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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130. The initial mass function
What is the mass distribution when stars are born
The 'present-day mass function' describes the distribution of actual masses in any given stellar population. From it can be inferred the more fundamental 'initial mass function', intimately related to the original star formation processes. Its precise form, and whether it is universal, have been difficult to establish in the absence of accurate distances.
26 June 2023
129. Stars within 25 pc: the CNS5
A leap in understanding our neighbourhood
In essay 33 I described Gaia's census of stars within 100 pc, the GCNS. A recent result, also from EDR3, is the Fifth Catalogue of Nearby Stars, the CNS3. With 5931 objects out to 25 pc, it covers a smaller volume than GCNS, but it is much more complete. It comprises 4946 main-sequence, 20 red giants, 264 white dwarfs, and 701 brown dwarfs.
19 June 2023
128. The enormous dwarf galaxy Antlia II
Discovery of one of the largest of the Milky Way's satellite galaxies
The past 20 years has seen the discovery a new class of dwarf galaxy, the ultra-faint dwarfs, of which some 60 are known today. They include some of the most dark matter-dominated systems known, with core-density profiles challenging predictions of structure formation. A striking discovery with Gaia has been Antlia II, similar in spatial extent to the LMC.
12 June 2023
127. Gould's Belt and the Radcliffe Wave
Goodbye to Gould's Belt, hello to the Radcliffe Wave
The structure and origin of the grouping of bright stars known as the Gould Belt has been studied for more than a century. Distances from Gaia rule out it being a physical ring-like structure. But the 3D arrangement of nearby molecular clouds has instead revealed a more complex and more mysterious Galactic-scale distribution of dense gas, the Radcliffe Wave.
5 June 2023
126. OurSun's height above the Galaxy disk
Where are we in the Galaxy?
It sounds like a simple enough question: how far is the Sun from our Galaxy's mid-plane? But studies over the past century have failed to reach a consensus answer, with estimates still ranging from as little as 5 parsec to as much as 20 parsec. And even though distances are central in deriving such a linear quantity, Gaia has so far failed to resolve the debate.
29 May 2023
125. The Besançon Galaxy model
Modelling the star counts seen by Gaia
The Besançon Galaxy model is a stellar population synthesis model replicating many important features of our Galaxy. It played a key part in the preparation of the Gaia mission, and is now being used, by comparison with the latest data releases, as a probe of properties such as our Galaxy's stellar initial mass function, star formation rate, and gravitational potential.
22 May 2023
124. Planetary nebulae
Known distances have gone from 40 to 2000
Planetary nebulae are the dust and gas shells ejected during the late asymptotic giant branch phase for intermediate mass stars, the ejecta being ionised by the radiation from the hot, evolving central star. Their modelling has been constrained by poor knowledge of their distances. With Gaia DR3, the number with good distances has gone from some 40, to around 2000.
15 May 2023
123. Using ADS citations in a Latex file
Do you struggle managing your citations?
I am deviating from my usual topic of scientific results from Gaia, and offer some suggestions about one aspect of using the ADS (Astrophysics Data System) bibliography in Latex documents. Whether to use, for citations, the 19-character code created by ADS, or your own ad hoc code. If you struggle with the management of citations, do read!
8 May 2023
122. The cosmic distance ladder
The big picture of where we stand at the moment
I give a big picture of where we are in establishing the cosmic distance scale, noting the `tension' that exists between the `early' and `late Universe' estimates of the Hubble constant. This, in turn, seems to imply that there is a problem with the late Universe distance `ladder'. I point to the areas where we are expecting Gaia to contribute in the future.
1 May 2023
121. Modelling stellar evolution
How Gaia is helping to improve our models
I will look briefly at the big picture involved in modelling stellar structure and evolution. I will point to some of the areas of stellar physics where Gaia is providing new constraints, and which are contributing to a number of ongoing model refinements. Playing their part are advances in software infrastructure, computer processing power, and data storage.
24 April 2023
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