Dazzling New Views of a Familiar Cluster

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.
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!

When Is a Star Not a Star?

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.

Read more: The Secret Origin Story of Brown Dwarfs

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.
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’.”

Dr. Todd Henry, RECONS Director

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.

Read more here.

Nearby Ancient Star is Almost as Old as the Universe

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.

Age estimate for HD 140283 is 14.46+-0.80 Gyr.  On the y-axis is the star's pseudo-luminosity, on the x-axis its temperature.  An evolutionary track was applied to infer the age (credit: adapted by D. Majaess from Fig 1 in Bond et al. 2013, arXiv).
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 globular clusters were previously larger than that inferred for the Age of the Universe from the Hubble constant (NASA, R. Gilliland (STScI), D. Malin (AAO))
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.

“Proplyd-like” Objects Discovered in Cygnus OB2

Hubble image of a Proplyd-like object in Cygnus OB2. Credit: Z. Levay and L. Frattare, STScI


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.

NSV 11749 – Born Again and Grown Old


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.

Guest Post by Author Peter Shaver: Cosmic Time Scales


Editor’s note: Peter Shaver is the author of the new book “Cosmic Heritage – Evolution from the Big Bang to Conscious Life.” Find out here how you can win a copy!

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.

“Impossible” Star Exists in Cosmic Forbidden Zone


Astronomers say a newly found star should not exist and is in the “forbidden zone” of a widely accepted theory of star formation. The star, called SDSS J102915+172927, is composed almost entirely of hydrogen and helium, with only remarkably small amounts of other chemical elements in it. And how should this star be classified? We suggest either ‘easy listening’ or ‘jazz,’ as this star is certainly not heavy metal! But it may be one of the oldest stars ever found.

This faint star is located in the constellation of Leo (The Lion), and has the lowest amount of elements heavier than helium (what astronomers call “metals”) of all stars yet studied. It has a mass smaller than that of the Sun and is probably more than 13 billion years old.

“A widely accepted theory predicts that stars like this, with low mass and extremely low quantities of metals, shouldn’t exist because the clouds of material from which they formed could never have condensed,” said Elisabetta Caffau (Zentrum für Astronomie der Universität Heidelberg, Germany and Observatoire de Paris, France), lead author of the paper appearing in this week’s edition of Nature. “It was surprising to find, for the first time, a star in this ‘forbidden zone’, and it means we may have to revisit some of the star formation models.”

The team found the star with the X-shooter and UVES instruments on the Very Large Telescope. This allowed them to measure how abundant the various chemical elements were in the star. They found that the proportion of metals in SDSS J102915+172927 is more than 20,000 times smaller than that of the Sun.

“The star is faint, and so metal-poor that we could only detect the signature of one element heavier than helium — calcium — in our first observations,” said Piercarlo Bonifacio (Observatoire de Paris, France), who supervised the project. “We had to ask for additional telescope time from ESO’s Director General to study the star’s light in even more detail, and with a long exposure time, to try to find other metals.”

This picture shows the distribution of the light of different colours coming from the remarkable star SDSS J102915+172927 after it has been split up by the X-Shooter instrument on the ESO VLT. Credit: ESO/E. Caffau

The prevailing theory is that hydrogen and helium were created shortly after the Big Bang, together with some lithium, while almost all other elements were formed later in stars. Supernova explosions spread the stellar material into the interstellar medium, making it richer in metals. New stars form from this enriched medium so they have higher amounts of metals in their composition than the older stars. Therefore, the proportion of metals in a star tells us how old it is.

“The star we have studied is extremely metal-poor, meaning it is very primitive. It could be one of the oldest stars ever found,” adds Lorenzo Monaco from ESO, also involved in the study.

Also very surprising was the lack of lithium in SDSS J102915+172927. Such an old star should have a composition similar to that of the Universe shortly after the Big Bang, with a few more metals in it. But the team found that the proportion of lithium in the star was at least fifty times less than expected in the material produced by the Big Bang.

“It is a mystery how the lithium that formed just after the beginning of the Universe was destroyed in this star.” Bonifacio added.

Is this star one-of-a-kind? Probably not, the researchers say. “We have identified several more candidate stars that might have metal levels similar to, or even lower than, those in SDSS J102915+172927. We are now planning to observe them with the VLT to see if this is the case,” said Caffau.

Read the team’s paper (pdf file)

Source: ESO

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”.

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

Another Ceasing Cepheid


Earlier this year, I wrote an article about a Cepheid variable star named V19 in M31. This Cepheid was one that once pulsated strongly and was one of the variables Hubble first used to find the distance to the Andromeda galaxy. But today, V19 is a rare instance of a Cepheid that has seemingly, stopped pulsating. Another example of this phenomenon is that of Polaris, which has decreased in the amplitude of brightnesses by nearly an order of magnitude in the past century, although some reports indicate that it may be beginning to increase again. Meanwhile, a new paper is looking to add another star, HDE 344787, to this rare category and according to the paper, it may be “even more interesting than Polaris”.

The star in question, HDE 344787, is a F class supergiant. Although the variations in brightness have been difficult to observe, due to their small amplitude, astronomers have revealed two fundamental pulsation modes corresponding to 5.4 days and 3.8 days. But perhaps even more interesting, is that the 5.4 day period seems to be growing. Careful analysis of the data suggests that this period is growing by about 13 seconds per year. This finding is in strong agreement with what is predicted by models of stellar evolution for stars with metallicity similar to the sun passing through the instability strip for the first time.

HDE 344787 is similar in Polaris in that both stars share the same spectral type. However, the existence of two modes of pulsation is not seen in Polaris. The lengthening of the period of pulsation, however, is seen. For Polaris, its variation is growing by 4.5 seconds per year. Another similarity is that, like Polaris and V19, has been decreasing in the amplitudes of its brightness since at least 1890.

While the addition of this star to the collection of Cepheids that have decreased their amplitude, it does little to solve the mystery of why they might do so. Currently, both Polaris and HDE 344787 lie near the middle of the instability strip and, as such, are not simply evolving out of the region of instability. However, the confirmation of second pulsational mode may lend support to the notion that a change in one of these modes may serve to dampen the other, creating an effect known as the Blazhko Effect.

Ultimately, this star will require further observations to understand its nature better. Due the the faintness of this star (~10th magnitude) as well as the small change in brightness from the pulsations and the dense stellar field on which it lies, observations have been notoriously challenging.