The stars of autumn

Ken Tapping's weekly column on astrophysics from the Herzberg Institute

After the short nights of summer, with the Sun only a few degrees below the horizon during the night, the skies of autumn come as a bit of surprise. Suddenly the nights are longer, darker and clearer.

Low in the west in the early evening is the bright, golden star Arcturus. This star is visible in the east in late winter evenings, and is a herald of coming spring. Now, half a year later, and the Earth halfway around the Sun, we see it in the western sky. It will set earlier and earlier every night until it disappears, to return in the eastern sky next spring.

However, at this time of year the dominant feature of a dark, clear sky is the Milky Way, which crosses the heavens like a great arch from the northeastern horizon and down into the southwest, broadening as it goes. A pair of binoculars will show it to be made of countless stars. In some places there are dark gaps, where clouds of cold gas and dust block out the light of the stars. In other places, especially down towards the southern end of the Milky Way, there are little silvery or pinkish blobs. This is where new stars have been born and their ultraviolet radiation is making the surrounding cloud glow.

Almost overhead is the fairly bright star Deneb, the brightest star in the constellation of Cygnus, “The Swan”, flying along the Milky Way. Westward, still high in the sky, you’ll see Vega, brighter than Deneb. Vega is the brightest star in Lyra “The Lyre”. Draw a line southward from the middle of the line joining Deneb and Vega, and you’ll arrive at Altair, in the constellation of Aquila “The Eagle”. These three stars make up the “Summer Triangle”, which is best visible – of course  –  in the autumn. Vega might look brighter, but actually Deneb is radiating far more heat and light. It is about 200,000 times more luminous than the Sun, but much further away. Our Sun will manage to sustain its energy output for about eight billion years. Since Deneb is only about 20 times more massive than the Sun, but radiating 200,000 times the amount of heat and light, it will last only a fraction of that time.

Vega is high in the sky in summer evenings, but now it is moving down in the west, making room for Capella, which will take its place in the winter. These evenings Capella lies in the northeast, but it’s getting higher in the sky every night.

Low in the southeast glimmers Fomalhaut, the brightest star in the constellation of Piscis Austrinus, “The Southern Fish”. This is the southernmost bright star visible from our latitudes, and is the “Herald of Autumn”.

However, this year, the biggest evening light show is the planet Jupiter, which rises over the eastern hills around 9 p.m. It is the brightest object in the sky other than the Moon, until around 4 a.m., when Venus rises. Venus is utterly unmissable.

In the eastern sky there is a little silvery patch of stars. This is the Pleiades star cluster, and looks like a necklace dropped by a careless goddess. Binoculars will show it to be a cluster of hundreds of bluish stars – all siblings from the same birth cloud.

In the late evening, scan to the right (south) from the Pleiades, and youíll find a bright, red star. This is Aldebaran, the angry eye of Taurus the Bull, one of the main winter constellations. Winter is coming. The Moon will reach first quarter on the 21st.

Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astro-physical Observatory, Penticton.