The Milky Way is a magnificent spectacle in the night sky. Among the myriad stars of the galaxy, there exist visible "dark nebulae" (Figure 1). These "dark clouds" are composed of neutral gas and solid particles beyond the stars and serve as the birthplaces of the next generation of stars. These solid particles are commonly referred to as interstellar dust.
What is interstellar dust? Interstellar dust refers to solid particles distributed in interstellar gas, which may consist of icy substances such as silicates, water, and methane.
Interstellar dust is mainly generated in the later stages of stellar evolution, accompanying stellar winds or supernova explosions, where the metals accumulated by stars throughout their lifetimes are ejected back into interstellar space. Interstellar dust absorbs energy in the ultraviolet and visible light wavelengths (forming dark clouds) and emits in the infrared wavelength. Dust not only aids in cooling interstellar gas to form the next generation of stars but is also fundamental to the formation of planets and life. Although interstellar dust typically accounts for less than one-thousandth of the total mass of a galaxy, it plays a crucial role in the formation and evolution of galaxies, stars, and planetary systems.
So how is interstellar dust distributed in the Milky Way? The spatial distribution of dust in galaxies can provide crucial information about galaxy structure and evolution. Observations of external galaxies have revealed that dust is abundant in disk-like galaxies similar to the Milky Way, forming a prominent disk-like structure. Generally, compared to the disk of stars, the dust disk appears more extended radially and thinner in thickness ( Figure 2). In famous galaxies like the Sombrero Galaxy (M104), the extension and thinness of the dust disk far exceed that of the disk composed of stars.
What is the impact of interstellar dust on starlight?
Scattering: Interstellar dust scatters starlight, a phenomenon known as Rayleigh scattering. When light encounters dust particles, it scatters in different directions depending on the size and shape of the particles. Dust particles typically scatter light with wavelengths comparable to their size, thus tending to scatter shorter wavelengths of blue light rather than longer wavelengths of red light. This is why the daytime sky appears blue, as dust in the atmosphere scatters blue light. At night, dust also scatters starlight, making distant stars appear dimmer than they actually are.
Absorption: Besides scattering, interstellar dust also absorbs starlight. When light passes through a layer of dust, some of it is absorbed, causing the starlight to dim. Larger dust particles absorb more short-wavelength light, while smaller particles tend to absorb longer-wavelength light. This absorption effect varies with the concentration of dust and the wavelength of light. Therefore, interstellar dust can alter the color and intensity of starlight propagating from celestial bodies to Earth.
Emission: Interstellar dust can also emit light. When cosmic dust is struck by high-energy particles, they emit light in a process called fluorescence. The color of this emitted light is often related to the composition of the dust.
In the vastness of the cosmos, interstellar dust plays an indispensable role, serving as both the "behind-the-scenes worker" among the stars and the creator of the universe's beauty. Though small, they shape the universe in unparalleled ways, shaping our perception of the cosmos. May our journey of exploration never cease as we, together with interstellar dust, unravel the mysteries of the universe and appreciate its endless charm.
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