Friday, November 18, 2011

Weekly SkyWatcher's Forecast: November 19-25, 2011

Written By Tammy Plotner

Friday, November 19 - Tonight let’s head less than a degree south-southeast of Delta Ceti (RA 02 43 40.83 Dec -00 00 48.4) to have a look at a galaxy grouping that features the magnificent M77.

Discovered on October 29, 1780 by Pierre Mechain, Messier cataloged it as #77 around six weeks later as a “nebulous cluster” – an accurate description for a small telescope. It wasn’t until 1850 when Lord Rosse uncovered its spiral nature that we began to view it as the grand structure seen in today’s modern telescopes.

Around 47 million light-years away, larger instruments will reveal its wide spiral arms where the older stars call home, and the concentrated core region where gigantic gas clouds move rapidly and new stars are being formed – a core which contains such a massive energy source that it emits spectrum of radio waves. After decades of study, the highly active nucleus of this Seyfert galaxy is known to have a mass equaling 10 million suns and a 5 light-year wide disc which rotates around it, which has intense star forming regions. This is one of the brightest known, and was cataloged by Arp as number 37 on his list of peculiar galaxies.

While even binoculars can spot the core, and modest scopes can reveal M77′s glory, larger telescopes will also spy 10th magnitude, edge-on NGC 1055 about half a degree north-northwest and 11th magnitude, face-on NGC 1073 about a degree north-northeast. Enjoy them tonight!

Saturday, November 20 - Today celebrates another significant astronomer's birth - Edwin Hubble. Born 1889, Hubble became the first American astronomer to identify Cepheid variables in M31 - which in turn established the extragalactic nature of the spiral nebulae. Continuing with the work of Carl Wirtz, and using Vesto Slipher's redshifts, Hubble then could calculate the velocity-distance relation for galaxies. This has become known as "Hubble's Law" and demonstrates the expansion of our Universe.

Tonight we're going to head just a little more than a fistwidth west of the westernmost bright star in Cassiopeia to have a look at Delta Cephei (RA 22 29 10.27 Dec +58 24 54.7). This is the most famous of all variable stars and the granddaddy of all Cepheids. Discovered in 1784 by John Goodricke, its changes in magnitude are not due to a revolving companion - but rather the pulsations of the star itself.

Ranging over almost a full magnitude in 5 days, 8 hours and 48 minutes precisely, Delta's changes can easily be followed by comparing it to nearby Zeta and Epsilon. When it is its dimmest, it will brighten rapidly in a period of about 36 hours - yet take 4 days to slowly dim again. Take time out of your busy night to watch Delta change and change again. It's only 1000 light-years away, and doesn't even require a telescope! (But even binoculars will show its optical companion...)

Sunday, November 21 - Tonight let's continue our stellar studies with the central-most star in the lazy "W" of Cassiopeia - Gamma...

At the beginning of the 20th century, the light from Gamma appeared to be steady, but in the mid-1930s it took an unexpected rise in brightness. In less than 2 years it jumped by a magnitude! Then, just as unexpectedly, it dropped back down again in roughly the same amount of time. A performance it repeated some 40 years later!

Gamma Cassiopeiae isn't quite a giant and is still fairly young on the evolutionary scale. Spectral studies show violent changes and variations in the star's structure. After its first recorded episode, it ejected a shell of gas which expanded Gamma's size by over 200% - yet it doesn't appear to be a candidate for a nova event.

The best estimate now is that Gamma is around 100 light-years away and approaching us a very slow rate. If conditions are good, you might be able to telescopically pick up its disparate 11th magnitude visual companion, discovered by Burnham in 1888. It shares the same proper motion - but doesn't orbit this unusual variable star. For those who like a challenge, visit Gamma again on a dark night! Its shell left two bright (and difficult!) nebulae, IC 59 and IC 63, to which we will return at the end of the month.

Monday, November 22 - Tonight’s astronomical adventure will be about exploring an ancient and well renowned star cluster — the Pleiades! Easily found from a modestly dark site with the unaided eye, the Pleiades can be spotted well above the north-eastern horizon within a couple of hours of nightfall. To average skies, many of the 7 bright components will resolve easily without the use of optical aid, but to telescopes and binoculars? The M45 is stunning…

First let’s explore a bit of history. The recognition of the Pleiades dates back to antiquity and it’s known by many names in many cultures. The Greeks and Romans referred to them as the “Starry Seven”, the “Net of Stars”, “The Seven Virgins”, “The Daughters of Pleione” and even “The Children of Atlas”. The Egytians referred to them as “The Stars of Athyr”, the Germans as “Siebengestiren” (the Seven Stars), the Russians as “Baba” after Baba Yaga, the witch who flew through the skies on her fiery broom. The Japanese call them “Suburu”, Norsemen saw them as packs of dogs and the Tonganese as “Matarii” (the Little Eyes). American Indians viewed the Pleiades as seven maidens placed high upon a tower to protect them from the claws of giant bears, and even Tolkien immortalized the stargroup in the “Hobbit” as “Remmirath”. The Pleiades have even been mentioned in the Bible! So, you see, no matter where we look in our “starry” history, this cluster of seven bright stars has been part of it.

Tuesday, November 23 - Tonight in 1885, the very first photograph of a meteor shower was taken. Also, the weather satellite TIROS II was launched on this day in 1960. Carried to orbit by a three-stage Delta rocket, the "Television Infrared Observation Satellite" was about the size of a barrel, testing experimental television techniques and infrared equipment. Operating for 376 days, Tiros II sent back thousands of pictures of Earth's cloud cover and was successful in its experiments to control the orientation of the satellite spin and its infrared sensors. Oddly enough, a similar mission - Meteosat 1 - also became the first satellite put into orbit by the European Space Agency, in 1977 on this day.

Where is all this leading? Why not try observing satellites on your own! Thanks to wonderful on-line tools from NASA you can be alerted by e-mail whenever a bright satellite makes a pass for your specific area. It's fun!

Wednesday, November 24 - Tonight let’s have a look at Beta Perseii – the most famous of all eclipsing variable stars. Now, identify Algol and we’ll learn about the “Demon Star”.

Ancient history has given this star many names. Associated with the mythological figure, Perseus, Beta was considered to be the head of Medusa the Gorgon, and was known to the Hebrews as Rosh ha Satan or “Satan’s Head”. 17th century maps labeled Beta as Caput Larvae, or the “Spectre’s Head”, but it is from the Arabic culture that the star was formally named. They knew it as Al Ra’s al Ghul, or the “Demon’s Head”, and we know it as Algol. Because these medieval astronomers and astrologers associated Algol with danger and misfortune, we are led to believe that Beta’s strange visual variable properties were noted throughout history.

Italian astronomer Geminiano Montanari was the first to note that Algol occasionally “faded” and its methodical timing was cataloged by John Goodricke in 1782, who surmised that it was being partially eclipsed by a dark companion orbiting it. Thus was born the theory of the “eclipsing binary” and it was proved spectroscopically in 1889 by H.C. Vogel. At 93 light years away, Algol is the nearest eclipsing binary of its kind and is treasured by the amateur astronomer for it requires no special equipment to easily follow its stages. Normally Beta Persii holds a magnitude of 2.1, but approximately every three days it dims to magnitude 3.4 and gradually brightens again. The entire eclipse only lasts about 10 hours!

Although Algol is known to have two additional spectroscopic companions, the true beauty of watching this variable star is not telescopic – but visual. The constellation of Perseus is well placed this month for most observers and appears like a glittering chain of stars that lay between Cassiopeia and Andromeda. To help further assist you, locate Gamma Andromedae (Almach) east of Algol. Almach’s visual brightness is about the same as Algol’s at maxima.

Thursday, November 25 - While Cassiopeia is in prime position for most northern observers, let's return tonight for some additional studies. Starting with Delta, let's hop to the northeast corner of our "flattened W" and identify 520 light-year distant Epsilon. For larger telescopes only, it will be a challenge to find this 12" diameter, magnitude 13.5 planetary nebula I.1747 in the same field as magnitude 3.3 Epsilon!

Using both Delta and Epsilon as our "guide stars" let's draw an imaginary line between the pair extending from southwest to northeast and continue the same distance until you stop at visible Iota. Now go to the eyepiece...

As a quadruple system, Iota will require a telescope and a night of steady seeing to split its three visible components. Approximately 160 light-years away, this challenging system will show little or no color to smaller telescopes, but to large aperture, the primary may appear slightly yellow and the companion stars a faint blue. At high magnification, the 8.2 magnitude "C" star will easily break away from the 4.5 primary, 7.2" to the east-southeast. But look closely at that primary: hugging in very close (2.3") to the west-southwest and looking like a bump on its side is the B star!

Dropping back to the lowest of powers, place Iota to the southwest edge of the eyepiece. It's time to study two incredibly interesting stars that should appear in the same field of view to the northeast. When both of these stars are at their maximum, they are easily the brightest of stars in the field. Their names are SU (southernmost) and RZ (northernmost) Cassiopeiae and both are unique! SU is a pulsing Cepheid variable located about 1000 light-years away and will show a distinctive red coloration. RZ is a rapidly eclipsing binary that can change from magnitude 6.4 to magnitude 7.8 in less than two hours. Wow!

Enjoy your observations and we'll see you next week!

No comments:

Post a Comment