Chapter 160: Galaxy

During his intervals of browsing through the information feed, Hua Feng recounted the entire sequence of events to Bai Feng and Yun Meng. Once reassured, Bai Feng returned to her room, while Yun Meng remained by Hua Feng's side, slowly engaging in her cultivation.

Sagittarius A possesses an extremely small scale, comparable only to the size of an ordinary star, with a radio radiation emission intensity of $2 imes 10^{34}$ ergs per second. It is located within 0.2 light-years of the galactic dynamic center.

Surrounding it are ionized gases moving at velocities up to 300 kilometers per second, as well as a strong infrared radiation source. The activities of all known stellar-scale celestial bodies fail to explain the peculiar characteristics of Sagittarius A; therefore, Sagittarius A appears to be the prime candidate for a massive black hole. However, due to the current lack of conclusive evidence regarding massive black holes, astronomers cautiously avoid using definitive terminology to refer to it as such.

Our Milky Way galaxy contains approximately two hundred billion stars, among which roughly one hundred billion are stars in the strict sense, with the Sun being a typical example. The Milky Way is a fairly large barred spiral galaxy consisting of three parts: the galactic disk containing the spiral arms, the centrally bulging galactic bulge, and the halo component.

The spiral galaxy M83 is very similar in both size and shape to our Milky Way. Outside the galactic disk lies a spherical body composed of sparse stars and interstellar matter called the galactic halo, with a diameter of about 160,000 light-years.

The Milky Way also rotates. The solar system revolves around the galactic center at a speed of 250 kilometers per second, completing one revolution in approximately 220 million years. The Milky Way has two companion galaxies: the Large Magellanic Cloud and the Small Magellanic Cloud.

Astronomer Maria Germanta suggested through the study of the disk of star clusters in the Milky Way that the stellar clusters in the inner regions of the galaxy are older, whereas the outer stars are much younger. It can thus be inferred that the formation process of the Milky Way began from the interior and gradually evolved outward to a diameter exceeding 100,000 light-years.

Scientists state that recent surveys have also found new evidence indicating that the Milky Way has swallowed many smaller galaxies during its growth, with celestial bodies from other galaxies merging into its interior. Stephen Hawking once claimed that his observations indicated the center of the Milky Way is a giant black hole.

In June 2013, NASA released the clearest map of the Milky Way to date, boasting 160 megapixels and a file size of 457 MB.

The primary mass of the Milky Way forms a thin disk known as the galactic disk. The centrally bulged, approximately spherical region at the center of the disk is called the nuclear bulge, within which stars are densely packed.

At the center of the nuclear bulge lies a very small, dense region known as the galactic nucleus. Outside the galactic disk is a much larger, nearly spherical region where the density of matter is vastly lower than that of the disk, known as the galactic halo. Beyond the galactic halo lies the galactic corona, whose distribution of matter is also roughly spherical.

In 2005, the structure of the Milky Way's spiral arms was observed. According to the Hubble classification, the Milky Way should be a giant barred spiral galaxy SBc (a barred spiral galaxy with loosely wound arms), with a total mass ranging from 0.6 trillion to 3 trillion solar masses and containing approximately 100 billion stars.

Since the 1980s, astronomers have suspected that the Milky Way is a barred spiral galaxy rather than an ordinary spiral galaxy. In 2005, the Spitzer Space Telescope confirmed this suspicion and further verified that the bar structure at the galactic core is larger than anticipated.

The estimated diameter of the galactic disk is 98,000 light-years, and the distance from the Sun to the galactic center is about 26,000 light-years, with the disk bulging outward at the center.

The center of the Milky Way possesses a massive weight and a compact structure, leading to suspicions of a supermassive black hole, as numerous galaxies are already believed to harbor supermassive black holes at their cores.

Like many typical galaxies, the orbital speeds of celestial bodies orbiting the center of the Milky Way are not determined by the distance from the center and the distribution of the galaxy's mass. Beyond the central bulge or in the outer regions, the typical speeds of stars range between 210 and 240 kilometers per second.

Consequently, the period of these stars in orbiting the galaxy depends solely on the length of their orbits. This differs from the solar system, where varying distances correspond to different orbital speeds.

The bar structure of the Milky Way is approximately 27,000 light-years long and stretches between the Sun and the galactic center at an angle of $44 pm 10$ degrees. It is composed primarily of red stars, most of which are elderly.

Each of the inferred and observed spiral arm structures of the Milky Way is assigned a corresponding numerical designation (like the arms of all spiral galaxies), which can be divided into roughly one hundred segments. Four major spiral arms originate from the galactic core, including:

- 3-kiloparsec arm and Perseus arm.
- Norma arm and Cygnus arm (together with the recently discovered extension).
- Crux arm and Scutum-Centaurus arm.
- Carina arm and Sagittarius arm.

There are also two minor spiral arms or branches, including:
- Orion arm (containing the Sun and the solar system).

Recent research suggests that the Milky Way may possess only two major spiral arms—the Sagittarius arm and the Norma arm—with the vast majority consisting of gas and only a small number of scattered stars.

The Gould Belt (Local Association) is a belt of concentrated bright stars extending from one end of the Orion arm. Its primary members are B2 to B5 type stars, along with some O-type stars, diffuse nebulae, and several stellar associations, the closest OB association being the Scorpius-Centaurus association, located about four hundred light-years from the Sun.

Outside the major spiral arms lies the outer ring, also known as the Monoceros Ring, proposed by astronomers Brian Yanny and Heidi Jo Newberg. It is a ring composed of stars surrounding the Milky Way, which includes stars and gas born from interactions with other galaxies billions of years ago.

The galactic disk is surrounded by a spherical galactic halo with a diameter of 250,000 to 400,000 light-years. Because the gas and dust on the disk absorb electromagnetic radiation of certain wavelengths, the compositional structure of the halo remains unclear. The disk (especially the spiral arms) is an active region of star birth, whereas such activity is absent in the halo, and open clusters appear predominantly on the disk.

It is generally believed that stars in the Milky Way are mostly binary or multiple star systems. A 2006 discovery indicated that two-thirds of the main-sequence stars in the Milky Way are single stars. The vast majority of the Milky Way's matter is dark matter, forming a dark galactic halo with a mass of 0.6 trillion to 3 trillion solar masses, concentrated around the galactic nucleus.

New discoveries have enhanced our understanding of the Milky Way's structure and dimensions beyond what was previously obtained from the disk of the Andromeda Galaxy (M31). Newly discovered evidence confirms that the outer ring extends from the Cygnus arm, providing definitive support for the possibility that the galactic disk extends outward. The discovery of the Sagittarius Dwarf Spheroidal Galaxy and galactic debris in orbits surrounding the galactic poles indicates that it has been torn apart due to interactions with the Milky Way. Similarly, interactions between the Canis Major Dwarf Galaxy and the Milky Way have caused its remnants to loop around the galaxy within the disk.

On January 9, 2006, Mario Juric and associates from Princeton University announced that the Sloan Digital Sky Survey had discovered a giant cloud structure in the northern hemisphere sky (spanning an area roughly equivalent to five thousand full moons) located within the Milky Way, yet seemingly inconsistent with all current galactic models. It congregates stars along a line perpendicular to the disk containing the spiral arms, a possible explanation being the result of a merger between a small dwarf galaxy and the Milky Way. This structure is located in the direction of the constellation Virgo at a distance of about 30,000 light-years and has been tentatively named the Virgo Stellar Stream.

On May 9, 2006, Daniel Zucker and Vasily Belokurov announced that the Sloan Digital Sky Survey had discovered two additional dwarf galaxies in the constellations Canes Venatici and Boötes.

The English name of the Milky Way, "milky," originates from its appearance as a dim luminous band stretching across the night sky. The name "Milky Way" is translated from the Latin *via lactea*, which in turn was translated from the Greek *γαλαξίας κύκλος* (*galaxías kýklos*, "milky circle"). In 1610, Galileo used a telescope to first resolve this band into individual aggregated stars.

In 1785, William Herschel became the first to study the structure of the Milky Way. Using the star-counting method, he concluded that the distribution of stars in the Milky Way is flat and disk-shaped, with the Sun located at the center of the disk.

In 1918, Harlow Shapley studied the spatial distribution of globular clusters and established a lens-shaped model of the Milky Way, with the Sun not situated at the center.

In the 1920s, the Shapley model gained widespread acceptance. However, because interstellar extinction was not accounted for, the numerical values of the Shapley model were inaccurate. Traditionally, studying the structure of the Milky Way relied on optical methods, which possess certain limitations. In recent decades, radio methods and infrared technology have developed into powerful tools for studying the galactic structure. Building upon the Shapley model, we have gained a profound understanding of the Milky Way's structure.

The galactic disk is the primary component of the Milky Way, containing ninety percent of all detectable matter within the galaxy. The galactic disk resembles a thin lens, distributed axisymmetrically around the galactic center. Its central thickness is about 10,000 light-years—though this accounts for the slightly bulging nuclear bulge, while the thickness of the disk itself is only 2,000 light-years—and its diameter is nearly 160,000 light-years, making the galactic disk exceptionally thin overall.

Aside from the galactic nucleus within a 1-kiloparsec radius, which rotates as a rigid body around the galactic center on a fixed axis, the rest of the galactic disk undergoes differential rotation around the center, meaning the farther a region is from the center, the slower it rotates.

Matter in the galactic disk exists primarily in the form of stars. Interstellar matter accounts for less than 10% of the Milky Way's total mass, and the vast majority of it is also scattered within the galactic disk.

Among interstellar matter, aside from ionized hydrogen, molecular hydrogen, and various interstellar molecules, there is also 10% interstellar dust. These solid microparticles, with diameters of about 1 micrometer, are the primary cause of interstellar extinction and are mostly concentrated near the galactic plane.