Minkowski 2-9 (M2-9, the Twin Jet Nebula, Minkowski’s Butterfly, Wings of a Butterfly Nebula or just Butterfly Nebula) is a bipolar planetary nebula some 1.4 light-years across, located about 2,100 light-years away in the constellation Ophiuchus.
Objects such as M2-9 are called planetary nebulae due to a mistake made by early astronomers who discovered these objects while sweeping the sky with small telescopes. Many of these nebulae have the color, shape and size of Uranus and Neptune, so they were dubbed planetary nebulae. The name persists despite the fact that these nebulae are now known to be distant clouds of material that are shed by stars about the size of our sun undergoing upheavals during their final life stages.
Although the M2-9 nebular material is flowing out from a spherical star, it is extended in one dimension, appearing as the wings of a butterfly or hourglass. Planetary nebulae with such shapes are created when a red giant blew away the outer layers at over 300 kilometers per second, and the disk deflected the materials in two directions. Astronomers have dubbed this object as the Twin Jet Nebula because of the polar jets believed to cause the shape of the lobes.
M2-9 represents the spectacular “last gasp” of a binary star system at the nebula’s center that orbit each other inside a gaseous disk just 10 times the orbit of Pluto about once every 120 years. The primary component of this binary is the hot core of a Sun-like star that reached the end of its life. It ejected most of its outer layers and became a red giant, and is now contracting into a white dwarf.
(Most white dwarfs gradually cool. If, however, the mass of a such a star exceeds 1.4 solar masses – the so-called Chandrasekhar limit – the force of its own gravity make it collapse further and explode as a supernova.)
The second, smaller star of the binary orbits may have been engulfed by the other’s expanding stellar atmosphere with the resulting interaction, creating the nebula. Astronomers theorize that the gravity of one star pulls some of the gas from the surface of the other and flings it into a thin, dense disk extending into space. Such a disk can successfully account for the jet-exhaust-like appearance of M2-9.
Minkowski 2-9 has inflated dramatically due to a fast stellar wind, blowing out into the surrounding disk and inflating the large, wispy hourglass-shaped wings perpendicular to the disk, producing the butterfly appearance. The outer shell is estimated to be about 1,200 years old, and will fade gradually over thousand of years.