Detectives in the center of the Milky Way: We found the missing young stars

For more than a decade, it has been assumed that over the past 10 million years, the center of our galaxy, the Milky Way, has seen the formation of a large number of stars with a mass equivalent to nearly a million suns. However, the extreme properties of this region made it impossible to detect these stars. They were considered lost stars. It took our own detective work to find for the first time a large number of young stars in Sgr B1, the region at the center of the galaxy that we see.

View of the center of the galaxy from Earth

The center of the Milky Way is the closest galactic nucleus to the Earth and the only one in which individual stars can be observed with great accuracy.

Situated on Only 26,000 light-years away, this is the fundamental model for understanding how the centers of galaxies work and how they are connected to each other. However, serious observational problems make it difficult to study the stars it contains. Due to the large number of stars it contains, it is very difficult to distinguish one from the other. Even with large telescopes with 8-10m mirrors, such as the ESO-VLT or the WM Keck telescopes, we can only observe the brightest of them, which are just the tip of the iceberg of the entire stellar population present.

Also, the problem is that from the Earth we observe the core of the galaxy from the inside of the galaxy itself. The light emitted by stars passes through the galactic disk, reaches us, and is quickly dissipated by interstellar gas and dust in the plane of the Milky Way. Thus, the observation of stars is limited to the infrared region of the spectrum, where there is less loss of light.

In the infrared, stars have very similar colors, making it difficult to distinguish between stars that are similar in brightness but fundamentally different, such as a red giant with a mass of one solar mass and an age of billions of years, and a young star with only a few million years, but with a mass ten times more than the Sun.

The largest factory of young stars in the galaxy

The galactic center consists of the so-called nuclear disk, a nuclear star cluster, and a supermassive black hole, Sagittarius A*. The volume of this region is only about 0.5% of the entire galaxy. However, it contains almost 10% of all molecular gas, the raw material from which stars are formed.

The rate of new star formation throughout the Milky Way is on the order of one solar mass per year, but at its center it is about 0.1 solar mass per year. This means that, in a normalized volume, ten times more new stars are being formed in the galactic center than in the rest of the Milky Way.

Calculations said they were there

In the past, the rate of star formation could have reached much higher values. Thanks to indirect measurements, we know that the center of the galaxy should contain several million solar masses of young stars with ages ranging from 0 to several tens of millions of years. However, due to observational difficulties, it has not yet been possible to distinguish these young stars from the rest of the old stars that dominate this region. Only two young massive clusters are known, with a mass of about 10,000 solar masses each.

Several dozen young stars have also been discovered that do not appear to be associated with any young clusters. This created a serious problem. Where are the young stars at the center of the galaxy?

why did they disappear

To begin detective work, it was necessary to deal with several known factors.

Stars, especially very massive ones at the galactic center, usually form from collapsing clouds of molecular gas. This collapse leads to the emergence of associations and young star clusters that rotate around the galactic center for several million years.

Throughout their journey, newborn stars move in a gravitational field that causes tidal effects (like the Moon on Earth). They often collide with dense and very massive molecular clouds, the gas mass of which is ten to one hundred times the mass of the star cluster. This meeting produces what we call tidal thrusts.

The stars also meet each other. All these effects lead to the rapid dissolution of the star cluster near the galactic center. For this reason, we cannot detect associations or clusters of stars older than a few million years simply by looking for regions with a high density of stars.

Amid all this, young stars hide among the millions of old stars that exist in the galactic center and “disappear”. We can find some of them, but only the brightest ones.

In search of young stars

The region of special interest at the galactic center is Sagittarius B1. Characterized by an intense release of ionized hydrogen, the presence of six known young stars and a nearby supernova remnant, it contains fundamental ingredients that indicate the possible presence of young stars in its interior. Thus, we decided to characterize the stellar population of this region in a study recently published in Astronomy of nature.

We rely on how the number of discovered stars changes depending on their brightness in the Sagittarius B1 region. Comparing this luminosity function with theoretical models, we were able to determine the presence of stellar populations of different ages in the region.

Our study showed the presence of a significant number of young stars (less than 60 million years old) in Sagittarius B1, the mass of which we estimate to be several hundred thousand solar masses.

To test our results, we conducted a similar study in a control field far enough away from the target region, but within the galactic center. The presence of young stars in the control region was about six times less than in Sagittarius B1. Therefore, we can say that Sagittarius B1 has an unusual number of young stars in relation to other areas of the center of the galaxy.

There are more hidden areas

The discovery of a large number of young stars in Sagittarius B1 helps us understand the problem of missing stars. The dissolving effect of young stellar associations or clusters is clearly visible in Sagittarius B1, where the stars we found formed several million years ago and have already managed to circle the center of the galaxy several times.

Thus, in addition to finding an important population of young stars that were missing, our study suggests that there are more hidden regions like Sagittarius B1 that contain other young stars waiting to be discovered.

Francisco Nogueras Lara, Humboldt Fellow, Max Planck Institute for Astronomy and Rainer Schedel, Observational Astronomy, High Angular Resolution, Infrared, Milky Way Center, Massive Black Holes, Andalusian Institute of Astrophysics (IAA-CSIC)

This article was originally published on The Conversation. Read the original.

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