A reprint from American Scientist the magazine of Sigma Xi, The Scientific Research Society This reprint is provided for personal and noncommercial use. For any other use, please send a request to Permissions, American Scientist, P.O. Box 13975, Research Triangle Park, NC, 27709, U.S.A., or by electronic mail to [email protected]. ©Sigma Xi, The Scientific Research Society and other rightsholders The Galactic Environment of the Sun The heliosphere appears to protect the inner solar system from the vagaries of the interstellar medium Priscilla C. Frisch ieces of interstellar matter are con- Part of the answer must lie with the so- Some supershells are also threaded by Pstantly passing through our solar lar wind—the hot, ionized gas that magnetic fields, which trap ions that system. These galactic visitors—atomic blows out from the sun. As it happens, may be in the vicinity. It is this com- particles and bits of dust—flow through the solar wind modulates what can bined image of various activities that interplanetary space and may collide (and cannot) flow into the solar system. should be held in mind when we con- with the major bodies in the solar sys- Given that the solar wind is itself a vari- sider our local galactic environment. tem—the earth and the other planets. able phenomenon (changing in strength Our sun is also in motion. Relative to Although each particle is microscopic, periodically with an 11-year solar cycle), the average motion of the most com- their total mass in the solar system is the relation between the solar wind and monly measured nearby stars, the sun enormous. Indeed, about 98 percent of the invading particles from the inter- moves with a speed of about 16.5 kilo- the gaseous fraction in the heliosphere— stellar medium is in constant flux. Sort- meters per second, or nearly 50 light- the volume of space filled by the solar ing out the complex interplay between years per million years. The sun’s path wind—consists of interstellar material! these dynamic phenomena has become is inclined about 25 degrees to the plane How do these particles interact with a a fascinating area of research. Here we of the galaxy and is headed toward a re- planet’s environment? Do they have a describe what space scientists and as- gion in the constellation of Hercules significant impact on a planet’s atmos- trophysicists have learned about the near its border with Lyra. The sun oscil- phere? No one knows. sun’s galactic environment and its inter- lates through the plane of the galaxy These questions take on special signifi- action with the solar system. with an amplitude of about 230 light- cance in light of the fact that the interstel- years, crossing the plane every 33 mil- lar medium—the sun’s galactic environ- The Solar Neighborhood lion years. However, the sun’s motion ment—is not a homogeneous substrate. We tend to think of our neighborhood relative to the local stellar neighborhood Astronomers have discovered that inter- in the Milky Way galaxy as motionless: should not be confused with its move- stellar material is organized into clouds of Various nebulae and dust clouds look ment around the center of the galaxy, dust and gas with elaborate features re- pretty much the same as when as- since the whole solar neighborhood (in- sembling filaments, worms, knots, loops tronomers first photographed them cluding the sun) orbits the galactic cen- and shells. Within a relatively small region more than a century ago. But this per- ter once every 250 million years. Just as of space, just a few thousand light-years ception of a tableau frozen in space is we do not include the earth’s velocity across, the interstellar medium may deceiving. Over the course of millions around the sun when calculating the exhibit a broad range of temperatures, of years, interstellar clouds form and speed of an airplane (we are only inter- densities and compositions. Indeed, as- dissipate as stars and supernovae stir ested in the ground-speed), astrono- tronomers’ understanding of the inter- up the interstellar matter. Fierce stellar mers do not include the sun’s galactic stellar medium has been up-ended in the winds produced during star formation orbital velocity when describing its local past decade as the physical and structural evacuate cavities in the molecular motion. complexity of interstellar material has un- clouds in which the stars are born. The interstellar cloud currently sur- folded. It is now evident that the sun must When subsequent supernovae explode rounding the solar system—often re- have experienced a broad range of galactic in these cavities, violent shock fronts are ferred to as the Local Interstellar Cloud— environments in its 5-billion-year history. produced that ram into surrounding is warm, tenuous and partially ionized. How does the solar system respond material and sweep interstellar gas into Like all interstellar clouds, our local to a change in its galactic environment? “supershells” that may break out of the cloud is made of dust and gas, with the parent molecular clouds and propagate dust fraction making up about one per- outward through the low-density re- cent of the cloud’s mass. The elemental Priscilla Frisch is a Senior Research Associate in the Department of Astronomy and Astrophysics at gions of space. The atoms within these composition of interstellar clouds is the University of Chicago. Address: Department of clouds are partially ionized by stellar ra- much like that of the sun, about 90 per- Astronomy and Astrophysics, 5640 S. Ellis Ave., diation and collisions with each other, cent hydrogen and 9.99 percent helium. Chicago, IL 60637. Internet: and by x rays from shocked gas within The heavier elements make up the re- [email protected] the evacuated “superbubble” cavities. maining 0.01 percent. © 2000 Sigma Xi, The Scientific Research Society. Reproduction 52 American Scientist, Volume 88 with permission only. Contact [email protected]. to galact ic center Scorpius-Centaurus Association R M Orion Association S A G G I T A R I U S A sun s ar M -ye A R ht R I O N 0 lig O 3,00 rs ea t-y gh 0 li 6,00 Figure 1. Our “neighborhood” in the Milky Way lies just inside one of the galaxy’s great spiral arms, the Orion Arm. The majority of the brightest stars are distributed along a section of sky known as Gould’s Belt (dashed ellipse), which also marks the distribution of the nearby star-forming re- gions in the Orion spiral arm. Stellar winds from these star-forming regions—such as the Scorpius-Centaurus association—push “shells” of in- terstellar material into the sun’s path. The sun is currently on the edge of such a shell (not visible at this scale). The actions of interstellar winds and the sun’s own motion through the galaxy may alter the sun’s local galactic environment on time scales as brief as a few thousand years. The author discusses what space scientists know about the interaction between the solar system and its changing galactic environment. (Courtesy of the Na- tional Geographic Society. Adapted from the supplement to the October 1999 issue of National Geographic.) The sun is on the edge of what is The Local Bubble lies within a ring of from which stars are formed are both sometimes called the Local Bubble, a great young stars and star-forming regions cooler (less than 100 degrees) and denser void in the distribution of interstellar gas known as Gould’s Belt. The Belt is evi- (over 1,000 atoms per cubic centimeter) in the nearby galactic neighborhood. As dent in the night sky as a band of very than the Local Interstellar Cloud. voids go, the Local Bubble interior is one bright stars that sweeps in a great circle A plot of the sun’s course through of the most extreme vacuums yet dis- from the constellations Orion to Scor- our galactic locale shows that the sun covered. The very best laboratory vacu- pius, inclined about 20 degrees relative has been traveling through the Gould’s um is about 10,000 times denser than a to the galactic plane. The north pole of Belt interior in a region of very low av- typical interstellar cloud, which in turn Gould’s Belt lies close to the Lockman erage interstellar density for several mil- is thousands of times denser than the Hole, a region in the sky with the least lion years. The sun is unlikely to have Local Bubble. The Local Bubble is not amount of intervening interstellar gas encountered a large, dense interstellar only relatively empty (with a density of between the sun and extragalactic space. cloud in this relatively benign region less than 0.001 atoms per cubic centime- Star formation regulates the distribution during this time. Although our solar ter); it is also quite hot, about one million of interstellar matter, including the system is in the process of emerging degrees kelvin. By comparison, the in- boundaries of the Local Bubble. The from the Local Bubble, the sun’s trajec- terstellar cloud around the solar system closest star-forming region on the out- tory suggests that it will probably not is merely warm, about 7,000 degrees, skirts of the Local Bubble is about 400 encounter a large, dense cloud for at with a density of about 0.3 atoms per cu- light-years away in the Scorpius-Cen- least several more million years. The bic centimeter. taurus association. The molecular clouds consequences of such an encounter for © 2000 Sigma Xi, The Scientific Research Society.