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A Colorful Gathering of Middle-aged Stars
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Name: NGC 3532, Caldwell 91, Wishing Well Cluster
Description: Open Star Cluster
Position (J2000): RA 11h 5m 35.21s Dec -58° 44' 2.31"
Constellation: Carina
Visual magnitude: 3.0
Angular dimensions: Diameter of 55 arc-min
Distance: 1321 light years
Field of view: 34.34 x 33.19 arcminutes
Orientation: North is 0.0° left of vertical
Image Credit: ESO/G. Beccari
Release date: November 26, 2014
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The MPG/ESO 2.2-meter telescope at ESO's La Silla Observatory in Chile has captured a richly colorful view of the bright star cluster NGC 3532. Some of the stars still shine with a hot bluish color, but many of the more massive ones have become red giants and glow with a rich orange hue. It consists of approximately 150 stars of 7th magnitude or fainter, including seven red giants and seven white dwarfs..

NGC 3532 is a bright open cluster located some 1300 light-years away in the constellation of Carina (The Keel of the ship Argo). It is informally known as the Wishing Well Cluster, as it resembles scattered silver coins which have been dropped into a well. It is also referred to as the Football Cluster, although how appropriate this is depends on which side of the Atlantic you live. It acquired the name because of its oval shape, which citizens of rugby-playing nations might see as resembling a rugby ball.

This very bright star cluster is easily seen with the naked eye from the southern hemisphere. It was discovered by French astronomer Nicolas Louis de Lacaille while observing from South Africa in 1752 and was catalogued three years later in 1755. It is one of the most spectacular open star clusters in the whole sky.

NGC 3532 covers an area of the sky that is almost twice the size of the full Moon. It was described as a binary-rich cluster and one of the finest irregular clusters in the John Herschel who observed "several elegant double stars" here during his stay in southern Africa in the 1830s. Of additional, much more recent, historical relevance, NGC 3532 was the first target to be observed by the NASA/ESA Hubble Space Telescope, on May 20, 1990.

This grouping of stars is about 300 million years old. This makes it middle-aged by open star cluster standards [1]. The cluster stars that started off with moderate masses are still shining brightly with blue-white colors, but the more massive ones have already exhausted their supplies of hydrogen fuel and have become red giant stars. As a result the cluster appears rich in both blue and orange stars. The most massive stars in the original cluster will have already run through their brief but brilliant lives and exploded as supernovae long ago. There are also numerous less conspicuous fainter stars of lower mass that have longer lives and shine with yellow or red hues. NGC 3532 consists of around 400 stars in total.

The background sky here in a rich part of the Milky Way is very crowded with stars. Some glowing red gas is also apparent, as well as subtle lanes of dust that block the view of more distant stars. These are probably not connected to the cluster itself, which is old enough to have cleared away any material in its surroundings long ago. NGC 3532 lies near the Eta Carinae nebula (NGC 3372) and several other prominent open clusters like NGC 3293 and IC 2581.

This image of NGC 3532 was captured by the Wide Field Imager instrument at ESO's La Silla Observatory in February 2013.


[1] Stars with masses many times greater than the Sun have lives of just a few million years, the Sun is expected to live for about ten billion years and low-mass stars have expected lives of hundreds of billions of years - much greater than the current age of the Universe.