March’s talk: Studies of nearby star clusters with the Gaia data

March 2020, and luckily our early end to our season came with a very mind-expanding talk came from Dr Floor van Leeuwen from the institute of Astronomy in Cambridge.  He worked on the Hipparcos star position data and his talk was “the nearby Open Clusters as observed in the Second Gaia data release”.

The Gaia satellite continues to make very accurate observations of star positions and uses the Earth’s orbit as a baseline so that it can record their parallaxes.  By recording star positions over six month intervals over time it can also record stars’ proper motions across the sky.  Its optics have a folded focal length of 35m and has two mirror units separated by 106° whose images are projected onto the same focal plane.  All this provides excellent triangulations and very accurate stellar parallaxes down to ten micro-arcseconds.

The detectors are 100CCDs which are elongated; this enables them to scan to a high or a low resolution depending on the direction of scan.

They need to be kept to a very accurate temperature and cannot scan in the plane of the ecliptic for fear of looking at the Sun and being fried.  In practice Gaia is kept facing 45° from the Sun so the best coverage is of the zone within 45° of the poles although all sky coverage has been completed.

The positions of around 1.7 billion stars was released on 25/4/18.  This was the second data release after the first one in 2016.

The data contained positions of open clusters and he has based his research on those clusters within 240 parsec of us.  These include our familiar friends the Hyades, the Alpha Persei group, Pleiades, Praesepe and less familiar ones such as Blanco 1.  It is able to detect stars down to magnitude 20.

In the case of the Hyades, 480 stars have been found to be part of the group and their average distance is 47.6pc.  Van Leeuwen used data from 300 of the stars.  They appear to be moving away from us towards a converging point.  They display a proper motion, especially those nearer to us.  There are also stars moving through the cluster but their proper motions don’t fit.  There are also stars that are right on the converging point so they don’t have a proper motion, just a parallax (line of sight effect), as they move directly away from us.  Using the data he was able to construct a 3D view of the cluster, and that it appears flattened along the plane of the Milky Way.

Gathering data depends on the distance.  Anything beyond 250pc is hard to see.  If the cluster members are not moving quickly through space that also makes movement harder to detect.  Praesepe is such an example.

Coming back to the Hyades, he placed its members onto the HR diagram and they produce a lovely main sequence line with a thin second line running along just above it.  These are double stars so their combined light makes them appear .75 of a magnitude brighter.  There are already some white dwarfs.  The Hyades are only about 790 million years old.

When van Leeuwen plotted Praesepe along with the Hyades he found their velocities suggested they were of the same origin.  Praesepe’s distance averages 186pc and its age is around 710 million years.  It contains at least 771 stars.  Unfortunately a lot of the stars are too faint for Gaia to obtain enough data.

In the case of the Pleiades and Blanco 1 their double stars also follow the main sequence line on the HR diagram, slightly above the main line.

In total, van Leeuwen used nine clusters in his study, and he combined them all onto the HR diagram. Although these clusters are all quite young, you can see the big stars have already been leaving the main sequence and turning into white dwarfs (at the bottom left) but on the bottom right are small stars that haven’t yet evolved onto the main sequence line.

Other findings are that the Pleiades members are spread out to 11° in diameter.  All we see from the ground is about 6°.

Sizes of globular clusters can also be scanned out to be much bigger than first thought: Omega Centauri is 3—4° and 47 Tucanae is much bigger than the Moon in our sky.

The Gaia mission is hoping to continue up to 2025, with two more data releases in the pipeline.  The longer we can observe these local clusters, the clearer the proper motions of their stars will become.

Bear in mind that these data releases will be made available to citizen science, and that you can access these archives yourself should you wish.  You just have to register.  Beware though that there is so much stuff that you would take a day to download it and your computer would need to have petabytes of capacity.  The best Gaia website is

(The nine star clusters used in the study are: Hyades, Pleiades, Praesepe, Alpha Persei group, Coma Berenices, NGC 2451, Blanco 1, IC 2602, IC 2391)

Our after tea talk was Dan Larkins who informed us of various websites where you can have free access to astronomy books and magazines.

The best links are epubBoooks, project Gutenberg and Internet Archive.

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