Two stars are better than one
Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. His column, Atlantic Skies, appears every two weeks.
Until the summer constellations make an appearance in the night sky in a couple of months, I thought we'd look at some of the other interesting celestial objects that can be readily seen in small telescopes and even in binoculars.
Many of the stars that appear in the night sky, although we see them as single points of light, are, in fact, double stars (called binaries), and, in some cases, actually consist of three or more star components. It is estimated that as many as 50 per cent of the known stars are binaries. The most interesting thing about these binary systems (and with other multi-star systems) is that many of the stars in them are different colours.
There are two kinds of binary star systems — optical doubles, where, as a result of the line-of-sight effect, the two stars appear close to each other (as seen from Earth), although they are many millions of kilometres apart, and visual doubles (the most common sort), where the two component stars revolve around each other, sharing a common centre of gravity (referred to as being gravitationally bound).
A star's colour depends primarily on its surface temperature: the hottest stars appear blue (or blue-white) in colour, while the coolest stars appear red. Stellar temps are measured in degrees Kelvin (temperatures are measured upward from absolute zero, about -273 C). Without going into detail regarding stellar evolution and thermonuclear reactions in the core of stars (both of which contribute to a star's surface temperature), it's sufficient to say that it is the spectral wavelength that the light radiating from a star peaks at that determines its colour. Light peaking at long wavelengths from a cool star (low surface temperature) makes the star appear red, while light radiating at shorter wavelengths from a hot star (high surface temperature) makes the star appear blue/white. Stars in between the high and low temp ranges can appear a variety of colours.
For example, our sun, with an intermediate temp of about 5,800K, appears as yellow; Betelgeuse (in Orion), with a surface temp of 3,500K, appears red; and Bellatrix (also in Orion), with a surface temp of 21,500K, appears blue.
Viewing colourful doubles in binoculars and small telescopes is an interesting and challenging hobby. The number of doubles that can be seen in either instrument is determined by a number of factors: the light-gathering aperture (lens size) of the instrument, the magnification used, the limiting magnitude of the instrument (the faintest stars that can be seen for a given aperture size), quality of the optics, atmospheric clarity, the age of the observer (which determines the exit pupil size of the observer's eyes), and dark adaptation (letting your eyes to sufficiently adapt to the darkness).
There are countless double stars in the night sky (far too many to list here), many of them very colourful.
THE NIGHT SKY
Our evening star Venus becomes visible high in the western sky around 8:30 p.m. the week of April 27, with the waning, crescent moon to its upper left.
On April 28, Venus shines at its brightest (magnitude -4.7) for the year, referred to as its greatest illuminated extent, when the illuminated portion of the planet covers the greatest square area of the sky.
Jupiter (magnitude -2.1) rises in the southeastern sky around 2 a.m., before fading from sight shortly before 6 a.m.
Saturn (magnitude +0.6) is visible in the southeastern sky around 2:30 a.m., followed by Mars (magnitude +0.5) around 3:30 a.m., with both planets fading from view by about 5:30 a.m. Mercury is too close to the sun to be seen.
Until next week, clear skies.
EVENTS
April 28 — Venus at brightest of the year
April 30 — First quarter moon (second of April)