Gaia: Studying Star Clusters in the Milky Way
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Since eternity, humankind has been observing the sky with the naked eye and monitoring the various luminous stars in it. Long ago, people observed that stars formed patterns that recurred at specific times during the year or year-round; hence, the naming of constellations. As our knowledge of the universe's physics grew and observation tools improved, we realized that the stars in these constellations are only temporarily grouped together. Each star eventually separates from the others to go its own way. Thus, these clusters are merely visual and their components have no physical connection. On the other hand, some clusters are tied together by gravity.
Up until just 500 years ago, it was not possible to differentiate between star clusters and nebulae; as such, many clusters were mistakenly categorized as nebulae. Galileo's invention of the telescope in 1610 revolutionized astronomy, signaling the start of a new era of celestial exploration. Since then, telescopes have witnessed continued refinements. By the late 18th century, William Herschel had embarked on an ambitious project to observe the entire visible sky, classifying his observations into nebulae and star clusters.
The capabilities of telescopes advanced and numerous catalogs of celestial objects were published, such as the catalog of Giovanni Battista Hodierna (1675), the catalog of Nicolas Louis de Lacaille (1755), the catalog of Charles Messier (1784), and the catalog of William Thomas Shapley (1916), in which the term "star clusters" appeared for the first time. These catalogs included nebulae, galactic star clusters, and open clusters. Cosmic clusters are massive collections of stars held together by intense gravity, whereas open clusters are smaller groupings of stars with weaker gravitational bonds. Subsequently, several catalogs were produced, with WEBDA in 1966 standing out as the most significant.
In late 2013, the European Space Agency (ESA) launched the Gaia space observatory mission, which aims to create the largest 3D map of the universe. The telescope has managed to observe more than one billion celestial objects to date. The Gaia observatory was positioned in an orbit around the Sun-Earth L2 Lagrange point, situated approximately 1.5 million kilometers from the Sun. The balance of gravitational forces at this point and the fact that it has a similar orbit to Earth's provide a clear view of our planet. The Gaia mission is scheduled to end in 2025.
Gaia measures the proper motion and parallax of celestial objects with a precision 104 times higher than its predecessor, Hipparcos, which was launched by ESA in 1994. This has enabled the discovery of new clusters and determining their photometric, spectroscopic, astrometric, and kinematic properties. Scientists rely on astronomical data from the Gaia observatory to identify stars that belong to star clusters and determine their distances from us.
The latest Gaia data release has provided astronomers with an overwhelming amount of information that it has altered the way many of them approach their work. As stated by Anthony Brown (2021), Chair of the Gaia Data Processing and Analysis Consortium, the observatory is now "fully integrated into the astronomical landscape".
The photometric, astrometric, and spectroscopic data collected by Gaia directly contributes to determining the precise speed of stars and clusters in the solar neighborhood; thus, enabling the estimation of its dynamic and chemical properties. By identifying the stars that belong to clusters using high-precision astrometric data, it becomes possible to accurately determine the astrophysical properties of these clusters as well.
Stellar clusters are fundamental building blocks in galaxy formation. Understanding their formation, evolution, and properties, as well as the characteristics of their member stars directly contributes to our comprehension of galaxy and universe formation and evolution.
References
Hunt, E. L., Reffert, S., 2023, Improving the Open Cluster Census. II. An All-Sky Cluster Catalogue with Gaia DR3, Astronomy and Astrophysics.
He, Z., Wang, K., Luo, Y., Li, J., Liu, X., Jiang, Q., 2022, A Blind All-sky Search for Star Clusters in Gaia EDR3: 886 Clusters within 1.2 kpc of the Sun, The Astrophysical Journal Supplement Series.
Herschel, W., 1864, Catalogue of Nebulae and Clusters of Stars, Philosophical Transactions of the Royal Society of London.
Harris, W. E., 2010, A New Catalog of Globular Clusters in the Milky Way, eprint arXiv.
Cover image: The Gaia 1 open star cluster next to Sirius/wikimedia.org