What is the Milky Way?

The Milky Way is a typical galaxy, consisting of 15-2 billion stars and countless interstellar matter. The main part of the Milky Way galaxy is called the silver disk, with a diameter of 8.5×14l.y (light-years) (1 L.Y. = 94,6× 18 km), and the part with an approximate spherical bulge in the center is called the nuclear sphere, with a diameter of 14 ~ 1.3× 14 L.Y. and a thickness of about 14 L.Y., which is a highly dense area of stars. The center of the nuclear sphere is called the silver nucleus, which is the center of mass of the Milky Way. The Milky Way seen by the naked eye is the projection of the main body of the Milky Way on the celestial sphere. The periphery of the silver disk is surrounded by a oblate silver halo with sparse star density, with a diameter of 1×14l.y (Figure 2-1, left; 1×13 parsec (PC) = 3261.6 L.Y.).

from the direction perpendicular to the plane of the milky way galaxy, the distribution of stars and interstellar matter in the silver disk is not uniform under the influence of magnetic field and density wave, but is a vortex structure composed of four spiral arms extending outward from the nuclear sphere (fig. 2-1, right). The spiral arm is a dense part of stars and interstellar matter in the Milky Way.

The sun is an ordinary member of many stars in the Milky Way. It is located near the central plane of the silver disk (galactic plane) and the inner edge of a spiral arm (Orion spiral arm), about 2.7×14l.y away from the silver nucleus.

(2) the movement of the sun in the Milky Way

The vortex structure of the Milky Way reflects its own rotation movement, that is, the stars, nebulae and interstellar matter in the Milky Way all revolve around the silver core. The speed of the sun's rotation around the silver nucleus is 25 km/s, and the rotation is about 2.5× 18 ~ 3× 18 y, which is called the year of the Milky Way.

There are also complicated motions among different stars in the Milky Way. It has been suggested that there may be two periodic changes in the rotation of the sun. One is that from the side of the Milky Way, it fluctuates up and down in galactic plane, crossing galactic plane roughly every 35 million years. On the other hand, from the plane of the Milky Way, due to the different rotational speeds of different stars, the sun and the four spiral arms of the Milky Way are not synchronized and parallel, and generally cross the spiral arms every 75 million years. The above hypothesis needs to be further verified in the field of astronomical research.

2. Galactic motion and total galaxy

(1) The motion of the Milky Way and the extragalactic galaxy

The Milky Way is moving towards the direction of the constellation Unicorn at a speed of 214 km/s. In recent years, astronomical research has found that there are 5 billion stellar systems similar to the Milky Way in space, and their own diameters are about 1, light years, while billions to billions of light years away from the Milky Way, which are called extragalactic galaxies. If the visible universe is compared to the vast ocean, they are just islands scattered in it, also known as the world island.

(2) The 1,-light-year-scale galaxies from the cluster to the total galaxy

are not evenly distributed in space, and they tend to form clusters, which can form clusters. The scale and shape of galaxy clusters are different, and the typical spatial scale reaches the order of 1 million light years (18km), and the total mass reaches the order of 147g g. The distance between galaxies in a cluster is about one million light years. The Milky Way, neighboring Andromeda and Magellanic Cloud form a smaller group, which is called the local galaxy group.

the spatial distribution of galaxy clusters is also uneven, and many galaxy clusters can further form supercluster, with a typical spatial scale of 1 million light years. The local supercluster, composed of the local cluster and the Virgo cluster, is about 1-25 million light years in diameter, and its total mass is trillions of times that of the sun.

The visible universe that human beings can observe now is called the master galaxy, with a typical spatial scale of 15 billion light years, an age of 1 billion light years and a total mass of 156 grams.

3. The Big Bang Theory and the Origin of the Universe

(1) The red shift of spectral lines and the visible universe

When the roaring train approaches us, the frequency of sound waves increases and the tone becomes higher; When driving away, the frequency of sound waves decreases and the tone becomes lower (Doppler effect). Similarly, when the luminous star approaches the observer, the spectral line of the star moves in the direction of blue light with high frequency, which is called blue shift; When leaving the observer, it moves in the direction of red light with low frequency, which is called red shift.

Hubble (E．P．Hubble, 1929) discovered through a large number of practical observations that the light from different galaxies showed some systematic redshift phenomenon. According to the comparison between the spectral line of light emitted by a specific atom in the galaxy and the light emitted by the same atom in the laboratory on earth, the regression speed of the light source galaxy leaving the observer can be obtained. Then, according to the brightness comparison of the same type of stars, the distance between the light source stars and us is calculated. This leads to the conclusion that "the farther the light source is, the faster the star will leave us", which is the famous Hubble Law (Figure 2-2).

Hubble's law reveals that distant galaxies are "escaping" from us, and the whole galaxy is in the process of expansion and change, which has become the knowledge of people today. On the other hand, the spectral line analysis of different stars in the Milky Way proves that there are also many blue shifts, reflecting that the interior of the galaxy is still attractive. In 1996, the Hubble Space Telescope also photographed a super collision between galaxies 63×14 light years away from the Earth (south of the crow's nest). Therefore, the expansion of the universe seems to mainly occur in the rapid expansion of the space between galaxy clusters, and the scale of the galaxy itself has not changed much, similar to what is seen on the surface of the balloon when it is inflated (Figure 2-3). It is known that the density characteristics of different parts of the universe may also be related to it (Table 2-1).

(2) The Big Bang Cosmology Theory

In the hypothesis of the origin of the contemporary universe, the Big Bang Cosmology Theory is the most influential theory. The theory was put forward in the 194s, and it is constantly developing and perfecting itself. The main contents are as follows:

Before the Big Bang, the universe was in a state of extremely high temperature and density, and matter and antimatter, as well as matter and energy, were in a state of balance. Under certain physical conditions, the Big Bang started. After the birth of the universe for 1-44s, its volume exploded rapidly, and it expanded by about 1,1 times in 1-34s, and its density decreased accordingly. However, the temperature is still as high as 132K to 11K within one second, and neither atoms nor molecules can exist. At that time, the existing form and behavior of matter in the universe can't be simulated in the laboratory at present. It is speculated that there may be radiation energy, electrons, neutrinos (a basic particle unaffected by electric, magnetic and nuclear forces, which was proved to have a very small rest mass in 1998) and proton and neutron basic particles. At present, it has been observed that the earliest nucleus left from the early universe was formed one second after the explosion. Therefore, this 1s can be regarded as a watershed in the study of the history of the universe.

after the explosion lasted for 3min minutes, the temperature dropped below 19K, and the nuclear reaction started. After the protons and neutrons were fused into the lightest elements of deuteron, helium and lithium nuclei, they could not collapse (Figure 2-4). At that time, helium accounted for about 22%, hydrogen accounted for 78%, and there were very few deuterium and lithium.

around a million years ago, the temperature dropped to 17 ~ 6k, and the universe was filled with plasma composed of light element nuclei, electrons and protons. After 25 million years, when the temperature drops to 13K, the radiation weakens, neutral atoms are formed, and the plasma recombines into normal gas. Galaxies began to form around 1 billion years ago, the first stars began to appear around 5 billion years ago, and the solar system formed around 1 billion years ago.

BIGBANG's theory is supported by most international scholars, but there are still different opinions on the cause of the Big Bang, whether the Big Bang will go on forever or will be transformed into contraction in the later stage, whether the Big Bang starts from a singularity or every point in the whole space can be regarded as the center of expansion, and there are no recognized conclusions on fundamental issues such as the form and behavior of matter in the first second of the Big Bang, and there are still different opinions on the determination of Hubble radius and the age of the universe.

The discussion of BIGBANG's various models is bound to involve a series of fundamental philosophical issues such as whether time and space exist forever. For example, some people think that in the 1-43s (Planck period) at the beginning of the Big Bang, the scale of the visible universe at that time was smaller than its quantum wavelength, and the whole universe became dominated by quantum uncertainty, and there was no "clock" or "ruler" to measure it, that is, the concept of general relativity time and space failed, and it was a physical world without time and space. It is necessary to explore the origin of known time and space forms through the quantization of time and space. This is also a shock to the traditional philosophical thought that the universe has no boundaries, no beginning and no end, and it is also of great significance to promote the modernization of philosophical concepts.