Interstellar Medium and Nebulae:
NGC 3370 is a spiral galaxy similar in size and structure to our own Milky Way Galaxy. In visible wavelengths, the image is dominated by the stars and clouds of gas and dust that reside in and define the spiral arm structure. Not obvious in the image are the dust grains, and atomic and molecular gases that comprise the tenuous interstellar medium (ISM) interspersed between the stars. The extremely low average density of the interstellar medium - about one atom per cubic centimeter - is nearly a perfect vacuum; however, due to the enormous amount of space between the stars, the ISM constitutes ~20-30% of the mass of a galaxy. The interstellar medium is primarily hydrogen and helium created during the Big Bang, enriched with heavier elements from the nuclear fusion of elements in the cores of the following generations of stars. The interstellar medium is immersed in radiation from stars, magnetic fields, and cosmic ray particles, and has an average temperature of ~1,000,000 Kelvin (K).
The interstellar dust particles are extremely small – usually less than about one thousandth (1/1000th) of a millimeter across – and composed mostly of H, C, O, Si, Mg and Fe in the form of silicates, graphite, ices, metals and organic compounds. The size of the dust grains is the same size as the wavelength of the blue portion of the visible spectrum; therefore, the dust grains scatter blue light. Since the light that reaches Earth from distant objects is depleted in blue wavelengths by the dust, the resultant transmitted light appears redder than it actually is. This is called interstellar reddening. The dust particles also absorb incident light, heat up, and emit in the infrared - resulting in the dimming of starlight. This is called interstellar extinction, and dims the light from deep sky objects.
Nebulas are denser agglomerations of interstellar gas and dust; the main types of nebulas are diffuse, reflection, and absorption. An emission nebula produces an emission spectrum because of energy that has been absorbed from one or more hot luminous stars that excite the hydrogen gas. The ultraviolet (UV) radiation from the massive hot stars ionizes the hydrogen - it strips electrons from the hydrogen atoms - by the process of photoionization. The free electrons combine with protons, forming hydrogen atoms, and emit a characteristic series of emission lines as they cascade down through the energy levels of the atoms. The visible radiation in these lines imparts to these regions their beautiful reddish-colored glows. These regions of ionized hydrogen gas (called HII regions) have typical temperatures of ~10,000 - 20,000 K, and a densityof ~10 atoms/cm3. In the Tony Hallas image to the right is the emission nebul M42, located in the constellation of Orion. The hot luminous stars to within the nebula are ionizing the interstellar hydrogen, and protons and electrons are recombining and emitting red light.
A nebula that is mainly composed of cool interstellar dust that reflects and scatters light from nearby stars is called a reflection nebula. They are usually blue because the scattering is more efficient for blue light by the dust particles. The Witch Head Nebula to the left is a reflection nebula, and is also glowing due to the ultraviolet radiation from the nearby hot, blue massive star Rigel in the constellation of Orion. Absorption nebulas are physically very similar to reflection nebulas; they look different only because of the geometry of the cloud of dust, the light source and Earth. Absorption, (dark) nebulas, are simply blocking the light from the source behind them. The Horsehead Nebula (Barnard 33) is visible only because it is silhouetted against the emission nebula behind it. Emission, reflection, and absorption nebulas are often seen within the same field of view. The image of NGC 6559 below, a bright red emission nebula, also contains a large region of nebulosity surrounding the two hot young stars located in the lower left portion of the image. The image also contains dark clouds and filaments, highlighted against the bright emission nebula. Emission and reflection nebulas are often associated with star formation regions as they are caused by ultraviolet emissions from hot, young stars; however, stars do not form within these types of nebulas. Emission and reflection nebulas are too warm and diffuse to support star formation.