Salaris, ms astro. Received: 15 August Accepted: 2 October We determine ages of 71 old Open Clusters by a two-step method: we use main-squence fitting to 10 selected clusters, in order to obtain their distances, and derive their ages from comparison with our own isochrones used before for Globular Clusters.
We then calibrate the morphological age indicator , which can be obtained for all remaining clusters, in terms of age and metallicity. Particular care is taken to ensure consistency in the whole procedure. The resulting Open Cluster ages connect well to our previous Globular Cluster results. From the Open Cluster sample, as well as from the combined sample, questions regarding the formation process of Galactic components are addressed.
We determine a delay by Gyr between the start of the halo and thin disk formation, whereas thin and thick disk started to form approximately at the same time. We do not find any significant age—metallicity relationship for the open cluster sample. The cumulative age distribution of the whole open cluster sample shows a moderately significant level departure from the predictions for an exponentially declining dissolution rate with timescale of 2. The cumulative age distribution does not show any trend with galactocentric distance, but the clusters with larger height to the Galactic plane have an excess of objects between 2—4 and 6 Gyr with respect to their counterpart closer to the plane of the Galaxy.
What is a pulsar? They are are particularly useful to astronomers as they provide a way to study and model stellar evolution and ages. The two basic categories of stellar clusters are open clusters, also known as galactic clusters, and globular clusters. Open clusters are so-named due to the fact that the individual component stars are easily resolved through a telescope. Some examples such as the Hyades and Pleiades are so close that the individual stars can be clearly distinguished by the naked eye.
They are sometimes called galactic clusters due to their location on the dusty spiral arms on the plane of spiral galaxies. Stars in an open cluster have a common origin - they formed from the same initial giant molecular cloud.
Clusters typically contain a few hundred stars though this can vary from as low as a few dozen up to a few thousand. Stars within an open cluster are only loosely bound by gravity. As the cluster rotates around the galaxy it eventually disperses due to gravitational perturbations with other other objects in the galaxy. Whilst our Sun is likely to have initially formed in an open cluster there is now no discernable clustering with nearby stars.
Open clusters are therefore usually relatively young objects. Some, such as the Pleiades still show evidence of nebulosity suggesting their recent formation. Open cluster stars belong to Population I, they are young and have high metallicities.
Clusters range from a couple to 20 or so parsecs across. Globular clusters contain several thousand to one million stars in spherical, gravitationally-bound system.
Located mostly in the halo surrounding the galactic plane they comprise the oldest stars in the galaxy. These Population II stars are highly evolved but with low metallicities.
Clusters are so old that any star higher than a G or F-class will have already evolved off the main sequence. There is little free dust or gas found in globular clusters so no new star formation is taking place in them.
Stellar densities within the inner regions of a globular cluster are very high compared with regions such as those around the Sun. As with open clusters, stars in globular clusters probably had a common origin. Unlike open clusters, globular clusters normally remain gravitationally coherent throughout their lives. The stars within them are not dispersed out of the cluster.
Our Milky Way has about globular clusters. Prominent examples include 47 Tuc, M4 and Omega Centauri although there is some debate as to whether this may in fact be a captured dwarf spheroidal galaxy.
Star clusters are particularly important because they allow astronomers to check models of stellar evolution and the ages of stars. Let us look firstly at open clusters to understand why this is so. Stars in an open cluster have a common origin from a given nebula. The open star cluster NGC left contains young, bright stars roughly 30 million years old that are loosely gravitationally bound and will spread apart over time.
By contrast, the globular cluster 47 Tucanae NGC contains millions of stars about 13 billion years old that have been bound in the spherical cluster their entire lives. Globular clusters are old clusters of stars that have remained in a gravitationally bound system. These clusters are usually roughly spherical and can contain anywhere from a few thousand to a million stars.
They are typically on the order of 13 billion years old, meaning they contain some of the oldest stars in our galaxy. Open clusters are much younger and smaller than globular clusters. They are the recent birthplaces of new stars, which form out of clouds of dust and gas, and contain only hundreds or thousands of stars.
Though the stars in an open cluster formed together out of the same material, they do not remain gravitationally bound over time and spread out, scattering their stars far and wide.
Because they are not gravitationally bound, these clusters can have random and irregular shapes. They are located in the plane of our galaxy, along the spiral arms where the gas and dust in the Milky Way resides.
Because open clusters are both younger and shorter lived, astronomers use them to study young stars and the processes of star formation, as well as the dynamics of how material spreads out in our galaxy. Receive news, sky-event information, observing tips, and more from Astronomy's weekly email newsletter.
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