Corona Borealis or the Northern Crown is a spring constellation with a very noticeable curve shape of stars, it was the only stellar crown known to the Greek astronomers including Eratosthenes, but they referred to it as a wreath. The legend which appears to be most common today is that Bacchus the Roman god of wine gave it as a present to his new wife Ariadne, after her death it was cast by him into the night sky for ever. This story first seems to have appeared around 500 BCE and was recorded by the Greek philosopher Pherecydes. The constellation consists of an arc of seven stars the brightest of which is alpha or Alphecca.
The Arab astronomer called it Azophi Parmi or the Shield,
whereas most other Arabic astronomers called it Al Iklil al Shamaliyyah which
by the time of Ulug Beg had been shortened to Illil which means Corona
Borealis.
In the Alphonsine Tables the constellation was referred to
as Al Muni al Khatar or The Bright One of the Dish although other references
give it as Al Malf al Katar or the Loop of the Wreath.
The Hebrews called it Ataroth or Crown while the Syrians
called it Ashtaroth or their goddess Astarte who was worshipped as an ancient
near eastern goddess sometimes linked with Venus.
The Shawnee Indians of North America knew this area of the
sky as the Celestial Sisters, the fairest of them being the wife of the hunter
White Hawk, which is our star Arcturus.
In Celtic mythology Corona was Caer Arianrod, The House of
Arianrod or Entlenn, the sister of Gwydyon and daughter of Don, The Fairy King,
this name bearing a singular resemblance to that of the classical owner of the
crown.
The brightest star is alpha or Alphecca was known to the Arab astronomers as the Bright One of the Dish. Alpha is very occasionally called Gemma which means The Pearl of the Crown. Alphecca has a magnitude of 2.2 and lies at a distance of 75 light years and is an A class star making it hotter than the Sun.
The other six stars in the crown are Theta, Beta, Gamma,
Delta, Epsilon and iota, all can be seen on a clear night, but none are as
easily seen as Alphecca.
Although a small constellation it does play host to two very
important variable stars.
R Coronae Borealis is a yellow supergiant which usually is
just at the limit of naked eye visibility at magnitude 6.0 but at unpredictable
times it will fade to as faint as magnitude 14. When the star is this faint a
reasonably powerful telescope is needed to find it. Astronomers believe the
reason for these light variations are caused by carbon rich particles building
up in the atmosphere blocking the light from the star reaching Earth. These
particles then disappear, and the star then brightens once again. This cycle in
light variations can often take many years.
There are only about 100 of these stars known. R itself was
discovered by Edward Piggot in 1795 while living in the city of Bath.
Edward Pigott worked with John Goodricke in York between
1781 and 1786, they were what I called the Fathers of Variable Star Astronomy
because of the work they undertook in this field of astronomy.
R Coronae Borealis is the prototype of the R Coronae
Borealis class of variable stars. It is one of only two R Coronae Borealis
variables bright enough to be seen with the naked eye, along with RY
Sagittarius.
R lies 4,560 light years away the maximum recorded
brightness of the star varies between magnitude 5.7 to 14.8 and it is a G class
star and is hotter than the Sun.
The other variable star is T Coronae Borealis also known as the Blaze Star, it’s a nova
which erupted in 1866 reaching magnitude 2.2 and going novae again in 1946
reaching magnitude 3.0. It is called the Blaze Star because until around 20
years ago it was one of the few stars that had been observed to go novae more
than once. Astronomers know today that all novae erupt more than once on times
scales of decades or centuries.
The term Nov was used by astronomers in the medieval period
to describe a star which suddenly appears from apparently nowhere, these were
in days before the invention of the telescope. The term nova means new; this is
because astronomers thought that these were new stars being created. We now
know that it is a star going through a period of its timeline of evolution.
A nova is a binary system of two stars one a small hot white
dwarf star the other a larger cooler giant star. The white dwarf is the remains
of a much older star that has blown away all its outer gases and all that is
left is the core of the star. This will happen to our Sun in around 4 billion
years’ time when it expands from its present-day yellow dwarf state to that of
a red giant. After that phase all that
will be left is the core or a white dwarf.
The small white dwarf which is made of material that is
incredibly compressed together. It is so compressed that a tablespoonful of
white dwarf material would weigh over ten tons. This is called degenerate
material.
The white dwarf pulls material from the giant cooler star
towards it; this material then forms a disc around the white dwarf. Material
from this disc then cascades onto the surface of the white dwarf, as this gas
is cooler than the white dwarf when it hits the surface the gas kind of sizzles
and throws a shell of gas into space. This then makes the star become bright
for a period of time, it becomes a nova. After a while it will fade from view.
According to astronomers who study this star it was expected
to go nova again in either 2024 or 2025, so far as least as far as the summer
of 2025 is concerned nothing has yet happened. I am sure that astronomers will
be carefully watching this star for the next few years.
Corona Borealis contains a cluster of around 400 distant
galaxies over 1 billion light years away, the galaxies can only be seen with
large telescopes.
www.theramblingastronomer.co.uk
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