A.2 Magnitude System
Magnitude that have been discussed above is measured by the sensitivity of the eye that is within the range λ = 5500 Ao (light yellow - green), so-called magnitude visual magnitude (mv). Since the end of the 19th century, photography began flourish astronomers began to use it to determine the magnitude stars.Compared with the eye, photographic emulsions are more sensitive to blue light - purple which is at λ = 4500 Ao region. Photographic magnitudes are measured is called the magnitude of photography (mfot).
Magnitude that have been discussed above is measured by the sensitivity of the eye that is within the range λ = 5500 Ao (light yellow - green), so-called magnitude visual magnitude (mv). Since the end of the 19th century, photography began flourish astronomers began to use it to determine the magnitude stars.Compared with the eye, photographic emulsions are more sensitive to blue light - purple which is at λ = 4500 Ao region. Photographic magnitudes are measured is called the magnitude of photography (mfot).
Measurement with visual magnitude and the magnitude of photography for the same star will have differences. As an example we compare between the stars Rigel and Betelgeuse star in the constellation Orion. Star Rigel is blue and the red star Betelgeuse. If observed by photography, star Rigel would appear much brighter than when observed visually. This happens because the blue star Rigel will emit more blue light, in this case mv > mfot (the brighter the star, the smaller magnitude). Conversely, the red star Betelgeuse will be more bright if observed visually rather than in photography, because the star emits more yellow light than blue light, thereby mv < mfot. So in general, for every star, mv prices vary with mfot. The second difference magnitude, namely photographic magnitudes reduced visual magnitude, called the Color Index (mfot - mv). The more heat or a blue star, the smaller the color index.Because of this difference, there should be standardization of both magnitudes are zero. From equation (3) is obtained:
mv = -2.5 log Ev + Cv
mfot = -2.5 log Efot + Cfot (5)
mv = -2.5 log Ev + Cv
mfot = -2.5 log Efot + Cfot (5)
with Cv and Cfot is constant (constant). Cfot constant Cv and this can be taken in such a way that
mv = mfot (6)
mv = mfot (6)
To meet this condition, use the star Vega (spectral class AOV) as a standard star.Thus the color of the star Vega index is zero.
(Mfot - mv) vega = 0
Based on this definition, for a more blue stars of Vega, the color index would be negative. For example, photographic magnitude star Rigel mfot = -0.03 and its visual magnitude is mv = 0.14, so the color index of star Rigel is mfot - mv = -0.17. In contrast to the more red stars of Vega, a negative color index. For example, Betelgeuse star who has a photographic magnitude mfot = 2.14 and mv = 0.70, so the color index is mfot - mv = 1.44.
Initially, only photographic plates sensitive to blue-violet light. With the development of photography, it can be a sensitive portrait plates in other wavelength regions, such as the yellow, red and even infrared. Then, by combining portraits with a filter plate, can be obtained various magnitude systems.
In 1951, H.L. Johnson and W.W. Morgan introduced a system called the system UBV magnitudes, namely:
U = magnitude apparent in the purple area (mU) with λef = 3600 Ao
B = apparent magnitude in the blue area (mB) with λef = 4300 Ao
V = apparent magnitude in the visual area (mV) by Ao λef = 5500
Figure 1. Regional magnitude Measurement Sensitivity
In this system, the color index U - B and B - V. for the standard star Vega, U = B = V or U - B = B - V = 0. Thus, stars that have a price (B - V) > 0, then the red labih star of Vega, by contrast, stars with (B - V) < 0, will be more blue than Vega. Figure 1.Showing sensitivity function for magnitudes UBV system.
Today photometry observations no longer use film plate, but performed using a CCD camera (digital), so as to determine the magnitude of the various systems is determined by the filter only uses only. In addition to magnitude system discussed above, there's more other magnitude systems as shown in Table. 2.
(Mfot - mv) vega = 0
Based on this definition, for a more blue stars of Vega, the color index would be negative. For example, photographic magnitude star Rigel mfot = -0.03 and its visual magnitude is mv = 0.14, so the color index of star Rigel is mfot - mv = -0.17. In contrast to the more red stars of Vega, a negative color index. For example, Betelgeuse star who has a photographic magnitude mfot = 2.14 and mv = 0.70, so the color index is mfot - mv = 1.44.
Initially, only photographic plates sensitive to blue-violet light. With the development of photography, it can be a sensitive portrait plates in other wavelength regions, such as the yellow, red and even infrared. Then, by combining portraits with a filter plate, can be obtained various magnitude systems.
In 1951, H.L. Johnson and W.W. Morgan introduced a system called the system UBV magnitudes, namely:
U = magnitude apparent in the purple area (mU) with λef = 3600 Ao
B = apparent magnitude in the blue area (mB) with λef = 4300 Ao
V = apparent magnitude in the visual area (mV) by Ao λef = 5500
Figure 1. Regional magnitude Measurement Sensitivity
In this system, the color index U - B and B - V. for the standard star Vega, U = B = V or U - B = B - V = 0. Thus, stars that have a price (B - V) > 0, then the red labih star of Vega, by contrast, stars with (B - V) < 0, will be more blue than Vega. Figure 1.Showing sensitivity function for magnitudes UBV system.
Today photometry observations no longer use film plate, but performed using a CCD camera (digital), so as to determine the magnitude of the various systems is determined by the filter only uses only. In addition to magnitude system discussed above, there's more other magnitude systems as shown in Table. 2.
Table. 2 Various magnitude System
frequently used in the study of Astronomy
Magnitude System | Magnitude | Colour | Effective λ (Ao) | Ribbon Width (Ao) |
UGR system from Becker | U | Ultraviolet | 3.690 | 500 – 700 |
G | Green | 4.680 | ||
R | Red | 6.380 | ||
Sistem UBV dari Johnson dan Morgan | U | Ultraviolet | 3.500 | 800 – 1000 |
B | Blue | 4.350 | ||
V | Yellow | 5.550 | ||
Sistem Stromgren (Sistem ubvy) | u | Ultraviolet | 3.500 | ~ 200 |
v | Violet | 4.100 | ||
b | Blue | 4.670 | ||
y | Green | 5.470 | ||
Sistem Stebbins dan Withford | U | Ultraviolet | 3.550 | 600 - 1500 |
V | Violet | 4.200 | ||
B | Blue | 4.900 | ||
G | Green | 5.700 | ||
R | Red | 7.200 | ||
I | Infrared | 10.300 |
From Astronomi 2009 book
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