Slowing Down Stars

Forming Star's Magnetic Field Interacting With Disc Credit: NASA/JPL-Caltech/R. Hurt (SSC).


One of the long standing challenges in stellar astronomy, is explaining why stars rotate so slowly. Given their large masses, as they collapsed to form, they should spin up to the point of flying apart, preventing them from ever reaching the point that they could ignite fusion. To explain this rotational braking, astronomers have invoked an interaction between the forming star’s magnetic field, and forming accretion disc. This interaction would slow the star allowing for further collapse to take place. This explanation is now over 40 years old, but how has it held up as it has aged?

One of the greatest challenges to testing this theory is for it to make predictions that are directly testable. Until very recently, astronomers were unable to directly observe circumstellar discs around newly formed stars. In order to get around this, astronomers have used statistical surveys, looking for the presence of these discs indirectly. Since dust discs will be warmed by the forming star, systems with these discs will have extra emission in the infrared portion of the spectra. According to the magnetic braking theory, young stars with discs should rotate more slowly than those without. This prediction was confirmed in 1993 by a team of astronomers led by Suzan Edwards at the University of Massachusetts, Amherst. Numerous other studies confirmed these general findings but added a further layer to the picture; stars are slowed by their discs to a period of ~8 days, but as the discs dissipate, the stars continue to collapse, spinning up to a period of 1-2 days.

Another interesting finding from these studies is that the effects seem to be most pronounced for stars of higher mass. When similar studies were conducted on young stars in the Orion and Eagle nebulae, researchers found that there was no sharp distinction between stars with or without disks for low mass stars. Findings such as these have caused astronomers to begin questioning how universal the magnetic disc braking is.

One of the other pieces of information with which astronomers could work was the realization around 1970 that there was a sharp divide in rotational speeds between high mass stars and lower mass ones at around the F spectral class. This phenomenon had been anticipated nearly a decade earlier when Evry Schatzman proposed that the stellar wind would interact with the star’s own magnetic field to create drag. Since these later spectral class stars tended to have more active magnetic fields, the braking effect would be more important for these stars.

Thus astronomers now had two effects which could serve to slow rotation rates of stars. Given the firm theoretical and observational evidence for each, they were both likely “right”, so the question became which was dominant in which circumstance. This question is one with which astronomers are still struggling.

To help answer the question, astronomers will need to gather a better understanding of how much each effect is at work in individual stars instead of simply large population surveys, but doing so is tricky. The main method employed to examine disc locking is to examine whether the inner edge of the disc is similar to the radius at which an object in a Keplarian orbit would have a similar angular velocity to the star. If so, it would imply that the star is fully locked with the disc’s inner edge. However, measuring these two values is easier said than done. To compare the values, astronomers must construct thousands of potential star/disc models against which to compare the observations.

In one recent paper astronomers used this technique on IC 348, a young open cluster. Their analysis showed that ~70% of stars were magnetically locked with the disc. However, the remaining 30% were suspected to have inner disc radii beyond the reach of the magnetic field and thus, unavailable for disc braking. However, these results are somewhat ambiguous. While the strong number of stars tied to their discs does support the disc braking as an important component of the rotational evolution of the stars, it does not distinguish whether it is presently a dominant feature. As previously stated, many of the stars could be in the process of evaporating the discs, allowing the star to again spin-up. It is also not clear if the 30% of stars without evidence of disc locking were locked in the past.

Research like this is only one piece to a larger puzzle. Although the details of it aren’t fully fleshed out, it is readily apparent that these magnetic braking effects, both with discs and stellar winds, play a significant effect on slowing the angular speed of stars. This runs completely contrary to the frequent Creationist claim that “[t]here is no know [sic] mechanical process which could accomplinsh [sic] this transfer of momentum”.

Ovation For A Stellar Senior


Residing in space 6500 light-years away in the constellation of Cepheus, an aged star designated as IRAS 22036+5306 is making its final curtain call. Its stellar play is ending and its making the transition through the protoplanetary, or preplanetary, nebula phase. This isn’t an unusual occurrence, but considering we’ve only been able to witness perhaps a few hundred such events out of the millions of stars we’ve observed – it is a rare visual example. Behold a red giant turning into a white dwarf…

Kudos go to the watchful eye of the Hubble Space Telescope for capturing this ancient celestial oddity. Inside the elaborate enclosure of expelled material is an exposed stellar core – burning hotter than the aspirations of a young actor. Encircling it is a diaphanous cloak of composites – everything from comets to small, rocky bodies. Gases and clumps of material ten thousand times the mass of Earth rocket outward from the poles at speeds of up to 800 000 kilometres per hour. It is the last hurrah.

When its time has passed, IRAS 22036+5306 will transform into a planetary nebula. Intense ultraviolet radiation will ionise the cast-off gases and it will kindle the colorful spectacle which signals the low, slow cool-down until its next evolution. “Studying rarities such as IRAS 22036+5306 provides astronomers with a window into the short and poorly understood phase of stellar evolution when bloated red giant stars pare down to small white dwarfs.” says the ESA/Hubble/NASA team. “For example, mysteries remain about how exactly the dusty torus and jets form.”

Transforming into a planetary nebula may be what awaits the star of our own solar system play – as it is thought to be the eventual destiny of most medium-sized stars. But, our stellar actor might not exit with such splendor since IRAS 22036+5306 is roughly four times the size of the Sun.

And it’s a round of applause we’ll be waiting on for another 5 billion years…

Stellar Super Soaker


Located in the constellation of Perseus and just a mere 750 light years from Earth, a young protostar is very busy spewing forth copious amounts of water. Embedded in a cloud of gas and dust, the hundred thousand year old infant is blasting out this elemental life ingredient from both poles like an open hydrant – and its fast moving droplets may be seeding our Universe…

“If we picture these jets as giant hoses and the water droplets as bullets, the amount shooting out equals a hundred million times the water flowing through the Amazon River every second,” said Lars Kristensen, a postdoctoral astronomer at Leiden University in the Netherlands and lead author of the new study detailing the discovery, which has been accepted for publication in the journal Astronomy & Astrophysics.. “We are talking about velocities reaching 200,000 kilometers [124,000 miles] per hour, which is about 80 times faster than bullets flying out of a machine gun.”

To capture the the quicksilver signature of hydrogen and oxygen atoms, the researchers employed the infrared instruments on-board the European Space Agency’s Herschel Space Observatory. Once the atoms were located, they were followed back to the star where they were formed at just a few thousand degrees Celsius. But like hitting hot black top, once the droplets encounter the outpouring of 180,000-degree-Fahrenheit (100,000-degree-Celsius) gas jets, they turn into a gaseous format. “Once the hot gases hit the much cooler surrounding material – at about 5,000 times the distance from the sun to Earth – they decelerate, creating a shock front where the gases cool down rapidly, condense, and reform as water.” Kristensen said.

Like kids of all ages playing with squirt guns, this exciting discovery would appear to be a normal part of a star “growing up” – and may very well have been part of our own Sun’s distant past. “We are only now beginning to understand that sun-like stars probably all undergo a very energetic phase when they are young,” Kristensen said. “It’s at this point in their lives when they spew out a lot of high-velocity material – part of which we now know is water.”

Just like filling summer days with fun, this “star water” may well be enhancing the interstellar medium with life-giving fundamentals… even if that “life” is the birth of another star. The water-jet phenomenon seen in Perseus is “probably a short-lived phase all protostars go through,” Kristensen said. “But if we have enough of these sprinklers going off throughout the galaxy – this starts to become interesting on many levels.”

Skip the towel. I’ll let the Sun dry me off.

Original Story Source: National Geographic.

Light Blows Away Giant Molecular Clouds


Although they only make up about one percent of the interstellar medium, giant molecular clouds are a rather formidable thing. These dense masses of gas can reach tens of parsecs in diameter and we know them as star forming regions. But, what we didn’t know is that light from massive stars can tear them apart.

New findings presented by Dr. Elizabeth Harper-Clark and Prof. Norman Murray of the Canadian Institute for Theoretical Astrophysics (CITA) show that radiation pressure is not a thing which should be discounted. It has widely been theorized that supernovae accounted for GMC disruption, but “Even before a single star explodes as a supernova, massive stars carve out huge bubbles and limit the star formation rates in galaxies.”

Galaxies harbor stellar nurseries and, as stars are born, the galaxy evolves. It is our understanding that stellar birth occurs within giant molecular clouds where low temperatures, high density and gravity work together to ignite the stellar process. It happens at a smooth and steady rate – a pace which we surmise occurs from the outflow of energy from other stars and possibly black holes. But just what exactly is the life expectancy of a GMC?

To understand a giant molecular cloud is to understand the mass of the stars contained within it. This is key to star formation rates. “In particular, the stars within a GMC can disrupt their host and consequently quench further star formation.” says Harper-Clark. “Indeed, observations show that our own galaxy, the Milky Way, contains GMCs with extensive expanding bubbles but without supernova remnants, indicating that the GMCs are being disrupted before any supernovae occur.”

What’s happening here? Ionization and radiation pressure are blending together within the gases. Electrons are being forced out of atoms during ionization… an action which happens incredibly fast, heating up the gases and increasing pressure. The often over-looked radiation is far more subtle. “The momentum from the light is transferred to the gas atoms when light is absorbed.” says the team. “These momentum transfers add up, always pushing away from the light source, and produce the most significant effect, according to these simulations.”

The simulations performed by Harper-Clark are just the beginning of new studies. The work shows calculations of the effects of radiation pressure on GMCs and reveal they are capable of not only disrupting star-forming regions, but completely blowing them apart, cutting off further formation when about 5 to 20% of the clouds mass had been converted to stars. “The results suggest that the slow rate of star formation seen in galaxies across the Universe may be the result of radiative feedback from massive stars,” says Professor Murray, Director of CITA.

So what of supernovae? Incredibly enough, it would seem they are simply unimportant to the equation. By calculating the results both with and without star light radiation, supernova events didn’t change star formation nor did they alter the GMC. “With no radiation feedback, supernovae exploded in a dense region leading to rapid cooling. This robbed the supernovae of their most effective form of feedback, hot gas pressure.” says Dr. Harper-Clark. “When radiative feedback is included, the supernovae explode into an already evacuated (and leaky) bubble, allowing the hot gas to expand rapidly and leak away without affecting the remaining dense GMC gas. These simulations suggest that it is the light from stars that carves out nebulae, rather than the explosions at the end of their lives.”

Original Story Source: Canadian Astronomical Society More information on Dr. Harper-Clark’s work can be found here.

Revealing A Hybrid Star Cluster


Almost a century ago, astronomers Shapley and Melotte began classifying star clusters. This rough, initial go-around took in the apparent number of stars and the compactness of the field – along with color. By 1927, these “classes” were again divided to include both open and globular clusters. But there are some that simply defy definition.

According to Johns Hopkins astronomer Imants Platais, there is one case which has puzzled astronomers for decades: a well-known, seemingly open star cluster in the constellation of Lyra, named NGC 6791.

“This cluster is about twice the age of the sun and is unusually metal rich (at least twice the Sun’s metallicity),” said Platais, of the Henry A. Rowland Department of Physics and Astronomy’s Center for Astrophysical Sciences. “A couple of decades ago, it was also found that NGC 6791 contains a handful of very hot but somewhat dim stars, called hot subdwarfs. The presence of such stars in an open cluster is rare, though not unique.”

Why are these hot subdwarfs an anomaly? The facts about star clusters as we know them are that globular clusters are notoriously metal poor, while open clusters are metal rich. “The massive stars that create much of the metals live for only a short time, and when they die, they spit out or eject the metals they have created.” says the team. “The expelled metals become part of the raw material out of which the next stars are formed. Thus, there is a relationship between the age of a star and how much metal it contains: old stars have a lower metallicity than do younger ones. Less massive stars live longer than higher mass stars, so low mass stars from early generations still survive today and are studied extensively.”

A team led by Platais and Kyle Cudworth from The University of Chicago’s Yerkes Observatory set out to solve the mystery of NGC 6791 by taking a census of its stars. Their findings revealed several luminous stars in the horizontal branch of the HR diagram… Stars that would normally be found in globular clusters. The hot subdwarfs were confirmed to be genuine cluster members, but they now “appear to be simply the bluest horizontal branch stars”. What’s wrong with this picture? NGC 6791 contains simultaneously both red and very blue horizontal branch stars – making it both old and metal rich. Quite simply put, studying star clusters is key to understanding stellar evolution – unless the cluster starts breaking the rules.

“Star clusters are the building blocks of galaxies and we believe that all stars, including our own sun, are born in clusters. NGC 6791 is a real oddball among about 2,000 known open and globular star clusters in the Milky Way and as such provides a new challenge and a new opportunity, to our understanding of how stars form and evolve,” said Platais, who presented this work last week at the 218th meeting of the American Astronomical Society in Boston.

So… what about star clusters in other galaxies? Three hybrids have been discovered (2005) in the Andromeda Galaxy – M31WFS C1, M31WFS C2, and M31WFS C3. They have the same basic population and metallicity of a globular cluster, but they’re expanded hundreds of light years across and are equally less dense. Are they extended? Or perhaps a dwarf spheroidal galaxy? They don’t exist (as far as we know) in the Milky Way, but there’s always a possibility these hybrid clusters may call other galaxies home.

Until then, we’ll just keep learning.

Original Story Source: John Hopkins University.

Old Star Clusters Shed New Light on Starbirth


Hovering about the galactic plane and locked in the embrace of a spiral galaxy’s arms, open star clusters usually contain up to a few hundred members and generally span around thirty light years across. Most are young, up to a few tens of millions of years old – with a few rare exceptions as old as a few billion years. We understand that over time the members of a galactic cluster slowly drift apart to form loose associations. But what we don’t understand is exactly how their stars formed.

“The net effect of this is that their stars eventually become redistributed throughout the Galaxy,” said Nathan Leigh, a PhD student at McMaster University and lead author for a study being presented this week at the CASCA 2011 meeting in Ontario, Canada. “This is how we think most of the stars in the Milky Way came to be found in their currently observed locations.”

One of the reasons we’re not able to probe deeply into the construction and evolution of galactic clusters is because they are typically hidden by a dense veil of gas and dust. Beautiful to look at… But nearly impossible to cut through in visible light. This means we can’t directly observe the process of starbirth. To help understand this process, astronomers have combined their observations of star clusters so old they date back to the beginning of the Universe itself . And, thanks to modern computing, they are also able to generate state-of-the-art simulations for stellar evolution.

“Unfortunately, most star clusters take so long to dissolve that we cannot actually see it happening. But we now understand how this process occurs, and we can look for its signatures by examining the current appearances of clusters,” said Nathan Leigh. “We have gone about this by matching up the clusters we make with our simulations to the ones we actually observe. This tells us about the conditions at the time of their formation.”

These simulations have given Leigh and collaborators the stimulus they needed to re-trace the histories of real star clusters, giving us new clues about formation. To complete their studies, they relied upon highly sophisticated observations recently taken with the Hubble Space Telescope.

“Remarkably, we are finding that all star clusters more or less share a common history, extending all the way back to their births,” said Leigh. “This came as a big surprise to us since it suggests that the problem could be much simpler than we originally thought. Our understanding of not only how stars form, but also the history of our Galaxy, just took a much bigger step forward than we were expecting.”

Source: Canadian Astronomical Society.

MOST… Cutting To The Heart Of A Wolf-Rayet Star


In 1867, astronomers using the 40 cm Foucault telescope at the Paris Observatory, discovered three stars in the constellation Cygnus (now designated HD191765, HD192103 and HD192641), that displayed broad emission bands on an otherwise continuous spectrum. The astronomers’ names were Charles Wolf and Georges Rayet, and thus this category of stars became named Wolf–Rayet (WR) stars. Now using the Canadian MOST microsatellite, a team of researchers from the Universite de Montreal and the Centre de Recherche en Astrophysique du Quebec have made a stunning observation. They probed into the depth of the atmospheric eclipses in the Wolf-Rayet star, CV Serpentis, and observed a never before seen change of mass-loss rate.

Thanks to the service of MOST – Canada’s first space telescope and its high precision photometry – the team has observed significant changes in the depth of the atmospheric eclipses in the 30-day binary WR+O system. The equipment is aboard a suitcase-sized microsatellite (65 x 65 x 30 cm) which was launched in 2003 from a former ICBM in northern Russia. It is on a low-Earth polar orbit and has long outlived its original estimated life expectancy, offering Canadian astronomers almost eight years (and still counting) of ultra-high quality space-based data. Now this data gives us a huge insight into the heart of Wolf-Rayet stars.

Intrinsically luminous, WR stars can be massive or mid-sized, but the most interesting stage is arguably the last 10% in the lifetime of the star, when hydrogen fuel is used up and the star survives by much hotter He-burning. Towards the end of this phase, the copious supply of carbon atoms head for the stellar surface and are ejected in the form of stellar winds. WR stars in this stage are known as WC stars… and their production of carbon dust is one of the greatest mysteries of the Cosmos. These amorphous dust grains range in size from a few to millions of atoms and astronomers hypothesize their formation may requires high pressure and less than high temperatures.

“One key case is undoubtedly the sporadic dust-producing WC star in CV Ser. MOST was recently used to monitor CV Ser twice (2009 and 2010), revealing remarkable changes in the depths of the atmospheric eclipse that occurs every time the hot companion’s light is absorbed as it passes through the inner dense WC wind.” says the researchers. “The remarkable, unprecedented 70% change in the WC mass-loss rate might be connected to dust formation.”

And all thanks to the MOST tiny little satellite imaginable…

Original Story Source: AstroNews and excerpt from Wikipedia.

Coming To A Theatre Near You… Extreme Neutron Stars!


They came into existence violently… Born at the death of a massive star. They are composed almost entirely of neutrons, barren of electrical charge and with a slightly larger mass than protons. They are quantum degenerates with an average density typically more than one billion tons per teaspoonful – a state which can never be created here on Earth. And they are absolutely perfect for study of how matter and exotic particles behave under extreme conditions. We welcome the extreme neutron star…

In 1934 Walter Baade and Fritz Zwicky proposed the existence of the neutron star, only a year after the discovery of the neutron by Sir James Chadwick. But it took another 30 years before the first neutron star was actually observed. Up until now, neutron stars have had their mass accurately measured to about 1.4 times that of Sol. Now a group of astronomers using the Green Bank Radio Telescope found a neutron star that has a mass of nearly twice that of the Sun. How can they make estimates so precise? Because the extreme neutron star in question is actually a pulsar – PSR J1614-2230. With heartbeat-like precision, PSR J1614-2230 sends out a radio signal each time it spins on its axis at 317 times per second.

According to the team; “What makes this discovery so remarkable is that the existence of a very massive neutron star allows astrophysicists to rule out a wide variety of theoretical models that claim that the neutron star could be composed of exotic subatomic particles such as hyperons or condensates of kaons.”

The presence of this extreme star poses new questions about its origin… and its nearby white dwarf companion. Did it become so extreme from pulling material from its binary neighbor – or did it simply become that way through natural causes? According to Professor Lorne Nelson (Bishop’s University) and his colleagues at MIT, Oxford, and UCSB, the neutron star was likely spun up to become a fast-rotating (millisecond) pulsar as a result of the neutron star having cannibalized its stellar companion many millions of years ago, leaving behind a dead core composed mostly of carbon and oxygen. According to Nelson, “Although it is common to find a high fraction of stars in binary systems, it is rare for them to be close enough so that one star can strip off mass from its companion star. But when this happens, it is spectacular.”

Through the use of theoretical models, the team hopes to gain insight as to how binary systems evolve over the entire lifetime of the Universe. With today’s extreme super-computing powers, Nelson and his team members were able to calculate the evolution of more than 40,000 plausible starting cases for the binary and determine which ones were relevant. As they describe at this week’s CASCA meeting in Ontario, Canada, they found many instances where the neutron star could evolve higher in mass at the expense of its companion, but as Nelson says, “It isn’t easy for Nature to make such high-mass neutron stars, and this probably explains why they are so rare.”

Original story source at


Globular Clusters Are Real Oddballs


Hanging onto the outskirts of our Milky Way galaxy like cockle burs on a shaggy dog’s coat, globular clusters contain over hundreds of thousands of stars. Estimated to be up to ten billion years old, these spherical stellar seed pods are gravitationally bound together and tend to be more dense towards their cores. We’ve long known all the stars contained within a globular cluster to be about the same age and the individual members most likely formed at the same time as the parent galaxy – but what we weren’t expecting was change.

“We thought we understood these clusters very well”, says Dr. Alison Sills, Associate Professor of Physics & Astronomy. She is presenting new findings at this week’s CASCA 2011 meeting in Ontario, Canada. “We taught our students that all the stars in these clusters were formed at the same time, from one giant cloud of gas. And since that time, the individual stars may have evolved and died, but no new stars were born in the cluster.”

In 1953, astronomer Allan Sandage was performing photometry of the stars in the globular cluster M3 when he made an incredible discovery – blue stragglers. No, it’s not a down-his-luck musician waiting for a coin in his instrument case… but a main sequence star more luminous and more blue than stars at the main sequence turn-off point for the cluster. They shouldn’t belong where they are, but with masses two to three times that of the rest of the main sequence cluster stars, blue stragglers seem to be exceptions to the rule. Maybe they are a product of interaction… grappling together… pulling material from one another… and eventually merging.

Image of NGC 6397 taken by the Hubble Space Telescope, with evidence of a number of blue stragglers.

“Astronomers expect that the stars get too close to each other because of the complicated dance that stars perform in these dense clusters, where thousands of stars are packed into a relatively small space, and each star is moving through this cluster under the influence of the gravity of all the other stars. Somewhat like a traffic system with no stop lights, there are a lot of close encounters and collisions,” explains Sills.

By taking a closer look at globular clusters, the Hubble Space Telescope has given us evidence for two generations of star formation. The first is our accepted rule, but the second generation isn’t like anything else found in our Galaxy. Instead of being created from an earlier generation of expended stars, the second generation in globular clusters appears to have formed from material sloughed off by the first generation of stars. An enigma? You bet.

“Studying the normal stars in clusters was instrumental in allowing astronomers to figure out how stars lived and died”, says Dr. Sills, “but now we can look even further back, to when they were born, by using the oddballs. It pays off to pay attention to the unusual individuals in any population. You never know what they’ll be able to tell you.”

At the CASCA conference, Dr. Sills is presenting her work – a link between these two unusual forms of globular clusters. Blue stragglers and the second generation of stars would appear to have identical properties, including where they are concentrated in the cluster, and that both are.. well.. a little more “blue” than we would expect. She is investigating how the close encounters and collisions could affect the formation of this strange second generation and link the two phenomena we see in these complicated systems.

Real oddballs…

Original story soucre at

Early Stars Were Whirling Dervishes


Even though some of the first stars in the early universe were massive, they probably lived fast and furious lives, as they likely rotated much faster than their present-day counterparts. A new study on stellar evolution looked at a 12-billion-year-old star cluster and found high levels of metal in the stars – a chemical signature that suggests that the first stars were fast spinners.

“We think that the first generations of massive stars were very fast rotators – that’s why we called them spinstars,” said Cristina Chiappini of the Astrophysical Institute Potsdam in Germany, who led the team of astronomers.

These first generation stars died out long ago, and our telescopes can’t look back in time far enough to actually see them, but astronomers can get a glimpse of what they were like by looking at the chemical makeup of later stars. The first stars’ chemical imprints are like fossil records that can be found in the oldest stars we can study.

The general understanding of the early universe is that soon after the Big Bang, the Universe was made of essentially just hydrogen and helium. The chemical enrichment of the Universe with other elements had to wait around 300 million years until the fireworks started with the death of the first generations of massive stars, putting new chemical elements into the primordial gas, which later were incorporated in the next generations of stars.

Using data from ESO’s Very Large Telescope (VLT), the astronomers reanalyzed spectra of a group of very old stars in the Galactic Bulge. These stars are so old that only very massive, short-living stars with masses larger than around ten times the mass of our Sun should have had time to die and to pollute the gas from which these fossil records then formed. As expected, the chemical composition of the observed stars showed elements typical for enrichment by massive stars. However, the new analysis unexpectedly also revealed elements usually thought to be produced only by stars of smaller masses. Fast-rotating massive stars on the other hand would succeed in manufacturing these elements themselves.

“Alternative scenarios cannot yet be discarded – but – we show that if the first generations of massive stars were spinstars, this would offer a very elegant explanation to this puzzle!” said Chiappini.

A star that spins more rapidly can live longer and suffer different fates than slow-spinning ones. Fast rotation also affects other properties of a star, such as its colour, and its luminosity. Spinstars would therefore also have strongly influenced the properties and appearance of the first galaxies which were formed in the Universe. The existence of spinstars is now also supported by recent hydrodynamic simulations of the formation of the first stars of the universe by an independent research group.

Chiappini and her team are currently working on extending the stellar simulations in order to further test their findings. Their work is published in a Nature article on April 28, 2011.

Source: University of Potsdam, Nature