Panning across "The Dancer in Dorado": Spiral Galaxy NGC 1566 | Hubble

Panning across "The Dancer in Dorado": Spiral Galaxy NGC 1566 | Hubble

This vibrant and dynamic-looking image features the spiral galaxy NGC 1566. This is sometimes informally referred to as the ‘Spanish Dancer Galaxy’. NGC 1566 is a weakly-barred or intermediate spiral galaxy, meaning that it does not have either a clearly present or a clearly absent bar-shaped structure at its center. The galaxy owes its nickname to the vivid and dramatic swirling lines of its spiral arms. This could evoke the shapes and colors of a dancer’s moving form. NGC 1566 lies around 60 million light-years from Earth in the constellation Dorado, and is also a member of the Dorado galaxy group. 

Galaxy groups are assemblages of gravitationally bound galaxies. Groups differ from galaxy clusters in size and mass: galaxy clusters may contain hundreds of galaxies and groups may contain several tens of galaxies. Nevertheless, there is not a precise delineation between the definition of a galaxy group and a galaxy cluster. Astronomers have proposed that the definitions be sharpened up, with one suggestion that galaxy aggregations with less mass than 80 trillion Suns should qualify as galaxy groups.

The Dorado group has had a fluctuating membership over the past few decades, with scientific papers changing its list of constituent galaxies. To understand why it is so challenging for astronomers to pin down members of groups such as the Dorado group, we can imagine a photograph of an adult human and a large oak tree. We have foreknowledge of the approximate size of the person and the tree, so if we were to see a photo where the person appeared roughly the same size as the tree, then we would be able to guess that, in reality, the person was positioned much closer to the camera than the tree was, giving the false impression that they were the same size. When working out members of a galaxy group, astronomers are not necessarily equipped with the knowledge of the size of the individual galaxies, and so have to work out whether galaxies really are relatively close together in space, or whether some of them are actually much closer or much further away. This has become easier with more sophisticated observation techniques, but still sometimes presents a challenge.

Credit: European Space Agency (ESA)/Hubble & NASA, D. Calzetti and the LEGUS team, R. Chandar

Duration: 30 seconds

Release Date: Nov. 13, 2023

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