Posted on by Tammy Plotner

The Way Cool Clouds Of The Carina Nebula

The APEX observations, made with its LABOCA camera, are shown here in orange tones, combined with a visible light image from the Curtis Schmidt telescope at the Cerro Tololo Interamerican Observatory. The result is a dramatic, wide-field picture that provides a spectacular view of Carina’s star formation sites. The nebula contains stars equivalent to over 25 000 Suns, and the total mass of gas and dust clouds is that of about 140 000 Suns.

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It’s beautiful…. But it’s cold. By utilizing the submillimetre-wavelength of light, the 12 meter APEX telescope has imaged the frigid, dusty clouds of star formation in the Carina Nebula. Here, some 7500 light-years away, unrestrained stellar creation produces some of the most massive stars known to our galaxy… a picturesque petri dish in which we can monitor the interaction between the neophyte suns and their spawning molecular clouds.

By examining the region in submillimetre light through the eyes of the LABOCA camera on the Atacama Pathfinder Experiment (APEX) telescope on the plateau of Chajnantor in the Chilean Andes, a team of astronomers led by Thomas Preibisch (Universitäts–Sternwarte München, Ludwig-Maximilians-Universität, Germany), in close cooperation with Karl Menten and Frederic Schuller (Max-Planck-Institut für Radioastronomie, Bonn, Germany), have been able to pick apart the faint heat signature of cosmic dust grains. These tiny particles are cold – about minus 250 degrees C – and can only be detected at these extreme, long wavelengths. The APEX LABOCA observations are shown here in orange tones, combined with a visible light image from the Curtis Schmidt telescope at the Cerro Tololo Interamerican Observatory.

This amalgamate image reveals the Carina nebula in all its glory. Here we see stars with mass exceeding 25,000 sun-like stars embedded in dust clouds with six times more mass. The yellow star in the upper left of the image – Eta Carinae – is 100 times the mass of the Sun and the most luminous star known. It is estimated that within the next million years or so, it will go supernova, taking its neighbors with it. But for all the tension in this region, only a small part of the gas in the Carina Nebula is dense enough to trigger more star formation. What’s the cause? The reason may be the massive stars themselves…

With an average life expectancy of just a few million years, high-mass stars have a huge impact on their environment. While initially forming, their intense stellar winds and radiation sculpt the gaseous regions surrounding them and may sufficiently compress the gas enough to trigger star birth. As their time closes, they become unstable – shedding off material until the time of supernova. When this intense release of energy impacts the molecular gas clouds, it will tear them apart at short range, but may trigger star-formation at the periphery – where the shock wave has a lesser impact. The supernovae could also spawn short-lived radioactive atoms which could become incorporated into the collapsing clouds that could eventually produce a planet-forming solar nebula.

Then things will really heat up!

Original Story Source: ESO News Release.

Posted on by Nancy Atkinson

Gas, Not Galaxy Collisions Responsible for Star Formation in Early Universe

Artist concept of how a galaxy might accrete mass from rapid, narrow streams of cold gas. These filaments provide the galaxy with continuous flows of raw material to feed its star-forming at a rather leisurely pace. Credit: ESA–AOES Medialab

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Was the universe a kinder, gentler place in the past that we have thought? The Herschel space observatory has looked back across time with its infrared eyes and has seen that galaxy collisions played only a minor role in triggering star births in the past, even though today the birth of stars always seem to be generated by galaxies crashing into each other. So what was the fuel for star formation in the past?

Simple. Gas.

The more gas a galaxy contained, the more stars were born.

Scientists say this finding overturns a long-held assumption and paints a nobler picture of how galaxies evolve.

Astronomers have known that the rate of star formation peaked in the early Universe, about 10 billion years ago. Back then, some galaxies were forming stars ten or even a hundred times more vigorously than is happening in our Galaxy today.

In the nearby, present-day Universe, such high birth rates are very rare and always seem to be triggered by galaxies colliding with each other. So, astronomers had assumed that this was true throughout history.

GOODS-North is a patch of sky in the northern hemisphere that covers an area of about a third the size of the Full Moon. Credit: ESA/GOODS-Herschel consortium/David Elbaz

But Herschel’s observations of two patches of sky show a different story.

Looking at these regions of the sky, each about a third of the size of the full Moon, Herschel has seen more than a thousand galaxies at a variety of distances from the Earth, spanning 80% of the age of the cosmos.

In analyzing the Herschel data, David Elbaz, from CEA Saclay in France, and his team found that even though some galaxies in the past were creating stars at incredible rates, galaxy collisions played only a minor role in triggering star births. The astronomers were able to compare the amount of infrared light released at different wavelengths by these galaxies, the team has shown that the star birth rate depends on the quantity of gas they contain, not whether they are colliding.

They say these observations are unique because Herschel can study a wide range of infrared light and reveal a more complete picture of star birth than ever seen before.

However, their work compliments other recent studies from data from the Spitzer Space Telescope and the Very Large Telescope which found ancient galaxies fed on gas,not collisions

“It’s only in those galaxies that do not already have a lot of gas that collisions are needed to provide the gas and trigger high rates of star formation,” said Elbaz.

Today’s galaxies have used up most of their gaseous raw material after forming stars for more than 10 billion years, so they do rely on collisions to jump-start star formation, but in the past galaxies grew slowly and gently from the gas that they attracted from their surroundings.

This study was part of the GOODS observations with Herschel, the Great Observatories Origins Deep Survey.

Read the team’s paper in Astronomy & Astrophysics: GOODS–Herschel: an infrared main sequence for star-forming galaxies’ by D. Elbaz et al.

Source: ESA

Posted on by Vanessa Janek

Ancient Galaxies Fed On Gas, Not Collisions

The Sombrero Galaxy. Credit: ESO/P. Barthe

[/caption]The traditional picture of galaxy growth is not pretty. In fact, it’s a kind of cosmic cannibalism: two galaxies are caught in ominous tango, eventually melding together in a fiery collision, thus spurring on an intense but short-lived bout of star formation. Now, new research suggests that most galaxies in the early Universe increased their stellar populations in a considerably less violent way, simply by burning through their own gas over long periods of time.

The research was conducted by a group of astronomers at NASA’s Spitzer Science Center in Pasadena, California. The team used the Spitzer Space Telescope to peer at 70 distant galaxies that flourished when the Universe was only 1-2 billion years old. The spectra of 70% of these galaxies showed an abundance of H alpha, an excited form of hydrogen gas that is prevalent in busy star-forming regions. Today, only one out of every thousand galaxies carries such an abundance of H alpha; in fact, the team estimates that star formation in the early Universe outpaced that of today by a factor of 100!

This split view shows how a normal spiral galaxy around our local universe (left) might have looked back in the distant universe, when astronomers think galaxies would have been filled with larger populations of hot, bright stars (right). Image credit: NASA/JPL-Caltech/STScI

Not only did these early galaxies crank out stars much faster than their modern-day counterparts, but they created much larger stars as well. By grazing on their own stores of gas, galaxies from this epoch routinely formed stars up to 100 solar masses in size.

These impressive bouts of star formation occurred over the course of hundreds of millions of years. The extremely long time scales involved suggest that while they probably played a minor role, galaxy mergers were not the main precursor to star formation in the Universe’s younger years. “This type of galactic cannibalism was rare,” said Ranga-Ram Chary, a member of the team. “Instead, we are seeing evidence for a mechanism of galaxy growth in which a typical galaxy fed itself through a steady stream of gas, making stars at a much faster rate than previously thought.” Even on cosmic scales, it would seem that slow and steady really does win the race.

Source: JPL

Posted on by Jon Voisey

Star Forming Density – How Low Can You Go?

Star formation in the Eagle Nebula

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The general picture of star formation envisions stars emerging in clusters, having condensed from cores of gas under self gravity after having passed a critical density threshold. Perhaps the cloud was pushed over the threshold by the shockwave of a supernova or the tidal twisting of a nearby object. How it happens isn’t important since the methods are likely to be many and diverse. What is important is understanding what that threshold is so we may know when it is reached. It is generally referred to as the Jeans mass and observations have generally been well in line with densities predicted by this formulation. However, over the past several years, astronomers have discovered some objects amongst a new class that form in regions and densities not readily explained by the Jeans mass criterion.

The first of this new class, named IRAM 04191, was discovered in 1999 in the Taurus molecular cloud. This object, originally discovered in the radio portion of the spectrum with the Very Large Array, was a tiny forming protostar. The discoverers announced that the object was undergoing gravitational collapse, still disassociating the molecular hydrogen in the cloud from which it formed. While this object fit the traditional picture of star formation it was unique in that it was exceptionally dim. As more of these were discovered, it established a new class of objects that are now being called Very Low Luminosity Objects or VeLLOs.

The launch of the Spitzer infrared telescope allowed for the discovery of more objects. The first one from this telescope was discovered in 2004 and named L1014-IRS. Others have included L1521F-IRS, L328-IRS, and L1148-IRS. These objects are not yet well understood but have the general characteristics of having less than a tenth of the mass of the sun, seem to be accreting heavily (as indicated by outflows), and be only on the order of tens of thousands of years old.

Among these, L1148-IRS has been an oddity. While still low in overall light output, this object was relatively bright in the infrared when compared to other VeLLOs. Studies of the object and its surrounding gas suggested that the object was forming in an unusually empty region, one in which the usual scenario doesn’t seem to fit. A new paper by the original discoverers of this object, suggest that there may be some peculiarities that may be related to this puzzle. In particular, the region doesn’t seem to be collapsing uniformly. Different portions appear to be collapsing at different rates.

Regardless of how this protostar came to collapse, L1148-IRS is an unusual case and expected to form a very low mass star or brown dwarf. Since there are so few VeLLOs, the formation of such early stages of star formation, especially for low mass stars is not well understood and future detection of similar objects will likely greatly contribute to the understanding of low-mass objects in relative isolation.

Posted on by Jason Major

Dust in the Wind

WISE image of the "Elephant Trunk" nebula. NASA/JPL-Caltech/WISE Team.

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The stellar wind, that is! This beautiful image, taken by NASA’s Wide-Field Infrared Explorer (WISE) shows a vast ring of interstellar dust and gas being forced outwards by the wind and radiation from a massive star.

The star, HR8281, is located in the center of the image, the topmost star in a small triangular formation of blue stars to the upper left of the tip of a bright elongated structure – the end of the “elephant trunk” that gives the nebula its name. The star may not look like much, but HR8281’s powerful stellar wind is what’s sculpting the huge cloud of dust into the beautiful shapes seen in this infrared image.

Located 2,450 light-years from Earth, the Elephant’s Trunk Nebula spans 100 light-years. The “trunk” itself is about 30 light-years long. (That’s about, oh… 180 trillion miles!)

Structures like this are common in nebulae. They are formed when the stellar wind – the outpouring of ultraviolet radiation and charged particles that are constantly streaming off stars – blows away the gas and dust near a star, leaving only the densest areas. It’s basically erosion on a massive interstellar scale.

The tip of the "trunk" and the triangle of stars, the topmost of which is HR8281.

It’s not just a destructive process, though. Within those dense areas new stars can form… in fact, in the bright tip of the trunk above a small dark spot can be seen. That’s an area that’s been cleared by the creation of a new star. When a baby star “ignites” and its nuclear fusion factory turns on, its stellar wind clears away the dust and gas in the cloud it was formed from. Nebulae aren’t just pretty clouds in space… they’re stellar nurseries!

The red-colored stars in this image are other newborn stars, still wrapped in their dusty “cocoons”.

The colors used in this image represent specific wavelengths of infrared light. Blue and cyan (blue-green) represent light emitted at wavelengths of 3.4 and 4.6 microns, which is predominantly from stars. Green and red represent light from 12 and 22 microns, respectively, which is mostly emitted by dust.

Read more about this image on the WISE site here.

Image Credit: NASA/JPL-Caltech/WISE Team

Posted on by Jon Voisey

Cyanoacetylene in IC 342

IC 342 - Ken and Emilie Siarkiewicz/Adam Block/NOAO/AURA/NSF
IC 342 - Ken and Emilie Siarkiewicz/Adam Block/NOAO/AURA/NSF

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Star formation is an incredible process, but also notoriously difficult to trace. The reason is that the main constituent of stars, hydrogen, looks about the same well before a gravitational collapse begins, as it does in the dense clouds where star formation happens. Sure, the temperature changes and the hydrogen glows in a different part of the spectrum, but it’s still hydrogen. It’s everywhere!

So when astronomers want to search for denser regions of gas, they often turn to other atoms and molecules that can only form or be stimulated to emit under these relatively dense conditions. Common examples of this include carbon monoxide and hydrogen cyanide. However, a study published in 2005, led by David Meier at the University of Illinois at Urbana-Champaign, studied inner regions of the nearby face-on spiral by tracing eight molecules and determined that the full extent of the dense regions is not well mapped by these two common molecules. In particular, cyanoacetylene, an organic molecule with a chemical formula of HC3N, was demonstrated to correlate with the most active star forming regions, promising astronomers a peek into the heart of star forming regions and prompting a follow-up study.

The new study was conducted from the Very Large Array in late 2005. Specifically, it studied the emissions due to 5-4, 10-9, and 16-15 transitions which each correspond to different levels of heating and excitation. The dense regions uncovered by this study were consistent with the ones reported in 2005. One, discovered by the previous survey from another tracer molecule, was not found by this most recent study, but the new study also discovered a previously unnoticed giant molecular cloud (GMC) through the presence of HC3N.

Another technique that can be applied is examining the ratios of various levels of excitation. From this, astronomers can determine the temperature and density necessary to produce such emission. This can be performed with any type of gas, but using additional species of molecules provides independent checks on this value. For the area with the strongest emission, the team reported that the gas appeared to be a cool 40 K (-387°F) with a density of 1-10 thousand molecules per cubic centimeter. This is relatively dense for the interstellar medium, but for comparison, the air we breathe has approximately 1025 molecules per cubic centimeter. These findings are consistent with those reported from carbon monoxide.

The team also examined several of the star forming cores independently. By comparing the varying strengths of tracer molecules, the team was able to report that one GMC was well progressed in making stars while another was less evolved, likely still containing hot cores which had not yet ignited fusion. In the former, the HC3N is weaker than in the other cores explored, which the team attributes to the destruction of the molecules or dispersal of the cloud as fusion begins in the newly formed stars.

While using HC3N as a tracer is a relatively new approach (these studies of IC 342 are the first conduced in another galaxy), the results of this study have demonstrated that it can trace various features in dense clouds in similar fashions to other molecules.

Posted on by Nancy Atkinson

Thick Stellar Disk Isolated in Andromeda

Schematic representation of a thick disc structure. The thick disc is formed of stars that are typically much older than those in the thin disc, making it an ideal probe of galactic evolution (Credit: Amanda Smith, IoA graphics officer)

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From the Institute of Astronomy at Cambridge University press release:

A team of astronomers from the UK, the US and Europe have identified a thick stellar disc in the nearby Andromeda galaxy for the first time. The discovery and properties of the thick disc will constrain the dominant physical processes involved in the formation and evolution of large spiral galaxies like our own Milky Way.

By analyzing precise measurements of the velocities of individual bright stars within the Andromeda galaxy using the Keck telescope in Hawaii, the team have managed to separate out stars tracing out a thick disc from those comprising the thin disc, and assess how they differ in height, width and chemistry.

Optical image of The Andromeda galaxy (M31) (credit Robert Gendler)

Spiral structure dominates the morphology of large galaxies at the present time, with roughly 70% of all stars contained in a flat stellar disc. The disc structure contains the spiral arms traced by regions of active star formation, and surrounds a central bulge of old stars at the core of the galaxy. “From observations of our own Milky Way and other nearby spirals, we know that these galaxies typically possess two stellar discs, both a ‘thin’ and a ‘thick’ disc,” explains the leader of the study, Michelle Collins, a PhD student at Cambridge’s Institute of Astronomy. The thick disc consists of older stars whose orbits take them along a path that extends both above and below the more regular thin disc. “The classical thin stellar discs that we typically see in Hubble imaging result from the accretion of gas towards the end of a galaxy’s formation, whereas thick discs are produced in a much earlier phase of the galaxy’s life, making them ideal tracers of the processes involved in galactic evolution.”

Currently, the formation process of the thick disc is not well understood. Previously, the best hope for comprehending this structure was by studying the thick disc of our own Galaxy, but much of this is obscured from our view. The discovery of a similar thick disk in Andromeda presents a much cleaner view of spiral structure. Andromeda is our nearest large spiral neighbor — close enough to be visible to the unaided eye — and can be seen in its entirety from the Milky Way. Astronomers will be able to determine the properties of the disk across the full extent of the galaxy and look for signatures of the events connected to its formation. It requires a huge amount of energy to stir up a galaxy’s stars to form a thick disc component, and theoretical models proposed include accretion of smaller satellite galaxies, or more subtle and continuous heating of stars within the galaxy by spiral arms.

Ages and orientations of the stellar components of disc galaxies. The halo (or spheroid) contains the oldest populations, followed by the thick stellar disc. The thin disc typically contains the youngest generations of stars. (Credit: RAVE collaboration)

“Our initial study of this component already suggests that it is likely older than the thin disc, with a different chemical composition” commented UCLA Astronomer, Mike Rich. “Future more detailed observations should enable us to unravel the formation of the disc system in Andromeda, with the potential to apply this understanding to the formation of spiral galaxies throughout the Universe.”

“This result is one of the most exciting to emerge from the larger parent survey of the motions and chemistry of stars in the outskirts of Andromeda,” said fellow team member, Dr. Scott Chapman, also at the Institute of Astronomy. “Finding this thick disc has afforded us a unique and spectacular view of the formation of the Andromeda system, and will undoubtedly assist in our understanding of this complex process.”

This study was published in Monthly Notices of the Royal Astronomical Society by Michelle Collins, Scott Chapman and Mike Irwin from the Institute of Astronomy, together with Rodrigo Ibata from L’Observatoire de Strasbourg, Mike Rich from University of California, Los Angeles, Annette Ferguson from the Institute for Astronomy in Edinburgh, Geraint Lewis from the University of Sydney, and Nial Tanvir and Andreas Koch from the University of Leicester.

This study is published in Monthly Notices of the Royal Astronomical Society:
* http://arxiv.org/abs/1010.5276
* http://www.ast.cam.ac.uk/~mlmc2/M31thickdisc.html

Posted on by Jon Voisey

A Peek Inside NGC 7538

The active star forming region NGC 7538. Image by Fred Calvert/Adam Block/NOAO/AURA/NSF
The active star forming region NGC 7538. Image by Fred Calvert/Adam Block/NOAO/AURA/NSF

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Often overshadowed by the more famous Bubble Nebula which lies nearby, NGC 7538 is an exciting emission and reflection nebula located in Cepheus. While it is often overlooked by amateur astronomers, professionals looking to study stellar formation find it an exciting target as it is the host to ongoing star formation, including the largest known protostar.

Because of the dusty nature of this region, studies targeting the nebula are frequently conducted in longer wavelengths, ranging from the infrared to the radio. Previous studies have put the age of the forming stars at around ~1-4 million years and at a distance of ~2.8 kiloparsecs. Within it, several individual sub groups of star formation seem to have occurred. Among some of the more interesting individual forming stars are NGC 7538S and MM 1.

Observations from earlier this year targeted NGC 7538S. This protostar is embedded in a collapsing core of approximately 85 – 115 solar masses and hosts a rotating accretion disc as well as large outflows of material. Although the star has not finished forming, the conditions are right for it to form into a high mass B star and is undergoing accretion at an unusually high rate of 1/1000th of a solar mass per year.

More recently another paper explores several other forming stars in the region including the massive MM 1. This star is already estimated to have accumulated 20-30 solar masses and be well on the way to forming an O class star. But it’s not done yet. Radial velocity measurements of molecules in the protostar’s vicinity indicate it’s still undergoing large amounts of accretion, mostly from its equatorial plane. Numerous studies have shown that this massive star is creating powerful jets.

In addition, this new study identifies an additional eight cores forming into young stars near MM 1. These cores are interesting because they exist in regions where the density and temperature were not expected to be sufficiently high to induce star formation. This suggests that their formation was not uniquely due to a self induced collapse, but rather, triggered by shock waves or magnetic fields. Although no studies have searched for the signs of magnetic fields in the region, there are indications that numerous shock waves exist. Additionally, four of these cores have mass available to them similar to that of MM 1 which may allow them to form into a grouping of high mass stars similar to the famous Trapezium in Orion. These stars all exist in a narrowly confined region of about 1 light year, which is also similar to the separation of the Trapezium. Many of the newly discovered cores have large outflows and maser emission as well.

Further studies on this region will certainly uncover new protostars and assist astronomers in understanding how clusters of stars form. Already, astronomers have used it to help probe the Initial Mass Function which describes the number of stars forming for various masses. Additionally, with small clusters of stars like the Trapezium being common, catching one in the act of forming may help astronomers determine just how they form.

Posted on by Nancy Atkinson

Big or Small, All Stars Form the Same Way

IRAS 13481-6124 (upper left is about twenty times the mass of our sun and five times its radius. It is surrounded by its pre-natal cocoon. Image credit: NASA/JPL-Caltech/ESO/Univ. of Michigan

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How do massive stars form? This has been one of the more hotly debated questions in astronomy. Do big stars form by accretion like low-mass stars or do they form through the merging of low mass protostars? Since massive stars tend to be quite far away and usually are surrounded by a shroud of dust, they are difficult to observe, said Stefan Kraus from the University of Michigan. But Kraus and his team have obtained the first image of a dusty disc closely encircling a massive baby star, providing direct evidence that, big or small, all stars form the same way.

“Our observations show a disc surrounding an embryonic young, massive star, which is now fully formed,” said Kraus. “It’s the first time something like this has been observed, and the disk very much resembles what we see around young stars that are much smaller, except everything is scaled up and more massive.”

Not only that, but Kraus and his team found hints at a potential planet-forming region around the nascent star.

Using ESO’s Very Large Telescope Interferometer Kraus and his team focused on IRAS 13481-6124, a star located about 10,000 light-years away in the constellation Centaurus, and about 20 times more massive than our sun. “We were able to get a very sharp view into the innermost regions around this star by combining the light of separate telescopes,” Kraus said, “basically mimicking the resolving power of a telescope with an incredible 85-meter (280-foot) mirror.”

Kraus added that the resulting resolution is about 2.4 milliarcseconds, which is equivalent to picking out the head of a screw on the International Space Station from Earth, or more than ten times the resolution possible with current visible-light telescopes in space.

They also made complementary observations with the 3.58-meter New Technology Telescope at La Silla. The team chose this region by looking at archived images from the Spitzer Space Telescope as well as from observations done with the APEX 12-meter submillimeter telescope, where they discovered the presence of a jet.

“Such jets are commonly observed around young low-mass stars and generally indicate the presence of a disc,” says Kraus.

Astronomers have obtained the first clear look at a dusty disk closely encircling a massive baby star, providing direct evidence that massive stars do form in the same way as their smaller brethren -- and closing an enduring debate. This artist's concept shows what such a massive disk might look like. Image credit: ESO/L. Calçada

From their observations, the team believes the system is about 60,000 years old, and that the star has reached its final mass. Because of the intense light of the star — 30,000 times more luminous than our Sun — the disc will soon start to evaporate. The disc extends to about 130 times the Earth–Sun distance — or 130 astronomical units (AU) — and has a mass similar to that of the star, roughly twenty times the Sun. In addition, the inner parts of the disc are shown to be devoid of dust, which could mean that planets are forming around the star.

“In the future, we might be able to see gaps in this and other dust disks created by orbiting planets, although it is unlikely that such bodies could survive for long,” Kraus said. “A planet around such a massive star would be destroyed by the strong stellar winds and intense radiation as soon as the protective disk material is gone, which leaves little chance for the development of solar systems like our own.”

Kraus looks forward to observations with the Atacama Large Millimeter/submillimeter Array (ALMA), currently under construction in Chile, which may be able to resolve the disks to an even sharper resolution.

Previously, Spitzer detected dusty disks of planetary debris around more mature massive stars, which supports the idea that planets may form even in these extreme environments. (Read about that research here.) .

Sources: ESO, JPL

Posted on by Steve Nerlich

Astronomy Without A Telescope – Stellar Archaeology

Artist's impression of Population 3 stars born over 13 billion years ago - the earliest, oldest and presumably now extinct star types. Credit: NASA.

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Although, as we look further and deeper into the sky, we are always looking into the past – there are other ways of gaining information about the universe’s ancient history. Low mass, low metal stars may be remnants of the early universe and carry valuable information about the environment of that early universe.

The logic of stellar archaeology involves tracking generations of stars back to the very first stars seen in our universe. Stars born in recent eras, say within the last five or six billion years, we call Population I stars – which includes our Sun. These stars were born from an interstellar medium (i.e. gas clouds etc) that had been seeded by the death throes of a previous generation of stars we call Population II stars.

Population II stars were born from an interstellar medium that existed maybe 12 or 13 billion years ago – and which had been seeded by the death throes of Population III stars, the first stars ever seen in our universe.

And when I say death throes seeding the interstellar medium this includes average sized stars blowing off a planetary nebula at the end of their red giant phase – or bigger stars exploding as supernovae.

So for example, the low metal spectral signature of HE 0107-5240 matches that predicted for a very early low mass Population II star built from the end-products of a Population III supernova.

This is about as close as we can get gathering any information about Population III stars. Telescopes that can look deeper into space (and hence look further back in time) may eventually spot one – but it’s unlikely that any still exist. Theory has it that Population III stars formed from a homogenous interstellar medium of hydrogen and helium. The homogeneity of this medium meant that any stars that formed were all massive – in the order of hundreds of solar masses.

Stars of this scale, not only have short life spans but explode with such a force that the star literally blows itself to bits as a ‘pair-instability’ supernova – leaving no remnant neutron star or black hole behind. Supernova SN2006gy was probably a pair-instability supernova – mimicking the last gasps of Population III stars that lived more than 13 billion years ago.

Recipe for a pair instability supernova. In very massive stars, gamma rays radiating from the core become so energetic that they can undergo pair production after interaction with a nucleus. Essentially, the gamma ray creates a paired particle and antiparticle (commonly an electron and a positron). The loss of radiation pressure as gamma rays convert to particles results in gravitational collapse of the star's core - and kaboom! Credit: chandra.harvard.edu

It was only after Population III stars had seeded the interstellar medium with heavier elements that fine structure cooling resulted in disruption of thermal equilibrium and fragmentation of gas clouds – enabling smaller, and hence longer lived, Population II stars to be born.

Around the Milky Way, we can find very old Population II stars in orbiting dwarf galaxies. These stars are also common in the galactic halo and in globular clusters. However, in ‘the guts’ of the galaxy we find lots of young Population I stars.

This all leads to the view that the Milky Way is a gravitational hub nearly as old as the universe itself – which has been steadily growing in size and keeping itself looking young by maintaining a steady diet of ancient dwarf galaxies – which, deprived of such a diet, have remained largely unchanged since their formation in the early universe.

Further reading:

A. Frebel. Stellar Archaeology – Exploring the Universe with Metal-Poor Stars http://arxiv4.library.cornell.edu/abs/1006.2419