Astronomy

Advances in Astronomy in the 19th Century

Picture of a solar flare taken in mid 2012 by NASA's Solar Dynamics Observatory. Image-credit:nasa.gov

Significant advances in celestial spectroscopy were made in the early 19th century in England where William Huggins (1824-1910), an amateur astronomer looked at deep sky objects with a spectroscope . He made the first photograph of the stellar spectra. He showed that the absorption lines in the photo really indicated the chemical elements in the stars. He confirmed fraunhofer’s ideas and held that stars exhibit largely the same spectral line patterns as the sun does, through the relative prominence often differ.

In 1864 Huggins made a key discovery: the spectra of some nebulae had only emission lines, as if they were from hot gas. The feeble light from unresolved stars were later attributed to island universes. He studied the spectrum of 4,000 stars and concluded that the same elements are in the stars as in the earth. Another notable’s scientist in this field was Pietro Secchi (1818-1878), who applied spectroscopic techniques to astronomy.

It was latter observed that the spectral lines proved a measure of a star’s line-of-sight(or radial) motion in space in 1842, the Austrian physicist , Christian Doppler (1803-1853), described a principles since known as the Doppler effect, which states that the wavelength of light would be altered by the velocity of the emitting object. The Doppler Effect soon provided a key to understanding binary stars. An Englishman, William Henry fox Talbot predicated (in 1871) that the periodic oscillation of spectral lines in binary stars might indeed reveal orbital motion.

An interesting and significant observation of the sun was made in 1859. An English astronomer, Richard Carrington, discovered the differential rotation periods of the sun’s disc viz. 27.5 days at the equator and 25 days at the Polar Regions. He was also arguably the first to point out that solar flares (though they were then not known by that name) were linked to sunspots and to terrestrial magnetic field disturbance and auroras. His sighting of a solar flare temporarily coincided with an impulsive change in the direction of compass needles at a nearby magnetic observatory, followed by an aurora of great intensity.

Picture of a solar flare taken in mid 2012 by NASA's Solar Dynamics Observatory. Image-credit:nasa.gov

Picture of a solar flare taken in mid 2012 by NASA’s Solar Dynamics Observatory. Image-credit:nasa.gov

An amateur astronomer in Germany, Heinrich Schwabe (1789-1875) had by then discovered the 11 year cycle of sunspots. Even in 1802, an English chemist, William Hyde Wollaston (1766-1828) observed dark lines in the solar spectrum, which led to the discovery of various elements in the sun. Chemists in Europe studied spectra of light and found that the yellow line showed up virtually in every substance.

Robert Bunsen (1811-1899), famous for his Bunsen burner, and Gustavo Kirchoff (1824-1887), a physicist, developed the most sensitive spectrometer of their time and applied it to celestial objects. The scientists confirmed that the d-line (dark line) found by fraunhofer, was due to the presence of sodium and discovered cesium and rubidium as well in 1861. Their work showed that the chemical composition of the sun was imprinted in the very light it emits. Bunsen and kickoff were the first to explain the mechanism the gives rise to emission or absorption spectra (when the spectral line is viewed against an incandescent background).

Bunsen found that different wavelength of light refracted differently. They showed this feature in their spectroscopic experiments with various elements. Sodium vapor, for instant when heated to incandescence, produced double yellow line. He concluded that when light passed through gas, it absorbed those wavelengths which it would emit in an incandescent state. An analysis of the spectrum would reveal the elements present in the Sun’s surface. The spectrum of hydrogen contains only a few lines, while iron has thousands of lines. The two scientists showed that atoms of chemical elements have distant absorption spectra that match their emission element spectra.

In 1875 Jules Janssen perfected high-resolution photography and used the spectrograph to study the sun he even used a balloon for the first time for astronomical research. It was not, however, followed up. Ground-based device were used to discover new features of the sun. in 1890, George Ellery Hale, an American (1868-1938), built the first spectroheliography, which combined a spectrograph with photography, for studying the moment of solar gasses .He also built the largest refractor telescope in the world at that time. Anther American, Albert Michelson gazed at the red giant, Betelgeuse and measured the star’s diameter using stellar interferometer, which combines light from multiple source in an optical instrument to make precise measurement.

Though scientists were aware of the spectral lines of a substance seen while being burnt in the laboratory, some of the lines from the sun were puzzling, as there were no corresponding colours of known substances.The puzzle was explained by India’s Meghnad Saha (1893-1986). He gave his famous ionizing theory. The atoms of gases in the sun, he pointed out, would break up under enormous heat and pressure and would no longer be electrically neutral. They would become ionized. If the state of ionization of an element is known, it would be easy to identify its presence. His calculations showed the degree of ionizing that could be expected at various temperatures in stars.

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