Precisely dating a star can have important consequences for understanding stellar evolution and any circling exoplanets. But it’s one of the toughest plights in astronomy with only a few existing techniques.
One method is to find a star with radioactive elements like uranium and thorium, whose half-lives are known and can be used to date the star with certainty. But only about 5 percent of stars are thought to have such a chemical signature.
Another method is to look for a relationship between a star’s age and its ‘metals,’ the astronomer’s slang term for all elements heavier than helium. Throughout cosmic history, the cycle of star birth and death has steadily produced and dispersed more heavy elements leading to new generations of stars that are more heavily seeded with metals than the generation before. But the uncertainties here are huge.
The latest research is providing a new technique, showing that protostars can easily be dated by measuring the acoustic vibrations — sound waves — they emit.
Stars are born deep inside giant molecular clouds of gas. Turbulence within these clouds gives rise to pockets of gas and dust with enough mass to collapse under their own gravitational contraction. As each cloud — protostar — continues to collapse, the core gets hotter, until the temperature is sufficient enough to begin nuclear fusion, and a full-blown star is born.
Our Sun likely required about 50 million years to mature from the beginning of collapse.
Theoretical physicists have long posited that protostars vibrate differently than stars. Now, Konstanze Zwintz from KU Leuven’s Institute for Astronomy, and colleagues have tested this prediction.
The team studied the vibrations of 34 protostars in NGC 2264, all of which are less than 10 million years old. They used the Canadian MOST satellite, the European CoRoT satellite, and ground-based facilities such as the European Southern Observatory in Chile.
“Our data show that the youngest stars vibrate slower while the stars nearer to adulthood vibrate faster,” said Zwintz in a press release. “A star’s mass has a major impact on its development: stars with a smaller mass evolve slower. Heavy stars grow faster and age more quickly.”
Each stars’ vibrations are indirectly seen by their subtle changes in brightness. Bubbles of hot, bright gas rise to the star’s surface and then cool, dim, and sink in a convective loop. This overturn causes small changes in the star’s brightness, revealing hidden information about the sound waves deep within.
You can actually hear this process when the stellar light curves are converted into sound waves. Below is a video of such singing stars, produced by Nature last year.
“We now have a model that more precisely measures the age of young stars,” said Zwintz. “And we are now also able to subdivide young stars according to their various life phases.”
One has never been spotted for sure in the wild jungle of strange stellar objects out there, but astronomers now think they have finally found a theoretical cosmic curiosity: a Thorne-Zytkow Object, or TZO, hiding in the neighboring Small Magellanic Cloud. With the outward appearance of garden-variety red supergiants, TZOs are actually two stars in one: a binary pair where a super-dense neutron star has been absorbed into its less dense supergiant parter, and from within it operates its exotic elemental forge.
First theorized in 1975 by physicist Kip Thorne and astronomer Anna Zytkow, TZOs have proven notoriously difficult to find in real life because of their similarity to red supergiants, like the well-known Betelgeuse at the shoulder of Orion. It’s only through detailed spectroscopy that the particular chemical signatures of a TZO can be identified.
Portrait of the Small Magellanic Cloud, made by NASA’s Spitzer Space Telescope
Observations of the red supergiant HV 2112 in the Small Magellanic Cloud*, a dwarf galaxy located a mere 200,000 light-years away, have revealed these signatures — unusually high concentrations of heavy elements like molybdenum, rubidium, and lithium.
While it’s true that these elements are created inside stars — we are all star-stuff, like Carl Sagan said — they aren’t found in quantity within the atmospheres of lone supergiants. Only by absorbing a much hotter star — such as a neutron star left over from the explosive death of a more massive partner — is the production of such elements presumed to be possible.
“Studying these objects is exciting because it represents a completely new model of how stellar interiors can work,”said Emily Levesque, team leader from the University of Colorado Boulder and lead author on the paper. “In these interiors we also have a new way of producing heavy elements in our universe.”
Definitive detection of a TZO would provide direct evidence for a completely new model of stellar interiors, as well as confirm a theoretically predicted fate for massive star binary systems and the existence of nucleosynthesis environments that offer a new channel for heavy-element and lithium production in our universe.
– E.M. Levesque et al., Discovery of a Thorne-Zytkow object candidate in the Small Magellanic Cloud
One of the original proposers of TZOs, Dr. Anna Zytkow, is glad to see her work resulting in new discoveries.
“I am extremely happy that observational confirmation of our theoretical prediction has started to emerge,” Zytkow said. “Since Kip Thorne and I proposed our models of stars with neutron cores, people were not able to disprove our work. If theory is sound, experimental confirmation shows up sooner or later. So it was a matter of identification of a promising group of stars, getting telescope time and proceeding with the project.”
The findings were first announced in January at the 223rd meeting of the American Astronomical Society. The paper has now been accepted for publication in the Monthly Notices of the Royal Astronomical Society Letters, and is co-authored by Philip Massey, of Lowell Observatory in Flagstaff, Arizona; Anna Zytkow of the University of Cambridge in the U.K.; and Nidia Morrell of the Carnegie Observatories in La Serena, Chile. Read the team’s paper here.
Source: University of Colorado, Boulder. Illustration by ‘Digital Drew.’
__________________________ *In the paper the team notes that it’s not yet confirmed that HV 2112 is part of the SMC and could be associated with our own galaxy. If so it would rule out it being a TZO, but would still require an explanation of its observed spectra.
NASA's NuSTAR is revealing the mechanics behind Cassiopeia A's supernova explosion (Image credit: NASA/JPL-Caltech/CXC/SAO)
Supernovas are some of the most energetic and powerful events in the observable Universe. Briefly outshining entire galaxies, they are the final, dying outbursts of stars several times more massive than our Sun. And while we know supernovas are responsible for creating the heavy elements necessary for everything from planets to people to power tools, scientists have long struggled to determine the mechanics behind the sudden collapse and subsequent explosion of massive stars.
Now, thanks to NASA’s NuSTAR mission, we have our first solid clues to what happens before a star goes “boom.”
The image above shows the supernova remnant Cassiopeia A (or Cas A for short) with NuSTAR data in blue and observations from the Chandra X-ray Observatory in red, green, and yellow. It’s the shockwave left over from the explosion of a star about 15 to 25 times more massive than our Sun over 330 years ago*, and it glows in various wavelengths of light depending on the temperatures and types of elements present.
Artist’s concept of NuSTAR in orbit. (NASA/JPL-Caltech)
Previous observations with Chandra revealed x-ray emissions from expanding shells and filaments of hot iron-rich gas in Cas A, but they couldn’t peer deep enough to get a better idea of what’s inside the structure. It wasn’t until NASA’s Nuclear Spectroscopic Telescope Array — that’s NuSTAR to those in the know — turned its x-ray vision on Cas A that the missing puzzle pieces could be found.
And they’re made of radioactive titanium.
Many models have been made (using millions of hours of supercomputer time) to try to explain core-collapse supernovas. One of the leading ones has the star ripped apart by powerful jets firing from its poles — something that’s associated with even more powerful (but focused) gamma-ray bursts. But it didn’t appear that jets were the cause with Cas A, which doesn’t exhibit elemental remains within its jet structures… and besides, the models relying on jets alone didn’t always result in a full-blown supernova.
As it turns out, the presence of asymmetric clumps of radioactive titanium deep within the shells of Cas A, revealed in high-energy x-rays by NuSTAR, point to a surprisingly different process at play: a “sloshing” of material within the progenitor star that kickstarts a shockwave, ultimately tearing it apart.
Watch an animation of how this process occurs:
The sloshing, which occurs over a time span of a mere couple hundred milliseconds — literally in the blink of an eye — is likened to boiling water on a stove. When the bubbles break through the surface, the steam erupts.
Only in this case the eruption leads to the insanely powerful detonation of an entire star, blasting a shockwave of high-energy particles into the interstellar medium and scattering a periodic tableful of heavy elements into the galaxy.
In the case of Cas A, titanium-44 was ejected, in clumps that echo the shape of the original sloshing asymmetry. NuSTAR was able to image and map the titanium, which glows in x-ray because of its radioactivity (and not because it’s heated by expanding shockwaves, like other lighter elements visible to Chandra.)
“Until we had NuSTAR we couldn’t really see down into the core of the explosion,” said Caltech astronomer Brian Grefenstette during a NASA teleconference on Feb. 19.
Illustration of the pre-supernova star in Cassiopeia A. It’s thought that its layers were “turned inside out” just before it detonated. (NASA/CXC/M.Weiss)
“Previously, it was hard to interpret what was going on in Cas A because the material that we could see only glows in X-rays when it’s heated up. Now that we can see the radioactive material, which glows in X-rays no matter what, we are getting a more complete picture of what was going on at the core of the explosion.”
– Brian Grefenstette, lead author, Caltech
Okay, so great, you say. NASA’s NuSTAR has found the glow of titanium in the leftovers of a blown-up star, Chandra saw some iron, and we know it sloshed and ‘boiled’ a fraction of a second before it exploded. So what?
“Now you should care about this,” said astronomer Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics. “Supernovae make the chemical elements, so if you bought an American car, it wasn’t made in Detroit two years ago; the iron atoms in that steel were manufactured in an ancient supernova explosion that took place five billion years ago. And NuSTAR shows that the titanium that’s in your Uncle Jack’s replacement hip were made in that explosion too.
“We’re all stardust, and NuSTAR is showing us where we came from. Including our replacement parts. So you should care about this… and so should your Uncle Jack.”
And it’s not just core-collapse supernovas that NuSTAR will be able to investigate. Other types of supernovas will be scrutinized too — in the case of SN2014J, a Type Ia that was spotted in M82 in January, even right after they occur.
“We know that those are a type of white dwarf star that detonates,” NuSTAR principal investigator Fiona Harrison responded to Universe Today during the teleconference. “This is very exciting news… NuSTAR has been looking at [SN2014J] for weeks, and we hope to be able to say something about that explosion as well.”
Previous imaging with Chandra (left, middle) is combined with new data from NuSTAR (right) to make a complete image of a supernova remnant. (NASA/JPL-Caltech/CXC/SAO)
One of the most valuable achievements of the recent NuSTAR findings is having a new set of observed constraints to place on future models of core-collapse supernovas… which will help provide answers — and likely new questions — about how stars explode, even hundreds or thousands of years after they do.
“NuSTAR is pioneering science, and you have to expect that when you get new results, it’ll open up as many questions as you answer,” said Kirshner.
Launched in June of 2012, NuSTAR is the first focusing hard X-ray telescope to orbit Earth and the first telescope capable of producing maps of radioactive elements in supernova remnants.
Massive stars can devastate their surroundings, unleashing hot winds and blasting radiation. With a mass over 100 times heavier than the Sun and a luminosity a million times brighter than the Sun, Eta Carinae clocks in as one of the biggest and brightest stars in our galaxy.
The enigmatic object walks a thin line between stellar stability and tumultuous explosions. But now a team of international astronomers is growing concerned that it’s leaning toward instability and eruption.
In the 19th Century the star mysteriously threw off unusually bright light for two decades in an event that became known as the “Great Eruption,” the causes of which are still up for debate. John Herschel and others watched as Eta Carinae’s brightness oscillated around that of Vega — rivaling a supernova explosion.
We now know the star ejected material in the form of two big globes. “During the eruption the star threw off more than 10 solar masses, which can now be observed as the surrounding bipolar nebula,” said lead author Dr. Andrea Mehner from the European Southern Observatory. Miraculously the star survived, but the nebula has been expanding into space ever since.
Eta Carinae has been observed at the South African Astronomical Observatory — a 0.75m telescope outside of Cape Town — for more than 40 years, providing a wealth of data. From the start of observations in 1976 until 1998, astronomers saw an increase across the J, H, K and L bands — filters, which allow certain wavelength ranges of infrared light to pass through.
“This data set is unique for its consistency over a timespan of more than 40 years,” Mehner told Universe Today. “It provides us with the opportunity to analyze long-term changes in the system as Eta Carinae still recovers from its Great Eruption.”
In order to understand the longterm overall increase in light we have to look at a more recent discovery noted in 2005 when scientists discovered that Eta Carinae is actually two stars: a massive blue star and a smaller companion. The temperature increased for 15 years until the companion came very close to the massive star, reaching periastron.
This increase in brightness is likely due to an overall increase in temperature of some component of the Eta Carinae system (which includes the massive blue star, its smaller companion, and the shells of gas and dust that now enshroud the system).
After 1998, however, the linear trend changed significantly and the star’s brightness increased much more rapidly in the J and H bands. It’s getting bluer, which in astronomy, typically means it’s getting hotter.
However, it’s unlikely the star itself is getting hotter. Instead we are seeing the effect of dust around the star being destroyed rapidly. Dust absorbs blue light. So if the dust is getting destroyed, more blue light will be able to pass through the nebulous globes surrounding the system. If this is the case, then we’re really seeing the star as it truly is, without dust absorbing certain wavelengths of its light.
While the nebula is slowly expanding and the dust is therefore dissipating, the authors do not think it’s enough to account for the recent brightening. Instead Eta Carinae is likely rotating at a different speed or losing mass at a different rate. “The changes observed may imply that the star is becoming more unstable and may head towards another eruptive phase,” Mehner told Universe Today.
Perhaps Eta Carinae is heading toward another “Great Eruption.” Only time will tell. But in a field where most events occur on a timescale of millions of years, it’s a great opportunity to watch the system evolve on a human time scale. And when Eta Carinae reaches periastron in the middle of this year, tens of telescopes will be collecting its light, hoping to see a sudden turn of events that may help us explain this exotic system.
The paper has been accepted for publication in Astronomy & Astrophysics and is available for download here.
Wow. It’s always amazing to get new views of familiar sky targets. And you always know that a “feast for the eyes” is in store when astronomers turn a world-class instrument towards a familiar celestial object.
Such an image was released this morning from the European Southern Observatory (ESO). Astronomers turned ESO’s 2.2-metre telescope towards Messier 7 in the constellation Scorpius recently, and gave us the star-studded view above.
Also known as NGC 6475, Messier 7 (M7) is an open cluster comprised of over 100 stars located about 800 light years distant. Located in the curved “stinger” of the Scorpion, M7 is a fine binocular object shining at a combined magnitude of about +3.3. M7 is physically about 25 light years across and appears about 80 arc minutes – almost the span of three Full Moons – in diameter from our Earthly vantage point.
One of the most prominent open clusters in the sky, M7 lies roughly in the direction of the galactic center in the nearby astronomical constellation of Sagittarius. When you’re looking towards M7 and the tail of Scorpius you’re looking just south of the galactic plane in the direction of the dusty core of our galaxy. The ESO image reveals the shining jewels of the cluster embedded against the more distant starry background.
Messier 7 is middle-aged as open clusters go, at 200 million years old. Of course, that’s still young for the individual stars themselves, which are just venturing out into the galaxy. The cluster will lose about 10% of its stellar population early on, as more massive stars live their lives fast and die young as supernovae. Our own solar system may have been witness to such nearby cataclysms as it left its unknown “birth cluster” early in its life.
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Other stars in Messier 7 will eventually mature, “join the galactic car pool” in the main sequence as they disperse about the plane of the galaxy.
But beyond just providing a pretty picture, studying a cluster such as Messier 7 is crucial to our understanding stellar evolution. All of the stars in Messier 7 were “born” roughly around the same time, giving researchers a snapshot and a chance to contrast and compare how stars mature over there lives. Each open cluster also has a unique spectral “fingerprint,” a chemical marker that can even be used to identify the pedigree of a star.
For example, there’s controversy that the open cluster Messier 67 may actually be the birth place of our Sun. It is interesting to note that the spectra of stars in this cluster do bear a striking resemblance in terms of metallicity percentage to Sol. Remember, metals in astronomer-speak is any element beyond hydrogen and helium. A chief objection to the Messier 67 “birth-place hypothesis” is the high orbital inclination of the open cluster about the core of our galaxy: our Sun would have had to have undergone a series of improbable stellar encounters to have ended up its current sedate quarter of a billion year orbit about the Milky Way galaxy.
Still, this highlights the value of studying clusters such as Messier 6. It’s also interesting to note that there’s also data in what you can’t see in the above image – dark gaps are thought to be dust lanes and globules in the foreground. Though there is some thought that this dust is debris that may also be related to the cluster and may give us clues as to its overall rotation, its far more likely that these sorts of “dark spirals” related to the cluster have long since dispersed. M7 has completed about one full orbit about the Milky Way since its formation.
Another famous binocular object, the open cluster Messier 6 (M6) also known as the Butterfly Cluster lies nearby. Messier 7 also holds the distinction as being the southernmost object in Messier’s catalog. Compiled from Parisian latitudes, Charles Messier entirely missed southern wonders such as Omega Centauri in his collection of deep sky objects that were not to be mistaken for comets. We also always thought it curious that he included such obvious “non-comets” such as the Pleiades, but missed fine northern sky objects as the Double Cluster in the northern constellation Perseus.
Finding Messier 6: the view from latitude 30 degrees north before dawn in mid-February. Credit: Stellarium.
Messier 7 is also sometimes called Ptolemy’s Cluster after astronomer Claudius Ptolemy, who first described it in 130 A.D. as the “nebula following the sting of Scorpius.” The season for hunting all of Messier’s objects in an all night marathon is coming right up in March, and Messier 7 is one of the last targets on the list, hanging high due south in the early morning sky.
Interested in catching how Messier 7 will evolve, or might look like up close? Check out Messier 45 (the Pleiades) and the V-shaped Hyades high in the skies in the constellation Taurus at dusk to see what’s in store as Messier 7 disperses, as well as the Ursa Major Moving Group.
And be sure to enjoy the fine view today of Messier 7 from the ESO!
Got pics of Messier 7 or any other deep sky objects? Send ’em, in to Universe Today!
Artist's impression of a Y-dwarf, the coldest known type of brown dwarf star. (NASA/JPL-Caltech)
When it’s a brown dwarf — but where do we draw the line?
Often called “failed stars,” brown dwarfs are curious cosmic creatures. They’re kind of like swollen, super-dense Jupiters, containing huge amounts of matter yet not quite enough to begin fusing hydrogen in their cores. Still, there has to be some sort of specific tipping point, and astronomers (being the scientists that they are) would like to know: when does a brown dwarf stop and a star begin?
Researchers from Georgia State University now have the answer.
From a press release issued Dec. 9 from the National Optical Astronomy Observatory (NOAO):
For most of their lives, stars obey a relationship referred to as the main sequence, a relation between luminosity and temperature – which is also a relationship between luminosity and radius. Stars behave like balloons in the sense that adding material to the star causes its radius to increase: in a star the material is the element hydrogen, rather than air which is added to a balloon. Brown dwarfs, on the other hand, are described by different physical laws (referred to as electron degeneracy pressure) than stars and have the opposite behavior. The inner layers of a brown dwarf work much like a spring mattress: adding additional weight on them causes them to shrink. Therefore brown dwarfs actually decrease in size with increasing mass.
As Dr. Sergio Dieterich, the lead author, explained, “In order to distinguish stars from brown dwarfs we measured the light from each object thought to lie close to the stellar/brown dwarf boundary. We also carefully measured the distances to each object. We could then calculate their temperatures and radii using basic physical laws, and found the location of the smallest objects we observed (see the attached illustration, based on a figure in the publication). We see that radius decreases with decreasing temperature, as expected for stars, until we reach a temperature of about 2100K. There we see a gap with no objects, and then the radius starts to increase with decreasing temperature, as we expect for brown dwarfs. “
Dr. Todd Henry, another author, said: “We can now point to a temperature (2100K), radius (8.7% that of our Sun), and luminosity (1/8000 of the Sun) and say ‘the main sequence ends there’ and we can identify a particular star (with the designation 2MASS J0513-1403) as a representative of the smallest stars.”
The relation between size and temperature at the point where stars end and brown dwarfs begin (based on a figure from the publication) Image credit: P. Marenfeld & NOAO/AURA/NSF.
“We can now point to a temperature (2100K), radius (8.7% that of our Sun), and luminosity (1/8000 of the Sun) and say ‘the main sequence ends there’.”
Aside from answering a fundamental question in stellar astrophysics about the cool end of the main sequence, the discovery has significant implications in the search for life in the universe. Because brown dwarfs cool on a time scale of only millions of years, planets around brown dwarfs are poor candidates for habitability, whereas very low mass stars provide constant warmth and a low ultraviolet radiation environment for billions of years. Knowing the temperature where the stars end and the brown dwarfs begin should help astronomers decide which objects are candidates for hosting habitable planets.
The data came from the SOAR (SOuthern Astrophysical Research) 4.1-m telescope and the SMARTS (Small and Moderate Aperture Research Telescope System) 0.9-m telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile.
A billion years after the big bang, hydrogen atoms were mysteriously torn apart into a soup of ions. Credit: NASA/ESA/A. Felid (STScI)).
A metal-poor star located merely 190 light-years from the Sun is 14.46+-0.80 billion years old, which implies that the star is nearly as old as the Universe! Those results emerged from a new study led by Howard Bond. Such metal-poor stars are (super) important to astronomers because they set an independent lower limit for the age of the Universe, which can be used to corroborate age estimates inferred by other means.
In the past, analyses of globular clusters and the Hubble constant (expansion rate of the Universe) yielded vastly different ages for the Universe, and were offset by billions of years! Hence the importance of the star (designated HD 140283) studied by Bond and his coauthors.
“Within the errors, the age of HD 140283 does not conflict with the age of the Universe, 13.77 ± 0.06 billion years, based on the microwave background and Hubble constant, but it must have formed soon after the big bang.” the team noted.
Metal-poor stars can be used to constrain the age of the Universe because metal-content is typically a proxy for age. Heavier metals are generally formed in supernova explosions, which pollute the surrounding interstellar medium. Stars subsequently born from that medium are more enriched with metals than their predecessors, with each successive generation becoming increasingly enriched. Indeed, HD 140283 exhibits less than 1% the iron content of the Sun, which provides an indication of its sizable age.
HD 140283 had been used previously to constrain the age of the Universe, but uncertainties tied to its estimated distance (at that time) made the age determination somewhat imprecise. The team therefore decided to obtain a new and improved distance for HD 140283 using the Hubble Space Telescope (HST), namely via the trigonometric parallax approach. The distance uncertainty for HD 140283 was significantly reduced by comparison to existing estimates, thus resulting in a more precise age estimate for the star.
HD 140283 is estimated to be 14.46+-0.80 billion years old. On the y-axis is the star’s pseudo-luminosity, on the x-axis its temperature. Computed evolutionary tracks (solid lines ranging from 13.4 to 14.4 billion years) were applied to infer the age (image credit: adapted from Fig 1 in Bond et al. 2013 by D. Majaess, arXiv).
The team applied the latest evolutionary tracks (basically, computer models that trace a star’s luminosity and temperature evolution as a function of time) to HD 140283 and derived an age of 14.46+-0.80 billion years (see figure above). Yet the associated uncertainty could be further mitigated by increasing the sample size of (very) metal-poor stars with precise distances, in concert with the unending task of improving computer models employed to delineate a star’s evolutionary track. An average computed from that sample would provide a firm lower-limit for the age of the Universe. The reliability of the age determined is likewise contingent on accurately determining the sample’s metal content. However, we may not have to wait long, as Don VandenBerg (UVic) kindly relayed to Universe Today to expect, “an expanded article on HD 140283, and the other [similar] targets for which we have improved parallaxes [distances].”
As noted at the outset, analyses of globular clusters and the Hubble constant yielded vastly different ages for the Universe. Hence the motivation for the Bond et al. 2013 study, which aimed to determine an age for the metal-poor star HD 140283 that could be compared with existing age estimates for the Universe. The discrepant ages stemmed partly from uncertainties in the cosmic distance scale, as the determination of the Hubble constant relied on establishing (accurate) distances to galaxies. Historical estimates for the Hubble constant ranged from 50-100 km/s/Mpc, which defines an age spread for the Universe of ~10 billion years.
Age estimates for the Universe as inferred from globular clusters and the Hubble constant were previously in significant disagreement (image credit: NASA, R. Gilliland (STScI), D. Malin (AAO)).
The aforementioned spread in Hubble constant estimates was certainly unsatisfactory, and astronomers recognized that reliable results were needed. One of the key objectives envisioned for HST was to reduce uncertainties associated with the Hubble constant to <10%, thus providing an improved estimate for the age of the Universe. Present estimates for the Hubble constant, as tied to HST data, appear to span a smaller range (64-75 km/s/Mpc), with the mean implying an age near ~14 billion years.
Determining a reliable age for stars in globular clusters is likewise contingent on the availability of a reliable distance, and the team notes that “it is still unclear whether or not globular cluster ages are compatible with the age of the Universe [predicted from the Hubble constant and other means].” Globular clusters set a lower limit to the age of the Universe, and their age should be smaller than that inferred from the Hubble constant (& cosmological parameters).
In sum, the study reaffirms that there are old stars roaming the solar neighborhood which can be used to constrain the age of the Universe (~14 billion years). The Sun, by comparison, is ~4.5 billion years old.
The team’s findings will appear in the Astrophysical Journal Letters, and a preprint is available on arXiv. The coauthors on the study are E. Nelan, D. VandenBerg, G. Schaefer, and D. Harmer. The interested reader desiring complete information will find the following works pertinent: Pont et al. 1998, VandenBerg 2000, Freedman & Madore (2010), Tammann & Reindl 2012.
Hubble image of a Proplyd-like object in Cygnus OB2. Credit: Z. Levay and L. Frattare, STScI
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The well known Orion Nebula is perhaps the most well known star forming regions in the sky. The four massive stars known as the trapezium illuminate the massive cloud of gas and dust busily forming into new stars providing astronomers a stunning vista to explore stellar formation and young systems. In the region are numerous “protoplanetary disks” or proplyds for short which are regions of dense gas around a newly formed star. Such disks are common around young stars and have recently been discovered in an even more massive, but less well known star forming region within our own galaxy: Cygnus OB2.
Ten times more massive than its more famous counterpart in Orion, Cygnus OB2 is a star forming region that is a portion of a larger collection of gas known as Cygnus X. The OB2 region is notable because, like the Orion nebula, it contains several exceptionally massive stars including OB2-12 which is one of the most massive and luminous stars within our own galaxy. In total the region has more than 65 O class stars, the most massive category in astronomers classification system. Yet for as bright as these stars are, Cygnus OB2 is not a popular target for amateur astronomers due to its position behind a dark obscuring cloud which blocks the majority of visible light.
But like many objects obscured in this manner, infrared and radio telescopes have been used to pierce the veil and study the region. The new study, led by Nicholas Wright at the Harvard-Smithsonian Center for Astrophysics, combines infrared and visual observations from the Hubble Space telescope. The observations revealed 10 objects similar in appearance to the Orion proplyds. The objects had long tails being blown away from the central mass due to the strong stellar winds from the central cluster similar to how proplyds in Orion point away from the trapezium. On the closer end, the objects were brightly ionized.
Yet despite the similarities, the objects may not be true proplyds. Instead, they may be regions known as “evaporating gaseous globules” or EGGs for short. The key difference between the two is whether or not a star has formed. EGGs are overdense regions within a larger nebula. Their size and density makes them resistant to the ionization and stripping that blows away the rest of the nebula. Because the interior regions are shielded from these dispersive forces, the center may collapse to form a star which is the requirement for a proplyd. So which are these?
In general, the newly discovered objects are far larger than those typically found in Orion. While Orion proplyds are nearly symmetric across an axis directed towards the central cluster, the OB2 objects have twisted tails with complex shapes. The objects are 18-113 thousand AU (1 AU = the distance between the Earth and Sun = 93 million miles = 150 million km) across making them significantly larger than the Orion proplyds and even larger than the largest known proplyds in NGC 6303.
Yet as different as they are, the current theoretical understanding of how proplyds work doesn’t put them beyond the plausible range. In particular, the size for a true proplyd is limited by how much stripping it feels from the central stars. Since these objects are further away from OB2-12 and the other massive stars than the Orion proplyds are from the trapezium, they should feel less dispersive forces and should be able to grow as large as is seen. Attempting to pierce the thick dust the objects contain and discover if central stars were present, the team examined the objects in the infrared and radio. Of the ten objects, seven had strong candidates central stellar sources.
Still, the stark differences make conclusively identifying the objects as either EGGs or proplyds difficult. Instead, the authors suggest that these objects may be the first discovery of an inbetween stage: old, highly evolved EGGs which have nearly formed stars making them more akin to young proplyds. If further evidence supports this, this finding would help fill in the scant observational details surrounding stellar formation. This would allow astronomers to more thoroughly test theories which are also tied to the understanding of how planetary systems form.
In 1996, a Japanese amateur astronomer discovered a new star in the constellation Sagittarius. Dubbed V4334 Sgr, astronomers initially expected it to be a typical novae, but closer examination revealed it to be a previously predicted but unseen event known as a “Very Late Thermal Pulse” (VLTP), the last hurrah of a white dwarf as hydrogen from the exterior of the star is carried to lower depths where one last gasp of fusion occurs. Astronomers then identified a second star, V605 Aql, that had been caught undergoing a VLTP in 1919. Recently, astronomers from the National University of La Plata, in Argentina, have claimed to have uncovered a third star undergoing this rare event.
It has been estimated that roughly one star every year ends its main sequence life and heads down the path of making a planetary nebula. Many of them won’t become convective white dwarfs that could turn into stars that should undergo a VLTP, but conservative estimates suggest that roughly 10% should. At such a rate, there should be roughly one star every decade that undergoes this phase. Since the stars have already shed their outer layers, the rejuvenated fusion is not diminished by them, and these stars shine exceptionally brightly making them detectable through most of the galaxy. Yet prior to this new identification, only two have been discovered which suggests that many objects historically identified as novae may truly have been stars similar to V4334 Sgr and V605 Aql.
In 2005, David Williams, a member of the American Association of Variable Star Observers, gathered images from the Harvard College Astronomical Plate collection. This massive collection of over 500,000 photographic plates, was the result of an early and long running survey that photographed great portions of the sky repeatedly from 1885 until 1993. This collection allowed him to reconstruct the changes in brightness the star NSV 11749 underwent during its outburst.
The star first became visible on the photographic plates in 1899. It peaked in brightness in 1903 and remained at that brightness for several years, until 1907 when it began to fade away again. The amount of time it took to brighten as well as the total change in brightness were similar to the previously identified VLTP stars. Over the 15 years since it first became detectable, it disappeared from images several times, another feature seen in V4334 Sgr and V605 Aql. The sudden disappearance has been explained by ejections of carbon from the star which cools and forms small dust grains which are effective at blocking light in the visible portion of the spectrum until they disperse.
However, two key differences stands out: The overall time before the NSV 11749 faded was roughly twice as long as for V605 Aql and V4335 Aql. The authors suggest that this may be due to a different mass of the white dwarf behind the outbursts. If the two previously identified VLTP stars were close in mass, they would likely have similar properties, while NSV 11749 could potentially have a different mass. The second discrepancy was the presence of a young planetary nebula. In both of the previously identified cases, the stars were the center of nebulae, but infrared images of the star did not reveal any nebula or remaining dust from the previous outburst. Authors again attribute this to a different evolutionary timescale due to the star’s potentially different mass.
While this tentative new classification is hardly conclusive, it is a reminder that astronomers have only just begun to understand this phase of stellar evolution and there is a great need for further examples to help refine models. The evolution of V4334 Sgr moved roughly 100 times faster than simulations had predicted, prompting revisions to the models. Certainly, similar changes will be necessary as more VLTP stars are discovered. This era of a star’s life is important to astronomers because the light obscuring carbon ejection is expected to be a major source of this important element.
This single all-sky image, captured by the Planck telescope, simultaneously captured two snapshots that straddle virtually the entire 13.7 billion year history of the universe. Credit: ESA
The universe has gone through a number of distinct phases. The first part of the first second is speculative, but the physics of the latter part is well know to us. In the first several minutes the lightest elements (hydrogen and helium) were formed.
Over the next 380,000 years the universe was a hot (but always cooling) plasma of electrons, nuclei and photons. At 380,000 years it was cool enough for electrons and nuclei to combine into atoms, in a process called recombination. The photons were freed from the plasma, and the universe became transparent for the first time. As the universe was opaque before recombination and transparent after, we see this epoch as a ‘wall’, and it is known as the cosmic microwave background.
What followed was a period known as the ‘cosmic dark ages’. The only light was that of the fading afterglow of the Big Bang, and the matter was comprised of the primordial elements and the exotic ‘dark matter’. During this time gravitational accretion slowly but surely produced larger and larger concentrations of matter, and when these became sufficiently dense, nuclear reactions could form and the first stars and galaxies were born. These lit up and ionized the universe again, some 400-500 million years after the Big Bang, in what is known as the ‘reionization epoch’.
The activity increased exponentially, culminating in the ‘quasar epoch’ 2-4 billion years after the Big Bang, a frenetic period of chaotic star and galaxy formation, galaxy interactions, monster quasars and radio galaxies. This activity eventually began to drop off, although it still continues today; the incidence of quasars today is a thousand times less than it was at the peak of the quasar epoch. At 13.7 billion years, the universe has now reached a ‘dignified middle age’.
The ‘heavy elements’ such as carbon and oxygen, essential for life as we know it, are all produced in stars, and this process has been going on ever since the first stars formed. Each generation of stars ejects more heavy elements into the intergalactic medium, so the abundances of the heavy elements have been built up over time.
By the time the Sun and Earth were formed 4.6 billion years ago, over 8.4 billion years of star and planet formation had already taken place in the universe. Star formation still takes place today, so in total there have been over 13 billion years of star and planet formation.
Zooming in now to our planet, life started not long after the Earth itself formed, sometime between 3.8 and 3.5 billion years ago (bya). But for almost half the age of the Earth, the only forms of life were microorganisms such as bacteria. More complex life forms started to appear about 1-2 bya. Invertebrates, which appeared some 600 million years ago (mya), were the earliest multicellular life forms, and vertebrates appeared about 500 mya. Life invaded the land about 400 mya. The dinosaurs dominated from 240 mya until their extinction 66 mya, and then mammals gradually took over. Many species came and went. Our closest living relatives are the chimpanzees, which split off from our ancestral line 5-6 mya; our more recent relatives have all become extinct.
It is amazing to think how recently humans appeared on the cosmic scene. Our species only appeared about 200,000 years ago, our ancestors emerged out of Africa just 50,000 years ago, agriculture started 10,000 years ago, and we have had modern technology for only the last 100 years or so! We are newcomers to the universe.
We now know that there are planets orbiting other stars like our Sun, probably billions of them in our galaxy alone, and billions more in the billions of other galaxies. Given the huge timescale of the universe, any life on those planets is bound to be millions or billions of years more or less advanced than life on Earth. If it is less advanced, it would certainly not be able to communicate with us. If it is more advanced, its technology would probably be totally unrecognisable to us. Nevertheless, we are probably not alone in the universe.
Of course the timescales discussed above only cover the ‘conventional’ universe from the Big Bang to now. If there was a ‘preexisting’ multiverse, we have no idea how far back any ‘before’ may extend. And as the expansion of the universe is accelerating, the future of the universe may be very long indeed: trillions upon trillions of years.
Peter Shaver obtained a PhD in astrophysics at the University of Sydney in Australia, and spent most of his career as a senior scientist at the European Southern Observatory (ESO), based in Munich. He has authored or co-authored over 250 scientific papers, and edited six books on astronomy and astrophysics.
