Download Our Star - the Sun

Our Star - the Sun
The Sun’s energy is generated by thermonuclear
reactions in its core
• The energy released in a
nuclear reaction
corresponds to a slight
reduction of mass
according to Einstein’s
equation E = mc2
• Thermonuclear fusion
occurs only at very high
temperatures; for example,
hydrogen fusion occurs
only at temperatures in
excess of about 107 K
• In the Sun, fusion occurs
only in the dense, hot core
The Sun’s energy is produced by hydrogen
fusion, a sequence of thermonuclear
reactions in which four hydrogen nuclei
combine to produce a single helium nucleus
deuterium
A theoretical model of the Sun shows how energy
gets from its center to its surface
• Hydrogen fusion takes
place in a core extending
from the Sun’s center to
about 0.25 solar radius
• The core is surrounded by
a radiative zone extending
to about 0.71 solar radius
– In this zone, energy travels
outward through radiative
diffusion
• The radiative zone is
surrounded by a rather
opaque convective zone of
gas at relatively low
temperature and pressure
– In this zone, energy travels
outward primarily through
convection
The photosphere is the lowest of three main layers
in the Sun’s atmosphere
• The Sun’s atmosphere
has three main layers: the
photosphere, the
chromosphere, and the
corona
• Everything below the
solar atmosphere is
called the solar interior
• The visible surface of the
Sun, the photosphere, is
the lowest layer in the
solar atmosphere
The spectrum of the photosphere is similar to that of a
blackbody at a temperature of 5800 K
Convection in the photosphere produces
The chromosphere is characterized by spikes
of rising gas
• Above the
photosphere is a
layer of less dense
but higher
temperature gases
called the
chromosphere
• Spicules extend
upward from the
photosphere into the
chromosphere along
the boundaries of
supergranules
• The outermost
layer of the solar
atmosphere, the
corona, is made
of very hightemperature
gases at
extremely low
density
• The solar corona
blends into the
solar wind at
great distances
from the Sun
The corona ejects mass into space to form the solar wind
~106km
Activity in the corona includes coronal mass ejections and coronal holes
UV Corona
(SOHO)
Sunspots are low-temperature regions in
the photosphere
~1 rotation / 4 weeks
Differential rotation
Sunspots are produced by a 22-year cycle
in the Sun’s magnetic field
•
•
•
•
The Sun’s surface features vary in an 11-year cycle
This is related to a 22-year cycle in which the surface magnetic field
increases, decreases, and then increases again with the opposite polarity
The average number of sunspots increases and decreases in a regular cycle
of approximately 11 years, with reversed magnetic polarities from one 11year cycle to the next
Two such cycles make up the 22-year solar cycle
Effect of B-fields
Hα image
656 nm
UV image
SOHO
30.4 nm
The Sun’s magnetic field also produces other
forms of solar activity
• A solar flare is a
brief eruption of hot,
ionized gases from a
sunspot group
• A coronal mass
ejection is a much
larger eruption that
involves immense
amounts of gas from
the corona
X-ray
and
UV image
FIN
The Nature of the Stars
The Population of Stars; Luminosity function
• Stars of relatively low luminosity are more common than more
luminous stars
• Our own Sun is a rather average star of intermediate luminosity
Astronomers often use the magnitude scale
to denote brightness
• The apparent magnitude
scale is an alternative
way to measure a star’s
apparent brightness
• The absolute magnitude
of a star is the apparent
magnitude it would have
if viewed from a distance
of 10 parsecs
A star’s color depends on its surface temperature
Photometry and Color Ratios
•
•
•
Photometry measures the apparent brightness of a star
The color ratios of a star are the ratios of brightness values obtained through
different standard filters, such as the U, B, and V filters
These ratios are a measure of the star’s surface temperature
b
apparent brightness = observed flux
The spectra of stars reveal their chemical
compositions as well as surface temperatures
• Stars are classified
into spectral types
(subdivisions of the
spectral classes O, B,
A, F, G, K, and M),
based on the major
patterns of spectral
lines in their spectra
The spectral class and type of a star is directly
related to its surface temperature: O stars are the
hottest and M stars are the coolest
• Most brown dwarfs are in even cooler spectral
classes called L and T
• Unlike true stars, brown dwarfs are too small to
sustain thermonuclear fusion
Hertzsprung-Russell (H-R) diagrams reveal
the different kinds of stars
• The H-R diagram is a
graph plotting the
absolute magnitudes of
stars against their
spectral types—or,
equivalently, their
luminosities against
surface temperatures
• The positions on the H-R
diagram of most stars are
along the main sequence,
a band that extends from
high luminosity and high
surface temperature to
low luminosity and low
surface temperature
On the H-R diagram,
giant and supergiant
stars lie above the
main sequence, while
white dwarfs are below
the main sequence
By carefully examining a star’s spectral lines,
astronomers can determine whether that star is a
main-sequence star, giant, supergiant, or white
dwarf
Using the H-R diagram
and the inverse square
law, the star’s luminosity
and distance can be
found without
measuring its stellar
parallax
Spectroscopic Analysis
A Binary Star System
Binary star systems provide crucial information
about stellar masses
• Binary stars are important because they allow
astronomers to determine the masses of the two stars
in a binary system
• The masses can be computed from measurements of
the orbital period and orbital dimensions of the system
Mass-Luminosity Relation for MainSequence Stars
• Main sequence stars are stars like the Sun
but with different masses
• The mass-luminosity relation expresses a
direct correlation between mass and
luminosity for main-sequence stars
• The greater the mass of a main-sequence
star, the greater its luminosity (and also
the greater its radius and surface
temperature)
FIN
47
Spectroscopy makes it possible to study binary
systems in which the two stars are close together
•
•
•
•
Some binaries can be
detected and analyzed,
even though the system
may be so distant or the two
stars so close together that
the two star images cannot
be resolved
A spectrum binary appears
to be a single star but has a
spectrum with the
absorption lines for two
distinctly different spectral
types
A spectroscopic binary has
spectral lines that shift back
and forth in wavelength
This is caused by the
Doppler effect, as the orbits
of the stars carry them first
toward then away from the
Earth
Binary Stars
• Binary stars, in which two stars are held in orbit
• around each other by their mutual gravitational
attraction, are surprisingly common
• Those that can be resolved into two distinct star
images by an Earth-based telescope are called
visual binaries
• Each of the two stars in a binary system moves
in an elliptical orbit about the center of mass of
the system
Light curves of eclipsing binaries provide detailed
information about the two stars
• An eclipsing binary is a
system whose orbits
are viewed nearly edgeon from the Earth, so
that one star
periodically eclipses the
other
• Detailed information
about the stars in an
eclipsing binary can be
obtained from a study of
the binary’s radial
velocity curve and its
light curve
Parallax
Careful measurements of the parallaxes of stars
reveal their distances
•
•
•
Distances to the nearer stars can be determined by parallax, the
apparent shift of a star against the background stars observed as
the Earth moves along its orbit
Parallax measurements made from orbit, above the blurring effects
of the atmosphere, are much more accurate than those made with
Earth-based telescopes
Stellar parallaxes can only be measured for stars within a few
hundred parsecs
The magnetic-dynamo model suggests that many
features of the solar cycle are due to changes in
the Sun’s magnetic field
These changes are caused by convection
and the Sun’s differential rotation
Rotation of the Solar Interior
If a star’s distance is known, its luminosity can be
determined from its brightness
• A star’s luminosity (total light output), apparent brightness,
and distance from the Earth are related by the inversesquare law
• If any two of these quantities are known, the third can be
calculated
Relationship between a star’s luminosity, radius, and
surface temperature
Stars come in a wide variety of sizes
Finding Key Properties of Nearby Stars