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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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
80. Neutron stars and pulsars
Invisible objects and advances with Gaia
Apart from the Crab, pulsars are not bright enough to be observed by Gaia. But more than 20 binary pulsar companions have been identified in Gaia DR2. Astrometry provides crucial input to models of supernova core collapse and their equation of state, their orbital decay and spin-down rates, and their birth sites.
11 July 2022
79. More insights into non-single stars
Insights from non-single stars
Gaia DR3 was accompanied by new insights into the nature of non-single stars in the Gaia survey. In Essay 78, I looked at the first results on exoplanet companions discovered by astrometry. Here, I look at some other results on stellar masses, brown dwarf and white dwarf companions, and some of the more exotic variables.
4 July 2022
78. Gaia's first exoplanets
The first of thousands?
Pre-Gaia DR3, the NASA exoplanet archive tabulated more than 5000 exoplanets, with just one discovered from astrometry. With just over 34 months of data, Gaia DR3 is accompanied by 130,000 astrometric orbit solutions, including 1843 brown dwarf companion candidates, and 72 exoplanet candidates.
27 June 2022
77. The Galactic escape velocity
Constraints on the halo mass
What is the total mass of our Galaxy? How far out does our Galaxy halo extend? The distribution of stellar velocities, and in particular the ‘escape’ velocity from the solar neighbourhood, holds a number of clues. Estimates from Gaia are converging on a Milky Way mass of about 10^(12) times the mass of our Sun.
20 June 2022
76. Data Release 3
Gaia's latest stunning data release
Gaia is now almost seven years into a possible 10-year data collection phase. Today marks the latest data release, Gaia DR3. For the same stretch of time and the same set of observations as EDR3 (Early Data Release 3), DR3 presents a stunning wealth of new data products derived from this first 3 years of mission data.
13 June 2022
75. The local mass density
Gaia as a dark matter detector
Stars in the Galactic disk 'bounce' slowly up and down around its mid-plane as a result of the force exerted by the matter comprising the disk itself. The detailed stellar motions depend on the total disk mass, both visible and dark matter. The Gaia data are throwing new light on the disk structure and its dark matter content.
6 June 2022
74. Open clusters with Gaia
More sites of recent star formation
Gaia is revolutionising the study of Galactic open clusters. High-quality distances allow cluster membership to be refined, space motions convey details of their dynamics and dispersion, and its unprecedented multi-epoch multi-colour photometry further contributes to classifying membership and chemistry.
30 May 2022
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