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The Milky Way loses its arms

The Milky Way loses its arms

230 million year revolution

Galaxies are disc-shaped collections of stars. The solar system and Earth are part of the Milky Way, an estimated galaxy 100 billion stars.

Astronomers estimate that there are between 100 and 200 billion galaxies in the part of the universe they can observe, but the number – like the universe itself – is probably infinite.

The entire galaxy revolves around its center. In the same way that the Earth orbits the Sun, our star – and all the other stars in the galaxy – orbits the black hole Sagittarius A* at the center of the Milky Way.

At a speed of 828 thousand kilometers per hour, this solar system takes about 230 million years to revolve one revolution around the center of the galaxy. So the last time we were in this specific place in the galaxy was during the Triassic geological period, when the first dinosaurs were just appearing.

Because we are so deeply immersed in our galaxy, we cannot take a picture of it. Therefore, it has been difficult for astronomers to determine the exact shape of the Milky Way.

Although there is broad consensus that we live in a so-called spiral galaxy, the number of arms in a spiral galaxy has caused problems – until now.

Four unusual arms

In the early 1950s, American astronomer William Wilson Morgan used the method of spectroscopic parallax to observe… Measure distance To high-mass stars. The calculations led Morgan to conclude that the Milky Way has three smaller spiral arms.

Later, in 1976, Another research group came So the Milky Way Galaxy has four large spiral arms. They did this by studying the distances to so-called H II regions, where star formation has recently occurred.

But it now appears that this conclusion needs to be reconsidered. It was painted by Professor Xu Yi at the Purple Mountain Observatory in China The most detailed image yet From our galaxy.

The Milky Way Galaxy is a so-called multi-armed spiral galaxy. About 83% of spiral galaxies observed in the Universe have two identical inner arms and a number of smaller outer arms. Only about two percent A multi-armed spiral galaxy has four inner arms.

“Until now, it is generally accepted that the Milky Way has four connected spiral arms, extending from the interior of the galaxy to its outer parts. If that were the case, the Milky Way would be an atypical galaxy in the universe.” Illustrated Science.

Young stars gossip

The most common way to determine the number of arms in a spiral galaxy is to focus on small objects such as stars and star-forming regions. That is, the density of matter is greater in spiral arms, making them the most important places for the birth of new stars.

In other words, researchers can track the number of spiral arms by measuring the distance to young stars. The more defined the regions of dense star formation in a galaxy, the greater the number of arms it will have.

This task was previously difficult to solve with high precision, but in recent years, new telescopes and methods have made the work of astronomers easier.

One such method is VLBI (The basic interferometry is too long), Xu Yi and colleagues used distance measurements to 204 maser sources, which are regions of hot gas near star-forming regions that emit microwaves.

The measurements were made using radio telescopes in Europe, Australia and elsewhere, and their high accuracy helped researchers show that stars previously placed in separate spiral arms actually belong to the same arm, the Norma Arm.

Like William Wilson Morgan, who identified three arms in the 1950s, Xu Yi and his colleagues also looked closely at high-mass stars, analyzing 23,807 massive OB2 stars. The stars were discovered with the help of the Gaia space telescope, which since 2013 has built a 3D map of just over a billion stars.

OB2 stars are relatively young, barely 20 million years old, and thus have not had time to travel far from their birthplaces. Since star formation occurs mainly in spiral arms, young stars serve as orientation points for where and how many arms there are.

Finally, the researchers selected 981 young stars from so-called open clusters, where stars of the same age are held together by gravity.

With the masers, OB2 stars, and open clusters in place, the researchers used so-called parallax calculations to build an accurate picture of the Milky Way’s spiral arms.

Using this method, astronomers can calculate the distance to the star based on observations of the angles between the Earth, the Sun, and the star in summer and winter.

By observing the star half a year apart, astronomers can read the displacement that the star appears to have when observed from two different locations. This phenomenon is called parallax.

It’s the same principle as when you hold up your thumb with an outstretched arm and look at it with alternate left and right eyes closed. It then appears as if the thumb is moving relative to the background. The distance between the two positions corresponds to the view. The smaller the distance, the farther away the star.

Calculations show that the Milky Way consists of two main inner spiral arms, the Norma Arm and the Perseus Arm. It extends from the center of the galaxy, where there are many stars, to the outer parts, where there are several long, irregular branches of arms.

The fallout likely arose when the Milky Way long ago collided with other galaxies, disrupting the spiral shape. These collisions may also explain another property of the galaxy: its skewness.

Simplicity shows the way

The Milky Way is not shaped like a perfect flat pancake, but looks more like a warped vinyl record. In 2020, a group of Italian researchers proposed it theory The anomaly is the result of a collision with the Sagittarius dwarf galaxy.

This galactic aberration is also a feature that astronomers now know is relatively common in spiral galaxies like the Milky Way, and something they are trying to learn more about.

At the same time, researchers are also getting closer to answering why all galaxies are different. Data It appears from the SOFIA Flying Telescope that dust and gas collect within galaxies and thus contribute more to star formation in some regions than others.

Perhaps this is why spiral galaxies can have different numbers of arms.

The fact that the Milky Way has had its arms severed and has thus become significantly more common is actually good news from a scientific perspective.

According to Xu Yi, the new knowledge about the shape of the Milky Way fits well with the old scientific ideal of not making nature more complex than it actually is.

“Our galaxy is like most other multi-armed galaxies in the universe. Both Newton and Einstein argued that nature is simple, and therefore simplicity is the scientific evidence of truth. From this perspective, what we have found seems reasonable.”

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