Hubble observations of HH 30 show a pair of thin jets streaming away from the center of a dusty disk. The disk, which is over 40 billion miles (64 billion kilometers) in diameter, is seen almost edge-on. Like a thin, dark cloud moving in front of the Sun, the disk blocks any direct view of its central star. All that is seen are the top and bottom sides of the dusty disk reflecting light from the star, like the "silver lining" of a cloud. The jets reveal the hidden star's location. Astronomers are interested in the disk because it is probably similar to the one from which the Sun and the planets in our solar system formed. HH 30's disk and jet show dramatic changes in the six years covered by the time-lapse movie. The jets are easiest to explain: as in XZ Tauri, material is being ejected along the magnetic poles of the star at speeds of between 200,000 and 600,000 miles per hour (320,000 and 960,000 kilometers per hour). Every few months a compact clump of gas, called a knot, is ejected, and may eventually merge with other clumps downstream. However, astronomers aren't sure why the knots in the upper jet are moving only about half as fast as in the fainter, lower one.
The changes in the disk are quite peculiar: patterns of light appear to be moving around within it. Astronomers believe this effect is similar to distant clouds being illuminated by the beam from a lighthouse: As the light rotates, the clouds seem to brighten and then fade. In the case of HH 30, the lighthouse is the star and the inner part of its disk, which throws bright rays and casts dark shadows on the outer part of the disk. This "lighthouse" in HH 30 appears to be rotating between once every few days and once a year. Astronomers hope more observations will narrow down that cycle and thus show whether the light patterns are shadows cast by material in the disk or beams of light from hot spots on the star.
Credits: Alan Watson (Universidad Nacional Autonoma de Mexico, Mexico), Karl Stapelfeldt (NASA Jet Propulsion Laboratory), John Krist (Space Telescope Science Institute), and Chris Burrows (European Space Agency/ Space Telescope Science Institute)