Bakersfield Night Sky – October 15, 2011
By Nick Strobel
The MESSENGER spacecraft (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) completed its first Mercury solar day a couple of weeks ago and has another Mercury solar day to go. Notice that I'm careful to say "solar day" that is the time from when the Sun crosses the meridian (due south or due north position if standing on its surface) to the next time it crosses the meridian. Mercury's spin rate is once per 58.8 Earth days and it takes 88 Earth days to orbit the Sun. These roughly equal time lengths produce a bizarre path of the Sun on Mercury. The Sun rises in the east, makes an arc upward toward the south, pauses, goes slightly backward, pauses, the proceeds toward the west again making an arc downward to the west. All of this takes about 176 Earth days. See the link to an animation of this on the MESSENGER website. One surprising finding from MESSENGER is that Mercury's magnetic field is offset far to the north of the planet's center by nearly 20% of Mercury's radius, a much larger offset than for any other planet. Another new finding is that our view of how Mercury's extemely thin atmosphere is created is too simplified. The basics are correct: the atmosphere particles are blasted off of Mercury's surface by solar wind, solar photons, and micrometeorite impacts but different types of atoms in its atmosphere are put there by varying amount of the three mechanisms. Mercury's magnetic field is too weak to protect the surface from the solar wind. The Earth's much stronger magnetic field protects our atmosphere from being blasted by the solar wind. More than 6% of Mercury's surface has volcanic plains, though the peppering of the plains by impacts is making it difficult to figure out the geologic history of those parts of the surface. Another surprise is the apparent lack of iron in the surface rocks. All these findings and more will be used to figure out Mercury's formation and evolution. Besides the intrinsic value of understanding Mercury, that understanding will help us better understand Earth's formation and subsequent evolution.
The Dawn spacecraft now orbiting the asteroid Vesta is returning a lot of fantastic information about this previously unexplored world. Vesta is large enough to be almost spherical and it has a rich trove of geologic features besides the expected impact craters including lava flows, giant ridges several miles high, valleys several miles deep, some mountains that are much taller than those on Earth, a diverse mineralogy, and a differentiated interior: a crust, mantle, and fairly large iron core. There are more craters in the north hemisphere than the south hemisphere. That may be because the south hemisphere had a huge impact near the end of the heavy bombardment about 3.8 billion years ago that greatly changed the topography of the southern hemisphere. The southern hemisphere seems to be on average about a billion years younger than the northern hemisphere based on crater counts but that is a preliminary number that will undoubtedly get revised with more data and re-examining the assumptions behind the estimate. Dawn has moved four times closer than its first survey orbit to the high altitude mapping orbit 420 miles above the mean surface. It will be in this orbit until the end of October and then spiral even closer for about another month to its closest position of the Vesta part of its mission. After completing its exploration of Vesta next year, Dawn will head on over to the largest asteroid, Ceres, an asteroid so large that it is classified as a dwarf planet.
In our night sky we see super-bright Jupiter rising just a little after sunset. In Aries, Jupiter continues its retrograde motion back towards Pisces. On October 28th and 29th, Jupiter will be at opposition which means it is directly opposite the Sun in our sky. At that time Jupiter will rise at sunset. Even brighter Venus is now becoming visible low in the west just after sunset and its appearance will improve over the weeks as it climbs up higher from the Sun. On Halloween, Venus will set an hour after the Sun but tonight it sets about 45 minutes after the Sun. In tonight's sky, Mercury will be down and to the right of Venus, setting 30 minutes after sunset. Both Venus and Mercury are between Virgo and Libra. On Halloween they both will be at the left side of Libra and Mercury will have moved to being straight below Venus—see the second star chart below. Tonight the Moon is a Waning Gibbous near the head of Taurus rising a little after 9 PM. The trick-or-treaters will see a thin Waxing Crescent in the southwest next to Sagittarius that will set around 10:30 PM. To complete the planetary line-up, orange-red Mars will rise at about 2:15 AM and we'll see it about half-way between the dim stars of Cancer and the brighter stars of Leo. By Halloween, Mars will be just under the upper end star of the Sickle of Leo. Mars will pass just above Regulus, the bright star at the bottom end of the Sickle of Leo November 9th and 10th. That should be a pretty sight in binoculars!
Patterns of stars like the Sickle of Leo, the Teapot of Sagittarius, and the Big Dipper of Ursa Major are called "asterisms". The Kern Astronomical Society's president, Rod Guice, gave a nice presentation on another asterism that encompasses several constellations and that we'll see in the east after 1:30 AM, called the Winter Hexagon. The first star chart below shows the Winter Hexagon. It is formed by connecting in clockwise order: Capella in Auriga, Aldebaran at the eye of Taurus, Rigel at the knee of Orion, Sirius at the neck or nose of Canis Major, Procyon at the hindquarters of Canis Minor, and Pollux or Castor at the heads of Gemini. Capella is actually a quadruple star system made of binary systems orbiting each other. One of the binary systems is responsible for practically all of the light we see and is made of two yellow giants orbiting each other every 104 days. The giants are 13.6 and 8.3 times larger in diameter than the Sun and about 3 and 2.6 times as massive as the Sun. They spent most of their lives as much hotter blue stars but are now swelling in size as they begin to fuse the helium in their cores to make carbon and oxygen while the outer layers puff up to make red giants. The other binary is made of much fainter red dwarf stars that are about 250 times farther away from the yellow giant pair than Pluto is from the Sun. At this distance the red dwarf binary takes about 400 years to orbit the yellow giant pair.
Other giants in the Winter Hexagon are red giant Pollux at about 9 times the diameter of the Sun and a mass 1.8 times the Sun and red giant Aldebaran at 44 times the diameter of the Sun and a mass 1.7 times that of the Sun. Aldebaran is expected to get even larger in diameter as its helium core contracts heats up. Sirius, Procyon, Rigel, and Castor are all normal adult stars still fusing hydrogen in their cores, though they are all larger, hotter, and more massive than the Sun. Sirius is 1.75 times larger in diameter and 2.1 times more massive than the Sun. It has a very small but very dense companion orbiting it, a white dwarf just 90% the diameter of the Earth but slightly more massive than the Sun. The white dwarf is the dead, exposed core of star that was once 5 to 7 times the mass of the Sun. Besides having that white dwarf orbiting it, Sirius' other claim to fame for Bakersfield observers, is that it is the closest star outside the solar system that we can see without a telescope. Observers a bit further south of us (Mexico, etc.) can see Alpha Centaurus that is even closer to us. Procyon is about 1.4 times the mass of the Sun and is 2.1 times the diameter. It too has a white dwarf companion that is about 60% the mass of the Sun and is 1.35 times the diameter of the Earth. Castor is actually a sextuplet with three pairs of binaries orbiting each other. I'll run out of room describing all of them, so see the link on the Planetarium's version of this column to get its stats. The last of the Winter Hexagon is definitely not the least. Rigel appears slightly dimmer than Capella but that is because of its large distance of 860 light years. Rigel actually produces about 85,000 times as much light as the Sun and is a blue-white supergiant about 73 times the diameter of the Sun (about the size of Mercury's orbit) with nearly 18 times the Sun's mass. Rigel is swelling as it turns into a red supergiant that will be about as large as Betelgeuse on the other side of Orion. Rigel has a binary system orbiting it made of two blue-hot stars that take about 25,000 years to orbit Rigel A. Around those three is a cool orange star that takes about 250,000 years to orbit all three. To finish the asterisms in the east, you can also connect the bright red star inside the Winter Hexagon, Betelgeuse, to Sirius and Procyon to make an equilateral triangle called the Winter Triangle (as opposed to the Summer Triangle of Deneb, Vega, and Altair visible in our evening sky high in the west). Whew!
Want to see more of the stars at night and save energy? Shield your lights so that the light only goes down toward the ground. See www.darksky.org for how.
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com