The Milky Way Galaxy is the name given to our home in the cosmos. We have never imaged the Galaxy from the outside, only from our position in the solar system. As a result, much of what we know about the structure has to be inferred from clever and detailed studies. While there are many fast fact pages I can write about the Galaxy, within this article I have chosen four important ones for understanding our home location.
Jackie Faherty, astrophysicist, Senior Scientist with the American Museum of Natural History and Editor at dailygalaxy.com. Jackie was formerly a NASA Hubble Fellow at the Carnegie Institution for Science. Aside from a love of scientific research, she is a passionate educator and can often be found giving public lectures in the Hayden Planetarium. With over 100 peer-reviewed articles in astrophysical journals, Faherty has been an invited speaker at universities and conferences across the globe. Jackie received the 2020 Vera Rubin Early Career Prize from the American Astronomical Society, an award that recognizes scientists who have made an impact in the field of dynamical astronomy, and the 2021 Robert H Goddard Award for science accomplishments.
Fast fact #1: The Milky Way Can Be Described With a Thin and Thick Disk and a Halo
Our Galaxy is often described as a spiral with three major components, a thin disk, a puffed up thick disk, and a halo. The thin disk is considered the main, flat, plane easily identified in the nighttime sky. It has a very small scale height relative to its width. The thin disk probably stretches ~1000 light years high while it stretches ~14,000 light years wide. It is the location of much of the young star forming areas as it also contains dense areas of the Galaxy and young stars do not have enough time to leave their natal environment. However the thin disk also houses a wide range of stars at different ages. The thick disk of the Galaxy differentiates from the thin disk in that it contains a population of stars that are more metal-poor, kinematically faster (with larger overall velocities) hence older. The thick disk extends above the Galactic plane by several light years (~3000 – 15000 light years). There are multiple theories on how the thick disk formed with ideas varying from a collision with another Galaxy to kinematic heating of the thin disk. The halo of the Galaxy is thought to be the oldest component and it stretches in a sphere around the Galactic center with a radius of ~100,000 light years. Halo stars are typically the fastest in the Galaxy and orbit widely with (at times) erratic motions. Recent studies from the precise distance and motion catalog of the European Space Agency’s Gaia observatory suggest that the halo did not form in situ. Rather it is the result of a merger that happened in the infancy of the Milky Way when it collided with another Galaxy called Gaia Enceladus.
Fast fact #2: The Milky Way is a Spiral Galaxy
Most people can easily state that we live in a spiral galaxy. However we have never imaged the Milky Way from beyond our local position so we have to infer that it has spiral arms. In order to do so, Astronomers look at the density of interstellar gas and dust as well as the location of star forming regions which trace the spiral arm structure. But given that we have not been able to take a photograph of the Milky Way, the exact structure is uncertain and can be a source of debate amongst Astronomers. From images taken of other nearby spirals, we know that arms can take on highly complex features with twists, bumps, merges, etc. In one well accepted model of the Milky Way Galaxy, there are four major arms with common names of (1) Perseus Arm, (2) Norma Arm, (3) Scutum-Centaurus Arm, and (4) Carina-Sagittarius Arm. There are also smaller arms or spurs which include the Orion-Cygnus arm that our solar system belongs to. We continue to map the locations of molecular HII and star forming regions to inform our map of the shape of our Galaxy.
Fast fact #3: The Milky Way is On a Collision Course with the Andromeda Galaxy
The Andromeda Galaxy is ~2.5 million light years away from the Milky Way but it is on a collision course which will bring it closer and closer over the next few billion years. Under our current predictions, the Milky Way and Andromeda will collide in ~4 billion years. While we can not be fully certain when this will occur (or if it is a definite collision) it is clear that the two Galaxies are bound to have strong interactions with each other in the coming eons. Looking across the cosmos, galaxy interactions and collisions are a relatively common occurrence. When the Milky Way-Andromeda collision does occur, stellar collisions will not be the driving issue. Stars are so far from each other in space that it is an extremely rare occurrence for them to directly collide into each other. Primarily the driving explosion of this collision will be the merging of the supermassive black hole at the center of each and the compression of gas which could lead to new star formation. At present there is some debate on how much gas will be left throughout the two Galaxies therefore it is unclear what shape the new Galaxy might take on (elliptical vs. gigantic spiral) and how much “starburst” might occur.
Fast fact #4: The Milky Way is Engulfing Other Smaller Galaxies
The Andromeda galaxy is not the only one that the Milky Way is destined to interact with. In fact the Milky Way has been colliding with a much smaller galaxy for the past several billion years. That object called the Sagittarius dwarf galaxy is an elliptical loop shaped object approximately 78,000 light years away. Recent work has shown that Sagittarius has been swept up and passed through the plane of the Milky Way in at least three separate encounters over the past 6 billion years. Each time it has done so, we find that star formation rates in the Galaxy seem to increase — a direct consequence of the interaction. In fact, one such collision with Sagittarius seems to line up with the time that the solar system formed therefore it has been suggested that our very existence is a consequence of the interaction between this dwarf galaxy and our own. As evidence for the violent passes of this satellite galaxy through our own is a stream of stars that loops from Sagittarius around the Milky Way in a tidal stream. Eventually, the Sagittarius dwarf galaxy will be completely engulfed by the Milky Way.
The stunning ESO photograph at the top of the page shows some of the antennas comprising the Atacama Large Millimeter/submillimeter Array (ALMA), all observing a panoramic view of the Milky Way’s center. The array can span distances of up to 16 kilometers and is formed of 66 individual antennas. Some features visible in the sky include Crux (The Southern Cross) just above and to the right of the nearest antenna, and the Carina Nebula slightly further to the right.