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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186. Moving groups and traceback ages
New clues about cluster formation and gas dispersal processes
An independent method of estimating the ages of nearby stellar associations makes use of their expanding space motions to yield what are referred to as dynamical or traceback ages. The difference between various methods is providing important clues about the cluster formation and gas dispersal processes.
22 July 2024
185. Mira variables
Another sort of distance indicator
As evolved stars cool and expand on the red giant or asymptotic giant branch, they become pulsationally unstable. Miras are AGB stars with large variability amplitude and long pulsation period. Here, I will look at some Gaia results regarding their use as distance indicators and probes of Galactic structure.
15 July 2024
184. Stellar streams and sub-halos
An important test of Cold Dark Matter cosmology with Gaia
A specific application of the study of stellar streams is in placing constraints on the existence and nature of the numerous dark matter sub-halos (halos within halos) that are predicted, in standard Cold Dark Matter cosmology, to exist surrounding the Milky Way. The stream GD-1 shows some tantalising prospects.
8 July 2024
183. CCDs and CTE
What makes Gaia's CCDs special
Gaia presented a number of new and demanding requirements for its CCDs, which in turn required an extensive development program pre-launch. I will describe some of these, and expand on why the issue of radiation damage was so critical for Gaia, and what measures were taken to minimise the effects of the solar cycle.
1 July 2024
182. The YORP effect
A second-order Yarkovesky-like force
The YORP effect is a second-order Yarkovsky-like effect, influencing the spin rate and spin axis orientation of sub-km sized irregular asteroids. Like the Yarkovsky force, its effects accumulate over long periods, resulting in some rather remarkable changes in the properties of the asteroid population, seen clearly in DR3.
24 June 2024
181. The Yarkovsky effect
A curious but important phenomenon in solar system dynamics
The Yarkovsky effect is important in solar system dynamics. For a small rotating body illuminated by the Sun, re-radiated thermal emission lags behind the incident radiation, contributing a component of force in the direction of its orbital motion. I explain why it is important, and how Gaia is contributing to its understanding.
17 June 2024
180. The spectra of solar system objects
What sunlight reflected from astroids tells us about their history
Photometry of minor solar system bodies provides information about their shape and rotation. Reflected sunlight encodes information about their composition and taxonomic classification, from which information about their origin and evolution can be deduced. DR3 gives reflectance spectra for more than 60,000 such bodies.
10 June 2024
179. Stellar masses from SB2 binaries
Gaia's contribution to fundamental stellar masses
Masses are one of the most fundamental stellar properties, crucial in determining their structure and evolution. Yet ways of determining accurate masses are strictly limited and, even today, only a couple of hundred are known to better than 1-2 per cent. I summarise how Gaia is contributing.
3 June 2024
178. Bifurcation in the white dwarf HRD
New evolutionary models inspired by Gaia's colour-magnitude diagram
The prominent bifurcation in the white dwarf colour-magnitude diagram, seen for the first time in Gaia DR2, was tentatively attributed to different evolutionary tracks for hydrogen- and helium-dominated atmospheres. Subsequent studies have focussed on trace amounts of `dredged-up' carbon being the cause.
27 May 2024
177. An intermediate black hole in M4?
Hints of an intermediate-mass black hole in our nearest globular cluster
Intermediate-mass black holes lie between the masses of stellar mass black holes, formed by single star collapse, and supermassive black holes, formed in the high-density environment of galaxy centres. Plausible formation mechanisms, but no definitive candidates, are known. I describe some insights from Gaia in the case of our nearest globular cluster, M4.
20 May 2024
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