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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90. Astrophysics of our Galaxy
Examples of Gaia's astrophysical parameters
I continue with the subject of my previous essay, and give some examples of the way in which the derived astrophysical data are providing new insights in understanding the structure, formation, and evolution of huge numbers of stars in our Galaxy. They give just a flavour of what Gaia is providing.
19 September 2022
89. A revolution in stellar astrophysics
How the astrophysical parameters are derived
The recently published Data Release 3 includes a wealth of `extracted' astrophysical data, of staggering extent, including stellar spectroscopic and evolutionary parameters for up to 470 million sources. A dozen refereed papers detail the underlying computations. Here, I provide a synopsis of the methods, and results.
12 September 2022
88. Pinpointing exoplanets
Some other areas where Gaia assists
Continuing with the study of exoplanets, I focus here on three areas where Gaia is helping to vet, and to pinpoint, these other worlds: rejecting false positives from transit searches, establishing masses from radial velocity minimum estimates, and identifying accelerating systems for imaging searches.
5 September 2022
87. Radial velocities: results from DR3
First science with RVS
Gaia Data Release 3 provides radial velocities for more than 33 million stars down to about 14 mag. Here I look at some of the first scientific results from these radial velocities: their distribution across our Galaxy, their high-velocity star content, and their distribution in the globular cluster 47 Tuc.
29 August 2022
86. Radial velocities: their acquisition
How are radial velocities measured
Of the 1.8 billion sources in Gaia Data Release 3, more than 33 million have published radial velocities. More than 100 million are expected in DR4. I describe how the radial velocities are acquired on board, how they are processed, and the additional data that is being extracted from the spectra.
22 August 2022
85. Radial velocities: what wavelength?
The wavelength choice for radial velocities
I look back to the period leading up to the selection of Gaia in 2000, and recall why the decision was made to acquire radial velocity observations on-board the satellite itself. And I recall the various considerations which influenced the choice of spectral range chosen for the Gaia radial velocity spectrometer.
15 August 2022
84. Gaia's microlensing events
A new chapter in microlensing
Photometric microlensing has been responsible for the many thousands of events that have been discovered to date, including more than 350 exoplanets. Gaia is opening a new chapter in these studies, with its all-sky coverage, and its 3-colour sampling. Data Release 3 has identified 363 such events, 90 of them new.
8 August 2022
83. The Andromeda photometric survey
A test region for epoch photometry
The Gaia Andromeda Photometry Survey contains 3-colour epoch photometry for a million stars in the region of the Andromeda Galaxy. It was selected, as representative of the sky scanning and range of stellar densities, to provide a test region in advance of the full-sky epoch photometry planned for DR4.
1 August 2022
82. Gaia's galaxy survey
First results on Gaia's galaxies
Gaia Data Release 3 contains the first treatment of sources considered to be extended. Out of nearly one million galaxies, profile fitting yields robust parameter solutions for more than 900,000 mostly elliptical systems. Out of more than a million known quasars, a host galaxy has been detected around more than 60,000.
25 July 2022
81. Supernova remnants
Furthering our knowledge of supernovae
Continuing with the theme of neutron stars and pulsars, I look here at some well-known supernova remnants and the search for runaway stars escaping from them. I also look at Gaia's distance to the Crab Pulsar, to Gaia's photometry of its synchrotron-induced spin down, and to a class of stars that... disappear!
18 July 2022
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