Solar Physics Information


What is the history of solar physics?

Solar physics began with the invention of the telescope at the beginning of the 17th century. Using this new device in the period from 1610 to 1613, Galileo studied the properties of dark sunspots and established their existence as features move on the visible disk of the Sun as it rotates. Thus began observations to lead eventually to the discovery of the 11 year cycle of solar activity by the amateur astronomer S. Schwabe, in 1843.

Another major scientific advance came in 1817 with Fraunhofer's discovery of 'dark lines' in the spectrum of light from the Sun. Coupled with the understanding of atomic energy levels, studies of the solar spectrum gave us a scientific method to learn the atomic composition of the sun and the temperatures in its outer layers. Solar spectroscopy advanced rapidly late in the 19th century to the development of specialized instruments showing the existence of magnetic fields on the Sun at Mt. Wilson Observatory in 1908.

Solar astronomers continued fruitful studies of the Sun from the ground until the space age dawned in 1946 with rocket observations and then accelerated in 1957 with the launch of Sputnik. Rocket instruments and orbiting satellites allowed observation of the ultraviolet and x-ray regions of the solar spectrum for the first time (since these wavelengths are normally absorbed by the earth's atmosphere and do not reach the ground) and thus opened the way for high temperature astrophysics to be applied to the solar atmosphere where temperature can exceed 1 million degrees. Although our understanding of solar-type stars required ultraviolet radiation from the Sun, such short wavelength emission cannot be observed from the ground because of absorption by the Earth's atmosphere.

It is this instrumental heritage to record the radiation from the Sun that gives us the tools we use today in the continuing study of the Sun and its variation with time.

Some great moments in the history of solar physics:

ca. 200 BC The distance to the Sun

1543 The Sun moves to center stage

1609 The Sun in focus

1610 First telescopic observations of sunspots

1644 The Sun as a star

1645-1715 The Maunder minimum

1687 The mass of the Sun

1774-1801 The physical nature of sunspots

1817 Solar spectroscopy is born

1843 The sunspot cycle

1852 The sunspot cycle is linked to geomagnetic activity

1859 First observation of a solar flare

1860 First observations of a coronal mass ejection

1908-1919 The magnetic nature of sunspots

Suggested further readings on the history of solar physics

Does the Sun have a surface?

The Sun visible to our eyes does not have a solid surface such as that of the Earth or the Moon. The visible Sun is a hot gas with a characteristic temperature of 5700 deg. K, well beyond the melting points of material on Earth. Nevertheless, we see only its very outer layers because the gas is opaque. The effect is the same as that for a cloud which we know is composed of water molecules but which appears to have a fluffy surface. This outer visible surface is only a few hundred kilometers thick on the Sun and is called the Photosphere. This layer is the top of the solar convection zone where the solar energy is carried to the outer surface by convective gas motions over the last quarter of the solar radius. Further inside lies the radiative zone where the energy is carried principally by radiation, not convection. At the very center is the nuclear core generating energy by fusion of hydrogen to helium at temperatures of 20 million deg. K.

Above the photosphere are two additional layers, the chromosphere and corona, which were first identified at eclipses of the Sun by the Moon. The chromosphere is an inhomogeneous layer extending 10,000 km above the photosphere. It is best thought of as the transition from the photosphere to the corona. The very outer extent of the Sun proper is the tenuous corona which can extend several million kilometers into the interplanetary medium. Such extensions of the solar atmosphere produce the striking images seen at the times of solar eclipses.

Does the brightness of the Sun change over time?

Yes, modern measurements between 1978 and 1995 show that the "brightness" or total irradiance of the Sun fluctuates by a few tenths of a precent over the 11 year solar cycle. This small fluctuation reflects stability of the solar photosphere as seen in the visible spectrum which extends from the blue at 400 nanometers (nm) to the deep red at 800nm. Observations from space show increasing variation from the ultraviolet below 400nm to the x-ray region down to .1nm. However, the bulk of the output solar energy is in the visible spectrum; therefore, its variation dominates fluctuations on the Sun's "brightness".

How does the Sun work (what goes on inside)?

The Sun is a giant, natural thermonuclear reactor that converts hydrogen to helium in its core to produce the heat we sense on our faces as sunshine. Why does this reactor not explode as a thermonuclear bomb? The Sun is held together in an equilibrium state by the mutual gravitational attractions between all its atoms acting to compress the solar center and, thus, produce and contain the nuclear reactions taking place there. The solar atmosphere outside the energy generating core adjusts itself to carry the enormous amount of energy that emerges from the surface in the form of radiation. This is the basic idea behind the existence of all stars beginning with primordial gravitational attraction and compression to the beginning of nuclear energy generation and, finally, to the exhaustion of the nuclear fuel and death of the star as a truly self luminous object.

What are the key areas of solar research today?

Contemporary solar research falls in two basic areas: 1) studies of the outer solar atmosphere and its variation, and 2) studies of the inside of the Sun using seismological techniques similar to those employed in oil prospecting on the Earth. Studies of the solar interior reveal for the first time the motions and thickness of the various internal zones predicted by the theory of stellar interiors such as the nuclear core, the radiative zone, and the convective zone. The interface between the radiative and convective zones appears to be the shell where the Sun generates the magnetic fields eventually seen on the surface in sunspots and other structures associated with the 11 year solar cycle. Thus, understanding the inside of the Sun is crucial to understanding solar variability due to the effects of intense magnetic fields appearing at the visible surface.

It is the variation of the visible solar atmosphere that affects the Earth's atmosphere directly as a source of heat as well as a modifier of the its chemistry . If we are to anticipate the effects of solar change on short times of minutes to decades, we must have a clear understanding of how the Sun's atmosphere produces radiation at all wavelength from x-rays to the infrared. Since the solar surface is mottled by darks spots and bright vein-like features called faculae, their combination determines the net variability in the radiation reaching the earth.

The outer solar atmosphere also expels solar gas and magnetism into the interplanetary medium as a steady solar wind as well as transients called coronal mass ejections. This flow of mass from the sun results in variation in the amount of material trapped inside the dipolar magnetic field of the earth. Therefore, today we study the total solar output of radiation and material with the view to understanding its sources inside the Sun as well as their variation and visibility in the outer solar atmosphere. The parallel research to understand the detailed effects of this variability in changing the Earth's atmosphere gives the research strategy behind the larger study of coupling of the outer atmosphere of the Sun with that of the Earth.

What is a sunspot?

Sunspot and granulation (HAO slideset, #3)

The sunspot cycle (HAO slideset, #17)

Hale's sunspot polarity law (HAO slideset, #19)

What is a coronal mass ejection?

A coronal mass ejection (HAO slideset, #13)

Two more coronal mass ejections (HAO slideset, #14)


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 Return to HAO homepage.       -Written by orw@ucar.edu.

-Revised 3 March 2000 by cmwucar.edu.

Copyright 2000, NCAR.       - Approved by Paul Charbonneau -