In 1996, a bright “new” star was discovered in Sagittarius
by Japanese amateur astronomer Yukio Sakurai. It was found not to be a usual
nova, but instead was a star going through a dramatic evolutionary state,
re-igniting its nuclear furnace for one final blast of energy called the “final
helium flash.” It was only the second to be identified in the twentieth
century. A star like the Sun ends its active life as a white dwarf star
gradually cooling down into visual oblivion. Sakurai's Object had a mass a few
times that of the Sun. Its collapse after fusing most of its hydrogen fuel to
helium raised its temperature so much higher it began nuclear fusion of its
helium remains. This was confirmed using its light spectrum to identify the
elements present.
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| Sakurai's Object By ESO, [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons |
Sakurai's Object is a highly evolved post-asymptotic giant
branch star which has, following a brief period on the white dwarf cooling
track, undergone a helium shell flash (also known as a very late thermal
pulse). The star is thought to have a mass of around 0.6 M☉.
Observations of Sakurai's Object show increasing reddening and pulsing
activity, suggesting that the star is exhibiting thermal instability during its
final helium-shell flash.
Prior to its reignition V4334 Sgr is thought to have been cooling
towards a white dwarf with a temperature around 100,000 K and a luminosity
around 100 L☉. The luminosity rapidly increased about a
hundred-fold and then the temperature decreased to around 10,000 K. The star
developed the appearance of an F class supergiant (F2 Ia). The apparent
temperature continued to cool to below 6,000 K and the star was gradually
obscured at optical wavelengths by the formation of carbon dust, similar to an
R CrB star. Since then the temperature has increased to around 20,000 K.
The properties of Sakurai's Object are quite similar to that
of V605 Aquilae. V605, discovered in 1919, is the only other known star
observed during the high luminosity phase of a very late thermal pulse, and
Sakurai's Object is modeled to increase in temperature in the next few decades
to match the current state of V605.
During the second half of 1998 an optically thick dust shell
obscured Sakurai's Object, causing a rapid decrease in visibility of the star,
until in 1999 it disappeared from optical wavelength observations altogether.
Infrared observations showed that the dust cloud around the star is primarily
carbon in an amorphous form. In 2009 it was discovered that the dust shell is
strongly asymmetrical, as a disc with a major axis oriented at an angle of
134°, and inclination of around 75°. The disc is thought to be growing more
opaque due to the fast spectral evolution of the source towards lower
temperatures.
Sakurai's Object is surrounded by a planetary nebula created
following the star's red giant phase around 8300 years ago. It has been
determined that the nebula has a diameter of 44 arcseconds and expansion
velocity of roughly 32 km/s.
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