The "constellations" composed of stars are not used for fortune-telling!

We already know that the sky is filled with twinkling, shining "little" stars. These stars of different sizes and brightness actually have a common origin - molecular clouds!

On April 1, 1995, the Hubble Space Telescope captured an image of a column of interstellar gas and dust in the Eagle Nebula in the constellation Serpens, 6,500-7,000 light-years from Earth[1] (see Figure 1). Due to its spectacular appearance, it was rated as one of the "top ten best" photos taken by the Hubble Telescope and was also hailed as the top of the "ten incredible sights in the universe". Research has found that this is an incubator for the birth of many new stars[2], so it is called the "Pillars of Creation".

Figure 1. The Eagle Nebula - Pillars of Creation [HST, NASA]. It is huge, a typical giant molecular cloud: the leftmost pillar is about 4 light years long, and the finger-like protrusion at the top of the pillar is larger than our solar system.[3]

How are stars conceived and born in the universe?

The cradle of star birth—emptier than a vacuum

Scientists have observed that young stars are always inside or near interstellar clouds, and therefore infer that stars are formed in interstellar clouds. Interstellar clouds are places where interstellar matter is relatively concentrated in the universe. Its average density is several hundred to several thousand atoms per cubic centimeter, which is much higher than the average density of interstellar matter (1 atom per cubic centimeter, about 10-24 grams per centimeter), but 10 to 100 times lower than the density of the best "vacuum" in laboratories on Earth [4].

Interstellar clouds that form stars are usually cold, dark nebulae, whose conditions such as density, temperature and size allow the formation of molecules, so such nebulae are called molecular clouds.

The average temperature of a molecular cloud is very low, only a dozen degrees Kelvin. In addition to the main material components such as hydrogen, nitrogen, carbon monoxide, and dust particles, scientists have detected more than 100 molecules in molecular clouds [5]. Because of the extinction effect of dust, the central area of ​​a molecular cloud is difficult to detect in the visible light band, so it is usually called a dark nebula (as shown in Figure 2). Under the illumination of background stars, molecular clouds show different outlines, such as the "Pillars of Creation".

Molecular clouds can be divided into small and giant molecular clouds according to their size. Small molecular clouds are usually only a few light years in diameter and have a mass less than a few hundred times the mass of the sun. Because they were first discovered by American astronomer Bart Bock, they are often called "Bock globules" [6]. Giant molecular clouds have a typical diameter of 15-600 light years and a mass of thousands or even tens of millions of times the mass of the sun [7]. The latest research has found a number of newly born star structures with a scale far exceeding 600 light years, such as the "snake"-shaped giant young star "family" [8] (over 1,200 light years [9]), indicating that the scale of their parent molecular clouds may be far larger than the typical molecular cloud size currently observed.

Figure 2. Barnard 68 Nebula [11] (left: optical band image; right: near-infrared band image), about 400 light years away from us, with a diameter of about 0.5 light years and a mass of only 2 times that of the sun. It is a typical small molecular cloud, and the dust inside it blocks the visible light of the background stars. However, in the infrared band, the extinction effect of the dust is small, and the background stars are visible.

Giant molecular clouds often present complex substructures such as fibers, sheets, bubbles, and irregular clumps [10], as shown in Figure 3. The high-density parts of fibers and clumps are called molecular cloud cores, and their density can reach tens of thousands or even millions of atoms per cubic centimeter. Small molecular clouds have relatively independent structures and are small in size, and their high-density parts have a density similar to that of the cloud cores in giant molecular clouds. These dense cloud cores are the "seeds" for the formation of stars.

The internal structure of molecular clouds can be detected through the far-infrared radiation of dust particles or the microwave radiation of molecules. For example, the “Galactic Scroll”[16] led by the Purple Mountain Observatory of the Chinese Academy of Sciences is a large-scale sky survey based on CO and its isotopes to observe the distribution of molecular clouds in the Milky Way.

Figure 3. The Orion Molecular Cloud [12] - a giant star-forming region about 1,400 light-years from Earth. The blue background gas in the picture shows the filamentary structure of the molecular cloud (from the European Space Agency's Herschel Space Infrared Telescope), and the sub-images on both sides show nine young "protostars", of which the blue and orange sub-images are from the radio telescope arrays ALMA and VLA respectively. [ALMA/ESO/NAOJ NRAO N Karnath/ AUI/NSF B.Saxton/ S. Dagnello.]

How many conditions are needed for a “cloud” to become a star?

We can regard the process of a "cloud" forming a star as a process of "contraction (or collapse)". However, British astronomer Jeans pointed out in 1902 that not all molecular clouds can form stars. The formation of stars requires the satisfaction of two basic conditions:

(1) Mass: Under certain temperature and material density, there is a critical (i.e., "threshold") mass. Only when the mass of certain regions of the molecular cloud is greater than this "threshold" mass, that is, when the gravitational force of the matter in the region is greater than its own gas pressure, can contraction occur and further form stars. This "threshold" mass is called the Jeans mass. The size of the Jeans mass is related to the temperature of the molecular cloud and its material density. The higher the temperature, the greater the Jeans mass, that is, the higher the "threshold" for the molecular cloud to collapse; the higher the material density, the smaller the Jeans mass, that is, the lower the "threshold " for the molecular cloud to collapse. Therefore, only those molecular cloud cores with lower temperatures and higher densities can easily cross the "threshold" and collapse.

(2) Disturbance: Molecular clouds must experience some kind of disturbance, which causes the cloud core to break up and shrink. This disturbance can be when the molecular cloud passes through the asymmetric structure of the Milky Way (such as spiral arms), or when it is affected by the shock wave generated by the explosion of the death of a nearby star, or when molecular clouds collide with each other. Some local areas in the molecular cloud become denser due to the disturbance, and the Jeans mass decreases. In particular, the dense core in the molecular cloud continues to split and gravitationally shrink, eventually producing many clumps with masses ranging from 0.05 to more than 100 times the mass of the sun.

In addition to the above necessary conditions, the formation of stars also needs to meet the following conditions:

(1) Energy change: In the early stages of molecular cloud collapse, the nebular gas must radiate some of its energy to reduce the total energy. In this phase, the transitions between the energy levels of molecules in the nebular gas will produce long-wave (infrared) radiation, which can easily pass through the dense cloud layer and dissipate, causing the cloud to be in a rapid contraction stage.

(2) Change in angular momentum: Usually, the molecular cloud as a whole has a certain original angular momentum (i.e., the whole cloud is rotating). Because angular momentum will prevent the collapse of the molecular cloud, the overall angular momentum of the molecular cloud must be dispersed in some form. The overall angular momentum of the molecular cloud will be decomposed into each fragmented cluster and converted into their rotational angular momentum and orbital angular momentum. This is the secret of the rotation and orbital revolution of our sun and its eight planets.

(3) Magnetic field changes: The original molecular cloud generally still has a weak magnetic field (about 10-7 Gauss). As the molecular cloud continues to be compressed, the magnetic field strength will become very large. For example, according to theoretical calculations, the magnetic field strength of the sun may increase by 1016 times (i.e. 109 Gauss) from the scale of the original molecular cloud to its current size, which will hinder the molecular cloud from collapsing to form stars. At the same time, this is seriously inconsistent with the actual field strength on the surface of the sun (about 1 Gauss). Therefore, during the collapse of the molecular cloud, the magnetic energy in it must be lost through some mechanism.

(4) Recently, by using the "China Sky Eye" to observe a molecular cloud, Chinese astronomers discovered that the magnetic energy of the molecular cloud had been effectively dissipated to an extremely low level of micro-Gauss before it collapsed into a dense state [13], which overturned some of the academic community's understanding of the mechanism of magnetic energy disappearance.

The conception and birth of the sun

Similar to life on Earth, the birth process of a star can be divided into four stages. Let’s take the most familiar star, the Sun, as an example to explain (as shown in Figure 4).

The first step - from diffuse interstellar matter to "stellar eggs": Due to some disturbances, the very thin interstellar matter (mainly hydrogen and helium) in the universe gathers to form molecular clouds. Under the action of its own gravity, many dense clumps of varying masses and sizes are formed inside the molecular cloud, and the overall structure is a typical fibrous extension. Here we figuratively compare the dense clumps to stellar eggs, which are the "seeds" for the subsequent formation of stars.

The second step - from stellar egg to stellar embryo: The stellar egg has an extremely low density (10-19 g/cm3), a very large volume (the radius of a stellar egg with 1 times the mass of the sun is about 5 million times the radius of the sun), and a low temperature (less than 2000 Kelvin). As a result, the internal pressure is not high, and the weak infrared radiation produced by the molecules can easily penetrate the clouds, causing the energy to dissipate quickly. The stellar egg shrinks rapidly under its own gravity and its volume decreases rapidly.

When the radius shrinks to about 1000 times the radius of the sun, the density increases to 10-8 g/cm3, the thermal pressure gradually increases, and the contraction gradually slows down. The star egg forms an opaque "outer coat", which allows energy to accumulate inside it, and the temperature rises rapidly and a significant gradient appears. The temperature is higher towards the center. The internal energy converted from the gravitational potential energy of the outer layer of collapsed matter and conducted from the inner layer through convection significantly enhances the infrared radiation of the star egg, thus forming a "stellar embryo".

Figure 4. The process of star formation from a nebula[14]

The third step - from star embryo to stellar "fetus": Gravity drives the star embryo material to sink to the center like a "tightening spell", causing the size of the star embryo to continue to decrease, the density to continue to increase, and the temperature to rise rapidly. When the central temperature exceeds 7 million Kelvin, the nuclear fusion of a small amount of hydrogen is gradually ignited, the pressure increases rapidly, and the outer shell gradually becomes transparent, with not only infrared radiation but also high-energy X-ray radiation. At this time, the star embryo has evolved into a "fetus" with a radius of about 4 times that of the sun, which is called a "protostar" in astronomy (Figure 3 shows 9 protostars photographed by a telescope).

Step 4 - From "fetus" to newborn "baby": Driven by gravity, the outer material of the "fetus" star continues to accumulate, the range of hydrogen-helium nuclear fusion reaction rapidly expands, and the internal temperature becomes higher and higher. When the internal temperature reaches more than 10 million Kelvin, the hydrogen-helium thermonuclear reaction inside the "fetus" is almost fully ignited, continuously and stably providing energy, and the pressure and gravity reach a balance and stop contracting. This means that a "baby" sun of the same size as the current sun is born, and thus begins its 10 billion-year "life" journey. At this time, we say that the sun has reached the "zero-age main sequence" stage.

Star embryos of different masses take different amounts of time to reach the zero-age main sequence. The smaller the mass, the longer it takes. For example, a star embryo with a mass of 0.2 times that of the sun takes 1.7 billion years to reach the zero-age main sequence; a star embryo with a mass of 1 times that of the sun takes about 75 million years to reach the zero-age main sequence; and a star embryo with a mass of 15 times that of the sun takes only 60,000 years to reach the zero-age main sequence. Star embryos with a mass less than 0.08 times that of the sun will never reach the temperature required for nuclear reactions to begin. They will always be in a slow contraction stage, emitting very weak red light by converting gravitational potential energy. Such stars are called brown dwarfs.

Conclusion

Giant parent molecular clouds usually breed many stellar eggs of different masses, giving birth to a number of stars or star systems with very similar physical and chemical properties. Small parent molecular clouds usually form binary stars or simple multiple star systems [15]. In astronomical observations, many "sister" stars have been found to be gathered together in both the Milky Way and extragalactic galaxies, which are called star clusters in astronomy. Scientists have not yet found the sisters that were born with the sun. Perhaps our sun is an "only child" in the vast universe.

"Where do we come from and where do we go" has always been a question that people are concerned about. After learning about the birth process of the sun, we can't help but wonder, what will happen to the sun in the future? What kind of life will a star go through?

Stay tuned for the next episode to find out.

References:

[1]Clavin, Whitney. "'Elephant Trunks' in Space". Retrieved March 9, 2011.

[2]"A Stunning View Inside an Incubator for Stars – New York Times". Nytimes.com. 1995-11-03. Retrieved 2012-02-13.

[3]"NOVA | Origins | The Pillars of Creation image 1". PBS. Retrieved 2012-02-13.

[4]https://pages.uoregon.edu/jimbrau/astr122/Notes/Chapter18.html

[5] Craig Kulesa. Research Projects. Retrieved September 7, 2005.

[6]Bok, Bart J.; Reilly, Edith F. (March 1947). ApJ. 105: 255.

[7]Norman Murray, ApJ, 729 (2): 133. .

[8] Tian, ​​Hai-Jun 2020, ApJ, 904, 196.

[9] Wang, Fan., Tian, ​​Hai-Jun, et al. 2021, MNRAS, 513, 503

[10]Williams, JP; Blitz, L.; McKee, CF, (2000). Protostars and Planets IV. Tucson: University of Arizona Press. p. 97.

[11]Alves, JF, Lada, CJ, Lada, EA 2001, Nature, 409, 159

[12]John J. Tobin et al. 2020, ApJ 890, 130.

[13]Ching, T C., Li, D., Heiles, C. et al. Nature 601, 49–52 (2022)

[14] Su Yi, Astronomy in Humanities, Science Press, 2010.

[15] Launhardt, R.; Sargent, AI; Henning, T.; Zylka, R.; Zinnecker, H. (2000). Birth and Evolution of Binary Stars, Poster Proceedings of IAU Symposium No. 200 on The Formation of Binary Stars. p. 103. Bibcode:2000IAUS..200P.103L.

[16]https://mp.weixin.qq.com/s/8NvvVRLl-ltLlf4TSVlP2g

(Author: Tian Haijun, professor at Hangzhou Dianzi University, recipient of Hubei Province Outstanding Youth Fund, has focused on the research of celestial body proper motion measurement, wide-spaced binary stars, and the structure and evolution of the Milky Way in recent years, and won the second prize of Hubei Province Natural Science Award.)