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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240. Extreme nuclear transients
A Gaia science alert is the most luminous transient event discovered to date
The existence of supermassive black holes is implicit in various scientific areas being impacted by Gaia, including tidal disruption events which result from the tidal disruption of a star as it passes close to a supermassive black hole. The recent discovery of the highest-known luminosity transient events points to the disruption of intermediate-mass stars onto particularly massive black holes.
3 November 2025
239. Actinide-boost stars
The importance of the actinide elements in astronomy
Actinide-boost stars are a subset of stars enhanced with r-process elements. They display an over-abundance of actinide elements, characterised by the thorium/europium [Th/Eu] ratio. The availability of the Gaia DR3 distances and proper motions is providing some remarkable new insights into the physical mechanisms underlying the r-process enhancements, and the origin of the actinide-boost stars.
6 October 2025
238. Ophion: a most curious cluster
A new cluster in the process of rapid and complete disruption
A new determination of astrophysical parameters from a recalibration of the 220 million BP/RP spectra made available in Data Release 3 has extended the training set to emphasise pre-main sequence stars. An analysis of the resulting sky distribution of young stars has identified a new young association, Ophion, which appears to be in the process of its rapid and complete disruption.
1 September 2025
237. Beta Cephei variables
Asterosiesmology for the most massive pulsating stars
The Beta Cephei variables are the most massive pulsating stars on the main sequence, and are an important class for asteroseismology investigations. Recent studies have shown that the sparsely sampled Gaia photometry is nevertheless exceptionally accurate for detecting their dominant and secondary frequencies. I look at how these studies are furthering our understanding of their fundamental properties, and their detailed physics.
4 August 2025
236. More on interstellar extinction
New insights into the growth of interstellar dust
Recent studies of interstellar extinction, based on Gaia DR3 distances and low-resolution BP/RP spectra, have provided a substantial advance in mapping the total-to-selective extinction for 130 million lines of sight throughout our Galaxy. Their analysis throws new light on the processes of accretion and coagulation that are responsible for the growth of dust grains in the interstellar medium.
7 July 2025
235. Populations in globular clusters
New light in this puzzling problem in stellar astrophysics
The occurrence of multiple stellar populations in globular clusters has been described as one of the most puzzling open issues of stellar astrophysics. I describe the problem, and demonstrate how the combination of Gaia's astrometry, and the synthetic photometry derived from its low-resolution BP/RP spectra, is helping to identify its origin.
30 June 2025
234. Ultra-faint dwarf galaxies
The oldest and faintest end of the dwarf galaxy spectrum
The class of ultra-faint dwarf galaxies, the first of which were discovered only in 2005, represent the faintest, oldest, most metal-poor, and most dark-matter-dominated end of the dwarf galaxy spectrum. Providing important tests of LambdaCDM cosmology, Gaia is helping to refine their membership, clarify their morphologies (including evidence for tidal disruption), and define their orbits and orbital histories.
23 June 2025
233. Two more exoplanets: Gaia 4b and Gaia 5b
Bringing us up-to-date with Gaia's exoplanet discoveries
Today, NASA's Exoplanet Archive lists almost 6000 confirmed exoplanets. There are only five astrometric discoveries amongst them, with one found from VLT-FORS, and one from radio VLBA. The first Gaia astrometric discovery, HIP 66074, was reported in 2023. Here I describe the two latest discoveries, Gaia 4b and Gaia 5b. And I explain why the numbers discovered with Gaia are still rather small.
16 June 2025
232. Delta Scuti stars
New insights into their complex pulsations
Delta Scuti variables are a common class of pulsating star located at the intersection of the instability strip with the main sequence, showing a rich variety of pulsation behaviour. Gaia-enabled studies are being used to characterise their period-luminosity relation as a function of pulsation mode and metallicity. And a long-standing question being advanced by Gaia is why many of the stars in the Delta Scuti instability strip do not pulsate.
9 June 2025
231. Dwarf spheroidals: an update
More advances in understanding our remote galaxy companions
I describe the latest Gaia-based studies of the classical dwarf spheroidal galaxies of the Local Group. Gaia DR3 is being used to derive improved orbits and orbital histories, and these are shown to link to associated star formation bursts at they approach pericentre. There are new studies of their structure, internal kinematics, and bulk rotation, and of their associated globular cluster systems.
2 June 2025
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