Novae (3) ...
Some novae can exhibit much more complex behavior than the relatively simple
expanding shell of Nova Cygni 1992.
An Exploding Crab
The following figure shows
a Hubble Space Telescope image of a nebula that is several light years across and
several thousand light years away
in the southern constellation Centaurus. This nebula is called the "Southern Crab" because
of its shape.
The hourglass inside an hourglass shape is thought to be due to nova explosions from a white dwarf that
forms a binary system with a red giant (the stars are in the center of the nebula and not visible in this
image).
The red giant is emitting a strong wind, part of which
the white dwarf captures into an
accretion disk and then onto its surface. This accumulation eventually triggers a nova outburst on the
white dwarf. In one proposed scenario,
the accretion disk forces the expanding shell to be focused into cones above and
below the disk, producing the hourglass shape. If this is a correct picture, we may expect future
observations on other nova systems with accretion disks to exhibit similar focusing effects.
It is thought that there are
two hourglasses because of two separate nova explosions: one several thousand
years ago, producing the large hourglass, and one much more recent explosion
that has produced the smaller
hourglass (which has not had time to expand to large size yet).
Nova Persei 1901
The adjacent image shows an expanding nebula called
GK Persei. It is the remnant of Nova Persei 1901,
which exploded in the constellation Perseus in 1901.
The remnant is about 1500 light years distant and
the portion of the nebula shown is about 0.7 light years across.
The "firework burst" pattern is unusual for a nova remnant. It may be caused by
the nova shell expanding into an unusually dense medium, though the details are not well known.
