Everything is connected. Are you familiar with the butterfly effect? According to this idea small changes could have dramatic consequences. Well, I have been thinking about this lately as one question was forming in my mind: Could it be that the ultimate answer to why I am writing this blog post is intimately related to the Sagittarius dwarf galaxy? It might be, but I do not want to make of this scientific post a Science Fiction story, so let’s move to the facts and then we will play a bit with our imagination.
In previous posts I have already written about the research that we are carrying out – making use of Gaia data at the Instituto de Astrofísica de Canarias (IAC). Even if you are not familiar with those posts, I am convinced you have heard of Gaia. Gaia is the ESA (European Space Agency) space mission that is revolutionizing the knowledge on our Galaxy, the Milky Way. Because you know that the “blurry, cloudy” thing that sometimes you can see from dark skies is the disc of a spiral galaxy seen from inside, don’t you? Indeed, almost everything you can see with your eyes is part of the Milky Way, with the only exception of the Andromeda galaxy… another galaxy 😉 Back to Gaia! In April 2018, the Gaia team released information on brightness, positions, and much more information for more than a billlion stars in our Galaxy. It could be compared to knowing what everyone is doing in a given moment. As you can imagine, that would be a gold mine for sociologists, right? Well, Gaia is the particular gold mine for galactic archaeologists.
Right after this data release, we started to delve into this astonishing dataset and were able to unveil the early chain of events shaping our Milky Way 1. But this was just the start. In an article that has been recently published in Nature Astronomy 2, we obtained the most detailed star formation history ever gathered of the disc of the Milky Way. In other words, we have been able to characterize the rate at which stars have been formed in our Galaxy since its very formation up to now. How is that possible? Nope, sorry to disappoint you, but the answer does not imply time travel. Stars are considered galactic fossil records. In the same way an archaeologist can infer the way of living or the customs of ancient civilizations through old pieces of pottery, astrophysicists can unravel the past history of a galaxy through the analysis of its stellar populations. In this particular case, we exploit the fact that stars do not emit light randomly, but in a precise way determined by their masses, chemical composition and evolutionary phase. As such, we have been able to obtain stellar age distributions in a volume of nearly 6500 light-years around our Sun, from which the evolutionary history of the Milky Way was sketched.
Our Galaxy started forming stars around 13.5 billion years ago (in a configuration far from the current one) in a fierce manner and gradually decreased to the rate of nearly 1 star per year. However, on top of this predictable behaviour, we found evidence suggesting that violent star formations were triggered 5-6, 2 and 1 billion years ago as well as recently. Something dramatic should have happened at that point to estimulate such bursts of star formation. It was then when we decided to put the Milky Way in a global context because, as we know, our Galaxy is not isolated and external effects are able to alter the conditions of an otherwise quiescent system.
It is known that massive galaxies as our own grow via the merging of small systems called dwarf galaxies. In fact, in 1994, one of such small systems was discovered in the direction of the Sagittarius constellation, right in the process of merging with our Milky Way 3. Nowadays, through the combination of a careful characterisation of the position and movements of stars in the so-called Sagittarius dwarf galaxy, its surroundings and powerful theoretical modelling, we know that Sagittarius nearly collided with the Milky Way in three other occasions: 1, 2, and 5-6 billion years ago, precisely coinciding with the bursts of star formation reported in our analysis. Thus, the present work suggests that Sagittarius, in its repeated approaches to our Galaxy, was able to destabilize its equilibrium triggering recurrent star forming events.
But this is not all. Back to the speculation and the question that I have been pondering lately and remember, science and imagination is a powerful combination pushing our knowledge to its limits. “Shortly after the first approach of Sagittarius to our Galaxy, right before calm recovered its throne in the Milky Way, one last star was forged from the collapse of a huge cloud of dust: our Sun. However, it was not until 4.7 billion years passed that this very instant in the history of our planet was unveiled. Because, my dear friends, everything that surrounds us, each and every one of our memories, everyone we know and love, is nothing but the outcome of such a cosmic moment in which Sagittarius formed a part of our lives”. Did the first approach of Sagittarius to our Galaxy really trigger the formation of our Sun? So far this is pure speculation, an extract from a potential Science Fiction story. Fortunately, this is the power of Science, the ability to test our wildest ideas while moving towards a full understanding of the Universe around us.
Simulation showing the drastic episodes of star formation produced by the approach and interaction of the Sagittarius dwarf galaxy with the Milky Way. Credit: Sagittarius orbital evolution adapted from David R. Law (Dunlap Institute, University of Toronto). Gabriel Pérez Díaz, SMM (IAC).
- Gallart, C; Bernard, E. J.; Brook, C. B.; Ruiz-Lara, T.; Cassisi, S.; Hill, V.; Monelli, M. (2019) “Uncovering the birth of the Milky Way through accurate stellar ages with Gaia” Nature Astronomy doi: 10.1038/s41550-019-0829-5. Mapping Ignorance article. ↩
- Ruiz-Lara, T; Gallart, C; Bernard, E. J.; Cassisi, S. (2020) “The recurrent impact of the Sagittarius dwarf on the star formation history of the Milky Way” Nature Astronomy doi: 10.1038/s41550-020-1097-0 ↩
- Ibata, R. A., Gilmore, G., Irwin, M. J. (1994) “A dwarf satellite galaxy in Sagittarius” Nature doi: 10.1038/370194a0. ↩