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Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all.
If you want to see the Universe, you have to do more than merely open your eyes. Even with the advantage of large, powerful telescopes, even from far above the limitations of Earth’s atmosphere in space, there are still enormous portions of the Universe that are virtually invisible to our optical telescopes. The reason why? Because enormous portions of the Universe are blocked by cosmic dust: small, cold grains of atom-based matter that absorb and block the visible wavelengths of light that human eyes have adapted to see. They obscure enormous regions of the galactic plane, and hinder our ability to observe star-forming regions, planet-forming disks, and objects that lie behind and beyond the plane of the Milky Way.
Sure, we’ve developed many techniques, like multi-wavelength astronomy (particularly at longer wavelengths), to help peer through that cosmic dust, and to identify the objects that lie both inside and behind it, but the existence of cosmic dust itself has been a longstanding puzzle for astronomers. This week’s Ask Ethan question comes from High School teacher Allan Clark, who was puzzled by its presence, asking:
“[when] teaching grade[s] 9 and 10, I taught the following: all matter is made from atoms (grade 9), [and that] stars were formed as gas and dust coalesced, eventually producing heavier elements. As evidenced by my home, dust is a solid, i.e. matter; so my question is what does dust consist of and where does it come from?”
It’s kind of remarkable that we teach students that dust exists, that it’s a form of matter, and that it plays a role in the formation of stars, but we don’t actually teach what it is and where it comes from. Let’s start with what it actually is, and move on from there.
This image shows a laser guide star being created toward the galactic center of the Milky Way. While the Milky Way’s plane shines prominently overhead in this photo, optical observatories find very few spiral and elliptical galaxies close to the Milky Way’s plane compared to the large numbers they find elsewhere. This led to the Milky Way’s plane being known as the “Zone of Avoidance,” a mystery that wasn’t solved until the 1960s with the development of infrared astronomy. The dark bands silhouetting the plane of the Milky Way are dust clouds, which are outstanding at blocking visible light.
Credit: G. Hüdepohl / atacamaphoto.com / ESO
If you want to see the presence of cosmic dust for yourself, you don’t need anything other than a clear, dark (and, ideally, moonless) sky on a night where the plane of the Milky Way is visible. When you look at the Milky Way with even your naked eye, you’ll see more than just an uncountably large number of stars: the points of light that seem to merge together into a stream of illumination across the heavens. It looks like there are giant streams of material, akin to dark clouds, floating in front of enormous parts of what should be a much more uniform backdrop of brilliance. Astronomers, for hundreds of years at least, referred to these regions as “dark nebulae,” as though they already suspected there were stars in these regions as well, but something nebulous was preventing the light from getting through.
The 20-to-30 degrees of galactic latitude, both north and south of the galactic plane, were dubbed the Zone of Avoidance in the late 19th century, because while there were huge numbers of spiral nebulae elsewhere, all across the sky, there were none found in or near the plane of the Milky Way. Many had long suspected there was light-blocking material in the way, but it really wasn’t until the invention of longer wavelength astronomy, and of infrared astronomy in particular, that we truly solved the puzzle. When we looked at the Universe in longer wavelengths of light, those dark patches seemed to disappear, revealing stars behind (and sometimes within) them, and a Universe rich in galaxies roughly equally, and in all directions.
By viewing the Milky Way in infrared wavelengths of light, we can see through large amounts of the galactic dust and view the distribution of stars and star-forming regions behind them. As revealed by the 2 micron all-sky survey (2MASS), the densest collections of galactic dust can be seen tracing out our spiral arms, but the center of the plane of the Milky Way is where the dust is densest. Infrared and visible light views both showcase this, but in vastly different ways.
Credit: 2MASS/IPAC/Caltech & UMass
The culprit, of course, is cosmic dust. We can learn a lot about the dust that’s out there in the Universe simply from the observations we acquire here on Earth, as well as from samples that we can collect by traveling up above the atmosphere into space and measuring their composition directly. Dust is produced in all sorts of ways in the Universe, including right here in our own Solar System. In fact, every time we experience a meteor shower (or see a “shooting star”) here down on terrestrial ground, it’s because there are tiny grains of dust — now known to originate from the debris streams of comets and asteroids — that smash into the atmosphere, at relative speeds of thousands or even tens of thousands of kilometers-per-second.
When we examine this dust, we find that it’s made of the same stuff that everything else is: mostly normal matter, and largely of small, simple molecules:
rocky material like silicates,
icy volatiles like carbon monoxide, ammonia-ice, carbon dioxide, and water-ice,
and occasionally heavier elements like metals,
while lighter ices, like nitrogen and possibly even hydrogen ice, are strongly suspected to exist farther away from the Sun.
We have performed sample collections on the dust within our Solar System, and in particular dust that arises from the tails of comets, and determined that this is very likely the composition of most dust within the Universe.
A view of many meteors striking Earth over a long period of time, shown all at once, from the ground (left) and space (right). If a comet’s path crosses Earth’s orbit twice, its debris stream can create up to two meteor showers per year, as Halley’s comet does. We have sampled the dust from the debris streams that cause meteor showers, enabling us to measure their composition.
Credit: Comenius University (L), NASA (R); Wikimedia Commons
We can also observe these nebulous, dusty regions that are out there in the greater Universe not only in visible light, but at longer and longer wavelengths as well. When we do, we can learn a lot about the dust that’s present, including about the typical sizes and particle densities of the dust grains that make up these clouds, simply by noting which types of light they absorb versus which types of light they allow to pass right through them.
This technique is incredibly important, and the science behind why some wavelengths of light behave as though the dust is transparent while others are completely absorbed away by it is the same science behind the tiny holes in the door to your microwave oven. Visible light is a short-wavelength form of light: between 400 and 700 nanometers, approximately. The holes in the microwave are larger — approximately millimeter-sized — and so the light that we see passes in and out of them easily. But microwaves are much longer in wavelength, at around 10 centimeters (a few inches), and so the light can’t pass through the holes. In the case of the microwave oven, that microwave radiation instead gets reflected off of the walls, which is why that energy is so efficiently transmitted into your food instead.
For cosmic dust, the dust comes in a variety of particle sizes: with larger numbers of small particles and smaller numbers of large particles. Typically, large particles are about the size of tiny pebbles or large grains of sand, while smaller particles might be only tens of nanometers in size: roughly the size of smoke particles. By observing which wavelengths of light get absorbed versus which ones get transmitted through, we can learn about the size of the particles that compose this cosmic dust.
This animation shows the Bok globule Barnard 68 in a variety of visible and infrared wavelengths. As the longer wavelengths reveal, this is not a hole in the Universe but simply a dusty cloud of gas, where the longer (redder) wavelengths of light penetrate and pass through the dust. As dust clouds form and dissipate, the dust density can be revealed by examining the light blocked and transmitted by fixed, background objects.
Credit: ESO
When the wavelength of light is smaller than the size of the dust particle, the dust behaves like a good absorber. Short-wavelength light gets absorbed by these dust particles, which then causes the particles to heat up and re-emit that light at much longer wavelengths. To an instrument that probes light of a specific (short) wavelength, it will look like the light has simply disappeared. When the wavelength of light is longer than the typical particle sizes, however, very little of that light gets absorbed, and most of it will simply continue on in its initial direction. In this way, cosmic dust behaves as a very efficient absorber of visible light, is only lightly absorbed by near-infrared light, and is practically perfectly transparent to mid-infrared (and longer) wavelengths of light.
It’s through these methods that we’ve learned that most of the dust in our Universe is made of small particles: particles that are perhaps one micron (a millionth of a meter) in size or smaller, and that large particles, of millimeter sizes and above, are extremely uncommon. Hydrogen and helium alone don’t typically make up this dust; dust requires heavier elements, but pretty much any heavy element (or sets of heavy elements) will do: carbon, nitrogen, oxygen, silicon, sulfur, magnesium, iron, etc. As far as we can tell, the abundance of the various elements in dusty regions pretty much matches the abundance of heavy elements (elements aside from hydrogen and helium) found elsewhere throughout the cosmos.
The elements of the periodic table, and where they originate, are detailed in this image above. While most elements originate primarily in supernovae or merging neutron stars, many vitally important elements are created, in part or even mostly, in planetary nebulae, which do not arise from the first generation of stars.
Credit: NASA/CXC/SAO/K. Divona
So that’s the scientific, data-driven answer to the first question: what dust consists of. But what about the second — arguably, more difficult — part of the question, which is where dust comes from? The answer definitively isn’t “from the Big Bang,” because although there are nuclear reactions that occur during the Big Bang, they only wind up producing the lightest elements and isotopes. In particular, the Big Bang creates a Universe whose atomic contents, by mass, are:
hydrogen-1 (75%),
helium-4 (25%),
deuterium (0.003%),
helium-3 (0.001%),
lithium-7 (0.000000003%),
and with only about 1-part-in-1013 being beryllium and 1-part-in-1015 being boron, carbon, or any heavier element.
With such low abundances of even the lightest heavy elements, there simply isn’t a way to form dust. This makes it exceedingly difficult for clouds of neutral matter to cool, to radiate heat away, and to gravitationally collapse. It’s for this very reason, a lack of dust, that the very first stars in the Universe to form out of this pristine material are thought to have been so great in mass: because they require enormously massive gas clouds to trigger gravitational collapse. Those stars live only for a short while, die, and finally enrich the Universe with the first significant abundances of heavier elements: mostly in the form of oxygen, then carbon, and then followed by nitrogen, silicon, neon, sulfur, magnesium, and iron.
This composite JWST image of the object Herbig-Haro 30 in the Taurus Molecular Cloud shows many features common to young, massive stars: a dusty disk (seen edge-on here), reflective dust grains above and below the disk, bipolar jets running perpendicular to the central disk, and conical outflows dovetailing into tail-like ejecta. Inside, planets are suspected to be forming around the central young star, which has only recently transitioned from the protostellar phase into the fusion-driven main sequence phase of its life. Although the outflows from Herbig-Haro objects very much inject dust into the Universe, it isn’t in sufficient amounts to explain what we see on a cosmic scale.
Credit: ESA/Webb, NASA & CSA, Tazaki et al.; Processing: E. Siegel
Once these heavier elements come into existence, they’ll participate in the next generation of stars that form, and their ability to radiate heat much more efficiently than hydrogen and helium alone leads to stars of lower masses and longer lifetimes, in general. Among those stars, some of them will become Herbig-Haro objects, which are massive stars that exhibit outflows: outflows that create cosmic dust. Unfortunately, they create, on average, less than one Earth mass worth of dust for each such star: not enough to explain what we see.
In the outskirts of stellar systems that form, there are low-mass, icy bodies: things like the analogues of Kuiper belt and asteroid belt objects in our own Solar Systems. When they get heated or ablated, whether by their parent star, another planet, or by traveling through the interstellar medium, they produce dust as well. But again, unfortunately, it’s not enough to explain what we see.
And when lower-mass stars die, they blow off their outer layers in a planetary nebula: another source of dust. Those lower-mass stars, unfortunately, mostly eject hydrogen and helium, and because these stars live a long time, they don’t generate very much dust very quickly. Combined, all of these mechanisms do admit the creation of significant amounts of dust, but not enough to explain, even all together, anywhere close to the amount of cosmic dust that we actually observe in the Universe.
This three-panel animation fades between visible light (Hubble) views, near-infrared (JWST NIRCam) views, and even cooler mid-infrared (JWST MIRI) views. This planetary nebula is one of the most well-studied in all of history, yet JWST can still reveal features never seen before. Many dusty features are present inside, but this nebula, and other planetary nebulae like it, cannot account for enough of the dust that exists in the Universe to fully explain the abundance that exists today.
Credit: ESA/Webb, NASA, CSA, M. Barlow, N. Cox, R. Wesson; NASA, ESA, and C. Robert O’Dell (Vanderbilt University); Animation: E. Siegel
Cosmic dust is incredibly important from our perspective, because it’s the solid material that gets incorporated into planets that form: making the cores of gas giant worlds and composing the majority of terrestrial-like worlds, including Mars, Earth, Venus, and Mercury. The origin of the Universe’s dust is an important question, because in a very real way, dust is the origin point for rocky planets and life as we know it. The story of dust, almost poetically, is the story of us.
But there is another potential candidate for an abundant source of dust in the Universe, one that generates dust relatively quickly in the scheme of cosmic history: the most massive, shortest-lived stars of all. Those stars burn through their fuel quickly, shine brightly, and then — overwhelmingly — end their lives in a core-collapse supernova. Sure, other fates are possible, like direct collapse to a black hole or explosions through the pair-instability mechanism, but a core-collapse supernova (also known as a type II supernova) represents the fate of the majority of the most massive stars ever to be born.
They could be the primary source of cosmic dust, but it would’ve taken a lot of dust per such supernova to make it happen: about a tenth of a solar mass (or about 30,000 Earth masses) worth of cosmic dust would need to be generated, and survive, from each such core-collapse supernova to explain the amount of dust that we presently see in the Universe.
Five different combined wavelengths show the true magnificence and diversity of phenomena at play in the Crab Nebula. The X-ray data, in purple, shows the hot gas/plasma created by the central pulsar, which is clearly identifiable in both the individual and the composite image. This nebula arose from a massive star that died in a core-collapse supernova back in 1054, when a bright light appeared worldwide, allowing us, at present, to reconstruct the elements created by this now-historical event. Large amounts of dust, mostly invisible at infrared and radio wavelengths, are also present here.
Credit: G. Dubner (IAFE, CONICET-University of Buenos Aires) et al.; NRAO/AUI/NSF; A. Loll et al.; T. Temim et al.; F. Seward et al.; Chandra/CXC; Spitzer/JPL-Caltech; XMM-Newton/ESA; and Hubble/STScI
This question was only settled, observationally, over the past ten years: when researchers studied large numbers of type II supernova remnants over time, to see how much dust was being ejected and generated by a significant population of these supernovae on one hand, and then to estimate how much of that dust would be subsequently destroyed by astrophysical processes in the aftermath of those dust creation events. Only by doing that could we learn the answer to whether these core-collapse supernovae make enough dust, and whether enough of that dust survives, to explain the amount of cosmic dust that we have in our Universe.
Two flagship-class ground-based telescopes, the NSF’s Gemini telescope and the ESO’s Very Large Telescope, were used to observe an impressive 31 separate supernova remnants at various stages of post-explosion evolution, enabling astronomers to measure the amount of dust that gets generated over roughly a thirty-year timescale after the supernova’s detonation. As the supernova remnant ages, the mass of dust:
steadily increases,
absorbs more and more of the heavier elements,
and becomes saturated at a mass of 0.42 solar masses (!!) per supernova, on average.
That means that each supernova generates roughly 130,000 Earth masses worth of dust, leading University College London professor Mike Barlow to declare, in 2022:
“This large mass implies that core-collapse supernovae were likely the dominant sources of dust in the early universe and may be the most important stellar source of dust in our local universe today.”
This image from NASA’s Chandra X-ray Observatory shows the location of different elements in the Cassiopeia A supernova remnant including silicon (red), sulfur (yellow), calcium (green), and iron (purple), as well as the overlay of all such elements (top). A supernova remnant expels heavy elements created in the explosion back into the Universe, and each element produces X-rays within a narrow energy range, allowing maps of their locations to be constructed. Only about 30-40% of the dust generated by this supernova will make it back into the interstellar medium intact: an important data point for understanding the generation of dust from type II supernovae.
Credit: NASA/CXC/SAO
Of course, 100% of that dust couldn’t possibly survive, or there would be roughly four times as much dust in the Universe from core-collapse supernovae, alone, as we actually observe there to be. But dust is also easy to destroy: photodissociation from subsequent star-formation, collision with other dust grains, and absorption by other astrophysical objects all play a role. By observing the long-ago Milky Way supernova remnant, Cassiopeia A, for example, we learned that only about 30-40% of dust grains generated by that event (also a core-collapse supernova) would survive passing through that supernova’s reverse shock.
But if that’s what occurs, that core-collapse supernovae generate enormous amounts of dust, and then a little over half of that dust gets destroyed subsequently (but the rest survives), that would account for the amount of dust that we observe — within the error bars and uncertainties, of course — throughout the cosmos. Although there are many sources of dust, it seems, at this point in time, that core-collapse supernovae really are the dominant origin source of cosmic dust. This is something, honestly, that you couldn’t have known prior to 2016, however, and it is still an open field of research in astrophysics and cosmology today, with our best conclusions always subject to revision based on the new data we’re collecting here in the JWST era. While some astrophysicists view this dust only as a source of noise, something to be avoided and looked through whenever possible, it’s a fascinating topic of study in its own right, reminding us that the old adage is still true, “one astronomer’s noise is another astronomer’s data!”
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Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all.