24.01.2021

High toward the south this week at around 9:30 p.m. local time, we can see a "Great Hexagon" of bright winter stars; a pattern that was first conceived by author Hans Augusto Rey (1898-1977) in his popular sky guide "The Stars — A New Way to See Them." 

Mr. Rey's chief claim to fame was as the creator of the mischievous little monkey, Curious George. But he was also a very assiduous amateur astronomer and both of his star guides (the other, a children's' book, "Find the Constellations") are considered enduring classics from the 1950s and have never been out of print since they were first published by Houghton Mifflin Co. of Boston. 

A hexagon is a polygon with six sides and our celestial hexagon is composed of six of the 21 brightest stars. To the south and a little east lies the bluish star Sirius, in Canis Major, the "Dog Star," the brightest of all stars. Sirius is roughly twice as massive as our sun and shines 25 times as luminous. But the chief reason it appears so dazzling is that it lies relatively close to us; a distance of 8.6 light-years away. It appears to be stud on the Big Dog's collar. 

Up to the west is blue-white Rigel, in the constellation of Orion, the hunter. A supergiant star, located 860 light-years from Earth and one of the most luminous of all the naked-eye stars. It is estimated to shine with a luminosity of approximately 120,000 times that of our sun. 

Still higher, is orange Aldebaran in the constellation of Taurus, the bull, a giant star 65 light-years away, with the distinction of having a planet estimated to be several times more massive than Jupiter circling it. The bull is charging toward Orion and a V-shaped pattern of stars represents his face — the Hyades star cluster — and we might think of Aldebaran as the bull's angry bloodshot eye,

Then at the north end of the Hexagon we would find yellow Capella in the constellation of Auriga. At 42 light-years distant, this is actually a quadruple star system composed of four stars. Capella is known as the "she-goat star" and if you look carefully below and to its right, you'll see three much fainter stars popularly known as the "kids," or young goats.

South and slightly east, we come to yellow-orange Pollux, the brighter of the two stars marking the heads of the Gemini twins. Slightly dimmer Castor is immediately adjacent to Pollux. 

Finally, south again to Procyon, the "Little Dog Star" in Canis Minor. Like Sirius, it is relatively close to us at only 11.5 light-years away. Procyon is a Greek name meaning "before the dog." And indeed, from mid-northern latitudes, it rises about 25 minutes prior to Sirius making its appearance above the east-southeast horizon. 

What's inside?

There are two things of interest that can be found inside of the Hexagon. First is a very bright reddish star, Betelgeuse, a star that was in the news quite a bit during 2020 because it went through an unexpected fading. 

Since Betelgeuse is a red supergiant star and such stars are close to the end of their life, there was some speculation that this star might soon end its life in a supernova explosion. However, scientists ultimately determined that the abnormal fading was most likely caused by an immense amount of hot material ejected into space, forming a dust cloud that temporarily blocked starlight coming from Betelgeuse's surface. 

Also, inside the Hexagon lies the intersection of the celestial and galactic equators. The celestial equator is simply the plane of the Earth's equator, projected on the sky. The galactic equator lies in the plane of rotation of the Milky Way. The two are inclined to each other at 62 degrees. That value, corresponding to the angular difference between Earth's axis and that of the galaxy, shows how far the Earth is tilted from the galactic viewpoint, and vice versa.

Other patterns

With so many bright stars encompassing a relatively small region of the sky, it is easy to envision other geometrical shapes. In fact, before Mr. Rey created his Great Hexagon, he used Sirius, Procyon, Pollux, Castor and Capella to form a sweeping semicircle which he called the "Great Arc of Sirius."

And we can also form a large, nearly equilateral triangle formed by the stars Betelgeuse, Sirius and Procyon, sometimes called the Winter Triangle. Like the Vega-Deneb-Altair Summer Triangle, it overlaps the Milky Way, though a much less conspicuous part of it. 

And yet others have turned Rey's Hexagon into something different: the heavenly "G." Just start at Aldebaran and sweep counterclockwise down and around until you come to Rigel. Then, instead of continuing back to Aldebaran, divert instead to Bellatrix, the star that marks Orion's left shoulder; the third brightest star of the Orion constellation. Then head straight east (left) to Betelgeuse, completing the outline of the sixth letter of the alphabet.

Stars for all seasons

Orion, Taurus, Gemini and the other surrounding star patterns are conventionally referred to as winter constellations, yet such seasonal designations are rather loose. They simply tell the season when a star pattern is best seen during convenient evening hours. 

Aside from the fact that these constellations are summertime ones for Australia and Brazil, they can also be seen during the autumn by those in the Northern Hemisphere who may look skyward between midnight and dawn. For example, the same stars and constellations that are now in the sky during the mid-evening hours can be seen at 5 a.m. local daylight saving time in mid-October. 

Precession is another factor that determines the "season" of a constellation. Precession refers to the wobble of the Earth's axis; a funnel-shaped motion, which describes one complete wobble in about 25,800 years. Currently, the star Polaris is the North Star. But back in the days of pyramids, some 5,000 years ago — when the star Thuban was the North Star — Orion, Taurus and Gemini were autumn constellations for the Northern Hemisphere. And when the brilliant star Vega becomes the North Star in 14,000 AD, they will be summer groupings. 

Quelle: SC