Stellar evolution and star formation are ongoing processes. You can see a cloud in which stars are forming today with your naked eye, namely, the middle “star” in Orion’s sword (see the figure opening this chapter). This isn’t a star at all! Called the Orion Nebula or Great Nebula of Orion, it is actually part of a gigantic interstellar cloud region called the Orion Molecular Cloud in which new stars and planets are coming into existence.

Detecting and studying newly forming stars and planets require observations in various parts of the electromagnetic spectrum outside the visible. Recall from Chapter 3 that visible light provides limited astronomical information; you can see in Figure 3-32c and Figure 4-2b that infrared images reveal even more of the matter distributed in this region. Likewise, ultraviolet images provide insights into star and planet formation unavailable from visible images. Observing with telescopes sensitive to infrared and ultraviolet wavelengths, we can see the newly forming stars and planets deep inside obscuring clouds of gas and dust. Furthermore, radio observations provide information about stellar rotation and the properties of magnetic fields surrounding planets outside the solar system.

As discussed in Section 4-2, astronomers have discovered stars in early stages of formation. Most of these protostars (conceptually analogous to the protosun) are surrounded by protoplanetary disks. These disks are similar to what the solar nebula was believed to have been like more than 4.5 billion years ago.

Along with disks of gas and dust, large numbers of comets and asteroids have been observed orbiting a few stars. The first system discovered to have comets was the young star Beta Pictoris. The comets were detected as a result of the gas they emitted when passing close to their stars.

The first star observed to have asteroids in orbit around it is Zeta Leporis, a young star just 70 ly from Earth. Its asteroid belt, located about the same distance from that star as our asteroid belt is from the Sun, contains an estimated mass 200 times greater than the mass of our asteroid belt. Although pieces of asteroid debris have not been directly observed, their presence has been inferred from the existence of hot (room-temperature) dust surrounding Zeta Leporis. The most likely explanation for this high-temperature dust is that it is being generated by the collisions of asteroids. As noted in Section 4-6, our solar system has one asteroid belt, located between Mars and Jupiter. Observations reveal that the star Epsilon Eridani has two asteroid belts, along with planets and a ring of icy debris. In 2013, astronomers discovered an asteroid belt around the bright star Vega, in the constellation Lyra.

Although astronomers had expected to find disks of gas and dust orbiting young stars, consistent with the theory explaining the formation of the solar system, they have also found disks of dusty debris orbiting stars as old as the Sun (so they are over 4 billion years old). These stars are all known to have planets, and the rubble surrounding them was most likely formed by the collisions of asteroids and comets with each other and perhaps with the planets in these systems.