Download The Sun – “Our” Star

The Sun – “Our” Star













The Sun is a typical star - a sphere of plasma, made self-luminous via
thermonuclear reactions in its core.
Currently the Sun exists in a stable state. The immense outward pressure
generated by energy released in the Sun’s core is balanced by the immense
inward pressure produced by gravity.
The Sun is the dominant object in the Solar System, with 330,000 times
Earth’s mass, and a radius = 110 Earth radii
The solar luminosity, defined as the amount of energy generated within the
Sun every second, is constant. The Sun emits radiation like a blackbody
with a surface temperature ~ 5800 K.
Astronomers have developed a “standard model” to explain the Sun's
interior structure and energy generation processes.
Four regions comprise the solar interior, differing in the way energy flows
through each region. These zones are the core, the radiation zone, the
convective zone, and the photosphere.
Three regions make up the Sun’s “atmosphere”, the chromosphere, the
transition region, and the corona. Surprisingly, the gas temperature
increases with height in the solar atmosphere.
The visible “surface” of the Sun is termed the photosphere. Absorption lines
seen the spectrum of the photosphere define the solar composition.
Besides electromagnetic radiation, hot gases flow from the Sun - the solar
wind. This wind can affect the Earth when the particles in the wind interact
with Earth’s magnetic field.
The Sun’s atmosphere is dynamic; Sunspots, prominences, and flares are
expressions of solar activity. This activity ranges from the gentle rise and
fall of gas blobs from the photosphere into the corona and back, to energetic
ejections of gas from the surface directly into the solar wind.
Solar activity varies in a 22-year cycle. The number of sunspots rises and
declines over this cycle, as does the number and intensity of solar flares, and
all related activity. These phenomena appear to powered by the twisting of
magnetic fields in the Sun’s outer layers during the cycle.
The Sun’s energy is generated by fusing hydrogen nuclei into helium within
the solar core. Some of the mass of the hydrogen is converted into energy,
according to Einstein’s famous formula: E = mc2. Nuclear fusion is the
primary source of energy in the universe.
The expected number of solar neutrinos, subatomic particles produced in
these fusion reactions, is not observed. This mismatch between theory and
observation, known as the “solar neutrino” problem, has been an
outstanding question in physics for 40 years. In the past several years, a
possible answer to this problem has been found, indicating that the standard
solar model is correct, and that neutrinos are more mysterious particles than
previously realized.