Object 127: NGC 188

Podcast release date: 24 June 2024

Right ascension: 00:47:11.5


Epoch: ICRS

Constellation: Cepheus

Corresponding Earth location: The Gakkel Ridge in the Arctic Ocean

This episode's coordinates point to NGC 188, which is one of the best-known open clusters in the sky that isn't in the Messier Catalogue. In most images, it basically looks like a loose, gravitationally-bound collection of hundreds of yellowish and reddish stars spread over an area about half the diameter of the Moon, although the GAIA spacecraft has found that the cluster is actually twice that size [1]. However, the cluster is about magnitude 8.1 [2], which is three magnitudes fainter than what can be seen without a telescope, although it doesn't take a very big telescope to see the cluster, as I'll describe later. The cluster is located in the constellation Cepheus and is actually very close to the North Celestial Pole. In fact, one of my references uses the technical description "way up near the North Celestial Pole" to describe its position in the Earth's sky [2]. In terms of its location in space, NGC 188 is located about 6430 light years (1970 pc) from Earth in a place outside the plane of of the Milky Way [1]. Also, the distance from the center of the Milky Way to the cluster is actually a bit larger than the distance from the center of the Milky Way to the Solar System [1].

Many other open clusters are either brighter or closer to Earth or both, which would make them easier for professional astronomers to study, yet NGC 188 is one of the most well-studied clusters in the night sky for a couple of reasons. First of all, NGC 188 is old. Very old. It's older than the internet, or electricity, or dinosaurs, or even the Solar System.

First, though, let me explain how astronomers determine the ages of cluster of stars. This is typically determined by making a scatter plot with the stars' brightnesses on the y-axis and the stars' colors ranging from red to blue on the x-axis. These diagrams are called color-magnitude diagrams or color-luminousity diagrams or color-brightness diagrams. When a cluster of stars initially form from an interstellar cloud of gas, the stars will all lie along a roughly straight line in this plot extending from the bright blue corner to the faint red corner. This is called the main sequence, and all of the stars on the main sequence will be powered by fusing hydrogen into helium in their cores.

As the stars age, their cores will fill up with helium, and then they will undergo transformations into red giants with other changes taking place later. In the color-brightness diagrams, they will move off the main sequence towards the bright red corner of the diagram. Really big, bright, blue stars have relatively short lifespans and leave the main sequence after just a few million years and eventually explode as supernovae. The Sun and other mid-brightness yellow main sequence stars like it have lifespans of about 10 billion years; after the red giant stage and a few stages after that, they eventually become white dwarfs. In contrast, red dwarf stars take billions of years longer than Sun-like stars to leave the main sequence and become red giants. Astronomers can therefore identify the age of a cluster of stars by looking at which stars have left the main sequence. If lots of blue stars are present, then it's a relatively young cluster, but if lots of yellow stars are beginning to die off, then it's a relatively old cluster.

In NGC 188, most of the main sequence stars are either mid-sized yellow stars, including stars about the same size as the Sun, or smaller and redder red dwarf stars that seem to live for extremely long times. The cluster seems to be close to devoid of bigger blue stars as well as many medium-large whitish stars; most of these stars have either transformed into red giants, evolved into other forms of giant stars, or died in a process in which they form supernovae or planetary nebulae. Based on this information, NGC 188 is estimated to be about 7 billion years old [3].

By open cluster standards, this is ancient. Most open clusters get ripped apart by tidal forces in the Milky Way or just by gravitational interactions between the cluster and whatever the cluster is passing through, so it is extremely surprising that NGC 188 has managed to stick together for 7 billion years. In general, only globular clusters get much older than this, and that is because globular clusters orbit outside the plane of the Milky Way where they don't gravitationally interact with the stuff in the plane of the Milky Way. It looks like NGC 188 may also be this old in part because, by chance, it also orbits outside the plane of the Milky Way.

What is extra unusual about NGC 188 is that, even though most of the blue main sequence stars seemed to have evolved into red giants and exploded billions of years ago, the cluster still contains 21 blue hydrogen burning main sequence stars [4]. These are stars that people are fairly certain are located within the cluster, too; they aren't stars that lie in front of or behind the cluster and just happen to be photobombing astronomers' images of the cluster. These stars are called blue stragglers. They look like stars that are too young, and I guess you would say that they are straggling in terms of growing up. Back in episode 115, Tim Erickson from Funk, Nebraska, asked me if I could talk about blue stragglers if the random number generator landed on one, and, well, we have a cluster with over 20 of them. So, let's dive into understanding the origins of blue stragglers.

First of all, just to be clear, NGC 188 does not contain any interstellar gas that it could have used to form new stars, so it would not have spontaneously formed a few new blue stars for some reason. It also seems really unlikely that the blue stragglers were somehow gravitaitonally captured by the cluster, especially since the cluster is currently in a place outside the plane of our Milky Way where new stars are not forming. Therefore, the smaller, cooler yellow and red stars within the cluster somehow had to be transformed into bigger, hotter blue stars. This could include smaller stars merging together somehow to form bigger blue stars or gas being transferred from one star to another to make the second star into a bigger star that would look hotter and bluer.

Observations of NGC 188 with the Hubble Space Telescope in 2012 demonstrated that the vast majority of the blue stragglers within this cluster exhibited some sort of periodic Doppler shifting that was consistent with the stars being in binary star systems [4]. This seemed to favor the scenario where gas was being transferred from one star to another within a binary system to create the blue straggler stars. In this scenario, one star in the system would first evolve into a red giant and then beyond that stage and would expand. In fact, the star would expand so much that the second star in the binary star system could gravitationally strip the outer gas layers from the giant star. The second star would then grow in size and also get hotter, which would make it bluer. Thus, the second star would transform into the blue straggler star. The first star would still be present in the system, but after its outer gas layers had been stripped away, all that would be left would be the inert core of the evolved star that would actually more-or-less be a white dwarf that would actually be hard to see next to the bigger blue star. These results from NGC 188 have been quite revolutionary in understanding the solving the question of where do blue stragglers come from, and since these observations, people have identified blue stragglers forming in similar ways in other clusters as well.

Now, if you want to look at NGC 188 yourself with an amateur telescope, the cluster is located a little to the east of a position about one third of the distance along a line between Polaris and Gamma Cephei, which is at the northern tip of the constellation Cepheus. (I hope that description isn't too complicated, but if it was, I recommend just finding a star chart showing the location of the object.) The cluster is faint and extended, which can make it difficult to find with a small telescope. In a 15 cm (6 inch) telescope, it will look like just a faint glow with potentially a few distinct stars [2, 5]. However, in a 25 cm (10 inch) telescope or larger, it's possible to see many of the brighter individual stars [5]. My telescope is too small to see NGC 188, so let me know if you can get access to a larger telescope and spot it.


[1] Cantat-Gaudin, T. and Anders, F., Clusters and mirages: cataloguing stellar aggregates in the Milky Way, 2020, Astronomy & Astrophysics, 633, A99

[2] Eicher, David J., The Universe from Your Backyard, 1988

[3] Sarajedini, Ata et al., WIYN Open Cluster Study. II. UBVRI CCD Photometry of the Open Cluster NGC 188, 1999, Astronomical Journal, 118, 2894

[4] Gosnell, Natalie M. et al., Implications for the Formation of Blue Straggler Stars from HST Ultraviolet Observations of NGC 188, 2015, Astrophysical Journal, 814, 163

[5] Neata, Emil, NGC 188 – Open Cluster in Cepheus, 2015, Starlust


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

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Image Viewer: Aladin Sky Atlas (developed at CDS, Strasbourg Observatory, France)