When I first saw this ghostly Hubble Space Telescope image, I assumed that faint blurry spiral was a lens flare or some other photographic anomaly. But on closer inspection, the details started to present themselves.
As imaged by the space telescope’s sensitive Advanced Camera for Surveys, this striking pattern is formed by material being ejected from a dying star. But this isn’t a lone star; there’s a second star — a binary partner — orbiting with it and modulating the expanding gas.
This binary system is called LL Pegasi and the surrounding “pre-planetary nebula” is known as IRAS 23166+1655.
Much like the jets of water being sprayed from a spinning sprinkler head, the result is an expanding pinwheel spiral when viewed from above.
Through blind cosmic luck, we are also looking LL Pegasi from above, so as the stars orbit, we can see a perfect gas spiral expand into space.
Planetary nebulae are created during the final stages of stellar evolution. For stars from half to eight-times the size of our sun, once they run out of hydrogen fuel, they start to burn heavier and heavier elements. During the latter stages of this process, they swell and the outer layers of the star are stripped away into space. This escaping gas and dust forms the nebula.
In this image, the central stars cannot be seen as they are smothered in obscuring material belching from the dying star, but the nebula has only just started to form.
According to the Hubble press release (where high resolution versions of this image can be found), the gas is being flung into space at a speed of 50,000 km/hour (about 30,000 miles/hour) and astronomers have calculated that the two stars must be orbiting each other with a period of 800 years.
Therefore, the distance between the “shells” of the spiral represent the time taken for one orbit. Similar to the rings in a tree stump (only inverted), the spacing in the spiral represents an 800 year step in time, getting older the further they move away from the center.
Although these spiral phenomena are rare, several pinwheel spirals are known, perhaps the most infamous being the spiral generated by WR 104.
WR 104 is a Wolf-Rayet star — far larger than the dying star in LL Pegasi — also releasing gas into space during the final stages of its life, but it’s not forming a comparatively peaceful planetary nebula.
As WR 104 is so massive, its Wolf-Rayet phase is frenzied and violent, potentially resulting in a powerful gamma-ray burst (GRB) — LL Pegasi is too small to explode as a GRB. What’s more, as WR 104 was thought to be pointing in our direction (i.e., we can see the perfect spiral), Earth could be “peering” down the barrel of a potentially devastating GRB should it explode.
Fortunately, when Discovery News spoke with Wolf-Rayet star expert Grant Hill last year, he had some good news for us: WR 104 might not be pointing in our direction after all.