It’s all taking shape
ON the night of July 12, 1764, a French astronomer named Charles Messier discovered an interesting patch of light in the constellation of Vulpecula, The Fox.
Writing in his native language, he noted that it “appears oval and contains no star’’. So began the story of the so-called planetary nebulae, many of which appear as roughly circular clouds of gaseous material in space.
Recently, we have found out more about the particular type of planetary nebula that Messier discovered: the bipolar type, which contain two “lobes’’ of material rather than being circular.
Before I discuss the recent exciting images astronomers have obtained, it’s important to mention something about the name given to this type of object.
Planetary nebulae have absolutely nothing to do with planets. It’s purely a historic name that was applied to them long ago. Because they generally appeared to be circular and somewhat resembled the discs of planets as seen through a telescope, they were described as “planetary’’. Indeed, one of my favourites is called the Ghost of Jupiter because it looks like a faint version of Jupiter’s disc. It’s really just a rather unfortunate name we give them, rather than being any kind of scientific description.
We now know that planetary nebulae are the expelled outer layers of a dying star. When a star like the sun reaches an extreme age, a complicated series of changes takes place that results in the core of the star eventually becoming a very small, dense object called a white dwarf, while its outer regions are blown off into space.
A classic example of such a nebula is the Helix Nebula in the constellation of Aquarius, which is a famous circular ring of material.
Indeed, one would normally expect them to be circular, but as I’ve mentioned some are not, and their shapes can be either weird or spectacular, depending on one’s opinion.
The reason for the odd shapes of some of them has been somewhat of a mystery, but some light had been shed on this problem recently by astronomers analysing images of a bipolar planetary nebula surrounding a star called L2 Puppis, in the southern-sky constellation of Puppis, the Poop (part of what was originally a much larger constellation called Argo, the Ship).
L2 Puppis is a red giant star, exactly the type that produces planetary nebulae as it expels its material. It is producing a fine example of a bipolar nebula, and from the images, made using the European Southern Observatory’s Very Large Telescope in South America, we can see that a disc of dust surrounding the star is an obvious component of the system.
However, there was more. Careful examination of the images showed a spot of light close to the dying star.
Astronomers had found a second star in the system, and a calculation of the expected effect of this somewhat less massive stellar companion in combination with the dusty material shows that this arrangement would indeed form a nebula with two distinct parts.
Although the physics is complicated, we do now appear to have an explanation for these bizarre objects.
I expect that as time goes on, we are likely to find that similar conditions have given rise to many of the amazingly shaped planetary nebulae, some of which adorn people’s computer screens as colourful wallpaper.
I am often asked whether our sun will explode. It won’t, although it too one day will form a planetary nebula. Our sun’s nebula will be of the circular type, but we shan’t see this happen. By then, billions of years from now, it will have been a long time since the Earth was habitable.