2025 August 14

M13: The Great Globular Cluster in Hercules
* Image Credit & Copyright: R. Jay Gabany
https://www.cosmotography.com/index.html

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 with our neighborhood of the Milky Way, the closest star to the Sun is over 4 light-years away. Early telescopic observers of the great globular cluster also noted a curious convergence of three dark lanes with a spacing of about 120 degrees, seen here just below the cluster center. Known as the propeller in M13, the shape is likely a chance optical effect of the distribution of stars viewed from our perspective against the dense cluster core.
http://www.messier.seds.org/m/m013.html
https://science.nasa.gov/missions/hubble/a-celestial-snow-globe-of-stars/
https://www.cosmotography.com/images/small_ngc6205.html

https://en.wikipedia.org/wiki/Globular_cluster
http://www.messier.seds.org/xtra/similar/halley_pt.html
https://www.bbc.co.uk/history/historic_figures/halley_edmond.shtml
https://skyandtelescope.org/observing/gobs-of-globs-guide-to-16-spring-globular-clusters/

https://apod.nasa.gov/apod/ap250814.html

2025 August 7

The Double Cluster in Perseus
* Image Credit & Copyright: Ron Brecher
https://astrodoc.ca/about-me/

Explanation:
This stunning starfield spans about three full moons (1.5 degrees) across the heroic northern constellation of Perseus. It holds the famous pair of open star clusters, h and Chi Persei. Also cataloged as NGC 869 (right) and NGC 884, both clusters are about 7,000 light-years away and contain stars much younger and hotter than the Sun. Separated by only a few hundred light-years, the clusters are both 13 million years young based on the ages of their individual stars, evidence that both clusters were likely a product of the same star-forming region. Always a rewarding sight in binoculars or small telescopes, the Double Cluster is even visible to the unaided eye from dark locations.
https://astrodoc.ca/double-cluster-2025/
https://science.nasa.gov/mission/hubble/science/explore-the-night-sky/hubble-caldwell-catalog/caldwell-14/
http://www.messier.seds.org/xtra/ngc/n0869.html
https://arxiv.org/abs/astro-ph/0205130

https://astrobackyard.com/double-cluster-in-perseus/
https://en.wikipedia.org/wiki/Perseus_(constellation)
http://www.messier.seds.org/open.html

https://apod.nasa.gov/apod/ap250807.html

2020 December 16

Sonified: The Matter of the Bullet Cluster
* Image Credit: X-ray: NASA/CXC/SAO
* Optical: NASA/STScI, Magellan/U.Arizona
* Lensing Map: NASA/STScI, ESO WFI, Magellan/U.Arizona
* Sonification: NASA/CXC/SAO/K.Arcand, SYSTEM Sounds (M. Russo, A. Santaguida)

Explanation:
What's the matter with the Bullet Cluster? This massive cluster of galaxies (1E 0657-558) creates gravitational lens distortions of background galaxies in a way that has been interpreted as strong evidence for the leading theory: that dark matter exists within. Different analyses, though, indicate that a less popular alternative -- modifying gravity-- could explain cluster dynamics without dark matter, and provide a more likely progenitor scenario as well. Currently, the two scientific hypotheses are competing to explain the observations: it's invisible matter versus amended gravity. The duel is dramatic as a clear Bullet-proof example of dark matter would shatter the simplicity of modified gravity theories. The featured sonified image is a Hubble/Chandra/Magellan composite with red depicting the X-rays emitted by hot gas, and blue depicting the suggested separated dark matter distribution. The sonification assigns low tones to dark matter, mid-range frequencies to visible light, and high tones to X-rays. The battle over the matter in the Bullet cluster is likely to continue as more observations, computer simulations, and analyses are completed.

https://apod.nasa.gov/apod/ap201216.html

2025 July 26

Globular Cluster Omega Centauri
* Image Credit & Copyright: Data acquisition - SkyFlux Team, Processing - Leo Shatz
https://app.astrobin.com/u/spinlock#gallery

Explanation:
Globular star cluster Omega Centauri packs about 10 million stars much older than the Sun into a volume some 150 light-years in diameter. Also known as NGC 5139, at a distance of 15,000 light-years 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. With a yellowish hue, Omega Centauri's red giant stars are easy to pick out in this sharp telescopic view. A two-decade-long exploration of the dense star cluster with the Hubble Space Telescope has revealed evidence for a massive black hole near the center of Omega Centauri.
https://app.astrobin.com/i/7na4mz
https://ui.adsabs.harvard.edu/abs/2021A%26A...653L...8L/abstract
https://en.wikipedia.org/wiki/Globular_cluster
https://earthsky.org/clusters-nebulae-galaxies/omega-centauri-milky-ways-prize-star-cluster/
https://esahubble.org/news/heic0809/

https://apod.nasa.gov/apod/ap250726.html

About Intermediate-Mass Black Hole in Omega Centauri:
https://defcon.social/@grobi/114918173169808417

[...]
The stars in stellar associations are so spread out that they defy the image of a "cluster." Visually, the stars in these clusters might not look bound together at all. This is because, while the stars in stellar associations formed together and continue to move together through space, they have become gravitationally unbound. As a result, stellar associations are the least stable variety of star cluster. However, many of the brightest stars in constellations like Scorpius, Lupus, Centaurus, and Crux as well as the bright star Antares are all a part of a stellar association located about 400 light-years away.

Because they are not as obvious as their counterparts, stellar associations can be hard to spot. But scientists can still determine if stars are part of the same cluster or have common origins based on key factors like the stars’ age, movement, and sometimes even their chemical composition.

* Text excerpt from https://science.nasa.gov/universe/star-clusters-inside-the-universes-stellar-collections/

CREDIT

* Chelsea Gohd
NASA Universe Web Team

Please see ALT-Text for more information
(image below)

Stellar Associations

While globular and open clusters are generally referred to as the two main types of star clusters, there is a third variety of stellar groupings, called stellar associations, which are the most dispersed of the bunch. In fact, the vast majority of stars are born as members of a stellar association before they move apart.

In the Milky Way, the highest concentration of stellar associations is in the galaxy's spiral arms. Stellar associations are very loose clusters of 10 to upwards of 10,000 stars. While they might hold fewer stars than other types of clusters, because the stars are so spread out, stellar associations can be very large, averaging 700 light-years across.

Stellar associations are categorized based on the types of stars they contain. OB associations are made up of O and B-type stars that are young and massive (B-type stars are typically 2 to 15 times more massive than the Sun and O-type stars are usually 15 to 90 times as massive); R associations have young, bright stars with a medium mass between 3 to 10 times the mass of the Sun; and T associations contain mostly T Tauri stars that are fairly cool, young stars with low mass similar to the mass of our Sun.
[...]
Please see the reply for more Information about Stellar Associations

* Text excerpt from https://science.nasa.gov/universe/star-clusters-inside-the-universes-stellar-collections/

CREDIT

* Chelsea Gohd
NASA Universe Web Team

Please see ALT-Text for more information about NGC 206
(image below)

Open Clusters

Smaller than globular clusters, open clusters are more loosely bound groups that typically contain anywhere from tens to thousands of stars. Open star clusters have a core that typically measures a few light-years across that is surrounded by a corona, or the outermost part of a star’s atmosphere, that can stretch about tens of light-years out from the center.

Found in irregular galaxies and spiral galaxies like the Milky Way, open clusters contain both old and young stars and are generally much younger than globular clusters – typically they are less than a billion years old. Stars in open clusters are also more spread out, so these clusters are not very stable and the stars have a tendency to disperse after a few million years.

More plentiful in our galaxy than globular clusters, there are thousands of open clusters in the Milky Way, though it's thought that there could be many, many more. In the Milky Way, these clusters can be spotted in our galaxy's disk, both in and between its spiral arms.

* Text excerpt from https://science.nasa.gov/universe/star-clusters-inside-the-universes-stellar-collections/

CREDIT

* Chelsea Gohd
NASA Universe Web Team

Please see ALT-Text for more information about Messier 2 (image below)

Globular Star Clusters

Ancient and giant, globular star clusters are stellar "dinosaurs" scattered throughout the universe. They can survive for billions of years, holding some of the oldest stars in the universe.

Globular star clusters contain anywhere from tens of thousands to millions of stars, packed tightly together in dense clumps ranging from 50 to 450 light-years across, and they can be found throughout the halo, or outer regions, of our own Milky Way galaxy. These stars formed within massive clouds of gas anywhere from about 8 to 13 billion years ago. Once formed, these roughly spherical star clusters were left without gas and dust to feed new star formation. There is, however, some evidence that some globular clusters can produce multiple generations of stars early in the cluster’s life.

Globular star clusters are associated with all types of galaxies. The Milky Way holds about 150 known globular clusters, many of which are in retrograde orbits, meaning they are orbiting the center of the galaxy in the opposite direction of most of the objects in the Milky Way. This indicates that our galaxy could have captured these clusters during an interaction with another galaxy. These clusters are born so dense and massive that they can remain a cluster even after being pulled apart by gravity over time.

Because they contain so many stars, globular clusters are often the most visibly obvious clusters. Sky watchers can see many of them with the unaided eye. The largest and brightest globular cluster in the Milky Way galaxy, Omega Centauri (NGC 5139), is one of the clusters visible without a telescope.

"Due to their strong compression of mass, they can also be used for gravitational lensing. https://defcon.social/@grobi/114653568101686376"

* Text excerpt from https://science.nasa.gov/universe/star-clusters-inside-the-universes-stellar-collections/

CREDIT
* Chelsea Gohd
NASA Universe Web Team

Please see ALT-Text for more information about Messier 2 (image below)