#globular_cluster

[phonemantra-blog]

Using computer models, scientists were able to simulate the formation of intermediate-mass black holes in star clusters. This opens up new ways to study mysterious objects The Universe is teeming with black holes from stellar masses to supermassive monsters. But there is one class that remains elusive: “average” black holes. They are called intermediate-mass black holes. How common are they, how are they formed and where are they located? To answer these questions, astronomers modeled possible formation scenarios. Intermediate-mass black holes lie in the mass gap between stellar mass and a supermassive black hole. They range from 100 to 100,000 solar masses. If they exist, do they indicate a hierarchical model of black hole formation and do small black holes form from the collapse of supermassive stars? If this is so, then intermediate-mass black holes will be a kind of “transition link” between stellar mass and supermassive black holes. If this idea is correct, then is it possible that intermediate-mass black holes could collide with each other and form the seeds of supermassive black holes? Astronomers need more observational data to answer all these questions. [caption id="attachment_68861" align="aligncenter" width="780"] star clusters[/caption] At the same time, astronomers have enough data on stellar-mass black holes. They are formed during the collapse of supermassive stars. Supermassive black holes located at the centers of galaxies are most likely formed through the accretion of matter, as well as mergers with other black holes. The existence of intermediate-mass black holes appears to be beyond doubt, but their observation presents certain difficulties. This doesn't mean they don't exist. Observers have found candidates for intermediate-mass black holes in the Milky Way. They also appear to be present in active galactic nuclei, where accretion effects are observed. Additionally, some ultraluminous X-ray sources may also have these "average" black holes. The Sloan Digital Sky Survey also found several potential candidates that emit strongly in the X-ray range. X-ray emission is one of the characteristics of activity around a black hole. One of the most interesting observations involves the gravitational waves emitted when two massive black holes merge. As a result, a black hole with a mass of about 150 solar masses was formed - exactly the kind of mass that can be classified as intermediate. Simulation reveals secret of origin of intermediate-mass black holes in star clusters Scientists are sure they exist, but still cannot determine exactly where and how they are formed. An international group led by Arca Sedda from the Gran Sassa Institute (Italy) modeled the possible mechanisms of their formation.  [caption id="attachment_68862" align="aligncenter" width="451"] star clusters[/caption] “Current observational limitations do not allow us to say anything definitive about the population of intermediate-mass black holes with masses between 1,000 and 10,000 solar masses, and they cause headaches for scientists regarding the possible mechanisms of their formation,” Sedda explained. Sedda and his team looked at star clusters as possible birthplaces for intermediate-mass black holes, and they created computer models that could simulate the formation of these mysterious objects using DRAGON-II data. This is a collection of 19 computer models representing dense clusters of up to a million stars each. Using these in further simulations, the team discovered that the objects they were interested in could form star clusters. This occurs due to a complex combination of three factors: mergers between stars much more massive than the Sun, accretion of matter from the star onto stellar-mass black holes, and mergers between stellar-mass black holes. "The latter process makes it possible to 'see' these phenomena through the detection of gravitational waves," Sedda explained. The team also hypothesized what happens after the birth of intermediate-mass black holes. They appear to be ejected from their clusters by complex gravitational interactions, or experience "relativistic recoil" when formed. This keeps them from gaining weight.  “Our models show that although seeds form naturally from interactions in star clusters, they are unlikely to become heavier than a few hundred solar masses unless the parent cluster is extremely dense or massive,” Sedda said. Finding out the history of the origin of these black holes still does not answer the question of whether they are the missing link between stellar and supermassive black holes. For a better understanding, we need two ingredients: one or more processes capable of forming black holes in the intermediate mass range and the ability to preserve such black holes. Our study places strict constraints on the first ingredient, providing clear insight into what processes may contribute to the formation of intermediate-mass black holes. Looking at more massive clusters containing more binary star systems in the future may be the key to obtaining the second ingredient. But this will require enormous effort from a technological and computational point of view.

[apod]

2021 May 20

M13: The Great Globular Cluster in Hercules Image Credit & Copyright: Martin Dufour

Explanation: In 1716, English astronomer Edmond Halley noted, "This is but a little Patch, but it shews itself to the naked Eye, when the Sky is serene and the Moon absent." Of course, M13 is now less modestly recognized as the Great Globular Cluster in Hercules, one of the brightest globular star clusters in the northern sky. Sharp telescopic views like this one reveal the spectacular cluster's hundreds of thousands of stars. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter. Approaching the cluster core upwards of 100 stars could be contained in a cube just 3 light-years on a side. For comparison, the closest star to the Sun is over 4 light-years away. The remarkable range of brightness recorded in this image follows stars into the dense cluster core. Distant background galaxies in the medium-wide field of view include NGC 6207 at the lower right.

∞ Source: apod.nasa.gov/apod/ap210520.html

[nebris]

A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of member stars.[2] Their name is derived from Latin globulus (small sphere). Globular clusters are occasionally known simply as "globulars".

Although one globular cluster, Omega Centauri, was observed in antiquity and long thought to be a star, recognition of the clusters' true nature came with the advent of telescopes in the 17th century. In early telescopic observations globular clusters appeared as fuzzy blobs, leading French astronomer Charles Messier to include many of them in his catalog of astronomical objects that he thought could be mistaken for comets. Using larger telescopes, 18th-century astronomers recognized that globular clusters are groups of many individual stars. Early in the 20th century the distribution of globular clusters in the sky was some of the first evidence that the Sun is far from the center of the Milky Way.

Globular clusters are found in nearly all galaxies. In spiral galaxies like the Milky Way they are mostly found in the outer spheroidal part of the galaxy – the galactic halo. They are the largest and most massive type of star cluster, tending to be older, denser, and composed of lower abundances of heavy elements than open clusters, which are generally found in the disks of spiral galaxies. The Milky Way has more than 150 known globulars, and there may be many more.

The origin of globular clusters and their role in galactic evolution are unclear. Some are among the oldest objects in their galaxies and even the universe, constraining estimates of the universe's age. Star clusters were formerly thought to consist of stars that all formed at the same time from one star-forming nebula, but nearly all globular clusters contain stars that formed at different times, or that have differing compositions. Some clusters may have had multiple episodes of star formation, and some may be remnants of smaller galaxies captured by larger galaxies.

https://en.wikipedia.org/wiki/Globular_cluster

[wigmund]

From SpaceTelescope.Org Picture of the Week; October 1, 2018:

Celestial Fairy Lights

This glittering ball of stars is the globular cluster NGC 1898, which lies towards the center of the Large Magellanic Cloud — one of our closest cosmic neighbors. The Large Magellanic Cloud is a dwarf galaxy that hosts an extremely rich population of star clusters, making it an ideal laboratory for investigating star formation.

Discovered in November 1834 by British astronomer John Herschel, NGC 1898 has been scrutinized numerous times by the NASA/ESA Hubble Space Telescope. Today we know that globular clusters belong to the oldest known objects in the Universe and that they are relics of the first epochs of galaxy formation. While we already have a pretty good picture on the globular clusters of the Milky Way — still with many unanswered questions — our studies on globular clusters in nearby dwarf galaxies just started. The observations of NGC 1898 will help to determine if their properties are similar to the ones found in the Milky Way, or if they have different features, due to being in a different cosmic environment.

This image was taken by Hubble’s Advanced Camera for Surveys (ACS) and Wide Field Camera 3 (WFC3). The WFC3 observes light ranging from near-infrared to near-ultraviolet wavelengths, while the ACS explores the near-infrared to the ultraviolet.

Credit: ESA/Hubble & NASA

[spacettf]

Messier 3 by Rüdiger Via Flickr: The night from March 17th to 18th was truly exceptionell. Good transparency, no dew and a seeing that left me undersampled(!) with my ATIK 9 at 0,97"/pixel. Never have experienced this before. The picture of globular cluster Messier 3 is made of RGB data with about 20x2min for each channel. No Luminance frames. Drizzle 2x2 (drop shrink 1.0) in PixInsight and downsampling to about 0.652"/pix later for the shown center crop.

[phonemantra-blog]

The absence of white dwarfs in the Hyades cluster is of interest to astronomers, and this case helps reconstruct the history of the cluster The Hyades star cluster is located about 153 light years away. At such a short distance it is visible to the naked eye in the constellation Taurus. THIS proximity makes it easier for professional astronomers to observe than many other objects. The Hyades cluster contains many stars with approximately the same age - about 625 million years, the same metallicity, and similar trajectories. But it lacks white dwarfs - there are only eight of them in the center of the cluster. The Hyades cluster is quite common. His research greatly helps in understanding star clusters. But such a feature as the almost complete absence of white dwarfs puzzles astronomers.  A new study has found one "escapee" from the cluster. This is a white dwarf whose mass is approaching the limiting value for this type of star. The study is called "A White Dwarf That Was Able to Escape the Hyades Star Cluster." [caption id="attachment_68737" align="aligncenter" width="769"] star cluster[/caption] An extremely massive white dwarf was able to escape the Hyades star cluster Clusters such as the Hyades are weakly gravitationally bound, and over time they lose stars through their interactions with gas clouds, other clusters, and between the cluster stars themselves. Study author David Miller of the Department of Physics and Astronomy at the University of British Columbia and his co-authors studied the phenomenon of the absence of white dwarfs in the Hyades to reconstruct the history of the cluster. If we can identify the stars that were expelled, especially white dwarfs in this case, then it is possible to reconstruct the history of the cluster. The European Space Agency's Gaia space telescope tracks more than 1 billion stars in the Milky Way, providing Miller and his colleagues with a huge amount of data. The team found three white dwarfs with trajectories indicating possible escape from the Hyades cluster. For two of them, the range of masses makes it unlikely that they originated in a cluster, but for the third object, it is possible. "We estimate there is a 97.8% probability that the candidate is a true native of the Hyades" White dwarfs have a mass comparable to the Sun, but their size is comparable to the Earth. They consist of degenerate matter and emit only residual thermal energy. This is the final state of about 97% of the stars in the Milky Way. Their mass is governed by the Chandrasekhar limit and at maximum can reach about 1.44 solar masses. Large white dwarfs are usually found in binary star systems and gain mass by pulling matter from a companion; such white dwarfs eventually explode in a type 1a supernova, and all their mass is dissipated. [caption id="attachment_68738" align="alignnone" width="780"] star cluster[/caption] The white dwarf that left the Hyades is called an ultramassive white dwarf. These have a mass of 1.10 or more solar masses. This is well below the Chandrasekhar limit, but well above the average mass of a white dwarf, which is about 0.6 solar masses. Such high-mass objects typically originate from two-parent stars in a binary system, where one star has "taken" material from the other, increasing its mass. However, the Hyades ultramassive white dwarf has a mass of 1.317 solar masses and an age consistent with only one parent star. It appears to be the most massive white dwarf to come from a single progenitor.

[phonemantra-blog]

A new release of Gaia data has revealed half a million faint stars in the globular cluster Omega Centauri. This discovery helps fill gaps in maps of the galaxy and will allow scientists to study the structure of the cluster. The European Space Agency (ESA) has unveiled new and improved data about our galaxy and outer space with the release of 5 new pieces of data collected by the Gaia space telescope. Among the mission's findings, the release identified half a million dim stars in the massive Omega Centauri cluster. The new stars discovered by Gaia inhabit one of the densest regions of the sky. The Gaia mission's previous third edition of observations provided information on more than 1.8 billion stars, providing a fairly comprehensive view of the Milky Way and beyond. However, gaps remained in the map of the galaxy. In those areas that are particularly densely “populated” with stars, the usual observing regime reached its limits, which left these areas poorly unexplored - Gaia did not notice dim stars. Globular clusters are a good example of such regions. These clusters, which are among the oldest objects in the Universe, are of particular interest to scientists who study the history of the cosmos. But their bright, star-filled cores can obscure telescopes. Thus, they remain invisible regions on maps of the Universe. [caption id="attachment_66850" align="aligncenter" width="780"] Omega Centauri[/caption] To fill the gaps in Gaia's maps, it chose Omega Centauri, the largest globular cluster visible from Earth and a good example of a "typical" cluster. Instead of focusing just on individual stars, in this survey Gaia used a special observing mode, creating 2D images using the Sky Mapper tool. New Gaia Data Release: Half a Million New Stars in Omega Centauri “In Omega Centauri, we discovered more than half a million new stars that Gaia had not seen before – and that’s just in one cluster,” says lead author Dr. Katja Weingrill, Gaia project leader at the Leibniz Institute for Astrophysics in Potsdam. “The new data has allowed us to discover stars that are so close to each other that they cannot be accurately detected using the regular Gaia survey. With the new data, we will be able to study the structure of the cluster, the distribution of its constituent stars and their motion, and create a complete overview of the Omega Centauri cluster. This was using Gaia’s capabilities to their full potential,” adds co-author and member of the Gaia Collaboration, Dr. Alexey Mints. The discovery exceeds Gaia's normal capabilities, as the Sky Mapper instrument was originally intended only for calibration. The team used an observing mode designed to ensure the smooth operation of all telescope instruments. And I didn’t plan to use it for scientific research. Gaia is now exploring eight more areas using this approach, the results of which will be included in Gaia Data Release 4. The data will help astronomers better understand what's going on inside these cosmic building blocks, helping data scientists pinpoint the age of our galaxy, accurately determine its center, find out how stars change throughout their lives, clarify models of the evolution of galaxies and clarify the age of the Universe. In addition to the major discovery, the new Gaia release also reveals more than 380 possible gravitational lenses, improves the orbits of more than 150,000 asteroids within the solar system, maps the disk of the Milky Way, and characterizes the dynamics of 10,000 binary stars.

[apod]

2021 February 7

Blue Straggler Stars in Globular Cluster M53 Image Credit: ESA/Hubble, NASA

Explanation: If our Sun were part of this star cluster, the night sky would glow like a jewel box of bright stars. This cluster, known as M53 and cataloged as NGC 5024, is one of about 250 globular clusters that survive in our Galaxy. Most of the stars in M53 are older and redder than our Sun, but some enigmatic stars appear to be bluer and younger. These young stars might contradict the hypothesis that all the stars in M53 formed at nearly the same time. These unusual stars are known as blue stragglers and are unusually common in M53. After much debate, blue stragglers are now thought to be stars rejuvenated by fresh matter falling in from a binary star companion. By analyzing pictures of globular clusters like the featured image taken by the Hubble Space Telescope, astronomers use the abundance of stars like blue stragglers to help determine the age of the globular cluster and hence a limit on the age of the universe. M53, visible with a binoculars towards the constellation of Bernice's Hair (Coma Berenices), contains over 250,000 stars and is one of the furthest globulars from the center of our Galaxy.

∞ Source: apod.nasa.gov/apod/ap210207.html

[apod]

2020 October 24

Globular Star Cluster 47 Tuc Image Credit & Copyright: Jose Mtanous

Explanation: Globular star cluster 47 Tucanae is a jewel of the southern sky. Also known as NGC 104, it roams the halo of our Milky Way Galaxy along with some 200 other globular star clusters. The second brightest globular cluster (after Omega Centauri) as seen from planet Earth, it lies about 13,000 light-years away and can be spotted naked-eye close on the sky to the Small Magellanic Cloud in the constellation of the Toucan. The dense cluster is made up of hundreds of thousands of stars in a volume only about 120 light-years across. Red giant stars on the outskirts of the cluster are easy to pick out as yellowish stars in this sharp telescopic portrait. Tightly packed globular cluster 47 Tuc is also home to a star with the closest known orbit around a black hole.

∞ Source: apod.nasa.gov/apod/ap201024.html

[apod]

2020 March 19

M13: The Great Globular Cluster in Hercules Image Credit & Copyright: Eric Coles and Mel Helm

Explanation: In 1716, English astronomer Edmond Halley noted, "This is but a little Patch, but it shews itself to the naked Eye, when the Sky is serene and the Moon absent." Of course, M13 is now less modestly recognized as the Great Globular Cluster in Hercules, one of the brightest globular star clusters in the northern sky. Sharp telescopic views like this one reveal the spectacular cluster's hundreds of thousands of stars. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter. Approaching the cluster core upwards of 100 stars could be contained in a cube just 3 light-years on a side. For comparison, the closest star to the Sun is over 4 light-years away. The remarkable range of brightness recorded in this image follows stars into the dense cluster core and reveals three subtle dark lanes forming the apparent shape of a propeller just below and slightly left of center. Distant background galaxies in the medium-wide field of view include NGC 6207 at the upper left.

∞ Source: apod.nasa.gov/apod/ap200319.html

[apod]

2019 August 24

Millions of Stars in Omega Centauri Image Credit & Copyright: Michael Miller, Jimmy Walker

Explanation: Globular star cluster Omega Centauri, also known as NGC 5139, is some 15,000 light-years away. The cluster is packed with about 10 million stars much older than the Sun within a volume about 150 light-years in diameter. It's the largest and brightest of 200 or so known globular clusters that roam the halo of our Milky Way galaxy. Though most star clusters consist of stars with the same age and composition, the enigmatic Omega Cen exhibits the presence of different stellar populations with a spread of ages and chemical abundances. In fact, Omega Cen may be the remnant core of a small galaxy merging with the Milky Way.

∞ Source: apod.nasa.gov/apod/ap190824.html

[apod]

2019 March 24

Zooming in on Star Cluster Terzan 5 Video Credit: Nick Risinger (skysurvey.org), DSS, Hubble, NASA, ESA, ESO; Music: Johan B. Monell

Explanation: Globular clusters once ruled the Milky Way. Back in the old days, back when our Galaxy first formed, perhaps thousands of globular clusters roamed our Galaxy. Today, there are less than 200 left. Over the eons, many globular clusters were destroyed by repeated fateful encounters with each other or the Galactic center. Surviving relics are older than any Earth fossil, older than any other structures in our Galaxy, and limit the universe itself in raw age. There are few, if any, young globular clusters in our Milky Way Galaxy because conditions are not ripe for more to form. The featured video shows what it might look like to go from the Earth to the globular cluster Terzan 5, ending with a picture of the cluster taken with the Hubble Space Telescope. This star cluster has been found to contain not only stars formed in the early days of our Milky Way Galaxy, but also, quite surprisingly, others that formed in a separate burst of star formation about 7 billion years later.

∞ Source: apod.nasa.gov/apod/ap190324.html

[apod]

2017 October 11

Star Cluster NGC 362 from Hubble Image Credit: Hubble WFC3, NASA, ESA, J. Heyl, I. Caiazzo, & Javiera Parada (UBC)

Explanation: If our Sun were near the center of NGC 362, the night sky would glow like a jewel box of bright stars. Hundreds of stars would glow brighter than Sirius, and in many different colors. Although these stars could become part of breathtaking constellations and intricate folklore, it would be difficult for planetary inhabitants there to see -- and hence understand -- the greater universe beyond. NGC 362 is one of only about 170 globular clusters of stars that exist in our Milky Way Galaxy. This star cluster is one of the younger globulars, forming likely well after our Galaxy. NGC 362 can be found with the unaided eye nearly in front of the Small Magellanic Cloud, and angularly close to the second brightest globular cluster known, 47 Tucanae. The featured image was taken with the Hubble Space Telescope to help better understand how massive stars end up near the center of some globular clusters.

∞ Source: apod.nasa.gov/apod/ap171011.html

[apod]

2017 June 20

The Massive Stars in Westerlund 1 Image Credit: ESA/Hubble & NASA

Explanation: Star cluster Westerlund 1 is home to some of the largest and most massive stars known. It is headlined by the star Westerlund 1-26, a red supergiant star so big that if placed in the center of our Solar System, it would extend out past the orbit of Jupiter. Additionally, the young star cluster is home to 3 other red supergiants, 6 yellow hypergiant stars, 24 Wolf-Rayet stars, and several even-more unusual stars that continue to be studied. Westerlund 1 is relatively close-by for a star cluster at a distance of 15,000 light years, giving astronomers a good laboratory to study the development of massive stars. The featured image of Westerlund 1 was taken by the Hubble Space Telescope toward the southern constellation of the Altar (Ara). Although presently classified as a "super" open cluster, Westerlund 1 may evolve into a low mass globular cluster over the next billion years.

∞ Source: apod.nasa.gov/apod/ap170620.html

[apod]

2017 May 12

M13: The Great Globular Cluster in Hercules Image Credit & Copyright: Adam Block, Mt. Lemmon SkyCenter, U. Arizona

Explanation: In 1716, English astronomer Edmond Halley noted, "This is but a little Patch, but it shews itself to the naked Eye, when the Sky is serene and the Moon absent." Of course, M13 is now less modestly recognized as the Great Globular Cluster in Hercules, one of the brightest globular star clusters in the northern sky. Telescopic views reveal the spectacular cluster's hundreds of thousands of stars. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter. Approaching the cluster core upwards of 100 stars could be contained in a cube just 3 light-years on a side. For comparison, the closest star to the Sun is over 4 light-years away. Along with the cluster's dense core, the outer reaches of M13 are highlighted in this sharp color image. The cluster's evolved red and blue giant stars show up in yellowish and blue tints.

∞ Source: apod.nasa.gov/apod/ap170512.html