A Roadtrip to the Edge of the Galaxy - How Scientists Found the Diameter of the Milky Way
Written by: Yonatan Najman-Licht '24
Edited by: Jason Johnston '23
It takes about 45 hours to drive across America, 51 hours to fly around the globe, and 3 days to reach the moon by rocket ship. When you look into the sky on a clear night, you might be able to see the greenish purple mist of stars that we call the Milky Way. It resembles a thin disk highlighted with a glow of captivating brightness. You might ask yourself: how long would it take me to travel across that massive cloud of stars?
Recently, scientists discovered that the Milky Way is actually much larger than the saucerous shape we can observe in the night sky. Previously, scientists measured the yellow, visible part of the milky way to be 120,000 light years across. However, that number doesn’t account for the total diameter of the Galaxy. Surrounding the frisbee shape of light are millions of square light years of dark matter . Dark matter is invisible matter that doesn’t emit or absorb light and thus cannot be detected by electromagnetic radiation. However, we do know dark matter exists because of its effect on other matter .
According to National Geographic, galaxies are “sprawling systems of dust, gas, dark matter, and anywhere from a million to a trillion stars that are held together by gravity” . The Milky Way consists of stars, planets, and suns, all revolving around the dense black hole in the center, Sagittarius A*. Finding the diameter of the light, or visible part of the Milky Way was easy. The challenge scientists embarked on a few years ago was to figure out a way to measure the immeasurable: millions of light years of dark matter.
To tackle this obstacle, astronomers might use a technique called “gravitational lensing” . According to Albert Einstein’s understanding of gravity, light bends when it encounters a mass which exerts a gravitational force on it. One may think a star is located in one place based on an observation, when in reality it may be located somewhere else; this is because the light that travels to one’s eye has been distorted by gravitational forces along its path. If light passes by a very large mass it will form a ring around the mass that distorted it. The large mass can be measured as a result—this approach is called gravitational lensing . Dark matter has mass, so it will distort light. It is important to note that dark matter does not bend or reflect light itself, it merely creates a curvature of spacetime that light follows. Therefore if light bends without us seeing any mass in the area, it is very possible that it was dark matter that bent it.
To find the bounds of our Galaxy, we can use gravitational lensing to measure the total mass of our galaxy, including dark matter. We know the mass of dark matter to be 2.241×10−27 kg/m3, which accounts for a quarter of the mass in the universe. Using gravitational lensing, we may obtain the total mass of the galaxy and subtract the mass of matter we can measure, leaving us with the total amount of dark matter in our galaxy.
Gravitational lensing helps to prove the existence of dark matter and the distorted shape of the galaxy, but it is still very difficult to determine the bounds of the universe. Where exactly does the shape of our galaxy end?
Alis Deason and her team at Durham University discovered the astonishing extent of the Milky Way—a whopping 1.9 (±0.4) million light years in diameter. To discover this enormous number, they looked at the nearest galaxy. They noticed that when this dwarf galaxy approached the Milky Way it began to slow down. Deason determined that at a certain point, the other galaxy interacts with the edge of the Milky Way, the last bit of dark matter in the galaxy, and slows down. When close to the galaxy, each galaxy has a gravitational pull on the other . At some point outside the Galactic Halo, the part of our galaxy beyond visibility, the velocity and acceleration decrease tremendously because they are not as close to the mass of our galaxy. The point of deceleration is where the Durham University team marked the edge of the Milky Way. Deason has been studying the Galactic Halo and dark matter for years, trying to probe dark matter and learn about the archaeology of our galaxy .
This discovery is a historic step in learning about our galaxy and others as well. Although the extent of the galaxy is intimidating and can make us feel small and insignificant, it is important to use a discovery such as this to appreciate the complexity and beauty of the universe. There is so much still to learn about the Milky Way, but thanks to the team from Durham University it seems that we have millions of lightyears worth within reach.
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