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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110. Variation of the gravitational constant
Is general relativity correct?
According to our present understanding of the laws of Nature, the gravitational constant (G) does not vary, neither across space, nor time. Measuring any variation over cosmological time would be important in confirming or challenging our present understanding. Measuring such changes is possible, and I explain how Gaia may contribute.
6 February 2023
109. The tidal tails of Palomar 5
Constraining the shape of our Galaxy's halo
Palomar 5 is a faint globular cluster 20 kpc from the Sun. Its two tidal tails, evidence that the cluster is experiencing heavy mass loss, have been known since 2001. With the form of the tails being sensitive to the shape of the Galactic halo, and with longer tails being more constraining, Gaia is contributing to their understanding.
30 January 2023
108. White dwarf physics
Insights into these bizarre objects
Continuing on the subject of white dwarfs, I look at two other aspects that Gaia is helping to characterise. The white dwarf sequence in the colour-magnitude diagram reveals new insights into their composition and mass, and also into the physical processes accompanying their cooling over cosmological time scales.
23 January 2023
107. The white dwarf mass-radius relation
Empirical confirmation of stellar evolution theory
Theories of white dwarf interiors predict a relation between mass and radius that depends on their core composition and temperature. But accurate empirical confirmation is challenging. Results from Gaia DR2 show good agreement with predictions, confirming stellar degeneracy, while raising new questions.
16 January 2023
106. The remarkable cluster Westerlund 1
Resolving the puzzle of its age
At a distance of about 4 kpc, the young compact star cluster Westerlund 1 contains many rare, evolved, high-mass stars, including red supergiants, yellow hypergiants, one of the largest known stars, and 24 Wolf-Rayet stars. Gaia is pinpointing its distance and its members, and resolving the puzzling question of its age.
9 January 2023
105. Wolf–Rayet stars
Some of the hottest and brightest stars
Wolf-Rayet stars are the final He-burning phase in the evolution of massive O stars, the last observable stage before core collapse. They are rare, but have an enormous influence on their environment. Distances were only poorly constrained before Gaia, which is now having a significant impact on their detailed understanding.
2 January 2023
104. Light deflection... by Jupiter
Measuring Jupiter's effects on space-time
Gravitational light-bending by the Sun during the solar eclipse of 1919, around 1.7 arcsec at the solar limb, was the first observational confirmation of general relativity. Hipparcos measured light bending over the entire celestial sphere. Gaia has now measured light bending due to Jupiter, at around 10 milli-arcsec.
26 December 2022
103. Stellar rotation... for 3 million stars
How is Gaia measuring them?
The study of stellar rotation has been transformed within the past decade. Twenty years ago, the number of stars with measured rotation periods stood at 11,000. Kepler made a major advance by characterising photometric modulation for some 60,000. With Gaia DR3, more than three million are now known.
19 December 2022
102. The heart of the Milky Way
Hints of the earliest structures
Compared to recent advances in understanding the details of our Milky Way's halo, the inner Galaxy has proven more elusive, complicated by the large distances involved. Amongst several advances being made with the Gaia data is the identification of the most ancient `proto-Galactic' component of our Galaxy.
12 December 2022
101. The nearest black hole
The first black hole discoveries
Gaia is enabling the discovery of two important classes of stellar-mass black holes in our Galaxy: isolated black holes, and those in binary systems which lack significant mass transfer. Amongst the latter, recently reported, is the nearest known black hole, at a distance of just 480 pc.
5 December 2022
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