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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170. Metrication in the UK
Some thoughts on the UK's move to the metric system
Metrication concerns the move from the historical use of feet and inches, of pounds and ounces, and of pints and gallons, to the coherent metric system in which units are inter-related. The UK's resistance to full adoption of the metric system, used by almost all other countries, has left society confused, and at an economic disadvantage.
1 April 2024
169. A billion radial velocities
A new catalogue of 125 million radial velocities
Gaia Data Release 3 includes 33 million radial velocities, obtained from its Radial Velocity Spectrometer, ten times as many as previously known. DR4, in 2025, should include some 100 million. A recent study has derived 125 million from the low-resolution BP/RP spectra, albeit of much lower accuracy, with perhaps 1–2 billion coming available in the future.
25 March 2024
168. S stars in the Wesenheit diagram
A powerful way of classifying AGB stars
S stars are cool luminous giants, lying between the O-rich M-type giants and the C-rich carbon stars on the AGB. With atmospheres enriched with s-process elements brought to the surface through the `third dredge-up', they have created around half the atomic nuclei heavier than iron. Gaia astrometry and photometry is advancing their understanding.
18 March 2024
167. Carbon stars
New insights into this important class of complex star
Carbon stars, which reside on the asymptotic giant branch, are an important phase of stellar evolution. Their very high luminosities makes them important for integrated light studies of galaxies, and they are being considered as potential standard distance indicators. Their large distances and complex physics provide numerous challenges for theoretical modelling.
11 March 2024
166. Hypervelocity stars: part 2
An update on hypervelocity stars
In essay 22 (May 2021) I described some of the early insights that Gaia was providing in the field of hypervelocity stars, a rare and exotic type of star, racing through our Galaxy with velocities of 500-1000 km per second or more. Here, I bring the Gaia results up-to-date, describing the latest searches, and more recent insights into their origin.
4 March 2024
165. Runaway stars
How these high velocity stars originate
Runaway stars are stars with such high space velocities that they must have been imparted by a particular formation process. Many insights into the favoured scenarios - binary-supernova and dynamical ejection - are being made as Gaia's astrometry yields their accurate distances and space motions.
26 February 2024
164. Sub-subgiants and (tiny) black holes
A new class of star, and a link with primordial black holes
I look at some recent advances in the numbers of blue stragglers identified by Gaia, and Gaia's contribution to the identification and understanding of the new class of sub-subgiant star. I discuss whether these offer a particularly interesting search sample for stars hosting primordial black holes in their centres.
19 February 2024
163. Dual active galactic nuclei
A remarkable contribution to Lambda CDM cosmology
Dual active galactic nuclei, we now know, provide an important probe of the physical processes that drive the in-spiralling of supermassive black hole pairs inside a single merged galaxy. Difficult to detect and characterise, Gaia is making a significant contribution to their discovery.
12 February 2024
162. Cosmology with Gaia's quasars
Two topical tests of Lambda CDM cosmology
The large number of quasars measured by Gaia contributes fundamentally to the determination of the quasi-inertial reference frame. Today, Gaia's quasar survey is also being applied to two topical observational questions in Lambda CDM cosmology: the kinematic dipole anomaly, and the S8 tension.
5 February 2024
161. Strongly lensed quasars
A new search for gravitational lenses in the Gaia data
In this last of five essays on ESA's October 2023 'Focused Product Release' topics, I look at a new search for strongly lensed quasars in the data interval used for Data Release 3. Exploiting Gaia's 0.18 arcsec angular resolution, this new analysis found 450 previously known systems but with 86 newly detected components, and 381 new lensed candidates.
29 January 2024
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