlier in the week, on Monday the 21st, Saturn ceases its retrograde, westerly, motion against the background stars, and resumes its normal easterly movement as it heads toward Scorpius. Number two in the Solar System, Saturn has a third the mass of Jupiter, but because of lower gravity puffs out almost as large, rendering it by far the least-dense of the Sun's family.
Farther away, Uranus begins retrograde on Tuesday the 22nd. The distant planet rises about as Mars sets.
Saturn and Mars dominate the early nighttime sky, Mars to the southwest as evening falls, somewhat dimmer Saturn not far to the east of the red planet, Mars in Virgo now a bit to the east of Spica, Saturn in the next zodiacal constellation over, Libra. Mars sets about midnight Daylight Time, while Saturn goes down an hour later.
The morning sky hosts the two inner planets, Venus, which rises as dawn starts to light the sky, then down and to the left of the bright planet, Mercury, which is now past its prime and hard to see.
In late evening, around 11 PM, two outstanding constellations of the Zodiac and Milky Way, Scorpius (to the right) and Sagittarius are crossing the sky to the far south. On the other side of the sky, the Big Dipper falls into the northwest, while the stars of the Summer Triangle (Vega in Lyra, Deneb in Cygnus, Altair in Aquila), climb high in the east.
STAR OF THE WEEK: 83 UMA (83 Ursae Majoris)
In a constellation known for its white class A stars and its orange class K giants, it's a surprise and a treat to come across something quite different.
Streaming just to the east of Mizar in Ursa Major's Big Dipper is a string of little stars that starts with Alcor (80 UMa, wedded to Mizar), then continues with Flamsteed's 81, 83, 84, and 86 Ursae Majoris. 82 UMa is a bit south of them. (The gap at 85 UMa is filled by Alkaid at the end of the Dipper's handle.)
As a group, the string points southeasterly toward the fingers of Bootes next door. 81, 86, and 82 are (surprise) rapidly rotating sixth magnitude white class A stars, while 84 UMa is a more interesting sixth magnitude magnetic star (like the variety's prototype, Cor Caroli in nearby Canes Venatici.)
The prize in the set is 83 UMa, a red class M (M2) giant, which fits not at all with the rest of the gang. Moreover, with a distance of 524 light years (give or take 19) it's the most distant of them, though at 486 light years, 86 UMa comes close. Numerous temperature measures of 83 UMa average out to a cool 3685 Kelvin, most of the star's radiation then falling into the infrared.
Using distance and the infrared correction, 83 UMa is seen to shine with the light of 3685 Suns, which with temperature gives a radius of 59 times the solar value, or 0.27 Astronomical Units, 71 percent the size of Mercury's orbit.
Theory applied to luminosity and temperature strongly suggests that 83 UMa is an ageing star that is brightening as a giant for the second time, now with a quiet carbon-oxygen core. (In the first brightening as giants, stars have dead helium cores that heat until they can fuse the helium into the C-O mix.
They then stabilize as the common orange giants of the sort we see in Ursa Major, and when the helium runs out they brighten again as 83 UMa seems to be doing now.) Such stars tend to be unstable.
As a "semiregular" variable it changes by just short of a tenth of a magnitude over a poorly known "period" (if such exists) estimated at 18 days or so.
The best estimate of mass gives about twice that of the Sun. It's hard to be exact as mature stars have a range of masses with closely similar properties.
Adopting two solar masses, it began evolving as a giant around 300 million years ago, and is now 1.4 billion years old. But don't take the numbers too seriously as age is very dependent on mass. Flamsteed's 83 is listed as a weak barium star, one that had a close companion that when evolving dumped freshly made elements onto the star we now see. The classification is probably spurious as real barium stars tend to be rather obvious.
83's most likely fate is to continue to brighten, and eject its outer envelope. The revealed hot core will then light up the ejecta as an expanding planetary nebula and die as a tiny white dwarf.
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