Chapter 10 Measuring the Stars

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Chapter 10 Measuring the Stars What are the
properties of Stars? What are the patterns among
Stars?
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• From the Sun weve learned
• stars are far away
• stars are bright
• stars are hot
• stars are massive

• How FAR AWAY? (measure stellar DISTANCES)
• How BRIGHT? (measure stellar LUMINOSITY)
• How HOT? (measure stellar SPECTRAL TYPE)
• How MASSIVE? (measure stellar MASS)
• we already know how to determine the composition

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Measuring Distances
Easy way use a ruler
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Parallax An apparent shift
in the relative positions of two objects when
viewed from different perspectives
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hyperphysics.phy-astr.gsu.edu/HBASE/Astro/para.htm
l
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??????? view
???????
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?uly view
?uly
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tan p B d d 1.A.U.
tan p
p
B
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For distances to stars comparable to the
Earth-Sun distance, parallax would be easily
measured
Parallax angle
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For distances to stars comparable to the
Earth-Sun distance, parallax would be easily
measured
tan p B d d 1.A.U.
tan p
1 A.U.
p (
Parallax angle
?
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For small angles tan p --gt p d B
1AU tan p tan p d 1 p
But stars are at much greater distances from the
solar system, and at great distances from one
another
June
December
The parallax of the nearest stars cannot be
measured without a telescope
Since the ancients were unable to observe
stellar parallax, they would not accept the Sun
as the center, even tough they considered the
possibility.
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Angular Measurements
Angles are measured in degrees
Degrees are divided into 60 arcminutes
Arcminutes are divided into 60
arcseconds

• Full circle 360º

• 1º 60? (arcminutes)

• 1? 60? (arcseconds)

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Measuring Distances
d 1 p
parsec parallax arcsec
?????sec is the distance to a star with a
parallax of 1 second of arc.
???? 1 1 arc sec
1 pc 206,265 AU 1 pc 3.2 ly
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Example Nearest star, a Centauri, has a parallax
of p 0.76 arc seconds
d 1 p d 1 1.3 pc 4.3 LY
0.76
This method does not work for stars farther away
than 50 pc.
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PARSEC is not a speed
PARSEC is a DISTANCE
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• From the Sun weve learned
• stars are far away
• stars are bright
• stars are hot
• stars are massive

• How FAR AWAY? DISTANCE
• How BRIGHT? LUMINOSITY
• How HOT? SPECTRAL TYPE
• How MASSIVE? MASS

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Stellar brightness -- what is it?
The brightness of an object depends on both
distance and energy output
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Amount of energy output a star radiates is called
the Luminosity (L) the energy per second
Amount of starlight that reaches Earth is called
the Apparent brightness (m)
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Luminosity passing through each sphere is the
same Divide luminosity by area to get brightness
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The amount of light received (Brightness) from
the source is proportional to its energy output
(Luminosity (L) )and inversely proportional to
the square of the distance (d)

Luminosity Brightness
4p (distance)2
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Two Kinds of Brightness

• Apparent Magnitude (m) How bright the object
  appears to us on Earth.

• Absolute Magnitude (M) How bright a star
  actually is, its intrinsic brightness

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Two Kinds of Brightness

• Apparent Magnitude How bright the object appears
  to us on Earth.

m 1 brightest m 6 dimmest
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As the Sun sets, some stars are visible. These
are the first magnitude stars. Later, when
twilight is over, more stars are visible. These
are the second magnitude stars, and so on
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Two Kinds of Brightness

• Apparent Magnitude How bright the object appears
  to us on Earth.

• Which would look brighter?
• Vega, m 0.03
• Antares, m 1.06
• Which would look brighter?
• Sirius, m -1.4
• Venus, m -4.4

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Two Kinds of Brightness

• Apparent Magnitude How bright the object appears
  to us on Earth.

• Which would look brighter?
• Vega, m 0.03
• Antares, m 1.06
• Which would look brighter?
• Sirius, m -1.4
• Venus, m -4.4

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Two Kinds of Brightness

• Absolute Magnitude How bright a star actually
  is, its intrinsic brightness

PROBLEM stars are at different distances from
Earth and so its hard to know which stars are
ACTUALLY brighter versus which APPEAR bright
The Absolute Magnitude that a star would have if
it were at a distance of 10 pc.
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Compare some stars
Absolute Apparent MSun 4.8 mSun
-26 MSirius 1.4 mSirius -1.46 MBetelgeuse
-5.6 mBetelgeuse 0.50
Which star looks brightest from Earth?Which
star is brightest?
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Star light, Star bright .
qualitatively
quantitatively
Magnitude apparent and absolute
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Most luminous stars 106 LSun Least
luminous stars 10-4 LSun (LSun is
luminosity of Sun)
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• From the Sun weve learned
• stars are far away
• stars are bright
• stars are hot
• stars are massive

• How FAR AWAY? DISTANCE
• How BRIGHT? LUMINOSITY
• How HOT? SPECTRAL TYPE
• How MASSIVE? MASS

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Temperature Color
AND
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These BLACKBODY Radiation curves will provides a
relation between TEMPERATURE and COLOR
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Hipparchus developed the magnitude scale in
ancient Greece. The brightest star apparent
magnitude 1 The faintest star apparent
magnitude 6
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The computors at Harvard finished the
classification process. (1890)
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Spectra of Stars
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Remember the Balmer Series?
The calculators used H lines in spectra
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Classification Scheme
A B C D E . . . S
Annie Jump Cannon (1863-1941)
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Classification Schemebased on the strength of
Balmer lines

• Originally
• A strongest H lines
• B less strong H lines
• O Weakest H lines
• 10,000 stars
• publishes in 1890

A B C D E . . . S
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A B C D E . . . S
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Antonia Maury (1866-1952)
Spectral classes might make more sense if
arranged by temperature
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temperature
strength
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Keep in mind that this classification is for the
stars SURFACE, only!!!!!
Summary of Spectral Classes
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Meghnad Saha offered the explanation (theory of
thermal ionization of atoms) which was confirmed
at Harvard by Cannon and Maurys work.
Cecelia Payne-Gaposchkin (1900-1979) First PhD in
Astronomy from Harvard/Radcliffe
Provided a convincing argument that stars are
mostly made of hydrogen.
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• From the Sun weve learned
• stars are far away
• stars are bright
• stars are hot
• stars are massive

• How FAR AWAY? (measure stellar DISTANCES)
• How BRIGHT? (measure stellar LUMINOSITY)
• How HOT? (measure stellar SPECTRAL TYPE)
• How MASSIVE? (measure stellar MASS)

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Direct mass measurements are possible only for
stars in binary star systems
About half of all stars are in binary systems
The orbit of a binary star system depends on
strength of gravity
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Binary Star system
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Measure the mass of this?????
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The binary star IW Tau is revealed through
adaptive optics. The stars have a 0.3 arc second
separation. The images were taken by Chas
Beichman and Angelle Tanner of JPL.
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Binary Stars
Stellar mass can be estimated from the orbital
motion.
center of mass balance point of the system
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Estimating Stellar Masses
Remember Keplers 3rd Law? Py2 aAU3
For binary stars with masses MA and MB
aAU3
MA MB
____
Py2
Example Binary system with period of P 32
years and separation of a 16
AU
163
____
4 solar masses
MA MB
322
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Types of Binary Stars
Visual Binary Eclipsing Binary Spectroscopic
Binary
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Visual Binary
We can directly observe the orbital motions of
these stars
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Eclipsing Binary
We can measure periodic eclipses
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Spectroscopic Binary
We determine the orbit by measuring Doppler shifts
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• How FAR AWAY? DISTANCE --gt Parallax
• How BRIGHT? LUMINOSITY --gt distance
• 
  brightness
• How HOT? SPECTRAL TYPE --gt Temp
• How MASSIVE? MASS --gt Keplers 3rd

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Stellar Properties

• Parallax tells us distances to the nearest stars
• If we measure a stars apparent brightness
• and distance, we can compute its
  luminosity
• A stars color and spectral type
• both reflect its temperature
• Newtons version of Keplers third law
• tells us the total mass of a binary system,
• if we can measure the orbital period (p)
• and average orbital separation of the system
  (a)

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• General properties of Stars
• a. brightness
• temperature
• mass

Is there a way we can use the general properties
of stars to get a snap shot about the stars????
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THE H-R DIAGRAM

• Enjar Hertzsprung and Henry Norris Russell
• graph of luminosity (or absolute magnitude)
  versus temperature (or spectral class)

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Herzsprung Russell Diagram
H-R diagram plots the luminosity and
temperature of stars
Luminosity brightness
temperature
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brightness
temperature
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Luminosity brightness
temperature
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Mass - Luminosity Relation
Luminosity brightness
L M4
S
mass
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Most stars fall somewhere on the main-sequence
of the H-R diagram
Main-sequence stars are fusing hydrogen into
helium in their cores like the Sun Luminous
main-sequence stars are hot (blue) Less luminous
ones are cooler (yellow or red)
Main Sequence
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Mass measurements of main-sequence stars show
that the hot, blue stars are much more massive
than the cool, red ones
High-mass stars
Short lived stars
Low-mass stars
Long lived stars
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So far Brightness Luminosity
(energy output rate) Color
Temperature Temp Lum HR diagram
main sequence
giants

white dwarfs Mass Lum
L M4
S
Now we know relative size, lifetimes, and
lifelines.
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End of Chapter 10
Now you know the basics of star measurements!