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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180. The spectra of solar system objects
What sunlight reflected from astroids tells us about their history
Photometry of minor solar system bodies provides information about their shape and rotation. Reflected sunlight encodes information about their composition and taxonomic classification, from which information about their origin and evolution can be deduced. DR3 gives reflectance spectra for more than 60,000 such bodies.
10 June 2024
179. Stellar masses from SB2 binaries
Gaia's contribution to fundamental stellar masses
Masses are one of the most fundamental stellar properties, crucial in determining their structure and evolution. Yet ways of determining accurate masses are strictly limited and, even today, only a couple of hundred are known to better than 1-2 per cent. I summarise how Gaia is contributing.
3 June 2024
178. Bifurcation in the white dwarf HRD
New evolutionary models inspired by Gaia's colour-magnitude diagram
The prominent bifurcation in the white dwarf colour-magnitude diagram, seen for the first time in Gaia DR2, was tentatively attributed to different evolutionary tracks for hydrogen- and helium-dominated atmospheres. Subsequent studies have focussed on trace amounts of `dredged-up' carbon being the cause.
27 May 2024
177. An intermediate black hole in M4?
Hints of an intermediate-mass black hole in our nearest globular cluster
Intermediate-mass black holes lie between the masses of stellar mass black holes, formed by single star collapse, and supermassive black holes, formed in the high-density environment of galaxy centres. Plausible formation mechanisms, but no definitive candidates, are known. I describe some insights from Gaia in the case of our nearest globular cluster, M4.
20 May 2024
176. Black holes in stellar streams
Stellar mass black holes influence the morphology of stellar streams
I look at how the existence of stellar mass black holes might affect the morphology and kinematics of the stellar streams that are now known to exist in the inner and outer halo of our Galaxy, and to what extent Gaia can help to distinguish between those that are rich in, or devoid of, stellar mass black holes.
13 May 2024
175. Black holes in open clusters
How we can infer that an open clusters hosts stellar mass black holes
The existence of stellar mass black holes has observable consequences on the dynamics of open clusters, which can in turn place useful constraints on their formation. I look at some early results for the Hyades open cluster, which suggest that the cluster should contain 2-3 stellar mass black holes in long-period binary systems.
6 May 2024
174. Proper motion anomalies
A fascinating subset of astrometric binaries
A subset of astrometric binaries evaded identification by Hipparcos, but they become recognisable from the difference in the proper motion determined by Hipparcos and that measured by Gaia. Recent work on these `proper motion anomalies' has underlined their ubiquity, and their scientific importance.
29 April 2024
173. The breathing motion of spiral arms
New insights into the dynamics and origin of spiral arms
In essay 114, I looked at advances being made in understanding our Galaxy's spiral arm structure. Here I will look at some remarkable insights into their kinematics. Gaia is providing confirmation of their `breathing modes' which, in turn, support some of the theoretical and numerical models being developed to understand their origin.
22 April 2024
172. The basic angle
Why was the basic angle 58 degrees for Hipparcos, and 106.5 degrees for Gaia?
For Hipparcos and Gaia, the `basic angle' is the angle between the instrument's two viewing directions on the sky. For Hipparcos, the basic angle was 58 degrees. For Gaia it is 106.5 degrees. What is the reason for the two fields of view in the first place? How is the angle between them chosen? And why is it so different for Hipparcos and Gaia?
15 April 2024
171. The Small Magellanic Cloud
A radically new view of one of our nearest neighbours
The Large and Small Magellanic Clouds are two of our nearest galaxy neighbours. The line-of-sight structure of the SMC is particularly complex, and the wealth of observational data has proven difficult to interpret. A recent study using Gaia DR3 suggests a radically new picture: that the SMC is composed of two distinct superimposed structures.
8 April 2024
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