Milky Way Detectives: Reveal ‘Missing’ Star Populations in the Galaxy’s Center

For more than a decade it has been suspected that in the last 10 million years the formation of a large number of stars, with a mass equivalent to almost a million suns, took place in the center of our galaxy: the Milky Way.

However, the extreme properties of this region made it impossible to detect these stars. They were considered “lost”.

It has taken some detective work of its own to find, for the first time, a large number of young stars in Sgr B1, the region in our sights in the galactic center.

The center of the galaxy seen from Earth

The center of the Milky Way is the closest galaxy core to Earth. and the only one in which it is possible to observe individual stars with great precision.

Located at 26.000 light yearsconstitutes a fundamental model to understand how the centers of galaxies work and how they are related to each other.

However, significant observational challenges complicate the study of the stars it hosts.

The large number of stars it contains makes it very difficult to distinguish one from another. Even with large telescopes with 8-10 meter mirrors, for example the ESO-VLT or the WM Keck telescopes, we are only able to observe the brightest ones, which are only the tip of the iceberg of the entire stellar population present.

It is also a problem that from Earth, we observe the core of the galaxy from within the galaxy itself. The light emitted by stars passes through the galactic disk to reach us and is quickly scattered by interstellar gas and dust in the plane of the Milky Way.

A) Yes, stargazing is restricted to the infrared range of the spectrum, where light loss is less.

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

The biggest factory of young stars in the galaxy

The galactic center is made up of the so-called nuclear disk, the nuclear star cluster, and the supermassive black hole Sagittarius A*. The volume of this region is only about 0.5% of the total galaxy.

But it contains nearly 10% of all molecular gas, the raw material from which stars form.

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 masses per year.

This means that, normalized by volume, in the galactic center ten times more new stars are formed than in all the rest of the Milky Way.

The calculations said they were there

In the past, the rate of star formation could reach much higher values. Thanks to indirect measurements, we know that the center of the galaxy must contain several million solar masses of young stars, with ages between 0 and 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 the region. Only two young massive clusters are known, of about 10,000 solar masses each.

Several dozen young stars have also been detected that do not appear to be associated with any young clusters.

This posed a major problem. Where are the young stars at the center of the galaxy?

why were they missing

To start the detective work, several known factors had to be dealt with.

Stars, particularly the very massive ones in the galactic center, typically form from collapsing clouds of molecular gas. This collapse gives rise to associations and young stellar clusters that are found orbiting around the galactic center on time scales of several million years.

Throughout their journey, newborn stars move in a gravitational field that causes tidal effects (much like the Moon on Earth).

They frequently undergo close encounters with dense and very massive molecular clouds, with between ten and one hundred times more mass in gas than a cluster of stars. This encounter produces what we call tidal shocks.

Also, the stars meet each other. All of these effects result in a star cluster rapidly dissolving in the vicinity of the galactic center. For this reason we cannot detect associations or clusters of stars with ages beyond a few million years simply by looking for regions with a high density of stars.

Against all this, the 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.

On the hunt for young stars

A region of particular interest in the galactic center is Sagittarius B1. Characterized by an intense emission of ionized hydrogen, the presence of six known young stars and a nearby supernova remnant, contains the fundamental ingredients that point to the possible presence of young stars in its interior.

For this reason we decided to characterize the stellar population of this region in a study recently published in Nature Astronomy.

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

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

To test our results, we carried out a similar study in a control field sufficiently far 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 extraordinary amount of young stars in relation to other regions of the center of the galaxy.

There are more hidden regions

The detection 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 had time to circle the center of the galaxy several times.

So, in addition to finding a significant population of young stars that were missing, our study suggests that there are more hidden regions, similar to Sagittarius B1, that contain the rest of the young stars that are waiting to be discovered.

Francisco Nogueras is Humboldt he is a researcher at the Max Planck Institute for Astronomy.

Rainer Schoedel is an expert in observational astronomy, high angular resolution, infrared, center of the Milky Way, massive black holes at the Institute of Astrophysics of Andalusia (IAA-CSIC).