February Sky Notes

Winter’s Mighty Hunter
The outline of Orion is one of the most recognizable constellations across the world, straddling the celestial equator as it does and visible from every inhabited part of the globe. Orion is not only blessed with bright stars, it also contains numerous nebulae and hidden gas clouds. The main body of Orion is quite distinct: a sloping line of three stars set amidst a large rectangle of four others.
The orange star Betelgeux (sometimes spelled Betelgeuse) marks the left shoulder of Orion.  It is a red super giant star nearing the end of its life, constantly readjusting its structure and therefore variable in brightness. Betelgeux is so gargantuan it would swallow up the whole orbit of Mars around the Sun! Recently Betelgeuse has noticeably dimmed giving rise to wild speculation in the media that it is about to go supernova. It will one day, astronomers know that for certain, but caution against the idea that it already has. Our best guess is that we have several thousands of years to wait before it catastrophically self destructs, seeding space with all the elements manufactured in its interior throughout its life and appearing as a dazzlingly bright guest star in our skies for several months.
Diagonally opposite Betelgeux, occupying the bottom right corner of the rectangle, sits sparkling blue/white Rigel, a high mass, extremely hot star still in the flush of youth. Rigel is over a thousand light years away, approximately twice the distance of Betelgeux, and is ranked 7th brightest star in the heavens. It is estimated to be over 60,000 times more luminous than our Sun and as such will squander its fuel reserves in tens of millions of years rather than billions. Rigel’s ultimate fate will be similar to that of Betelgeux.
Residing not too far below the three iconic belt stars hangs the sword of Orion, a fainter line of three stars. The middle star of these appears distinctly hazy to the naked eye and binoculars show it clearly as a nebulous smudge. Through a telescope the vista is memorable – one of heavens’ finest. This is the Great Orion nebula, a region in space some 1,300 light years distant in which stellar birth is occurring. The nebula is the brightest portion of a huge gas and dust cloud that encompasses much of the region surrounding the belt and sword stars. Most of this cloud is invisible to the eye, but in a few places young emerging stars ionize and illuminate the gas cloud. The source of the great nebula’s illumination can be observed with a telescope, a tiny cluster of four stars known as the Trapezium, the ‘bully boys’ of this particular stellar crèche, estimated to be only a few millions years old. These stars too will evolve to become stellar beacons, by which time the appearance of Orion will have drastically altered, a consequence of stellar evolution on the grandest of scales.
The Dog Star
Given a clear, dark, moonless night in February, the winter sky is presented at its best, with mighty Orion located at the hub of a glittering array of stars ranged across the south. The most eye-catching of all these stellar jewels, sparkling Sirius (the ‘Dog star’) is visible low above the southern horizon. Sirius is the brightest star in the night sky and lies in Canis Major – the Great Dog – a constellation which only just clears the horizon as seen from the UK. Because of the low altitude, Sirius often appears to twinkle vigorously (scintillation), an effect which can sometimes appear quite alarming to the uninitiated in stellar observations. It is purely the distortions of our own atmosphere which causes this, not a UFO!                                                                                                                                                          Throughout human history, Sirius, meaning, ‘sparkling or scorching one’, has been the most brilliant of the fixed stars and as such revered in many cultures. In ancient Egypt, around 3000 bc, Sirius was the ‘Nile star’ whose appearance just before dawn near the summer solstice heralded the impending rise of the Nile into flood, an event upon which Egyptian agriculture depended.                                                                                                          
Sirius may be twice the size and twenty-six times more luminous than our Sun, but its eminence in our night sky is really down to its proximity, a mere 8.6 light years away, making it the 5th nearest star. Sirius is accompanied by a tiny White Dwarf companion star, the degenerate core of less massive Sun-like stars. Dubbed the ‘Pup’, this object is less than 20,000 miles in diameter, but very dense, a teaspoon of ‘Pup’ material weighing as much as a ten bull elephants! The Pup is just visible in larger amateur scopes, but requires careful positioning to spot it, overwhelmed by the radiance of Sirius itself.
Pick a clear night later in the month and take a good look at Orion and the Dog Star.
Whitby & District Astronomical Society will be supporting the National Park’s Dark Sky Festival at events to be held at the Fox and Hounds, Ainthorpe, and the Danby Moors Centre on 21 and 22 February respectively.


Mark Dawson,
Whitby & District Astronomical Society

For more information about Whitby & District Astronomical Society see: www.whitby-astronomers.com