View from Mars Hill: Looking at the Summer Triangle and its brilliance

View from Mars Hill: Looking at the Summer Triangle and its brilliance

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Summer Triangle

A wide field image of the region of sky in which HD 189733b is located. In this image we can see the asterism of the "Summer Triangle" a giant triangle in the sky composed of the three bright stars Vega (top left), Altair (lower middle) and Deneb (far left). HD 189733b is orbiting a star very close to the centre of the triangle.

Astronomers divide the night sky into 88 regions, each representing a different grouping of stars called constellations and delineated — like countries on Earth — by specific boundaries. Besides these familiar astral federations, many less formal star groupings sprinkle the sky, consisting of patterns of stars that do not comprise entire constellations. The Big Dipper in the constellation Ursa Major is perhaps the best-known. Another, prominent in the current night sky, is the Summer Triangle.

The Summer Triangle lies in part of the sky marked by a white streak oriented in a northeast/southwest direction. This is the central portion of our Milky Way Galaxy.

The points of the Summer Triangle are represented by the brightest stars of three different constellations — Lyra, Cygnus and Aquila. Vega (in Lyra the Harp) is the brightest star in this asterism and the 5th brightest star in the night sky. It is visible in the northeastern skies just after dark, and by 11:00 p.m. will be nearly at zenith — the point directly overhead.

As an aside, if you were to draw an imaginary line passing through the zenith and connecting the poles of Earth, the resulting curved line defines the local meridian, which is used to define the familiar time-keeping terms “a.m.” and “p.m.” The former stands for ante meridian, meaning before the meridian. This refers to the sun being positioned ahead of the meridian, which occurs in the morning. As the sun appears to move across the sky, it passes through the meridian at about noon and then is past the meridian (post-meridian, “p.m.”) in the afternoon.

Vega is significant because it occasionally serves as the North Star. Earth spins on its axis once a day and revolves around the sun once a year. It also wobbles like a top, a motion known as precession. Each wobble takes about 26,000 years to complete. As this wobble progresses, the Earth’s polar axis points at different points in the sky, resulting in different pole stars.

Thirteen thousand years ago Vega was the North Star. During construction of the great pyramids 4,000 years ago, the star Thuban, in Draco (the Dragon) was the North Star. The honor currently goes to Polaris, the brightest star in Ursa Minor (the Little Bear).

The second point of the Summer Triangle is the star Altair, in Aquila (the Eagle) — an otherwise rather uninteresting constellation. The third point is Deneb, the brightest star in Cygnus (the Swan). Deneb means “tail” and refers to the tail of the swan. Just south of Deneb is a row of three prominent stars. The middle one represents the swan’s body, and the outside two symbolize the swan’s outstretched wings. An imaginary line drawn from Deneb through the middle star will continue to a fainter but still distinct star; this is Albireo, and represents the head of the swan.

Albireo is fascinating in its own right; through a telescope it’s revealed as a double star. One is yellow and the other blue, like the flowers of Navajo Tea and lupine that are now blooming across northern Arizona.

Thanks to the brilliance of Vega, Altair, and Deneb, the Summer Triangle is one of the easiest groups of stars to find. It will remain high in the early evening sky for the next few months.


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