What is stardust and how telescopes can see through it?

Cosmic dust is made up of sub-micron-sized solid particles that fill the interstellar medium of galaxies, and are likely made of either carbonaceous or silicate material (Figure 1). Usually, the size of dust grains does not exceed 0.1 mm, while their temperature is extremely cold: from 10 to 25 degrees above absolute zero. Cosmic dust makes up only a small fraction of the total mass of a galaxy (about 1%), yet its role is one of the most crucial to the physical and chemical evolution of galaxies. Dust grains act as catalysts for the formation of molecular hydrogen, regulate the heating and cooling of interstellar gas, and shield the molecular gas from the diffuse, harmful stellar radiation. This cooling of the interstellar gas allows the formation of dense molecular structures within galaxies, which act as "nurseries" of new stars and thus lead to the creation of solar systems like ours.

Figure 1: Images of dust grains from electron microscope scans. On the left side, an example of spherical graphite grain that was found in pre-solar meteorites (Clayton & Nittler 2004). Image Credit: S. Amari. On the right side, an example of a porous chondrite interplanetary dust particle collected from Earth's stratosphere. Image Credit: D. E. Brownlee and E. Jessberger.

The study of the properties of cosmic dust becomes possible through its two main effects on the stellar radiation. Due to the size of dust grains, the light coming from young hot stars is "blocked" so that when we observe areas within our own Galaxy or other galaxies in the ultraviolet and optical wavelengths, they look like dark patches in the sky (such as in the left panel of Figure 2). Essentially, dust grains have the same size as the wavelength of the radiation emitted by the stars, causing the radiation to either be scattered away from the observer's line-of-sight or absorbed by the dust itself. But if we observe the same areas of the Galaxy at longer wavelengths (e.g. the near-infrared) then we can see the stars behind the cloud of dust (as in the right panel of Figure 2).

Figure 2: Two views of the Barnard 68 dark nebula in the constellation of Ophiuchus. On the left side, a composite image of the cloud in visible and infrared light, where it looks like a dark patch in the sky. On the right side, a composite image but now in near-infrared where light pass through the dust of the cloud to reveal thousands of stars in the background of the image. Image source: ESO.

But as we know very well, energy is not lost nor can it be destroyed, it is simply converted into another kinds of energy. Thus, in the case of cosmic dust, the absorbed stellar radiation is re-emitted by the dust grains themselves at far-infrared wavelengths as thermal radiation, which we can measure and convert into useful quantities concerning the properties of the dust. For example, from the light emitted by the dust we can draw conclusions about its temperature and mass. Unfortunately, we can not observe the thermal radiation of dust with ground-based telescopes, as the far-infrared radiation is absorbed by the Earth's atmosphere. For this reason, various missions have been designed in the past to place telescopes in orbit around the Earth to observe the dust emission, e.g. IRAS (1983), Spitzer (2003-2020), AKARI (2006-2011), WISE (2009 to date), and others. The telescope that allowed us to study the structures formed by cold dust in incredible detail within our Galaxy and in many other galaxies, was the Herschel Space Observatory (2009-2013), literally opening a huge window to understanding the dust properties.

Figure 3: On the left side, the Andromeda galaxy as seen in the far-infrared and on the right as seen in the visible, with the composite image in the middle. Image source: Robert Gendler (visible); ESA / Herschel / SPIRE / HELGA (far-infrared).

Undoubtedly, the role of dust is one of the most important in the Universe. In essence, dust helps stars to form, and it is also the main material from which planets like Earth are formed. One could say that stardust is the 'salt and pepper' of the Universe, without its presence the Universe wouldn't have its current form.

The Greek version of this post has been originally published in the 2'science website. Please check their site and the post here : )