9. The Material Between the Stars


Fig9_1 Rossette Nebula

Fig9_1 Rossette Nebula

Fig. 9.1 . This large, circular emission nebula, or H II region, known as the Rosette Nebula, lies at a distance of about 5,200 light-years from Earth and is about 130 light-years in diameter. Parts of this region include nebulae designated as NGC 2237, NGC 2238, NGC 2239, as well as the open star cluster NGC 2244. The mass of the Rosette Nebula is estimated to be about 10,000 solar masses. Hot O and B stars in the core of the Rosette Nebula exert pressure on the nearby interstellar material, triggering star formation, and heat the surrounding gas to a temperature of about 6 million K, causing it to emit x-rays observed from the Chandra X-ray Observatory. (Courtesy of KPNO/CTIA.)


Fig9_2 Turbulent interstellar gas

Fig9_2 Turbulent interstellar gas

Fig. 9.2 . Glowing hydrogen gas in a small region of the Omega Nebula, which is also designated as M 17. The wave-like patterns of gas have been sculpted and illuminated by intense ultraviolet radiation from young, massive stars, which lie outside the picture to the upper left. The ultraviolet radiation heats the surface of otherwise cold, dark clouds of interstellar hydrogen. (A Hubble Space Telescope image, courtesy of NASA/ESA/J. Hester, ASU.)


Fig9_3 Spheres of ionization

Fig9_3 Spheres of ionization

Fig. 9.3 . Ultraviolet radiation, denoted by uv, from a hot star ionizes hydrogen and other atoms in its immediate vicinity, creating a nebulous region that radiates emission lines and contains abundant ionized hydrogen, denoted H II. They are known as emission nebulae or H II regions. In this figure, the size of the atoms, ions, electrons and protons are greatly exaggerated in order to visualize them. The ionization by the uv creates numerous free electrons and protons that are not attached to atoms. They subsequently recombine to make atoms in a process of continued disruption and reconciliation. The free electrons can emit two kinds of radiation, illustrated in Figure 9.4. At large distances from the star, its ultraviolet rays are all absorbed, and can travel no further into surrounding space. This limits the radius of the emission nebula, or H II region, to the Strömgren radius at about 30 light-years.


Fig9_4 Radiative interactions between electrons and protons

Fig9_4 Radiative interactions between electrons and protons

Fig. 9.4 . When an electron moves rapidly and freely outside an atom, it inevitably passes near a proton in the ambient gas. There is an electrical attraction between the electron and proton because they have equal and opposite charge, and this pulls the electron toward the proton. If the interaction is distant, it bends the electron’s trajectory and alters its speed. The electron then emits electromagnetic radiation known as bremsstrahlung from the German word for “braking radiation;” this is also called free-free radiation since the electron remains free and unattached to the proton. In a close encounter, the electron goes into orbit around the proton, forming a hydrogen atom and cascading down through allowed orbital energies. In this case, the electron emits recombination radiation, also known as free-bound radiation.


Fig9_5 Radio-frequency spectrum of the Orion Nebula

Fig9_5 Radio-frequency spectrum of the Orion Nebula

Fig. 9.5 . Radio-frequency spectrum of the Orion Nebula, also designated as M 42, showing the hydrogen, H, and helium, He, recombination lines from the a (m-n=1) and b (m-n=2) transitions at high quantum numbers n = 109 and n = 137. The hydrogen lines are more intense than the helium or carbon ones because of the greater abundance of hydrogen. (Adapted from K.R. Lang, Astrophysical Formulae, Heidelberg: Springer Verlag 1979.)


Fig9_6 Dark clouds Caroina Nebula

Fig9_6 Dark clouds Caroina Nebula

Fig. 9.6 . Molecular clouds in the Carina Nebula contain so much gas and dust that they are opaque to optically visible light, forming dark and dense structures where new stars may be born. Energetic stellar winds and intense radiation from nearby massive stars are sculpting the outer edges of the dark clouds. This image is a composite of observations taken in light emitted by hydrogen and oxygen atoms, using the Hubble Space Telescope. The Carina Nebula, also designated NGC 3372, is about 7,500 light-years away from Earth and spans over 300 light-years. (Courtesy of NASA/ESA/Hubble Heritage Project/STScI/AURA.)


FIg9_7 Cosmic eggs

FIg9_7 Cosmic eggs

Fig. 9.7 . A cloud of cool interstellar dust and molecular hydrogen gas may have embryonic, unseen stars embedded inside it. Ultraviolet light from nearby hot stars uncovers dark globules of gas at the top of the cloud, each about the size of our solar system. They have been called “eggs,” an acronym of evaporating gaseous globules. This cloud is located in the Eagle Nebula, also designated as M 16. (A Hubble Space Telescope image, courtesy of NASA/ESA/STScI/J. Hester and P. Schowen, ASU.)


Fig9_8 Polarized starlight

Fig9_8 Polarized starlight

Fig. 9.8 . The light from nearly 7,000 stars is polarized, with the strength and direction of polarization designated by the short lines. The observations are plotted in galactic coordinates where the galactic plane, or the Milky Way, runs horizontally across the middle of the figure. The starlight polarization has been attributed to dust grains elongated along the interstellar magnetic field. (Courtesy of D. S. Mathewson.)


Fig9_9 Nonthermal

Fig9_9 Nonthermal

Fig. 9.9 . When the intensity of thermal radiation is spread out as a function of wavelength, the resultant spectrum is most intense in a band of wavelengths that depends on the temperature. This peak occurs at visible wavelengths when the temperature is about 6,000 K. Such a hot gas emits radiation at longer wavelengths, but at a lower intensity. In contrast, non-thermal radiation is more intense at longer radio wavelengths. High-speed electrons emit non-thermal synchrotron radiation in the presence of a magnetic field (see Fig. 9.10).


Fig9_10 Synchrotron radiation

Fig9_10 Synchrotron radiation

Fig. 9.10 . Electrons moving at velocities near that of light emit a narrow beam of synchrotron radiation as they spiral around a magnetic field. This emission is sometimes called non-thermal radiation because the electron speeds are much greater than those of thermal motion at any plausible temperature. The name “synchrotron” refers to the man-made, ring-shaped synchrotron particle accelerator where this type of radiation was first observed; a synchronous mechanism keeps the particles in step with the acceleration as they circulate in the ring.