Star birth: Assembling clouds of gas and dust
Stars form from huge clouds of gas and dust called molecular clouds, which occupy part of interstellar space. Star formation in molecular clouds can take millions of years.
The molecular clouds in which stars are born can span hundreds of light-years across, and most star-forming sites are hidden within these dense dust clouds.However, radiation from bright newborn stars clears parts of the dust and illuminates the surrounding gas, and we recognize these star-forming regions as bright nebulae, such as the one in the constellation Serpens.eagle nebula(Eagle Nebula), the Orion Nebula, and many others.Sometimes we also see some dark clouds formed by the accumulation of dust and gas in molecular clouds, calledBao Keyun ball(Bok globule), where binary or multiple star systems often form.
star formation
For star formation to begin within a molecular cloud, something must trigger it—perhaps a nearby supernova explosion, or a nebula passing through a more crowded region of space, or a nearby star passing by. Under these conditions, the cloud is pushed and tidal forces and pressure waves kick in, compressing parts of the cloud until some regions become dense enough to form stars.
Gravity then does the rest of the star formation, gathering more and more matter onto the developing node and concentrating most of it in the center. As matter gets denser, random motion transforms into uniform rotation around a single axis, and particles jostle and collide in the nebula causing temperatures to rise—especially in the center, where newly formed stars begin to emit infrared (thermal) radiation.
At this stage, the protostar (the newly formed star) is very unstable and loses mass by throwing gas and dust into opposite jets emanating from the poles. The center eventually becomes so hot that nuclear fusion begins, and the protostar begins to stabilize as a main-sequence star as inward gravity and outward pressure begin to equalize.
Astronomers have calculated that, on average, the Milky Way produces regarding seven new stars each year, most of them smaller than our sun.
Black holes: One of the strangest celestial objects
A black hole is one of the most peculiar celestial bodies in the universe. The matter in this space region is squeezed into a small point or ring with infinite density, which we call “Singularity」（singularity）。
In the spherical region around the singularity, gravity toward the center is so strong that nothing—including light—can escape. The boundary of this region from which nothing can escape is called the “event horizon” (event horizon), and anything that crosses this boundary inward can never return. There are two main types of black holes. Stellar black holes are formed by the collapse of the core of a supergiant star at the end of its life in a supernova explosion. Supermassive black holes, which are much larger, are thought to exist at the centers of most galaxies.
How to detect black holes
Black holes don’t emit light, so we can’t directly observe or photograph them. But we can use the strong gravity of black holes to attract other matter to detect some black holes. Surrounding these black holes may be disks of gas and dust that are spiraling into the black holes, throwing out large amounts of X-rays or other radiation. The easiest to detect are black holes that produce jets of energetic particles from their poles.
Milky Way: Our Home
All the stars we see in the sky are located in our home galaxy, the Milky Way. This huge star system contains hundreds of billions of stars and is regarding 120,000 light-years across. It is a barred spiral galaxy with a complex structure.
The visible stars in the Milky Way form a disk with a bulge at the center, and although the diameter of the Milky Way is wide, the height of the disk averages only a thousand light-years. Looking from the earth along the disk of the Milky Way, you can see more stars than looking up or down from the disk into the space between galaxies, so the Milky Way we see is a line of countless faint distant stars. Blended milky white broad light band.
The central bulge of the Milky Way is dominated by low-mass red and yellow stars with high metallicity, but the surrounding galactic disk is filled with gas, dust, and younger stars. Like all spiral galaxies, the Milky Way’s stars are scattered throughout the disk, but the brightest stars are concentrated in the spiral arms. Stars orbit at different rates depending on their distance from the center, so the spiral arms cannot be permanent structures. Instead, the spiral arms are clearly visible because they are active regions of star formation. Stars are born in spiral arms, the largest and brightest of which end their brief lifetimes before orbiting into the wider galactic disk.
What is a “rotary arm”?
Astronomers have recently confirmed that the Milky Way is a barred spiral galaxy, with stars forming a rectangular bar-shaped structure regarding 27,000 light-years away in the center of the galaxy. The Milky Way’s spiral arms are caused by stars, gas and dust moving in and out of spiral-shaped “traffic zones” called density waves.
The latest evidence shows that the Milky Way has four spiral arms — two large and two small — with distinct differences in the stars in these two arms.
※ This article is excerpted from “Illustrated Solar Systems: The most authoritative guide map of stars, galaxies and constellations“, titled by editor Narutodo,Dashi International CultureAuthorized to publish.
《Illustrated Solar Systems: The most authoritative guide map of stars, galaxies and constellations》author: Robert.Robert Dinwiddie et al.Translator:Hu Jialingthe publisher:Dashi International CulturePublication date:2023/03/14
