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
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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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
136. Gaia and gravitational waves
How can Gaia contribute?
I describe three areas in which Gaia is contributing to the field of gravitational wave research: in helping to identify `verification binaries' for the LISA gravitational wave space mission, in searches for the electromagnetic counterparts of gravitational wave sources, and in providing constraints on the stochastic gravitational wave background.
7 August 2023
135. Triple star systems
Why are triple systems important?
How common are triple star systems? Why are they important to study, why they are so difficult to discover, and what can they tell us about the star formation process? Gaia is providing an unprecedented census of wide triple systems by virtue of its discovery depth and uniformity. Amongst them is the first resolved triple white dwarf system.
31 July 2023
134. Resolved binaries within 1 kpc
A huge sample of resolved binaries
Binary and multiple stars occur in a bewildering variety of configurations, and range from the very compact to wide, long-period systems. Here I look at a uniform sample of 1.3 million resolved binaries constructed from Gaia EDR3. It includes white dwarf–white dwarf binaries, main sequence–white dwarf binaries, and many `twin' binaries.
24 July 2023
133. Ellipsoidal variables and black holes
More searches for black holes
Ellipsoidal variables are close binaries with orbit inclinations too small to yield eclipses, but with components nonetheless distorted by their mutual gravitation as revealed by their light curves. More than 60,000 have been identified in Gaia DR3. Amongst them, various searches for dormant black hole or neutron star secondaries are ongoing.
17 July 2023
132. Variability, DR3, and Citizen Science
The content of DR3, and the role of Citizen Science
In essay 61, I looked at the identification and classification of variable stars from Gaia DR2, and some of the statistics of the resulting 550,000 variable stars. Here, I update the statistics for the 10 million variable stars identified in DR3. And I describe Gaia-Vari, the first Citizen Science contributions to Gaia which are assisting with the classification process.
10 July 2023
131. Double white dwarf mergers
Gaia is identifying some merger products
Gaia is making some fascinating contributions to the known occurrence of white dwarf--white dwarf binary systems, and on the observational consequences of the eventual merger of the two white dwarfs. The story involves gravitational waves, the unusual properties of `ultra-massive' white dwarfs, and the precursors of (some) type~Ia supernovae.
3 July 2023
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