Stars in Spiral Galaxies

Dark molecular clouds in spiral galaxies have a balance between gravity pulling inward and pressure pushing outward and might not ever collapse and form stars; however, a triggering event such as the merger of two clouds or a supernova provides a shock that causes the cloud to collapse and form stars.

Large stars form first, and their high energy radiation (UV) ionizes the remaining diffuse hydrogen gas in the nebula. The excited hydrogen gas glows and forms a pink H II emission nebulae (Figure 3‑7). H II refers to the fact that the gas is ionized. The hydrogen gas emits a blue color when it reflects (blue reflection nebulae) radiation from stars. For example, there is a blue color behind the large stars at the center of the Rosette Nebula. This is probably light reflected off the gas behind the stars. Stellar radiation also forms emission nebula at the surface of the remaining dense clouds in which star formation is yet to take place, as it ionizes gas at the surface. Below the surface, the clouds are dark because dust in the cloud protects the interior of the cloud from high energy radiation.

Frank Summers from the Space Telescope Science Institute describes the Pillars of Creation in the Eagle Nebula at https://youtu.be/JqZ2xtsJRGc. Dr. Summers has many excellent videos on Hubble images. The Eagle Nebula (Figure 3‑9), Messier object 16 (M16), is 6,800 light-years from Earth. It is near the constellation of Sagittarius, which is in the direction of the center of the Milky Way. The entire nebula has the shape of an eagle if you use your imagination. Many regions in this stellar nursery have already collapsed and formed stars, and the light from the stars excites the gas and forms a pink H II region (Figure 3‑9). A famous part of the nebula is an uncollapsed dark molecular cloud section called the Pillars of Creation (Figure 3‑10), which is several light-years in length

The possible existence of small unrevealed stars in dark molecular clouds was doubted after it was proposed by Bart Bok and Edith Reilly in 1947. Lin-Yun and Clemens finally proved it in 1990 when they observed a warm source inside a molecular cloud. Small sections of the cloud in Figure 3‑11 form individual stars, which do not reveal themselves immediately because their radiation has yet to blow away the molecular cloud envelope that remains around them. This is similar to the process in Bok globules in which individual stars form in relatively small clouds that are surrounded by H II emission nebulae that are already excited by the light from large stars. A complex process takes place inside these star-forming clouds in which there might be polar jets and a disk surrounding the protostar (Figure 3‑12), all concealed by the molecular cloud envelope. Some of the uncollapsed sections of the nebula are so small and under such intense radiation from large nearby stars that they will erode away before collapsing and forming stars.

The European Space Agency’s Herschel Telescope and NASA’s Spitzer telescope collected infrared and microwave images of forming stars. The telescopes observed small stars forming within filaments that are 1/3 light year in diameter (Figure 3‑13) and large stars forming at ridges where filaments intersected. Scientists have developed complex computer models of molecular clouds and analyze star formation scenarios with these models. Models are also showing that stars form in filament-like structures in the clouds (Figure 3‑14). The models run on massive supercomputers that simulate the complex physics and chemistry (gravity, thermal energy, hydrodynamics, turbulence, radiation) and chemistry of each part of interstellar clouds. The simulation of Figure 3‑14 required thousands of processors running for several months. Simulations include up to millions of cells (space steps) and millions of time steps.