2017: Autumn, Sidereal Signs of the Season


A year ago, I wrote about the positions the Big Dipper and Arcturus signalling autumn.  Another signal of the autumn is the Summer Triangle:  Vega, Deneb, and Altair.  Each star belongs to its own constellation, yet they are group together into a “super constellation” or better yet, an asterism.   Such groups are informal collections that make familiar shapes:  Big Dipper, Little Dipper, Teapot, Kite, Milk Dipper, Summer Triangle.  And the list goes on.

The Summer Triangle starts the night high in the south during the early evening hours of early autumn.  Deneb is nearly overhead.  Step outside around 8 a.m. on the next clear evening.  Face south.  Tilt your head back until you almost fall backwards.  That star at the top of the sky is Deneb, the tail of Cygnus the Swan.  The nose of the swan is marked by Alberio.

Deneb is one of the brightest stars in the night sky.  At 3,200 light years away, it shines with a brightness of over 100,000 suns.  Yet it appears in our sky as a bright star.  Think about it:  Our sun is so close it makes daytime on our planet.  Deneb is far brighter, but it appears as a dot of light because of its immense distance.

Alberio is a spectacular double star.  The star we see is yellow.  Through a telescope a dim blue companion star is visible.  It is one of the finest double stars in the sky because of its contrasting star colors.

Cygnus is inlaid in the Milky Way.  The swan seems to be flying southward along its length.  If you click the image to enlarge it, you can see a glow in the region around Cygnus.

Vega is the bright star in Lyra the harp.  It is about 100 times brighter than the sun, but it is about 250 light years away.  Again click the image to make it look larger.  Immediately above Vega is a star.   Look closely at it.  You will see that it’s two stars.  Through a telescope, at modest powers, each of those stars breaks into two stars.  This star is informally known as the “double-double,” as at low powers a double star is visible and then each individual resolves into another double star.

The final star is Altair, part of Aquila the Eagle.  This star is about 20 light years and a mere 10 times brighter than the sun.

Each year in early autumn, the Summer Triangle starts the evening high in the southern sky, a sidereal signal of the season.

For more information about sky watching events:

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