The Summer Triangle is the name given to an area of the
night sky that contains several features of interest for the amateur
astronomer.
Out On The Lawn
In 1933 W H Auden wrote a poem (“A Summer Night”) that
begins with the lines:
Vega conspicuous overhead
In the windless night of June …
In the windless night of June …
Leaving aside the merits of the poem – and there are many –
nobody could fault Auden’s astronomical knowledge. On a clear night in June, if
you lie looking up at the stars on an English lawn in June you will hardly fail
to be aware of the prominent star Vega, which is the fifth brightest star in
the entire sky.
Vega forms one of the extremities of what is known as the
Summer Triangle, the other points being Altair and Deneb. The Triangle is what
is termed an “asterism”, by which is meant an arrangement of stars other than
an officially recognised constellation. Altair forms the lowest point of the
triangle, with Vega to the top right and Deneb at the top left.
All three stars are also members of constellations: Vega is
in Lyra (The Lyre), Deneb in Cygnus (The Swan) and Altair in Aquila (The
Eagle). Each named star is the brightest in its constellation.
It is easy to imagine that the constellations and asterisms
seen with the naked eye represent actual associations of stars that are
relatively close to each other. However, this is rarely the case, because what
we are seeing are stars along certain lines of sight when viewed from Earth. A
star can be many times further away from us than its apparent neighbor, and the
Summer Triangle provides an excellent example of this fact.
The closest of the three stars to Earth is Altair. It is
16.7 light years away, which means that we are seeing it as it was 16.7 years
ago. Vega is 25 light years away but appears to be somewhat brighter than
Altair. That is because Vega is considerably more luminous than Altair, which
is a main sequence dwarf star – as is our own Sun. Altair is 11 times more
luminous than the Sun, but Vega is 52 times more luminous and therefore appears
brighter than Altair despite being considerably further away.
Even more striking is the case of Deneb. It appears to be
three times fainter than Vega, but that is because it is nowhere near as close.
It has been estimated that it could be between 1,550 and 2,600 light years
away, and would be invisible to the naked eye were it the same sort of star as
Vega or Altair. However, Deneb is a white supergiant star with a diameter 200
times greater than that of our Sun and it is 200,000 times more luminous. Were
Deneb to be at the same distance from us as is Vega, it would appear so bright
that it would cast shadows at night and be visible in daylight!
If the sky is dark and the Moon not shining, a viewer of the Summer Triangle will not fail to be impressed by the sweep of the Milky Way crossing the Triangle between Vega and Altair. It is even more impressive when viewed through a telescope.
This “river” is the combined light of the many millions of
stars that form part of our galaxy. The tilt of Planet Earth means that viewers
in the Northern Hemisphere can only look “outwards” whereas in the Southern
Hemisphere you would look “inwards” towards the heart of the galaxy. That is
not a great limitation as far as northern viewers are concerned, because our
Sun lies in one of the outer swirls of the galaxy (the Orion Arm) and there is
plenty to see that is in the same swirl as ourselves and in swirls even further
out, such as the Perseus and Cygnus Arms.
Albireo
This star is just to the left of a line drawn between Vega
and Altair and about halfway along that line. It can be seen with the naked
eye, but even a modest telescope will reveal it to be a double star with
strongly contrasting components in terms of color – Beta Cygni A is amber and
Beta Cygni B is blue-green. It has been suggested that Beta Cygni A is itself a
double star, making Alberio a triple-star system.
However, the jury is still out over whether Alberio
represents a true double/triple, with the components orbiting a common centre
of gravity, or whether this is an optical double with Beta Cygni A and Beta
Cygni B merely being seen along very close lines of sight.
If there are questions about the composition of Alberio,
there are none about Epsilon Lyrae, which lies quite close (visually) to Vega.
This is a genuine “double double”, with the two main components also being
doubles. You would need very good eyesight – or modest binoculars – to resolve
the main division, but something much stronger in terms of telescopic power to
see the system in all its glory – around 200x magnification in a four-inch
aperture telescope should do the trick!
The impression that these stars are sitting on top of each
other is easy to gain but somewhat misleading. The Epsilon Lyrae system (which
probably contains more than the four stars indicated above) is 162 light years
away, which is 6.5 times further away than Vega. The fact that it is possible
to resolve the components at all using standard ground-based equipment must mean
that they are at some distance from each other. Each of the two doubles are
around 120 AUs (astronomical units) apart. Given that an AU is the distance
from Earth to the Sun, that distance takes you considerably beyond the orbit of
any known object in the solar system – the most remote object discovered to
date is at 103 AUs and Pluto is almost cheek by jowl with the Sun at an average
distance of 39.5 AUs!
The distance between the two main pairs in Epsilon Lyrae is on
a completely different scale, at 10,500 AUs. When seen in the context of its
own star system that sounds like a vast distance; however, when one considers
that the distance to the Sun’s nearest star neighbor, Proxima Centauri, is
268,000 AUs, the Epsilon Lyrae stars sound to be almost touching each other!
Nebulas M57 and M27
Two impressive planetary nebulas can be seen in the Summer
Triangle. A nebula looks like a fuzzy star when seen through low-resolution
equipment, but a better telescope will reveal its true nature, namely the
expelled outer layers of an ancient red giant star that reached the end of its
life as a giant and continued as a white dwarf.
M57, which is known as the Ring Nebula, is about halfway
between Vega and Alberio, slightly to the right of an imaginary line between them.
A three-inch aperture or larger telescope will reveal the dramatic colors of
what has been described as a “celestial smoke ring”. M57 is 2,300 light years
away from us.
Alberio lies halfway between M57 and M27, which is known as
the Dumbbell Nebula. It is brighter and larger than M57 and closer to us at
1,360 light years. It is easier to spot than M57 and was in fact the first
planetary nebula to be identified. It has been calculated that the original red
giant star threw off its outer layers about 14,500 years ago, leaving its core
behind as a white dwarf star that is the largest such star discovered to date.
Time spent with a moderately powerful telescope pointed at
the Summer Triangle will be well rewarded!
© John Welford
No comments:
Post a Comment