Henize 3-1333, a planetary nebula in Ara

Henize 3-1333, a planetary nebula in Ara

Image Credit: ESA/Hubble & NASA

Henize 3-1333 (Hen 3-1333) is a planetary nebula in the H-shaped southern constellation of Ara, the Altar. It is named after the American astronomer and astronaut Karl Gordon Henize.

Despite their name, planetary nebulae have nothing to do with planets. The name of planetary nebulae arose because of the visual similarity between some round planetary nebulae and the planets Uranus and Neptune when viewed through early telescopes.

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 as a planetary nebula.

Over the next several thousand years, the nebula will gradually disperse into space, and then the star will cool and fade away for billions of years as a white dwarf. Our own Sun is expected to undergo a similar fate, but fortunately this will not occur until some 5 billion years from now.

The central star of Henize 3-1333 is the exposed core of a former red giant. Its strong stellar wind has formed the symmetric colourful hot gas clouds. The star has only about 60% of the mass of our Sun. Due to its high surface temperature it emits large amounts of energetic ultraviolet radiation, and thus stimulates the surrounding gas to glow in visible light.

The star’s stellar wind is blowing out material of about the mass of the Earth every year. That’s a tiny fraction of the star’s mass, but it blows out this much every year for thousands of years. Eventually that wind will turn off, and all that will be left is the very hot (30,000° Celsius) core of the star, which will then cool over the next few billion years as a white dwarf and only have approximately the size of the Earth. Long before then the expanding gas around it will dissipate, and all that’ll be left is a very diffuse cloud of material that will mix with the interstellar medium.

Unlike the Sun, the star’s apparent brightness varies substantially over time. This variability is probably caused by a thick disk of dust that surrounds the star, which lies almost edge-on when viewed from Earth, and periodically obscures the star. This disk is something like 30 billion kilometers across, far larger than our Solar System, and may also be responsible for shaping the outflow of the gas from the star.

This star is a Wolf–Rayet type star — a late stage in the evolution of Sun-sized stars. These are named after (and share many observational characteristics with) Wolf–Rayet stars, which are much larger. Why the similarity? Both Wolf–Rayet and Wolf–Rayet type stars are hot and bright because their helium cores are exposed: the former because of the strong stellar winds characteristic of these stars; the latter because the outer layers of the stars have been puffed away as the star runs low on fuel.

The exposed helium core, rich with heavier elements, means that the surfaces of these stars are far hotter than the Sun, typically 25,000 to 50,000 degrees Celsius (the Sun has a comparatively chilly surface temperature of just 5500 degrees Celsius).

So while they are dramatically smaller in size, the Wolf–Rayet type stars such as the one at the core of Henize 3-1333 effectively mimic the appearance of their much bigger and more energetic namesakes.

This visible-light image was taken by the high resolution channel of Hubble’s Advanced Camera for Surveys on the Hubble Space Telescope.

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