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.

Mars and Jupiter get close

Over the next few days Mars and Jupiter come close together in the early morning sky. The closest approach is on the morning of the 20th March, when Mars will be approx 43” south of Jupiter. The picture below shows the two planets at 05:00GMT.

Jupiter and Mars on the morning of March 20th 2020 at 05:00GMT

Also note that they are very low in the sky. Less than 10 degrees above the horizon! Sunrise over the next few days is shortly after 06:00GMT So you’ve got a bit of time to see them.

They should make a nice pairing in a small scope of binoculars.

Clear skies.

Uranus buzzes Venus

Over the next few nights Uranus comes to within a few degrees of Venus. This should make a nice view in binoculars. Venus will be very bright and Uranus very dim in comparison so it won’t be an easy observation.

Venus and Uranus, 8th March 2020, 20:00UT, from Abingdon

The two planets will be closest on the 8th March. After that Uranus will move past Venus. By Friday 13th March, Uranus will be just over 5 degrees south of Venus.

Look for the two planets in the evening sky just after 8pm when it gets properly dark.

Clear skies.

February’s talk: ‘The Grand Tour – Mission to the Giants’

Given by Rob Slack of Swindon Stargazers.

His talk ‘The Grand Tour – Mission to the Giants’, namely the Voyager missions took us back to the sixties and the space race and the two people who made the Grand Tour possible.  I will introduce the second person first: Gary Flandro, who worked out that we could use gravity assist on spacecraft to propel them from one gas giant to the other.   Also that an amazing planetary alignment would take place in September 1977 and that there would not be another chance for 175 years.

These gravity assists were a recent big thing and could be used to speed up or slow a vehicle to help it cross the interplanetary vastness of space.  This was actually part of Flandro’s postgraduate studies, so he wasn’t high on the academic ladder at that point.  Rob then introduced another person (person number one!) whose computations enabled Flandro to make his own amazing proposition: Michael Minovitch.  He was a mathematician who was set a challenge to work on the ‘three body problem’, in other words you have scenarios of the Sun, a planet, and a third object such as a comet, asteroid or spacecraft acting on each other.  He was given time on the massive IBM 7090 computer at UCLA in order to work out different trajectories and came up with gravity assists.  This was in 1961 and sadly the space race meant it got buried.

Luckily Flandro was around to build on it and in 1969 NASA wanted a ten year mission to the outer planets.

At the time it was not known how populated the asteroid belt was, and how strong was Jupiter’s radiation.  (James Alfred) Van Allen had done lots of work on the radiation belts around the Earth, but it was not known yet that Jupiter’s Van Allen belts are millions of times stronger.  (If you have a short wave radio you can hear Jupiter, mainly as Io moves through its magnetic field.)

Those with longer memories may already know that the two Pioneer craft (1972/73) did not even have proper cameras; they had photopolarimeters, which were brightness sensors and those wonderful Jupiter and Saturn pictures they returned were created by the spacecraft spinning and scanning in the brightness changes.  The energy to run everything was created by electricity coming from thermocouples wrapped around plutonium which was so radioactive that it was kept on booms away from the main craft.  Pioneer 11 gave us views of Saturn and Titan but its route via Jupiter had to be altered in order to miss the worst of Jupiter’s radiation belts.

By the time the Voyagers came along the use of gyros meant that the craft did not need to spin in order to take pictures.  There was a ten day launch window for the grand tour originally calculated by Flandro but now they were able to launch the two craft on 20th August and 5th September 1977 respectively.  (Voyager 2 was the first one to leave.)  Energy came from a radioisotope thermoelectric generator.  Fortran was the computer language.  The Jupiter closest approaches were V1 5/3/79 and V2 9/7/79. 

Their cameras were a kind of cathode ray colour tv tube.  Voyager 2 had a slightly better camera, and it was able to detect aurorae and lightning on Jupiter and lightning discharges from Io.  It photographed Jupiter’s ring.  Amazingly there had been no plans to photograph the Galilean satellites but sense prevailed and we now have visible records of the extreme volcanic activity on Io.  We now know Io’s surface changes day by day as it gets mangled by Jupiter’s effects.  Europa’s frozen watery landscape was seen to be riddled with lines.

Voyager 1’s arrival at Saturn was hastened in order to allow it to view Titan.  It then swung up out of the Solar System.  On 14/2/90 it looked down on the Solar System and took a ‘family portrait’ series of shots of the Sun and planets.  Remember the ‘pale blue dot’ (the Earth, less than a pixel size) and the poignant words of Carl Sagan, reminding us how anyone who’d ever lived had been on that tiny dot.  Voyager 1’s cameras were turned off after that.  Voyager 2 stayed on the ecliptic to continue to the other planets.

In 1980 the spokes on Saturn’s rings were detected, as were the ‘shepherd satellites’ that meandered along the ring edges.  Uranus and Neptune….Rob didn’t really talk about them but the pictures are well remembered: greenish tilted Uranus and the storms on blue Neptune.

Voyager 2 continues to send back data, although the signal is very weak.  In December 2018 the number of solar particles dropped and cosmic rays increased, indicating it has reached the heliopause and is entering interstellar space.  It is still generating enough heat to keep equipment going, although it is now functioning at much lower temperatures than expected.

The “Winter Alberio”

The double star HR2764 is sometimes referred to as the Winter Alberio after is similarity to the famous double in Cygnus. The double has a strong colour contrast of blue and orange. At 26.8 arcsecs separation the two components are easy to separate and should be visible in any small scope.

The pair can be found in the constellation of Canis Major, about 1.5 degrees North of NGC 2362 and approximately 3.5 degrees to the North east of the bright star Wezen. See the chart below.

HR 2764 from Abingdon on March 4th, 20:25UT.

Clear skies.

Mars on the shores of the Lagoon

On the morning of the 18th February, Mars will be about 3/4 degree above the Lagoon Nebula, M8, and approximately 3/4 degree below the Trifid nebula, M20. See the picture below:

True astronomical dark ends at 5:24UT that morning so you will have to be up early and have a clear horizon. You may have to juggle the ever brightening sky with altitude as Sunrise is at 7:15UT.

Clear skies.

Venus and Mercury in the evening sky

Over the first half of February both Venus and Mercury are well place in the evening sky. Venus is the brighter and higher of the two as you can see in the graphic below:

This shows the view from Abingdon at around sunset or just after.

Mercury will be at its furthest distance from the Sun (from our perspective here on Earth) on February 10th when it will get 15 degrees above the horizon.

Clear skies.