Galactic Archaeology

• Seeing the Milky Way
• Cosmic Mouse
• Dance of the Galaxies
• Meet the Neighbours
• Picture Credits
• Elsewhere

A review of “The Ins and Outs of the Milky Way”

The Vectis Astronomical Society public talk for May 2023 by Professor Sean Ryan

Seeing the Milky Way

Do we ever look up at the night sky and think how the view is never-changing, except for the wandering planets against a timeless backdrop of the “fixed stars”? In our May talk, Prof. Sean Ryan showed us a wide perspective in time and space and demonstrated how even the galaxies of billions of stars are constantly in motion, constantly interacting and evolving. Far from being a timeless backdrop, the Milky Way is a cauldron of flux and change.

Sean has been Professor of Astrophysics at the University of Hertfordshire for 17 years and before that worked on several exciting projects around the world. But he also has decades of experience as a keen amateur observer, going back to his roots in Christchurch, New Zealand. Even with the naked eye, the view of the Milky Way from the southern hemisphere provides clues about the nature of our Galaxy: most stars are concentrated in a narrow band; the Milky Way wraps around the whole sky (so we are inside this structure); there is a brighter concentration in one direction (so we are not at the centre); not everywhere is bright (there is dark stuff – dust); and there are separate concentrations of stars (the Magellanic Clouds, companion galaxies – so we are not alone). And with binoculars one can map out a halo of globular clusters around the centre of the Milky Way.

But to really get to grips with the history and changing structure of our Galaxy, we need more measurements: various methods give us the distances to stars, clusters and galaxies, and along with angular and radial motion, this provides velocities – everything is moving – and through study of spectra we can assess what distant stars are made of. Great work from the beginning of the 20^th century gives methods for estimating the age of populations of stars. These are the tools of what Sean terms “galactic archaeology”.

Cosmic Mouse

There is a galaxy far, far away… about 1.6 million light years away in fact, called NGC6822, and this is central to Sean’s archaeological tale. It was first discovered by E E Barnard in 1884 in the constellation of Sagittarius. Then nearly 100 years ago in 1925, Edwin Hubble identified it as “the first object assigned outside of the galactic system”. Sean introduced an entertaining analogy here: mice are a perennial problem for observatories, but the question is, if you see ONE mouse, how many others are out there? Similarly, if you see ONE satellite galaxy, how many others are out there…?

Most of the rest of Sean’s talk took us on a whirlwind tour of the galactic archaeological discoveries of the next 100 years. It could have been a blur of dates and names and ideas, but in the hands of an expert teacher we saw a developing picture of galaxies and dwarf galaxies passing close to each other, causing distortions, leaving tell-tale streams of stars with similar motions, giving rise to galactic spirals and saucer-like bulges.

We heard about:

• Nancy Roman in the 1940s and 50s identifying the first connected stream of stars with the same motion in Ursa Major
• Various researchers in the 1960s finding fast moving populations of stars that formed outside of the Milky Way, but “fell into” the main disk of stars
• In the 1970s there were the first pioneering computer simulations of galaxy formation, which showed the persuasive possibilities of spiral-like structures coming about from interaction with nearby galaxies; and in 1974 was the first direct observation of a tell-tale filament structure (a stream of stars) that joined the Magellanic Clouds to the Milky Way
• In the 1980s the fall of dwarf satellite galaxies through the Milky Way was related to characteristic orbital periods of some star populations

Dance of the Galaxies

The stunning finale of this presentation was a computer simulation which ran from the Big Bang, through initial Galaxy formation and then the dynamic whirl of dwarf galaxies coming and going, leaving traces of their path in the present-day structures of the Galaxy. This amazing work is being done by Sean’s colleague Prof. Chiaki Kobayashi at the University of Hertfordshire. (The links to this and other astronomical simulations can be found on her home page at: CHIAKI’s Research (herts.ac.uk) https://www.star.herts.ac.uk/~chiaki/research-e.html – this is well worth a visit!)

Meet the Neighbours

Meticulous observations and ingenious theory, combined with cutting-edge computer simulations, have convincingly pieced together the encounters between our near-neighbours in the Local Group of galaxies (did you know there are at least 80 members, mostly dwarf galaxies? That’s a lot of mice!)

And what about that first “mouse”, NGC6822? Further study has revealed even more about it: by characterising different populations of stars (oxygen-rich versus carbon-rich) a distribution of star velocities shows that there is a systemic rotation. So even in this small, relatively isolated galaxy, there is evidence of a complex dynamic history. Next time you look up at the night sky, remember that we are not seeing an eternal unchanging backdrop, but a seething mass of motion and interaction, an astronomically slow cosmic dance.

[This review was originally written for New Zenith, the newsletter of Vectis Astronomical Society]

Picture Credits

• Milky Way image: ESO / Y. Beletsky
• Prof. Sean Ryan: his own photo, used with permission
• NGC6822: ESO / wikimedia
• Prof. Chiaki Kobayashi: Twitter image, used with permission
• The Local Group image: Antonio Ciccolella / wikimedia (cropped)
• Mouse: Credit George Shuklin / wikimedia

Elsewhere on the bookshelf

Darken our Lightness – Welcome (grandpops-bookshelf.co.uk) – an appreciation of the night, and the call of the Milky Way

Our Past is in the Stars – Welcome (grandpops-bookshelf.co.uk) – the story of how the study of the night sky was a spur to the development of science and societies

Reaching for the sky – Welcome (grandpops-bookshelf.co.uk) – a biography of the pioneering Caroline Hershel, who was more familiar with the Milky Way than anyone of her generation

John D. Barrow & Frank J. Tipler, The Anthropic Cosmological Principle (Oxford University Press, 1986) – the pages of my copy are yellowing with age, but this remains a mind expanding book which tackles humanity’s place in the vastness of the Universe and tried to quantify the chances of life elsewhere.