THE SUN
• The Sun is the central object of the Solar System, around which
all the planets and other objects revolve.
• The Sun is 109 times Earth’s diameter (and 10 times the
diameter of Jupiter, the largest planet), and has more than
300,000 times Earth’s mass (1000 times Jupiter’s mass).
• The Sun is composed of about 90% hydrogen and 10% helium
(by number of atoms), with less than 1% of all heavier elements.
• The composition of the Sun is similar to those of other normal
stars, and of the interstellar medium.
• The Sun derives its energy by thermonuclear fusion, in which
four atoms of hydrogen are combined to make one atom of
helium.
• The Sun and solar system are about 4.6 billion years old, but
the total lifetime of the Sun (before depleting its supply of
hydrogen fuel) is about 10 billion years.
The Sun
Jupiter to Scale
Jupiter to Scale
Earth to Scale
Earth to Scale
THE ENERGY SOURCES OF THE SUN AND STARS
• Stars, including our Sun, derive their energy from a source
much more efficient than ordinary chemical reactions (such as
combustion): namely, thermonuclear fusion of the nuclei of
light atoms to form nuclei of heavier ones.
• The primary source of energy through the normal lifetimes of
stars is the fusion of four hydrogen atom nuclei (protons) to
form the nucleus of one helium atom (two protons and two
neutrons).
• Albert Einstein postulated, as part of his Theory of Relativity,
that matter can be converted to energy (and vice versa) by the
relationship, E = mc2 (where c = the speed of light = 300,000
kilometers per second).
• The helium nucleus is only slightly less massive than the four
protons combined to make it, but this difference results in the
release of an enormous amount of energy!
THE ENERGY SOURCES OF THE SUN AND STARS
• The mass of a Hydrogen Atom is 1.007825 AMU (Atomic
Mass Units)
• The mass of a Neutron is 1.008665 AMU
• The mass of 2 H + 2 n is 4.032980 AMU
• The mass of a Helium Atom is only 4.002603 AMU. We
are missing 0.030377 AMU!
• According to Einstein, the energy equivalent of matter is:
Energy = Mass x (Speed of Light)2, or E = mc2.
• Therefore, 1 AMU = 931.5 MeV (million electron volts) and
the mass deficiency of 4H
He = 28.3 MeV, compared
with the chemical reaction 2 H2 + O2
2 H2O = 5 eV!
• In actuality, particularly for our Sun, the process is not as
simple and direct as the above implies; there are several
steps in the process of converting hydrogen into helium.
The Solar Spectral Energy Distribution
(Ground-Accessible Wavelength Range)
Solar Visible-Light Spectra
Calcium
H and K
Sodium D
Sodium D
Day Sky Spectrum
Grating Angles
10°
11.5°
15°
Calcium H and K
Sodium D
Sodium D
High Resolution Solar Spectrum
(Showing Absorption Lines)
OBSERVING THE SUN
• Observations of the Sun in visible light reveal the presence of
darker regions (sunspots) which are highly variable in size,
number, and position on the solar surface.
• Observations in particular spectral wavelengths, such as the
Balmer- line of atomic hydrogen (wavelength 656.3
nanometers) also reveal that sunspots are also associated with
other activity in the solar atmosphere, such as prominences.
• During total solar eclipses, when the bright light of the
photosphere is hidden by the Moon, the lower atmosphere
(chromosphere) and upper atmosphere (corona) of the Sun can
be observed.
• It is found that these regions of the solar atmosphere, although
of very low density, are much hotter than the solar photosphere
(reaching temperatures exceeding a million degrees in the
corona).
• The study of the hot, outer regions of the solar atmosphere, and
regions of solar activity (associated with sunspots and solar
flares) are best done at far ultraviolet and X-ray wavelengths.
White Light Image of the Sun
Image of the Sun in Hydrogen Balmer- (656.3 nm)
Closeup View of Sunspots in H
Solar Prominence, in H Light (656.3 nm)
Solar Corona Observed in Total Eclipse
Temperature vs. Altitude Above Solar Photosphere
OBSERVING THE SUN FROM SPACE
• Observations of the Sun from space are beneficial for two
primary reasons:
o Because Earth’s atmosphere absorbs nearly all ultraviolet and Xray radiation from the Sun, observations and measurements in
these wavelength ranges can only be made from space vehicles.
o Because Earth’s lower atmosphere scatters visible sunlight (by the
process of Rayleigh scattering), it is very difficult to observe even
the visible-light solar corona except during total solar eclipses,
which are very rare at any location on Earth, are of very limited
duration, and are subject to interference by local weather
conditions.
• Although far-UV and X-rays constitute only a small part of the
Sun’s total energy output, these high-energy forms of radiation
are of practical importance because of their effects on Earth’s
upper atmosphere and ionosphere.
• Space-based observatories, such as the currently operating
Solar and Heliospheric Observatory (SOHO), can observe the
Sun continuously in ultraviolet and X-ray wavelengths, as well
as make visible-light observations of the solar corona (using a
coronagraph to block the light from the photosphere) at all times.
Solar Far-Ultraviolet Spectral Irradiance
Solar Far- and Extreme- Ultraviolet Spectral Irradiance
The Sun in He+ (30.4 nm) Emission
The Sun in Fe XII (19.5 nm) Emission
The Sun Imaged in X-Rays
Extreme UV Images of Solar Prominence Eruption
Transition Region and Coronal Explorer (TRACE)
EUV Images of Solar Filament Eruption
Solar Activity and the Sunspot Cycle
• The average number of sunspots which are observable at one
time, varies in a regular cycle (maximum, to minimum, and back to
maximum) of about 11 years.
• Large numbers of sunspots are also associated with high solar
activity, as observed in far-UV and X-ray wavelength ranges, and
frequent solar flares, prominences, and coronal mass ejections.
• There is significant variability from one solar cycle to the next, in
the number of sunspots and associated solar activity.
• Observations of the Sun from space, in visible as well as infrared,
ultraviolet, and X-ray wavelengths, have greatly improved the
accuracy and long-term, continuous measurement capability of the
Sun’s total radiation output, and its variations with time.
• One unexpected finding from these measurements, is that the
Sun’s total radiation output is slightly higher at sunspotmaximum (by about 0.1%) than at sunspot-minimum stages of the
11-year cycle.
Solar Activity and the Sunspot Cycle
• In addition to these energetic forms of electromagnetic radiation,
the Sun emits electrically charged particles (primarily ionized
hydrogen and helium, and electrons) as a result of solar flares
and coronal mass ejections.
• Solar activity is of practical importance to us here on Earth, and
to astronauts and spacecraft in near-Earth space, because of
the potentially harmful effects of solar X-rays, gamma rays, and
highly-energetic particles on radio communications, longdistance power transmission, and on spacecraft electronics and
humans in space.
• The study of the effects of solar activity on the near-Earth and
interplanetary regions of space constitutes the new field of
“space weather”.
Solar EUV Variation Over an 11-Year Solar Cycle
Intensity and Solar-Cycle Variability vs. Wavelength
in the Solar Spectrum
Solar Wind and Coronal Mass Ejections
• In addition to its emissions of electromagnetic radiation, the Sun
also emits material (mostly in the form of electrons, protons, and
helium nuclei) which flows outward into the solar system (some of it
reaching Earth’s vicinity).
• The major part of this mass ejection, especially in times of low solar
activity, is the solar wind, a steady flow of ionized gas outward
through the solar system, having low enough energy as to not have
major effects on the planets and their local environments.
• At Earth’s distance from the Sun, the solar wind has a typical density
of about 7 atoms/cm3 and typical velocity of 300-700 km/sec.
• Because of the angular momentum induced by the Sun’s rotation on
its axis, the solar wind travels outward in a spiral fashion, along solar
magnetic field lines.
• More significant, in terms of its effects, are coronal mass ejections,
mostly associated with active regions on the solar surface, which are
most frequent and energetic during times of high solar activity.
• Coronal mass ejections can result from solar flares and are often
associated with sunspots and their local surroundings.
The Solar Wind
Solar Wind Variations Due To Solar Rotation
Note, the Sun’s axis of rotation is not perpendicular to to the plane of
Earth’s orbit around the Sun. Also, the solar wind source is not confined
to the Sun’s equator, but is variable over a range of solar latitudes.
THE SOLAR WIND
Because of the Sun’s magnetic field and its rotation on its axis, solar
wind particles travel outward at much higher speeds along the
magnetic polar directions than along its magnetic equatorial plane.
Coronal Mass Ejection Observed with LASCO
in Visible Light
The image on the left is with a narrow-field coronagraph (dark occulting disk blocks direct view of the
Sun; white circle indicates size of the Sun image without disk). The image on the right is with a widefield coronagraph, taken nearly 6 hours later. (Red and blue are false colors.)
Comparison of Solar Mass Ejections with Chromospheric XUV Emissions
Effects of Solar Activity on the Near-Earth
Space Environment
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In addition to the heat and light that the Sun provides to us on the surface of
Earth (which is very stable over long periods of time), it also has much more
variable effects on Earth’s upper atmosphere and the near-Earth space
environment.
These latter effects are due to (1) far-ultraviolet and X-ray radiations from
the Sun, and (2) energetic solar particle (proton and electron) emissions,
traveling along interplanetary magnetic field lines and interacting with
Earth’s magnetic field in near-Earth space.
Although the far-UV and X-ray emissions of the Sun are only a small
percentage of its total output, they are responsible for creating the Earth’s
ionosphere by ionization of the upper atmosphere.
Solar flares greatly increase the X-ray radiation and high-energy particle
components of the Sun’s emissions.
The solar wind, and much more energetic coronal mass ejections, create
the Earth’s magnetosphere and (indirectly) Earth’s polar auroras.
In times of high solar activity, these energetic radiation and solar particles
can cause problems with communications and electronic equipment, on the
ground as well as in space, and can be hazardous to astronauts in nearEarth and interplanetary space.
Ground-Based Image of an Aurora
THE HELIOSPHERE
• The heliosphere is defined as the entire region of space in which the
Sun’s mass ejections (including solar wind) and magnetic field
predominate over those of the Galaxy (the interstellar medium and
the galactic magnetic field).
• By this definition, the heliosphere extends well beyond the outer
planets of our solar system.
• Studies of the heliosphere and its boundary with the interstellar
medium have been made by the two Voyager spacecraft, which flew
by the outer planets (Jupiter, Saturn, Uranus, and Neptune) as their
primary missions in the 1979-1989 time periods.
• Voyager 1 recently (about December 16, 2004) crossed the
boundary known as the “termination shock”, where the outgoing solar
wind transitions from supersonic to subsonic velocity, about 93 AU
from the Sun.
• This, in turn, resulted in an abrupt increase in the density, and count
rate, of solar wind particles.
Voyager 1 Crosses Heliospheric Terminal Shock, 2005
Note, the increased count rate of solar wind particles is due to the
crossing of the boundary between supersonic (closer to the Sun) and
subsonic (further from the Sun) velocities, resulting in higher density.
Solar Wind Termination Shock Analogy
Voyager 1 Crossing of the Heliospheric Termination Shock
THE HELIOSPHERE
• It is believed that the boundary between the solar wind and
interstellar medium, called the heliopause, is still further distant
from the Sun than is the termination shock.
• The region between the termination shock and the heliopause is
called the heliosheath.
• Between the heliopause and the general interstellar medium is a
region in which both solar wind and interstellar gas are combined,
and travel at subsonic velocity.
• The outermost feature is a “bow shock” in which incoming
supersonic interstellar gas impacts, and mixes with, the outgoing
solar wind particles.
• Outside of the bow shock is the interstellar medium, mostly
hydrogen and helium, which fills our entire Galaxy.
Voyager Trajectories and the Heliosphere
Heliosheath
SOLAR EVOLUTION
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The Sun, as well as the planets and other objects making up our solar
system, were created about the same time, some 4.6 billion years ago.
The Sun and its planetary system condensed from a cloud of interstellar
dust and gas, as a result of its self-gravitational force.
The material making up the Sun collapsed into a sphere which grew hotter
as it grew smaller, until it became hot enough at its center to generate
energy by the thermonuclear fusion of hydrogen to form helium.
When first created, the sun was larger in diameter but cooler and less
luminous than at the present time.
The Sun, currently, is very slowly shrinking, but increasing in luminosity.
When the hydrogen in the Sun’s core nears depletion, the Sun will expand
and become cooler, while continuing to increase in luminosity (it will become
a Red Giant star).
In the final stage of evolution, the Sun will blow off its outer layers, and its
central regions will collapse to form a White Dwarf star (about the size of
our Earth but still containing most of the present mass of the Sun) which,
lacking any internal thermonuclear energy source, will slowly cool off (over
billions of years).
The total lifetime of the Sun, from its origin to the white dwarf stage, is about
10 billion years.