NGC 6302, a planetary nebula in Scorpius

NGC 6302, a planetary nebula in Scorpius

Image Credit: NASA, ESA, and the Hubble SM4 ERO Team

NGC 6302 (better known as the Bug Nebula or the Butterfly Nebula) is a 2,200 years old 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, while is approaching us at approximately 35.7 kilometers per second.

When a star with a mass 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 – that was once about five times the mass of the Sun – will cool and fade away for billions of years. Our own Sun is expected to undergo a similar fate, but fortunately this will not occur until some 5 billion years from now.

What resemble the wings of the bug or 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!

Its central star, a white dwarf with a surface temperature of more than 200,000 K, is one of the hottest stars in the galaxy, implying that the star from which it formed must have been very large. The star has a current mass of around 0.64 solar masses.

The original mass of the star was much higher, but most was ejected in the event which created the planetary nebula. The luminosity and temperature of the star indicate it has ceased nuclear burning and is on its way to becoming a white dwarf, fading at a predicted rate of 1% per year.

The structure in the nebula is among the most complex ever observed in planetary nebulae. NGC 6302 is bipolar with two primary lobes, though there is evidence for a second pair of lobes that may have belonged to a previous phase of mass loss. A prominent dark lane runs through the waist of the nebula obscuring the central star at all wavelengths. This thick doughnut-shaped dust belt constricts the star’s outflow, creating the classic “bipolar” or hourglass shape displayed by some planetary nebulae.

This image also reveals a complex history of ejections from the star. The star first evolved into a huge red-giant star, with a diameter of about 1,000 times that of our Sun. It then lost its extended outer layers. Some of this gas was cast off from its equator at a relatively slow speed, perhaps as low as 20,000 miles (32,000 kilometers) an hour, creating the doughnut-shaped dust belt.

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.

We can also see numerous finger-like projections in this image pointing back to the star, which may mark denser blobs in the outflow that have resisted the pressure from the stellar wind.

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 or PAHs). Stars are usually either O-rich or C-rich, the change from the former to the latter occurring late in the evolution of the star due to nuclear and chemical changes in the star’s atmosphere. NGC 6302 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). The white blob with the crisp edge at upper right is an example of one of those shock waves.

This image was taken on July 27, 2009, with Hubble’s Wide Field Camera 3 in ultraviolet and visible light. Six filters that isolate emissions from oxygen, helium, hydrogen, nitrogen, and sulfur from the planetary nebula were used to create this composite image.

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