BRIGHTEST OF THEM ALL
Palahalli R Vishwanath
Astronomers have glimpsed the most powerful supernova ever seen.
A star in a galaxy billions of light-years away exploded with such
force it briefly shone nearly 600 billion times brighter than our Sun
. It challenges all known theories of supernovae.
Supernova is a phenomenon where a very bright star like object
appears suddenly in the sky , fades away in a few months and
eventually becomes invisible to the naked eye. Some of the ancient
civilizations, especially the Chinese ,kept a record of these events
. Four supernova visible to the naked eye has been well recorded in
the last millennium .The most well known of them all is the Crab
supernova which appeared in the skies on July 4 1054.
It was learnt in the last century that Supernovae represent
the end products of the death of a star. Fusion reaction where four
hydrogen atoms come together to create a Helium nucleus and lot of
energy makes a star start shining. The pressure due to this reaction
opposes the gravitational force which tries to contract the star and
keeps the star in tact. In big stars this fusion process goes on
till the element iron is produced. At this stage the balance between
pressure and gravity is upset and the
star begins to contract into a very small volume. The net result is
the formation of a small but dense star( neutron
star) in the stellar
core and the total disruption of the remainder of the star with the
liberation of enormous energy ( equivalent to what sun would release
in its lifetime) in neutrinos and photons . These myriads of
photons are responsible for the brightness of a supernova.
However, the fact that it continues to be bright for several months
is due to the decay of radioactive nuclei produced in the star. This
is called a type II supernova . When a
white dwarf star in a binary star system accretes material from its
companion star and if its mass exceeds the the so called
Chandrasekhar Mass Limit (1.44 solar mass)
it will
collapse and produce a Type Ia supernova. Type I and II supernovae
can be distinguished by the way they dim with time and their spectra
.
One of the most famous supernovae of recent times seen by the
naked eye appeared on Feb23, 1987. This supernova, termed SN1987A,
was seen in the Large Magellanic Cloud, a nearby galaxy. While the
discovery of pulsars in 1967 had already proved the existence of
neutron stars, there was the first time ever detection of neutrinos
from a supernova . Thus, observation of this supernova at
various steps confirmed several of the earlier theories on
supernovae. On average, only one supernova per galaxy per century
is expected to take place. Till recently the supernova of 2006 was
considered the brightest Supernova ever observed which was 100
times brighter than any earlier Supernova.
The recent supernova
An innocuous dot of light in an image of the southern skies was
spotted on June 15th 2015. The spectrum of this object taken later
showed a very high red shift indicating that it a had taken place
very far away, some 3.8 billion light-years away. This showed that
this was a very, very bright supernova and was termed ASASSN-15lh,
named after the All Sky Automated Survey for Super Novae . It was
found to be 20 times brighter than the combined light of Milky Way
, making it the brightest supernova ever observed. After several
further observations , this was reconfirmed in Jan 2016. If the
supernova took place in our own galaxy, it would be easily seen by
the naked eye even during the day; If it was at distance of ~ 9
Light years (like Sirius) , it would be as bright as the Sun. If it
were as close as Pluto, it would vaporize the whole solar system.
This supernova is 2-3 times brighter than the biggest one which
appeared in 2006. According to some theorists, this supernova could
really be the brightest that ever can be.
The
electromagnetic spectrum of a star tells us not only about its
distance but also about the elements that are present . The
progenitor of this supernova is thought to be a a massive, blue,
hot star, rotating rapidly. It must have shed its outer layers of
hydrogen and helium shortly before it died, because those elements
are absent from the spectrum It may have been a type of massive star
called a Wolf-Ra yet star,
There are two most likely explanations for such a huge energy
output . The supernova could be due to the death of a very big star.
An example is Eta Carinae which is a hyper giant star (about 130
times more massive than sun) located approximately 7,500 light years
from Earth. if Eta Carinae exploded in a similar fashion, it would be
bright enough that one could read by its light here on nights, and
would even be visible during the day time.
Another explanation is that is due to a
magnetar, which is the strongest magnet known in the universe. At
~1015
Gauss, the magnetic field is a thousand trillion times stronger than
that of the Earth. They would probably have to spin at a rate of
1,000 times a second to provide this huge amount of energy via a
magnetized wind which could produce enough shocks to emit this
enormous burst of light. Magnetars are formed in the same way as
other neutron stars. but with the core collapse of much more
massive stars. We will get more information about this supernova when
The Hubble telescope will be used for its study in the next
few months .
Pic:
This supernova as it might appear in the skies of a planet
about 10,000 light-years away
No comments:
Post a Comment