Uncovering the true face of the Milky Way: Exploring how we describe the entire picture of the Milky Way

How do we know what the Milky Way looks like? - It wasn't until the 19th century that we began to understand the true nature of our galaxy.

Our Milky Way Galaxy, the Milky Way seen sideways, with its arms and central bar in their roughly known locations. (Image credit: Mark Garlic/Science Photo Library/Getty Images)

While our telescopes have captured some truly stunning images of the Milky Way, astronomers have only a vague idea of ​​our home galaxy. It took a lot of work to even get that rough sketch, and it's amazing what we've been able to learn from our limited vantage point. From Earth's surface, the Milky Way appears to the naked eye as a camouflaged ribbon stretching across the sky.

While astronomers and philosophers have long debated the true nature and location of the Milky Way, the great astronomer, physicist, and all-around genius Galileo was the first to discover the Milky Way's true nature: countless stars so small that their light blends together. In the mid-17th century, philosopher Immanuel Kant correctly surmised that the Milky Way is a rotating disk of stars, and because we are embedded in that disk, it appears to us as a band. Decades later, astronomer William Herschel attempted to map the universe, but with little success.

It wasn't until the early 20th century that we really began to piece together the true nature of the Milky Way. It was then that astronomer Edwin Hubble discovered that the Andromeda Nebula was actually the Andromeda Galaxy, an "island universe" millions of light-years away. The Milky Way isn't just a disk of nearby stars. It forms most of our own Milky Way, so our galaxy is named after this familiar sky feature. Observations of other galaxies have helped us piece together what our home galaxy looks like. Most disk galaxies have spiral arms and a dense central bulge, so it's natural to assume that the Milky Way has these features, too. But directly mapping the Milky Way is an extremely challenging task. For one thing, it's huge - about 100,000 light-years at its widest point.

- Between 100 billion and 400 billion stars, hundreds of thousands of star-forming regions, and countless planets, black holes, neutron stars, and more. So surveying even a small portion of the Milky Way requires a massive amount of resources. And then there's the dust. In interstellar space, dust has the annoying property of dimming and scattering light. Because we're embedded in the Milky Way, the farther we look, the more our view becomes obscured by dust. Even the world's most powerful telescopes can't directly examine regions on the other side of the galaxy.

The arms of the Milky Way as seen in the night sky. (Credit: Getty Images)

So to map the Milky Way, researchers use many kinds of observations and combine those with comparisons of other galaxies and clever theoretical modeling to form a complete picture. For example, globular clusters orbit the center of the Milky Way in a roughly spherical arrangement. By plotting their positions in three-dimensional space, we can find where the center is - about 25,000 light-years away.

We can also examine the motions of the stars orbiting the centre and use our understanding of gravity to model what the core looks like. It's from this technique that we think our galaxy is a 'barred' spiral - the core is elongated, perhaps even peanut-shaped. This is confirmed by observations showing that a certain type of red giant star located near the core is separated into two populations, and that the infrared light from the core is symmetrical. The Gaia spacecraft, launched in 2013, has one main mission: to take a census of as many stars as possible, recording their distances, motions, brightnesses and colours. So far, the mission has catalogued nearly two billion stars, which, while impressive, still only represents about 1% of all the stars in the Milky Way.

Nonetheless, this gives astronomers a huge repository of information about our local region of the Milky Way. In addition to providing an accurate map of our galactic neighborhood, the data can be used as a baseline to compare and contrast other narrower but deeper surveys to look for anything suspicious that might help us build a larger map of the galaxy.

Despite their striking appearance, they are only about 10% denser than their surroundings. Instead, they appear visually striking because they are active star-forming regions, hosting many newly formed large, bright stars. Because we have a detailed map of our local galactic patch, which is not all actively star-forming, we can look for higher concentrations of star formation to outline the spiral arms. From these techniques, we know that the Milky Way has at least two prominent spiral arms, which are anchored to a large, S-shaped central bar, or peanut-shaped core.

Beyond that, however, things are a little fuzzy. The galaxy might have two additional moderately intense arms, or just a jumble of spurs and branches. Any "map" of the Milky Way you might come across is mostly guesswork, and is likely to change every few years as we improve our technology and gain a better understanding.

By: Paul Sutter

FY: He Danhuai

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