Gaia: science essays
In these short (mostly 2-page) weekly 'essays', I have picked out some of the scientific highlights of the Gaia mission as they are emerging, or as they caught my attention. They offer a snapshot of some of the discoveries that Gaia is making across all of astronomy. I've also included some essays on related topics, including the history of astrometry, and some more technical, managerial, or developmental aspects of both the Hipparcos and Gaia missions. In each, I have included a footnote DR1, DR2, EDR3, DR3, etc to indicate which of the (latest) data releases the essay refers to (described in essays #10 and #76), with DR0 signifying technical or historical material not connected with any specific data release. Who are they written for? Anyone who might have a general interest in science and astronomy, including amateur astronomers, young scientists starting out on their careers, mid-career scientists looking in on Gaia for the first time to get a feeling of what is possible, and specialists looking in from different areas of astronomy, or physics more generally. My thanks go to many people: to all those I worked with on the Hipparcos and Gaia projects over almost 30 years, to those now dedicating huge reserves of their time, energy, and skill to the ongoing data processing, and to those who have entered into the Gaia catalogue and published the results described here. Click on the access PDF icon to access the file. Only a few references are included, and these are 'discreetly' hyperlinked for those who want to read more... where references appear in the form (Einstein 1908) or www.gaia.com, clicking on the text (even though generally not highlighted!) should lead to the relevant online article. In a few cases, I've recorded an interview on the subject (see science interview page).
New: As of early July 2025, have converted Essays 1–130 into audio "discussion-type" podcasts, entirely using generative AI. They are available at my Gaia Essay YouTube channel, and I describe their construction in Essay 227.​​
As of July 2025, my essays will be monthly (on the first Monday of the month) until further notice.​​
This table page lists all essays, updated to the end of June 2025 (1–235 inclusive), in tabular form. It includes a simple search on the title field.​​​​
New: This Gaia Science Tree (v3.0, July 2025) presents essays 1–235 (Jan 2021–Jun 2025) as a hyperlinked "mind map"
* all end-nodes are hyperlinked to the given essay number (links are to "legacy" copies at the CERN-Zenodo site)
* catalogue content topics are at top right, background material at bottom left, otherwise moving "outwards" clockwise in the diagram
* I have prepared this as a didactic tool. Please feel free to make use of it as you wish
Please make use of this subscribe page to receive an email (usually Monday) when each new essay is published
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Essays through to the end of 2023 (1–156 inclusive) also appear in a hyperlinked indexed form in the Bulletin of the American Astronomical Society (BAAS Vol. 56, Issue 1, 15 March 2024): ADS 2024BAAS...56a.008P
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146. Benford's law - and astrometry
A curious mathematical property
A curious property of many collections of numbers, including naturally occurring data, is that the leading digits are not uniformly distributed, but are skewed toward smaller values. As Benford's law, it has been considered in many contexts, including identifying suspicious accounting. I consider it here in the context of the Gaia DR2 parallaxes.
16 October 2023
145. Spectroscopic binaries
New insights in orbit circularisation
Included in the DR3 data release are some 180,000 single-lined spectroscopic binaries, a colossal increase in numbers enabling the study of many aspects of short-period binaries. I look here at some new insights into orbit circularisation, where the Gaia data point to the process operating most efficiently in the pre-main sequence evolutionary phase.
9 October 2023
144. How many open clusters?
An explosion of new discoveries with Gaia
Gaia is transforming the study of open clusters. Pre-2016, some 3000 clusters had been identified. But Gaia has shown that more than half of these are unreal, being simply asterisms. In their place, from nearly 25,000 new discoveries reported with Gaia (some duplicated), there are today nearly 14,000 unique clusters known in our Galaxy.
2 October 2023
143. Gaia's maps of the Milky Way
Different views of our Galaxy seen with Gaia
The ESA-Gaia `Image of the Week', on the first anniversary of the 34-month Data Release~3 (DR3), 13 June 2023, was a remarkable multi-dimensional map of the Milky Way, produced by the many members of the Data Processing and Analysis Consortium (DPAC). I'm showing these 10 splendid sky plots again here.
25 September 2023
142. Gaia and the search for axions
Constraints on "axionic" dark matter
Gaia's contribution to understanding the distribution of dark matter follows from studies of the stellar halo population, Galactic kinematics, and stellar streams. Studies have also begun to place specific constraints on the properties of dark matter, for example if it is comprised of axions. I provide an overview of the various approaches.
18 September 2023
141. White dwarf pulsars
A remarkable new class of white dwarf
White dwarfs have featured in a number of my previous essays. Gaia is providing well-defined samples in the solar neighbourhood, with more than 260,000 from Gaia DR2. I look here at the recent discovery of the new type of `white dwarf pulsars'. With just two members of this exclusive class, Gaia is contributing to their understanding,
11 September 2023
140. Cataclysmic variables
Binary star evolution and accretion disks
Cataclysmic variables are interacting binaries containing a white dwarf accreting from a donor star. Their space density places strong constraints on models of their formation and evolution, and has revealed major discrepancies between observations and theory. New and improved space densities are becoming available with Gaia.
4 September 2023
139. Quadruple star systems
Some very special objects
How common are quadruple star systems? How is Gaia discovering and characterising new systems, and what do they tell us about the star-formation process? One specific quadruple system is providing insights into the cooling and crystallisation of white dwarf interiors, and another is providing hints about the origin of type Ia supernovae.
28 August 2023
138. Twin binaries - and our Sun
Might our own Sun have been one?
Gaia has confirmed the existence of 'twin binaries' with components of equal brightness and presumably equal mass. This has stimulated new investigations into their enigmatic origin, the possible implications for models of star formation, and perhaps some consequences for the origin of our own solar system
21 August 2023
137. Occultations and stellar diameters
A new approach to measuring star diameters
Gaia is revolutionising the study of stellar occultations because its dense grid of accurate star positions allows greatly improved prospects of predicting such occultation events. I mention some recent results on outer solar system bodies, and explain how the technique is also being used to measure the angular diameters of stars
14 August 2023
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