Physics: A Very Short Introduction by Sidney Perkowitz

Author:Sidney Perkowitz [Perkowitz, Sidney]
Language: eng
Format: epub
ISBN: 0198813945
Google: j0KjDwAAQBAJ
Amazon: B07SD5Y1H6
Goodreads: 44137578
Publisher: Oxford University Press
Published: 2019-07-24T23:00:00+00:00

To the stars

Though astrophysics originated in the ancient science of astronomy, today it is an especially active blended area. It uses physics tools and principles to extend classic observational astronomy, which has for centuries tracked heavenly bodies. Applied physics broadens astronomical observation in the design of advanced optical telescopes and other telescopes and detectors for radio waves, microwaves, infrared and ultraviolet light, X-rays, and gamma rays—along with cosmic rays, elementary particles that come from deep space. Physics applied to rocketry and space technology makes it possible to put some of these observational platforms such as the Hubble Telescope into Earth orbit to avoid atmospheric interference.

Physical methods also serve to analyse data gathered by these devices to determine the dynamics and composition of planets, stars, galaxies, and the universe itself, from the CMB to the interstellar medium, dark matter, and dark energy. Further, we need relativity and the Big Bang theory to interpret these results.

One essential astrophysical tool is spectroscopy, the study of how matter emits and absorbs electromagnetic waves and other radiation. Newton performed an early spectroscopic experiment when he passed sunlight through a glass prism and saw a continuous band of colour varying from red to violet, the sun’s blackbody spectrum spread out by wavelength. The German physicist Joseph Fraunhofer refined the method when he invented the spectroscope in 1814. This combination of a prism and a lens broke sunlight into its wavelengths at a higher resolution than Newton achieved, and showed many narrow dark lines overlaid on the colours of sunlight.

These lines were later shown to represent the composition of the source. A hot gas emits energy at definite wavelengths that come from quantized atomic transitions in the gas and provide a unique fingerprint for any atomic element such as hydrogen. Elements can also be identified in absorption spectra, when quantum transitions in the cooler outer layers of a star absorb radiation from its hotter interior and appear as dark lines where energy is missing at the characteristic wavelengths (Figure 9). Emission and absorption spectra tell us the make-up of astronomical bodies and have produced surprises. In 1868, a previously unknown emission line at 587.49 nanometres was observed in our sun’s spectrum. It was a signature of the element helium, a major component of active stars found only later on Earth.

9. Quantum transitions produce characteristic spectral lines that show the composition of celestial bodies.

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