Chapter 13
Solar Astronomy
Abstract The Sun is the most obvious astronomical object and the nearest star. It
is dynamically active, with many changing surface features. We discuss sunspots,
flares, prominences, and other solar features and, via a practical example, show how
to view the Sun in a safe manner. We conclude with a brief discussion of the nature
and observation of solar eclipses.
13.1 Introduction
With a diameter 109 times that of Earth and containing 99.89% of the mass of the solar
system, the Sun dominates the solar system and our daily lives. It provides almost
all the energy for biological processes on Earth, and its long-term, and arguably
short-term, evolution has a major impact on life and human civilisation.
The Sun’s composition is typical of a star of its age, 75% hydrogen, 24% helium,
and a smattering of other elements, including carbon and oxygen. High densities
and temperatures within the Sun’s core (150 kg m
−3 and 15 × 10
15 K) create the
environment for a sustained thermonuclear reaction, the conversion of hydrogen
into helium via the proton–proton chain. The first part of this chain, the creation of
deuterium, is the choke point in the process, and it is sensitive to conditions. When
the core is exceptionally hot or dense, more deuterium is created and more energy
released. The release of more energy causes a slight expansion in the core, dropping
the core temperature and pressure and thereby suppressing the fusion rate. Hence we
have a feedback loop whereby the radiation and thermal pressure within the Sun are
matched by the gravitational pressure; the Sun is in hydrostatic equilibrium.
The nuclear synthesis of helium from hydrogen results in the liberation of gamma
rays and neutrinos. Neutrinos weakly interact with matter and escape the Sun in a
few seconds. The gamma rays, however, are continually absorbed and reemitted as
they pass through the layer above the core—the radiative zone—and are converted
into lower-energy photons. Eventually, the radiative zone gives way to the convective
zone, and energy is now transported to the surface by the convection of hot gas. Once
the photosphere, the visible surface of the Sun, is reached, the gas temperature has
© Springer Nature Switzerland AG 2020
M. Gallaway, An Introduction to Observational Astrophysics,
Undergraduate Lecture Notes in Physics,
https://doi.org/10.1007/978-3-030-43551-6_13
183
Solar Astronomy
Abstract The Sun is the most obvious astronomical object and the nearest star. It
is dynamically active, with many changing surface features. We discuss sunspots,
flares, prominences, and other solar features and, via a practical example, show how
to view the Sun in a safe manner. We conclude with a brief discussion of the nature
and observation of solar eclipses.
13.1 Introduction
With a diameter 109 times that of Earth and containing 99.89% of the mass of the solar
system, the Sun dominates the solar system and our daily lives. It provides almost
all the energy for biological processes on Earth, and its long-term, and arguably
short-term, evolution has a major impact on life and human civilisation.
The Sun’s composition is typical of a star of its age, 75% hydrogen, 24% helium,
and a smattering of other elements, including carbon and oxygen. High densities
and temperatures within the Sun’s core (150 kg m
−3 and 15 × 10
15 K) create the
environment for a sustained thermonuclear reaction, the conversion of hydrogen
into helium via the proton–proton chain. The first part of this chain, the creation of
deuterium, is the choke point in the process, and it is sensitive to conditions. When
the core is exceptionally hot or dense, more deuterium is created and more energy
released. The release of more energy causes a slight expansion in the core, dropping
the core temperature and pressure and thereby suppressing the fusion rate. Hence we
have a feedback loop whereby the radiation and thermal pressure within the Sun are
matched by the gravitational pressure; the Sun is in hydrostatic equilibrium.
The nuclear synthesis of helium from hydrogen results in the liberation of gamma
rays and neutrinos. Neutrinos weakly interact with matter and escape the Sun in a
few seconds. The gamma rays, however, are continually absorbed and reemitted as
they pass through the layer above the core—the radiative zone—and are converted
into lower-energy photons. Eventually, the radiative zone gives way to the convective
zone, and energy is now transported to the surface by the convection of hot gas. Once
the photosphere, the visible surface of the Sun, is reached, the gas temperature has
© Springer Nature Switzerland AG 2020
M. Gallaway, An Introduction to Observational Astrophysics,
Undergraduate Lecture Notes in Physics,
https://doi.org/10.1007/978-3-030-43551-6_13
183