FEATURE ARTICLE
The Galactic Environment of the Sun
The heliosphere appears to protect the inner solar system from the vagaries of the interstellar medium
Priscilla Frisch
The Solar Neighborhood
We tend to think of our neighborhood in the Milky Way galaxy as motionless: Various nebulae and dust clouds look pretty much the same as when astronomers first photographed them more than a century ago. But this perception of a tableau frozen in space is deceiving. Over the course of millions of years, interstellar clouds form and dissipate as stars and supernovae stir up the interstellar matter. Fierce stellar winds produced during star formation evacuate cavities in the molecular clouds in which the stars are born. When subsequent supernovae explode in these cavities, violent shock fronts are produced that ram into surrounding material and sweep interstellar gas into "supershells" that may break out of the parent molecular clouds and propagate outward through the low-density regions of space. The atoms within these clouds are partially ionized by stellar radiation and collisions with each other, and by x rays from shocked gas within the evacuated "superbubble" cavities. Some supershells are also threaded by magnetic fields, which trap ions that may be in the vicinity. It is this combined image of various activities that should be held in mind when we consider our local galactic environment.
Our sun is also in motion. Relative to the average motion of the most commonly measured nearby stars, the sun moves with a speed of about 16.5 kilometers per second, or nearly 50 light-years per million years. The sun's path is inclined about 25 degrees to the plane of the galaxy and is headed toward a region in the constellation of Hercules near its border with Lyra. The sun oscillates through the plane of the galaxy with an amplitude of about 230 light-years, crossing the plane every 33 million years. However, the sun's motion relative to the local stellar neighborhood should not be confused with its movement around the center of the galaxy, since the whole solar neighborhood (including the sun) orbits the galactic center once every 250 million years. Just as we do not include the earth's velocity around the sun when calculating the speed of an airplane (we are only interested in the ground-speed), astronomers do not include the sun's galactic orbital velocity when describing its local motion.
The interstellar cloud currently surrounding the solar system—often referred to as the Local Interstellar Cloud—is warm, tenuous and partially ionized. Like all interstellar clouds, our local cloud is made of dust and gas, with the dust fraction making up about one percent of the cloud's mass. The elemental composition of interstellar clouds is much like that of the sun, about 90 percent hydrogen and 9.99 percent helium. The heavier elements make up the remaining 0.01 percent.
The sun is on the edge of what is sometimes called the Local Bubble, a great void in the distribution of interstellar gas in the nearby galactic neighborhood. As voids go, the Local Bubble interior is one of the most extreme vacuums yet discovered. The very best laboratory vacuum is about 10,000 times denser than a typical interstellar cloud, which in turn is thousands of times less dense than the Local Bubble. The Local Bubble is not only relatively empty (with a density of less than 0.001 atoms per cubic centimeter); it is also quite hot, about one million degrees kelvin. By comparison, the interstellar cloud around the solar system is merely warm, about 7,000 degrees, with a density of about 0.3 atoms per cubic centimeter.
The Local Bubble lies within a ring of young stars and star-forming regions known as Gould's Belt. The Belt is evident in the night sky as a band of very bright stars that sweeps in a great circle from the constellations Orion to Scorpius, inclined about 20 degrees relative to the galactic plane. The north pole of Gould's Belt lies close to the Lockman Hole, a region in the sky with the least amount of intervening interstellar gas between the sun and extragalactic space. Star formation regulates the distribution of interstellar matter, including the boundaries of the Local Bubble. The closest star-forming region on the outskirts of the Local Bubble is about 400 light-years away in the Scorpius-Centaurus association. The molecular clouds from which stars are formed are both cooler (less than 100 degrees) and denser (over 1,000 atoms per cubic centimeter) than the Local Interstellar Cloud.
A plot of the sun's course through our galactic locale shows that the sun has been traveling through the Gould's Belt interior in a region of very low average interstellar density for several million years. The sun is unlikely to have encountered a large, dense interstellar cloud in this relatively benign region during this time. Although our solar system is in the process of emerging from the Local Bubble, the sun's trajectory suggests that it will probably not encounter a large, dense cloud for at least several more million years. The consequences of such an encounter for the earth's climate are unclear; however, one wonders whether it is a coincidence that Homo sapiens appeared while the sun was traversing a region of space virtually devoid of interstellar matter.
Despite the absence of massive clouds within 100 light-years, it seems likely that the local galactic environment changes in subtle ways on much shorter time scales. The low density of the Local Bubble permits the products of supernova explosions—such as superbubbles and shock fronts—to expand easily into the void and sweep past the sun. Indeed, within the past 250,000 years the sun has entered the outward flow of material from the star-forming region of the Scorpius-Centaurus association. There is even some suspicion that the interstellar environment may have changed within the past 2,000 years! This is uncertain, however, because astronomers have an incomplete understanding of the structure of the local interstellar cloud complex.
The cloud around the solar system is part of the outflow of material from the Scorpius-Centaurus association. If we adopt the viewpoint of a person who is stationary with respect to the average motion of the nearest stars, then the motions of the sun through space and the Local Interstellar Cloud are seen to be nearly perpendicular. In other words, the interstellar cloud complex around the sun is sweeping past the solar system in a direction roughly perpendicular to the sun's movement with respect to the local solar neighborhood. The result of these two motions is that we observe interstellar material flowing toward the sun at about 26 kilometers per second from a direction close to the plane of the ecliptic and within about 15 degrees of the center of the galaxy. Because this material flows through the solar system, it has been dubbed the Local Interstellar Wind.
The origin of the Local Bubble and the Local Interstellar Cloud is still an open question. Some astronomers believe the void is a region of space between spiral arms of the galaxy that has been evacuated by shock waves of successive star-formation episodes in the Scorpius, Centaurus and Orion constellations and the Gum Nebula. Others believe that the void was created by a supernova explosion (in the Scorpius- Centaurus association) that evacuated a "hole" in a pre-existing low-density portion of the interstellar medium. (The term "Local Bubble" was originally coined with the idea that the solar system was sitting inside a supernova remnant.) So the Local Interstellar Cloud appears to be either material pushed aside by the winds of star formation or a supershell marking the edge of a superbubble.
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