A new study shows how massive young stars create the kind of organic molecules that are necessary for life.
A team of researchers used an airborne observatory to examine the inner regions around two massive young stars. Along with water, they found things like ammonia and methane. These molecules are swirling around in a disk of material that surrounds the young stars.
That material is the same stuff that planets form from, and the study presents some new insights into how the stuff of life becomes incorporated into planets.
Neutron stars are the end-state of massive stars that have spent their fuel and exploded as supernovae. There’s an upper limit to their mass, because a massive enough star won’t become a neutron star; it’ll become a black hole. But finding that upper mass limit, or tipping point, between a star that becomes a black hole and one that becomes a neutron star, is something astronomers are still working on.
Now a new discovery from astronomers using the National Science Foundation’s (NSF) Green Bank Telescope (GBT) have found the most massive neutron star yet, putting some solid data in place about the so-called tipping point.
A recent analysis of a star in the south hemisphere constellation of Centaurus has highlighted the role that amateurs play in assisting with professional discoveries in astronomy.
The find used of the European Southern Observatory’s Very Large Telescope based in the Atacama Desert in northern Chile — as well as data from observatories around the world — to reveal the nature of a massive yellow “hypergiant” star as one of the largest stars known.
The stats for the star are impressive indeed: dubbed HR 5171 A, the binary system weighs in at a combined 39 solar masses, has a radius of over 1,300 times that of our Sun, and is a million times as luminous. Located 3,600 parsecs or over 11,700 light years distant, the star is 50% larger than the famous red giant Betelgeuse. Plop HR 5171 A down into the center of our own solar system, and it would extend out over 6 astronomical units (A.U.s) past the orbit of Jupiter.
Researchers used observations going back over 60 years – some of which were collected by dedicated amateur astronomers – to pin down the nature of this curious star. A variable star just below naked eye visibility spanning a magnitude range from +6.1 to +7.3, HR 5171 A also has a relatively small companion star orbiting across our line of sight once every 1300 days. Such a system is known as an eclipsing binary. Famous examples of similar systems are the star Algol (Alpha Persei), Epsilon Aurigae and Beta Lyrae. The companion star for HR 5171 is also a large star in its own right at around six solar masses and 400 solar radii in size. The distance from center-to-center for the system is about 10 A.U.s – the distance from Sol to Saturn – and the surface-to-surface distance for the A and B components of the system are “only” about 2.8 A.U.s apart. This all means that these two massive stars are in physical contact, with the expanded outer atmosphere of the bloated primary contacting the secondary, giving the pair a distorted peanut shape.
“The companion we have found is very significant as it can have an influence on the fate of HR 5171 A, for example stripping off its outer layers and modifying its evolution,” said astronomer Olivier Chesneau of the Observatoire de la Côte d’Azur in Nice France in the recent press release.
Knowing the orbital period of a secondary star offers a method to measure the mass of the primary using good old Newtonian mechanics. Coupled with astrometry used to measure its tiny parallax, this allows astronomers to pin down HR 5171 A’s stupendous size and distance.
Along with luminous blue variables, yellow hypergiants are some of the brightest stars known, with an absolute magnitude of around -9. That’s just 16x times fainter than the apparent visual magnitude of a Full Moon but over 100 times brighter than Venus – if you placed a star like HR 5171 A 32 light years from the Earth, it would easily cast a shadow.
Astronomers used a technique known as interferormetry to study HR 5171 A, which involves linking up several telescopes to create the resolving power of one huge telescope. Researchers also culled through over a decade’s worth data to analyze the star. Though much of what had been collected by the American Association of Variable Star Observers (the AAVSO) had been considered to be too noisy for the purposes of this study, a dataset built from 2000 to 2013 by amateur astronomer Sebastian Otero was of excellent quality and provided a good verification for the VLT data.
The discovery is also crucial as researchers have come to realize that we’re catching HR 5171 A at an exceptional phase in its life. The star has been getting larger and cooling as it grows, and this change can be seen just over the past 40 year span of observations, a rarity in stellar astronomy.
“It’s not a surprise that yellow hypergiants are very instable and lose a lot of mass,” Chesneau told Universe Today. “But the discovery of a companion around such a bright star was a big surprise since any ‘normal’ star should at least be 10,000 times fainter than the hypergiant. Moreover, the hypergiant was much bigger than expected. What we see is not the companion itself, but the regions gravitationally controlled and filled by the wind from the hypergiant. This is a perfect example of the so-called Roche model. This is the first time that such a useful and important model has really been imaged. This hypergiant exemplifies a famous concept!”
Indeed, you can see just such photometric variations as the secondary orbits its host in the VLTI data collected by the AMBER interferometer, backed up by observations from GEMINI’s NICI chronograph:
These latest measurements place HR 5171 A firmly in the “Top 10” for largest stars in terms of size known, as well as the largest yellow hypergiant star known This is due mainly to tidal interactions with its companion. Only eight yellow hypergiants have been identified in our Milky Way galaxy. HR 5171 A is also in a crucial transition phase from a red hypergiant to becoming a luminous blue variable or perhaps even a Wolf-Rayet type star, and will eventually end its life as a supernova.
HR 5171 A is also known as HD 119796, HIP 67261, and V766 Centauri. Located at Right Ascension 13 Hours 47’ 11” and declination -62 degrees 35’ 23,” HR 5171 culminates just two degrees above the southern horizon at local midnight as seen from Miami in late March.
HR 5171 A is a fine binocular object for southern hemisphere observers.
But the good news is, there’s another yellow hypergiant visible for northern hemisphere observers named Rho Cassiopeiae:
Rho Cass is one of the few naked eye examples of a yellow hypergiant star, and varies from magnitude +4.1 to +6.2 over an irregular period.
It’s amusing read the Burnham’s Celestial Handbook entry on Rho Cass. He notes the lack of parallax and the spectral measurements of the day — the early 1960s — as eluding to a massive star with a “true distance… close to 3,000 light years!” Today we know that Rho Cassiopeiae actually lies farther still, at over 8,000 light years distant. Robert Burnham would’ve been impressed even more by the amazing nature of HR 5171 as revealed today by ESO astronomers!
Score another one for citizen science! In a study released just days ago, a new catalog containing over five thousand infrared bubble entries was added through the “Milky Way Project” website. The work was done independently by at least five participants who measured parameters for position, radius, thickness, eccentricity and position angle. Not only did their work focus on these areas, but the non-professionals were responsible for recovering the locations of at least 86% of additional bubble and HII catalogs. Cool stuff? You bet. Almost one third of the Milky Way Project’s studied bubbles are located at the edge of an even larger bubble – or have more lodged inside. This opens the door to further understanding the dynamics of triggered star formation!
Just what is the Milky Way Project? Thanks to the Galaxy Zoo and Zooniverse, scientists have been able to enlist the help of an extensive community of volunteers able to tackle and analyze huge amounts of data – data that contains information which computer algorithms might miss. In this case it’s visually searching through the Galactic plane for whole or broken ring-shaped structures in images done by Spitzer’s Galactic Legacy Infrared Survey Extraordinaire (GLIMPSE) project. Here the bubbles overlap and the structures are so complex that only humans can sort them out for now.
“The MWP is the ninth online citizen science project created using the Zooniverse Application Programming Interface
(API) tool set. The Zooniverse API is the core software supporting the activities of all Zooniverse citizen science projects.” says R. J. Simpson (et al). “Built originally for Galaxy Zoo 2, the software is now being used by 11 different projects. The Zooniverse API is designed primarily as a tool for serving up a large collection of `assets’ (for example, images or video) to an interface, and collecting back user-generated interactions with these assets.”
Through the interface, users mark the location of bubbles and other areas of significance such as small bubbles, green knots, dark nebulae, star clusters, galaxies, fuzzy red objects or simply unknowns. During this phase, the citizen scientist can make as many annotations as he or she wants before they submit their findings and receive a new assignment. Each annotated image is then stored in a database as a classification and the user can access their image again in an area of the website known as “My Galaxy”. However, images may only be classified once.
When identifying galactic bubbles, the user creates a circle around the area which can be scaled to size and stretched into an elliptical configuration. Initially as the object is identified and marked, the user can control the position and size of the bubble. Once annotated the parameters can be edited, such as the ellipticity, annular thickness and rotation. The program even allows for regions where no obvious emission is present, such as a broken or partial bubble. This allows the user to match the bubbles they find in individual images to achieve an accurate representation You can even mark a favorite or interesting configuration as well!
“In order to assist in the data-reduction process, users are given scores according to how experienced they are at drawing bubbles. We treat the first 10 bubbles a user draws as practice drawings and these are not included in the final reduction. Users begin with a score of 0 and are given scores according to the number of precision bubbles they have drawn.” explains the team. “Precision bubbles are those drawn using the full tool set, meaning they have to have adjusted the ellipticity, the thickness and the rotation. This is done to ensure that users’ scores reflect their ability to draw bubbles well. While only precision bubbles are used to score volunteers, all bubbles drawn as included in the data reduction. The scores are used as weights when averaging the bubble drawings to produce the catalogue.”
Now it’s time to combine all that data. As of October of last year, the program has created a database of 520,120 user-drawn bubbles. The information is then sorted out and processed – with many inclusions left for further investigation. However, not all bubbles make the cut. When it comes to this project, only bubbles that have been identified fifty times or more are included into the catalog. What remains is a “clean bubble” – one that has been verified by at least five users and picked out at least 10% of the time by the volunteers when displayed.
“It is not known how many bubbles exist in the Galaxy, hence it is impossible to quantify the completeness of the MWP catalogue. There will be bubbles that are either not visible in the data used on the MWP, or that are not seen as bubbles.” says the team. “Distant bubbles may be obscured by foreground extinction. Faint bubbles may be masked by bright Galactic background emission or confused with brighter nebular structures. Fragmented or highly distorted bubbles present at high inclination angles may not appear as bubbles to the observer.”
But don’t let it burst your bubble. This citizen science approach is an excellent idea from the the standpoint of observer objectivity and the final, reduced catalogue contains 5,106 visually identified bubbles. Of these, they are divided into a catalogue of 3,744 large bubbles identified by users as ellipses, and a catalogue of 1,362 small bubbles annotated by users at the highest zoom level images in the MWP.
And that’s not all… “In addition to the reduced bubble catalogue, a crowd sourced `heat map’ of bubble drawings has also been produced. The MWP `heat maps’ allow the bubble drawings to be explored without them needing to be reduced to elliptical annuli. Rather, the `heat maps’ allow contours of overlapping classifications to be drawn over regions of the Galactic plane reflecting levels of agreement between independent classifiers. In most cases the structures outlined in these maps are photo-dissociation regions traced by 8 um emission, but more fundamentally they are regions that multiple volunteers agree reflect the rims of bubbles.”
Yep. They are bubbles alright. Bubble produced around huge stars when an HII region is hollowed out by thermal overpressure, stellar winds, radiation pressure or a combination of them all. This impacts the surrounding, cold interstellar medium and creates a visible shell – or bubble. These regions serve as perfect observation points “to test theories of sequential, massive star formation triggered by massive star winds and radiation pressure” and to keep us forever fascinated…
Thanks to the incredible infra-red imagery of NASA’s Spitzer Space Telescope, we’re able to take a look into a tortured region of star formation. Infrared light in this image has been color-coded according to wavelength. Light of 3.6 microns is blue, 4.5-micron light is blue-green, 8.0-micron light is green, and 24-micron light is red. The data was taken before the Spitzer mission ran out of its coolant in 2009, and began its “warm” mission. This image reveals one of the most active and tumultuous areas of the Milky Way – Cygnus X. Located some 4,500 light years away, the violent-appearing dust cloud holds thousands of massive stars and even more of moderate size. It is literally “star soup”…
“Spitzer captured the range of activities happening in this violent cloud of stellar birth,” said Joseph Hora of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., who presented the results today at the 219th meeting of the American Astronomical Society in Austin, Texas. “We see bubbles carved out by massive stars, pillars of new stars, dark filaments lined with stellar embryos and more.”
According to popular theory, stars are created in regions similar to Cygnus X. As their lives progress, they drift away from each other and it is surmised the Sun once belonged to a stellar association formed in a slightly less extreme environment. In regions like Cygnus X, the dust clouds are characterized with deformations caused by stellar winds and high radiation. The massive stars literally shred the clouds that birth them. This action can stop other stars from forming… and also cause the rise of others.
“One of the questions we want to answer is how such a violent process can lead to both the death and birth of new stars,” said Sean Carey, a team member from NASA’s Spitzer Science Center at the California Institute of Technology, Pasadena, Calif. “We still don’t know exactly how stars form in such disruptive environments.”
Thanks to Spitzer’s infra-red data, scientists are now able to paint a clearer picture of what happens in dusty complexes. It allows astronomers to peer behind the veil where embryonic stars were once hidden – and highlights areas like pillars where forming stars pop out inside their cavities. Another revelation is dark filaments of dust, where embedded stars make their home. It is visions like this that has scientists asking questions… Questions such as how filaments and pillars could be related.
“We have evidence that the massive stars are triggering the birth of new ones in the dark filaments, in addition to the pillars, but we still have more work to do,” said Hora.
You may have seen one of these astronomical scale picture sequences, where you go from the Earth to Jupiter to the Sun, then the Sun to Sirius – and all the way up to the biggest star we know of VY Canis Majoris. However, most of the stars at the big end of the scale are at a late point in their stellar lifecycle – having evolved off the main sequence to become red supergiants.
The Sun will go red giant in 5 billion years or so – achieving a new radius of about one Astronomical Unit – equivalent to the average radius of the Earth’s orbit (and hence debate continues around whether or not the Earth will be consumed). In any case, the Sun will then roughly match the size of Arcturus, which although voluminously big, only has a mass of roughly 1.1 solar masses. So, comparing star sizes without considering the different stages of their stellar evolution might not be giving you the full picture.
Another way of considering the ‘bigness’ of stars is to consider their mass, in which case the most reliably confirmed extremely massive star is NGC 3603-A1a – at 116 solar masses, compared with VY Canis Majoris’ middling 30-40 solar masses.
The most massive star of all may be R136a1, which has an estimated mass of over 265 solar masses – although the exact figure is the subject of ongoing debate, since its mass can only be inferred indirectly. Even so, its mass is almost certainly over the ‘theoretical’ stellar mass limit of 150 solar masses. This theoretical limit is based on mathematically modelling the Eddington limit, the point at which a star’s luminosity is so high that its outwards radiation pressure exceeds its self-gravity. In other words, beyond the Eddington limit, a star will cease to accumulate more mass and will begin to blow off large amounts of its existing mass as stellar wind.
It’s speculated that very big O type stars might shed up to 50% of their mass in the early stages of their lifecycle. So for example, although R136a1 is speculated to have a currently observed mass of 265 solar masses, it may have had as much as 320 solar masses when it first began its life as a main sequence star.
So, it may be more correct to consider that the theoretical mass limit of 150 solar masses represents a point in a massive star’s evolution where a certain balancing of forces is achieved. But this is not to say that there couldn’t be stars more massive than 150 solar masses – it’s just that they will be always declining in mass towards 150 solar masses.
Having unloaded a substantial proportion of their initial mass such massive stars might continue as sub-Eddington blue giants if they still have hydrogen to burn, become red supergiants if they don’t – or become supernovae.
Vink et al model the processes in the early stages of very massive O type stars to demonstrate that there is a shift from optically thin stellar winds, to optically thick stellar winds at which point these massive stars can be classified as Wolf-Rayet stars. The optical thickness results from blown off gas accumulating around the star as a wind nebulae – a common feature of Wolf-Rayet stars.
Lower mass stars evolve to red supergiant stage through different physical processes – and since the expanded outer shell of a red giant does not immediately achieve escape velocity, it is still considered part of the star’s photosphere. There’s a point beyond which you shouldn’t expect bigger red supergiants, since more massive progenitor stars will follow a different evolutionary path.
Those more massive stars spend much of their lifecycle blowing off mass via more energetic processes and the really big ones become hypernovae or even pair-instability supernovae before they get anywhere near red supergiant phase.
So, once again it appears that maybe size isn’t everything.
It might be hard to believe, but massive stars are larger in their infant stage than they are when fully formed. Thanks to a team of astronomers at the University of Amsterdam, observations have shown that during the initial stages of creation, super-massive stars are super-sized. This research now confirms the theory that massive stars contract until they reach the age of equilibrium.
In the past, one of the difficulties in proving this theory has been the near impossibility of getting a clear spectrum of a massive star during formation due to obscuring dust and gases. Now, using the powerful spectrograph X-shooter on ESO’s Very Large Telescope in Chile, researchers have been able to obtain data on a young star cataloged as B275 in the “Omega Nebula” (M17). Built by an international team, the X-shooter has a special wavelength coverage: from 300 nm (UV) to 2500 nm (infrared) and is the most powerful tool of its kind. Its “one shot” image has now provided us with the first solid spectral evidence of a star on its way to main sequence. Seven times more massive than the Sun, B275 has shown itself to be three times the size of a normal main-sequence star. These results help to confirm present modeling.
When young, massive stars begin to coalesce, they are shrouded in a rotating gas disk where the mass-accretion process starts. In this state, strong jets are also produced in a very complicated mechanism which isn’t well understood. These actions were reported earlier by the same research group. When accretion is complete, the disk evaporates and the stellar surface then becomes visible. As of now, B275 is displaying these traits and its core temperature has reached the point where hydrogen fusion has commenced. Now the star will continue to contract until the energy production at its center matches the radiation at the surface and equilibrium is achieved. To make the situation even more curious, the X-shooter spectrum has shown B275 to have a measurably lower surface temperature for a star of its type – a very luminous one. This wide margin of difference can be equated to its large radius – and that’s what the results show. The intense spectral lines associated with B275 are consistent with a giant star.
Lead author Bram Ochsendorf, was the man to analyze the spectrum of this curious star as part of his Master’s research program at the University of Amsterdam. He has also began his PhD project in Leiden. Says Ochsendorf, “The large wavelength coverage of X shooter provides the opportunity to determine many stellar properties at once, like the surface temperature, size, and the presence of a disk.”
The spectrum of B275 was obtained during the X-shooter science verification process by co-authors Rolf Chini and Vera Hoffmeister from the Ruhr-Universitaet in Bochum, Germany. “This is a beautiful confirmation of new theoretical models describing the formation process of massive stars, obtained thanks to the extreme sensitivity of X-shooter”, remarks Ochsendorf’s supervisor Prof. Lex Kaper.
Around 1957 light years away, a dense molecular cloud resides beside an OB star cluster locked in a massive HII region. The hydrogen envelope is slowly beginning to billow out and separate itself from the molecular gas, but we’re not able to get a clear picture of the situation thanks to interfering dust. However, by engaging NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA), we’re now able to take one of the highest resolution mid-infrared looks into the heart of an incredible star-forming region known as W40 so far known to science.
Onboard a modified 747SP airliner, the Faint Object infraRed Camera for the SOFIA Telescope (FORCAST) has been hard at work utilizing its 2.5 meter (100″) reflecting telescope to capture data. The composite image shown above was taken at wavelengths of 5.4, 24.2 and 34.8 microns. Why this range? Thanks to the high flying SOFIA telescope, we’re able to clear Earth’s atmosphere and “get above” the ambient water vapor which blocks the view. Not even the highest based terrestrial telescope can escape it – but FORCAST can!
With about 1/10 the UV flux of the Orion Nebula, region W40 has long been of scientific interest because it is one of the nearest massive star-forming regions known. While some of its OB stars have been well observed at a variety of wavelengths, a great deal of the lower mass stars remain to be explored. But there’s just one problem… the dust hides their information. Thanks to FORCAST, astronomers are able to peer through the obscuration at W40’s center to examine the luminous nebula, scores of neophyte stars and at least six giants which tip the scales at six to twenty times more massive than the Sun.
Why is studying a region like W40 important to science? Because at least half of the Milky Way’s stellar population formed in similar massive clusters, it is possible the Solar System also “developed in such a cluster almost 5 billion years ago”. The stars FORCAST measures aren’t very bright and intervening dust makes them even more dim. But no worries, because this type of study cuts them out of dust that’s only carrying a temperature of a few hundred degrees. All that from a flying observatory!
Utilizing ESO’s giant telescopes located in Chile, researchers at the Niels Bohr Institute have been examining “antique” stars. Located at the outer reaches of the Milky Way, these superannuated stellar specimens are unusual in the fact that they contain an over-abundance of gold, platinum and uranium. How they became heavy metal stars has always been a puzzle, but now astronomers are tracing their origins back to our galaxy’s beginning.
It is theorized that soon after the Big Bang event, the Universe was filled with hydrogen, helium and… dark matter. When the trio began compressing upon themselves, the very first stars were born. At the core of these neophyte suns, heavy elements such as carbon, nitrogen and oxygen were then created. A few hundred million years later? Hey! All of the elements are now accounted for. It’s a tidy solution, but there’s just one problem. It would appear the very first stars only had about 1/1000th of the heavy-elements found in sun-like stars of the present.
How does it happen? Each time a massive star reaches the end of its lifetime, it will either create a planetary nebula – where layers of elements gradually peel away from the core – or it will go supernova – and blast the freshly created elements out in a violent explosion. In this scenario, the clouds of material once again coalesce… collapse again and form more new stars. It’s just this pattern which gives birth to stars that become more and more “elementally” concentrated. It’s an accepted conjecture – and that’s what makes discovering heavy metal stars in the early Universe a surprise. And even more surprising…
Right here in the Milky Way.
“In the outer parts of the Milky Way there are old ‘stellar fossils’ from our own galaxy’s childhood. These old stars lie in a halo above and below the galaxy’s flat disc. In a small percentage – approximately one to two percent of these primitive stars, you find abnormal quantities of the heaviest elements relative to iron and other ‘normal’ heavy elements”, explains Terese Hansen, who is an astrophysicist in the research group Astrophysics and Planetary Science at the Niels Bohr Institute at the University of Copenhagen.
But the study of these antique stars just didn’t happen overnight. By employing ESO’s large telescopes based in Chile, the team took several years to come to their conclusions. It was based on the findings of 17 “abnormal” stars which appeared to have elemental concentrations – and then another four years of study using the Nordic Optical Telescope on La Palma. Terese Hansen used her master’s thesis to analyse the observations.
“After slaving away on these very difficult observations for a few years I suddenly realised that three of the stars had clear orbital motions that we could define, while the rest didn’t budge out of place and this was an important clue to explaining what kind of mechanism must have created the elements in the stars”, explains Terese Hansen, who calculated the velocities along with researchers from the Niels Bohr Institute and Michigan State University, USA.
What exactly accounts for these types of concentrations? Hansen explains their are two popular theories. The first places the origin as a close binary star system where one goes supernova, inundating its companion with layers of heavier elements. The second is a massive star also goes supernova, but spews the elements out in dispersing streams, impregnating gas clouds which then formed into the halo stars.
“My observations of the motions of the stars showed that the great majority of the 17 heavy-element rich stars are in fact single. Only three (20 percent) belong to binary star systems – this is completely normal, 20 percent of all stars belong to binary star systems. So the theory of the gold-plated neighbouring star cannot be the general explanation. The reason why some of the old stars became abnormally rich in heavy elements must therefore be that exploding supernovae sent jets out into space. In the supernova explosion the heavy elements like gold, platinum and uranium are formed and when the jets hit the surrounding gas clouds, they will be enriched with the elements and form stars that are incredibly rich in heavy elements”, says Terese Hansen, who immediately after her groundbreaking results was offered a PhD grant by one of the leading European research groups in astrophysics at the University of Heidelberg.
May all heavy metal stars go gold!
Original Story Source: Niels Bohr Institute News Release. For Further Reading: The Binary Frequency of r-Process-element-enhanced Metal-poor Stars and Its Implications: Chemical Tagging in the Primitive Halo of the Milky Way.