Sky & Telescope August 1983 [antikvár]

Bipolar Gas Dets in Star-Forming RegionsJohn Bally, Bell Laboratories, Holmdel, New JerseyMuch progress has been made recently on the old and fundamental problem of how stars and planets form. Astronomers have suspected for over a century that stars result from the gravitational contraction of interstellar matter. However, direct study of protostars and their environments has been possible only in the last two decades since the advent of infrared and millimeter-wave astronomy and the discovery of molecules in the densest parts of the interstellar medium.New techniques have revealed that star formation occurs deep inside dusty and opaque molecular clouds. The clouds often contain nearly a million solar masses of cold gas. making them the largest gravita-tionally self-bound objects in our galaxy. Although their star-forming cores are hidden from view, infrared and radio emissions readily escape from these regions. Many infrared stars, completely invisible at optical wavelengths, have been found inside molecular clouds. Such stars are believed to have formed very recently, approximately within the last 100,000 years.The birth of stars is frequently accompanied by spectacular phenomena. Powerful natural masers, which pump nearly a solar luminosity of energy into a few extremely narrow radio frequencies, shine near many luminous protostars. High-ve-locity outflows of gas, and radiation from the shock waves formed when this gas strikes the surrounding molecular cloud,Two views of the bipolar H II region SI06. Left: Enlarged from a red-light National Geographic Society-Palomar Observatory Sky Survey photograph, this field is 6 arc minutes wide. Right: A near-infrared photograph taken with the 2.2-meter Calar Alto telescope by K. Birkle, enlarged to three times the scale of the photo at left. More detail can be seen in the bright inner parts of the nebula, but most of the outer regions are lost. Compare with the radio images at right.The bipolar reflection nebula LkHa 208, enlarged from a Palomar Sky Survey blue plate. The scarcity of stars in the left half of the picture indicates the presence of a dark interstellar cloud. The field is 30 arc minutes high; in all photographs with this article, north is up and east is to the left.94 Sky Telescope. August. 1983have also been observed in many star-forming regions. Sometimes the shock fronts are visible optically as so-called Her-big-Haro (HH) objects, which were discovered a quarter century ago and at first were themselves thought to be newly formed stars. These nebulosities, which may move through space at velocities of 100 kilometers per second or more, now are believed to be associated with high-velocity shock waves.Millimeter-wave radio telescopes are now sensitive enough to map the distribution and velocity of molecular gas surrounding newly formed stars. A remarkable pattern has emerged from these studies. Instead of finding evidence for gravitational collapse of a surrounding cloud, many observations of recently formed stars have detected energetic outflows of molecular gas! These are called "bipolar" because they often consist of two streams of gas ejected in opposite directions from the vicinity of an infrared star.Bipolar outflows have been found in vastly different celestial objects. Examples include the jets of ionized gas that emerge from some galaxies and quasars. The molecular jets from newborn stars are, of course, much smaller and less energetic than their extragalactic counterparts. Nevertheless, the structural similarities suggest that these phenomena may have similar hydrodynamic causes that operate on both large and small scales. This article will ex-plore only the nature of bipolar flows associated with star formation.BIPOLAR NEBULAEEmission and reflection nebulae often mark the place where a recently formed star has emerged from its parent molecular cloud. The bipolar structure that sometimes appears comes from two distinct lobes of emission that are separated by a dark lane, which usually conceals the star responsible for the nebula's light. At left is a photograph of a bipolar reflection nebula in Gemini known as LkHa208. At visual wavelengths a star is hidden at the center of the "hourglass," but in infrared light it becomes visible. The dust responsible for the obscuration is confined to a plane lying in our line of sight; light can leave the star only perpendicular to that plane. Thus, tenuous gas above and below the-dense obscuring plane is illuminated by the star and a double nebula results.While LkHa208 is associated with a newborn star, other examples of bipolar nebulae result from stellar death. When a star exhausts the hydrogen that sustains its thermonuclear fires, it becomes a red giant and eventually sheds a large fraction of its atmosphere back into space. As the star's hot nucleus is exposed, ultraviolet radiation ionizes the expanding gas and creates a planetary nebula. Messier 27 (the Dumbbell nebula) is a fine example of a planetary with two distinct emission lobes