Interstellar Medium and Nebulae:
Spiral Galaxy NGC 3370
Credit: Hubble Heritage Team, A. Riess (STScI) NASA
Animation: Virtual Tour of the Milky Way
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 gas 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 left over from the Big Bang, enriched with heavier elements from the nuclear fusion of elements in the cores of previous generations of stars. The interstellar medium is immersed in radiation, magnetic fields and cosmic ray particles, and has an average temperature of 1,000,000 K.
M42
Credit: Stephan Seip
The interstellar dust particles are extremely small - usually less than about one thousandths (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.
Witch Head Nebula
Credit: Gary Stevens
Nebulae are denser agglomerations of interstellar gas and dust; the main types of nebulae 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 density of ~10 atoms/cm3. In the image to the right is the emission nebula M42, located in the constellation of Orion. The hot luminous stars to the left of the nebula are ionizing the interstellar hydrogen, and protons and electrons are recombining and emitting red light.
Horsehead Nebula
Credit: USNO
NGC 6559
Credit: Adam Block, KPNO
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 nebulae are physically very similar to reflection nebulae; they look different only because of the geometry of the cloud of dust, the light source and Earth. Absorption, or dark nebulae, 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 nebulae are often seen within the same field of view. The image of NGC 6559 below, a bright red emission nebula, also contains a reflection nebulosity surrounding the two hot young stars located in the left central portion of the image. The image also contains dark clouds and filaments, highlighted against the bright emission nebula. Emission and reflection nebulae are associated with star formation regions since they are caused by ultraviolet emissions from hot, young stars; however, stars do not form in these types of nebulae. Emission and reflection nebulae are too warm and diffuse for stars to form.
Revised: March 19, 2008