Chapter 161: Seemingly Right Yet Wrong

After imparting fundamental theoretical knowledge, the Dawn Academy began issuing various practical training missions. Students during this period enjoyed a relatively high degree of freedom. Hua Feng, Yun Meng, and Bai Feng formed a team together.

It was a regulation of the academy that all trainees must voluntarily form small teams, and as long as their strength was sufficient, the academy did not care whether a team consisted of three members or two. Consequently, the Dawn Academy dispersed into thousands of small teams, each maintaining an average combat power hovering around the 61st rank of the Primordial Emperor realm.

With his schedule becoming more relaxed, Hua Feng settled affairs concerning both the farming household and his family before devoting himself even more intensively to Sun Wukong, learning various abstruse concepts. Among these, knowledge concerning galactic structures became increasingly detailed.

Because the Sun is located within the galactic disc, discerning the initial appearance of the galactic disc is no easy task. To explore the structure of the galactic disc, astronomers in the 1940s—based on the research of Baade and Mayall into the spiral arms of the spiral galaxy M31 (the Andromeda Galaxy)—identified the primary types of celestial bodies inhabiting spiral arms. By conducting a census of these celestial body types within the Milky Way, they discovered three parallel spiral arms in the vicinity of the Sun.

Due to interstellar extinction, optical observations fail to yield a comprehensive view of the galactic disc. Evidence indicates that spiral arms are regions where interstellar gas accumulates; therefore, the detection of interstellar gas can reveal the spiral arm structure. Moreover, the 21-centimeter radio spectral line of interstellar gas remains unobstructed by interstellar dust, reaching virtually the entire Milky Way. Both optical and radio observation results demonstrate that the galactic disc indeed possesses a spiral structure.

The galactic disc is primarily composed of Population I celestial bodies, such as G-K type main-sequence stars, giant stars, novae, planetary nebulae, RR Lyrae variables, long-period variables, and semi-regular variables.

The center of the Milky Way is the intersection point of the galaxy's axis of rotation and the galactic plane. The central bulge of the galaxy is a brilliant spherical protrusion with a diameter of about 20,000 light-years and a thickness of 10,000 light-years. This region is composed of high-density stars, predominantly elderly red stars aged approximately 10 billion years or more. Evidence indicates the presence of a massive black hole within the central region, accompanied by extremely violent galactic nucleus activity.

The galactic center lies in the direction of the constellation Sagittarius. For the 1950 epoch coordinates, its right ascension is 17° 42′ 29″, and its declination is -28° 59′ 18″.

Beyond serving as a geometric point, the galactic center also refers to the central region of the Milky Way. The Sun is situated approximately 10 kiloparsecs away from the galactic center and lies about 8 parsecs north of the galactic plane. Because the space between the galactic center and the solar system is filled with massive amounts of interstellar dust, observing the galactic center in the visible light spectrum using optical telescopes in the Northern Hemisphere is exceedingly difficult.

Only with the rise of radio astronomy and infrared observation techniques have humans been able to peer through the interstellar dust to detect information regarding the galactic center within the 2-micrometer to 73-centimeter wavebands. Observations of the neutral hydrogen 21-centimeter spectral line have revealed a hydrogen expansion arm located 4 kiloparsecs from the galactic center—the so-called "3-kiloparsec arm" (although the distance was initially miscalculated as 3 kiloparsecs and later corrected to 4 kiloparsecs, the traditional name has been retained).

Approximately 10 million solar masses of neutral hydrogen rush toward the solar system at a velocity of 53 kilometers per second. On the opposite side of the galactic center lies a roughly equivalent mass of a neutral hydrogen expansion arm, receding from the galactic center at 135 kilometers per second. These are believed to have been expelled asymmetrically from the galactic center 10 to 15 million years ago.

Within a celestial region 300 parsecs from the galactic center lies a rapidly rotating hydrogen gas disc, expanding outward at speeds ranging from 70 to 140 kilometers per second. Contained within this disc are molecular hydrogen clouds with an average diameter of 30 parsecs.

At a distance of 70 parsecs from the galactic center exists a violently disturbed ionized hydrogen region, expanding outward at high velocity. It is now known that not only does a massive volume of gas surge outward from the galactic center, but a strong radio source—Sagittarius A—also resides there, emitting intense synchrotron radiation. Probes using Very Long Baseline Interferometry indicate that the central region of the galactic center radio source is exceedingly small, even less than ten astronomical units, meaning it is no larger than the orbit of Jupiter around the Sun.

Infrared observation data at 12.8 micrometers indicate that the galactic nucleus, with a diameter of 1 parsec, possesses a mass equivalent to several million solar masses, of which approximately 1 million solar masses manifest in the form of stars.

The galactic center region harbors a massive, compact nucleus, potentially a black hole. Relativistic electrons flowing into the accretion disc of this compact core are accelerated within intense magnetic fields, generating synchrotron radiation.

Recent observations of the galactic center indicate that the absolute core of the Milky Way is composed almost entirely of white dwarfs, numbering at least around 100,000. The innermost core itself consists of roughly 70 relatively large white dwarfs. Regarding how to observe even more details, scientists note that this will rely on next-generation observation equipment, such as NASA's James Webb Space Telescope currently under construction.

The galactic halo is a spherical region diffusing around the galactic disc. The halo has a diameter of approximately 98,000 light-years, characterized by a very low stellar density and distributed globular clusters composed of older stars. Some theorists propose that beyond the galactic halo lies a colossal, spherical radio radiation zone known as the galactic corona, which extends at least 100 kiloparsecs or 320,000 light-years away from the galactic center.

The Milky Way is enveloped by a galactic halo with a diameter of approximately 30 kiloparsecs. The brightest members within the halo are globular clusters.

In astronomy, a corona refers to the gaseous envelope surrounding a celestial body. The scorching gas at the outermost layer of such an atmospheric body strongly resembles a cap worn upon a person's head, wrapping around the radiant "rounded face" of the star. The Chinese character for corona originally referred to a type of ceremonial hat worn by ancient emperors; because the hot gas surrounding the heavens resembles such a hat, this gas is now designated as the galactic corona. The corona of the Sun is widely known as the solar corona, while the coronae of other stars are termed stellar coronae.

The solar system resides on a spiral arm called the Orion Arm, situated approximately 26,400 light-years from the center of the Milky Way, revolving counterclockwise. A complete revolution around the galactic center takes roughly 250 million years.

The solar system is located near the inner edge of the Orion Arm, drifting within the Local Fluff, at a distance of 7.94 ± 0.42 kiloparsecs from the galactic center. The spiral arm we inhabit is separated from the neighboring Perseus Arm by about 6,500 light-years (determined by measuring the distance to a massive molecular cloud core roughly 6,370 light-years from Earth). Our solar system sits precisely within the so-called Galactic Habitable Zone.

The direction of the Sun's motion, also known as the apex of the Sun's way, indicates the trajectory of the Sun's journey within the Milky Way, generally pointing toward Vega near the constellation Hercules, offset from the galactic center by about 86 degrees. The Sun's orbit around the galaxy is roughly elliptical, though subject to minor fluctuations caused by spiral arms and uneven mass distribution. We are currently positioned at a location about one-eighth of an orbital cycle away from the peri-galactic center (the point where the Sun makes its closest approach to the galactic center).

The solar system completes one orbital circuit approximately every 225 to 250 million years, a duration that may be termed a galactic year. Thus, calculated by the Sun's age, the Sun has orbited the galaxy 20 to 25 times. The orbital velocity of the Sun is 217 kilometers per second—in other words, it covers 1 astronomical unit every 8 days, and traverses a distance of 1 light-year in 1,400 years.

Galactic Waves:
Scientists utilized photometric and spectroscopic data from the Sloan Digital Sky Survey to study the galactic disc of the Milky Way. The results upended the textbook portrayal of the galaxy, indicating that the galactic disc possesses a wavy structure and that its dimensions may also be significantly larger than traditionally believed.

Traditional views maintained that the galactic disc of the Milky Way should be a smooth plane, with density decreasing exponentially outward from the galactic center, and exhibiting mirror symmetry in density between the upper and lower (or northern and southern) sides of the disc. In 2002, Heidi Newberg and her colleagues at Rensselaer Polytechnic Institute in the United States discovered dense stellar clumps situated at the outermost edge of the galactic disc; this substructure was named the Monoceros Ring. Later, other astronomers discovered another similar substructure beyond the Monoceros Ring, designated as the Triangulum-Andromeda Star Stream.

A research team conducted further analysis on observational data from the 2002 Sloan Digital Sky Survey, discovering indications of two additional similar substructures. These two substructures are positioned between the Monoceros Ring and our Sun. The substructure closest to the Sun lies roughly 30,000 light-years from the galactic center, where stellar density north of the galactic disc exceeds expectations; the other substructure lies approximately 40,000 to 45,000 light-years from the galactic center, where stellar density south of the galactic disc exceeds expectations.

They were respectively named the Northern Near Structure and the Southern Middle Structure.

Methods for Estimating Galaxy Age:
Based on the decay times (namely half-lives) of known long-lived radioactive nuclei, the ages of the oldest stars in the Milky Way can be determined from the abundance data of certain radioactive neutron-capture elements, thereby establishing the age of the galaxy. This radioactive dating method is termed nuclear cosmochronology.

For instance, the half-life of thorium is approximately 13 billion years. Utilizing contemporary giant astronomical telescopes equipped with high-resolution spectrometers, astronomers are already capable of detecting thorium within stars and making corresponding age estimations.

According to various measurement methods, calculated from the Big Bang, the age of the universe is approximately 14 billion years. Assuming an interval of 1 billion years between the Big Bang and the formation of the Milky Way, the galactic age determined by nuclear cosmochronology is compatible with the age of the universe.

According to a research report by the European Southern Observatory (ESO), the age of the Milky Way is estimated to be approximately 13.6 billion years, making it nearly as old as the universe itself.

A team composed of numerous astronomers utilized the Ultraviolet and Visual Echelle Spectrograph on the Very Large Telescope (VLT) in 2004 to conduct research that for the first time detected beryllium within two stars belonging to the globular cluster NGC 6397.

This discovery allowed them to push back the transition period between first-generation and second-generation stars by 200 to 300 million years, thereby estimating the age of the globular cluster to be approximately 12.9 ± 0.5 billion years. Consequently, the age of the Milky Way cannot be less than 12.2 ± 0.8 billion years.