The Sun as a Guide to Stellar Physics

Oddbjørn Engvold, Jean-Claude Vial, Andrew Skumanich, Oddbjørn Engvold, Jean-Claude Vial, Andrew Skumanich

• Publisher: Elsevier
• Year: 2018
• ISBN:
  9780128143353
• Topic: Scienze fisiche
• Subtopic: Astronomia e astrofisica

The Sun as a Guide to Stellar Physics illustrates the significance of the Sun in understanding stars through an
examination of the discoveries and insights gained from solar physics research. Ranging from theories to modeling
and from numerical simulations to instrumentation and data processing, the book provides an overview of what
we currently understand and how the Sun can be a model for gaining further knowledge about stellar physics.
Providing both updates on recent developments in solar physics and applications to stellar physics, this book
strengthens the solar–stellar connection and summarizes what we know about the Sun for the stellar, space, and
geophysics communities.

• Applies observations, theoretical understanding, modeling capabilities and physical processes first revealed by the sun to the study of stellar physics
• Illustrates how studies of Proxima Solaris have led to progress in space science, stellar physics and related fields
• Uses characteristics of solar phenomena as a guide for understanding the physics of stars

Chapter 1

Discoveries and Concepts

The Sun's Role in Astrophysics

Jack B. Zirker¹, and Oddbjørn Engvold^{2 1}National Solar Observatory, Sunspot, NM, United States ²Rosseland Centre for Solar Physics, Institute of Theoretical Astrophysics, University of Oslo, Oslo, Norway

Abstract

The Sun has had an important role in the development of stellar astrophysics. The discoveries of solar magnetism, solar wind, and global acoustic vibrations, to name only a few, have launched completely new topics for research in stellar physics. In addition, concepts such as magnetic reconnection and neutrino mass first arose in attempts to explain puzzling solar phenomena.

This volume is intended to remind astronomers, physicists, and students of the Sun's key role, which is based in part on its proximity and its commonality with other stars. After a short survey of the subject, successive chapters will describe the status and future progress in several topics in solar physics that are relevant to stellar physics. We begin with the simplest characteristic of the Sun, its luminosity.

Keywords

Chemical composition; Coronal mass ejections; Earth–Sun connection; Magnetic fields; Solar cycle; Solar neutrinos

Chapter Outline

1. 1. The Solar Constant
2. 2. The Sun's Chemical Composition
   1. 2.1 Spectroscopic Methods
   2. 2.2 Modeling of the Sun's Atmosphere
   3. 2.3 Settling of Light Elements
3. 3. Internal Structure and Helioseismology
   1. 3.1 Detection of Oscillatory Pattern
   2. 3.2 Interpretation of Solar Oscillations
4. 4. The Magnetic Sun and Its Variability
   1. 4.1 Solar Cycle
   2. 4.2 Magnetic Fields
   3. 4.3 Internal Structure and Location of the Magnetic Dynamo
5. 5. The Solar Corona and Wind
   1. 5.1 The Temperature of the Corona
   2. 5.2 The Shape of the Corona
   3. 5.3 The Solar Wind
6. 6. Earth–Sun Connection
   1. 6.1 Aurora and Geomagnetic Storms
   2. 6.2 The Carrington Event
   3. 6.3 Solar Flares, X-Rays and Energetic Particles
   4. 6.4 Reconnection of Magnetic Fields
   5. 6.5 Coronal Mass Ejections
7. 7. Testing Two Concepts
   1. 7.1 Neutrino Oscillations in the Sun
   2. 7.2 Testing General Relativity
8. 8. Concluding Remarks
9. Acknowledgments
10. References

1. The Solar Constant

The amount of energy the Earth receives from the Sun is critically important to astronomers, physicists, and meteorologists. This constant is defined as the flux of energy (in watts/m²) above the Earth's atmosphere, at the mean distance of the Earth from the Sun's surface. The constant includes all electromagnetic radiation summed over all wavelengths.

The theory of stellar evolution predicts that the luminosity of the Sun changes only very slowly, over billions of years. The question is, what is its current value?

The main difficulty in determining the constant from measurements at the Earth's surface is the correction for the absorption of the atmosphere. In 1838, French physicist C. Pouillet obtained a value of 1.228 kW/m², which (perhaps by chance) was close to the best modern value. S.P Langley determined a value of 2.9 kW/m² at the top of Mount Whitney in 1884, in strong discord with Pouillet.

C.G. Abbot, who followed Langley as the director of the Smithsonian Astrophysical Observatory in 1907, spent 40 years in search of a reliable estimate. He established observing stations at high dry locations such as Mount Wilson; Bassour, Algeria; and Calama, Chile. His best estimates (1.318–1.548 kW/m²) were obtained with balloon sondes, some of which reached an altitude of 25 km. Abbot was convinced the Sun actually varied by such an amount within a few years.

Measurements of extreme precision became possible with the use of satellites and with the development of a sensitive detector, the Active Cavity Radiometer Irradiation Monitor (ACRIM). Richard C. Willson, a physicist at the National Aeronautics and Space Administration’s (NASA's) Jet Propulsion Laboratory, was principally responsible for its development.

The first series of measurements was made during the flight of the Solar Maximum Mission (1980–89). It showed a distinct decrease of about 0.06% (from 1366.5 to 1365.8 W/m² in 1980–85 and a return to 1366.6 W/m² in 1986–89) with a day-to-day “noise” of about 0.3%. This noise was actually the response to the appearance and disappearance of sunspots.

In a tour de force, Woodard and Hudson (1983) analyzed the first 10 months of ACRIM data and extracted 5-min oscillations of low degree (long horizontal wavelength). Frequencies, amplitudes, and line widths were obtained for individual pulsations. This result was a tribute to the precision and stability of ACRIM 1.

A succession of satellites carried improved versions of ACRIM detectors, and with extensive calibration and cross-comparisons, an 11-year record of genuine variations was pieced together (Fig. 1.1, from Foukal et al., 2006). The solar luminosity varies in step with the sunspot cycle (Willson and Hudson, 1991). The reasons for this correlation will be addressed in Chapter 8.

Figure 1.1 Active cavity radiometer irradiation monitor measurements of solar constant during 11-year sunspot cycles (Foukal et al., 2006).

2. The Sun's Chemical Composition

The chemical abundance of the Sun is a fundamental yardstick in astronomy. Knowing the Sun's chemical composition became essential for discovering energy generation in the Sun and stars. The final breakthrough came in 1936 with the discovery by Hans Bethe, Charles Crichfield, and Carl Friedrich von Weizäcker of nuclear reactions taking place under the extreme pressure and temperatures in the core of the Sun (Foukal, 2004).

2.1. Spectroscopic Methods

Spectral observations of the solar photosphere are currently possible and available with very high spectral resolution and signal-to-noise ratio because of the great brightness of the source, allowing the profiles of a multitude of weak or blended absorption lines to be measured accurately. Element abundances of essentially all astronomical objects are referenced to the solar composition and basically every process involving the Sun and stars depend on their compositions. The abundance of elements in the Sun has become more extensively and reliably known than in any other star.

The German optician Joseph von Fraunhofer was the first to observe and describe the multitude of dark lines in the emission spectrum of the Sun. He designated the principal absorption features with the letters A through K, and weaker lines with lowercase letters. Physicist Gustav Kirchoff, also from Germany, realized that the dark lines corresponded to the emission lines that he and his colleague Robert Bunsen observed in emission from heated gases. Kirchhoff concluded that the lines on the spectrum of the Sun were dark because they resulted from absorption by cooler layers of gas in the Sun's atmosphere above hotter layers where the continuous emission spectrum originated. Kirchhoff's formulated the following three laws that enabled solar scientists to exploit the potential of spectrometry in chemical analysis of the Sun and subsequently in stars: (1) A solid, liquid, or dense gas excited to emit light will radiate at all wavelengths and thus produce a continuous spectrum; (2) a low-density gas excited to emit light will do so at specific wavelengths, and this produces an emission spectrum; and (3) if light composing a continuous spectrum passes through a cool, low-density gas, the result will be an absorption spectrum.

The emission spectra of elements, which could be vaporized by the Bunsen burner, were examined and compared with solar absorption line spectra. This became a truly fundamental astrophysical tool and a breakthrough in the science of astronomy. Kirchoff and Bunsen discovered lines from cesium and rubidium in the Sun. Swiss mathematician and physicist Johann Jakob Balmer observed the visible line spectrum of hydrogen and determined its wavelengths. The dominant red Fraunhofer line C, at wavelength 6563 Å, is referred to by astronomers as Hα of the Balmer series.

A...