Star Properties. Apparent Magnitude System of Hipparchus Group of brightest stars1 m Stars about ½...
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Transcript of Star Properties. Apparent Magnitude System of Hipparchus Group of brightest stars1 m Stars about ½...
Star Properties
Apparent Magnitude
System of Hipparchus Group of brightest stars 1m
Stars about ½ as bright as 1m 2m
Stars about ½ as bright as 2m 3m
•
•
•
Naked Eye Limit 6m
Apparent Magnitude
19th century photographers learn how eye responds to light (Pogson) Doubling the brightness is not perceived as a
doubling by the eye Eye response is logarithmic
Ratio of 100 in brightness corresponds to a Difference of five magnitudes m of 5 100X in light m of 1 2.512X in light
Some Apparent Magnitudes
Sun -26.8 Full Moon -12.6 Venus at brightest -4.4 Sirius -1.5 Naked Eye Limit 6.0 Faintest Objects +30.0
Hubble
Learning the Brightness
Is a star bright... Because it really is a bright star? Because it is close to the Earth?
Stellar brightness depends on Luminosity Distance
Measuring Distance
Stellar Parallax
June
January
Sun
Stellar Parallax
June
January
Sun
1 AU
Parallax
Measuring Parallax
1 AU
1 parsec
1 arcsec
Stellar Parallax
pd
1
When p is measured in arcsecand d is measured in parsecs
One parsec:
206,265 AU
3.26 light years
Stellar Parallax
Nearest star to Sun (largest parallax) Cen p = 0.7 arcsec
Limit of accurate parallax 200 pcs (angles of 0.005 arcsec) Hipparcos satellite (120,000 stars measured to
0.001 arcsec)
Absolute Magnitude
The magnitude a star would have at 10 parsecs from the Sun.
The apparent (m) and absolute (M) magnitudes of a star at 10 pcs are the same.
M, m, and d are related. Knowing two allows you to compute the third.
Putting the Pieces into Place
Ejnar Hertsprung1911
Henry Norris Russell1913
Luminosity Classes
I Supergiants
II Bright Giants
III Giants
IV Subgiants
V Dwarfs
Luminosity Class implies Size
Consider the Sun and Capella
The SunG2V M=5 Capella
G2III M=0
Luminosity Class implies Size
Equal sized pieces of each star are equally bright
Capella is 100X brighter (5 magnitudes) Capella must have 100X as much area Surface area radius2
Capella must be 10X larger than Sun.
Luminosity Class in the Spectrum
A3Supergiant
A3Giant
A3Dwarf
Sun G2V
Vega A1V
Betelgeuse M1I
Which of these stars is hottest?
1. Sun G2V
2. Vega A1V
3. Betelgeuse M1I
4. Can’t compare
Which of these stars is brightest?
1. Sun G2V
2. Vega A1V
3. Betelgeuse M1I
4. Can’t compare
Which of these stars is smallest?
1. Sun G2V
2. Vega A1V
3. Betelgeuse M1I
4. Can’t compare
Which of these stars is most distant?
1. Sun G2V
2. Vega A1V
3. Betelgeuse M1I
4. Can’t compare
Spectroscopic Parallax
• Observe the spectrum and apparent magnitude of a star
• Classify the spectrum
• Plot it on the H-R Diagram
• Read off the M
• From m and M compute distance
Main Sequence
Color Index
0.00
0.50
1.00
1.50
2.00
2.50
3.00
0 500 1000
Wavelength (nm)
Re
lativ
e E
nerg
y
B V
12000 K
7000 K
*
*
* *
Color Index
Star Temperature mB mV .
1 12000 K 2.0 2.4
2 7000 K 3.0 3.1
Color Index = mB - mV = B-V
1 B-V = 2.0 - 2.4 = -0.4
2 B-V = 3.0 - 3.1 = -0.1
Spectroscopic Parallax
Can now get distances to any object whose spectrum can be measured. Limit 5000 pcs
Study Tools
Review 1 Review 2
The Advantage of Color Index
Measures temperature just like Spectral Type Much easier to obtain
requires two measurements of brightness spectral type requires getting the spectrum
Color-Magnitude Diagrams
M
Spectral Type
Standard H-R Diagram
mV
B-V
Color-Magnitude Diagram
Color-Magnitude Diagrams
Useful for star clusters Can substitute mV for MV since you know all the
stars are the same distance away. Star Clusters
Open (galactic) Globular
Structure of the Milky Way
Open Clusters
Irregular shape Few tens to few hundred stars In the plane of the galaxy Young stars
Open clusters
M16
M45
M37
Color-Magnitude DiagramM45
0
2
4
6
8
10
12
14
16
18
-0.5 0 0.5 1 1.5 2B-V
mV
Globular Clusters
Spherical in shape Hundreds of thousands of stars Halo distribution about galactic nucleus Old stars
Globular Clusters
M5 M3
SFA Observatory
Color-Magnitude DiagramM3