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Nova

Nova is a Latin word meaning new, and it describes the appearance of a seemingly new star in the sky, a brilliant object in a place where there was previously only a very faint star, or perhaps nothing at all. Astronomers estimate that novae (the plural of nova) occur about 20 to 60 times per year in the Milky Way galaxy.

A nova is a phenomenon that happens in a binary star system containing a white dwarf and a stable companion star. A white dwarf is the dead, collapsed core of a star that formerly was about the size of the sun. When the sun dies, it will become a white dwarf. Unlike the sun, however, many stars exist in binary systems, where two stars orbit one another. In many binaries, a distance much less than the distance from the sun to Earth may separate such stars.

Suppose astronomers have a binary system with a white dwarf and a companion star that is expanding to become a red giant star. As the surface of the red giant star expands, it gets progressively closer to the white dwarf. Eventually, the surface of the giant may reach a critical point between the two stars where the gravity of the white dwarf is actually stronger than the giants own gravity. If this happens, matter will begin streaming off the giants surface and onto the white dwarf. This is like overfilling a bucket with watereventually the water will overflow, and if there is an adjacent bucket, begin pouring into it. Likewise, the large star will begin to lose matter once its surface expands past its Roche lobe, the imaginary surface beyond which the giant stars gravity is no longer sufficient for it to retain its matter.

The white dwarf has tremendously strong gravity, because it is very massive and very small. Therefore, the companion stars matter, which is mostly hydrogen, is squashed into a dense, thin, hot layer on the white dwarfs surface. The more matter that streams onto the white dwarf, the hotter it gets, and eventually, thermonuclear fusion reactions begin. These reactions are just like those that occur in the center of a stable star like the sun, converting the hydrogen to helium with an accompanying enormous release of energy. In a brief but violent cataclysm, the hydrogen on the white dwarfs surface burns away, and while it does, the white dwarf brightens by as much as a factor of a million (15 magnitudes). This is a nova, and after reaching its peak brightness, it slowly fades over a period of weeks to months.

Because mass transfer in a binary system does not stop after a nova explosion, the white dwarf will start to reaccumulate matter. Novae therefore are recurrent, with the length of time between nova outbursts in a system depending on how fast the companion star is losing matter to the white dwarf. If the stream is just a trickle, it might be thousands of years until the next outburst. Other novae recur much more frequently. As a single star, the sun is unlikely ever to become a nova after it dies. It may accrete enough matter just from the interstellar medium to become a nova, but such novae are extremely rare events due to the low rate of accretion of matter.

Novae should never be confused with supernovae, which are not just big novae. Supernovae involve the explosion and destruction of a star or a white dwarf, while a nova is merely the conflagration of a surface layer of hydrogen on a white dwarf. Novae are much more common than supernovae, and they do not release nearly as much energy. Nevertheless, they are a good reason to be familiar with the sky: things do change in space. A person never knows when looking up into the night sky, and seeing a familiar constellation looking a bit different. Just this kind of thing happened as recently as December 1999, when a bright, nakedeye nova appeared in the constellation Aquila, the Eagle. At its maximum, it was as bright as many of the stars in the constellation, and for a few days at least, viewers were treated to the spectacle of a truly new star in an otherwise familiar constellation.

A reoccurring nova occurred in 2006. Approximately 5,000 lightyears away in the constellation Ophiuchus, RS Ophiuchi was seen exploding. The nova reached its maximum brightness on February 13, 2006. It has erupted in the later years of 1898, 1933, 1958, 1967, and 1985.

See also Stellar evolution; Supernova.

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Nova

Nova is a Latin word meaning new, and it describes the appearance of a seemingly "new star" in the sky, a brilliant object in a place where there was previously only a very faint star , or perhaps nothing at all.

A nova is a phenomenon that happens in a binary star system containing a white dwarf and a stable companion star. A white dwarf is the dead, collapsed core of a star that formerly was about the size of the Sun . When the Sun dies, it will become a white dwarf. Unlike the Sun, however, many stars exist in binary systems, where two stars orbit one another. In many binaries, these stars may be separated by a distance much less than the distance from the Sun to Earth .

Suppose we have a binary system with a white dwarf and a companion star that is expanding to become a red giant star . As the surface of the red giant star expands, it gets progressively closer to the white dwarf. Eventually, the surface of the giant may reach a critical point between the two stars where the gravity of the white dwarf is actually stronger than the giant's own gravity. If this happens, matter will begin streaming off the giant's surface and onto the white dwarf. This is like overfilling a bucket with water—eventually the water will overflow, and if there is an adjacent bucket, begin pouring into it. Likewise, the large star will begin to lose matter once its surface expands past its Roche lobe, the imaginary surface beyond which the giant star's gravity is no longer sufficient for it to retain its matter.

The white dwarf has tremendously strong gravity, because it is very massive and very small. Therefore, the companion star's matter, which is mostly hydrogen , is squashed into a dense, thin, hot layer on the white dwarf's surface. The more matter that streams onto the white dwarf, the hotter it gets, and eventually, thermonuclear fusion reactions begin. These reactions are just like those that occur in the center of a stable star like the Sun, converting the hydrogen to helium with an accompanying enormous release of energy . In a brief but violent cataclysm, the hydrogen on the white dwarf's surface burns away, and while it does, the white dwarf brightens by as much as a factor of a million (15 magnitudes). This is a nova, and after reaching its peak brightness, it slowly fades over a period of weeks to months.

Because mass transfer in a binary system does not stop after a nova explosion, the white dwarf will start to reaccumulate matter. Novae therefore are recurrent, with the length of time between nova outbursts in a system depending on how fast the companion star is losing matter to the white dwarf. If the stream is just a trickle, it might be thousands of years until the next outburst. Other novae recur much more frequently. As a single star, the Sun is unlikely ever to become a nova after it dies. It may accrete enough matter just from the interstellar medium to become a nova, but such novae are extremely rare events due to the low rate of accretion of matter.

Novae should never be confused with supernovae, which are not just "big novae." Supernovae involve the explosion and destruction of a star or a white dwarf, while a nova is merely the conflagration of a surface layer of hydrogen on a white dwarf. Novae are much more common than supernovae, and they do not release nearly as much energy. Nevertheless, they are a good reason to be familiar with the sky: things do change up there, and you never know when you might look up and see a familiar constellation looking a bit different. Just this kind of thing happened as recently as December, 1999, when a bright, naked-eye nova appeared in the constellation Aquila, the Eagle. At its maximum, it was as bright as many of the stars in the constellation, and for a few days at least, viewers were treated to the spectacle of a truly "new star" in an otherwise familiar constellation.

See also Stellar evolution; Supernova.

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Nova

The word nova, Latin for "new," was assigned by ancient astronomers to any bright star that suddenly appeared in the sky. A nova occurs when

one member of a binary star system temporarily becomes brighter. Most often the brighter star is a shrunken white dwarf, the cooling, shrunken core remaining after a medium-sized star (like our sun) ceases to burn. Its partner is a large star, such as a red giant, a medium-sized star in a late stage of its evolution, expanding and cooling.

As the companion star expands, it loses some of its mattermostly hydrogento the strong gravitational pull of the white dwarf. After a time, enough matter collects in a thin, dense, hot layer on the surface of the white dwarf to initiate nuclear fusion reactions. The hydrogen on the white dwarf's surface burns away, and while it does so, the white dwarf glows brightly. This is a nova. After reaching its peak brightness, it slowly fades over a period of days or weeks.

The transfer of matter does not stop after a nova explodes, but begins anew. The length of time between nova outbursts can range from several dozen to thousands of years, depending on how fast the companion star loses matter to the white dwarf.

A nova should not be confused with a supernova, which is the massive explosion of a relatively large star. A nova is much more common than a supernova, and it does not release nearly as much energy. Because novae (plural of nova) occur more often, they can change the way constellations in the night sky appear. For example, in December 1999, a bright, naked-eye nova appeared in the constellation Aquila, the Eagle. At its maximum, the nova was as bright as many of the stars in Aquila. For a few days at least, viewers were treated to the spectacle of a truly "new star" in an otherwise familiar constellation.

Words to Know

Binary star: Pair of stars in a single system that orbit each other, bound together by their mutual gravities.

Red giant: A medium-sized star in a late stage of its evolution. It is relatively cool and has a diameter that is perhaps 100 times its original size.

White dwarf: The cooling, shrunken core remaining after a medium-sized star ceases to burn.

[See also Binary star; Star; Supernova; White dwarf ]

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no·va / ˈnōvə/ • n. (pl. -vae / -vē; -ˌvī/ or -vas ) Astron. a star showing a sudden large increase in brightness and then slowly returning to its original state over a few months. See also supernova.

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nova (pl. novae) Faint star that undergoes unpredictable increases in brightness by several magnitudes, apparently due to explosions in its outer regions, and then slowly fades back to normal. See also variable star

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nova A new star.