Deep in the far south of the zodiacal constellation Aquarius lies NGC7293, the Helix Nebula. This object illustrates one of the best examples of the last stages in the evolutionary track of a star similar to our Sun.
It was discovered in 1824 by Karl Ludwig Harding during his early sky surveys, but missed by our own great star finders, Sir William Herschel and John Herschel, probably due to their using high-powered telescopes with narrow fields of view, whilst the object has a quite large surface area and low surface brightness.
Sir William Herschel first used the term planetary nebula because he thought these objects resembled the fuzzy disc shape of the outer planet Uranus, which he had recently discovered. We now know these nebulae are not related to planets in any way. Instead, they represent the far end of the evolutionary cycle of stars similar in mass to our Sun.
Our galaxy holds more than 200 billion stars, and likely only 10,000 are planetary nebulae. The answer to this rarity lies in the fact that, on a galactic timescale, these nebulae are like puffs of smoke, lasting only tens of thousands of years before dissipating.
The twisted appearance of the Helix Nebula in images leads to its name. From our earthly perspective, we seem to be looking into spreading shells of gas. As a result, the ring like structures of the nebula looks as if they are twisted.
A few years ago, the Hubble Space Telescope revealed an incredible amount of detail in the Helix. Because of that image, the nebula earned the nickname The Eye of God.
Fortunately, the Helix is one of the nearest planetary nebulae known, a mere 700 light years away allowing it to be imaged in unprecedented detail. Astronomers have discovered thousands of knotlike structures that appear to have
long ‘comet’ tails. These tails are much larger than our solar system’s vagabonds. Recent images resolved the knots as large as twice the size of our solar system, with tails stretching some 160 billion km (100 billion miles). Astronomers estimate there are more than 20,000 knots, and perhaps as many as 40,000.
Planetary nebulae represent the last stage in the life of a sun-like star. Think of the events that led up to the star’s transformation as a metamorphosis, like a butterfly emerging from its chrysalis. For about 10 billion years, a star like the Sun transforms hydrogen into helium at a fairly steady rate. Eventually the star begins to swell and becomes a red giant. After millions of years, the star will begin to shrink until the temperature and internal pressure become so great that the helium at the star’s core starts fusing into carbon. For about 100,000 years, the star yo-yos back and forth as it shrinks and swells. Near the end of this process, the star puffs off its outer layers of gas.
Ultraviolet radiation from the stars extremely hot core excites the hydrogen, nitrogen, and oxygen in the expanding shell. The gas glows from this radiation, which allows the observer to glimpse the thin layer of material that the stellar remnant is blowing off. When our Sun develops into a planetary nebula, it will engulf the inner planets, Mercury, Venus, Earth, and possibly Mars in this deadly transformation.
The progenitor star that created the Helix Nebula may have surged off its outer layers 12,000 years ago, and had another shedding about 6,000 years ago, so these multiple shells are now embedded in each other, which is why images show two distinct rings in the Helix.
In about 5 billion years, our Sun will transform itself into a planetary nebula, but first it will become a monstrous, bloated red giant star.
In his story The Time Machine, H.G. Wells placed his traveller on a beach during our star’s transformation: “I stopped very gently and sat upon the Time Machine, looking around…. Overhead it was a deep Indian red and starless, and south-eastward it grew brighter to a glowing scarlet where, cut by the horizon, lay the huge ball of the Sun, red and motionless”.
A striking image of our star during its final chapter. Melancholy, yes, but how fascinating it is that we can gaze into the night sky and see the future in the past.