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.
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* 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)