(Photo A) Disks and jets pervade the universe on many scales. Here, they seem to be a natural result of a rotating cloud of gas contracting to form a star. Matter falling onto the embryonic star creates a pair of high-speed jets of gas perpendicular to the star's flattened disk, carrying away heat and angular momentum that might otherwise prevent the birth of the star. This image shows a small region near the Orion Nebula known as HH1/HH2, whose twin jets have blasted outward for several trillion km (nearly half a light-year) before colliding with interstellar matter. (HH stands for Herbig-Haro, after the investigators who first cataloged such objects.) The next three photos show stellar jets ejected from three different very young stars. Reproduced here to scale, these images collectively depict the propagation of a jet through space.
(Photo B) This image of HH30, spanning approximately 250 billion km, or about 0.01pc, shows a thin jet (in red) emanating from a circumstellar disk (at left in grey) encircling a nascent star.
(Photo C) One of HH34's jets is longer, reaching some 600 billion km, yet remains narrow, with a beaded structure.
(Photo D) HH47 is more than a trillion km in length, or nearly 0.1pc. This photo shows one of its jets plowing through interstellar space, creating bow shocks in the process.
Studying this chapter will enable you to:
Discuss the factors that compete against gravity in the process of star formation.
Summarize the sequence of events leading to the formation of a star like our Sun.
Explain how the process of star formation depends on stellar mass.
Describe some of the observational evidence supporting the modern theory of star formation.
Explain the nature of interstellar shock waves, and discuss their possible role in the formation of stars.
We now move from the interstellar medium—the gas and dust among the stars—back to the stars themselves. The next four chapters discuss the formation and evolution of stars. We have already seen that stars must evolve as they consume their fuel supply, and we have extensive observational evidence of stars at many different evolutionary stages. With the help of these observations, astronomers have developed a good understanding of stellar evolution—the complex changes experienced by stars as they form, mature, grow old, and die. We begin by studying the process of star formation, through which interstellar clouds of gas and dust are transformed into the myriad stars we see in the night sky.
