SKY WATCH February 1996

by Harry Adams


Light is an astronomer's best friend and worst enemy. We spend hours peering through the eyepiece trying to glimpse the thin trickle of photons reaching us from distant galaxies. Others use computers and CCD cameras to capture the even more rarefied output of quasars lurking near the edge of the universe. The quest for photons even results in a few of us catching "aperture fever", the obsessive acquisition of ever larger telescopes with which to hunt down our elusive prey.

One would think that more starlight would be every astronomer's dream but this is not always desirable. If you spend a day at the beach or skiing in the light from our own star, this overdose of sunshine can degrade your ability to see the faint fuzzies for hours. Wearing sunglasses definitely helps but it is better to avoid this type of exposure for a couple of days before that long awaited dusk until dawn session at the telescope.

Street lighting and house lights both cause astronomers grief. The direct illumination from these sources interferes with night vision and stray light entering the telescope reduces contrast at the eyepiece. In addition, most street lights send some light directly up into the sky. The sky glow from these poorly designed fixtures further reduces contrast. As a result, many amateurs and some professional astronomers travel for hours to find a dark sky location from which to observe. I know people from Toronto who travel to Deep River just for that reason.

This month's star chart is for 21h EST. The chart shows the sky for the middle of the month, looking to the South, from the Horizon (curved line near the bottom) to the Zenith (the point directly overhead, labelled Z on the chart). The directions southeast (SE), south (S) and southwest (SW) are shown on the Horizon line. Brighter stars are represented by larger dots, fainter stars by smaller dots. When magnitudes are given, more negative values indicate brighter objects. All times are given in Eastern Standard Time.

A pair of open clusters and a pair of double stars are highlighted this month. M41 in Canis Major is a large loose cluster of stars about 4 degrees south of Sirius, the brightest star in the constellation. It is best viewed in binoculars. Can anyone see any coloured stars in the cluster?

In Cancer, look for M44, also known as the Beehive Cluster. Under dark skies, this is visible as a hazy patch to the unaided eye. With binoculars or a telescope individual stars appear. Again, can you see any coloured stars?

The star Iota in Cancer (iota Cancri) is a double star visible with 10x50mm binoculars. The separation is 31 seconds and the colours are yellow and blue. The star Gamma in Lepus (gamma Leporum) is another binocular double with a separation of 95 seconds. The colours are yellow and deep red.

The colours of double stars have been and still are the subject of some controversy. Some of the colours seen, such as green, are not easily accounted for as they are rarely seen in isolated stars. The thinking goes that at least some of the colours seen are due to contrast effects in the eye of the observer. These easily resolved and colourful doubles are favourites at star parties. About two-thirds of all stars are found in double or multiple star systems. Single stars, like our Sun are the exception rather than the rule.

The star Castor in Gemini is an example of a multiple system. It has three components known as A,B and C, each of which is a double star with components of roughly equal size. Castor A and B orbit each other once every 467 years, while Castor C is so far away from the other two that it takes over 10,000 years to orbit them. The stars of Castor A are much larger and hotter than the Sun, while those of Castor B are of intermediate brightness. Castor C is a pair of cool red dwarf stars that are a little smaller than the Sun. The Earth lies in the orbital plane of C and so the pair eclipse each other twice in every 19.5 hour orbit. During eclipse the star fades from magnitude 9.1 to 9.6.

Castor A and B are separated by about 3.5 seconds of arc and a telescope with moderately high magnification is requires to split them. Castor C is separated from A and B by about 72 seconds of arc and lies a little north of east from them. Look for its reddish colour.

Binoculars will just reach 9th magnitude but will not show the colour in stars this faint. Double stars separated by less than 30 seconds of arc are difficult to split with binoculars due to their low (10x) magnification.

The Sun rises at 7:13h on the 15th and at 6:50h on the 29th. Sunset occurs at 17:35h on the 15th and at 17:55h on the 29th.

Venus continues to climb higher in the sky this month. Shining at magnitude -4.1 it may be seen about 35 degrees above the southwest horizon at sunset all month, setting 3 to 4 hours after sunset.

Jupiter shines at magnitude -1.9 in the morning sky. Rising at 4:51h on the 15th, it may be seen 16 degrees above the southeast horizon at sunrise.

Saturn and Venus pass very close to on another during the first few days of February. Saturn, shining at magnitude 1.2, is about 5 magnitudes fainter than Venus or about 100 times less brilliant. The ringed planet is low in the southwest at sunset and sets about 20:00h midmonth On the evening of the 11th Earth crosses through the Saturn's ring plane from north to south. The next crossing does not occur until 2009.

Moon FM - 4th, LQ - 12th, NM - 18th, FQ - 26th

Back issues of Sky Watch are now available on the Deep River Astronomy Club's world wide web page at http://intranet.on.ca/~rbirchal/draco.html .

References: Burnham's Celestial Handbook, R.Burnham, Jr., General Publishing Co., Toronto. The Observer's Handbook of The Royal Astronomical Society of Canada, edited by Roy L. Bishop, University of Toronto Press Incorporated. Star chart produced using Guide 4.0 software by Project Pluto, with permission.


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