Download Star Basics

Stars !
Common Name
Scientific Name
Sun
Distance (light years)
Apparent Magnitude
Absolute Magnitude
Spectral Type
-
-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
6.0
9.54
13.2
M3.8V
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
10.8
3.73
6.1
K2Vc
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
11.2
12.18
14.5
Barnard's Star
Wolf 359
Epsilon Eri
Ross 128
Luyten 789-6
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
Measurement of Distances to Nearby Stars
Parallax Revisited
Parallax Angle
R

d
Tan  =
R
d
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
Measurement of Distances to Nearby Stars
Parallax Revisited
Parallax Angle
R

d
 (radians) =
R
d
For astronomical measurements R and d are measured in A.U.
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.)
d (in A.U.) =
=
 (radians)
R (in A.U.)
206265 R (in A.U.)
=
 (arc seconds)
 (arc seconds)
206265
Measurement of Distances to Nearby Stars
Parallax Revisited
One parsec is defined to be 206265 A.U.
1
d (in parsecs) =
 (arc seconds)
Measurement of Speeds of Nearby Stars
Radial Speed – Doppler Shift Revisited
Blue Shift toward Earth
Red Shift away from Earth
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.
Measurement of Speeds of Nearby Stars
Astrophysics and Cosmology
Radial Speed – Doppler Shift Revisited
Longer ,
lower f
Shorter ,
higher f
In general
Apparent Wavelength
True Wavelength
=
True Frequency
Apparent Frequency
=
Velocity of Source
1+
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.
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)
Measurement of Speeds of Nearby Stars
Transverse (sideways) Speeds
Measurement
made same time
during the year

w
d
 (radians) =
w
d
w = d x  (radians)
If the time interval between measurements is measured, then v = w/ t
Measurement of Speeds of Nearby Stars
v
vt
vR
Pythagorian Theorem:
v2 = vR2 + vt2
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
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 it’s surface. This is an intrinsic property of the star.
Apparent Brightness: The power per square meter as measured at the
location of the earth.
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
Luminosity (brightness) of a Star
Apparent Brightness 
Power (or Luminosity)
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.
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 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
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
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
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
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)
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!
Stellar Magnitude (brightness)
Star
Magnitude
How Much Brighter
than a Sixth Magnitude
Star
Logarithmic scale of
2.512 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
Stellar Magnitude (brightness)
Star Magnitude Table Showing How Much Dimmer
Other Magnitudes are as Compared to a -1 Magnitude Star
How Much Dimmer
than a -1 Magnitude Star
How Much Dimmer
than a -1 Magnitude Star
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
Star Magnitude
-1
Stellar Radii
Stefan’s 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 star’s luminosity
Stellar Radii
Stefan’s Law
Power Emitted per unit Area =  T4
Once the absolute luminosity and temperature is measured, the star’s radius
can be calculated.
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
Spectral Classes
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.
Rigel
Spica
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,
Aldebara
n
M
Red
under 3,500 K
0.3
0.4
0.04
(very faint)
Some molecular bands of titanium
oxide.
Betelgeus
e, Antares
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
B
A
F
G
K
M
30,000 - 60,000 K Blue stars
10,000 - 30,000 K Blue-white stars
7,500 - 10,000 K White stars
6,000 - 7,500 K Yellow-white stars
5,000 - 6,000 K Yellow stars (like the Sun)
3,500 - 5,000K Yellow-orange stars
< 3,500 K Red stars
The commonly used mnemonic for the sequence of these classifications is
"Oh Be A Fine Girl, Kiss Me".
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 lowmass 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
CLASS O
DARK BLUE
TEMPERATURE
28,000 - 50,000°K
COMPOSITION
Ionized atoms,
especially helium
EXAMPLE
Mintaka (01-3III)
CLASS B
BLUE
TEMPERATURE
10,000 - 28,000°K
COMPOSITION
Neutral helium, some
hydrogen
EXAMPLE
Alpha Eridani A (B3V-IV)
CLASS A
LIGHT BLUE
TEMPERATURE
7,500 - 10,000°K
COMPOSITION
Strong hydrogen, some
ionized metals
EXAMPLE
Sirius A (A0-1V)
CLASS F
WHITE
TEMPERATURE
6,000 - 7,500°K
COMPOSITION
Hydrogen and ionized
metals, calcium and iron
EXAMPLE
Procyon A (F5V-IV)
CLASS G
YELLOW
TEMPERATURE
5,000 - 6,000°K
COMPOSITION
Ionized Calcium, both
neutral and ionized
metals
EXAMPLE
Sol (G2V)
CLASS K
ORANGE
TEMPERATURE
3,000 - 5,000°K
COMPOSITION
Neutral Metals
EXAMPLE
Alpha Centauri (K0-3V)
CLASS M
RED
TEMPERATURE
2,500 - 3,500°K
COMPOSITION
Ionized atoms,
especially helium
EXAMPLE
Wolf 359 (M5-8V)
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.