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Transcript
Nova
Nova
Nova – Rapid increase in luminosity of a white dwarf in a binary system
Rotation
The Roche lobe is the region of space around a star in a binary system within
which orbiting material is gravitationally bound to that star.
Nova
Nova – Rapid increase in luminosity of a white dwarf in a binary system
Rotation
The Main Sequence star will eventually expend its fuel and leave the main
sequence while expanding.
Nova
Nova – Rapid increase in luminosity of a white dwarf in a binary system
Rotation
Accretion disk – material pulled into the Roche lobe of the white dwarf
which is eventually deposited on the surface of the white dwarf.
An artist's concept of a close Algol-type binary. The relative size of the Sun is illustrated by the small circle to the upper right of the figure. Illustration courtesy of M.
Richards.
In Algol-type binaries, one of the stars has evolved and expanded to fill a droplet-shaped potential surface, called the Roche lobe, within which material is gravitationally
bound to the star (see Figure 1). The Roche surface is, therefore, the surface along which the gravitational potential is common between the stars. Once a star fills its
Roche lobe, gas moves into the Roche lobe of the companion star and is pulled in toward that star. This process of mass transfer is referred to as Roche lobe overflow.
Binaries in this stage of mass transfer are called semi-detached binaries, because only one of the stars is actually in contact with its Roche surface. The subsequent
flow of gas between the stars is called the gas stream or mass transfer stream. During Roche lobe overflow, mass transfer feeds gas particles in the stream from the
inner Lagrangian point (L1), where the two Roche lobes touch. This gas stream free-falls onto the companion star, much like rocks dropped from a building. However,
the path of the gas stream becomes curved because it feels the orbital motion of the binary (or the Coriolis force) as it falls.
Nova
The outer shells of the growing companion is composed primarily of
hydrogen.
The hydrogen will be deposited on the surface of the white dwarf.
As more and more hydrogen is accumulated, gravity will increase the
pressure and temperature of the layer of hydrogen.
Eventually, when about 10-8 solar masses of hydrogen accumulates, the
hydrogen will fuse.
The dwarf’s luminosity will grow to about 100,000 solar luminosities, and the
resulting radiation pressure will blow away the surface layers.
The luminosity will fade after a few months.
Hubble Sees Changes in Gas Shell
around Nova Cygni 1992
The Hubble telescope has given
astronomers their best look yet at a rapidly
ballooning bubble of gas blasted off a star.
The shell surrounds Nova Cygni 1992,
which erupted Feb. 19, 1992. A nova is a
thermonuclear explosion that occurs on the
surface of a white dwarf star in a doublestar system. The image [right], taken after
Hubble's near-sightedness had been
corrected, reveals an elliptical and slightly
lumpy ring-like structure. The ring is the
edge of a bubble of hot gas blasted into
space by the nova. Another Hubble picture
taken 467 days after the explosion [left]
provided the first glimpse of the ring and a
mysterious bar-like structure. But the image
interpretation was severely hampered by
the telescope's blurred vision
Luminosity of Cygni 1992 reached
4.4 (visible to naked eye).b
Hubble Sees Changes in Gas Shell
around Nova Cygni 1992
The ring is the edge of a bubble of hot gas
blasted into space by the nova (after
Hubble corrections made). Another Hubble
picture taken 467 days after the explosion
[left] provided the first glimpse of the ring
and a mysterious bar-like structure. But the
image interpretation was severely
hampered by the telescope's blurred vision
Luminosity of Cygni 1992 reached
4.4 (visible to naked eye).b
Old photographs show
no evidence of the
nebula to the right. In
1992, a white dwarf
star in Cygnus blew off
its outer layers in a
classical nova
explosion: an event
called Nova Cygni
1992. Light flooded the
local interstellar
neighborhood,
illuminated this existing
gas cloud, excited the
existing hydrogen, and
hence caused the red
emission. The only gas
actually expelled by the
nova can be seen as a
small red ball just
above the photograph's
center. Eventually, light
from the nova shell will
fade, and this nebula
will again become
invisible!
Blobs in Space: The Legacy of a
Nova
Nova eruptions by dying stars were
thought to be simple, predictable acts
of violence. Astronomers could point a
telescope at the most recently
exploded novae and see an expanding
bubble of gaseous debris around each
star.
Scientists using the Hubble telescope,
however, were surprised to find that
some nova outbursts may not produce
smooth shells of gas, but thousands of
gaseous blobs, each the size of our
solar system. In this Hubble picture of
the nova T Pyxidis, the shells of gas
ejected by the star are actually more
than 2,000 gaseous blobs packed into
an area that is 1 light-year across
Nova
Observations
Novae events are not powerful enough to destroy the star system.
Since the mass of the companion star will well above the 10-8 solar masses,
the nova event can re-occur multiple times.
Re-occurrence can take anywhere from decades to thousands of years.