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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46. The iterative solution: formulation
How the catalogues are created in principle
Gaia gathers an enormous quantity of observations of a vast numbers of stars over several years. The goal of the data analysis on the ground is straightforward in principle: like solving a giant celestial jigsaw, the task is to find the astrometric parameters of each star best matching this gargantuan global set of observations.
15 November 2021
45. RR Lyrae variables
There are 140,000 of these important tracers
Like the Cepheid variables, although less luminous, the distinctive light curves of RR Lyrae stars allows their detection to large distances. Nearly 200 were included in the Hipparcos catalogue, although only one provided a meaningful distance. Gaia DR2 provides results for 140,784 RR Lyrae stars as faint as 20.7 mag.
8 November 2021
44. The Hubble constant from Cepheids
First hints concerning the 'Hubble tension'
The expansion of the Universe is a central concept in modern cosmology. Empirically, it is partly characterised by the local value of the Hubble constant. But 'early Universe' and 'late Universe' methods are in slight, but significant, disagreement. Gaia parallaxes of Cepheids are assisting the understanding of this so-called 'Hubble tension'.
1 November 2021
43. Cepheid variables
Hipparcos measured 280, Gaia 10,000
Cepheids are pulsating stars with periods of around 1–30 days. They play a crucial role as distance indicators, and they are important tracers of spiral arms and Galactic rotation. The Hipparcos catalogue contained just 280. The second Gaia data release contains 9575 Cepheids, of which 3767 are in the LMC, and 3692 are in the SMC.
25 October 2021
42. Surprises in the HR diagram
Remarkable gaps in the lower main sequence
The vast numbers of accurate star distances being measured by Gaia yield Hertzsprung–Russell diagrams with unprecedented detail. The lower parts of the HR diagram show several surprising new features, with the convective-radiative transition evident in red dwarfs, and plasma crystallisation evident in white dwarfs.
18 October 2021
41. The age of our Milky Way Galaxy
Signs of mergers long long ago
Accurate distances out to 2000 parsec are yielding large samples of stars from our Galaxy's halo and from its thick disk populations. Their strikingly similar ages, from early in the life of the Universe, point to a common origin in which an orbiting galaxy merged with our own more massive progenitor.
11 October 2021
40. The distance of Omega Centauri
A new milestone in distance determinations
Globular clusters are amongst the oldest Galactic populations, and their origin remains unclear. But they are too distant for direct estimates of their parallaxes and ages to have been made in the past. Determination of the distance of Omega Centauri with Gaia represents a new milestone of distance measurements in astronomy.
4 October 2021
39. The Galactic anti-centre
A new probe of our Galaxy's components
Observations towards the Galactic centre probe its densest and most complex regions, with multiple populations superimposed: the thin and thick disks, star clusters, spiral arms, a bar, and the central bulge. In the anti-centre direction, star densities and interstellar extinction are lower, and the disk and halo can be more easily resolved.
27 September 2021
38. The Magellanic Clouds
Advances in understanding our neighbours
The Magellanic Clouds are two 'nearby' irregular dwarf galaxies, visible to the unaided eye in the dark skies of the southern hemisphere. They may be orbital companions to our Milky Way, or on a 'first approach' at the start of a grand galactic merger event. Hipparcos observed less than 50 stars in both systems. Gaia is observing more than 10 million.
20 September 2021
37. Ultra-wide binaries
Stellar binaries at the edge of disruption
Gaia is identifying tens of thousands of very wide and ultra-wide binaries from their highly accurate space motions. The resulting discoveries, and their space motions, are allowing great progress in understanding their formation, evolution, and the various processes that contribute to their eventual disruption.
13 September 2021
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