Star Formation

An important avenue of research involves studying the cycle of star births and deaths in the galaxy. Formation of stars depletes the interstellar medium, since some of its gas goes into making the stars. But then, as a star shines, a small part of its matter escapes its gravity and returns to the interstellar medium. More importantly, massive stars return a large fraction of their matter to the interstellar medium when they explode and die. This cycle of depletion and replenishment is critically important in understanding the types of stars we see in the galaxy, and the evolution of the galactic system as a whole.

The advent of powerful new telescopes like the Hubble Space Telescope has opened astronomer's eyes to new stars that may require new theories of formation. In 1997 the brightest star ever seen was discovered at the core of our own galaxy, the Milky Way. Named the Pistol Star, it has the energy of 10 million Suns and would fill the distance of the Earth's orbit around the Sun. The Pistol Star is about 25,000 light-years from Earth; it is so turbulent that its eruptions create a gas cloud four light-years across. It had been thought that a star so big could not have formed without blowing itself apart, and so the Pistol Star may require astronomers to reexamine their ideas about stellar formation, especially of supermassive stars near the centers of galaxies.

While some astronomers study the galactic or the interstellar medium, others study newly forming protostars. Protostars are hot, condensing blobs of gas that have not quite yet achieved starhood, and they are hard to observe for two reasons. First, the phase of star formation is quite short by astronomical standards, so there are not nearly as many protostars as there are fully formed stars. Second, protostars are often thickly shrouded by the remnants of the cloud from which they are forming. This makes them appear much dimmer, and so much harder to observe and study.

Fortunately, newly forming stars do have some observable characteristics. A protostar may be girdled by a disk of dust and gas, and an exciting possibility is that these disks are protoplanetary systems. Our own solar system is thought to have formed from such a disk that surrounded the newly forming Sun, and disks around other stars such as Beta Pictoris may be current sites of planetary formation. Additionally, a protostar with a disk may produce two "beams" of gas that stream outward from its poles along the lines of magnetic field associated with the disk. These so-called bipolar outflows are classic signatures of a protostar with a disk.

It is not necessary to observe only our own Milky Way Galaxy to find newly forming stars. Modern telescopes, including the Hubble Space Telescope, are used to study star-forming regions in other galaxies. High-resolution observations can detect individual stars in the Milky Way's satellite galaxies and in some other nearby galaxies. In more distant galaxies, the regions of heated gas produced by new stars are visible. Observations of star formation in other parts of the Universe help confirm and give us a broader perspective on our theories regarding star formation in our own celestial neighborhood.