How are invisible massive baby stars formed?

Produced by: Science Popularization China

Author: Ma Yingxiu and Jiang Chenfeng (Xinjiang Astronomical Observatory, Chinese Academy of Sciences)

Producer: China Science Expo

I often hear people say that the universe is so romantic, and astronomers are so romantic. In fact, every time we gaze at the starry sky, we are also looking for our own origins. Our story is the story of the universe. Today, let's talk about those things about stars.

Stars are not permanent - they also have a life cycle

Looking up at the starry sky, the twinkling stars are all stars. They are named stars because they seem to be constant and will not disappear, nor will they grow larger or smaller or brighter or darker. In fact, stars also have their own life cycle, including embryonic stage, infancy, youth, adulthood and old age. Of course, they cannot escape death in the end.

Since stars have a life cycle, why can't we see these changes in stars with our eyes? This is because the lifespan of a star is generally between a million and 10 billion years. Compared with the lifespan of a human being (a hundred years), what we see is just a moment in the life of a star.

Figure 1 Starry sky

(Photo source: Veer Gallery)

The much-watched “birth”: How are stars formed?

The deep night sky is always mysterious. In addition to the stars visible to the naked eye, are there other things hidden that we cannot see? The answer is yes. Many beautiful nebulae and stars in their embryonic and infant stages cannot be seen by the human eye. This is because the light emitted by early stars is relatively weak and is wrapped in thick molecular clouds, so our eyes cannot see them directly.

However, astronomers are very interested in these early stars that are invisible to the naked eye because they are not only related to how stars are born, but also to the formation of planets and life.

Through infrared telescopes, we can see early stars wrapped in molecular clouds. Figure 2 shows a molecular cloud photographed by the Spitzer Space Telescope in the infrared band. Many of the glowing blue-white dots are already born stars. It is certain that stars are born in molecular clouds. But there is no unified answer to the question of how molecular clouds "give birth" to early stars.

This is because the early formation period in the life of a star is very short, especially for massive stars, the evolution of this stage is very fast. In addition, massive stars are formed in the dense core of a large molecular cloud, so the molecular envelope around them is relatively thick, even with infrared telescopes, it is difficult to observe.

Due to various reasons, we still don’t have enough understanding of the early formation process of massive stars. Therefore, research on the theory of massive star formation has always been a hot topic among scientists.

Figure 2 Molecular cloud photographed by the Spitzer Space Telescope in the infrared band

(Image credit: NASA)

Two different theoretical models

Regarding the formation of massive stars, there are currently two main popular theories, namely the competitive accretion theory and the single-core collapse theory.

The competitive accretion theory emphasizes the clustering of stars, just like a group of children born at the same time scramble for food, whoever eats more will grow faster and bigger. However, the shortcoming of the competitive accretion theory is that it cannot well explain the formation of isolated massive stars, while single, independent massive stars are very common in galaxies.

The single core collapse model emphasizes the independence of stars, that is, stars are relatively independent of each other. However, this theory is based on a premise that massive stars have a massive and dense molecular cloud mass (core) progenitor, and it is unclear how this dense molecular cloud mass (core) is produced. However, rapid external compression provides a mechanism for the rapid formation of massive and dense molecular masses (cores).

“Cloud-to-cloud collision” – a typical mechanism of rapid external compression

Galaxy is widely distributed with molecular clouds composed of a large amount of molecular gas. In the 1970s, astronomers proposed that collisions between molecular clouds can quickly form large mass dense clumps, and then form stars. This is the "cloud-cloud collision" theory. Through simulation, it is found that when two molecular clouds with different speeds collide, a dense compression layer with large mass and high density will be quickly formed (Figure 3). In this compression layer, due to the unstable gravity, it is easier to form large mass clumps (nuclei).

Figure 3 Astronomers simulated the morphology of two molecular clouds at different stages after the collision.

Image source: Takahira et al. (2014)

G323.18+0.15 - A perfect example of "cloud-cloud collision"

Although the cloud-cloud collision theory was proposed a long time ago, there are very few related observational studies. Fortunately, we discovered a "cloud-cloud collision" candidate G323.18+0.15 using infrared continuum data and 12CO and 13CO molecular spectral line data. G323.18+0.15 is located on the galactic plane, about 11,508 light-years away from us.

We found that "cloud-cloud collisions" can form massive, high-density molecular cloud clusters (cores), which in turn form massive stars in molecular cloud clusters/cores. According to calculations, the "cloud-cloud collision" in G323.18+0.15 may have occurred 1.59 trillion years ago. Although we have not witnessed the collision process with our own eyes, we can find traces left by the collision.

Trace 1: The "bent" shape, also called a U-shaped or arched structure, as shown by the white contour lines in (Figure 4), with a "bent" gap in the middle.

Trace 2: Two molecular clouds after the collision. They have different speeds, but they are well "inlaid" together like a key and a keyhole, as shown in the blue and white contour lines in (Figure 4). Of course, the two molecular clouds that collided may separate again after a few million years.

Figure 4 Molecular cloud complex G323.18+0.15. The contour lines of three different colors represent the three daughter molecular clouds. The background is the RGB three-color image of 24 microns, 8 microns, and 4.5 microns.

(Photo source: Xinjiang Astronomical Observatory)

Trace 3: The collision interaction region has a large mass and high density, and has formed dense massive molecular cloud clusters (nuclei), which meet the conditions for the formation of massive stars (Figure 5).

Figure 5 Column density distribution of molecular cloud complex G323.18+0.15

(Photo source: Xinjiang Astronomical Observatory)

Trace 4: Based on the molecular gas dynamics information provided by the 12CO and 13CO molecular spectral line data, we found that the gases of the two molecular clouds in the collision area were mixed together, which is consistent with the collision characteristics (Figure 6).

Figure 6 The morphology and spectral characteristics of the collision molecular cloud

(Photo source: Xinjiang Astronomical Observatory)

Trace 5: Stars are forming in the molecular cloud G323.18+0.15. Through the analysis of the gravitational equilibrium state of the molecular cloud, the mass of G323.18+0.15 is not enough to form stars, but young stars have been observed, which shows that it was the accidental collision that led to the formation of stars.

Conclusion

At present, only more than 50 samples of "cloud-cloud collisions" have been found through observation. Based on the existing research results, "cloud-cloud collisions" can trigger the formation of massive stars and can also explain why single, independent massive stars exist in galaxies. However, the probability of "cloud-cloud collisions" in the universe, the process of collisions, and the efficiency of triggering the formation of massive stars still need further study.

I believe that in the future, we will be able to find more "cloud-to-cloud collision" samples and clearly reveal the process of "cloud-to-cloud collision" through observations and studies in different bands and resolutions.

Figure 7 Simplified schematic diagram of cloud-cloud collision in the molecular cloud complex G323.18+0.15

(Photo source: Xinjiang Astronomical Observatory)

Editor: Ying Yike

[Note: This research result has been officially published in the international astronomical core journal "Astronomy & Astrophysics" (2022, A&A, 663, A97). 】