Rethinking everything we thought we knew about star clusters

ESA — Gaia Mission patch.

28 February 2019

ESA’s Gaia satellite is on a mission: to map and characterise more than one billion of the stars in the Milky Way. Many of these stars reside in complex, eye-catching clusters scattered throughout our Galaxy and, by studying these stellar groupings, Gaia is revealing much about the formation and evolution of stars in our cosmic home and surroundings.

Image above: Gaia’s view of the sky. Image Credits: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO.

The Milky Way is full of stars. Our Galaxy contains over a hundred billion of them, from dwarf to giant, populating its crowded centre and its spiralling disc.

Many of these stars are thought to have formed in the same way: from huge clouds of cool, condensing molecular gas, which collapse under the influence of gravity and fragment to form groups of hundreds to thousands of stars, known as star clusters. Some of these clusters last thousands of millions of years, while others disperse rapidly, releasing their stellar residents into the Milky Way’s disc.

It is likely that also our Sun formed in a cluster some 4.5 billion years ago, and the quest for solar siblings – stars that were born in the same cluster as the Sun and then went on different paths – will provide important information on the birth of our parent star.

Despite our growing knowledge, many open questions remain. For instance, how many clusters exist, how many are currently being formed, how many are falling apart – and at what pace?

The incredible diversity of stars and their birth clusters is currently being explored by ESA’s Gaia satellite.

Launched in December 2013, Gaia aims to map the nearby cosmos and execute the most extensive census of stars ever performed, tracking the positions, motions, and properties of more than one billion stars in the Milky Way and its surroundings. So far, the mission has released two packages of data: Data Release 1 (DR1) on 14 September 2016, and Data Release 2 (DR2) on 25 April 2018.

Comparison between Gaias first and second data releases

Video above: Comparison between Gaia’s first (left) and second (right) data releases. Click here for details and large versions of the video. Video Credits: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO.

«The first release was planned as more of a test release than a full database, and the second is still early days for Gaia,» says Carme Jordi of the University of Barcelona, Spain, a member of the Gaia Science Team. «Nonetheless, these datasets have already offered us unique insights into the stars within our Galaxy, and in particular into stellar clusters.»

Gaia DR1 contained the positions and brightnesses of 1.1 billion stars, and the parallaxes (a way of measuring distance) and proper motions (movement through the sky) for a subset of two million. Gaia DR2 raises these figures to nearly 1.7 billion stars in terms of positions and brightnesses, over 1.3 billion in terms of parallax and proper motion, and adds new data about stellar colours, line-of-sight velocities, surface temperatures, variability, radii, luminosities, and more.

With Gaia DR2, the mission has provided scientists with new tools to look at star clusters in the Sun’s neighbourhood and beyond.

New clusters unveiled

Alfred Castro-Ginard and colleagues used a statistical method on a subset of Gaia DR1 to discover 21 nearby clusters that had previously gone unnoticed, confirming their findings using the full DR2 data. So far, even if not unanimously, scientists had generally thought that all such clusters out to distances between 3200 and 6500 light-years from Earth had been identified – but this study suggests that there is still much to discover, even in our cosmic neck of the woods.

«It’s worth noting that this study only looked at a small part of the sky,» explains Jordi. «The discovery of new nearby clusters, which should be the easiest to detect, indicates that our knowledge of these clusters is really quite incomplete out to greater distances.»

Image above: Parallaxes in Gaia’s sky. Image Credits: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO.

This finding was soon followed and supported by a different study, led by Tristan Cantat-Gaudin, which re-analysed every previously reported cluster, known and putative alike, using data from Gaia DR2. This research confirmed earlier detections of about 1200 clusters, and determined their average distance and overall motion.

The scientists also serendipitously discovered 60 new potential clusters, but also surprisingly discarded clusters that had previously been identified. The study revealed that many such clusters are actually overlapping groups consisting of more than one cluster, while others are just asterisms – apparent patterns or groupings of stars that are in fact tricks of perspective on the two-dimensional sky.

Another study by Cantat-Gaudin and collaborators focussed on a nearby stellar group known as Vela OB2, which is somewhat looser than an ordinary cluster. Gaia’s precision allowed the scientists to study stellar motions within Vela OB2 in great detail, revealing that it comprises multiple smaller clumps of stars and that the overall complex is expanding. They used the data to unravel the history of this stellar group, which is associated with a large, expanding nearby shell of gas known as the IRAS Vela Shell and thought to have originated in a supernova explosion: according to Gaia, the powerful event that triggered this shell also sparked the formation of Vela OB2’s stars over 10 million years ago.

«Gaia’s extraordinary data are allowing us to revisit our existing stellar census and confirm, discard, and discover clusters,» says Jordi, who is a co-author on all three studies. «This is invaluable in helping us to characterise the distribution of clusters throughout the Milky Way’s disc, and their height in relation to the Galactic plane.»

Tracing clusters across the Galaxy

On a broader Galactic scale, the new cluster counts that scientist have started to put together with Gaia seems to indicate that clusters sitting high above the plane of the Milky Way are old, and located further from our Galaxy’s centre.

«It seems there aren’t old clusters at high altitudes in the inner part of the Galactic disc, so they must have dissolved – just as our models predict,» explains Jordi.

Looking specifically at clusters in higher-altitude areas of our Galaxy’s disc by combining positions and motions on the sky from Gaia DR2 with line-of-sight velocities from a ground-based survey, Janez Kos and colleagues discarded the existence of four out of the five analysed clusters.

In a different study, Caroline Soubiran and colleagues used line-of-sight velocities from Gaia DR2 to explore the kinematics of 861 stellar clusters, and found them to follow the velocity distribution of field stars – stars that are not associated to any cluster – in the solar neighbourhood.

Precise data from Gaia DR2 also contributed to revealing the ongoing evaporation of the nearest cluster to the Sun, the Hyades, in two independent studies led by Stefan Meingast and Siegfried Röser, respectively. This cluster was found to boast two well-defined tidal tails containing hundreds of stars, each extending from the cluster’s core in a distinct ‘S’ shape.

The Hyades cluster

Video above: Flythrough the Gaia data, towards the Hyades cluster. Click here for details and large versions of the video. Video Credits: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO.

«This unique finding opens a new window for studying how clusters, through their gradual demise under the influence of the Milky Way’s gravity, continuously feed the Galactic disc with stars,» says Jordi.

Clusters as stellar test-beds

Star clusters are not only tracers of how the disc of our Galaxy has evolved over time, but also excellent laboratories for studying stellar physics. With its unprecedented data, Gaia has started to reveal previously unseen details that have an impact on our understanding of the formation and evolution of stars.

By plotting the colour of stars against their brightness, astronomers have been using the so-called Hertzsprung-Russell (HR) diagram to study the evolution of stellar populations for over a century. In this diagram, most stars lie along a top-left to bottom-right diagonal line known as the ‘main sequence’ – which identifies stars in their prime, burning hydrogen fuel in their cores – while stars in later stages of their lives are found away from this sequence.

The Hertzsprung-Russell diagram

Video above: The Hertzsprung-Russell diagram. Click here for details and large versions of the video. Video Credits: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO.

In clusters, which were historically thought to contain a single, simple population of stars that formed all at the same time, the position in the diagram where the main sequence ‘turns off’ was customarily used to estimate the age of that particular stellar population. However, in recent years, scientists had found evidence that clusters may comprise more than one population of stars, based on the observation of multiple turn-off points in their HR diagrams.

Gaia is now calling for a rethink of this phenomenon, as several studies based on its latest dataset seem to indicate how the multiple turn-off points can be explained without invoking several populations of stars, but rather by properly including the effects of the rotation of cluster stars on their predicted colours. For example, Anna Marino and colleagues conducted two studies using photometric data from Gaia DR2 to suggest that this is a common feature in Milky Way clusters, while Beomdu Lim and colleagues found a similar result by combining Gaia proper motions with ground-based spectroscopic observations of M11, also known as the ‘Wild Duck’ cluster. Similarly, Giacomo Cordoni and colleagues used a mix of stellar velocity, photometry, and proper motion data to confirm that rotational effects of stars can affect their estimated colours and lifespans.

Gaia. Animation Credit: ESA

This is certainly not the final word on stellar clusters, and many more studies will follow in coming years and clarify, reopen, and perhaps even deepen these intriguing topics. Like every great mission or experiment, Gaia is providing scientists with precise and plentiful data that are increasing our understanding of many astronomical problems, giving rise to new and more profound questions as our knowledge grows.

«Gaia is unique and revolutionising all fields of astrophysics, with this recent research on star clusters being a good example,» says Jos de Bruijne, Gaia deputy project scientist at ESA.

«The mission has given us precise measurements of how stars move through space and their distances from us… and it has done this for over one billion stars. We’ve never had anything like this database before, and it’s invaluable in helping us study our Galaxy.

«The recent star-cluster discoveries make this a hugely exciting area of research – especially as we have more Gaia data releases to look forward to in coming years.»

More information:

ESA’s Gaia satellite was launched in 2013 to create the most precise three-dimensional map of more than one billion stars in the Milky Way. The mission has released two lots of data so far: Gaia Data Release 1 on 14 September 2016, and Gaia Data Release 2 on 25 April 2018. More releases will follow in coming years.

Related links:

Gaia Data Release 1: http://sci.esa.int/gaia/58275-data-release-1/

Gaia Data Release 2: http://sci.esa.int/gaia/60243-data-release-2/

Hertzsprung-Russell (HR) diagram: http://sci.esa.int/gaia-stellar-family-portrait/

ESA Gaia: http://sci.esa.int/gaia/

Images (mentioned), Videos (mentioned), Animation (mentioned), Text, Credit: European Space Agency (ESA).

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2018 excavations of the Pafos Agora Project completed

The Department of Antiquities, Ministry of Transport, Communications and Works, announced the completion of the 2018 excavations of the Department of Classical Archaeology of the Institute of Archaeology, Jagiellonian University, Kraków, Poland, within the framework of the Pafos Agora Project (pap). The Project aims to explore and study the Agora of the ancient city of Nea Pafos, the capital of Cyprus in the Hellenistic and Roman periods. The excavations are conducted under the direction of Professor Ewdoksia Papuci-Władyka.

2018 excavations of the Pafos Agora Project completed
Credit: Department of Antiquities, Republic of Cyprus

The first stage of the 2018 campaign was a study season, which took place between April and May 2018, with a small group of researchers, as well as a photographer and an illustrator. The excavation campaign was conducted between the 8th of August and the 23rd of September 2018 in various trenches in the Agora area (T.II and four Trial Trenches (TT): TT.III and TT.IV – a continuation from 2017, as well as TT.V and TT.VI). Trench TT.V was initiated south of the Agora to verify the results of the geomagnetic investigations conducted in 2017.
These prospections suggested that the South Portico of the Agora was a complex building consisting of at least three rows of rooms. TT.V was established at the intersection of two hypothetical lines of the building’s walls. In T.II, the area investigated during the previous season next to the eastern entrance to the Agora, was extended towards the east, in order to verify the hypothesis of the double-line design of the East Portico.

In the extended area, the remains of the foundation of the eastern wall of the portico were discovered, thus confirming this theory. Investigations in T.II also involved the northern part of the East Portico where a new, room (R.22) was discovered with an open channel inside (of an unknown, as yet, function). It can probably be dated to the age of Emperor Augustus. Further to the north, a well (S.233) dating to the early Roman period, adjacent to R.22, was uncovered and excavated. Another well was also investigated (S.144) in R.8 of the East Portico in 2012. This well contained human remains, along with stones, pottery sherds and other objects (as was the case with Well S.50 in T.I, dated ca. 150 years earlier).

2018 excavations of the Pafos Agora Project completed
Credit: Department of Antiquities, Republic of Cyprus

In TT.III, situated in the so-called fosse (moat) outside the NW city gate, the aim of the investigations was two-fold: firstly, to verify the geomagnetic surveys which indicated the presence of parallel masonry structures, presumably connected with the second harbour of Pafos. Secondly, to check the geoarchaeological drillings that indicated sedimentation layers (up to 60 cm). Both geomagnetic and geoarchaeological data have turned out to be incorrect.
The layers with archaeological material were very thick, the bedrock was reached at a depth of ca. 2.5m, and the abovementioned masonry structures were not found. The bedrock was found to have been worked, so it has once more been confirmed that this area had been used as quarry (as was the case with the area revealed in TT.I and TT.II during the previous year). The question therefore of the features of this moat (or fosse), remains open and requires further research. However, it is crucial to understand the geomorphological processes that led (as in the area further to the north, where TT.I and TT.II are located,) to this extensive area being covered with a thick layer of sand.

During the 2018 excavations, investigations were also carried out in the limits of TT. IV, comprised mainly of a lime kiln, probably dating to the Late Roman or Early Byzantine period. It was decided to extend the area under investigation to the west in order to reveal the inlet channel. Preliminary investigations indicated that the kiln must have been partially built into the slope. The construction of the furnace seems to have destroyed a very large wall running east-west, preserved in the form of a single line of stone blocks located directly on the bedrock. The wall’s execution is extremely meticulous. It is therefore possible that it is a remnant of the city wall, perhaps even from the Hellenistic period. However, further research is required in order to confirm the above.

2018 excavations of the Pafos Agora Project completed
Credit: Department of Antiquities, Republic of Cyprus

The second trial trench, TT.VI, is located northeast of the Agora, on the hypothetical course of one of the city’s main streets, as evidenced by the results of the geomagnetic research. Investigations, however, are at their initial stages since only the modern strata have been explored. Geophysical prospection and geoarchaeological research also continued, as well as the conservation works on the finds (on coins and metal objects and also on Roman shoes found in 2016).
In summary, the most important discovery of the 2018 season was the evidence that demonstrates that the Agora was much larger in than had been previously assumed. The finds from both TT.V and T.II indicate that the stoai associated with the Agora were much larger than originally thought: the East Portico was double the size and the South Portico — triple. The entire Agora, with its accompanying structures, can now be described as square, its sides measuring around 160 metres and not 97 m, as suggested by scholars in the past. It seems to have occupied an area of about 2.5 ha, and not 1 ha, as assumed previously.

This places the Pafos Agora amongst some of the largest of its kind in the Hellenistic world. Secondly, in light of the team’s research, the interpretation of the architectural complexes of the so-called Asklepieion and Odeon, unveiled in the western part of Agora in the 1970s, should be revisited.

Source: Department of Antiquities, Republic of Cyprus [February 28, 2019]

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Researchers discover oldest tattoo tool in western North America

Washington State University archaeologists have discovered the oldest tattooing artifact in western North America.

Researchers discover oldest tattoo tool in western North America
Close up of a 2,000-year-old cactus spine tattoo tool discovered by WSU archaeologist
Andrew Gillreath-Brown [Credit: Bob Hubner/WSU]

With a handle of skunkbush and a cactus-spine business end, the tool was made around 2,000 years ago by the Ancestral Pueblo people of the Basketmaker II period in what is now southeastern Utah.

Andrew Gillreath-Brown, an anthropology PhD candidate, chanced upon the pen-sized instrument while taking an inventory of archaeological materials that had been sitting in storage for more than 40 years.

He is the lead author of a paper on the tattoo tool which was published in the Journal of Archaeological Science: Reports.

His discovery pushes back the earliest evidence of tattooing in western North America by more than a millennium and gives scientists a rare glimpse into the lives of a prehistoric people whose customs and culture have largely been forgotten.

«Tattooing by prehistoric people in the Southwest is not talked about much because there has not ever been any direct evidence to substantiate it,» Gillreath-Brown, 33, said. «This tattoo tool provides us information about past Southwestern culture we did not know before.»

Tattooing is an artform and mode of expression common to many indigenous cultures worldwide. However, little is known about when or why the practice began.

Researchers discover oldest tattoo tool in western North America
The 2,000-year-old cactus spine tattoo tool discovered by WSU archaeologist Andrew Gillreath-Brown
[Credit: Bob Hubner/WSU]

This is especially the case in places like the southwestern United States, where no tattoos have been identified on preserved human remains and there are no ancient written accounts of the practice.

Instead, archaeologists have relied on visual depictions in ancient artwork and the identification of tattoo implements to trace the origins of tattooing in the region.

Previously, bundled and hafted, or handled, cactus spine tattoo tools from Arizona and New Mexico provided the best archaeological examples of early tattoo implements from the Southwest. The earliest of these have been dated to between AD 1100-1280.

So when Gillreath-Brown came across a very similar looking implement from a site in Utah that is 1,000 years older, he knew he had found something special.

«When I first pulled it out of the museum box and realized what it might have been I got really excited,» said Gillreath-Brown, who himself wears a large sleeve tattoo of a turtle shell rattle, mastodon, water, and forest on his left arm.

The tool consists of a 3 ½ inch wooden skunkbush sumac handle bound at the end with split yucca leaves and holding two parallel cactus spines, stained black at their tips.

Researchers discover oldest tattoo tool in western North America
Close up of a 2,000-year-old cactus spine tattoo tool discovered by WSU archaeologist
Andrew Gillreath-Brown [Credit: Bob Hubner/WSU]

«The residue staining from tattoo pigments on the tip was what immediately piqued my interest as being possibly a tattoo tool,» Gillreath-Brown said.

Encouraged by Aaron Deter-Wolf, a friend and co-author of the study who had done ancient tattooing and edited several books on the subject, Gillreath-Brown analyzed the tips with a scanning electron microscope, X-ray florescence and energy dispersive ray spectroscopy. For good measure, he did several test tattoos using a replica on pig skin.

He saw the crystalline structure of pigment and determined it likely contained carbon, a common element in body painting and tattooing.

The find, said Gillreath-Brown, «has a great significance for understanding how people managed relationships and how status may have been marked on people in the past during a time when population densities were increasing in the Southwest.»

Author: Will Ferguson | Source: Washington State University [February 28, 2019]

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