Our sun is headed for a rather surprising end, but to understand it we need to look at what happens to stars much larger and smaller than our own.
Very large stars -- the bright giants that stand out in the night sky -- live short, fast lives. They burn their fuel so rapidly that they come to a swift end, and their final act can be quite spectacular: a cataclysmic explosion that rips the star apart and for a short time causes the star to outshine entire galaxies. This dazzling but destructive spectacle is what we know as a supernova.
A supernova generates such a tremendous blast of energy that if one occurred in our part of the galaxy it could well wipe out all life on Earth. Fortunately, a supernova near us any time soon is unlikely.
Very small stars -- stars much smaller than our sun -- do not have such spectacular ends, and in fact they could almost be said not to end at all.
Small stars consume their fuel very slowly, so slowly that the estimated life span of the smallest stars exceeds the known life span of the universe.
The end of our sun
Our sun is an in-between star, smaller than the giants but larger than the small dwarfs. As with all stars, it is powered by a continuous nuclear reaction very similar to the one that powers a hydrogen bomb. In this reaction, hydrogen is converted to helium, releasing energy in the process. This reaction is so powerful that the conversion of only a tiny amount of hydrogen to helium is enough to power a multi-megaton hydrogen bomb.
Our sun is powered the same way, but it converts 4 million tons of hydrogen to helium every second. The sun is so large that it can maintain this prodigious energy output for billions of years, but eventually the hydrogen will run out. It will then swell to become a red giant, its surface temperature cooling as its atmosphere expands to an enormous size -- so large that it will envelop (and burn to a crisp) all the inner planets, including ours.
The beautiful bubble
Eventually the sun will shrink to become a tiny dwarf star, but between its red giant phase and its end state as a tiny dwarf there will be a time when our sun will be at its most beautiful: It will eject a shell of glowing gas, and for a relatively short time our sun will be enshrined in a planetary nebula.
The name "planetary nebula" is very misleading. It came about because in early telescopes these small nebulae appeared as small disks, much as a distant planet would appear in such telescopes. But the term has nothing to do with planets. Instead, it refers to a "bubble" or shell of glowing gas that surrounds a small dying star.
Planetary nebulae are very common in the sky, with hundreds of them known.
Most are small, requiring very powerful telescopes to see them, but for their size they are typically quite bright, and always colorful. Like celestial snowflakes, no two are alike, each shining with a unique and fleeting beauty.
Planetary nebulae do not last long. The beautiful glowing bubble is ejected from the star at high speed. Within a short time (a few tens of thousands of years -- a blink in time for a star) the bubble dissipates into space. Some stars appear to have ejected several bubbles in succession, with the faint remaining wisps of the last one still faintly visible beyond the current bubble.
The Hubble breakthrough
Planetary nebulae have long been an area of research for astronomers, but the study of these beautiful objects was revolutionized by the Hubble Space Telescope. With its superb reach and resolution, the Hubble has been able to take images of planetary nebulae that far surpass anything previously seen.
The details made visible by the Hubble have not only been a great boon to astrophysicists and astronomers, but also to those of us who just enjoy the beauty of celestial objects. Under the Hubble's powerful eye, many planetary nebulae too small to be well seen from Earth have revealed their beauty in glorious detail, displaying intricate wisps and delicate colors never before revealed.
The Hubble has also revealed new mysteries, such as that many planetary nebulae are not bubble-like, but divided into halves that are mirror images of one another (what astronomers call "bipolar symmetry"). Some look like hourglasses, others like butterflies and a variety of shapes in between. One theory is that the symmetrical division of such nebulae is due to something orbiting the star, such as an unseen companion star, or perhaps planets. But that is only one theory, and it remains unproven.
The research continues, but a definitive answer may be difficult to come by, not least because each planetary offers its own unique twist to the mystery.
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