Of Particular Significance

Mapping the Unknown, Times Three

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 01/17/2022

I took a short break from other projects this weekend. Poking around, I found three particularly lovely science stories, which I thought some readers might enjoy as well. They all involve mapping, but the distances involved are amazingly different.

Starbirth Near to Home

The first one has been widely covered in the media (although a lot of the articles were confused as to what is new news and what is old news.) Our galaxy is full of stars, but also of gas (mainly hydrogen and helium) and dust (tiny grains of material made mainly from heavier elements forged in stars, such as silicon). That gas and dust, referred to as the “interstellar medium”, is by no means uniform; it is particularly thin in certain regions which are roughly in the shape of bubbles. These bubbles were presumably “blown” by the force of large stellar explosions, i.e. supernovas, whose blast waves cleared out the gas and dust nearby.

It’s been known for several decades that the Sun sits near the middle of such a bubble. The bubble and the Sun are moving relative to one another, so the Sun’s probably only been inside the bubble for a few million years; since we’re just passing through, it’s an accident that right now we’re near its center. Called simply the “Local Bubble”, it’s an irregularly shaped region where the density of gas and dust is 1% of its average across the galaxy. If you orient the galaxy in your mind so that its disk, where most of the stars lie, is horizontal, then the bubble stretches several hundred light-years across in the horizontal direction, and is elongated vertically. [For scale: a light-second is 186,000 miles or 300,000 km; the Sun is 8 light-minutes from Earth; the next-nearest star is 4 light-years away; and our Milky Way galaxy is about 100,000 light years across.] It’s been thought for some time that this bubble was created some ten to twenty million years ago by the explosion of one or more stars, probably siblings that were born close by in time and in space, and which at their deaths were hundreds of light-years from the Sun, far enough away to do no harm to Earthly life. [For scale: recall the Sun and Earth are about 5 billion years old.]

Meanwhile, it’s long been suggested that explosion debris from such supernovas can sweep up gas and dust like a snowplow, and that the compression of the gas can lead it to start forming stars. It’s a beautiful story; large stars live fast and hot, and die young, but perhaps as they expire they create the conditions for the next generation. [A star with 40 times the raw material as the Sun has will burn so hot that it will last only a million years, and most such stars die by explosion, unlike Sun-like stars.]

If this is true, then in the region near the Sun, most if not all the gas clouds where stars are currently forming should lie on the current edge of this bubble, and moreover, all relatively young stars, less than 10 million years old, should have formed on the past edge of the bubble. Unfortunately, this has been hard to prove, because measuring the locations, motions and ages of all the stars and clouds isn’t easy. But the extraordinary Gaia satellite has made this possible. Using Gaia’s data as well as other observations, a team of researchers (Catherine Zucker, Alyssa A. Goodman, João Alves, Shmuel Bialy, Michael Foley, Joshua S. Speagle, Josefa Groβschedl, Douglas P. Finkbeiner, Andreas Burkert, Diana Khimey & Cameren Swiggum) has claimed here that indeed the star-forming regions lying within a few hundred light-years of the Sun all lie on the Local Bubble’s surface, and that nearby stars younger than ten million years were born on the then-smaller shell of the expanding bubble. Moreover they claim that the bubble probably formed 14-15 million years ago, at a time when the Sun was about 500 light-years distant.

The one exception to this rule appears to be a star-forming region on the backside of a second bubble, adjoining to the Local Bubble. The discovery of this “Per-Tau superbubble,” and the fact that two star-forming regions lie on opposite sides of it, was announced by the (nearly) same team just a few months ago; in fact this earlier paper gives the first direct evidence of the explosion-snowplow-to-star-formation effect. (You can see a 3d image of that adjoining bubble, created by the research team, here. and you can see its relation to the Local Bubble in the right-hand-side of this image (via universetoday, credit C. Zucker) [sorry, an earlier link was blocked] where the Local Bubble is marked in purple and the adjoining “Per-Tau” bubble is marked in green. Other visuals and captions are at this site.) The supernova or supernovas that may have created this bubble also probably went off between 5 and 20 million years ago, based on the size of the bubble and the slow motion of the bubble walls.

That stars can reproduce by generating new ones from the debris of the old is reminiscent of a similar effect in thunderstorms; when a thunderstorm complex is mature and loses its powerful updrafts, its cold air drops downward and then flows outward, and the resulting outflow can generate new storms. (See this blog post for some detailed discussion.) Perhaps there are other examples of natural engines generating offspring in a similar way; do you know of any? I have long wondered whether life could have gotten its start through a mechanism like this one, a rudimentary form of imperfect physical reproduction on which evolution might then have begun to act.

Galaxies Across the Cosmos

Meanwhile the Dark Energy Spectroscopic Instrument (DESI), seeking to understand the history of the universe with precision, has been engaged in a similar mission to Gaia but on a different scale and with different methods. It is mapping distances not to nearby stars in our own galaxy but to other galaxies, covering a very substantial fraction of the visible universe. Its data has now allowed the creation of the largest ever 3D map of a region of the cosmos, extending across part of the sky out to distances reaching five billion light years from Earth. (I’m particularly fond of this type of map, because my first scientific paper was an attempt, led by Jerry Ostriker, to make sense of the structures seen in the first 2D slice map of the universe, made by Valerie de Lapparent, Margaret Geller and John Huchra, scarcely reaching out 200 million light-years.) The 3D map, seen as a movie that sweeps through 2D slices of the map, can be found here. Each dot on the map is a galaxy; our own galaxy is located at the lower left corner. Within the map (ignoring dark wedges which presumably arise from regions that are blocked from view by nearby objects in the sky) you can see emptier regions threaded by dense filaments and other features.

And DESI has much more to tell us; it has measured far more galaxies than appear in this map; it has measured the distance to many galaxies extending out to 10 billion light years, most of the distance across the visible universe; and it’s just getting started, with many more years of mapping left to do. By the way, the technology behind DESI is pretty amazing; I leave it to you to read about it on the DESI website.

The Ocean Deep

Finally I bring us closer to home, to another frontier where humans still have remote mapping to do: the bottom of the sea. The story begins with partial failure in an experiment to explore the deepest region of the ocean, the Challenger Deep, which extends below sea level 25% further than Mount Everest extends above it. In 2014 a research team put two thick glass spheres full of scientific instruments into the water above the Challenger Deep, expecting one to reach the very bottom and the other to remain just above it. Unfortunately the first one must have had a small but fatal flaw in its construction, because when still roughly 2 km (1.2 miles) above the sea floor, it imploded under the immense pressure. One can only begin to imagine the disappointment of those who built the instrument and were expecting reams of data from it. At least the other sphere survived.

But now comes a story of scientists at their most creative, making lemonade from this lemon. The implosion that destroyed the first instrument created a shock wave which reverberated all across the ocean floor, back to the ocean surface, yet again back to the floor, and so on; and those reverberations were detected by the surviving instrument. By carefully investigating all these echoes from the disaster, a team of three scientists has recently claimed to have obtained the most precise measurement ever of the depth of the Challenger Deep: 10,983 meters (36033 feet) with an uncertainty of only 6 meters (20 feet) up or down, one fourth the uncertainty of any previous measurement. While I’m in no position to evaluate the validity of this claim, it represents an inspiring effort to make the very best of bad news, and to treat an experiment’s failure as an experiment in its own right.

Now if someone could just boldly go and map the internet

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10 Responses

    1. Not really. The sun exists in its own bubble, the heliosphere, caused by its solar wind. This means that the environment around even, say, Jupiter will largely be independent of whether or not we’re in a cloud or void.

  1. “and you can see its relation to the Local Bubble in the right-hand-side of this image, ” link is broken

  2. Very interesting. Always a pleasure to read you. I hope you will retire soon and have more time to write posts 😉 Thank you

  3. Very interesting. Lovely to see you posting.

    Given that space is a near vacuum, I’ve always been surprised that explosion debris would be capable of sweeping up gas and dust. Naively, I would of thought it would be like trying to use a very holely snowplough to sweep up extremely fine snow.

    1. Interesting point. It’s not obvious, I agree. I don’t have the relevant estimates at the tips of my fingers; maybe a more expert reader can help us understand how the numbers work out. By the way, I oversimplified things for brevity. In fact, some of the snowplowing takes place before the supernovas occur; the gas is already pushed outward just by the stellar winds of these bright, hot stars.

    2. Two way of looking at this can help. The first is to consider the total mass flow as the wall of the bubble moves past.

      The gas contains around 1-10 particles per cubic centimeter. (The plasma becomes more diffuse at it spreads, but it also picks up more material from the interstellar medium). A light year (10 trillion Ks) of the lowest density will thus have about 1’000 quadrillion particles move through such a volume. The local bubble’s wall is, if I recall correctly, about 4 parsecs or 13Ly thick. 13 quintillion particles crossing a square centimeter of area is comparable to sea level air pressure. So what we’re dealing with in total is less a snowplow and more like a solid gust of wind, which absolutely will move snow.

      The second way to look at it is to note that a bubble wall of several Ly in thickness contains billions or trillions of times the mass of the static gas in any given volume of space it passes through. On any scale having that ration of mass interact is going to severely affect the static gas.

      The forces *are* very small, but applied for millions of years over quadrillions of square miles. It involves energies that would detonate our sun, but applied over distances that would reduce our star to a pinprick. In the moment a human’s breath would overpower it, but it is an event that lasts longer than our species has existed.

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