Astronomy

Solar Astronomy in the 19th Century

The chemistry of the sun continued to be analyzed by its light. a Swedish physicist , Anders Jonas Angstrom (1814-1874) detected the presence of hydrogen in the sun in 1862. In 1929, an american astronomer Henry Norris Russel (1877-1957) worked out the sun’s composition in detail. One element, helium, was first discovered on the sun during the total solar eclipse before it was found in the laboratory. Its discovery on the sun was from India. The emission spectrum of the new element was discovered in the solar chromospheres in 1868.

A device which has greatly advanced the study of the sun in the spectroheliograph. The device makes it possible to photograph the light of a single spectral line of the sun. All the parts of the image produced by the telescope are scanned with the help of a filter. The light to be analyzed is passed through a slit of the spectroscope and its image obtained. The invention in 1894 is attributed to a Frenchman, Deslanders, but it was independently developed earlier by George Ellery Hale.

Hale was able to photograph the sun the light of calcium. In 1924, he modified the instrument and photographed the Sun in hydrogen light and saw the prominence’s full of hydrogen. He detected strong magnetic fields in sunspots. He also discovered that the temperature of the sunspots is lower than the surrounding photosphere.

The sun as seen through different spectrum's. Image-credit:nasa.gov

The sun as seen through different spectrum’s. Image-credit:nasa.gov

A separate instrument was needed to study the corona, the outer part of the sun, which can be normally seen only during a solar eclipse. A compact corona is seen when solar activity is at the maximum. In contrast, when solar activity is at the minimum, the corona is seen as clouds with irregular shapes and sharp edges. A French astromer, Bernard Lyot (1897-1952) invented in the 1930s the coronagraph, which enables astronomers to study the inner corona at any time by creating an artificial eclipse. Outer corona studies would of course need a total eclipse or a space observatory. Coronagraphs enable scientists to study feature such as flares and the solar magnetic fields.

In 1885, Swiss school mathematics teacher, Johann Jakob Balmer (1825-1898) discovered a formula involving whole numbers, which predicted almost exactly the frequencies of the four visible hydrogen lines (in Red, Green. Blue, and Violet) and others in the ultraviolet region. A later day comparison of blamer’s values with the actual measurements confirmed the remarkable accuracy of his calculations. His work suggested that vote emission or absorption of light from an atom must correspond to a decrease or increase in the atom’s energy.

Most of the work on solar activity is done with a filter tuned to a smaller part of the red region of the spectrum –the Hydrogen Alpha line produced by atomic hydrogen. It is a red spectral line (at 6562.8 Angstroms) emitted by a hydrogen atom, when its electron falls from the third lowest energy level to the second lowest energy level.

The same line appears in absorption, when electrons are raised from the lower to the higher level. Many solar feature like prominences show up best in H-alpha. The H-alpha line universally used for solar observations including solar flares.

The spectroscope also revealed the connection between magnetism and light. Pieter Zeeman (1865-1943) discovered in 1886 that in the spectrum from a sodium flame; place between magnets, the emission lines were broadened or split into two or more lines. This was useful in the detection and measurement of magnetism on the sun, based on the analysis of the solar light.

Almost all activity as seen in sunspots, prominences and flares is related to solar magnetic fields. They are created shaped and destroyed by the magnetic force. From the furious flares having the energy of a million hydrogen bombs to the apparently flower-like coronal atmosphere, the magnetic lines of force determine the level of activity.

The shape of the spectrum is the key to the physical conditions of the sun. If the temperature is high, atoms have a high velocity and the lines are broadened. If there are strong magnetic fields, the spectral lines are spilt as mentioned above, while strong electric fields broaden the lines.

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