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CHAPTER 5
SUN LIGHT AND SUN ATOMS
Topic Summaries
spectral line A line in a spectrum
5. 1 The Sun-Basic Characteristics
at a specific wavelength produced by
How do you know the distance, size, mass, and density of the sun?
•
the absorption or emission of light by
Observing the parallax shift of Venus in transit against the sun's disk from opposite sides of Earth was the original method for finding the distance to the sun.
The diameter of the sun can be calculated from its distance and its angular size.
certain atoms.
transits of Venus Rare occasions
when Venus can be seen as a tiny dot
Newton's laws and the motion of the planets as they orbit the sun allow the
mass of the sun to be determined. Its average density can be easily calculated
from its mass and diameter.
directly between Earth and the sun.
density Mass per volume.
atom The smallest unit of a chemical
5. 2 The Origin of Sunlight
element. consisting of a nucleus
containing protons and neutrons plus
How does matter produce light?
a surrounding cloud of electrons.
Motion among charged particles in a solid, a liquid, or a dense gas causes the
emission of blackbody radiation. Pure blackbody radiation is a continuous
spectrum.
nucleus The central core of an atom
containing protons and neutrons that
• The hotter an object is, the more energy it radiates and the shorter is its wavelength of maximum intensity, A-max· This allows astronomers to estimate the
temperature of the sun and other stars from their colors.
5.3 The Sun's
carries a net positive charge.
proton A positively charged atomic
particle contained in the nucleus of an
T;}. -
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What do astronomers see when they observe the sun?
• The photosphere is the level in the sun fri
escape. Its temperature is about S,800 K.
which visible photons most easily
Energy flowing outward from the sun's in erior trave s as rismg currents of
hot gas and sinking currents of cool gas in the convective zone just below the
photosphere.
The granulation of the photosphere is produced by convection currents of gas
rising from below.
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proton.
electron Low-mass atomic particle
carrying a negative charge.
molecule Two or more atoms
bonded together.
heat Energy stored in a material as
agitation among its particles.
5.4 Light, Matter, and Motion
How does matter interact with light to produce spectral lines?
•
Electrons in an atom may occupy various permitted orbits around the nucleus
but not orbits in between. The size of an electron's orbit depends on the energy
stored in the electron's motion.
An electron may be excited to a higher orbit during a collision between atoms,
or it may move from one orbit to another by absorbing or emitting a photon of
the proper energy.
Shifts in the wavelengths of features in spectra of the sun and stars provide
clues to the motions of their atoms. The Doppler effect, which reveals the radial
velocity of the gas, the part of its velocity directed toward (blueshift) or away
from (redshift) Earth.
temperature A measure of the
agitation among the atoms and
molecules of a material.
Kelvin temperature scale A
temperature scale using Celsius
degrees and based on zero being
equal to absolute zero.
absolute zero The theoretical
lowest possible temperature at which
a material contains no extractable
heat energy. Zero on the Kelvin
temperature scale.
5.5 The Sun's Atmosphere
What can you learn from the sun's spectrum?
Because orbits of only certain energies are permitted in an atom, photons of
only certain wavelengths can be absorbed or emitted. Each kind of atom has its
own structure and therefore its own characteristic set of spectral lines.
blackbody radiation Radiation
emitted by a hypothetical perfect
radiator. The spectrum is continuous,
and the wavelength of maximum
emission depends on the body's
temperature.
Key Terrns
Topic Surnrnaries
wavelength of maximum
intensity The wavelength at which
If light from a blackbody such as the sun's photosphere passes through a
low-density gas such as the sun's atmosphere on its way to your spectrograph, the gas can absorb photons of certain wavelengths, producing an
absorption spectrum.
a perfect radiator emits the maximum
amount of energy. Depends only on
The sun's spectrum can tell you its chemical composition through the
presence of spectral lines of a certain element. However, you must proceed
with care because the strengths of lines also depend on the temperature of
the gas.
the object's temperature.
Wien's Law A law stating that the
hotter a glowing object is, the shorter
will be its wavelength of maximum
intensity, inversely proportional to its
temperature.
If you look at a low-density gas that is excited to emit photons, you see an
emission spectrum.
The solar atmosphere consists of two layers of hot, low-density gas, the
chromosphere and the corona.
Stefan-Boltzmann Law A law
stating that hotter objects emit more
energy than cooler objects of the
same size, in proportion to the fourth
power of temperature.
photosphere The bright visible
surface of the sun.
sunspot Relatively dark spot on the
sun that contains intense magnetic
fields.
The chromosphere is most easily visible during total solar eclipses. Its pink
color is caused by the Balmer emission lines in its spectrum.
•
Filtergrams of the chromosphere reveal spicules and filaments.
The corona is the sun's outermost atmospheric layer. It is composed of a
very-low-density, very hot gas extending far from the visible sun. Astronomers have evidence that its high temperature-2,000,000 K or more-is
maintained by effects of the magnetic carpet.
Parts of the corona give rise to the solar wind.
ol r..Activity
granulation The fine structure
of bright grains with dark edges
covering the sun's surface.
convection Circulation in a fluid
driven by heat. Hot material rises and
cool material sinks.
Coulomb force The electrostatic
force of repulsion or attraction
between charged bodies.
ion An atom that has lost or gained
one or more electrons.
ionization The process in which
atoms lose or gain electrons.
binding energy The energy
needed to pull an electron away from
its atom.
Wh~
r-':
oes th sun hav a cycle ot ctivit
becau~they
• Suns ots s em dak
are sli htly eeo er than the rest of the
photosphere. The average sun pot is about twice the size of Earth and contai s mag etie<fie1 1s..a fewth] sant:l times tro'hger than Earth's.
ooo~s
o~~~nsity, and temperature of gases
Solar as
c.9n stud . t e mot
inside the sun through helioseismologl : )
Astronomers can measure magnetic fields on the sun by measuring the
splitting of some spectral lines caused by the Zeeman effect.
The average number of sunspots varies over a period of about 11 years and
appears to be related to a magnetic cycle. Alternate sunspot cycles have
reversed magnetic polarity, which is explained by the Babcock model.
• The sunspot cycle does not repeat exactly, and the Maunder minimum
seems to have been a time when solar activity was very low and Earth's
climate was slightly colder.
The sun rotates differentially with regions far from the equator rotating
slower than equatorial regions.
quantum mechanics The study
Spectroscopic observations of other stars reveal that many have spots and
magnetic fields that follow long-term cycles like the sun's.
of the behavior of atoms and atomic
particles.
Prominences occur in the chromosphere; their arched shapes show that
they are formed of ionized gas trapped in the magnetic field.
permitted orbit One of the unique
orbits that an electron may occupy in
an atom.
isotopes Atoms that have the same
number of protons but a different
number of neutrons.
energy level One of a number
of states an electron may occupy in
an atom, depending on its binding
energy.
Flares are sudden eruptions of X-ray, ultraviolet, and visible radiation
plus high-energy atomic particles produced when magnetic fields on the
sun interact and reconnect. Flares are important because they can have
dramatic effects on Earth, such as communications blackouts and auroras.
Spacecraft images show long streamers extending from the corona out into
space. CMEs can produce auroras and other phenomena if they strike Earth.
CHAPTER 5
SUN LIGHT AND SUN ATOMS-CONTINUED
Review Questions
1. How was the distance to the sun
first determined?
2. How is the mass of the sun
determined?
3. Define density. How is the
density of the sun determined?
4. Why do hot stars look bluer
than cool stars?
S. Why does the amount of
blackbody radiation emitted
depend on the temperature of
the object?
6. Why can't you see deeper into
the sun than the photosphere?
7. What evidence can you give
that granulation is caused by
convection?
Key Terms
18. What evidence can you give
that the corona has a very high
temperature?
19. What heats the chromosphere
and corona to high
temperatures?
excited atom An atom in which an
electron has moved from a lower to a
higher energy level.
ground state The lowest permitted
electron energy level in an atom.
20. How are astronomers able to
explore the layers of the sun
below the photosphere?
quantum leap Jumps of electrons
21. What evidence can you give
that sunspots are magnetic?
Doppler effect The change in
22. How does the Babcock model
explain the sunspot cycle?
relative radial motion of source and
23. What does the shape of a
prominence reveal?
24. How can solar flares affect Earth?
from one orbit or energy state to
another.
the wavelength of radiation due to
observer.
blueshift A Doppler shift toward
shorter wavelengths caused by a
velocity of approach.
8. Why is the binding energy of an
electron related to the size of its
redshift A Doppler shift toward
orbit?
9. Why do different atoms have
different lines in their spectra?
velocit>{ ofi-ecession.
4t
10. Describe two ways a atom can
become excited.
11. What is the difference between
an isotope and an ion?
12. Explain why ionized calcium
can form absorption lines,
but ionized hydrogen
cannot.
13. What kind of spectrum does a
neon sign produce?
14. How can the Doppler effect
explain wavelength shifts in
both light and sound?
15. If a nebula contains mostly
hydrogen excited to emit
photons, what kind of spectrum
would you expect it to produce?
16. Explain why the absence of
spectral lines of a given element
in the solar spectrum would not
necessarily mean that element
is absent from the sun.
17. How can astronomers
detect structure in the sun's
chromosphere?
longer wavelengths caused by a
radial elocity (Vr) That
component of an object's velocity
directed away from or toward Earth.
~
chromosphere Bright gases just
above the photosphere of the sun.
corona The faint outer atmosphere
of the sun, com posed of low-density,
high temperature gas.
continuous spectrum A spectrum
in which there are no absorption or
emission lines.
absorption spectrum (dark-line
spectrum) A spectrum that conta ins
absorption lines.
absorption line A dark line in a
spectrum produced by the absence
of photons absorbed by atoms or
molecules.
emission spectrum (bright-line
spectrum) A spectrum produced by
photons emitted by an excited gas.
emission line A bright line in a
spectrum caused by the emission of
photons from atoms.
Kirchhoff's Laws A set of laws that
describe the absorption and emission
of light by matter.
Key Terms
transition The movement of an
helioseismology The study of
electron from one atomic energy level
to another.
the interior of the sun by the analysis
of its modes of vibration.
Lyman series Spectral lines in
the ultraviolet spectrum of hydrogen
produced by transitions whose lowest
energy level is the ground state.
Maunder butterfly diagram A
graph showing the latitude of
sunspots versus time, first plotted by
Babcock model A model of
the sun's magnetic cycle in which
the differential rotation of the sun
winds up and tangles the solar
magnetic field. This is thought to be
responsible for the sunspot cycle.
W. W. Maunder in 1904.
prominence Eruption on the solar
Balmer series A series of spectral
Zeeman effect The splitting
lines produced by hydrogen in the
near-ultraviolet and visible parts of
the spectrum. The three longest-
of spectral lines into multiple
components when the atoms are in
a magnetic field.
surface. Visible during total solar
eclipses.
flare A violent eruption on the
wavelength Balmer lines are visible to
the human eye.
Maunder minimum A period
between 1645 and 1715 of less
numerous sunspots and other solar
reconnection On the sun, the
merging of magnetic fields to
release energy in the form of flares.
activity.
aurora The glowing light display
that results w hen a planet's
magnetic field guides charged
particles toward the north and south
Paschen series Spectral lines in
the infrared spectrum of hydrogen
produced by transitions whose lowest
energy level is the third.
filtergram A photograph (usually of
the sun) taken in the light of a specific
region ofthe spectrum-for example,
an H0 filtergram.
filament A solar eruption, seerl from
above, silhouetted against the bright
photosphere.
active region A magnetic region
on the solar surface that includes
sunspots, prominences, flares, and
the like.
differential rotation The
rotatlPn o a bOdy in hi:~ different
partf of the bodY have i~erent
periods of rotation. This is true of the
sun, t pe Jovian planets, and the disk
J
spicule A small, flamelike projection
of the gal xy:
in the chromosphere of the sun.
dynamo effecLThe rocess
coronagraph A telescope designed
which a rotating, convecting body
of conducting matter, such as Earth's
core, can generate a magnetic field.
to capture images of faint objects
such as the corona of the sun that are
near relatively bright objects.
magnetic carpet The network of
small magnetic loops that covers the
solar surface.
solar wind Rapidly moving atoms
and ions that escape from the solar
corona and blow outward through the
solar system.
convective zone The region
inside a star where energy is carried
outward as rising hot gas and
sinking cool gas.
sun's surface.
magnetic poles, where they strike
the upper atmosphere and excite
atof s-te em'it photons.
coronal mass ejection
(CME) Matter ejected from the
sun's corona in powerful surges
go·aeCf by magnetic fields.
coronal hole An area of the
solar surface that is dark at Xray wavelengths, thought to be
associated with divergent magnetic
fields and the source of the solar
wind.