ALMAGAL survey seeks to uncover the origin of stars

Collage of many of the young star clusters observed with ALMA as part of the ALMAGAL survey.
Credit: ALMA (ESO/NAOJ/NRAO)/S. Molinari et al.

The ALMA antennas observe the Milky Way high up in the Chajnantor Plateau in Chile’s Atacama Desert.
Credit: ESO/Y. Beletsky

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Stars are massive spheres of plasma, nuclear reactors that illuminate the universe. But where do they come from? We know they form in vast clouds of gas and dust, which collapse into smaller fragments. However, the details of how this happens remain unclear. A groundbreaking survey of 1,000 stellar nurseries is helping to answer this question.

Look up at the night sky and observe our galaxy, the Milky Way. Among the countless stars are clouds of gas and dust that work tirelessly like true factories to create new stars.

Much like some of the factories on Earth, these clouds use simple building blocks (hydrogen, helium, and small amounts of heavier elements) to make more complex pieces like stars. However, not all factories operate the same way. Some produce stars at different rates, with varying masses and compositions. What is happening behind the scenes?

Astronomers have long studied individual stellar nurseries, but how do their findings apply on a larger scale? We need a broader view to develop a universal model of star formation. That is where ALMAGAL comes in.

A Thousand Star-Forming Regions Under the Lens

The ALMAGAL survey, using the Atacama Large Millimeter/submillimeter Array (ALMA), examines more star-forming regions than ever before: three to four times more than all previous surveys combined, with remarkable detail.

The survey has observed 6,000 cores and 800 clumps, revealing key insights:

• Not all star-forming regions are the same.
• More material leads to higher star production. Massive clumps produce more and larger cores, fueling star growth.
• Clumps evolve over time. Initially circular, they become more intricate as they fragment into cores. Some massive clumps remain unbroken, likely because they are still young.

What’s Next?

While ALMAGAL has already deepened our understanding of star birth, many questions remain. Researchers now aim to explore how the gas flows from clumps into cores and how newborn stars influence this process. Thanks to ALMAGAL’s extensive dataset, we are closer than ever to unraveling the secrets of star formation and even the origins of planetary systems.

Additional information

The description of the ALMAGAL survey and the first results are described in the following papers published in Astronomy & Astrophysics:

• ALMAGAL I. The ALMA evolutionary study of high-mass protocluster formation in the galaxy. Presentation of the survey and early results
• ALMAGAL II. The ALMA evolutionary study of high-mass protocluster formation in the galaxy. ALMA data processing and pipeline
• ALMAGAL III. Compact source catalog: Fragmentation statistics and physical evolution of the core population

This post is based on the original published by the European Southern Observatory (ESO), an ALMA partner on behalf of Europe.

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Southern Observatory (ESO), the U.S. National Science Foundation (NSF), and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science and Technology Council (NSTC) in Taiwan, and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of ALMA's construction, commissioning, and operation.

Source: ALMA (Atacama Large Millimeter/submillimeter Array)/Science Blog

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Contacts:

Nicolás Lira
Education and Public Outreach Coordinator
Joint ALMA Observatory, Santiago - Chile
Phone: +56 2 2467 6519
Cel: +56 9 9445 7726
Email: nicolas.lira@alma.cl

Bárbara Ferreira
ESO Media Manager
Garching bei München, Germany
Phone: +49 89 3200 6670
Email: press@eso.org

Jill Malusky
Public Information Officer
NRAO
Phone: +1 304-456-2236
Email: jmalusky@nrao.edu

Yuichi Matsuda
ALMA EA-ARC Staff Member
NAOJ
Email: yuichi.matsuda@nao.ac.jp

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NASA's Webb Peers into the Extreme Outer Galaxy

Digel Cloud 2S

NASA’s James Webb Space Telescope observed the outskirts of our Milky Way galaxy. Known as the Extreme Outer Galaxy, this region is located more than 58,000 light-years from the Galactic Center.

To learn more about how a local environment affects the star formation process within it, a team of scientists directed the telescope’s NIRCam (Near-Infrared Camera) and MIRI (Mid-Infrared Instrument) toward a total of four star-forming areas within Digel Clouds 1 and 2: 1A, 1B, 2N, and 2S.

In the case of Cloud 2S, shown here, Webb revealed a luminous main cluster that contains newly formed stars. Several of these young stars are emitting extended jets of material from their poles. To the main cluster’s top right is a sub-cluster of stars, a feature that scientists previously suspected to exist but has now been confirmed with Webb. Additionally, the telescope revealed a deep sea of background galaxies and red nebulous structures that are being carved away by winds and radiation from nearby stars.

Compass Image of Digel Cloud 2S

Annotated image of Digel Cloud 2S captured by Webb's NIRCam (Near-Infrared Camera) and MIRI (Mid-Infrared Instrument), with compass arrows, a scale bar, color key, and graphic overlays for reference.

The north and east compass arrows show the orientation of the image on the sky. Note that the relationship between north and east on the sky (as seen from below) is flipped relative to direction arrows on a map of the ground (as seen from above).

The scale bar is labeled in light-years and arcseconds. One light-year is equal to about 5.88 trillion miles or 9.46 trillion kilometers. One arcsecond is equal to 1/3600 of one degree of arc. (The full Moon has an angular diameter of about 0.5 degrees.) The actual size of an object that covers one arcsecond on the sky depends on its distance from the telescope.

This image shows invisible near- and mid-infrared wavelengths of light that have been translated into visible-light colors. The color key shows which NIRCam and MIRI filters were used when collecting the light. The color of each filter name is the visible light color used to represent the infrared light that passes through that filter.

In the main cluster are five white arrows, which highlight the paths of five protostar jets. To learn more about how a local environment affects the star formation process within it, a team of scientists directed the telescope’s NIRCam (Near-Infrared Camera) and MIRI (Mid-Infrared Instrument) toward a total of four star-forming areas within Digel Clouds 1 and 2: 1A, 1B, 2N, and 2S.

In the case of Cloud 2S, shown here, Webb revealed a luminous main cluster that contains newly formed stars. Several of these young stars are emitting extended jets of material from their poles. To the main cluster’s top right is a sub-cluster of stars, a feature that scientists previously suspected to exist but has now been confirmed with Webb. Additionally, the telescope revealed a deep sea of background galaxies and red nebulous structures that are being carved away by winds and radiation from nearby stars. Credits: Image: NASA, ESA, CSA, STScI, Michael Ressler (NASA-JPL)

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Astronomers have directed NASA’s James Webb Space Telescope to examine the outskirts of our Milky Way galaxy. Scientists call this region the Extreme Outer Galaxy due to its location more than 58,000 light-years away from the Galactic Center. (For comparison, Earth is approximately 26,000 light-years from the center.)

A team of scientists used Webb’s NIRCam (Near-Infrared Camera) and MIRI (Mid-Infrared Instrument) to image select regions within two molecular clouds known as Digel Clouds 1 and 2. With its high degree of sensitivity and sharp resolution, the Webb data resolved these areas, which are hosts to star clusters undergoing bursts of star formation, in unprecedented detail. Details of this data include components of the clusters such as very young (Class 0) protostars, outflows and jets, and distinctive nebular structures.

These Webb observations, which came from telescope time allocated to Mike Ressler of NASA’s Jet Propulsion Laboratory in California, are enabling scientists to study star formation in the outer Milky Way in the same depth of detail as observations of star formation in our own solar neighborhood.

“In the past, we knew about these star forming regions but were not able to delve into their properties,” said Natsuko Izumi of Gifu University and the National Astronomical Observatory of Japan, lead author of the study. “The Webb data builds upon what we have incrementally gathered over the years from prior observations with different telescopes and observatories. We can get very powerful and impressive images of these clouds with Webb. In the case of Digel Cloud 2, I did not expect to see such active star formation and spectacular jets.”

Stars in the Making

Although the Digel Clouds are within our galaxy, they are relatively poor in elements heavier than hydrogen and helium. This composition makes them similar to dwarf galaxies and our own Milky Way in its early history. Therefore, the team took the opportunity to use Webb to capture the activity occurring in four clusters of young stars within Digel Clouds 1 and 2: 1A, 1B, 2N, and 2S.

For Cloud 2S, Webb captured the main cluster containing young, newly formed stars. This dense area is quite active as several stars are emitting extended jets of material along their poles. Additionally, while scientists previously suspected a sub-cluster might be present within the cloud, Webb’s imaging capabilities confirmed its existence for the first time.

“We know from studying other nearby star-forming regions that as stars form during their early life phase, they start emitting jets of material at their poles,” said Ressler, second author of the study and principal investigator of the observing program. “What was fascinating and astounding to me from the Webb data is that there are multiple jets shooting out in all different directions from this cluster of stars. It’s a little bit like a firecracker, where you see things shooting this way and that.”

The Saga of Stars

The Webb imagery skims the surface of the Extreme Outer Galaxy and the Digel Clouds, and is just a starting point for the team. They intend to revisit this outpost in the Milky Way to find answers to a variety of current mysteries, including the relative abundance of stars of various masses within Extreme Outer Galaxy star clusters. This measurement can help astronomers understand how a particular environment can influence different types of stars during their formation.

“I’m interested in continuing to study how star formation is occurring in these regions. By combining data from different observatories and telescopes, we can examine each stage in the evolution process,” said Izumi. “We also plan to investigate circumstellar disks within the Extreme Outer Galaxy. We still don’t know why their lifetimes are shorter than in star-forming regions much closer to us. And of course, I’d like to understand the kinematics of the jets we detected in Cloud 2S.”

Though the story of star formation is complex and some chapters are still shrouded in mystery, Webb is gathering clues and helping astronomers unravel this intricate tale.

These findings have been published in the Astronomical Journal.

The observations were taken as part of Guaranteed Time Observation program 1237.

The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).

Source: NASA's James Webb Space Telescope/News

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About This Release

Credits:

Media Contact:

Abigail Major
Space Telescope Science Institute, Baltimore, Maryland

Christine Pulliam
Space Telescope Science Institute, Baltimore, Maryland

Permissions: Content Use Policy

Contact Us: Direct inquiries to the News Team.

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Dwarf galaxies stripped of their stars prove to be the missing link in the formation of rare ultra-compact dwarf galaxies

This illustration shows a dwarf galaxy in the throes of transitioning to an ultra-compact dwarf galaxy as it’s stripped of its outer layers of stars and gas by a nearby larger galaxy. Ultra-compact dwarf galaxies are among the densest stellar groupings in the Universe. Being more compact than other galaxies with similar mass, but larger than star clusters — the objects they most closely resemble — these mystifying objects have defied classification. The missing piece to this puzzle has been a lack of sufficient transitional, or intermediate objects to study. A new galaxy survey, however, fills in these missing pieces to show that many of these enigmatic objects are likely formed from the destruction of dwarf galaxies. Credit:NOIRLab/NSF/AURA/M. Zamani, download Large JPEG

A continuum of galaxies captured at different stages of the transformation process from a dwarf galaxy to an ultra-compact dwarf galaxy (UCD). These objects are located near the supergiant elliptical galaxy M87, the dominant member of the neighboring Virgo Cluster. Credit: NOIRLab/NSF/AURA/NASA/R. Gendler/K. Wang. download Large JPEG

NGC 3628 and an example of an ultra-compact dwarf galaxy (no annotations)

ANGC 3628, sometimes nicknamed the Hamburger Galaxy or Sarah's Galaxy, is an unbarred spiral galaxy about 35 million light-years away in the constellation Leo. Extending to the left of NGC 3628 for around 300,000 light-years is a ‘tidal tail’ — an elongated region of stars that arises as a result of gravitational interaction with another galaxy. Embedded within this tidal tail is the ultra-compact dwarf galaxy known as NGC 3628-UCD1. Credit: CTIO/NOIRLab/DOE/NSF/AURA. Image processing: T.A. Rector (University of Alaska Anchorage/NSF’s NOIRLab), M. Zamani (NSF’s NOIRLab), & D. de Martin (NSF’s NOIRLab). download Large JPEG

NGC 3628 and an example of an ultra-compact dwarf galaxy (annotated)

NGC 3628, sometimes nicknamed the Hamburger Galaxy or Sarah's Galaxy, is an unbarred spiral galaxy about 35 million light-years away in the constellation Leo. Extending to the left of NGC 3628 for around 300,000 light-years is a ‘tidal tail’ — an elongated region of stars that arises as a result of gravitational interaction with another galaxy. Embedded within this tidal tail is the ultra-compact dwarf galaxy known as NGC 3628-UCD1. Credit: CTIO/NOIRLab/DOE/NSF/AURA. Image processing: T.A. Rector (University of Alaska Anchorage/NSF’s NOIRLab), M. Zamani (NSF’s NOIRLab), & D. de Martin (NSF’s NOIRLab).  download Large JPEG

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Astronomers using the Gemini North telescope, one half of the International Gemini Observatory operated by NSF’s NOIRLab, have captured the eroding remains of more than 100 dwarf galaxies as they transition into ultra-compact dwarf galaxies, objects with masses much greater than star clusters yet much smaller than dwarf galaxies. These findings confirm that many ultra-compact dwarf galaxies are likely the fossil remains of normal dwarf galaxies that have been stripped of their outer layers.

Ultra-compact dwarf galaxies (UCDs) are among the densest stellar groupings in the Universe. Being more compact than other galaxies with similar mass, but larger than star clusters — the objects they most closely resemble — these mystifying objects have defied classification. The missing piece to this puzzle has been a lack of sufficient transitional, or intermediate objects to study. A new galaxy survey, however, fills in these missing pieces to show that many of these enigmatic objects are likely formed from the destruction of dwarf galaxies.

The idea that UCDs are remnants of disrupted dwarf galaxies has been proposed since they were discovered over two decades ago. However, previous searches have not revealed the large population of galaxies-in-transition that you would expect to find. So an international team of astronomers conducted a systematic search for these intermediate-stage objects around the Virgo Cluster, a grouping of thousands of galaxies in the direction of the constellation Virgo. Using the Gemini North telescope near the summit of Maunakea in Hawaiʻi, the team identified more than 100 of these missing-link galaxies that show every stage of the transformation process.

“Our results provide the most complete picture of the origin of this mysterious class of galaxy that was discovered nearly 25 years ago,” said NOIRLab astronomer Eric Peng, a co-author on the paper describing these results appearing in the journal Nature. “Here we show that 106 small galaxies in the Virgo cluster have sizes between normal dwarf galaxies and UCDs, revealing a continuum that fills the ‘size gap’ between star clusters and galaxies.”

The team compiled their sample by first looking at images from the Next Generation Virgo Cluster Survey, taken with the Canada-France-Hawaiʻi Telescope. And though they were able to identify hundreds of candidate UCD progenitors, they were unable to confirm their true nature. The obstacle was that UCDs that are surrounded by envelopes of stars are indistinguishable from normal galaxies that are located farther away beyond the Virgo Cluster.

To distinguish the candidate UCD progenitors from the background galaxies, the team performed follow-up spectroscopic studies with Gemini North to obtain more concrete measurements of their distances. These observations allowed the astronomers to eliminate all of the background galaxies from their samples until only the UCDs within the Virgo Cluster remained.

Scattered among this vast survey are many dwarf galaxies that contain ultra-compact central star clusters. These galaxies represent the early stages of the transformation process and suggest that after neighboring massive galaxies strip these dwarfs of their outer layers of stars and gas, what remains will be an object identical to the late-stage UCDs that have already been identified.

The researchers also found many objects with very extended and diffuse stellar envelopes around them, indicating that they are currently in the throes of transitioning as their stars and dark matter is stripped away. Within their extensive sample the team identified objects at several other stages of the evolutionary process that, when placed in sequence, tell a compelling story of the morphology of UCDs. Furthermore, nearly all the candidates were near to massive galaxies, suggesting that their local environment plays an important role in their formation.

“Once we analyzed the Gemini observations and eliminated all the background contamination, we could see that these transition galaxies existed almost exclusively near the largest galaxies. We immediately knew that environmental transformation had to be important,” said Kaixiang Wang, a PhD student at Peking University and lead author of the paper.

Besides identifying the environment UCDs live in, these results also lend valuable insight into how many of these objects there are and what the full sequence of their evolutionary change looks like. “It’s exciting that we can finally see this transformation in action,” said Peng. “It tells us that many of these UCDs are visible fossil remnants of ancient dwarf galaxies in galaxy clusters, and our results suggest that there are likely many more low-mass remnants to be found,” he added.

“This study illustrates how large surveys can improve our understanding of the biggest questions in astronomy, like galaxy evolution,” says Chris Davis, NSF Program Director for NOIRLab. “NSF’s NOIRLab is a world leader in supporting astronomical surveys and — importantly — providing community and public access to the data and the amazing resulting discoveries.”

Source: Gemini Observatory

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More information

This research was presented in a paper appearing in Nature. DOI: 10.1038/s41586-023-06650-z

The team is composed of K. Wang (Peking University), E. W. Peng (NSF’s NOIRLab), C. Liu (Shanghai Jiao Tong University), J. Christopher Mihos (Case Western Reserve University), P. Côté (National Research Council of Canada), L. Ferrarese (National Research Council of Canada), M. Taylor (University of Calgary), J. P. Blakeslee (NSF’s NOIRLab), J. Cuillandre (Universite! Paris Diderot), P. Duc (Université de Strasbourg), P. Guhathakurta (University of California Santa Cruz), S. Gwyn (National Research Council of Canada), Y. Ko (Korea Astronomy and Space Science Institute), A. Lançon (Université de Strasbourg), S. Lim (Yonsei University), L. A. MacArthur (Princeton University), T. Puzia (Pontificia Universidad Católica de Chile), J. Roediger (National Research Council of Canada), L. V. Sales (University of California), R. Sanchez-Janssen (Royal Observatory Edinburgh), C. Spengler (Pontificia Universidad Católica de Chile), E. Toloba (University of the Pacific), H. Zhang (University of Science and Technology of China), & M. Zhu (Peking University).

NSF’s NOIRLab, the US center for ground-based optical-infrared astronomy, operates the International Gemini Observatory (a facility of NSF, NRC–Canada, ANID–Chile, MCTIC–Brazil, MINCyT–Argentina, and KASI–Republic of Korea), Kitt Peak National Observatory (KPNO), Cerro Tololo Inter-American Observatory (CTIO), the Community Science and Data Center (CSDC), and Vera C. Rubin Observatory (operated in cooperation with the Department of Energy’s SLAC National Accelerator Laboratory). It is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with NSF and is headquartered in Tucson, Arizona. The astronomical community is honored to have the opportunity to conduct astronomical research on Iolkam Du’ag (Kitt Peak) in Arizona, on Maunakea in Hawai‘i, and on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence that these sites have to the Tohono O’odham Nation, to the Native Hawaiian community, and to the local communities in Chile, respectively.

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Links

• Research Paper
• Photos of Gemini North
• Videos of Gemini North

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Contacts:

Eric Peng
NSF’s NOIRLab
Email: eric.peng@noirlab.edu

Kaixiang Wang
Peking University
Email: kaixiang.wang@pku.edu.cn

Josie Fenske
NSF’s NOIRLab Communications
Email: josie.fenske@noirlab.edu

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On the edge of the Lagoon On the edge of the Lagoon

NGC 6544
Credit: ESA/Hubble & NASA, W. Lewin, F. R. Ferraro

A cluster of stars in warm and cool colours. The whole view is filled with small stars, which become much denser and brighter around a core just right of centre. Most of the stars are small, but some are larger with a round, brightly-coloured glow and four sharp diffraction spikes. Behind the stars, a dark background can be seen

The teeming stars of the globular cluster NGC 6544 glisten in this image from the NASA/ESA Hubble Space Telescope. This cluster of tightly bound stars lies more than 8000 light-years away from Earth and is — like all globular clusters — a densely populated region of tens of thousands of stars.

This image of NGC 6544 combines data from two of Hubble’s instruments — the Advanced Camera for Surveys and Wide Field Camera 3 — as well as two separate astronomical observations. The first observation was designed to find a visible counterpart to the radio pulsar discovered in NGC 6544. A pulsar is the rapidly spinning remnant of a dead star, emitting twin beams of electromagnetic radiation like a vast astronomical lighthouse. This pulsar rotates particularly quickly, and astronomers turned to Hubble to help determine how this object evolved in NGC 6544.

The second observation which contributed data to this image was also designed to find the visible counterparts of objects detected at other electromagnetic wavelengths. Instead of matching up sources to a pulsar, however, astronomers used Hubble to search for the counterparts of faint X-ray sources. Their observations could help explain how clusters like NGC 6544 change over time.

NGC 6544 lies in the constellation Sagittarius, close to the vast Lagoon Nebula, a hazy labyrinth of gas and dust sculpted by the fierce winds of newly born stars. The Lagoon Nebula is truly colossal — even by astronomical standards — and measures 55 light-years across and 20 light-years from top to bottom. Previous Hubble images of the nebula incorporated infrared observations to reveal young stars and intricate structures that would be obscured at visible wavelengths by clouds of gas and dust.

Source: ESA/Hubble/potw

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NGC 3293: Chandra Sees Stellar X-rays Exceeding Safety Limits

NGC 3293
Credit: X-ray: NASA/CXC/Penn State Univ./K. Getman et al.;
Infrared: ESA/NASA JPL-Caltech/Herschel Space Observatory/JPL/IPAC; NASA JPL-Caltech/SSC/Spitzer Space Telescope;

JPEG (469.7 kb) - Large JPEG (5.2 MB) - Tiff (18.6 MB) - More Images

A Tour of NGC 3293 - More Animations

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Astronomers have made the most extensive study yet of how magnetically active stars are when they are young. This gives scientists a window into how X-rays from stars like the Sun, but billions of years younger, could partially or completely evaporate the atmospheres of planets orbiting them.

Many stars begin their lives in “open clusters,” loosely packed groups of stars with up to a few thousand members, all formed roughly at the same time. This makes open clusters valuable for astronomers investigating the evolution of stars and planets, because they allow the study of many stars of similar ages forged in the same environment.

A team of astronomers led by Konstantin Getman of Penn State University studied a sample of over 6,000 stars in 10 different open clusters with ages between 7 million and 25 million years. One of the goals of this study was to learn how the magnetic activity levels of stars like our Sun change during the first tens of millions of years after they form. Getman and his colleagues used NASA’s Chandra X-ray Observatory for this study because stars that have more activity linked to magnetic fields are brighter in X-rays.

This composite image shows one of those clusters, NGC 3293, which is 11 million years old and is located about 8,300 light-years from Earth in the Milky Way galaxy. The image contains X-rays from Chandra (purple) as well as infrared data from ESA’s Herschel Space Observatory (red), longer-wavelength infrared data from NASA’s retired Spitzer Space Telescope (blue and white), and optical data from the MPG/ESO 2.2-meter telescope at ESO’s La Silla Observatory in Chile appearing as red, white and blue.

The researchers combined the Chandra data of the stars’ activity with data from ESA’s Gaia satellite — not shown in the new composite image — to determine which stars are in the open clusters and which ones are in the foreground or background. The team identified nearly a thousand members of the cluster.

They combined their results for the open clusters with previously published Chandra studies of stars as young as 500,000 years old. The team found that the X-ray brightness of young, Sun-like stars is roughly constant for the first few million years, and then fades from 7 to 25 million years of age. This decrease happens more quickly for heftier stars.

To explain this decline in activity, Getman’s team used astronomers’ understanding of the interior of the Sun and Sun-like stars. Magnetic fields in such stars are generated by a dynamo, a process involving the rotation of the star as well as convection, the rising and falling of hot gas in the star's interior.

Around the age of NGC 3293, the dynamos of Sun-like stars become much less efficient because their convection zones become smaller as they age. For stars with masses smaller than that of the Sun, this is a relatively gradual process. For more massive stars, a dynamo dies away because the convection zone of the stars disappears.

How active a star is directly affects the formation processes of planets in the disk of gas and dust that surrounds all nascent stars. The most boisterous, magnetically active young stars quickly clear away their disks, halting the growth of planets.

This activity, measured in X-rays, also affects the potential habitability of the planets that emerge after the disk has disappeared. If a star is extremely active, as with many NGC 3293 stars in the Chandra data, then scientists predict it will blast planets in its system with energetic X-rays and ultraviolet light. In some cases, this high-energy barrage could cause an Earth-sized rocky planet to lose much of its original, hydrogen-rich atmosphere through evaporation within a few million years. It might also strip away a carbon dioxide-rich atmosphere that forms later, unless it is protected by a magnetic field. Our planet possesses its own magnetic field that prevented such an outcome for Earth.

A paper describing these results was published in the August issue of The Astrophysical Journal and is available online. Coauthors of the paper are Eric D. Feigelson and Patrick S. Broos from Penn State University, Gordon P. Garmire from the Huntingdon Institute for X-ray Astronomy, Michael A. Kuhn from the University of Hertsfordshire, Thomas Preibisch from Ludwig-Maximilians-Universitat, and Vladimir S. Airapetian from NASA’s Goddard Space Flight Center.

NASA's Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.

Tour: Chandra Sees Stellar X-rays Exceeding Safety Limits

Source: NASA’s Chandra X-ray Observatory

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Fast Facts for NGC 3293:

Scale: Image is about 22 arcmin (53 light-years) across.
Category: Normal Stars & Star Clusters
Coordinates (J2000): RA 10h 35m 52.8 | Dec -58° 13´ 52"
Constellation: Carina
Observation Date: October 7, 2015
Observation Time: 19 hours 41 minutes
Obs. ID: 16648
Instrument: ACIS
References: Getman, K.V., et al., 2022, ApJ, 935, 43; arXiv:2203.02047
Color Code: X-ray: purple; Infrared: red, blue, white; Optical: red, green, blue;
Distance Estimate: About 8,300 light-years

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Magnetized gas flows feed a young star cluster

Composite image of the Serpens South Cluster. Magnetic fields observed by SOFIA are shown as streamlines over an image from the Spitzer Space Telescope. SOFIA indicate that gravity can overcome some of the strong magnetic fields to deliver material needed for new stars. The magnetic fields have been dragged into alignment with the most powerful flows, as seen in the lower left where the streamlines are following the direction of the narrow, dark filament. This is accelerating the flow of material from interstellar space into the cloud, and fueling the collapse needed to spark star formation. © NASA/SOFIA/T. Pillai/J. Kauff

Polarimetric observations with SOFIA/HAWC+ show the orientation of magnetic field lines

Observations of magnetic fields in interstellar clouds made of gas and dust indicate that these clouds are strongly magnetized, and that magnetic fields influence the formation of stars within them. A key observation is that the orientation of their internal structure is closely related to that of the magnetic field.

To understand the role of magnetic fields, an international research team led by Thushara Pillai, Boston University & Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, observed the filamentary network of the dense gas surrounding a young star cluster in the solar neighboorhood, with the HAWC+ polarimeter on the airborne observatory SOFIA at infrared wavelengths. Their research shows that not all dense filaments are created equal. In some of the filaments the magnetic field succumbs to the flow of matter and is pulled into alignment with the filament. Gravitational force takes over in the denser parts of some filaments and the resulting weakly magnetized gas flow can feed the growth of young stellar clusters like a conveyor belt.

The results are published in this week’s issue of “Nature Astronomy“.

The interstellar medium is composed of tenuous gas and dust that fills the vast amount of emptiness between stars. Stretching across the Galaxy, this rather diffuse material happens to be a significant mass reservoir in Galaxies. An important component of this interstellar gas are the cold and dense molecular clouds which hold most of their mass in the form of molecular hydrogen. A major finding in the last decade has been that extensive network of filaments permeate every molecular cloud. A picture has emerged that stars like our own sun form preferentially in dense clusters at the intersection of filaments.

The researchers observed the filamentary network of dense gas around the Serpens South Cluster with HAWC+, a polarization-sensitive detector onboard the airborne observatory SOFIA, in order to understand the role of magnetic fields. Located about 1,400 light-years away from us, the Serpens South cluster is the youngest known cluster in the local neighborhood at the center of a network of dense filament.

The observations show that low–density gaseous filaments are parallel to the magnetic field orientation, and that their alignment becomes perpendicular at higher gas densities. The high angular resolution of HAWC+ reveals a further, previously unseen twist to the story. “In some dense filaments the magnetic field succumbs to the flow of matter and and is pulled into alignment with the filament”, says Thushara Pillai (Boston University and MPIfR Bonn), the first author of the publication. “Gravitational force takes over in the more opaque parts of certain filaments in the Serpens Star Cluster and the resulting weakly magnetized gas flow can feed the growth of young stellar clusters like a conveyor belt”, she adds.

It is understood from theoretical simulations and observations that the filamentary nature of molecular clouds actually plays a major role in channeling mass from the larger interstellar medium into young stellar clusters whose growth is fed from the gas. The formation and evolution process of stars is expected to be driven by a complex interplay of several fundamental forces — namely turbulence, gravity, and the magnetic field. In order to get an accurate description for how dense clusters of stars form, astronomers need to pin down the relative role of these three forces. Turbulent gas motions as well as the mass content of filaments (and therefore gravitation force) can be gauged with relative ease. However, the signature of the interstellar magnetic field is weak, also because it is about 10,000–times weaker than even our own Earth’s magnetic field. This has made measurements of magnetic field strengths in filaments a formidable task.

"The magnetic field directions in this new polarization map of Serpens South align well with the direction of gas flow along the narrow southern filament. Together these observations support the idea that filamentary accretion flows can help form a young star cluster”, adds Phil Myers from the Harvard-Smithsonian Center for Astrophysics, a co-author of the paper.

A small fraction of a molecular cloud’s mass is made up by small dust grains that are mixed into the interstellar gas. These interstellar dust grains tend to align perpendicular to the direction of the magnetic field. As a result, the light emitted by the dust grains is polarized — and this polarization can be used to chart the magnetic field directions in molecular clouds.

Recently, the Planck space mission produced a highly sensitive all–sky map of the polarized dust emission at wavelengths smaller than 1 mm. This provided the first large–scale view of the magnetization in filamentary molecular clouds and their environments. Studies done with Planck data found that filaments are not only highly magnetized, but they are coupled to the magnetic field in a predictable way. The orientation of the magnetic fields is parallel to the filaments in low–density environments. The magnetic fields change their orientation to being perpendicular to filaments at high gas densities, implying that magnetic fields play an important role relative in shaping filaments, compared to the influence of turbulence and gravity.

This observation pointed towards a problem. In order to form stars in gaseous filaments, the filaments have to lose the magnetic fields. When and where does this happen? With the order of magnitude higher angular resolution of the HAWC+ instrument in comparison to Planck it was now possible to resolve the regions in filaments where the magnetic filament becomes less important.

"Planck has revealed new aspects of magnetic fields in the interstellar medium, but the finer angular resolutions of SOFIA’s HAWC+ receiver and ground-based NIR polarimetry give us powerful new tools for revealing the vital details of the processes involved", says Dan Clemens, Professor and Chair of the Boston University Astronomy Department, another co-author.

"The fact that we were able to capture a critical transition in star formation was somewhat unexpected. This just shows how little is known about cosmic magnetic fields and how much exciting science awaits us from SOFIA with the HAWC+ receiver”, concludes Thushara Pillai.

SOFIA, the Stratospheric Observatory for Infrared Astronomy. The HAWC+ polarimeter onboard SOFIA was used for the observations of the magnetic field in the Serpens South region. @ NASA/C. Thomas.

Source: Max Planck Institute for Radio Astronomy/News

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Background Information

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a Boeing 747SP jetliner modified to carry a 106-inch diameter telescope. It is a joint project of NASA and the German Aerospace Center, DLR. NASA’s Ames Research Center in California’s Silicon Valley manages the SOFIA program, science and mission operations in cooperation with the Universities Space Research Association headquartered in Columbia, Maryland, and the German SOFIA Institute (DSI) at the University of Stuttgart. The aircraft is maintained and operated from NASA’s Armstrong Flight Research Center Building 703, in Palmdale, California.

The High-resolution Airborne Wideband Camera Plus (HAWC+), SOFIA’s newest instrument, +, uses far-infrared light to observe celestial dust grains, which align perpendicular to magnetic field lines. From these results, astronomers can infer the shape and direction of the otherwise invisible magnetic field. Far-infrared light provides key information about magnetic fields because the signal is not contaminated by emission from other mechanisms, such as scattered visible light and radiation from high-energy particles. The HAWC+ instrument was developed and delivered to NASA by a multi-institution team led by the Jet Propulsion Laboratory in Pasadena, California.

The research team comprises Thushara Pillai, Dan P. Clemens, Stefan Reissl, Philip C. Myers, Jens Kauffmann, Enrique Lopez-Rodriguez, Felipe de Oliveira Alves, Gabriel A. P. Franco, Jonathan Henshaw, Karl M. Menten, Fumitaka Nakamura, Daniel Seifried, Koji Sugitani, and Helmut Wiesemeyer. Thushara Pillai, the first author, and also Karl Menten and Helmut Wiesemeyer have an affiliation with the MPIfR.

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Contact

Dr. Thushara Pillai
Boston University, Boston, USA

Prof. Dr. Karl M. Menten
Direktor und Leiter der Forschungsabteilung "Millimeter- und Submillimeter-Astronomie"
Phone:+49 228 525-297
Max-Planck-Institut für Radioastronomie, Bonn

Dr. Norbert Junkes
Press and Public Outreach
Phone:+49 228 525-399
Max-Planck-Institut für Radioastronomie, Bonn

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Original Paper

Magnetized filamentary gas flows feeding the young embedded cluster in Serpens South
T. Pillai et al., Nature Astronomy (August 17, 2020). DOI: 10.1038/s41550-020-1172-6

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Links

Millimeter- und Submillimeter-Astronomie
Research Department "Millimeter and Submillimeter Astronomy" at MPIfR, Bonn, Germany

Boston University
Institute for Astrophysical Research, Boston University

SOFIA
Stratospheric Observatory for Infrared Astronomy (SOFIA), NASA Web Page

HAWC+
High-resolution Airborne Wideband Camera Plus (HAWC+)

DSI
German SOFIA Institute (DSI), Stuttgart

DLR - SOFIA
Deutsches Zentrum für Luft- und Raumfahrt (DLR) – SOFIA Web Page

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Mystery of the Universe’s Expansion Rate Widens With New Hubble Data

This is a ground-based telescope's view of the Large Magellanic Cloud, a satellite galaxy of our Milky Way. The inset image, taken by the Hubble Space Telescope, reveals one of many star clusters scattered throughout the dwarf galaxy. The cluster members include a special class of pulsating star called a Cepheid variable, which brightens and dims at a predictable rate that corresponds to its intrinsic brightness. Once astronomers determine that value, they can measure the light from these stars to calculate an accurate distance to the galaxy. When the new Hubble observations are correlated with an independent distance measurement technique to the Large Magellanic Cloud (using straightforward trigonometry), the researchers were able to strengthen the foundation of the so-called "cosmic distance ladder." This "fine-tuning" has significantly improved the accuracy of the rate at which the universe is expanding, called the Hubble constant. Credits: NASA, ESA, A. Riess (STScI/JHU) and Palomar Digitized Sky Survey. Hi-res image

Astronomers using NASA's Hubble Space Telescope say they have crossed an important threshold in revealing a discrepancy between the two key techniques for measuring the universe's expansion rate. The recent study strengthens the case that new theories may be needed to explain the forces that have shaped the cosmos.

A brief recap: The universe is getting bigger every second. The space between galaxies is stretching, like dough rising in the oven. But how fast is the universe expanding? As Hubble and other telescopes seek to answer this question, they have run into an intriguing difference between what scientists predict and what they observe.

Hubble measurements suggest a faster expansion rate in the modern universe than expected, based on how the universe appeared more than 13 billion years ago. These measurements of the early universe come from the European Space Agency's Planck satellite. This discrepancy has been identified in scientific papers over the last several years, but it has been unclear whether differences in measurement techniques are to blame, or whether the difference could result from unlucky measurements.

The latest Hubble data lower the possibility that the discrepancy is only a fluke to 1 in 100,000. This is a significant gain from an earlier estimate, less than a year ago, of a chance of 1 in 3,000.

These most precise Hubble measurements to date bolster the idea that new physics may be needed to explain the mismatch.

"The Hubble tension between the early and late universe may be the most exciting development in cosmology in decades," said lead researcher and Nobel laureate Adam Riess of the Space Telescope Science Institute (STScI) and Johns Hopkins University, in Baltimore, Maryland. "This mismatch has been growing and has now reached a point that is really impossible to dismiss as a fluke. This disparity could not plausibly occur just by chance."

Tightening the bolts on the 'cosmic distance ladder'

Scientists use a "cosmic distance ladder" to determine how far away things are in the universe. This method depends on making accurate measurements of distances to nearby galaxies and then moving to galaxies farther and farther away, using their stars as milepost markers. Astronomers use these values, along with other measurements of the galaxies' light that reddens as it passes through a stretching universe, to calculate how fast the cosmos expands with time, a value known as the Hubble constant. Riess and his SH0ES (Supernovae H0 for the Equation of State) team have been on a quest since 2005 to refine those distance measurements with Hubble and fine-tune the Hubble constant.

In this new study, astronomers used Hubble to observe 70 pulsating stars called Cepheid variables in the Large Magellanic Cloud. The observations helped the astronomers "rebuild" the distance ladder by improving the comparison between those Cepheids and their more distant cousins in the galactic hosts of supernovas. Riess's team reduced the uncertainty in their Hubble constant value to 1.9% from an earlier estimate of 2.2%.

As the team's measurements have become more precise, their calculation of the Hubble constant has remained at odds with the expected value derived from observations of the early universe's expansion. Those measurements were made by Planck, which maps the cosmic microwave background, a relic afterglow from 380,000 years after the big bang.

The measurements have been thoroughly vetted, so astronomers cannot currently dismiss the gap between the two results as due to an error in any single measurement or method. Both values have been tested multiple ways.

"This is not just two experiments disagreeing," Riess explained. "We are measuring something fundamentally different. One is a measurement of how fast the universe is expanding today, as we see it. The other is a prediction based on the physics of the early universe and on measurements of how fast it ought to be expanding. If these values don't agree, there becomes a very strong likelihood that we're missing something in the cosmological model that connects the two eras."

How the new study was done

Astronomers have been using Cepheid variables as cosmic yardsticks to gauge nearby intergalactic distances for more than a century. But trying to harvest a bunch of these stars was so time-consuming as to be nearly unachievable. So, the team employed a clever new method, called DASH (Drift And Shift), using Hubble as a "point-and-shoot" camera to snap quick images of the extremely bright pulsating stars, which eliminates the time-consuming need for precise pointing.

This illustration shows the three basic steps astronomers use to calculate how fast the universe expands over time, a value called the Hubble constant. All the steps involve building a strong "cosmic distance ladder," by starting with measuring accurate distances to nearby galaxies and then moving to galaxies farther and farther away. This "ladder" is a series of measurements of different kinds of astronomical objects with an intrinsic brightness that researchers can use to calculate distances. Among the most reliable for shorter distances are Cepheid variables, stars that pulsate at predictable rates that indicate their intrinsic brightness. Astronomers recently used the Hubble Space Telescope to observe 70 Cepheid variables in the nearby Large Magellanic Cloud to make the most precise distance measurement to that galaxy. Astronomers compare the measurements of nearby Cepheids to those in galaxies farther away that also include another cosmic yardstick, exploding stars called Type Ia supernovas. These supernovas are much brighter than Cepheid variables. Astronomers use them as "milepost markers" to gauge the distance from Earth to far-flung galaxies. Each of these markers build upon the previous step in the "ladder." By extending the ladder using different kinds of reliable milepost markers, astronomers can reach very large distances in the universe. Astronomers compare these distance values to measurements of an entire galaxy's light, which increasingly reddens with distance, due to the uniform expansion of space. Astronomers can then calculate how fast the cosmos is expanding: the Hubble constant. Credits: NASA, ESA and A. Feild (STScI). Hi-res image

Download Hubble Constant Infographic as PDF

"When Hubble uses precise pointing by locking onto guide stars, it can only observe one Cepheid per each 90-minute Hubble orbit around Earth. So, it would be very costly for the telescope to observe each Cepheid," explained team member Stefano Casertano, also of STScI and Johns Hopkins. "Instead, we searched for groups of Cepheids close enough to each other that we could move between them without recalibrating the telescope pointing. These Cepheids are so bright, we only need to observe them for two seconds. This technique is allowing us to observe a dozen Cepheids for the duration of one orbit. So, we stay on gyroscope control and keep 'DASHing' around very fast."

The Hubble astronomers then combined their result with another set of observations, made by the Araucaria Project, a collaboration between astronomers from institutions in Chile, the U.S., and Europe. This group made distance measurements to the Large Magellanic Cloud by observing the dimming of light as one star passes in front of its partner in eclipsing binary-star systems.

The combined measurements helped the SH0ES Team refine the Cepheids' true brightness. With this more accurate result, the team could then "tighten the bolts" of the rest of the distance ladder that extends deeper into space.

The new estimate of the Hubble constant is 74 kilometers (46 miles) per second per megaparsec. This means that for every 3.3 million light-years farther away a galaxy is from us, it appears to be moving 74 kilometers (46 miles) per second faster, as a result of the expansion of the universe. The number indicates that the universe is expanding at a 9% faster rate than the prediction of 67 kilometers (41.6 miles) per second per megaparsec, which comes from Planck's observations of the early universe, coupled with our present understanding of the universe.

So, what could explain this discrepancy?

One explanation for the mismatch involves an unexpected appearance of dark energy in the young universe, which is thought to now comprise 70% of the universe's contents. Proposed by astronomers at Johns Hopkins, the theory is dubbed "early dark energy," and suggests that the universe evolved like a three-act play.

Astronomers have already hypothesized that dark energy existed during the first seconds after the big bang and pushed matter throughout space, starting the initial expansion. Dark energy may also be the reason for the universe's accelerated expansion today. The new theory suggests that there was a third dark-energy episode not long after the big bang, which expanded the universe faster than astronomers had predicted. The existence of this "early dark energy" could account for the tension between the two Hubble constant values, Riess said.

Another idea is that the universe contains a new subatomic particle that travels close to the speed of light. Such speedy particles are collectively called "dark radiation" and include previously known particles like neutrinos, which are created in nuclear reactions and radioactive decays.

Yet another attractive possibility is that dark matter (an invisible form of matter not made up of protons, neutrons, and electrons) interacts more strongly with normal matter or radiation than previously assumed.

But the true explanation is still a mystery.

Riess doesn't have an answer to this vexing problem, but his team will continue to use Hubble to reduce the uncertainties in the Hubble constant. Their goal is to decrease the uncertainty to 1%, which should help astronomers identify the cause of the discrepancy.

The team's results have been accepted for publication in The Astrophysical Journal.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu

Adam Riess
Space Telescope Science Institute, Baltimore, Md.
and Johns Hopkins University, Baltimore, Md.
410-338-6707
ariess@stsci.edu

Claire Andreoli
NASA's Goddard Space Flight Center, Greenbelt, Md.
301-286-1940
claire.andreoli@nasa.gov

Editor: Rob Garner

Source: NASA/Hubble

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Cosmic Collisions: SOFIA Unravels the Mysterious Formation of Star Clusters

Illustration of a star cluster forming from the collision of turbulent molecular clouds, which appear as dark shadows in front of the background galactic star field.Credits: NASA/SOFIA/Lynette Cook. Hi-res image

Illustration of the molecular clouds surrounded by atomic envelopes, in green, which have been detected by SOFIA via emission from ionized carbon. The spatial offset and motions of these envelopes confirm predictions of simulations of cloud collisions.Credits: NASA/SOFIA/Lynette Cook. Hi-res image

The sun, like all stars, was born in a giant cold cloud of molecular gas and dust. It may have had dozens or even hundreds of stellar siblings – a star cluster – but these early companions are now scattered throughout our Milky Way galaxy. Although the remnants of this particular creation event have long since dispersed, the process of star birth continues today within our galaxy and beyond. Star clusters are conceived in the hearts of optically dark clouds where the early phases of formation have historically been hidden from view. But these cold, dusty clouds shine brightly in the infrared, so telescopes like the Stratospheric Observatory for Infrared Astronomy, SOFIA, can begin to reveal these long-held secrets. 

Traditional models claim that the force of gravity may be solely responsible for the formation of stars and star clusters. More recent observations suggest that magnetic fields, turbulence, or both are also involved and may even dominate the creation process. But just what triggers the events that lead to the formation of star clusters?

Astronomers using SOFIA’s instrument, the German Receiver for Astronomy at Terahertz Frequencies, known as GREAT, have found new evidence that star clusters form through collisions between giant molecular clouds.

The results were published in the Monthly Notices of the Royal Astronomical Society.

"Stars are powered by nuclear reactions that create new chemical elements," said Thomas Bisbas, a postdoctoral researcher at the University of Virginia, Charlottesville, Virginia, and the lead author on the paper describing these new results. "The very existence of life on earth is the product of a star that exploded billions of years ago, but we still don't know how these stars — including our own sun — form."

Researchers studied the distribution and motion of ionized carbon around a molecular cloud where stars can form. There appear to be two distinct components of molecular gas colliding with each other at speeds of more than 20,000 miles per hour. The distribution and velocity of the molecular and ionized gases are consistent with simulations of cloud collisions, which indicate that star clusters form as the gas is compressed in the shock wave created as the clouds collide.

“These star formation models are difficult to assess observationally,” said Jonathan Tan, a professor at Chalmers University of Technology in Gothenburg, Sweden, and the University of Virginia, and a lead researcher on the paper. “We’re at a fascinating point in the project, where the data we are getting with SOFIA can really test the simulations.”

While there is not yet scientific consensus on the mechanism responsible for driving the creation of star clusters, these SOFIA observations have helped scientists take an important step toward unraveling the mystery. This field of research remains an active one, and these data provide crucial evidence in favor of the collision model. The authors expect future observations will test this scenario to determine if the process of cloud collisions is unique to this region, more widespread, or even a universal mechanism for the formation of star clusters.

“Our next step is to use SOFIA to observe a larger number of molecular clouds that are forming star clusters,” added Tan. “Only then can we understand how common cloud collisions are for triggering star birth in our galaxy.”

SOFIA is a Boeing 747SP jetliner modified to carry a 106-inch diameter telescope. It is a joint project of NASA and the German Aerospace Center, DLR. NASA’s Ames Research Center in California’s Silicon Valley manages the SOFIA program, science and mission operations in cooperation with the Universities Space Research Association headquartered in Columbia, Maryland, and the German SOFIA Institute (DSI) at the University of Stuttgart. The aircraft is maintained and operated from NASA’s Armstrong Flight Research Center Hangar 703, in Palmdale, California.

Media Point of Contact

Nicholas A. Veronico
Nicholas.A.Veronico@nasa.gov • SOFIA Science Center
NASA Ames Research Center, Moffett Field, California

Editor: Kassandra Bell

Source:  NASA/Stars

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NGC 6231: Stellar Family Portrait in X-rays

NGC 6231
Credit X-ray: NASA/CXC/Univ. of Valparaiso/M. Kuhn et al; IR: NASA/JPL/WISE

JPEG (1.6 MB) - Large JPEG (1.6 MB) - Tiff (13.6 MB) - More Images

Tour of NGC 6231 - More Animation

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In some ways, star clusters are like giant families with thousands of stellar siblings. These stars come from the same origins — a common cloud of gas and dust — and are bound to one another by gravity. Astronomers think that our Sun was born in a star cluster about 4.6 billion years ago that quickly dispersed.

By studying young star clusters, astronomers hope to learn more about how stars — including our Sun — are born. NGC 6231, located about 5,200 light years from Earth, is an ideal testbed for studying a stellar cluster at a critical stage of its evolution: not long after star formation has stopped.

The discovery of NGC 6231 is attributed to Giovanni Battista Hodierna, an Italian mathematician and priest who published observations of the cluster in 1654. Sky watchers today can find the star cluster to the southwest of the tail of the constellation Scorpius.

NASA's Chandra X-ray Observatory has been used to identify the young Sun-like stars in NGC 6231, which have, until recently, been hiding in plain sight. Young star clusters like NGC 6231 are found in the band of the Milky Way on the sky. As a result, interloping stars lying in front of or behind NGC 6231 greatly outnumber the stars in the cluster. These stars will generally be much older than those in NGC 6231, so members of the cluster can be identified by selecting signs of stellar youth.

Young stars stand out to Chandra because they have strong magnetic activity that heats their outer atmosphere to tens of millions of degrees Celsius and causes them to emit X-rays. Infrared measurements assist in verifying that an X-ray source is a young star and in inferring the star's properties.

This Chandra X-ray image of NGC 6231 shows a close-up of the inner region of the cluster. Chandra can detect a range of X-ray light, which has been split into three bands to create this image. Red, green, and blue represents the lower, medium, and high-energy X-rays. The brightest X-ray emission is white.

The Chandra data, combined with infrared data from the Visible and Infrared Survey Telescope for Astronomy (VISTA) Variables in the Vía Lactéa survey has provided the best census of young stars in NGC 6231 available. An infrared image from NASA’s Wide-field Infrared Survey explorer is shown on the left.

There are an estimated 5,700 to 7,500 young stars in NGC 6231 in the Chandra field of view, about twice the number of stars in the well-known Orion star cluster. The stars in NGC 6231 are slightly older (3.2 million years on average) than those in Orion (2.5 million years old). However, NGC 6231 is much larger in volume and therefore the number density of its stars, that is, their proximity to one another, is much lower, by a factor of about 30. These differences enable scientists to study the diversity of properties for star clusters during the first few million years of their life.

Chandra studies of this and other young star clusters, have allowed astronomers to build up a sample from which cluster evolution can be studied. These clusters come from dozens of star-forming regions, but NGC 6231 adds a crucial piece to this puzzle because it shows how a cluster looks after the end of star formation. A comparison of the ages, sizes and masses of clusters in this sample implies that NGC 6231 has expanded from a more compact initial state, but it has not expanded sufficiently fast for its stars to break free from the cluster’s gravitational pull. Astronomers are not sure what will happen next: will it remain held together by gravity? Or will its constituents one day disperse as our Sun’s ancestral cluster once did?

Nearby star-forming regions frequently contain multiple star clusters, most of which are individually less massive than NGC 6231. The simple structure of NGC 6231, along with its relatively high mass, suggests that NGC 6231 was built up by mergers of several star clusters early its lifetime, a process known as "hierarchical cluster assembly".

Two papers describing recent studies of NGC 6231, both led by Michael Kuhn while at the Universidad de Valparaíso in Chile, have been published and are available online at https://arxiv.org/abs/1706.00017 and https://arxiv.org/abs/1710.01731.

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.

A Quick Look at NGC 6231

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Fast Facts for NGC 6231:

Scale: Infrared image is ~5 degrees across (about 452 light years); Right: Xray image is ~16 arcmin across (about 24 light years)
Category: Normal Stars & Star Clusters
Coordinates (J2000): RA 16h 54m 08.51s | Dec -41° 49' 36.0"
Constellation: Scorpius
Observation Date: July 2005
Observation Time: 33 hours 30 minutes
Obs. ID: 5372, 6291
Instrument: ACIS
References: Kuhn, M. et al. 2017, AJ, 154, 87; arXiv:1706.00017 Kuhn, M. et al. 2017, AJ, 154, 214; arXiv:1710.01731
Color Code: Left: Infrared (red, yellow, green, cyan, blue); Right: X-ray (red, green, blue).
Distance Estimate: About 1.59 kpc (5,190 light years)

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Source: NASA’s Chandra X-ray Observatory