Nova

photo: Artist's conception of a white dwarf star accreting hydrogen from a larger companion

A nova (pl. novae or novas) is a cataclysmic nuclear explosion caused by the accretion of hydrogen onto the surface of a white dwarf star. Novae are not to be confused with Type Ia supernovae, or another form of stellar explosion first announced by Caltech in May 2007, Luminous Red Novae.

Occurrence rate, and astrophysical significance:

Astronomers estimate that the Milky Way experiences roughly 30 to 60 novae per year, with a likely rate of about 40. The number of novae discovered each year is much lower, probably due to great distance and observational biases.By comparison, the number of novae discovered each year in the nearby Andromeda Galaxy is much lower; roughly ½ to ⅓ that of the Milky Way.

Spectroscopic observation of nova ejecta nebulae has shown that they are enriched in elements such as helium, carbon, nitrogen, oxygen, neon, and magnesium. The contribution of novae to the interstellar medium is not great; novae supply only 1/50th the amount of material to the Galaxy as supernovae, and only 1/200th that of red giant and supergiant stars.

Recurrent novae like RS Ophiuchi (those with periods on the order of decades) are rare. Astronomers theorize however that most, if not all, novae are recurrent, albeit on time scales ranging from 1,000 to 100,000 years.The recurrence interval for a nova is less dependent on the white dwarf's accretion rate than on its mass; with their powerful gravity, massive white dwarfs require less accretion to fuel an outburst than lower-mass ones. Consequently, the interval is shorter for high-mass white dwarfs.

Historical significance:

The astronomer Tycho Brahe observed the supernova SN 1572 in the constellation Cassiopeia, and described it in his book de stella nova (Latin for "concerning the new star"), giving rise to the name nova. In this work he argued that a nearby object should be seen to move relative to the fixed stars, and that the nova had to be very far away. Though this was a supernova and not a classical nova, the terms were considered interchangeable until the 1930s.

Novae as distance indicators:

Novae have some promise for use as standard candles. For instance, the distribution of their absolute magnitude is bimodal, with a main peak at magnitude -7.5, and a lesser one at -8.8. Novae also have roughly the same absolute magnitude 15 days after their peak (-5.5). Comparisons of nova-based distance estimates to various nearby galaxies and galaxy clusters with those done with Cepheid variable stars have shown them to be of comparable accuracy.

Bright novae since 1890:

Year Nova Maximum brightness
1891 T Aurigae +3.8
1898 V1059 Sagittarii +4.5
1899 V606 Aquilae +5.5
1901 GK Persei +0.2
1903 DM Geminorum +4.8
1905 V604 Aquilae +7.3
1910 DI Lacertae +4.6
1912 DN Geminorum +3.5
1918 V603 Aquilae −1.4
1919 HR Lyrae +6.5
1919 V849 Ophiuchi +7.4
1920 V476 Cygni +2.0
1920 T Pyxidis +6.4
1925 RR Pictoris +1.2
1927 EL Aquilae +5.5
1927 XX Tauri +5.9
1933 RS Ophiuchi +4.3
1934 DQ Herculis +1.4
1936 CP Lacertae +2.1
1936 V368 Aquilae +5.0
1939 BT Monocerotis +4.5
1942 V450 Cygni +7.0
1942 CP Puppis +0.3
1943 V500 Aquilae +6.1
1944 T Pyxidis +7.1
1945 V528 Aquilae +7.0
1946 T Coronae Borealis +3.0
1948 CT Serpentis +6.0
1948 V465 Cygni +7.3
1950 DK Lacertae +5.0
1956 RW Ursae Minoris +6.0
1958 RS Ophiuchi +5.0
1960 V446 Herculis +2.8
1963 V533 Herculis +3.0
1964 QZ Aurigae +6.0
1967 T Pyxidis +6.7
1967 HR Delphini +3.7
1967 RS Ophiuchi +5.0
1968 LV Vulpeculae +5.2
1970 FH Serpentis +4.4
1970 V1229 Aquilae +6.7
1970 V1330 Cygni +7.5
1971 IV Cephei +7.0
1975 V1500 Cygni +1.7
1975 V373 Scuti +6.0
1976 NQ Vulpeculae +6.0
1977 HS Sagittae +7.2
1978 V1668 Cygni +6.0
1982 V1370 Aquilae +6.0
1984 PW Vulpeculae +6.4
1984 QU Vulpeculae +5.2
1985 RS Ophiuchi +5.4
1986 V842 Centauri +4.6
1986 OS Andromedae +6.3
1987 V827 Herculis +7.5
1987 QV Vulpeculae +7.0
1991 V838 Herculis +5.0
1992 V1974 Cygni +4.2
1993 V705 Cassiopeiae +5.8
1999 V382 Velorum +2.6
1999 V1494 Aquilae +4.0
2006 RS Ophiuchi +4.5
2007 V1280 Scorpii +3.9