Many years ago, the astronomer Carl Sagan
famously said that there are more stars in
the universe than grains of sand on Earths
beaches. It is impossible to know exactly
how many stars are out there in the cosmos,
but it is estimated that the universe contains
at least 1 quadrillion, which is 1 followed
by 24 zeros! (British system) And they come
in a variety of shapes and sizes. Some are
fairly small but extremely heavy, such as
the exotic Neutron star. While others are
low mass, much cooler and common throughout
the galaxies, such as red dwarfs. The star
that we see and feel every single day, the
Sun is technically classed as a yellow dwarf,
and compared to the Earth, it is enormous,
but on a stellar scale, it is actually fairly
average because the universe contains some
truly colossal stars, such as the mighty UY
Scuti. This ginormous ball of glowing plasma
is classed as a red supergiant and was once
considered the biggest star ever discovered.
Located in the constellation Scutum, UY Scuti
was originally estimated to be 1700 times
larger than the sun. For many years it was
believed to be the largest in the known universe,
however now UY Scuti doesn't even make it
into the list of 10 top. The reason for this
sudden downgrade is because it is actually
a lot closer to Earth than originally estimated,
and more recent and accurate measurements
have found that UY Scuti is more likely to
be 775 times the size of the Sun. Still making
it a gigantic star, but nowhere near as big
as many others that have already been discovered.
So what is the largest known star in the universe?
Well, that title currently belongs to Stephenson
2-18. Stephenson 2-18 is truly enormous, with
an estimated radius of 2150 times the size
of the Sun. In fact, if we could replace the
Sun with this colossal star, then it would
easily engulf the orbits of Earth, Mars, Jupiter
and even Saturn, which is on average 886 million
miles away, or 1.4 billion kilometres away.
The enormous star is apart of a relatively
small cluster called Stephenson 2, which is
located around 20 thousand light-years away.
The cluster also contains 26 other confirmed
red supergiants, far more than any other known
cluster in the universe but none of them seems
to be as large as Stephenson 2-18. This newly
titled "largest star" is also very young,
around 14-20 million years old and according
to the current understanding of stellar evolution,
Stephenson 2-18 may even continue to grow
bigger, possibly one day becoming what is
known as a yellow hyper-giant. Just a few
million years from now this gigantic glowing
ball of plasma may also enter into the latter
stages of its life as it quickly burns through
its fuel and eventually explodes in a catastrophic,
but magnificent supernova, possibly even leaving
behind a black hole as a reminder of Stephenson
2-18s once extreme parameters.
Of course, stellar sizes are only estimates
based on measurements from great distances
and so we will have to wait until further
studies are completed to find out if Stephenson
2-18 really is the king of the stars, or whether
another is waiting to take that top spot.
