Aquila the Eagle – Summer Sky
Aquila the Eagle is
one of the original 48 Greek constellations. Altair often represents a bird in mythology that was a
companion to Jupiter and often carried his thunderbolts.
The constellation is
one of the most prominent in the sky. Altair is one of the summer
triangle of stars that grace our sky during the summer months. The
others are Vega and Deneb. Altair is the most southerly of the
triangle stars.
Alpha Aquila or
Altair is from the Arabic meaning 'Flying Eagle' is one of the so
called 1st magnitude stars. Any star that is brighter than
magnitude 1.5 is classified as a 1st magnitude star.
Altair is fairly close to us being only 17 light years away. It is
the closest of the triangle stars. (Vega 25ly, Deneb 2200ly). The
12th brightest star in the sky Altair has a magnitude of
0.7. Although it is bright this is due to being fairly close to us.
Altair is an A class star being hotter than the Sun.
Altair is flanked by
two other reasonably bright stars.
Beta Aquilae whose
name is Alshain is just below Altair, the meaning of Alshain is not
certain but one Arabic source refers to Alshain as ‘The Raven’s
Neck’. The star is a yellow G8 star of magnitude 3.7 and is
located 45 light-years from Earth. Alshain is slightly cooler than
the Sun. Again we have the second lettered star of a constellation
not being the second brightest. That goes to gamma.
Gamma Aquilae whose
proper name is Tarazed, the meaning of which is unknown. has a magnitude of
2.7 and is a K3 giant which means it appears orange and is therefore
cooler than the Sun. It is 390 light years away
If you draw a line
from Altair to the right and slightly down you will come across delta Aquilae
whose name Deneb Okab means ‘The Tail of the Eagle’ which is
rather strange because it does not mark the tail of the eagle and is
located at the centre of the bird. It is an F0 star and is hotter
than the Sun and is 50 light years distant.
Using Altair and
delta it is possible to form a triangle with a third star eta which
would be the most southerly star of this triangle of stars. Eta has an average magnitude 3.9. I say average because the brightness of eta
varies in brightness between magnitude 3.5 and 4.3 in a period of
just over 7 days.
Stars that change in
brightness are called variable stars, there are many different types
of variable stars. Eta was discovered by Edward Piggot on September
10th 1784 in York, on the very same night also in York the
deaf astronomer John Goodricke discovered that the star beta Lyrae
also varied in brightness. I christened Goodricke and Piggot ‘The
Fathers of Variable Star Astronomy’ because of the work they did in
York between 1781-1786. With both Piggot and Goodricke discovering
two variables stars on the same night, something that had never
happened before I termed September 10th 1784 as ‘A Night
to Remember’. Eta is a Cepheid type variable the first Cepheid
which was delta Cepheus was discovered by Goodricke from York!! These
Cepheid variables are used by astronomers to work out the distance to
other galaxies.
Eta varies between
being a F5 to a G2 supergiant star when as it becomes brighter and
fainter, and lies about 1200
light years away.
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