Sky Diary

Although still a cold month, February brings longer hours of daylight. On the first of the month the Sun rises at 8.09am and sets at 4.42pm but by the end of the month sunrise is an hour earlier at 7.07am and sunset at 5.42pm, giving us 10 hours and 35 minutes of daylight. Nights are properly dark from about 7.30pm and there is much to see in the February sky. By mid-evenings Orion has moved towards the south-west and Gemini, the Twins, is high in the south. Below Gemini you can't fail to notice the brightest star in the sky, Sirius. Sirius is the brightest star of the constellation Canis Major, the Great Dog, and is bright because it lies only 8.6 light years distant. It is twice the mass of our Sun and hotter, so appearing white, but Sirius is actually a binary star with a white dwarf companion which was even more massive than its companion but burned its fuel more quickly and collapsed to a white dwarf about 120 million years ago. Visitors to Mills Observatory often ask about the apparent scintillation and colour changes of Sirius but these effects are due to the low elevation of the star and warm air currents from the city below.

Looking back up to Gemini, the two brightest stars, Castor and Pollux, form the heads of the twins. The brighter star is Pollux which is an orange giant star nine times the diameter of the Sun and 34 light years distant. Castor, the companion of Pollux, is 51 light years distant from Earth and 18 light years from Pollux. Castor is a very different star having six gravitationally bound components. The two brightest and main stars are more like Sirius than our Sun as they are hotter and larger than the Sun. The open star cluster Messier 35 can be seen at the western extreme of Gemini. Binoculars are ideal for spotting this cluster and the best plan is to slowly scan upwards from Betelgeuse to the gap in bright stars between Castor and Pollux and the lowest star of the hexagon shape of Auriga. The stars of M35 are scattered over an area about the size of the full Moon and close by you may see a smaller and tighter cluster of stars which is known as NGC 2158. This cluster is 11,000 light years distant, about 8,000 light years further than M35. As you look at these clusters just think how long light from them has travelled across space to reach your eyes!

Looking east at 10pm during February you will see the distinctive shape of Leo the Lion rising to take prominence in a month or so. Following round the horizon towards east north-east, the bright star Arcturus will be close to the horizon and can be identified by following the curve of the handle of the Plough down towards the horizon.