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Ghostly green eye

Planetary nebula NGC 6826

NGC 6826 is a planetary nebula, the dying stages of a star's life.

THIS AMAZING IMAGE shows the ghostly “eye-like” planetary nebula NGC 6826, located 2,200 light-years from Earth.

Despite their name, planetary nebulae have nothing to do with planets. They got their name because, through early telescopes, they looked more like planets than stars.

In fact, a planetary nebula is a complex cloud of gas produced in the dying stages of certain stars’ lives.

A star’s life ends when the fuel available to its thermonuclear engine runs out. When the star is about to expire, it becomes unstable and ejects its outer layers, forming a planetary nebula and leaving behind a tiny, but very hot, stellar remnant, known as white dwarf.

Schematic of NGC 6826

Anatomy of planetary nebula NGC 6826

At NGC 6826’s centre, the white dwarf is driving a fast “wind” of gas into older gas material, forming a hot interior bubble that pushes the older gas ahead of it to form a bright rim. The faint green of the eye is believed to be gas that made up almost half of the star’s mass for most of its life.

The red blobs at the edges are called FLIERs, or Fast Low-Ionisation Emission Regions. They’re thought to be dense regions of gas either flung off by the star, or floating in space and caught up in the outflowing rush of the stellar wind.

Stellar evolution theory predicts that our Sun will experience a similar fate to NGC 6826 in about five billion years (out of an estimated overall lifespan of some ten billion years).

And by the way, what does the NGC in its name stand for? The New General Catalogue is a huge list of more than 7,800 “deep space” objects compiled in 1880s by the Danish-Irish astronomer J.L.E. Dreyer.

Downloadable wallpaper image: 1280 x 1280

Adapted from information issued by Bruce Balick (University of Washington), Jason Alexander (University of Washington), Arsen Hajian (U.S. Naval Observatory), Yervant Terzian (Cornell University), Mario Perinotto (University of Florence, Italy), Patrizio Patriarchi (Arcetri Observatory, Italy) and NASA/ESA.

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Wolf in sheep’s clothing

The Crescent Nebula

The Crescent Nebula, a huge cloud of gas illuminated from within by the intense radiation of a Wolf-Rayet star.

THIS IMAGE OF the Crescent Nebula or NGC 6888 was obtained using the Wide Field Camera on the Isaac Newton Telescope in the Canary Islands.

The Crescent Nebula is a cloud of gas illuminated by a central, massive Wolf-Rayet star, WR 136, whose intense outpouring of ultraviolet radiation is responsible for heating and ionising most of the gas ejected by the star earlier in its life.

Strong winds blown by WR 136 are interacting with the previously expelled material and as a result, the nebula shows a complex structure which resembles a crescent red Moon.

The image is a three-colour composite made from data collected using filters to isolate the light emitted by hydrogen alpha (H-alpha) and doubly ionised oxygen (OIII) atoms, and coded in the image as red, green (25% H-alpha and 75% OIII) and blue.

You can see the full-size, high-resolution image here (will open in a new window or tab)

Information courtesy IAC. Image obtained and processed by members of the IAC astrophotography group—A. Oscoz, D. López, P. Rodríguez-Gil and L. Chinarro.

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The Ring Nebula

The Ring Nebula

The Ring Nebula, a classic planetary nebula located 2,300 light-years from Earth.

THIS IMAGE OF the Ring Nebula or Messier 57 was obtained using the Wide Field Camera on the Isaac Newton Telescope in the Canary Islands.

The Ring Nebula is often regarded as the prototype of a “planetary nebula“. But despite the name, a planetary nebula has nothing to do with planets. The moniker was given because such objects did not look sharp and star-like when seen through early telescopes; rather, they appeared as distinct blobs or discs, and in that sense their appearance resembled that of planets.

A planetary nebula forms when an elderly star puffs off its outer gas layers. Afterwards, intense radiation from the exposed core of the star heats up the gas, making it glow. It is the final stage in the life of Sun-like stars, and results in the remnant stellar core becoming a white dwarf.

Many planetary nebulae are full shells of gas surrounding the star on all sides. But observations have shown that the Ring Nebula is, most probably, actually a ring (torus) surrounding its central star, and not a spherical (or ellipsoidal) shell.

This nebula is located approximately 2,300 light-years from Earth. Astronomers estimate that the gas cloud has been expanding away from the star for about 1,600 years (plus or minus about 240 years).

The image is a three-colour composite made from data collected using filters to isolate the light emitted by hydrogen alpha (H-alpha), doubly ionised oxygen (OIII) and ionised sulphur (SII) atoms, and coded in the image as red, green and blue respectively.

See the full-size, high-resolution image here (will open in a new window or tab).

Text adapted from Information issued by IAC. Image obtained and processed by members of the IAC astrophotography group (A. Oscoz, D. López, P. Rodríguez-Gil and L. Chinarro).

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Cosmic eyes staring back at us

Eskimo Nebula

The Eskimo Nebula, so-called for its resemblance to a face inside a hooded parka.

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 Spirograph Nebula

The Spirograph 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 Boomerang Nebula

The Boomerang Nebula

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 Butterfly Nebula

The Butterfly Nebula

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.

The Ring Nebula

The Ring Nebula

NGC 6751

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.

NGC 6751

Planetary nebula NGC 6751

NGC 3132

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.

NGC 3132

Planetary nebula NGC 3132

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 Engraved Hourglass Nebula

The Engraved Hourglass Nebula

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 Cat's Eye Nebula

The Cat's Eye Nebula

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.

The Helix Nebula

The Helix Nebula

Image credits:

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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Cosmic jellyfish afloat in starry sea

NGC 1514

It looks like a deep-sea creature, but it's actually a dying star, or planetary nebula, known as NGC 1514.

  • Object is known as the ‘Crystal Ball’ nebula
  • Comprises a pair of stars surrounded by gas rings
  • Indicates the dying stage of a stars’ life

A new image from NASA’s Wide-field Infrared Survey Explorer shows what looks like a glowing jellyfish floating at the bottom of a dark, speckled sea.

In reality, this critter belongs to the cosmos—it’s a dying star surrounded by fluorescing gas and two very unusual rings.

“I am reminded of the jellyfish exhibition at the Monterey Bay Aquarium—beautiful things floating in water, except this one is in space,” said Edward (Ned) Wright, the principal investigator of the WISE mission at UCLA, and a co-author of a paper on the findings, reported in the Astronomical Journal.

The object, known as NGC 1514 and sometimes the ‘Crystal Ball’ nebula, belongs to a class of objects called planetary nebulae, which form when dying stars toss off their outer layers of material.

NGC 1514 is located 800 light-years away, in the direction of the constellation Taurus.

Ultraviolet light from a central star, or in this case a pair of stars, causes the gas to fluoresce with colourful light. The result is often beautiful—these objects have been referred to as the butterflies of space.

NGC 1514 was discovered in 1790 by Sir William Herschel, who noted that its “shining fluid” meant that it could not be a faint cluster of stars, as originally suspected.

Herschel had previously coined the term planetary nebulae to describe similar objects with circular, planet-like shapes. In reality they have nothing to do with planets.

Visible light and infrared views of NGC 1514

Two views of NGC 1514. On the left is the view from a ground-based, visible-light telescope; the view on the right shows the object in infrared light, as seen by NASA's Wide-field Infrared Survey Explorer, or WISE, satellite.

Dying star’s last gasp

Planetary nebulae with asymmetrical wings of nebulosity are common. But nothing like the newfound rings around NGC 1514 had been seen before. Astronomers say the rings are made of dust ejected by the dying pair of stars at the centre of NGC 1514. This burst of dust collided with the walls of a cavity that was already cleared out by stellar winds, forming the rings.

“I just happened to look up one of my favourite objects in our WISE catalogue and was shocked to see these odd rings,” said Michael Ressler, a member of the WISE science team at NASA’s Jet Propulsion Laboratory, and lead author of the paper.

Ressler first became acquainted with the object years ago while playing around with his amateur telescope on a desert camping trip. “It’s funny how things come around full circle like this.”

WISE was able to spot the rings for the first time because their dust is being heated and glows with the infrared light that WISE can detect. In visible-light images, the rings are hidden from view, overwhelmed by the brightly fluorescing clouds of gas.

Artist's impression of the WISE space telescope

Artist's impression of the WISE space telescope, which studies the cosmos at infrared wavelengths.

“This object has been studied for more than 200 years, but WISE shows us it still has surprises,” said Ressler.

Unexpected finding

Serendipitous findings like this one are common in survey missions like WISE, which comb through the whole sky. WISE has been surveying the sky in infrared light since January 2010, cataloguing hundreds of millions of asteroids, stars and galaxies.

In late September 2010, after covering the sky about one-and-a-half times, as planned it ran out of the frozen coolant needed to chill its longest-wavelength detectors.

The mission, now called NEOWISE, is still scanning the skies with two other of its infrared detectors, focusing primarily on comets and asteroids, including near-Earth objects, which are bodies whose orbits pass relatively close to Earth’s orbit around the sun.

The WISE science team says that more oddballs like NGC 1514 are sure to turn up in the plethora of WISE data—the first batch of which will be released to the astronomical community in spring 2011.

Adapted from information issued by NASA / JPL-Caltech / UCLA / DigitiSed Sky Survey / STScI.

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A dying star’s farewell show

Planetary nebula NGC 6210

Planetary nebula NGC 6210 in the constellation Hercules. A planetary nebula is a complex cloud of gas given off during the dying stages of a Sun-like star's life.

The NASA/ESA Hubble Space Telescope has taken a striking high-resolution image of the curious planetary nebula NGC 6210.

Located about 6,500 light-years away, in the constellation of Hercules, NGC 6210 was discovered in 1825 by the German astronomer Friedrich Georg Wilhelm Struve. Although through a small telescope it appears only as a tiny disc, it is fairly bright as planetary nebulae go.

Despite their name, planetary nebulae have nothing to do with planets. They got their name because, through early telescopes, they looked more like planets than stars.

In fact, a planetary nebula is a complex cloud of gas produced in the dying stages of certain stars’ lives.

In this instance, NGC 6210 is the last gasp of a star slightly less massive than our Sun. Multiple shells of gas ejected by the dying star are superimposed on one another in different orientations, giving NGC 6210 its odd shape.

See a full-size, high-resolution wallpaper image here (new window).

This sharp image shows the inner region of this planetary nebula in unprecedented detail, where the central star is surrounded by a thin, bluish bubble that has a delicate filamentary structure. This bubble is superposed onto an asymmetric, reddish gas complex where holes, filaments and pillars are clearly visible.

A star’s life ends when the fuel available to its thermonuclear engine runs out. The estimated lifetime for a Sun-like star is some ten billion years. When the star is about to expire, it becomes unstable and ejects its outer layers, forming a planetary nebula and leaving behind a tiny, but very hot, remnant, known as white dwarf.

This compact object, visible at the centre of the image, cools down and fades very slowly. Stellar evolution theory predicts that our Sun will experience the same fate as NGC 6210 in about five billion years.

Adapted from information issued by ESA / Hubble / NASA.

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The striking Ring Nebula

The Ring Nebula

A colour-composite image of the Ring Nebula, M57, which is 2,300 light-years from Earth. The nebula is a torus, or donut, of gas surrounding a white dwarf star.

The Ring Nebula is often considered the leader of a class of celestial objects known as “planetary nebulae”. The image above was produced by astronomers from the Isaac Newton Group of Telescopes in the Canary Islands.

Despite the term, planetary nebulae have nothing to do with planets. They got their name from observations using early telescopes, through which they looked like small blobs instead of pinprick stars. In this respect, they resembled the discs or faces of planets, and the name has stuck ever since.

In reality, a planetary nebula is a huge cloud of gas that has been “puffed” off by a star in the final stages of its life. Our Sun will eventually go through this phase.

The Ring Nebula, also known as M57 (being the 57th entry in the catalogue compiled by the 18th-19th century French astronomer Charles Messier) is 2,300 light-years from Earth. At its centre is a white dwarf star…the “burned out” remains of a normal type of star.

Some planetary nebula are spherical-shaped clouds that completely surround their central star. The Ring Nebula, though, is thought instead to be a ring (or torus, like a donut) surrounding the white dwarf and fortuitously seen face-on by Earth-bound astronomers.

The image above is a false-colour composite that shows emission from certain specific types of gas in the nebula: hydrogen (shown as red), doubly ionised oxygen (green) and ionised sulphur (blue).

See the full-size, high-resolution version here (new window).

The photos below give two other views. First, a Hubble Space Telescope image that shows the nebula in approximately true colour; and second, an infrared view from the Spitzer Space Telescope.

The Ring Nebula

A Hubble Space Telescope image of the Ring Nebula, showing it in approximately true colours.

The Ring Nebula.

A Spitzer Space Telescope infrared image of the Ring Nebula gives a very different picture, bringing out lots of detail not seen at other wavelengths.

Top image courtesy of the Isaac Newton Group of Telescopes; image obtained and processed by members of the IAC astrophotography group (A. Oscoz, D. López, P. Rodríguez-Gil and L. Chinarro).

Middle image courtesy The Hubble Heritage Team (AURA / STScI / NASA).

Bottom image courtesy NASA / JPL-Caltech / J. Hora (Harvard-Smithsonian CfA).

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