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ING scientific news release
7 June, 2021

Discovery of a Massive Open Cluster Hiding in Full Sight

An international team of astronomers led by Ignacio Negueruela (Universidad de Alicante, Spain) has discovered one of the most massive star clusters in the solar neighbourhood, signposted by dozens of stars sufficiently bright to be seen with a small backyard telescope.

The Valparaíso 1 open cluster, as the astronomers named it, had gone unnoticed until now because there is a plethora of stars in the same field that do not belong to this cluster. They are either in front of or behind it, and they are placed all around and between cluster members. Searches by eye, or with the sort of mathematical grouping algorithms that had been used before, simply failed to detect it.

Thanks to ESA's Gaia unique capability of measuring a huge number of positions and motions of stars with high accuracy, and distances to very faraway stars, stellar astronomers can now find groups liying at the same distance and moving together in the mathematical space of astrometric parameters, before they can spot them on the sky. Valparaíso 1 was found in this way.

With the aim of characterising the newly discovered cluster, the astronomers applied to use the Intermediate Dispersion Spectrograph (IDS) on the Isaac Newton Telescope. They obtained spectra of the brightest cluster members from which they could measure radial velocities, and thus, complement Gaia's information. From other measured stellar parameters and chemical composition, they could eventually estimate the cluster's age and total mass.

IDS spectra of the brightest cluster members reveal a rich population of evolved stars, including a much redder and brighter object, labelled as star F, which could be the first detection of a massive AGB star in an open cluster. Large format: JPG.

Open clusters are groups of stars that formed together from the same molecular cloud, and have remained gravitationally bound since then. Stars in an open cluster share the same chemical composition and astronomers use them as laboratories for testing several effects. The more stars in a cluster, the more useful it is for statistics and for finding stars in rare evolutionary stages.

For this reason, astronomers keep searching for massive clusters in our own Galaxy. At present, a dozen very young massive open clusters, less than 25 million years old, and a few very old ones, with ages of billions of years, descendants of similar objects in our Galaxy, have been found. However, there is a dearth of massive clusters with intermediate ages. Is this lack a consequence of some physical process in place or does it simply reflect our limitation to detect them?

The discovery of the Valparaíso 1 cluster at a relatively small distance from the Sun, suggests that many other similar clusters may lie hidden among lots of unrelated stars in the crowded regions of the inner Milky Way.

Journal article
I. Negueruela, A-N. Chené, H. M. Tabernero, R. Dorda, J. Borissova, A. Marco, R. Kurtev, "A massive open cluster hiding in full sight", MNRAS, DOI: Paper: ADS.

More information
"Astronomers discover a massive star cluster, of intermediate age, in the constellation Scutum", Instituto de Astrofisica de Canarias press release, 2 June 2021.

Research contact
Ignacio Negueruela (Universidad de Alicante, Spain)

Press contact
Javier Méndez (ING PR Officer)

About the Isaac Newton Telescope

Based on observations made with the Isaac Newton Telescope (INT) operated on the island of La Palma by the Isaac Newton Group of Telescopes (ING) in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias (IAC). The ING is funded by the Science and Technology Facilities Council (STFC-UKRI) of the United Kingdom, the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) of the Netherlands, and the IAC in Spain. IAC's contribution to ING is funded by the Spanish Ministry of Science, Innovation and Universities.

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Last modified: 04 June 2021