Planetary nebulae are the most beautiful objects in the night sky. Their gossamer shells of gas are otherworldly and evocative. They captivate the eye, and viewers need no scientific knowledge to get drawn in.
How are they created, and why do they look so beautiful?
Planetary nebulae are some of the most beautiful objects in the galaxy, spanning a variety of shapes and sizes. They’re created in the death throes of stars like the sun, and new research sheds light into how they get their distinctive and unique shapes. The answer: anything unlucky enough to orbit that dying star.
Some stars die a beautiful death, ejecting their outer layers of gas into space, then lighting it all up with their waning energy. When that happens, we get a nebula. Astronomers working with the Gemini Observatory just shared a new image of one of these spectacular objects.
The life cycle of our Sun began roughly 4.6 billion years ago. In roughly 4.5 to 5.5 billion years, when it depletes its supply of hydrogen and helium, it will enter into its Red Giant Branch (RGB) phase, where it will expand to several times its current size and maybe even consume Earth! And then, when it has reached the end of its life-cycle, it is believed that it will blow off its outer layers and become a white dwarf.
Until recently, astronomers were not certain how this would take place and whether or not our Sun would end up as a planetary nebula (as most other stars in our Universe do). But thanks to a new study by an international team of astronomers, it is now understood that our Sun will end its life-cycle by turning into a massive ring of luminous interstellar gas and dust – known as a planetary nebula.
Roughly 90% of all stars end up as a planetary nebula, which traces the transition they go through between being a red giant and a white dwarf. However, scientists were previously unsure if our Sun would follow this same path, as it was thought to not be massive enough to create a visible planetary nebula. To determine if this would be the case, the team developed a new stellar, data-model that predicts the lifecycle of stars.
This model – which they refer to as the Planetary Nebula Luminosity Function (PNLF) -was used to predict the brightness of the ejected envelope for stars of different masses and ages. What they found was that our Sun was just massive enough to end up as a faint nebula. As Prof. Zijlstra explained in a Manchester University press release:
“When a star dies it ejects a mass of gas and dust – known as its envelope – into space. The envelope can be as much as half the star’s mass. This reveals the star’s core, which by this point in the star’s life is running out of fuel, eventually turning off and before finally dying. It is only then the hot core makes the ejected envelope shine brightly for around 10,000 years – a brief period in astronomy. This is what makes the planetary nebula visible. Some are so bright that they can be seen from extremely large distances measuring tens of millions of light years, where the star itself would have been much too faint to see.”
This model also addressed an enduring mystery in astronomy, which is why the brightest nebulae in distant galaxies all appear to have the same luminosity. Roughly 25 years ago, astronomers began to observe this, and found that they could gauge the distance to other galaxies (in theory) by examining their brightest planetary nebulae. However, the model created by Gesicki and his colleagues contradicted this theory.
In short, the luminosity of a planetary nebula does not come down to the mass of the star creating it, as was previously assumed. “Old, low mass stars should make much fainter planetary nebulae than young, more massive stars,” said Prof. Zijlstra. “This has become a source of conflict for the past for 25 years. The data said you could get bright planetary nebulae from low mass stars like the Sun, the models said that was not possible, anything less than about twice the mass of the sun would give a planetary nebula too faint to see.”
Essentially, the new models demonstrated that after a star ejects its envelope, it will heat up three times faster than what older models indicated – which makes it much easier for low mass stars to form a bright planetary nebula. The new models also indicated that the Sun is almost exactly at the lower cut off for low mass stars that will still produce a visible, though faint, planetary nebula. Anything smaller, Prof. Zijlstra added, will not produce a nebula:
“We found that stars with mass less than 1.1 times the mass of the sun produce fainter nebula, and stars more massive than 3 solar masses brighter nebulae, but for the rest the predicted brightness is very close to what had been observed. Problem solved, after 25 years!”
In the end, this study and the model the team produced has some truly beneficial implications for astronomers. Not only have they indicated with scientific confidence what will happen to our Sun when it dies (for the first time), they have also provided a powerful diagnostic tool for determining the history of star formation for intermediate-age stars (a few billion years old) in distant galaxies.
It’s also good to know that when our Sun does reach the end of lifespan, billions of years from now, whatever progeny we leave behind will be able to appreciate it – even if they are looking across the vast distances of space.
The planetary nebula Fleming 1, as seen with ESO’s Very Large Telescope. Credit: ESO/H. Boffin
The neat thing about planetary nebulae is that they are like snowflakes: no two are quite the same. Some look like pools of hot water, some look like glowing eyes in the night and others, like this image of Fleming 1, have twin jets of material spiraling outward from the center resembling a huge cosmic sprinkler.
And for the first time, astronomers with the European Southern Observatory have paired new Very Large Telescope images of Fleming 1 with computer models to explain how the intricate dance between two dead stars result in these bizarre nebulae that appear to be flinging material out into space. The team’s findings were published in the November 9, 2012 issue of the journal Science.
“The origin of the beautiful and intricate shapes of Fleming 1 and similar objects has been controversial for many decades,” says team leader Henri Boffin in a press release. “Astronomers have suggested a binary star before, but it was always thought that in this case the pair would be well separated, with an orbital period of tens of years or longer. Thanks to our models and observations, which let us examine this unusual system in great detail and peer right into the heart of the nebula, we found the pair to be several thousand times closer.”
The team using ESO’s VLT to study Fleming 1’s central star, toward the constellation Centaurus, found not one but two white dwarfs at its core. The two white-hot dead stars slightly smaller than our Sun circle each other every 1.2 days. Binary stars have been found at the heart of planetary nebulae before, but two white dwarfs circling each other is very rare, say the scientists.
Planetary nebulae have nothing to do with planets. Astronomers in the eighteenth century likened these glowing bubbles of light to planets because they resembled the distant orbs Uranus and Neptune in their small telescopes. Planetary nebulae are actually a brief stage at the end of a sun-like star’s life. As a star with a mass up to eight times that of our Sun nears the end of its life, it sloughs off its outer shells in an immense bubble. As more and more mass is lost to space, the white-hot stellar core is exposed. This white dwarf gives off a stiff solar wind that pushes the bubble ever wider. Blistering ultraviolet radiation from the dead star excites atoms in the expanding cloud causing it to glow.
This animation shows how the two stars at the heart of a planetary nebula like Fleming 1 can control the creation of the spectacular jets of material ejected from the object. Credit:ESO/L. Calçada. Music: delmo “acoustic”
Gazing into a planetary nebulae rarely reveals a quiet environment. Complex knots and filaments form intricate patterns. For cosmic sprinklers like Fleming 1 material seems to be shooting from both poles with an S-shaped pattern between the star and the outermost wavefront. Scientists say that as the stars aged, they expanded and one sucked material from its companion; a kind of starry vampire, forming a spinning disk of material. As they rapidly orbited each other, the pair began to wobble like a spinning top, a type of motion called precession. The team’s study shows that precessing accretion disks within binary star systems form the symmetrical arcs of material in planetary nebulae like Fleming 1.
The VLT images revealed even more surprises about Fleming 1, named after Scottish astronomer Williamina Fleming in 1910. Scientists found a knotted ring of material within the inner nebula of Fleming 1. Scientists look for these rings as a sign of a binary system.
Planetary nebulae come in a dazzling array of shapes, from spherical shells of gas, to blobby structures barely containing symmetry at all. Controversy has surrounded the cause for this diversity. Could it be magnetic fields, high rotation rates, unseen companions, or something else entirely? Recently, there has been a growing consensus that binary companions are the main culprit for the most irregular of these nebulae, but exploring the connection is only possible with a statistically significant sample of planetary nebulae with binary cores can be found, giving hints as to what properties they may, or may not, create.
Currently, astronomers recognize over 3,000 planetary nebulae within our own galaxy. Only ~40 are known to harbor binary stars at their core but astronomers are uncertain of just how many truly due. The difficulty lies in the amount of time it takes to search for a companion. Typically, companions can be discovered with spectroscopic measurements in the same way astronomers discover planets by detecting a wobble. Alternatively, binary companions can be teased out through eclipses but both methods require frequent monitoring and, until recently, were best suited for single target studies.
With the recent popularity of wide field survey missions, possibilities to detect more binary companions has increased greatly. These surveys are ideally suited for capturing eclipses or microlensing events. In each case, they will preferentially discover companions with tight orbits and short orbital periods which are suspected to have the greatest effect on the shape of the nebulae.
Stars that orbit close together are expected to have a strong effect because, as the primary star enters its post-main sequence lifetime, it is likely that the secondary star will become engulfed in the envelope of the primary, essentially sharing the outer layers. This creates large differences in density along the equator which leads to uneven ejection of the material as the primary star sheds its outer layers, forming the nebula. These temporary overdensities would serve to funnel material and could be responsible for the presence of polar outflows or jets.
A recent study has added two more planetary nebulae to the list of those with known binary centers: NGC 6326 (shown right) and NGC 6778. Collimated outflows and jets were discovered in both cases. The authors also note that both nebulae have filaments with low ionization. Such structures have been noted previously, but their cause has remained uncertain. A 2009 study suggested that they may be the result of tight binaries, a hypothesis that is strengthened by the the new discovery. The overall shape of NGC 6326 is mostly elliptical while NGC 6778 is bipolar.
Determining the chemical distribution of the galaxy is a tricky business. The ideal method is spectroscopy but since high quality spectroscopy takes bright targets, the number of potential targets is somewhat reduced. Stars seem like logical choices, but due to differential separation during formation, they don’t provide a true description of the interstellar medium. Clouds of gas and dust are the best choice, but must be illuminated by star formation. Another option is to search for newly formed planetary nebulae which are in the process of enriching the interstellar medium.
A new paper does just this, discovering a new planetary nebula in hopes of mapping the chemical abundance of the galaxy. The new nebula is almost the exact opposite direction of the galactic center when viewed from Earth. It lies at a distance of about 13 kpc (42,400 lightyears) from Earth making it one of the most distant planetary nebulae from the galactic center for which a distance has been determined and currently, the furthest with a measured chemical abundance.
The nebula was originally recorded on images taken by the INT Photometric Hα Survey (IPHAS) in 2003 but the automated program for detecting such objects initially missed the nebula due to its relatively large angular size (10 arcseconds). It was subsequently caught on visual inspection of the mosaics. Follow-up spectroscopy was conducted from 2005 to 2010 and reveal that the nebula is quite regular for planetary nebula, containing strong emission from hydrogen, nitrogen, oxygen, and silicon. The rate of expansion combined with its physical size suggests an age of nearly 18,000 years.
This newly discovered nebula provides a rare data point for the chemical abundance for the outer portions of the galaxy. While the galaxy is known to be enriched towards the galactic center, there has been debate about how quickly, if at all, it falls towards the galactic edge where star formation, and thus, enrichment, is less common. While there aren’t enough known nebulae to determine just yet (only four others are known at similar distances), this planetary nebula suggests that the abundance levels off in the galactic outskirts.
The authors also note that this nebula, as well as potentially the others, aren’t native to the Milky Way. They lie near a structure known as the Monoceros Ring, which is a stream of stars believed to be stretched out as the Milky Way devours the Canis Major Dwarf Galaxy.
The Hubble Space Telescope’s Advanced Camera for Surveys has captured a remarkable image of a spiral in space. No, not a spiral galaxy, (and not another Norway Spiral!) but the formation of an unusual pre-planetary nebula in one of the most perfect geometrical spirals ever seen. The nebula, called IRAS 23166+1655, is forming around the star LL Pegasi (also known as AFGL 3068) in the constellation of Pegasus.
The image shows what appears to be a thin spiral pattern of amazing precision winding around the star, which is itself hidden behind thick dust. Mark Morris from UCLA and an international team of astronomers say that material forming the spiral is moving outwards at a speed of about 50,000 km/hour and by combining this speed with the distance between layers, they calculate that the shells are each separated by about 800 years.
The spiral pattern suggests a regular periodic origin for the nebula’s shape, and astronomers believe that shape is forming because LL Pegasi is a binary star system. One star is losing material as it and the companion star are orbiting each other. The spacing between layers in the spiral is expected to directly reflect the orbital period of the binary, which is estimated to be also about 800 years.
A progression of quasi-concentric shells has been observed around a number of preplanetary nebulae, but this almost perfect spiral shape is unique.
Morris and his team say that the structure of the AFGL 3068 envelope raises the possibility that binary companions are responsible for quasi-concentric shells in most or all of the systems in which they have been observed, and the lack of symmetry in the shells seen elsewhere can perhaps be attributed to orbital eccentricity, to different projections of the orbital planes, and to unfavorable illumination geometries.
Additionally – and remarkably — this object may be illuminated by galactic light.
This image appears like something from the famous “Starry Night” painting by Vincent van Gogh, and reveals what can occur with stars that have masses about half that of the Sun up to about eight times that of the Sun. They do not explode as supernovae at the ends of their lives, but instead can create these striking and intricate features as their outer layers of gas are shed and drift into space. This object is just starting this process and the central star has yet to emerge from the cocoon of enveloping dust.