1. Supernova: The Heavenly Wizardry of Light!
Supernova! I wonder who came up with this term at first. The term signifies the death of a high mass 1
star. Historically nova was the name used for an apparently new star! What an irony. The death of a star
has been given the name whose last part signifies the birth of a star. If you give it another look it is not
that ironic though! The death of a star as supernova may actually trigger the birth of another star, but that
is another story. Lets just talk about supernovae.
High mass stars end their life in a cataclysmic event, we now cal supernova. During this phase the star
releases more energy than it has released in its entire life span by the nuclear fusion reactions. Also, since
elements having atomic number greater than that of Iron cannot be manufactured in the stellar core (as
they are endothermic) , the huge amount of energy heat during this time assists in synthesizing all the
heavier elements that we see today in the process called necleosynthesis!
The amount of energy released during this period is really enormous. To get a feel of that lets have a look
at the magnitude of the numbers involved.
Let us consider a typical heavy mass star of mass (MS) = 10Mʘ 2
The luminosity of the sun is Lʘ= 3.85x1026 J/sec
Now, the relation between luminosity and mass is
ʘ ʘ
ʘ
Also the relation between the life time of a star and its mass is
Thus, comparing with solar values, we get
ʘ
Here, we have assumed that the life time of the sun is 10 billion years, which comes from a different set
of calculations.

2. Hence the total amount of energy liberated by a 10Mʘ star over its entire life time by the nuclear process
is
Now, as the star goes supernova, lets see what happens to the outer layers of the star and how much is the
energy liberated in the process!
That is, we consider the energy budget of the star. Clearly, the energy source of a supernova explosion is
gravitational: the collapse of a core of mass MC= 1.5Mʘ from an initial white dwarf radius RC ( ʘ)
to the final radius Rnc 20 km ( RC) of the neutron core releases an amount of gravitational energy of
the order of
This the amount of energy released! From our calculations done above, it is clear that the energy
produced by a star during its life time is not even 1% of this value.
The energy absorbed in nuclear processes amounts to
about one tenth of . There remains ample energy for ejecting all the material outside the core, for
imparting it enormous velocities and for producing the huge luminosities observed. The radiated energy
may be estimated by assuming a typical luminosity LSN 1037 J/sec for a typical period of on year:
This is clearly an overestimate, since most of the supernova remains that bright only for a short interval of
time of the order of hours or days or at max of the order of week. Still it is only few percent of the
released energy. A similar amount would be required for the ejection of the loosely bound envelope,
assuming total stellar mass M 10Mʘ, and a comparable amount would suffice for supplying the high
expansion velocities of the ejecta:
adopting vexp 10,000 km/sec, as derived from observations.

3. When we combine all the mechanisms of energy dissipation, two questions immediately arise: first, if
such a small fraction of released energy is sufficient for powering a supernova explosion, where does the
bulk of the energy go? Second- the question that has puzzled astrophysicists for decades- what is the
mechanism that deposits the required energy in the envelope? The answers to these questions are linked
and involve one of the major factors affecting the entire supernova process. These are the neutrinos,
which take part in any weak interaction so that the lepton number is conserved! But that is another story
and shall be addressed somewhere else.
The purpose of this article was to give the readers a feel of the huge numbers involved in a supernova
explosion and the enormity of the scale at which they occur. I hope I succeeded in that!
1: Stars having core mass greater than 1.46 Mʘ are considered to be high mass stars.
2: The symbol ʘ stands for solar values, for example Mʘ represents the sun’s mass and so on.
Reference: An introduction to the Theory of Stellar Structure and Evolution by Dina Prialnik