Friday, December 16, 2011

Weekly SkyWatcher's Forecast - December 16-22, 2011

Written by Tammy Plotner

Greetings, fellow SkyWatchers! It's going to be another great week. We have a host of unusual stars for you to explore - along with another meteor shower, a bit of history and even the upcoming solstice! Come on outside and join us... Because here's what's up!

Friday, December 16 - Tonight we’ll try our skills at observing an interesting variable star. RT (star 48) Aurigae is a bright Cepheid that is located roughly halfway between Epsilon Geminorum and Theta Aurigae. This perfect example of a pulsating star follows a precise timetable of 3.728 days and fluxes by close to one magnitude. Located 1600 light-years away, RT was first discovered in 1905 by T. H. Astbury of the British Astronomical Association. Like all Cepheids, it expands and contracts rhythmically - for reasons science is not completely sure of. Yet, we do know that it takes about 1.5 days for it to expand to its largest and brightest and 2.5 days for it to contract, cool, and dim.

Saturday, December 17 - Want a challenging double this evening? Then let's have a look at Theta Aurigae located on the east side of the pentagonal shape of this constellation. Located about 110 light-years away, 2.7 magnitude Theta is a four star system, whose members range in magnitude from 2.7 to 10.7. Suited even to a small telescope, the brightest member - Theta B - is itself a binary at magnitude 7.2, and was first recorded by Otto Struve in 1871. The pair moves quite slowly, and may take as long as 800 years to orbit each other at their separation of about 110 AU. The furthest member of this system was also noted by Struve as far back as 1852, but it is not a true member - the separation only occurring thanks to Theta's own proper motion. While you are there, be sure to note Theta's unusual color. While it will appear "white," look closely at the diffraction caused by our own atmosphere which acts much like a prism... You'll notice a lot more purple and blue around this star than many others of the same spectral type. Why? Theta is a silicon star!

Sunday, December 18 - Tonight let's head for Alpha Persei (Mirfak). While there's nothing particularly interesting about this 570 light-year distant star, what is incredible is the field in which it resides! Take a look at lowest power with a rich field telescope or binoculars and be prepared to be blown away... This is the Alpha Persei moving group - a fantastic field of main sequence stars that contains a little over 100 members. Even though it will take 90,000 years before any perceptible change is seen in this bright collection, they are happily moving at a pace of about 16 kilometers per second towards Beta Tauri! Enjoy this fine group also known as Melotte 20...

Monday, December 19 - Tonight is the peak of the Delta Arietid meteor shower. While most showers are best after midnight, this is an early evening shower that must be viewed before the radiant sets. The fall rate is modest - about 12 per hour. Today marks the founding of Mt. Wilson Solar Observatory. It officially opened its doors in 1904. We also celebrate the birth of Walter S. Adams on this date. Born in 1876, Adams was the astronomer at Mt. Wilson who revealed the nature of Sirius B, the first known white dwarf star. Sirius B was first seen by Alvan Clark in 1862 and most recently, the Hubble Space Telescope precisely measured the mass of B for the first time.

While Sirius is far too low at an early hour to study its white dwarf, we can have a look at a similar star when we view Omicron 2 Eridani located roughly a handspan west of Rigel. As the southernmost of the Omicron pair, it is sometimes known as 40 Eridani, and you'll find it to be an interesting multiple star system that's very worthy of your time. Discovered by William Herschel in 1783, this 16 light-year distant system is the eighth nearest of the unaided visible stars. Well spaced from the primary, the companion star is also a double for high powers and will reveal a red dwarf discovered by Otto Struve. Now, look closely at the 9th magnitude B star. This is the only white dwarf that can be considered "easy" for the backyard telescope. Its diameter is only about twice the size of Earth and its mass is about that of our Sun. Power up and locate the 11th magnitude companion - for it's one of the least massive stars known! And this white dwarf may be the smallest stellar object visible in an amateur telescope - it would be like spotting a tennis ball...on the Moon!

Tuesday, December 20 -While we're out tonight, let's have a look at one of the best known double stars in the night - Gamma Arietis (RA 01 53 31.81 Dec +19 17 37.9). Also known as Mesarthim, this combined magnitude 4 beauty was unintentionally discovered in 1664 by Robert Hooke who was following a comet. While no real change has been spotted in the more than 343 years since that time, there has been a slight difference detected in the components' radial velocities. Roughly 160 light-years away, you'll enjoy this almost matched-magnitude pair of white stars - but look carefully: in 1878, S. W. Burnham found a third star nearby that might not be a physical member, but is also a double!

Wednesday, December 21 - Up early? Fantastic! In the pre-dawn hours of this morning, I have a treat for you - the Ursid meteor shower! Cruising around the Sun about every thirteen and a half years, Comet 8P/Tuttle sheds a little skin. Although it never passes inside of Earth's orbit, some six years later we pass through its debris stream. Not so unusual? Then think again, because it takes as much as six centuries before the meteoroid trail is affected enough by Jupiter's gravitation to deflect the stream into our atmosphere. With little interference from the Moon while watching this circumpolar meteor shower, the hours before dawn could see activity of up to 12 per hour. By keeping watch on the constellation of Ursa Major, you just might spot one of these slow moving, 600 year old travelers that make their path only halfway between us and Selene!

Today marks Winter Solstice - for the northern hemisphere, the shortest day and the longest night of the year - and the point when the Sun is furthest south. Now is a wonderful time to demonstrate for yourself our own movements by choosing a "solstice marker." Anything from a fence post to a stick in the ground will suffice! Simply measure the shadow when the Sun reaches the zenith and repeat your experiment in the weeks ahead and watch as the shadow grows shorter...and the days grow longer!

Thursday, December 22 - Tonight we’ll look at a tremendous star as we head 150 light-years away to Menkar... Better known as Alpha Ceti, you'll find this nearly second magnitude giant orange beauty just west of Orion's "bow" (RA 03 02 16.77 Dec +04 05 23.0). With even a small telescope, you will also see 5th magnitude 93 Ceti in the eyepiece as well! Although they are not a true physical pair (the blue 93 is 350 light-years further away), they make a wonderful color contrast which is well worth your time. Just think... If 93 were as close as Menkar, it would be 250% brighter. But up the magnification and see if you can spot another true double in the field!

Until next week? May all your journeys be at light speed...

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