| The
constellations Lyra and Cygnus |
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This month the constellations
Lyra and Cygnus are seen almost overhead as darkness falls with their
bright stars Vega, in Lyra, and Deneb, in Cygnus, making up the "summer
triangle" of bright stars with Altair in the constellation Aquila
below. (see sky chart above)
Lyra
Lyra is dominated
by its brightest star Vega, the fifth brightest star in the sky. It
is a blue-white star having a magnitude of 0.03, and lies 26 light years
away. It weighs three times more than the Sun and is about 50 times
brighter. It is thus burning up its nuclear fuel at a greater rate than
the Sun and so will shine for a correspondingly shorter time. Vega is
much younger than the Sun, perhaps only a few hundred million years
old, and is surrounded by a cold,dark disc of dust in which an embryonic
solar system is being formed!
There is a lovely
double star called Epsilon Lyrae up and to the left of Vega. A pair
of binoculars will show them up easily - you might even see them both
with your unaided eye. In fact a telescope, provided the atmosphere
is calm, shows that each of the two stars that you can see is a double
star as well so it is called the double double!
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Epsilon Lyra - The
Double Double |
| Between
Beta and Gamma Lyra lies a beautiful object called the Ring Nebula. It
is the 57th object in the Messier Catalogue and so is also called M57.
Such objects are called planetary nebulae as in a telescope they show
a disc, rather like a planet. But in fact they are the remnants of stars,
similar to our Sun, that have come to the end of their life and have blown
off a shell of dust and gas around them. The Ring Nebula looks like a
greenish smoke ring in a small telescope, but is not as impressive as
it is shown in photographs in which you can also see the faint central
"white dwarf" star which is the core of the original star which
has collapsed down to about the size of the Earth. Still very hot this
shines with a blue-white colour, but is cooling down and will eventually
become dark and invisible - a "black dwarf"! Do click on the
image below to see the large version - its wonderful! |
M57 - the Ring Nebula
Image: Hubble Space telescope |
| M56 is
an 8th magnitude Globular Cluster visible in binoculars roughly half way
between Albireo (the head of the Swan) and Gamma Lyrae. It is 33,000 light
years away and has a diameter of about 60 light years. It was first seen
by Charles Messier in 1779 and became the 56th entry into his catalogue. |
M56 - Globular Cluster
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| Cygnus
Cygnus, the Swan,
is sometimes called the "Northern Cross" as it has a distinctive
cross shape, but we normally think of it as a flying Swan. Deneb,the
arabic word for "tail", is a 1.3 magnitude star which marks
the tail of the swan. It is nearly 2000 light years away and appears
so bright only because it gives out around 80,000 times as much light
as our Sun. In fact if Deneb where as close as the brightest star in
the northern sky, Sirius, it would appear as brilliant as the half moon
and the sky would never be really dark when it was above the horizon!
The star, Albireo,
which marks the head of the Swan is much fainter, but a beautiful sight
in a small telescope. This shows that Albireo is made of two stars,
amber and blue-green, which provide a wonderful colour contrast. With
magnitudes 3.1 and 5.1 they are regarded as the most beautiful double
star that can be seen in the sky.
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Alberio: Diagram
showing the colours and relative brightnesses |
| Cygnus
lies along the line of the Milky Way, the disk of our own Galaxy, and
provides a wealth of stars and clusters to observe. Just to the left of
the line joining Deneb and Sadr, the star at the centre of the outstretched
wings, you may, under very clear dark skys, see a region which is darker
than the surroundings. This is called the Cygnus Rift and is caused by
the obscuration of light from distant stars by a lane of dust in our local
spiral arm. the dust comes from elements such as carbon which have been
built up in stars and ejected into space in explosions that give rise
to objects such as the planetary nebula M57 described above.
There is a beautiful
region of nebulosity up and to the left of Deneb which is visible with
binoculars in a very dark and clear sky. Photographs show an outline
that looks like North America - hence its name the North America Nebula.
Just to its right is a less bright region that looks like a Pelican,
with a long beak and dark eye, so not surprisingly this is called the
Pelican Nebula. The photograph below shows them well.
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The North America
Nebula |
| Brocchi's Cluster
An easy object to spot with binoculars in Cygnus is "Brocchi's Cluster",
often called "The Coathanger",although it appears upside down
in the sky! Follow down the neck of the swan to the star Albireo, then
sweep down and to its lower left. You should easily spot it against the
dark dust lane behind. |
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Mecury
SuperMoon
