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Scientists have known for some time that stars much more massive than our Sun (about 20 times heavier) undergo a spectacular end when they explode as Supernovae. The star literally collapses under its own weight and then bounces back in an after-shock that breaks the star apart in a tremendous explosion. What is left behind is a small dense core barely 10 km across, called a neutron star. (If the original star were much more massive, the explosion may leave behind a massive black hole).
Such supernova burn very bright for weeks and months after they explode and the nearby ones can be seen with the naked eye. In fact, they have been observed over many centuries, and have been documented by many different civilisations. 
What has been seen and documented is the visible light after the star becomes supernova. However, the actual moment of the star burst has not been seen: since this lasts only a few minutes, and stars are normally visible only at night, a person has to be very lucky indeed to witness the actual moment of the death of a star.
Scientists think such shock breakouts – a shockwave and flash of light that rocks a massive star just before it explodes into a “supernova” – allow the stars to finally explode, spewing out all the heavy atoms that exist in the universe. But actually watching that process occur and seeing how it progresses has proved elusive, leaving scientists guessing about exactly how it happens.
Such a "shock break-out" has been seen with visible light for the first time with the help of NASA’s Kepler space telescope by a group of International scientists. It has however provided a fresh mystery for astronomers: the problem is, it seemed to happen in only one of two exploding stars that were observed.

In data collected in 2011, they found two supernovae begin, potentially capturing the crucial moment. However only one star seemed to have the shockwave. An author on the paper, Brad Tucker from the Australian National University, said that was a mystery. He said the shockwave was thought to ripple across the surface and actually allow the supernova to explode.

“We’ve always thought that this is the physical mechanism that allows the star to blow up,” he said. “So gravity collapses the core down, and once the pressure is too much, you create a neutron star or sometimes a black hole, the rest of the energy rebounds and causes the star to blow up.

“It’s been this fundamental thing that we’ve always thought occurs but we’ve never seen it take place.” Tucker said it had been seen by chance with x-ray telescopes before, but not in great detail.

The fact one of the supernova they saw
with Kepler had the shock breakout and one
didn’t, means there’s something to learn, he
said.
The one where they didn’t see the
shockwave was a bigger star – about 500
times the size of Earth’s sun. This could mean
that the shock wave actually happened but it wasn’t strong enough to escape the star’s
gravity.

Tucker said it was also possible that
something like dust was blocking the view
of the shockwave or, because it was further
away (2,000 times further than the smaller
one), it was just fainter and they missed it.
“It’s telling us something but we just
don’t know what it is,” he said.
“That is the puzzle of these results,” said
Peter Garnavich, an astrophysics professor
at the University of Notre Dame in Indiana.
“You look at two supernovae and see two
different things. That’s maximum diversity.”
The shock breakout itself lasted only
about 20 minutes, so catching the flash of
energy was a milestone for astronomers,
where things usually happen on the
timescale of years, centuries or millennia.
“In order to see something that happens
on timescales of minutes, like a shock
breakout, you want to have a camera
continuously monitoring the sky,” said
Garnavich. “You don’t know when a
supernova is going to go off, and Kepler’s
vigilance allowed us to be a witness as the
explosion began.”
Tucker said that as they push through
more data from the Kepler missions, they
will almost certainly see more of these
events. He said from 500 galaxies they
watched in the original Kepler mission, they
found six supernovae, including these two.
Kepler’s second mission – called K2 – aims
to watch 5,000 galaxies, so should increase
the odds, he said.
“While Kepler cracked the door open on
observing the development of these
spectacular events, K2 will push it wide
open observing dozens more supernovae,”
said Tom Barclay, director of the Kepler
mission at Nasa Ames. “These results are a
tantalising preamble to what’s to come from
K2.”
Steve Howell, project scientist for Nasa’s
Kepler and K2 missions, said: “All heavy
elements in the universe come from
supernova explosions. For example, all the
silver, nickel and copper in the earth and
even in our bodies came from the explosive
death throes of stars. Life exists because of
supernovae.”
That’s not quite as poetic as the way US
astronomer Carl Sagan famously put it:
The nitrogen in our DNA, the calcium in
our teeth, the iron in our blood, the carbon
in our apple pies were made in the interiors
of collapsing stars. We are made of starstuff.
The findings have been accepted for
publication in the Astrophysical Journal