The
Summer Triangle is
not one of the official 88 constellations, but it is nonetheless a prominent and
memorable shape made by stars (an "asterism"). It is comprised of the brightest stars of 3 constellations. You can't miss them; they are the brightest
stars east of Arcturus, which is the brightest star in last month's
Constellation of the Month: Bootes.
The brightest of the 3 stars – Vega, in the constellation
of Lyra (the Harp) – is the easternmost one.
To the west and north is Deneb, in the constellation Cygnus (the Swan,
also known as the Northern Cross). To
the west and south of Vega is Altair in the constellation Aquila, the Eagle.
The Summer Triangle is a good "asterism" to get to know. It is visible about half the year soon after
sunset, and it can be useful as a starting point for finding other
constellations -- as we'll describe in coming months.
Vega is an important star, because it was
used to define the zero point in the scale used to specify the apparent
brightness of stars – the magnitude scale. I
say "apparent" brightness, because it describes how bright a star
appears to us on earth. A very bright
star (one with a large "absolute" brightness) can appear to us as
less bright than a star of average brightness which is much closer to earth.
So, Vega was defined as having a magnitude of
0. And the brightness of other stars was
measured in relation to that. In order
to avoid very large numbers, the magnitude scale is logarithmic, like the
Richter scale for earthquake intensity.
A magnitude 1 star is 2.5 times fainter than a magnitude 0 star. And a magnitude 2 star is 2.5 times fainter
than that. So a magnitude 2 star is 2.5
x 2.5 (6.25) times fainter than a magnitude 0 star.
Stars brighter than 1.5 magnitude are called
1st magnitude, from 1.5 – 2.5 are 2nd magnitude and so
on. The naked eye in Ridgefield can
probably see no farther than 4th magnitude due to light pollution. The
other two stars in the Summer Triangle – Altair and Deneb – are magnitudes 0.77
and 1.2, respectively. So they are also 1st
magnitude stars. By way of comparison, the
stars in the Big Dipper are magnitude 2 stars.
South and west of Arcturus (0.13) in Bootes
are Spica (1.0) in the constellation of Virgo and two planets – Mars (currently
1.09) and Saturn (currently 0.93) –
which have been visiting in that area in recent weeks.
The apparent magnitude of planets varies – in some cases considerably – over the
course of weeks and months, since their distances from earth can vary greatly as they make their ways around the Sun.
Andy
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