in the news a few weeks ago, when it produced a detailed map of the Galaxy (pictured above). But images like these don’t really reflect Gaia’s main mission, which is to measure accurate positions and velocities for millions of individual stars. That’s something I was interested in 30 years ago, and in fact I co-wrote a paper on the subject in 1986. Tucked away in an appendix to that paper is my one and only all-out attempt at scientific “debunking”. It probably isn’t of much interest to anyone but me – but then I thought last week’s post (about a popular video game) would have mass appeal, and it only got 81 views. So I really don’t care any more. I’m going to indulge myself.
The paper in question was co-written with James Binney, who was my boss at the time, and quite an authority on galactic dynamics (he co-authored the standard textbook on the subject, and I think he’s now involved in the Gaia project itself). After I’d dug out a hard copy of May & Binney (1986), and carefully scanned it into my computer, I discovered that all my astronomical papers are freely available online. There’s one about Black Holes from The Astrophysical Journal, and seven others from Monthly Notices of the Royal Astronomical Society. Among the latter, there’s one called “Solar-neighbourhood observations and the structure of the Galaxy” – and that’s the one I’m talking about here.
The title may sound odd, because the “solar neighbourhood” is usually taken to be about a thousand parsecs across, while the Galaxy as a whole is fifty times that size. But it’s only in the solar neighbourhood that we can measure the velocities of other stars with any accuracy. You have to extrapolate from these measurements to work out what’s happening elsewhere in the Galaxy. That’s helped by the fact that the oldest stars in the solar neighbourhood – sometimes called “halo stars” or “Population II” – tend to move on wide-ranging orbits that take them all over the place.
Back in the 1960s and 70s, an American astronomer named Olin Eggen claimed that some of the halo stars in the solar neighbourhood were moving on very similar orbits, even though they were physically separate from each other – not gravitationally bound in a tight cluster. That’s known to occur with some young stars in the Galactic disc – they’re called “moving groups”, made up of stars formed in the same gas cloud which haven’t had time to disperse – but the idea of equivalent moving groups of halo stars is something else altogether. It’s not “bad astronomy” in the Velikovsky league, but it’s the kind of brash, attention-grabbing claim that really needs to be examined closely. And that’s what we did in Appendix B of our paper (at this distance of time, I can’t remember if it was my idea or James’s – probably his, though in either case it would have been me who did all the calculations).
Like I said at the start, we ended up debunking the whole idea, using a mixture of statistics and dynamical calculations. You can see the whole appendix in the scan at the bottom of this post. I have no idea if anyone actually read this part of the paper, or paid any attention to it (according to Google Scholar, the paper has only collected 41 citations in the last 30 years). I don’t know what the current thinking about “halo moving groups” is, either. My guess is that (with the Galaxy being a more complex place than it used to be) they’re not as a-priori impossible as we thought – although I’d still put money on the specific halo groups “found” by Eggen being nothing more than wishful thinking.
I just had a quick look at Wikipedia’s article about Olin Eggen. It says “He first introduced the now-accepted notion of moving groups of stars” ... but I imagine that refers to Population I groups, not Population II. I did pick up another interesting snippet from that article, though. It says “After his death he was found to have been in possession of highly significant historical files and documents that had apparently gone missing for decades from the Royal Greenwich Observatory”.