Posted on by Steve Nerlich

Astronomy Without A Telescope – Coloring In The Oort Cloud

A very distant and very red Sedna. Credit: NASA, JPL, Caltech.

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It’s possible that if we do eventually observe the hypothetical objects that make up the hypothetical Oort cloud, they will all be a deep red color. This red coloring will probably be a mix of ices, richly laced with organic compounds – and may represent remnants of the primordial material from which the solar system was formed.

Furthermore, the wide range of colors found across different classes of trans-Neptunian objects may help to determine their origins.

The current observable classes of trans-Neptunian objects includes Pluto and similar objects called plutinos, which are caught in a 2:3 orbital resonance with Neptune towards the inner edge of the Kuiper belt. There are other Kuiper belt objects caught in a range of different resonant orbital ratios, including two-tinos – which are caught in a 1:2 resonance with Neptune – and which are found towards the outer edge of the Kuiper belt.

Otherwise, the majority of Kuiper belt objects (KBOs) are cubewanos (named after the first one discovered called QB1), which are also known as ‘classical’ KBOs. These are not obviously in orbital resonance with Neptune and their solar orbits are relatively circular and well outside Neptune’s orbit. There are two fairly distinct populations of cubewanos – those which have little inclination and those which are tilted more than 12 degrees away from the mean orbital plane of the solar system.

Beyond the Kuiper belt is the scattered disk – which contains objects with very eccentric elliptical orbits. So, although it may take hundreds of years for them to get there, the perihelions of many of these objects’ orbits are much closer to the Sun – suggesting this region is the main source of short period comets.

The trans-Neptunian landscape. Classical Kuiper belt objects have relatively circular orbits that never stray within the orbit of Neptune (yellow circle) - while plutinos and scattered disk objects have eccentric orbits that may. Classical objects with low inclinations (see ecliptic view) tend to have the deepest red coloration. Objects with higher inclination - and those with eccentric solar orbits which take them closer to the Sun - appear faded.

Now, there are an awful lot of trans-Neptunian objects out there and not all of them have been observed in detail, but surveys to date suggest the following trends:

  • Cubewanos with little inclination or eccentricity are a deep red color; and
  • Plutinos, scattered disk objects and highly inclined cubewanos are much less red.

Beyond the scattered disk are detached objects, that are clearly detached from the influence of the major planets. The best known example is Sedna – which is… yep, deep red (or ultra-red as the boffins prefer to say).

Sedna and other extreme outer trans-Neptunian objects are sometimes speculatively referred to as inner Oort cloud objects. So if we are willingly to assume that a few meager data points are representative of a wider (and hypothetical) population of Oort cloud objects – then maybe, like Sedna, they are all a deep red color.

And, looking back the other way, the ‘much less red’ color of highly inclined and highly eccentric trans-Neptunian objects is consistent with the color of comets, Centaurs (comets yet to be) and damocloids (comets that once were).

On this basis, it’s tempting to suggest that deep red is the color of primordial solar system material, but it’s a color that fades when exposed to moderate sunlight – something that seems to happen to objects that stray further inward than Neptune’s orbit. So maybe all those faded objects with inclined orbits used to exist much nearer to the Sun, but were flung outward during the early planetary migration maneuvers of the gas giants.

And the primordial red stuff? Maybe it’s frozen tholins – nitrogen-rich organic compounds produced by the irradiation of nitrogen and methane. And if this primordial red stuff has never been irradiated by our Sun, maybe it’s a remnant of the glowing dust cloud that was once our Sun’s stellar nursery.

Ah, what stories we can weave with scant data.

Further reading: Sheppard, S.S. The colors of extreme outer solar system objects.

Posted on by Tammy Plotner

Weekend SkyWatcher’s Forecast: July 9-11, 2010

Greetings, Fellow SkyWatchers! Is it hot enough for you where you live? Not if you’re in the southern hemisphere… But this weekend the southern hemisphere is the place to be if you’re interested in catching a total solar eclipse! If you can’t travel that close, then let’s travel far, far away as we take a look at the season’s globular clusters… from easy to challenging! Be sure to keep an eye on Saturn and Mars as they draw closer together and look for bright Jupiter in the morning skies! Whenever you’re ready? Grab your optics and I’ll see you in the backyard…

July 9, 2010 – On this date in 1979, Voyager 2 quietly made its closest approach to Jupiter. How about if we take a close approach before dawn as well? Enjoy the waltz of the Galileans and all the fine details! If you enjoy watching the planets swim against the night sky, then be sure to keep an eye on the early evening visage of Saturn as Mars “back strokes” its way towards the Ring King!

Tonight let’s head on out toward two more close objects that appear differently from the rest (and each other)—same-field binocular pair M10 and M12. Located about half a fist-width west of Beta Ophiuchi, M12 (RA 16 47 14 Dec –01 56 52) is the northern most of this pair. Easily seen as two hazy round spots in binoculars, let’s go to the telescope to find out what makes M12 tick.


Since this large globular is much more loosely concentrated, smaller scopes will begin to resolve individual stars from this 24,000-light-year-distant Class IX cluster. Note that there is a slight concentration toward the core region, but for the most part the cluster appears fairly even. Large instruments will resolve out individual chains and knots of stars.

Now let’s drop about 3.5 degrees southeast and check out Class VII M10 (RA 16 57 08 Dec –04 05 57). What a difference in structure! Although they seem to be close together and similar in size, the pair is actually separated by some 2,000 light-years. M10 is a much more concentrated globular, showing a brighter core region to even the most modest of instruments. This compression of stars is what differentiates one type of globular cluster from another and is the basis of their classification. M10 appears brighter, not because of this compression but because it is about 2,000 light-years closer than M12.

July 10, 2010 – Today we celebrate the 1832 birth on this date of Alvan Graham Clark. An astronomer himself, Clark was also a member of a famous American family of telescope makers. He helped to create the largest refractor in the world—the lenses for the 40″ Yerkes Telescope. Perhaps the stress of worrying for their safety took its toll on Alvan, for he died shortly after their first use. Tonight let’s honor Clark’s work by studying a globular cluster suitable for all optics, M4. All you have to know is Antares!

Just slightly more than a degree west (RA 16 23 35 Dec –26 31 31), this major 5th magnitude Class IX globular cluster can even be spotted unaided from a dark location. In 1746 Philippe Loys de Cheseaux happened upon this 7,200-light-year-distant beauty, one of the nearest to us. It was also included in Lacaille’s catalog as object I.9 and in Messier’s in 1764. Much to Charles’s credit, he was the first to resolve it!


As one of the loosest, or most ‘‘open,’’ globular clusters, M4 would be tremendous if we were not looking at it through a heavy cloud of interstellar dust. To binoculars, it is easy to pick out a very round, diffuse patch, yet it will begin to resolve with even a small telescope. Large telescopes will also easily see a central ‘‘bar’’ of stellar concentration across M4’s core region, which was first noted by Herschel. As an object of scientific study, in 1987, the first millisecond pulsar was discovered within M4, which turned out to be ten times faster than the Crab Nebula pulsar. Photographed by the Hubble Space Telescope in 1995, M4 was found to contain white dwarf stars—the oldest in our galaxy—with a planet orbiting one of them! A little more than twice the size of Jupiter, this planet is believed to be as old as the cluster itself. At 13 billion years, it would be three times the age of the Solar System!

July 11, 2010 – Today marks the 1732 birth on this date of Joseph Jerome Le Francais de Lalande, who determined the Moon’s parallax and published a comprehensive star catalog in 1801. While we might not be determining the Moon’s parallax against the background stars, we’re certainly going to see its effects against the background Sun! Right now the southern hemisphere is the place to be if you’re interested in catching a total solar eclipse – but this eclipse isn’t going to be an easy one to observe unless you’re on the water.


Starting roughly 2000 kilometers northeast of New Zealand at 18:15 UT, totality will begin at local sunrise over the ocean. Minutes later the shadow pass will actually cross land as it encounters the island of Mangaia for about 3 minutes total time. Totality will brush by Tahiti, encompass the uninhabited atolls of the Tuamotu Archipelago and slide its way across the mysterious Easter Island. The Moon’s shadow will take once again to the water for another 3700 kilometers where it will reach its end at the very southernmost tip of South America. For those of you who have the great fortune to eclipse chase? We wish you the very best of skies and luck!

For hard-core observers, tonight’s globular cluster study will require at least a mid-aperture telescope, because we’re staying up a bit later to go for a same-low-power-field pair—NGC 6522 (RA 18 03 34 Dec –30 02 02) and NGC 6528 (RA 18 04 49 Dec –30 03 20). You will find them easily at low power just a breath northwest of Gamma Sagittarii, better known as Al Nasl, the tip of the ‘‘teapot’s’’ spout. Once located, switch to higher power to keep the light of Gamma out of the field, and let’s do some study.


The brighter, and slightly larger, of the pair to the northeast is Class VI NGC 6522. Note its level of concentration compared to the Class V NGC 6528. Both are located around 2,000 light years away from the galactic center and are seen through a very special area of the sky known as ‘‘Baade’s Window’’—one of the few areas toward our galaxy’s core region not obscured by dark dust.

Although each is similar in concentration, distance, etc., NGC 6522 has a slight amount of resolution toward its edges, while NGC 6528 appears more random. Although both NGC 6522 and NGC 6528 were discovered by Herschel on July 24, 1784, and both are the same distance from the galactic core, they are very different. NGC 6522 has an intermediate metallicity. At its core, the red giants have been depleted, or stripped tidally by evolving into blue stragglers. It is possible that core collapse has already occurred. NGC 6528, however, contains one of the highest metal contents of any known globular cluster collected in its bulging core!

Until next time? Keep reaching for the stars!

This week’s awesome images are: M10, M12, M4, NGC 6522 and NGC 6528 from Palomar Observatory, courtesy of Caltech. Alvan Clark historical image and eclipse information courtesy of NASA. We thank you so much!

Posted on by Nancy Atkinson

Astronomy Cast Ep. 193: Astronomy With the Unaided Eye

Full Moon

We talk a lot about telescopes here on Astronomy Cast, but you really don’t need any special equipment to appreciate what the night sky has to offer. Just head outside with some sky charts, maybe a planisphere, some friends and hot chocolate, and you’re good to go. Let’s talk about what kinds of things you can see with just your eyes.

Click here to download the episode.

Or subscribe to: astronomycast.com/podcast.xml with your podcatching software.

Astronomy With the Unaided Eye shownotes and transcript.

Posted on by Steve Nerlich

Astronomy Without A Telescope – Animal Astronomy

Avian astronomers at work. Credit: abc.net.au.

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In the 1950s, the Sauer research team locked some birds in Olbers planetarium and started messing with them. First they projected a northern hemisphere autumn sky and the birds flew ‘south’ – away from Polaris and keeping Betelgeuse to the left (‘east’). Then they projected a spring night sky and the birds flew ‘north’ towards Polaris with Betelgeuse again to their left, albeit this time in the ‘west’. The position of Betelgeuse appeared to be significant, perhaps because it’s one of the brighter stars in the northern hemisphere and just to the north of the celestial equator.

Later experiments with Indigo Buntings demonstrated that birds raised with no experience of the night sky didn’t have a clue what to do when released into a planetarium. However, birds that were raised with the night sky visible would fly ‘south’ away from the sky’s axis of rotation, whether that was Polaris or an artificial arbitrary axis created within the planetarium.

From this work, researchers concluded that it was unlikely that birds were born with a genetic star map, but instead learned to orientate themselves with respect to the rotating night sky by reference to other directional cues – like the position of the Sun and the Earth’s magnetic field.

It’s thought that many migratory birds closely monitor sunrise and sunset – allegedly when you see a line of birds on a power line, most will be facing east in the morning and west in the evening, recalibrating their internal compasses. Checking for a north-south plane of polarized light at sunrise and sunset may help them determine their latitude – by indicating how far off due east or west the Sun is when it’s at the horizon.

Pigeons have well developed magnetoreception that they can use as an alternative to solar navigation. For example, they can ‘home’ even with a heavily overcast sky – but get them to wear a little magnetized helmet that screws up their perception of the Earth’s magnetic field and they get lost. On the other hand, if it’s a clear day with the Sun visible they can find home just fine – even with a little magnetized helmet on.

As well as the birds – bacteria, bees, termites, lobsters, salamanders, salmon, turtles, mole rats and bats have all been shown to possess magnetoreception.

Magnetotactic bacteria manufacture their own magnetite crystals – building chains of crystals that mimic a compass needle. The bacteria appear to use their magnetite crystals for the simple purpose of determining which way is down – since a straight line to magnetic north will pass through the Earth’s surface.

Magnetospirillum with a line of synthesized magnetite crystals visible. Credit: www.microbiologybytes.com

It’s yet to be determined how a complex nervous system might interface with magnetite or whether magnetite is the primary mechanism in larger multicellular animals. Magnetite crystals have been isolated from bees and termites – and are apparently synthesized by them. However, in larger animals it’s harder to tell – as these crystals are tiny and difficult to find or visualize in vivo. An alternate magnetoreception mechanism based on photochemicals in the retina has been proposed for migratory birds – although a role for magnetite, particularly in pigeons which have relatively large concentrations of it in their beaks, can’t be ruled out.

Humans have traces of magnetite in their brains – although the court is still out on whether this gives us any capacity for direction finding by magnetoreception. Some research suggests a few individuals may have some very minor ability – but not enough for anyone to consider preferring this to their GPS.

Posted on by Tammy Plotner

Weekend SkyWatcher’s Forecast: July 2-4, 2010

Greetings, fellow SkyWatchers! Hopefully the rains have passed in your area and you’re ready for some dark skies and a double-dip… Double stars that is! This weekend we’ll take a look at some of the most colorful and interesting binary stars of the summer. Need more? Then hang tight as we take a look at one of the most concentrated globular clusters aroumd! Whenever you’re ready, I’ll see you in the backyard…

July 2, 2010 – This date marks the 1820 passing of British optician Peter Dollond, inventor of the triple achromatic lens. Dollond’s improvements to the refracting telescope included placing convex lenses of crown glass on either side of a biconcave flint glass lens to make the achromatic triplet lens we know today!

Now turn binoculars or telescopes toward magnitude 2.7 Alpha Librae, the second brightest star in the celestial ‘‘Scales.’’ Its proper name is Zuben El Genubi, and, as Star Wars as that sounds, the ‘‘Southern Claw’’ is actually quite close to home at a distance of only 65 light-years. No matter what size optics you are using, you’ll easily see Alpha’s widely spaced 5th magnitude companion, which shares the same proper motion. Alpha itself is a spectroscopic binary, as was verified during an occultation event, and its inseparable companion is only a half-magnitude dimmer according to the light curves. Enjoy this easy pair tonight!

July 3, 2010 – Tonight let’s go deep south and have look at an area that once held something almost half a bright as tonight’s later Moon and over four times brighter than Venus. Only one thing could light up the skies like that—a supernova.

According to historical records from Europe, China, Egypt, Arabia, and Japan, 1,003 years ago the very first supernova event was noted. Appearing in the constellation of Lupus, it was at first believed to be a comet by the Egyptians, yet the Arabs saw it as an illuminating ‘‘star.’’


Located less than a finger-width northeast of Beta Lupi (RA 15 02 48 Dec –41 54 42) and half a degree east of Kappa Centaurus, no visible trace is left of a once-grand event that spanned 5 months of observation, beginning in May and lasting until it dropped below the horizon in September 1006. It is believed that most of the star was converted to energy, and very little mass remains. In the area, a 17th magnitude star that shows a tiny gas ring and radio source 1459-41 remains our best candidate for pinpointing this incredible event.

Why you’re at it, try a challenging double star—Upsilon Librae (RA 15 37 01 Dec –28 08 06). This beautiful red star is right at the limit for a small telescope, but quite worthy, as the pair is a widely disparate double. Look for the 11.5-magnitude companion to the south in a very nice field of stars!

July 4. 2010 – Tonight let’s have a look at 400-lightyear-distant Rasalgethi—Alpha Herculis (RA 17 14 38 Dec +14 23 25). Known as the ‘‘Head of the Kneeling One,’’ it’s an easily resolved double and is noted for its fine color contrast. At magnitude 3.5, the variable bright primary is one of the largest known stars, with a diameter four times the Earth–Sun distance. Rasalgethi’s photospheric temperature is so low (3,000 Kelvin) that it barely glows a warm reddish orange. Meanwhile, its 5.4-magnitude companion is a yellow giant with a temperature twice the primary. The two together make Rasalgethi A seem a deeper red, while Rasalgethi B takes on a lovely yellow-green hue.

Need some fireworks? Then check out a single small globular—M80 (RA 16 17 02 Dec –22 58 30). Located about 4 degrees northwest of Antares (about two finger-widths), this little globular cluster is a powerpunch. Located in a region heavily obscured by dark dust, M80 will shine like an unresolvable star to small binoculars, but reveal itself to be one of the most heavily concentrated globulars in the telescope. Discovered within days of each other by Messier and Mechain, respectively, in 1781, this intense Class I globular cluster is around 36,000 light-years distant.


In 1860, M80 became the first globular cluster that was known to host a nova. As stunned scientists watched, a centrally located star brightened to magnitude 7 over a period of days, becoming known as T Scorpii. The event then dimmed more rapidly than expected, making observers wonder exactly what they had seen. Since most globular clusters’ stars are all about the same age, the hypothesis was put forward that perhaps they had witnessed an actual collision of stellar members. Given that the cluster contains more than a million stars, the probability is that some 2,700 collisions of this type may have occurred during M80’s lifetime.

Have a super weekend!

This week’s awesome images are: Zuben El Genubi, Field of SN1006, Upsilon Librae, Rasalgethi and M80. All done by Palomar Observatory, courtesy of Caltech. We thank you so much!

Posted on by Nancy Atkinson

Mysterious Giant Gas Ring Explained

he Leo ring: deep image in the optical domain with the distribution of the gas in HI in yellow-orange. The thumbnails on the right are a three of the dense areas of the ring with their optical counterparts. © CFHT/Astron - P.A. Duc

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From a Canada-France-Hawaii Telescope press release:

An international team unveiled the origin of the giant gas ring in the Leo group of galaxies. With the Canada-France-Hawaii Telescope, the scientists were able to detect an optical signature of the ring corresponding to star forming regions. This observation rules out the primordial nature of the gas, which is of galactic origin. Thanks to numerical simulations made at CEA, a scenario for the formation of this ring has been proposed: a violent collision between two galaxies, slightly more than one billion years ago. The results will be published in the Astrophysical Journal Letters.

In the current theories on galaxy formation, the accretion of cold primordial gas is a key-process in the early steps of galaxy growth. This primordial gas is characterized by two main features: it has never sojourned in any galaxy and it does not satisfy the conditions required to form stars. Is such an accretion process still ongoing in nearby galaxies? To answer the question, large sky surveys are undertaken attempting to detect the primordial gas.

The Leo ring, a giant ring of cold gas 650,000 light-years wide surrounding the galaxies of the Leo group, is one of the most dramatic and mysterious clouds of intergalactic gas. Since its discovery in the 80s, its origin and its nature were debated. Last year, studies of the metal abundances in the gas led to the belief that the ring was made of this famous primordial gas.

Thanks to the sensitivity of the Canada-France-Hawaii Telescope MegaCam camera, the international team observed for the first time the optical counterpart of the densest regions of the ring, in visible light instead of radio waves. Emitted by massive young stars, this light points to the fact that the ring gas is able to form stars.

A ring of gas and stars surrounding a galaxy immediately suggests another kind of ring: a so-called collisional ring, formed when two galaxies collide. Such a ring is seen in the famous Cartwheel galaxy. Would the Leo ring be a collisional ring too?

In order to secure this hypothesis, the team used numerical simulations (performed on supercomputers at CEA) to demonstrate that the ring was indeed the result of a giant collision between two galaxies more than 38 million light-years apart: at the time of the collision, the disk of gas of one of the galaxies is blown away and will eventually form a ring outside of the galaxy. The simulations allowed the identification of the two galaxies which collided: NGC 3384, one of the galaxies at the center of the Leo group, and M96, a massive spiral galaxy at the periphery of the group. They also gave the date of the collision: more than a billion years ago!

The gas in the Leo ring is definitely not primordial. The hunt for primordial gas is still open!

Posted on by Nancy Atkinson

Where In The Universe #110

It’s time once again for another Where In The Universe Challenge. Test your visual knowledge of the cosmos by naming where in the Universe this image was taken and give yourself extra points if you can name the spacecraft/telescope responsible for this picture. Post your guesses in the comments section, and check back on later at this same post to find the answer. To make this challenge fun for everyone, please don’t include links or extensive explanations with your answer. Good luck!

UPDATE: The answer has now been posted below.

This is a composite image from the Chandra X-Ray Telescope of one of the many star-forming regions in W3, called W3 Main. The green and blue represent lower and higher-energy X-rays, respectively, while red shows optical emission. There are hundreds of X-ray sources here, and these bright point-like objects are an extensive population of several hundred young stars, many of which were not found in earlier infrared studies.

Find out more about this image at the Chandra website.

Posted on by Nancy Atkinson

R Coronae Australis: A Cosmic Watercolor

The nearby star-forming region around the star R Coronae Australis imaged by the Wide Field Imager (WFI) on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile.

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From an ESO press release:

This magnificent view of the region around the star R Coronae Australis was created from images taken with the Wide Field Imager (WFI) at ESO’s La Silla Observatory in Chile. R Coronae Australis lies at the heart of a nearby star-forming region and is surrounded by a delicate bluish reflection nebula embedded in a huge dust cloud. The image reveals surprising new details in this dramatic area of sky.

The star R Coronae Australis lies in one of the nearest and most spectacular star-forming regions. This portrait was taken by the Wide Field Imager (WFI) on the MPG/ESO 2.2-metre telescope at the La Silla Observatory in Chile. The image is a combination of twelve separate pictures taken through red, green and blue filters.

This image shows a section of sky that spans roughly the width of the full Moon. This is equivalent to about four light-years at the distance of the nebula, which is located some 420 light-years away in the small constellation of Corona Australis (the Southern Crown). The complex is named after the star R Coronae Australis, which lies at the centre of the image. It is one of several stars in this region that belong to the class of very young stars that vary in brightness and are still surrounded by the clouds of gas and dust from which they formed.

The intense radiation given off by these hot young stars interacts with the gas surrounding them and is either reflected or re-emitted at a different wavelength. These complex processes, determined by the physics of the interstellar medium and the properties of the stars, are responsible for the magnificent colours of nebulae. The light blue nebulosity seen in this picture is mostly due to the reflection of starlight off small dust particles. The young stars in the R Coronae Australis complex are similar in mass to the Sun and do not emit enough ultraviolet light to ionise a substantial fraction of the surrounding hydrogen. This means that the cloud does not glow with the characteristic red colour seen in many star-forming regions.

The huge dust cloud in which the reflection nebula is embedded is here shown in impressively fine detail. The subtle colours and varied textures of the dust clouds make this image resemble an impressionist painting. A prominent dark lane crosses the image from the centre to the bottom left. Here the visible light emitted by the stars that are forming inside the cloud is completely absorbed by the dust. These objects could only be detected by observing at longer wavelengths, by using a camera that can detect infrared radiation.

R Coronae Australis itself is not visible to the unaided eye, but the tiny, tiara-shaped constellation in which it lies is easily spotted from dark sites due to its proximity on the sky to the larger constellation of Sagittarius and the rich star clouds towards the centre of our own galaxy, the Milky Way.

For more images and videos see this ESO webpage.

Posted on by Nancy Atkinson

Finding the Origin of Milky Way’s Ancient Stars

Simulation showing a Milky Way-like galaxy around five billion years ago, when most satellite galaxy collisions were happening. Credit: Andrew Cooper, John Helly (Durham University)

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From the Royal Astronomical Society

Many of the Milky Way’s ancient stars are remnants of other smaller galaxies torn apart by violent galactic collisions around five billion years ago, according to researchers at Durham University, who publish their results in a new paper in the journal Monthly Notices of the Royal Astronomical Society.

Scientists at Durham’s Institute for Computational Cosmology and their collaborators at the Max Planck Institute for Astrophysics, in Germany, and Groningen University, in Holland, ran huge computer simulations to recreate the beginnings of our Galaxy.

The simulations revealed that the ancient stars, found in a stellar halo of debris surrounding the Milky Way, had been ripped from smaller galaxies by the gravitational forces generated by colliding galaxies.

Cosmologists predict that the early Universe was full of small galaxies which led short and violent lives. These galaxies collided with each other leaving behind debris which eventually settled into more familiar looking galaxies like the Milky Way.

The researchers say their finding supports the theory that many of the Milky Way’s ancient stars had once belonged to other galaxies instead of being the earliest stars born inside the Galaxy when it began to form about 10 billion years ago.

Simulation showing the stellar halo of a Milky Way-like galaxy in the present day. Credit: Andrew Cooper (Durham University)

Lead author Andrew Cooper, from Durham University’s Institute for Computational Cosmology, said: “Effectively we became galactic archaeologists, hunting out the likely sites where ancient stars could be scattered around the galaxy.

“Our simulations show how different relics in the Galaxy today, like these ancient stars, are related to events in the distant past.

“Like ancient rock strata that reveal the history of Earth, the stellar halo preserves a record of a dramatic primeval period in the life of the Milky Way which ended long before the Sun was born.”

The computer simulations started from shortly after the Big Bang, around 13 billion years ago, and used the universal laws of physics to simulate the evolution of dark matter and the stars.

These simulations are the most realistic to date, capable of zooming into the very fine detail of the stellar halo structure, including star “streams” – which are stars being pulled from the smaller galaxies by the gravity of the dark matter.

One in one hundred stars in the Milky Way belong to the stellar halo, which is much larger than the Galaxy’s familiar spiral disk. These stars are almost as old as the Universe.

Professor Carlos Frenk, Director of Durham University’s Institute for Computational Cosmology, said: “The simulations are a blueprint for galaxy formation.

“They show that vital clues to the early, violent history of the Milky Way lie on our galactic doorstep.

“Our data will help observers decode the trials and tribulations of our Galaxy in a similar way to how archaeologists work out how ancient Romans lived from the artefacts they left behind.”

The research is part of the Aquarius Project, which uses the largest supercomputer simulations to study the formation of galaxies like the Milky Way and was partly funded by the UK’s Science and Technology Facilities Council (STFC).

Aquarius was carried out by the Virgo Consortium, involving scientists from the Max Planck Institute for Astrophysics in Germany, the Institute for Computational Cosmology at Durham University, UK, the University of Victoria in Canada, the University of Groningen in the Netherlands, Caltech in the USA and Trieste in Italy.

Durham’s cosmologists will present their work to the public as part of the Royal Society’s 350th anniversary ‘See Further’ exhibition, held at London’s Southbank Centre until July 4th.

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