Stellar Evolution - Source Chandra X-Ray.pdf

Stellar Evolution - Source Chandra X-Ray.pdf - Stellar...

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Stellar Evolution – Cosmic Cycles of Formation and Destruction Interstellar Medium and Nebulas: 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 Spiral Galaxy NGC 3370 (Hubble) 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 (1/1000 th ) 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 density M42 (Tony Hallas) of ~10 atoms/cm 3 . In the Tony Hallas image to the right is the emission nebula 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.
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