Transcript Chap 11 Characterizing Stars v2

Neil F. Comins • William J. Kaufmann III
Discovering the Universe
Eighth Edition
CHAPTER 11
Characterizing Stars
Using Parallax to Determine Distance
Our eyes change the angle between their lines of sight as we look at things
that are different distances away. Our eyes are adjusting for the parallax of
the things we see. This change helps our brains determine the distances to
objects and is analogous to how astronomers determine the distance to
objects in space.
Using Parallax to Determine Distance
A nearby star appears to
shift its position against the
background of distant stars.
The star’s parallax angle (p)
is equal to the angle
between the Sun and Earth,
as seen from the star.
The closer the star is to us,
the greater the parallax angle
p. The distance to the star
(in parsecs) is found by taking
the inverse of the parallax
angle p (in arcseconds),
d = 1/p.
Parallax Equation
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Distance (in parsecs) = 1/parallax angle
(arcseconds)
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Or:
D = 1/p
Star Brightness
Apparent Magnitude (m)
-How bright a star appears to be
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-Lower magnitude number = brightest stars
-Higher magnitude number = dimmer
Apparent Magnitude Scale
Several stars in and around
the constellation Orion labeled
with their names and apparent
magnitudes.
Astronomers denote the
brightnesses of objects in the sky
by apparent magnitudes. Stars
visible to the naked eye have
magnitudes between m = –1.44
and about m = +6.
Star Brightness
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Does apparent magnitude really tell us how
bright a star actually is?
Answer: No
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Apparent magnitude is dependant on how far
away stars are
The Inverse-Square Law
The same amount of radiation from a light source must illuminate
an ever-increasing area as the distance from the light source
increases. The decrease in brightness follows the inverse-square
law, which means, for example, that tripling the distance
decreases the brightness by a factor of 9.
The Inverse-Square Law
The car is seen at distances of 10 m, 20 m, and
30 m, showing the effect described in the previous
image.
Absolute Magnitude
Absolute Magnitude (M)- Describes the true brightness of
a star no matter the distance
-absolute magnitude describes the brightness of stars if
they were moved to the standard distance of 10 pc
-Range from -10 for brightest stars and +17 for the dimmest
-allows astronomers to compare energy outputs of stars
Absolute Magnitude
So, a star with the apparent
magnitude = absolute
magnitude would be at a
distance of 10 parsecs.
Luminosity
Luminosity – describes the total energy output by
stars
Luminosity units are Watts (J/sec)
Luminosity uses the sun as a comparison
-Sun Luminosity (1 L Θ)= 400 x 1024 W
- A luminosity of 2 L Θwould have twice the total energy output
**Luminosity is also dependent on the size of the star
GO TO EM Spectrum!
Temperature and Color
This beautiful Hubble Space Telescope
image shows the variety of colors of stars.
**(see class action UNL light module)
Temperature and Color
The range of visible wavelengths is indicated. Where the peak of a star’s
intensity curve lies relative to the visible light band determines the apparent
color of its visible light.
Stellar Classification
*Stars are classified based on their temperature and color:
Oh Be A Fine Girl and Kiss Me
*Each letter class has 10 subclasses (0-10 based on temperature)
Classification
Temperature
Max
Wavelength
O0
40,000 K
72.5 nm
Blue
B0
20,000 K
145 nm
Light Blue
A0
10,000 K
290 nm
White
F0
7,500 K
387 nm
Yellow-White
G0
5,500 K
527 nm
Yellow
K0
4,000 K
725 nm
Orange
M0
3,000 K
966 nm
Red-Orange
http://astro.unl.edu/naap/hr/hr.html
Color
A Hertzsprung-Russell (H-R) Diagram
Types of Stars
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Determined by size
Dwarf ~20,000 mi diameter
Sun ~800,000 mi diameter
Giant ~6,000,000 mi diameter
Supergiant ~500,000,000 mi diameter
A Hertzsprung-Russell Diagram
The Types of Stars and Their Sizes
The Mass-Luminosity Relation
On this H-R diagram, each
dot represents a mainsequence star. The number
next to each dot is the mass
of that star in solar masses
(M). As you move up the
main sequence from the
lower right to the upper left,
the mass, luminosity, and
surface temperature of mainsequence stars all increase.
Summary of Key Ideas
Magnitude Scales
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Determining stellar distances from Earth is the first step
to understanding the nature of the stars. Distances to the
nearer stars can be determined by stellar parallax, which
is the apparent shift of a star’s location against the
background stars while Earth moves along its orbit
around the Sun. The distances to more remote stars are
determined using spectroscopic parallax.
The apparent magnitude of a star, denoted m, is a
measure of how bright the star appears to Earth-based
observers. The absolute magnitude of a star, denoted M,
is a measure of the star’s true brightness and is directly
related to the star’s energy output, or luminosity.
Magnitude Scales
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The absolute magnitude of a star is the apparent
magnitude it would have if viewed from a
distance of 10 pc. Absolute magnitudes can be
calculated from the star’s apparent magnitude
and distance.
The luminosity of a star is the amount of energy
emitted by it each second.
The Temperatures of Stars
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Stellar temperatures can be determined from
stars’ colors or stellar spectra.
Stars are classified into spectral types (O, B, A,
F, G, K, and M) based on their spectra or,
equivalently, their surface temperatures.
Types of Stars
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The Hertzsprung-Russell (H-R) diagram is a graph on
which luminosities of stars are plotted against their
spectral types (or, equivalently, their absolute
magnitudes are plotted against surface temperatures).
The H-R diagram reveals the existence of four major
groupings of stars: main-sequence stars, giants,
supergiants, and white dwarfs.
The mass-luminosity relation expresses a direct
correlation between a main-sequence star’s mass and
the total energy it emits.
Distances to stars can be determined using their spectral
types and luminosity classes.
Stellar Masses
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Binary stars are surprisingly common. Those that can be
resolved into two distinct star images (even if it takes a
telescope to do this) are called visual binaries.
The masses of the two stars in a binary system can be
computed from measurements of the orbital period and
orbital dimensions of the system.
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 with a telescope.
Stellar Masses
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A spectroscopic binary is a system detected from the
periodic shift of its spectral lines. This shift is caused by
the Doppler effect as the orbits of the stars carry them
alternately toward and away from Earth.
An eclipsing binary is a system whose orbits are viewed
nearly edge-on from Earth, so that one star periodically
eclipses the other. Detailed information about the stars in
an eclipsing binary can be obtained by studying its light
curve.
Mass transfer occurs between binary stars that are close
together.
Key Terms
absolute magnitude
apparent magnitude
binary star
center of mass
close binary
eclipsing binary
giant star
Hertzsprung-Russell
(H-R) diagram
initial mass function
inverse-square law
light curve
luminosity
luminosity class
main sequence
main-sequence star
mass-luminosity relation
OBAFGKM sequence
optical double
photometry
radial-velocity curve
red giant
spectral types
spectroscopic binary
spectroscopic parallax
stellar evolution
stellar parallax
stellar spectroscopy
supergiant
visual binary
white dwarf
WHAT DID YOU THINK?
How near to us is the closest star other
than the Sun?
 Proxima Centauri is about 25 trillion mi (40
trillion km) away. Light from there will take
about 4 years to reach Earth.
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WHAT DID YOU THINK?
How luminous is the Sun compared with
other stars?
 The most luminous stars are about a
million times brighter, and the least
luminous stars are about a hundred
thousand times dimmer than the Sun.
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WHAT DID YOU THINK?
What colors are stars, and why do they
have these colors?
 Stars are found in a wide range of colors,
from red through violet as well as white.
They have these colors because they
have different temperatures.
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WHAT DID YOU THINK?
Are brighter stars hotter than dimmer
stars?
 Not necessarily. Many brighter stars, such
as red giants, are cooler but larger, than
hotter, dimmer stars, such as white
dwarfs.
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WHAT DID YOU THINK?
Compared to the Sun, what sizes are
other stars?
 Stars range from more than 1000 times
the Sun’s diameter to less than 1/100 the
Sun’s diameter.
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WHAT DID YOU THINK?
Are most stars isolated from other stars,
as the Sun is?
 No. In the vicinity of the Sun, one-third of
the stars are found in pairs or larger
groups.
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