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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193. More on wide binaries
Recent Gaia results throw new light on their formation
The origin of wide binary stars remains something of a mystery. In two earlier essays I looked at how Gaia detects them, and at suggestions that their orbits are inconsistent with Newtonian dynamics. Recent Gaia results throw new light on their formation, while researchers are still divided on the very long-period orbits.
9 September 2024
192. The structure of molecular clouds
Why their 3D structure is important
Great progress has been made in understanding the structure of Galactic molecular clouds with the availability of the Gaia data, as a probe of both star distances and interstellar extinction. I will give some background, explain why their 3D structure is relevant, and illustrate how Gaia is contributing.
2 September 2024
191. Extinction
Key to describing the distribution of gas and dust
A recurrent theme in the Gaia literature is interstellar extinction. This is important in the determination of stellar parameters through spectral modelling, and is central in describing the distribution of gas and dust in the solar neighbourhood. I will summarise what has been measured with Gaia so far.
26 August 2024
190. More phase-space features
Four more recent discoveries from Gaia
The number and variety of complex dynamical features in our Galaxy has been one of Gaia's greatest contributions to studies of its structure and evolution to date. Here I look at four more recent discoveries: one each related to the inner and outer halo, one to the central disk, and one to its cosmological evolution.
19 August 2024
189. More metallicities and gravities
New catalogues of metallicities and gravities
Data Release 3, in 2022, included a catalogue of metallicities, temperatures and gravities for 470 million sources. Since then, other community-generated catalogues of metallicities, temperatures and gravities have been made available. I will describe what is motivating these reductions, and how they are being used.
12 August 2024
188. The tip of the red giant branch
Another important distance indicator
Some important contributions are being made by Gaia in furthering the use of the `tip of the red giant branch' as a robust distance indicator. These sorts of new insights are becoming ever more crucial in the context of the ongoing `Hubble tension' debate.
5 August 2024
187. Gaia synthetic photometry
Photometry in any passband from Gaia
Gaia DR3 provides flux-calibrated low-resolution spectrophotometry for 220 million sources in the range 330-1050nm. Synthetic photometry can be obtained from them for any passband fully enclosed in this wavelength range. I will explain the principles, and the many applications which are being pursued.
29 July 2024
186. Moving groups and traceback ages
New clues about cluster formation and gas dispersal processes
An independent method of estimating the ages of nearby stellar associations makes use of their expanding space motions to yield what are referred to as dynamical or traceback ages. The difference between various methods is providing important clues about the cluster formation and gas dispersal processes.
22 July 2024
185. Mira variables
Another sort of distance indicator
As evolved stars cool and expand on the red giant or asymptotic giant branch, they become pulsationally unstable. Miras are AGB stars with large variability amplitude and long pulsation period. Here, I will look at some Gaia results regarding their use as distance indicators and probes of Galactic structure.
15 July 2024
184. Stellar streams and sub-halos
An important test of Cold Dark Matter cosmology with Gaia
A specific application of the study of stellar streams is in placing constraints on the existence and nature of the numerous dark matter sub-halos (halos within halos) that are predicted, in standard Cold Dark Matter cosmology, to exist surrounding the Milky Way. The stream GD-1 shows some tantalising prospects.
8 July 2024
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