Archive for GAIA

New Publication at the Open Journal of Astrophysics

Posted in OJAp Papers, Open Access, The Universe and Stuff with tags , , , , , , on February 3, 2023 by telescoper

The articles are coming in thick and fast at the Open Journal of Astrophysics. and why trying to get them refereed and published as quickly as we can. It’s time to announce yet another paper. This one was published officially yesterday (2nd February 2023) but I just found time to post about it here today before I go to my 9am tutorial.

The latest paper is the 4th paper in Volume 6 (2023) as well as the 69th in all. This one is in the Astrophysics of Galaxies folder.

The latest publication is entitled “Wide Binaries from GAIA EDR3: preference for GR over MOND?”.  The authors of this paper,  Charalambos Pittordis and Will Sutherland, are both based at Queen Mary, University of London. We published a related paper last month.

Here is a screen grab of the overlay which includes the  abstract:


You can click on the image to make it larger should you wish to do so. You can find the officially accepted version of the paper on the arXiv here.

R.I.P. James Lovelock (1919-2022)

Posted in Literature, The Universe and Stuff with tags , , on July 28, 2022 by telescoper

I heard this week of the death, on his 103rd birthday, of scientist and writer James Lovelock. He started out as a chemist but became what is now called an “independent scientist” and “futurist”. These terms are often applied to people who are simply cranks, but he wasn’t just that. Unorthodox he was, certainly, but there was depth to his thinking that mere cranks never reach.

James Lovelock was best known to me – and I suspect to many others – for his work on the Gaia Hypothesis, which is roughly speaking the idea that the system of life on Earth functions as a single organism that defines and maintains the conditions necessary for its own survival.

I’ve just had a rummage around my bookshelves and found my copy of his famous book on this topic, which I bought and read back in the 80s. The first edition was published in 1979, but the one I bought was the version published in 1987 after the topic had been featured on the BBC TV programme, Horizon:

The Gaia hypothesis has been widely criticized by biologists and ecologists but I remember finding it a very thought-provoking book, though I interpreted more as a metaphor than a mechanism. At any rate it seems to me to be a useful counter to the extreme reductionism of many prominent life scientists. It’s also very well written and definitely worth reading over 40 years after it was written. James Lovelock was as inventive and ingenious a thinker as he was unorthodox.

Rest in peace, James Ephraim Lovelock (1919-2022).

Gaia’s Third Data Release!

Posted in The Universe and Stuff with tags , , on June 13, 2022 by telescoper

It seems like only yesterday that I blogged about the second release of data from the European Space Agency’s Gaia mission but today sees the release of the third data set, known to its friends as DR3. This completes the set after some initial data were released early as EDR3 back in 2020.

Gaia on the Launchpad at Kourou, French Guyana, on 13th December 2013

In case you weren’t aware, Gaia, launched way back in 2013, is an ambitious space mission to chart a three-dimensional map of our Galaxy, the Milky Way, in the process revealing the composition, formation and evolution of the Galaxy. Gaia will provide unprecedented positional and radial velocity measurements with the accuracy needed to produce a stereoscopic and kinematic census of about one billion stars in our Galaxy and throughout the Local Group. This amounts to about 1 per cent of the Galactic stellar population.

Gaia is likely to operate until round about November 2024, so there’s a lot of data yet to come.

You can find a complete list of what is in DR3 here and if you want to go straight into the papers based on this dataset, go here. There is a nice promotional video here:

The Future of Extragalactic Observations from the Past

Posted in Biographical, The Universe and Stuff with tags , , , , , , , , , on December 27, 2021 by telescoper

The launch of the James Webb Space Telescope on Christmas Day triggered a memory that twenty years ago, in July 2001, I was an invited speaker at a Conference in Cape Town entitled The Early Universe and Cosmological Observations: a Critical Review. That meeting was preceded by the 16th International Conference on General Relativity and Gravitation in Durban which I also attended, but did not speak at. For the Cape Town meeting I was asked to give a talk about some of the things coming up in the future to do with observational extragalactic astronomy, though I was told to avoid the cosmic microwave background and galaxy redshift surveys as other speakers were covering those areas. At the time I was serving on the Astronomy Advisory Panel for the (now defunct) Particle Physics and Astronomy Research Council so I was keeping up with developments fairly well then.

Anyway, I wrote up my talk and it was published in 2002 in a special issue of Classical and Quantum Gravity, along with the other talks (which were more theoretical, as opposed to hypothetical). I never bothered to put in on the arXiv so if you want a copy you’ll have to get it from the publisher.

I’m not claiming it is a particularly insightful article – and I did refrain from giving specific timescales – but, looking back at it, it is interesting which projects I mentioned in the abstract actually did get completed in the following twenty years.

The European X-ray mission XEUS was never completed. It was proposed for a while to merge it with a rival US mission Constellation-X in the International X-ray Observatory (IXO), but that was cancelled in 2011/12 owing to budget constraints at NASA. An ESA X-ray mission, called ATHENA (Advanced Telescope for High ENergy Astrophysics, based to some extent on the XEUS concept, is pencilled in for launch in 2034.

At the time of writing the article, JWST was called the Next Generation Space Telescope (NGST) and was envisaged to be an 8m class telescope, though I did suggest in the article would probably be “de-scoped” to involve a smaller mirror “perhaps 6m or thereabouts”. As we now know, it was finally launched on December 25 2021 and has a mirror of diameter 6.5m.

GAIA was developed and launched in 2013 and will operate until next year; it has been a tremendous success.

The Overwhelming Large (OWL) Telescope was planned to be a huge ground-based telescope, with a 100m diameter mirror and a target timescale of around 2015, to be built by the European Southern Observatory in Chile. I remember in informal discussions we used to call it the FLT. It was eventually decided that was not technically feasible and it was downgraded to a merely Extremely Large Telescope, which has a 39m mirror, underwhelming in comparison. Construction is in progress and it should see first light in 2027.

As well as the ELT there are now also the Thirty Metre Telescope and the Giant Magellan Telescope, which will come into operation on a similar timescale.

The Atacama Large Millimetre Array (ALMA) consisting of 66 telescopes working as an interferometer was completed and has been fully operational since 2013. That too has been a great success.

The Square Kilometre Array (SKA) also had its share of cost overruns and technical delays and although initial construction plans have been developed it is not expected to be operational until 2027.

Probably the most notable omission from my list is the Large Synoptic Survey Telescope (LSST) now called the Vera Rubin Observatory. That wasn’t really within my horizon in 2001, although its planning phase had started then. It really got under way around 2008 and is now nearing completion. I certainly would have mentioned it had I known more about it at the time!

P.S. In case you’re wondering, the Euclid Mission due to be launched in early 2023 was very far from the drawing board in 2001 so I don’t apologize for not mentioning it!

Our Solar Neighbourhood

Posted in The Universe and Stuff with tags , , , on December 3, 2020 by telescoper

Here’s a very nice movie showing the stars in the Solar neighbourhood (defined to be within 100 parsecs of the Sun) with positions and colours mapped by the European Space Agency’s Gaia mission.

P.S. The video is timed to coincide with Gaia’s third data release: for more information about DR3 see here.

The Gaia video!

Posted in The Universe and Stuff with tags , on October 24, 2019 by telescoper

I’ve blogged quite a few times about the European Space Agency’s Gaia mission (see this tag). Social media brought this nice new video to my attention this morning so I couldn’t resist adding it to the collection. I don’t think it accompanies any new data release or scientific results but it’s very impressive anyway!

The Gaia Sausage

Posted in The Universe and Stuff with tags , , , , , on March 10, 2019 by telescoper

I had to undertake a top secret mission on Friday, which turned out to be much less exciting than I’d hoped, but at least it gave me an excuse to catch some of the Royal Astronomical Society Open Meeting followed by dinner at the RAS Club. I actually sat next to the Club Guest Michael Duff, the eminent theoretical physicist Michael Duff who gave a nice after-dinner speech.

An artist’s impression of the Gaia Sausage. The Gaia fork has not yet been proved to exist.

The last talk at the RAS Meeting was by Neil Wyn Evans of Cambridge University in the Midlands on the subject of the `Gaia Sausage‘ (which, as you can see, has its own Wikipedia page). The Gaia Sausage is so named because it is consists of a marked anisotropy of the velocity distribution of stars in Milk Way, which is elongated in the radial direction (like a sausage) indicating that many stars are on near-radial (i.e. low angular momentum orbits). This feature has been revealed by studying the second data release from Gaia.

The work Wyn described in his talk is covered by a nice press release from Cambridge University which links to no fewer than five articles on it and related topics, which can all be found on the arXiv here, here, here, here and here.

The most plausible explanation of the Gaia Sausage is that it is a consequence of a major collision between the Milky Way with a smaller galaxy containing about 109 stars about 8-10 billion years ago, as illustrated in this simulation.

I vote that this explanation of the velocity structure of the Milky Way should henceforth be called the Big Banger Theory.


I’ll get my coat.

Stars Dance to the Music of Parallax

Posted in The Universe and Stuff with tags , , on May 3, 2018 by telescoper

I thought I’d share this cute video from the European Space Agency about the Gaia mission I blogged about last week. It shows the effect of parallax, as measured by Gaia, on the positions of stars on the sky. As the Earth orbits the Sun stars do a dance in the sky; the shift in their position greater for closer stars rather than distant ones. To make the video, parallaxes measured by Gaia have been exaggerated by a factor 100,000 and proper motions have been speeded up by one trillion (1012). The effect is rather hypnotic, and gives a sense of the three-dimensional nature of the distribution of stars. At the end of the video you can see the effect of proper motions too, i.e. the change in position of a star due to its actual motion rather than that of the observer.

Gaia’s Second Data Release!

Posted in The Universe and Stuff with tags , , , on April 26, 2018 by telescoper

It seems like only yesterday that I was blogging excitedly about the first release of data (DR1) from the European Space Agency’s Gaia Mission. In fact it was way back in 2016! Anyway, yesterday came another glut of Gaia goodness in the form of the second release of data, known to its friends as DR2.

In case you weren’t aware, Gaia is an ambitious space mission to chart a three-dimensional map of our Galaxy, the Milky Way, in the process revealing the composition, formation and evolution of the Galaxy. Gaia will provide unprecedented positional and radial velocity measurements with the accuracy needed to produce a stereoscopic and kinematic census of about one billion stars in our Galaxy and throughout the Local Group. This amounts to about 1 per cent of the Galactic stellar population.

You can find links to all the DR2 science papers here, a guide to how to use the data here, and of course a link to the full Gaia Archive here.

Here’s a (brief!) list of the contents of DR2:

  • The five-parameter astrometric solution – positions on the sky (α, δ), parallaxes, and proper motions – for more than 1.3 billion (109) sources, with a limiting magnitude of G = 21 and a bright limit of G ≈ 3. Parallax uncertainties are in the range of up to 0.04 milliarcsecond for sources at G < 15, around 0.1 mas for sources with G=17 and at the faint end, the uncertainty is of the order of 0.7 mas at G = 20. The corresponding uncertainties in the respective proper motion components are up to 0.06 mas yr-1 (for G < 15 mag), 0.2 mas yr-1 (for G = 17 mag) and 1.2 mas yr-1 (for G = 20 mag). The Gaia DR2 parallaxes and proper motions are based only on Gaia data; they do no longer depend on the Tycho-2 Catalogue.
  • Median radial velocities (i.e. the median value over the epochs) for more than 7.2 million stars with a mean G magnitude between about 4 and 13 and an effective temperature (Teff) in the range of about 3550 to 6900 K. This leads to a full six-parameter solution: positions and motions on the sky with parallaxes and radial velocities, all combined with mean G magnitudes. The overall precision of the radial velocities at the bright end is in the order of 200-300 m s-1 while at the faint end the overall precision is approximately 1.2 km s-1 for a Teff of 4750 K and about 2.5 km s-1 for a Teff of 6500 K.
  • An additional set of more than 361 million sources for which a two-parameter solution is available: the positions on the sky (α, δ) combined with the mean G magnitude. These sources have a positional uncertainty at G=20 of about 2 mas, at J2015.5.
    G magnitudes for more than 1.69 billion sources, with precisions varying from around 1 milli-mag at the bright (G<13) end to around 20 milli-mag at G=20. Please be aware that the photometric system for the G band in Gaia DR2 is different from the photometric system as used in Gaia DR1.
  • GBP and GRP magnitudes for more than 1.38 billion sources, with precisions varying from a few milli-mag at the bright (G<13) end to around 200 milli-mag at G=20.
  • Full passband definitions for G, BP and RP. These passbands are now available for download.
  • Epoch astrometry for 14,099 known solar system objects based on more than 1.5 million CCD observations. 96% of the along-scan (AL) residuals are in the range -5 to 5 mas, and 52% of the AL residuals are in the range of -1 to 1 mas. The transit observations are part of Gaia DR2 and have also been delivered to the Minor Planet Center (MPC).
  • Subject to limitations (see below) the effective temperatures Teff for more than 161 million sources brighter than 17th magnitude with effective temperatures in the range 3000 to 10,000 K. For a subset of about 87 million sources also the line-of-sight extinction AG and reddening E(BP-RP) are given and for a part of this subset (around 76 million sources) the luminosity and radius are available as well.
  • Classifications for more than 550,000 variable sources consisting of Cepheids, RR Lyrae, Mira and Semi-Regular Candidates as well as High-Amplitude Delta Scuti, BY Draconis candidates, SX Phoenicis Candidates and short time scale phenomena.
  • Planned cross-matches between Gaia DR2 sources on the one hand and Hipparcos-2, Tycho-2, 2MASS PSC, SDSS DR9, Pan-STARRS1, GSC2.3, PPM-XL, AllWISE, and URAT-1 data on the other hand.

There’s much more to Gaia than pictures, but here’s a map of the stars in our galaxy to give you an idea:

I remember first hearing about Gaia about 17 years ago when I was on a PPARC advisory panel and was immediately amazed  by the ambition of its objectives. As I mentioned above, Gaia is a global space astrometry mission, which will make the largest, most precise three-dimensional map of our Galaxy by surveying more than a billion stars. In some sense Gaia is the descendant of the Hipparcos mission launched in 1989, but it’s very much more than that. Gaia monitors each of its target stars about 70 times over a five-year period. It is expected to discover hundreds of thousands of new celestial objects, such as extra-solar planets and brown dwarfs, and observe hundreds of thousands of asteroids within our own Solar System. The mission is also expected to yield a wide variety of other benefits, including new tests of the  General Theory of Relativity.

For the brighter objects, i.e. those brighter than magnitude 15, Gaia  measures their positions to an accuracy of 24 microarcseconds, comparable to measuring the diameter of a human hair at a distance of 1000 km. Distances of relatively nearby stars are measured to an accuracy of 0.001%. Even stars near the Galactic Centre, some 30,000 light-years away, have their distances measured to within an accuracy of 20%.

It’s an astonishing mission that will leave an unbelievably rich legacy not only for the astronomers working on the front-line operations of Gaia but for generations to come.

New: Top Ten Gaia Facts!

Posted in Astrohype, The Universe and Stuff with tags , , , on September 14, 2016 by telescoper

After today’s first release of data by the Gaia Mission, as a service to the community, for the edification of the public at large, and by popular demand, here is a list of Top Ten Gaia Facts.

Gaia looks nothing like the Herschel Space Observatory shown here.

Gaia looks nothing like the Herschel Space Observatory shown here.


  1. The correct pronunciation of GAIA is as in “gayer”. Please bear this in mind when reading any press articles about the mission.
  2. The GAIA spacecraft will orbit the Sun at the Second Lagrange Point, the only place in the Solar System where the  effects of cuts in the UK science budget can not be felt.
  3. The data processing challenges posed by GAIA are immense; the billions of astrometric measurements resulting from the mission will be analysed using the world’s biggest Excel Spreadsheet.
  4. To provide secure backup storage of the complete GAIA data set, the European Space Agency has commandeered the world’s entire stock of 3½ inch floppy disks.
  5. As well as measuring billions of star positions and velocities, GAIA is expected to discover thousands of new asteroids and the hiding place of Lord Lucan.
  6. GAIA can measure star positions to an accuracy of a few microarcseconds. That’s the angle subtended by a single pubic hair at a distance of 1000km.
  7. The precursor to GAIA was a satellite called Hipparcos, which is not how you spell Hipparchus.
  8. The BBC will be shortly be broadcasting a new 26-part TV series about GAIA. Entitled WOW! Gaia! That’s Soo Amaazing… it will be presented by Britain’s leading expert on astrometry, Professor Brian Cox.
  9. Er…
  10. That’s it.