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.