Ursa Major, the Great Bear, culminates at midnight tonight. This means the constellation is at its best visibility for the year. For observers in the northern hemisphere, the seven brightest stars of the bear make up an asterism known as the Big Dipper in North America and the Plough in the United Kingdom. (An asterism is a group of stars that appears to form a shape or pattern. Typically, the term refers to a group of stars within a larger group – for example, it can also refer to a small group of stars that appears to form a shape or pattern within an open star cluster.)
For observers in the northern hemisphere, this asterism can help us to find other stars and constellations in the area. Most famously, Dubhe and Merak point to Polaris, the Pole star, in Ursa Minor. Dubhe and Megrez point west to Capella, the brightest star in Auriga, the Charioteer. Megrez and Merak point south-west to the Castor and Pollux, the two stars that identify Gemini the Twins. Dubhe and Merak point south towards Leo, the Lion, while Merak and Megrez point toward Hercules, the Hero, and the rising summer constellations in the east.
Lastly, follow the curve of the bear’s tail (Alioth, Mizar and Alkaid) south-east to the orange giant star, Arcturus, in Bootes the Herdsman. If you continue the curved line even further, you’ll come to Spica, the blue giant star of Virgo, the Virgin.
This is asterism is one of the best known and most useful group of stars known to astronomers and is only rivalled by the winter constellation Orion, the Hunter. But unlike Orion, Ursa Major is visible throughout almost the entire year and is nearly always available as a signpost to the stars.
(N.B. Observers in the southern hemisphere will barely see Ursa Major above the northern horizon. Observers in the United Kingdom will see a very similar view to those in North America, except that the constellation will be a little higher in the sky. For observers in northern Europe – such as the United Kingdom – Ursa Major appears to be circumpolar, meaning it never sets, no matter what time of year it is. Therefore, it will always be visible in the night sky and can always be relied upon to act as a guide for those learning their way amongst the stars.)
(Click on an image to enlarge.)
Images courtesy of Mobile Observatory.
The constellation Ursa Major (the Great Bear) culminates at midnight tonight. (All night. See image above.)
The barred spiral galaxy M108 culminates at midnight tonight. (Magnitude 10.7. Leo, all night.)
Learn more about every upcoming astronomical event in 2015 An Astronomical Year (Kindle eBook) and The Astronomical Almanac (2015-2019) (Kindle eBook) and The Astronomical Almanac (2015-2019) (Paperback Edition).
The Amateur Astronomer’s Notebook allows astronomers to log 150 observing sessions and includes an appendix of hundreds of suggested deep sky objects.