Starship Asterisk*

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It’s well known that the Universe is changeable: even the stars that appear static and predictable every night are subject to change.

This image from the NASA/ESA Hubble Space Telescope shows planetary nebula Hen 3-1333. Planetary nebulae are nothing to do with planets — they actually represent the death throes of mid-sized stars like the Sun. As they puff out their outer layers, large, irregular globes of glowing gas expand around them, which appeared planet-like through the small telescopes that were used by their first discoverers.

The star at the heart of Hen 3-1333 is thought to have a mass of around 60% that of the Sun, but unlike the Sun, its apparent brightness varies substantially over time. Astronomers believe this variability is caused by a disc of dust which lies almost edge-on when viewed from Earth, which periodically obscures the star.

It 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 Hen 3-1333 effectively mimic the appearance of their much bigger and more energetic namesakes: they are sheep in Wolf–Rayet clothing.

This visible-light image was taken by the high resolution channel of Hubble’s Advanced Camera for Surveys. The field of view is approximately 26 by 26 arcseconds.

Credit: NASA/ESA/Hubble

Zoomable Image

I love a good coincidence. On Monday morning, I posted a gorgeous picture of the planetary nebula Abell 31, an object formed when a dying star blows off its outer layers in a series of winds which collide with each other. I mentioned that these nebulae are usually symmetric — Abell 31 happens not to be because it’s moving rapidly through space, and the gas through which it moves is compressing one side of it. But events like that are not the norm; most planetaries show stunning symetric features… like Henize 3-1333, as you can see in this nice Hubble image of it: (see above)

It looks like a flower, doesn’t it? The petals you see are actually sculpted lobes of gas. I’m guessing it undergoes periodic episodes where it blows out gas in focused beams, which then move outward and form those features as they plow into gas previously blown out by the star. It’s a guess, but it fits what’s known about the inner regions of the cloud near the central star. There’s a thick disk of material surrounding the central star, something like 30 billion km (20 billion miles) across, far larger than our solar system. Every six years or so, the central star appears to dim, which may be due to the inner part of the disk itself becoming unstable and puffing up, blocking the light. This disk may also be responsible for shaping the outflow of the gas from the star, forming those petals.

If you’re wondering just how much material the star is blowing out, it turns out to be 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° C or about 55,000° F) core of the star, which will then cool over the next few billion years. 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 ethereally thin matter between the stars.

Well that, plus images like this one. And, of course, the knowledge we’ve gained studying how stars die.

• Astronomy & Astrophysics 528 A39 (Apr 2011) DOI: 10.1051/0004-6361/201015469
  arXiv.org > astro-ph > arXiv:1101.4959 > 25 Jan 2011