We have imaged millions of galaxies, but we have so far not been able to take an image of our home galaxy as seen from the outside. So, how do we know what it actually looks like? And how it came to be and has evolved? With the combined power of ground- and space-based telescopes, astronomers can map and do archaeological studies of the Milky Way, to unveil its dramatic past, and peer into its future.
How do we study the galaxy where we live? It’s a bit like being asked to draw a map of the city you live in, without being able to leave your house. You might be able to peer through the windows and see some features like streets and tall buildings, but most will be hidden by nearby houses. It is quite similar when trying to image the Milky Way. Voyager 1, the most distant human-made object, has been travelling for 45 years and is now more than 20 billion kilometres away. Still, that’s pretty much next door in galactic terms: you would need to be several million times further away to get a clear view of the Milky Way from the outside.
We sit within the Milky Way, and as we look up in the sky we see the hazy stellar band of our galaxy, and clouds of dust obscuring the inner parts of the galaxy, making galactic mapping even harder. Follow along as we draw the map of the Milky Way, starting from the first draft.
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One of the first ever attempts to make a map of the Milky Way was in 1785 by the astronomers, and siblings, Caroline and William Herschel. The Herschels counted the stars they could see in the night sky, and they assumed the galaxy was deeper in the directions where there were more stars. The regions shielded by dust and gas towards the constellation of Sagittarius are already evident in this map, in the white space between the two reaching arms on the right. We now know that this region is the centre of the Milky Way, but back then it was supposed to be one of the edges, as stars to the other side were blocked from our view.
The first map of the Milky Way produced by astronomers Caroline and William Herschel in 1785, with the Sun roughly in the middle.
We have only known for about 100 years that galaxies outside of the Milky Way exist, but this discovery was not without controversy. In the early 1900, Harlow Shapley measured the distribution of globular clusters – old, massive collections of stars – in the Milky Way. He found them to be in a spherical arrangement around, what he correctly assumed, is the centre of the galaxy. This allowed him to both estimate the actual size of the Milky Way, as well as to place the Sun within it (and it was not in the centre!).
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At the same time, Heber Curtis was measuring the optical spectrum of the, at the time, spiral-shaped “Andromeda nebula”, arguing for its resemblance to the Milky Way, leading to his conclusion that there existed galaxies outside of our own. This led to a hefty discussion among astronomers, regarding the status of Andromeda as a galaxy of its own or as a nebulous cloud within the Milky Way. This discussion in 1920 was so pivotal that it has been given the name “The Great Debate”. Even though Shapley argued that nothing could be as large as the Milky Way, the debate was settled in Curtis favour when Edwin Hubble managed to measure the distance to the stars in Andromeda, and concluded them to be 10 times further away than the most distant stars in the Milky Way. This marked the beginning of a new era in astronomical research, recognising that the observable size of the Universe was much bigger than previously envisioned.
The leap in mapping the Milky Way came in the early 1990s when Lennart Lindegren, together with Michael Perryman and the European Space Agency (ESA), proposed the Gaia space telescope mission, the successor to the Hipparcos mission (1989-1993). Launched in 2013, the Gaia telescope has provided invaluable information about the appearance of our galaxy.
Early 1950, the astronomer Knut Lundmark commissioned Martin and Tatjana Kesküla to paint a map of our galaxy, known as the Lund Panorama of the Milky Way. By hand, they added the positions of about 7000 individual stars to create an, at the time, unprecedented drawing of the Milky Way. The 2 by 1 -metre map took two years to paint and can still be seen at Lund Observatory, Sweden.
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Gaia is an expert at measuring the position and velocities of the stars, with so far having mapped almost 2 billion stars in the Milky Way, which is still only 1% of all stars in the galaxy. This allows astronomers to trace the structures of the Milky Way. They can also model the motions of the stars into the past as well as into the future, giving clues as to what the galaxy used to look like, and what’s up ahead.
Illustration of how the parallax method works. The apparent position of a nearby star as seen from Earth changes as the Earth orbits around the Sun, allowing astronomers to measure the distance to it.
But how can Gaia measure the distance to a star? It does so by observing how the star moves with respect to background stars, using stellar parallax. You can try this yourself by sticking out your finger in front of you, and now close one eye, and then switch. Notice how your finger seems to move with respect to the background. If you move your finger further away, it will appear to move less, or the
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Is smaller. For the stars, we can use the same principle, but instead of having our two eyes, we now use Earth’s orbit around the Sun as our two viewing points. Using the angle of the apparent shift and the distance between the Sun and Earth, the distance to the star can be measured.
Galactic archaeologists study the history of our galaxy and the stars in the sky are their fossils, which provide clues to the Milky Way’s past. To do so, astronomers need to measure the chemical fingerprint of stars, which tells astronomers what they are made of, and in turn what the gas cloud they formed from consisted of, giving clues to their age and origin. However, stars tend to move away from their birthplace, making the puzzle much harder to solve. The chemical fingerprint is obtained through spectroscopy, which just so happens to be one of the many areas of expertise of ’s telescopes and instruments. Gaia on the other hand excels at tracking stellar positions and motions, which makes the synergy between Gaia and uniquely suited to unravel how our galaxy formed.
The synergy of and Gaia spans many fields. First, keeps track of the Gaia telescope, making sure that it stays where it’s supposed to be in space. Secondly, the Gaia- public survey uses ’s facilities to obtain the chemical information of stars tracked by Gaia, with the goal of further unravelling the mystery of the Milky Way.
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The Gaia- survey uses the Fibre Large Array Multi Element Spectrograph (FLAMES) on ’s Very Large Telescope (VLT) based in ’s Paranal site in the Chilean desert. This instrument allows astronomers to gather spectra for more than 100 stars simultaneously, thanks to optical fibres that are carefully arranged by a robot at the locations of the target stars. Over the course of 6 years the Gaia- survey collected spectra for over 100 000 stars. The science output from this survey is immense; at the time of writing close to 200 studies have been published using Gaia- data. A more thorough background on Gaia- can be found in this previous blog post, but for now, let’s go deeper into the Milky Way and see what we can learn about its history with the help of Gaia-.
The robotic fibre positioner at the FLAMES/GIRAFFE instrument at the VLT that is used in the Gaia- survey. The instrument has two plates where fibres are arranged. While one plate is used for observations, the other one is being prepared by the positioner for the next observations. This allows for many spectra to be obtained over the course of a good observational night. Credit:
Composite image of the Gaia space telescope (artist’s impression, top) and ’s VLT (bottom). Together they have mapped the Milky Way, providing clues to its structure and evolutionary history. Credit: ESA/
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In simplified terms, to build a galaxy, we first need gas and dark matter. Thanks to gravity, the gas will collapse into stars, which are gravitationally bound and swirl together creating a galaxy. Over time, the galaxy will change and evolve, partly due to the evolution of the stars themselves. During their lifetime, stars fuse hydrogen and helium into heavier elements that are then expelled into the surrounding gas. Subsequent generations of stars that form out
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