The Butterfly Nebula

NGC 6302, also called the Bug or Butterfly Nebula, is a bipolar planetary nebula of some 3 light-years across, located within our Milky Way galaxy at about 3,400 light-years distance in the constellation of Scorpius. It is approaching the Solar System at approximately 35.7 kilometers per second.

When a star with a mass of up to eight times that of the Sun runs out of fuel at the end of its life, it blows off its outer shells and begins to lose mass. This allows the hot, inner core of the star (collapsing from a red giant to a white dwarf) to radiate strongly, causing this outward-moving cocoon of gas to glow brightly. These glowing clouds of gas can show complex structures, as the ejection of mass from the star is uneven in both time and direction. Over the next several thousand years, NGC 6302 will gradually disperse into space, and then the white dwarf will cool and fade away for billions of years.

The spectrum of NGC 6302 shows that its central star is one of the hottest stars in the galaxy, with a surface temperature in excess of 200,000 K, implying that the star from which it formed must have been very large. The central star, a white dwarf, has a current mass of around 0.64 solar masses, is surrounded by a particularly dense equatorial disc composed of gas and dust. This dense disc is postulated to have caused the star’s outflows to form a bipolar structure similar to an hour-glass. A dark lane runs through the waist of the nebula obscuring the central star at all wavelengths.

What resemble the wings of the butterfly are actually cauldrons of gas heated to more than 36,000 degrees Fahrenheit (2,225 degrees Celsius), which is unusually hot compared to a typical planetary nebula. The gas is tearing across space at more than 600,000 miles (1 million kilometers) an hour—fast enough to travel from Earth to the Moon in 24 minutes! Other gas was ejected perpendicular to the dust belt at higher speeds, producing the elongated “wings” of the butterfly-shaped structure. Later, as the central star heated up, a much faster stellar wind, a stream of charged particles traveling at more than 2 million miles (3 million kilometers) an hour, plowed through the existing wing-shaped structure, further modifying its shape.

One of the most interesting characteristics of the dust in NGC 6302 is the existence of both oxygen-rich material (i.e. silicates) and carbon-rich material (i.e. poly-aromatic-hydrocarbons). The nebula belongs to a group of objects where hydrocarbon molecules formed in an oxygen-rich environment. The nebula’s reddish outer edges are largely due to light emitted by nitrogen, which marks the coolest gas visible in the picture. The white-colored regions are areas where light is emitted by sulfur. These are regions where fast-moving gas overtakes and collides with slow-moving gas that left the star at an earlier time, producing shock waves in the gas (the bright white edges on the sides facing the central star).