Spring Triangle – a Seasonal Asterism
The Big Dipper (known as the Plough in the UK) is probably the best known asterism. It's part of the constellation Ursa Major. Of the other asterisms, the Spring Triangle is one associated with the seasons. Although I've said the Spring Triangle, there are actually two versions of it. We'll have a look at both of them.
Both versions of the Spring Triangle contain three stars, each from a different constellation: Boötes (the Herdsman), Leo (the Lion) and Virgo (the Maiden).
Have a look at the triangles outlined in red in this diagram of the Spring Triangle. Both layouts of the Spring Triangle include Arcturus (in Boötes) and Spica (in Virgo). But if Regulus (in Leo) is the third star, you get more of an isosceles triangle, i.e., one with two sides of equal length. All three of these stars are very bright. If Denebola (also in Leo) is the third star, the triangle is almost equilateral (all sides equal), but Denebola isn't as bright as the other two stars.
Finding the Spring Triangle
The Spring Triangle is visible in most of the northern hemisphere in the southeastern evening sky between March and May. The easiest way to find it is by first locating the Big Dipper (Plough). Look at the two stars in the “bowl” that are nearest the handle. A line running southward through them takes you to Regulus which looks like a dot at the bottom of a backwards question mark.
To find Arcturus, follow the curve of the Dipper's handle around to a bright and yellowish star. This is Arcturus at the bottom of the constellation Boötes which looks like an elongated kite. Extending the curve about the same amount again into the southeastern sky, you can find Spica.
Denebola comes from the Arabic meaning the lion's tail. If you look back to the last diagram, you can see that there's a little triangle in Leo. Denebola is at the tail end of that triangle.
The stars of the Spring Triangle
Arcturus was the Bear Guard in Latin and Greek mythology. It's the brightest star in the northern sky and has been important in the astronomy and mythology of many cultures. Visible in both hemispheres, it was an important Polynesian navigation star. Arcturus is an orange giant 37 light years away from us and about 25 times bigger than the Sun.
Virgo represents an agricultural goddess and the name Spica refers to an ear of corn. Although it's 250 light years away, Spica is one the brightest stars in the sky. However it's not one star, but two. They're so close together that they appear as one in a telescope. The primary star of the pair is a blue giant, but both of them are bigger and brighter than the Sun. They're much closer to each other than Mercury is to the Sun, and they orbit each other every four days. Their closeness and the pull of their mutual gravity would make Spica look like one big egg-shaped star if you were close enough to see its shape.
Like the other two stars of the Spring Triangle, Regulus (the Little King) is one of the 25 brightest stars in the sky. It represents the heart of Leo the Lion. Regulus looks like a blue-white star, and through a telescope we can see it's a binary system. It takes a bigger telescope and a stellar spectrum to see that each of the two stars is itself binary. The primary star of the main pair is a blue-white star over three times the mass of the Sun. Its companion is probably a white dwarf, the remains of a sunlike star that has exhausted its nuclear fuel. The second pair of stars is quite faint.
Denebola is about 36 light years away, almost double the mass of the Sun and around fifteen times as luminous. It's easy to see without optical aids, but isn't as bright as the other Spring Triangle stars. Interestingly, it has a dusty disk from which planets could form, but so far none have been discovered. Here is the Spring Triangle with Denebola instead of Regulus. (Please note that although Saturn and Mars are shown in the diagram, they aren't always seen in those positions.)
Deep sky objects
The Spring Triangle is well away from the Milky Way, but the Virgo cluster of galaxies is there. It needs big telescopes to see most of the two thousand galaxies of the cluster. However fifteen of them are Messier objects, i.e., objects listed in the catalog of Charles Messier. Those, and many others, are visible in amateur telescopes.
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