Stars !

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Stars !
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Common Name Scientific Name Distance (light years) Apparent Magnitude Absolute Magnitude Spectral Type
Sun - -26.72 4.8 G2V
Proxima Centauri V645 Cen 4.2 11.05 (var.) 15.5 M5.5Vc
Rigil Kentaurus Alpha Cen A 4.3 -0.01 4.4 G2V
Alpha Cen B 4.3 1.33 5.7 K1V
Barnard's Star 6.0 9.54 13.2 M3.8V
Wolf 359 CN Leo 7.7 13.53 (var.) 16.7 M5.8Vc
BD 36 2147 8.2 7.50 10.5 M2.1Vc
Luyten 726-8A UV Cet A 8.4 12.52 (var.) 15.5 M5.6Vc
Luyten 726-8B UV Cet B 8.4 13.02 (var.) 16.0 M5.6Vc
Sirius A Alpha CMa A 8.6 -1.46 1.4 A1Vm
Sirius B Alpha CMa B 8.6 8.3 11.2 DA
Ross 154 9.4 10.45 13.1 M3.6Vc
Ross 248 10.4 12.29 14.8 M4.9Vc
Epsilon Eri 10.8 3.73 6.1 K2Vc
Ross 128 10.9 11.10 13.5 M4.1V
61 Cyg A (V1803 Cyg) 11.1 5.2 (var.) 7.6 K3.5Vc
61 Cyg B 11.1 6.03 8.4 K4.7Vc
Epsilon Ind 11.2 4.68 7.0 K3Vc
BD 43 44 A 11.2 8.08 10.4 M1.3Vc
BD 43 44 B 11.2 11.06 13.4 M3.8Vc
Luyten 789-6 11.2 12.18 14.5
Procyon A Alpha CMi A 11.4 0.38 2.6 F5IV-V
Procyon B Alpha CMi B 11.4 10.7 13.0 DF
BD 59 1915 A 11.6 8.90 11.2 M3.0V
BD 59 1915 B 11.6 9.69 11.9 M3.5V
CoD -36 15693 11.7 7.35 9.6 M1.3Vc
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Measurement of Distances to Nearby Stars Parallax
Revisited
Parallax Angle
R
?
d
R
Tan ?
d
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Measurement of Distances to Nearby Stars Parallax
Revisited
Parallax Angle
R
?
d
R
d
For small angles (valid for stellar
measurements) Tan ? ? ? where ? is measured in
radians
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Measurement of Distances to Nearby Stars Parallax
Revisited
Parallax Angle
R
?
d
R
? (radians)
d
For astronomical measurements R and d are
measured in A.U.
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Measurement of Distances to Nearby Stars Parallax
Revisited
R (in A.U.)
d (in A.U.)
? (radians)
A convenient variation 1 radian 206265 arc
seconds
R (in A.U.)
R (in A.U.)
R (in A.U.)
206265
d (in A.U.)


? (radians)
? (arc seconds)
? (arc seconds)
206265
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Measurement of Distances to Nearby Stars Parallax
Revisited
One parsec is defined to be 206265 A.U.
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d (in parsecs)
? (arc seconds)
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Measurement of Speeds of Nearby Stars Radial
Speed Doppler Shift Revisited
Blue Shift toward Earth
Red Shift away from Earth
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Measurement of Speeds of Nearby Stars Radial
Speed Doppler Shift Revisited
Doppler shifts are caused by line of sight
velocities (called radial velocity) of the
source. Sources moving away from the earth are
red shifter. Sources moving toward the earth are
blue shifted.
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Measurement of Speeds of Nearby Stars Radial
Speed Doppler Shift Revisited
Astrophysics and Cosmology
Longer ?, lower f
Shorter ?, higher f
In general
Apparent Wavelength
True Frequency
Velocity of Source


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True Wavelength
Apparent Frequency
Wave Speed
Note If the source and detector are moving
apart, the Velocity of the Source is POSITIVE. If
the source and detector are toward one another,
the Velocity of the Source is NEGATIVE.
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Measurement of Speeds of Nearby Stars Transverse
(sideways) Speeds
Proper motion is defined to be the transverse
motion of the star across the sky
Motion of Barnards Star captured left 1997 (Jack
Schmidling), right 1950 (POSS)
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Measurement of Speeds of Nearby Stars Transverse
(sideways) Speeds
Measurement made same time during the year
?
w
d
w
? (radians)
d
d x ? (radians)
w
If the time interval between measurements is
measured, then v w/ t
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Measurement of Speeds of Nearby Stars
v
vt
vR
Pythagorian Theorem v2 vR2 vt2
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Measurement of Speeds of Nearby Stars
A very recent animation of the historical motion
of thousands of currently nearby stars
http//www.spacedaily.com/news/milkyway-04b.html
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Luminosity (brightness) of a Star
Luminosity is the amount of energy per second
(Watts) emitted by the star Recall The
luminosity of the sun is about 4 x 1026
W Absolute Brightness The luminosity per
square meter emitted by the star at its surface.
This is an intrinsic property of the
star. Apparent Brightness The power per square
meter as measured at the location of the earth.
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Luminosity (brightness) of a Star
Note
Power (or Luminosity)
Absolute Brightness
Surface Area of star
Also Note Because of conservation of energy,
the energy per second radiated through the area
of a sphere of any radius must be a constant.
Therefore
Power (or Luminosity)
Apparent Brightness
Surface Area of sphere of radius equal to the
distance between the star and the earth
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Luminosity (brightness) of a Star
Power (or Luminosity)
Apparent Brightness ?
d2
Apparent brightness can be measured at the earth
with instruments. d is measured using parallax.
These pieces of information can be used to
measure the luminosity of the star.
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Temperature of a Star Photometry Revisited
Photometer An instrument which measure the
brightness of an object Will measure the TOTAL
brightness of an object, which might be difficult
to interpret. However, when combined with
filters, can be used to measure the amount of
light produced over a narrow range of
frequencies. This can be compared with standard
Blackbody radiation curves to determine the
temperature of the object
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Temperature of a Star Photometry Revisited
Photometer An instrument which measure the
brightness of an object Will measure the TOTAL
brightness of an object, which might be difficult
to interpret. However, when combined with
filters, can be used to measure the amount of
light produced over a narrow range of
frequencies. This can be compared with standard
Blackbody radiation curves to determine the
temperature of the object
Intensity
X
Wavelength
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Temperature of a Star Photometry Revisited
Photometer An instrument which measure the
brightness of an object Will measure the TOTAL
brightness of an object, which might be difficult
to interpret. However, when combined with
filters, can be used to measure the amount of
light produced over a narrow range of
frequencies. This can be compared with standard
Blackbody radiation curves to determine the
temperature of the object
Intensity
X
Wavelength
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Temperature of a Star Photometry Revisited
Photometer An instrument which measure the
brightness of an object Will measure the TOTAL
brightness of an object, which might be difficult
to interpret. However, when combined with
filters, can be used to measure the amount of
light produced over a narrow range of
frequencies. This can be compared with standard
Blackbody radiation curves to determine the
temperature of the object
Temperature of object is 7000 K
Intensity
X
Wavelength
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Temperature of a Star Photometry Revisited
Different typical filters used B (blue) Filter
380 480 nm V (visual) filter 490 590 nm
(range of highest sensitivity of the eye) U
(ultraviolet) filter near ultraviolet
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Stellar Magnitude (brightness) Magnitude is the
degree of brightness of a star. In 1856, British
astronomer Norman Pogson proposed a quantitative
scale of stellar magnitudes, which was adopted by
the astronomical community. Each increment in
magnitude corresponds to an increase in the
amount of energy by 2.512, approximately. A fifth
magnitude star is 2.512 times as bright as a
sixth, and a fourth magnitude star is 6.310 times
as bright as a sixth, and so on. Originally,
Hipparchus defined the magnitude scale of stars
by ranking stars on a scale of 1 through 6, with
1 being the brightest and six the dimmest. Using
modern tools, it was determined that the range of
brightness spanned a range of 100, that is, the
magnitude 1 stars were 100 times brighter than
magnitude 6. Therefore, each change in magnitude
corresponds to a factor of 2.512 change in
brightness, since (2.512)5 100 (to within
roundoff)
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Stellar Magnitude (brightness) The naked eye,
upon optimum conditions, can see down to around
the sixth magnitude, that is 6. Under Pogson's
system, a few of the brighter stars now have
negative magnitudes. For example, Sirius is 1.5.
The lower the magnitude number, the brighter the
object. The full moon has a magnitude of about
12.5, and the sun is a bright 26.51!
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Stellar Magnitude (brightness)
Star Magnitude How Much Brighterthan a Sixth Magnitude Star Logarithmic scale of2.512 X between magnitude levels Starting at Sixth Magnitude
1 100 Times 2.51 x 2.51 x 2.51 x 2.51 x 2.51
2 39.8 Times 2.51 x 2.51 x 2.51 x 2.51
3 15.8 Times 2.51 x 2.51 x 2.51
4 6.3 Times 2.51 x 2.51
5 2.51 Times 2.51 x
6    
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Stellar Magnitude (brightness)
Star Magnitude Table Showing How Much
DimmerOther Magnitudes are as Compared to a -1
Magnitude Star
Star Magnitude How Much Dimmerthan a -1 Magnitude Star How Much Dimmerthan a -1 Magnitude Star
-1    
0 1/2.51 0.398
1 1/6.31 0.158
2 1/15 0.063
3 1/39 0.0251
4 1/100 0.0100
5 1/251 0.00398
6 1/630 0.00158
7 1/1,584 0.000630
8 1/3,981 0.000251
9 1/10,000 0.000100
10 1/25,118 0.0000398
11 1/63,095 0.0000158
12 1/158,489 0.00000631
13 1/398,107 0.00000251
14 1/1,000,000 0.00000100
15 1/2,511,886 0.000000398
16 1/6,309,573 0.000000158
17 1/15,848,931 0.000000063
18 1/39,810,717 0.000000025
19 1/100,000,000 0.000000010
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Stellar Radii
Stefans Law
Power Emitted per unit Area ? T4

• 5.67 x 10-8 W / m2 K4
  (Stefan-Boltzmann constant)
• Note The power in this expression is the stars
  luminosity

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Stellar Radii
Stefans Law
Power Emitted per unit Area ? T4
Once the absolute luminosity and temperature is
measured, the stars radius can be calculated.
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Stellar Classifications
Star Type Color Approximate Surface Temperature Average Mass (The Sun 1) Average Radius (The Sun 1) Average Luminosity (The Sun 1) Main Characteristics Examples
O Blue over 25,000 K 60 15 1,400,000 Singly ionized helium lines (H I) either in emission or absorption. Strong UV continuum. 10 Lacertra
B Blue 11,000 - 25,000 K 18 7 20,000 Neutral helium lines (H II) in absorption. RigelSpica
A Blue 7,500 - 11,000 K 3.2 2.5 80 Hydrogen (H) lines strongest for A0 stars, decreasing for other A's. Sirius, Vega
F Blue to White 6,000 - 7,500 K 1.7 1.3 6 Ca II absorption. Metallic lines become noticeable. Canopus, Procyon
G White to Yellow 5,000 - 6,000 K 1.1 1.1 1.2 Absorption lines of neutral metallic atoms and ions (e.g. once-ionized calcium). Sun, Capella
K Orange to Red 3,500 - 5,000 K 0.8 0.9 0.4 Metallic lines, some blue continuum. Arcturus, Aldebaran
M Red under 3,500 K 0.3 0.4 0.04(very faint) Some molecular bands of titanium oxide. Betelgeuse, Antares
Spectral Classes

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Stellar Classifications
Stellar Spectral Types Stars can be classified
by their surface temperatures as determined from
Wien's Displacement Law, but this poses practical
difficulties for distant stars. Spectral
characteristics offer a way to classify stars
which gives information about temperature in a
different way - particular absorption lines can
be observed only for a certain range of
temperatures because only in that range are the
involved atomic energy levels populated. The
standard classes are Type Temperature O
30,000 - 60,000 K Blue stars B 10,000 - 30,000
K Blue-white stars A 7,500 - 10,000 K White
stars F 6,000 - 7,500 K Yellow-white stars G
5,000 - 6,000 K Yellow stars (like the Sun) K
3,500 - 5,000K Yellow-orange stars M lt 3,500 K
Red stars The commonly used mnemonic for the
sequence of these classifications is "Oh Be A
Fine Girl, Kiss Me".
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O-Type Stars The spectra of O-Type stars shows
the presence of hydrogen and helium. At these
temperatures most of the hydrogen is ionized, so
the hydrogen lines are weak. Both HeI and HeII
(singly ionized helium) are seen in the higher
temperature examples. The radiation from O5
stars is so intense that it can ionize hydrogen
over a volume of space 1000 light years across.
One example is the luminous H II region
surrounding star cluster M16. O-Type stars are
very massive and evolve more rapidly than
low-mass stars because they develop the necessary
central pressures and temperatures for hydrogen
fusion sooner. Because of their early
development, the O-Type stars are already
luminous in the huge hydrogen and helium clouds
in which lower mass stars are forming. They light
the stellar nurseries with ultraviolet light and
cause the clouds to glow in some of the dramatic
nebulae associated with the H II region
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                               CLASS O DARK BLUE
                               TEMPERATURE  28,000 - 50,000K
                               COMPOSITION  Ionized atoms, especially helium
                               EXAMPLE  Mintaka (01-3III)
                                  
                                  
     
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                               CLASS B BLUE
                               TEMPERATURE  10,000 - 28,000K
                               COMPOSITION  Neutral helium, some hydrogen
                               EXAMPLE  Alpha Eridani A (B3V-IV)
                                  
                                  
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                               CLASS A LIGHT BLUE
                               TEMPERATURE  7,500 - 10,000K
                               COMPOSITION  Strong hydrogen, some ionized  metals
                               EXAMPLE  Sirius A (A0-1V)
                                  
                                  
     
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                               CLASS F WHITE
                               TEMPERATURE  6,000 - 7,500K
                               COMPOSITION  Hydrogen and ionized metals,  calcium and iron
                               EXAMPLE  Procyon A (F5V-IV)
                                  
                                  
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                               CLASS G YELLOW
                               TEMPERATURE  5,000 - 6,000K
                               COMPOSITION  Ionized Calcium, both neutral and ionized metals
                               EXAMPLE  Sol (G2V)
                                  
                                  
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                               CLASS K ORANGE
                               TEMPERATURE  3,000 - 5,000K
                               COMPOSITION  Neutral Metals
                               EXAMPLE  Alpha Centauri (K0-3V)
                                  
                                  
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                               CLASS M RED
                               TEMPERATURE  2,500 - 3,500K
                               COMPOSITION  Ionized atoms, especially helium
                               EXAMPLE  Wolf 359 (M5-8V)
                                  
                                  
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Each Spectral class is divided into 10
subclasses, ranging from 0 (hottest) to 9
(coolest). Stars are also divided into six
categories according to luminosity 1a (most
luminous supergiants), 1b (less luminous
supergiants), II (luminous giants), III (normal
giants, IV (subgiants), and V (main sequence and
dwarfs). For instance, Sol is classified as a
G2V, which means that it is a relatively hot
G-classed main sequence star.