Although we are in the Milky Way, we know the true face of the Milky Way

Produced by: Science Popularization China

Author: Dong Zhichuan (Nanjing Institute of Astronomical Optics and Technology, Chinese Academy of Sciences)

Producer: China Science Expo

"The Altair is far away, the Milky Way is bright." "The Milky Way is clear and shallow, how far apart are they?"

In the Han Dynasty folk song "Distant Altair", the splendor of the Milky Way is vividly depicted.

The Milky Way has sparked so much inspiration and worry for mankind. Although we humans are in this enormous vortex of star stream, we are "not afraid of the clouds blocking our view" and can strive to piece together a panoramic picture of it with our own wisdom alone. What an amazing achievement this is.

Although we have been trying to understand this milky white bright band across the starry sky, the interstellar mysteries hidden in the Milky Way are really not exhaustible overnight. Whether you know a little bit or the whole picture, let's talk about the Milky Way we live in!

Why does the Milky Way look like this when we observe it from Earth?

In fact, the Milky Way can be seen all year round, but we see the brightest and most spectacular part of it at the turn of summer and autumn .

Why does the Milky Way we see appear in a belt shape? Because we are inside the Milky Way and can only see it from the side .

Galaxy

(Image source: Wikipedia)

The Earth "wanders" around the Sun in the Milky Way, and the Sun orbits the center of the Milky Way (with a period of about 220 million years). In winter, the Earth moves to the outside of the Sun, that is, the side away from the center of the Milky Way. Then the starry sky on the side of the Sun cannot be seen, and only the stars at the edge of the Milky Way can be seen at night, so the Milky Way is dim.

In the summer, the Earth moves to the inner side of the Sun, that is, the side close to the center of the Milky Way. The densest part of the Milky Way can be seen in the night sky. The multiple internal arms overlap, so it is clearer to see and it is easier to observe the obvious band distribution.

In July and August every year, the weather is clear and the Earth is in an excellent position. At this time, under the blessing of the subtropical high pressure in the northern hemisphere, the number of sunny days increases, the visual contrast is enhanced, and the night sky light is weakened, which is why we can see the Milky Way more clearly in July and August.

Two ways to “see” the entire Milky Way

For us who are in the Milky Way, if we want to see the spiral bar and the four spiral arms of the Milky Way clearly, do we have to fly vertically upwards from the Milky Way disk? How far do we have to fly to see the whole picture of the Milky Way?

Let's do a simple calculation. The comfortable field of vision of human eyes is about 124 degrees. Assuming that the diameter of the Milky Way is 150,000 light years (this is the average value of the latest data), then according to trigonometric functions, we can calculate that the distance to visually see the entire Milky Way is: 39,900 light years, which is farther than the distance between us and the center of the Milky Way (26,000 light years).

This means that if we start from Earth, we will need to travel at the speed of light for about 40,000 years to reach a place where we can visually see the entire Milky Way.

At present, this distance is out of reach. So, is there any other way for humans?

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There are at least two ways to help us achieve our goal of seeing the Milky Way. One is to use others as a mirror, and the other is to see the leopard through a tube.

“Use others as a mirror”

"Using others as a mirror" means that we can indirectly infer what our Milky Way looks like by observing other distant galaxies (such as our neighbor the Andromeda Galaxy).

It's like we live in a community where the buildings have highly consistent appearances. Although we are in our own house and cannot go out, and cannot see the whole picture of the unit building, we can still understand the general style of our own unit building by observing other unit buildings.

Among the galaxies known to mankind, the UGC 12158 galaxy, located 380 million light-years away, looks very similar to our Milky Way in appearance and is considered to be the galaxy that looks most like the Milky Way. Like the Milky Way, it also has four or five spiral arms and is also a "barred spiral" galaxy.

The appearance of the UGC 12158 galaxy (left) and the Milky Way (right)

(Image source: Wikipedia)

However, this method also faces a new challenge, which is how to screen out galaxies similar to the Milky Way from massive amounts of data .

Because manual screening is very time-consuming and inefficient, astronomers have begun to use the power of artificial intelligence to solve this problem.

They used more than 100,000 galaxy images collected by the European Space Agency (ESA) Herschel Space Observatory as training data, and tested the calibrated AI on another 20,000 galaxy images. The results showed that the AI’s classification accuracy reached more than 90%.

Using galaxy images classified by artificial intelligence, astronomers further screened out 1,000 galaxies that are most similar to the Milky Way and conducted a detailed analysis of them. After research, it was found that 45% of these galaxies are "flat", 12% are "curved", and 43% are "distorted".

This result means that the Milky Way is also likely to be a "distorted" galaxy , that is, the two ends of the Milky Way disk are bent upward and downward, forming an "S" shape.

Surprisingly, this guess coincidentally agrees with the picture of the Milky Way we obtained using the second method below.

The Milky Way's disk is "warped"

(Image source: Wikipedia)

“Seeing the leopard through the tube”

The second method is a kind of "seeing the whole picture from a small part" technology created by us humans, or a "blind men touching an elephant" technology, namely "big data sky survey technology."

Specifically, the application of this technology is similar to using a high-efficiency scientific survey instrument like the LAMOST Guo Shoujing Telescope to reconstruct and obtain the fine structure information of the entire Milky Way by taking frame-by-frame images of the Milky Way and accumulating massive amounts of data.

The principle of this "portrait" of the Milky Way is very simple. Astronomers mark the position, luminosity, spectrum and other information of each star "touched" by a large telescope, and obtain the chemical composition, distribution, age and movement of the stars. Then, through the accumulation of data day by day, they are able to reconstruct the overall picture of the Milky Way and fit its structural characteristics.

The general structure of the Milky Way

(Image source: Wikipedia)

Of course, we have to mention the first person who did this. He was not a contemporary astronomer, but a senior pioneer in the history of astronomy, the famous Sir William Herschel two hundred years ago (the Herschel Space Telescope mentioned above was named after him to commemorate his great achievements).

Sir William Herschel is known as the founder of stellar astronomy and is credited as the father of stellar astronomy . His "Cannon" telescope is world famous, with which Herschel pioneered modern astronomical observations, and he even discovered Uranus in his own yard (with another telescope).

Sir William Herschel

(Image source: Wikipedia)

It was this legendary British astronomer of the 18th century who, after long-term, persistent and precise observations, was the first to discover that the distribution of stars in the Milky Way is a flat disk structure by applying the observation method of counting stars. Based on this, he outlined the world's first outline model of the Milky Way . For this purpose, he even went to South Africa for further observation and verification.

Many scientific research institutions have launched similar big data work, such as the "Digital Milky Way" project of the National Astronomical Observatory of the Chinese Academy of Sciences, and the "Galactic Scroll" project of the Purple Mountain Observatory of the Chinese Academy of Sciences.

The method of big data sky survey, the idea of ​​"blind men touching an elephant and building a tower from sand grains", is not only an effective way for us to obtain a complete picture of the Milky Way, but also the only choice for us humans to explore the large-scale structures of other universes.

In the past decade or so, scientists have made many remarkable discoveries using this method. For example, Chinese and German scientists have collaborated to use a similar method to discover the ultra-large-scale network structure of dark matter in the vast universe.

Another group of scientists, through big data statistical analysis of a large amount of galaxy data, discovered the existence of the "Great Attractor" and the large-scale cosmic structure of the Laniakea Supercluster, which appears as a fiber/feather and spans 520 million light-years.

Laniakea Supercluster

(Image source: Wikipedia)

Conclusion

Being in the Milky Way, you can still recognize the true face of the Milky Way. Although the galaxies in the universe are very far away from us, it does not mean that humans cannot truly understand them. The universe is full of attraction. It is far away but approachable, which makes people full of infinite reverie...