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The emission spectrum from Supernova 1993J changed last week, suggesting it
represents a ‘missing link’ between the two main types of supernova. For the
first weeks after its discovery in late March, the exploding star emitted
the strong hydrogen lines that mark a Type II supernova (New Å®ÉúСÊÓƵ,
Science, 24 April). But on 11 May, Alex Filippenko of the University of
California at Berkeley found that the hydrogen lines had faded, helium lines
were stronger, and the object was ‘gradually transforming itself into a
Type 1B supernova’. The transformation had been hidden by the light of the
full Moon, which blocked observations for several days earlier this month.

This kind of transformation has been seen once before – Filippenko observed
it in the spectrum of Supernova 1987K, several months after it exploded.

Most supernovas explode when their cores collapse to form neutron stars. The
collapse of the interior releases energy which is picked up by the outer
layers and ejected into space. These expanding layers form the supernova.
The spectra from Type II and Type IB explosions differ because of the
different surface layers. Type II supernovas have a thick outer layer of
hydrogen, creating the hydrogen emission. Because Type IB supernovas have
lost this layer, we see the helium layer beneath.

Stan Woosley of the University of California at Santa Cruz says Supernova
1993J is a ‘missing link’ between Types II and IB. He believes the star
which exploded began with a mass of 10 to 15 solar masses and a smaller
companion star orbiting around it. As the larger star aged to the red giant
stage, expansion of its outer hydrogen layer brought it within the
gravitational reach of the companion. This then stripped off about 10 solar
masses of gas. By the time of the explosion, says Woosley, the star had only
about 0.1 solar mass of hydrogen left in its outer shell, and its total mass
was reduced to between 3 and 4 solar masses – almost entirely in its core.
The companion survived the explosion, and should eventually reappear as the
supernova remnant expands and thins.

This theory would also explain why the supernova’s brightness peaked twice.
Woosley says hot hydrogen produced the first peak. The second came from the
decay of radioactive nickel and cobalt, formed by the explosion, seen
through the tenuous hydrogen shell.

Supernova specialist Robert Kirshner of the Harvard-Smithsonian Center for
Astrophysics says Woosley’s explanation is ‘plausible’, but adds that the
changes may indicate supernovas are more complex events than had been
thought. ‘It does make you wring your hands a bit to see that all the
well-observed cases are a bit peculiar.’