Do stars live forever? No they don’t. Like everything else, they are born, go through middle age, get old and finally expire.
The manner of a star’s passing is determined by how big it was to begin with, with stars of different masses having wildly different destinies.
For very large stars—ones more than 8 times the mass of our Sun—the end comes in a violent supernova explosion, during which the remnant of the star can be squashed to form a neutron star or a black hole.
But for your average star, such as our Sun, a different fate awaits.
All stars go through their lives doing battle between two forces—the force of their own weight, which makes them want to squash in on themselves, and the force of their own heat (produced by nuclear fusion reactions in the core) which makes them want to expand.
Stars like these spend most of their lives in an uneasy equilibrium. But when their normal nuclear fuel—hydrogen—runs out, the heat drops and gravity makes the star’s outer layers squeeze in. But ironically, this squeezing in increases the pressure in the core and makes it heat up again, now fusing helium instead of hydrogen.
As the heat becomes too much, the star’s outer gas layers are blasted off, producing a huge expanding cloud. The exposed core of the star continues to radiate out enormous energy, which is absorbed up by the cloud, causing it to glow.
The result is a stunningly pretty nebula.
When astronomers first started spotting these nebulae, their early telescopes were not good enough to show them for what they really are. It was reasonably clear that they were stars surrounded by some sort of nebulosity, but through those early telescopes they looked quite tiny. In fact, they looked just like a distant planet might look—roundish and pale, rather than point-like and bright like a star.
So they were given the name “planetary nebulae“.
Here, we present our selection of the Top 10 prettiest and most spectacular planetary nebulae. All of the images were taken with the Hubble Space Telescope. In many cases the colours are not “real”; rather, they reflect the fact that the imagery was done with special filters to emphasise the light given off at particular wavelengths by certain gases.
The Eskimo Nebula
With some astronomical objects, you wonder how they ever got their names. But not this one. The picture at the top of the page shows the Eskimo Nebula, and it really does look like a face surrounded by a big, fleecy parka hood, doesn’t it? (It has also been called the Clownface Nebula.) Located almost 2,900 light-years away, it was discovered by English astronomer William Herschel in 1787 (although he didn’t quite realise what it was at the time).
The Spirograph Nebula
Two thousand light-years away and 0.3 light-years in width, the Spirograph Nebula (also known as IC 418) is so-named for its geometric patterns, which resemble those produced by the Spirograph toy. Once a red giant star, its outer layers have been puffed off and are now being illuminated by the hot radiation from the remnant white dwarf star, visible at the heart of the nebula.
The Boomerang Nebula
The Boomerang isn’t the most spectacular planetary nebula, but it does have a major claim to fame—it is the coldest natural body found so far in the entire universe! That’s right, the coldest natural body (colder conditions have been produced in the laboratory) at a whopping minus 272 degrees…that’s just one degree above absolute zero. An incredibly strong stellar wind is blowing from the central star, pushing gas out at a speed of 500,000 kilometres per hour. As the gas expands, it cools, leading to the frigid temperature.
The Butterfly Nebula
Also known as the Bug Nebula and NGC 6302, the Butterfly’s delicate appearance belies its real nature. The wings are actually clouds of gas sizzling at nearly 20,000 degrees Celsius, rushing outwards at almost one million kilometres per hour! At its heart is a star with a surface temperature of 220,000 degrees Celsius, one of the hottest known. (By comparison, our Sun’s surface temperature is a measly 5,500 degrees C.) The Butterfly is about 3,800 light-years away and is two light-years wide—that’s enough to stretch from Earth to the next nearest star system, Proxima Centauri, four light-years away.
The Ring Nebula
Long a favourite of amateur astronomers, the Ring Nebula (also known as M57) is 2,000 light-years from Earth and is about one light-year wide. The clouds of gas were sloughed off by the central star thousands of years ago; that star, now a white dwarf, can still be seen in the centre.
Located about 6,500 light-years from Earth, NGC 6751 is a little less than one light-year wide. Its central white dwarf star has a scorching surface temperature of around 140,000 degrees Celsius.
Also known as the Eight-Burst or Southern Ring nebula, NGC 3132 is located about 2,000 light-years from Earth. Unlike some of the other planetary nebulae shown here, this one’s gas cloud is expanding at a very sedate 24 kilometres per second. If you look closely, at its heart you’ll see two stars. The smaller of the two is the white dwarf; earlier in its life it was a much bigger star, and was the one responsible for puffing off its outer gas layers to produce the nebula.
The Engraved Hourglass Nebula
Not to be confused with another nebula called the Hourglass Nebula, this one—also known as MyCn 18—is 8,000 light-years from Earth. When it was discovered in 1940, its striking shape was not apparent, although it was clearly identified as a planetary nebula. It was only with the advent of modern telescopes that its arresting geometry became obvious. It is thought that the star’s equator is surrounded by a donut or torus of thick, gassy material. A stellar wind blowing from the star finds it hard to get through that torus, and instead “blows” out the top and bottom, expanding into the two halves of the hourglass shape.
The Cat’s Eye Nebula
The Cat’s Eye was the very first planetary nebula to be discovered, and it also turns out to be one of the most interesting and complex. Unlike some planetaries, which have a symmetrical and even appearance, the Cat’s Eye has loops and twists and knots. The faint concentric rings are thought to be shells of gas emitted by the star in the distant past. It is possible that the twisted shape of the rest of the nebula is the result of it having two central stars rather than just one. But so far, the putative second star has not been found. The Cat’s Eye is around 3,300 light-years from Earth.
The Helix Nebula
The Helix is one of the closest planetary nebulae to Earth, at just 700 light-years away. It’s also what our Sun might look like a few billion years from now. The Helix is the remains of a Sun-like star that sloughed off its outer gas layers thousands of years ago, just as our Sun will do one day.
Helix Nebula: NASA / ESA / C.R. O’Dell (Vanderbilt University) / M. Meixner, P. McCullough, and G. Bacon (STScI)
Eskimo Nebula: NASA / ESA / Andrew Fruchter (STScI) / ERO team (STScI + ST-ECF)
Cat’s Eye Nebula: ESA / NASA / HEIC / The Hubble Heritage Team (STScI / AURA)
Hourglass Nebula: Raghvendra Sahai, John Trauger (JPL) / WFPC2 science team / NASA / ESA
NGC 3132: Hubble Heritage Team (STScI / AURA / NASA / ESA)
NGC 6751: NASA / ESA / The Hubble Heritage Team (STScI / AURA)
Ring Nebula: Hubble Heritage Team (AURA / STScI / NASA / ESA)
NGC 6302: NASA / ESA / The Hubble SM4 ERO Team
Boomerang Nebula: ESA / NASA
IC 418: NASA / ESA / The Hubble Heritage Team (STScI / AURA)
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