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13.7: Cosmos And Culture
Tue September 17, 2013
The Bubble At The Edge Of The Solar System
Science at its most fundamental level is not made of experiments or math, copper tubing or silicon chips. Science, at its most fundamental level, is made up of stories because that's how human beings understand themselves and their place in the cosmos.
As you read this the Voyager spacecraft, launched 36 years ago, is pushing out into interstellar space. This summer it officially punctured the bubble at the edge of the solar system. But that simple statement of fact contains many stories.
On one extreme, there's the narrative of how it even measures and detects the solar system's edge. On the other extreme, there is the cultural meaning. It's a symbol of something that a machine a bunch of recently self-conscious mammals built could be tossed so far out into space.
Today I want to touch on another story, one near and dear to my own heart. This is the story I've spent much of my professional life studying: stellar winds and the bubbles they blow.
All stars drive loose mass by expelling atoms into space. These "stellar winds" come in many forms and vary in strength over time (meaning how much mass is lost and how fast). The sun is something like middle-aged (5 billion years old) and cool (a surface temperature of 5,700 kelvins). Its wind is fairly weak, driving only a hundred trillionth of its own mass back into space every year. But five billion years from now the sun will into age into a red giant getting so big its atmosphere will engulf the Earth. At that point its wind will also change, becoming so dense that it will strip off as much as one ten-thousandth of the sun's mass every year (or, in other words, 10,000 years of this wind would strip the sun down completely).
Such dense winds are also generated by hot massive stars, though these behemoths drive their mass into space at much higher speeds.
The physics behind these winds differs with each kind of star. For the sun it's all about heat generated in the upper solar atmosphere, where powerful magnetic fields get tangled and short out. Hot solar gas inflates away from the sun, creating a 500 km/s wind streaming out into space.
For massive hot stars it's the intense radiation field emerging from the stellar surface that creates the wind. For very young stars, rapid rotation and magnetic fields combine to fling gas outward as powerful winds.
However the wind forms, once it escapes from the star it begins a long journey through space. It charges through its neighborhood — whizzing into and past planets, if there are any — and out into interstellar space. But interstellar space is already full of gas and dust.
Most of this material was ejected ages ago via ancient stellar winds or the titanic explosions of massive stars called supernova. The winds coming from stars today have to push out into this so-called "interstellar medium" sweeping it up like snow pushed by a snowplow.
Seen from far away, a stellar wind blasting out into its interstellar neighborhood appears as a bubble — a wind blown bubble — with the shell of swept-up gas appearing as a ghostly, glowing rim. (Below is a wind-blown bubble simulation, calculated just for you by Martin Huarte Espinosa, a post-doc in my research group).
That's where Voyager is right now. For decades it sailed through the interior of the bubble blown by the sun's wind. This summer, as it reached a distance of more than 10 billion miles from the sun, it finally made its way through the rim of the sun's wind-blown bubble and out into the interstellar medium.
The study of stellar-wind bubble's is fascinating in its own right, providing insights into the evolution of stars and the complex physics of charged hypersonic gas flows in space.
They are also remarkably beautiful objects, a kind of stellar sculpture, etched in atoms stretching across lightyears. Across the span of my own career I have stared at hundreds of images of these bubbles and wondered what stories were encoded in their forms.
So, when it comes to Voyager's new step into the true depths of space, there are many stories we can tell. But the one which resonates for me is quite simple. All my life I've have studied these bubbles from afar, using only the light our telescopes could catch. Now, for the first time, we have gone beyond such passive observation. Now, for the first time, we have pushed out far enough to touch one of these gossamer bubbles directly. After literally thousands of years of simply watching, we humans have begun to truly explore the universe.
I feel lucky to live in this moment and can only hope it is just the beginning.