Minggu, 06 Februari 2011
About the Magnitude
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
Sabtu, 05 Februari 2011
Materi tentang Bintang bag. 2
A.2 SISTEM MAGNITUDO
READ MORE - Materi tentang Bintang bag. 2
Magnitudo yang telah dibicarakan sebelumnya diukur berdasarkan kepekaan mata yang berada pada daerah λ = 5500 Ao (cahaya kuning – hijau), karena itu magnitudonya disebut magnitudo visual (mv). Sejak akhir abad ke-19, fotografi mulai berkembangdan para astronom mulai menggunakannya untuk menentukan magnitudo bintang. Dibandingkan dengan mata, emulsi fotografi lebih peka pada cahaya biru – ungu yang berada pada daerah λ = 4500 Ao. Magnitudo yang diukur secara fotografi ini disebut magnitudo fotografi (mfot).
Pengukuran dengan magnitudo visual dan magnitudo fotografi untuk bintang yang sama akan terdapat perbedaan. Sebagai contoh kita bandingkan antara bintang Rigel dan bintang Betelgeuse yang berada di rasi Orion. Bintang Rigel berwarna biru dan bintang Betelgeuse berwarna merah. Jika diamati secara fotografi, bintang Rigel akan tampak lebih terang daripada jika diamati secara visual. Hal ini terjadi karena bintang Rigel yang berwarna biru akan lebih banyak memancarkan cahaya biru; dalam hal ini mv > mfot (makin terang suatu bintang, makin kecil magnitudonya). Sebaliknya, bintang Betelgeuse yang berwarna merah akan lebih terang jika diamati secara visual daripada secara fotografi, karena bintang ini lebih banyak memancarkan cahaya kuning daripada cahaya biru, dengan demikian mv < mfot.
Jadi pada umumnya, untuk setiap bintang, harga mv berbeda dengan mfot. Selisih kedua magnitudo itu, yaitu magnitudo fotografi dikurangi magnitudo visual, disebut Indeks Warna (mfot – mv). Makin panas atau biru suatu bintang, makin kecil indeks warnanya. Karena adanya selisih ini, perlu diadakan pembakuan titik nol kedua magnitudo tersebut. Dari persamaan (3) diperoleh :
mv = -2,5 log Ev + Cv
mfot = -2,5 log Efot + Cfot (5)
dengan Cv dan Cfot adalah konstanta (tetapan). Tetapan Cv dan Cfot ini dapat diambil sedemikian rupa sehingga
mv = mfot (6)
Untuk memenuhi kondisi ini, digunakan bintang Vega (kelas spektrum AOV) sebagai bintang standar. Dengan demikian indeks warna bintang Vega adalah nol.
( mfot - mv )vega = 0
Berdasarkan definisi ini, untuk bintang yang lebih biru dari Vega, indeks warnanya akan negatif. Sebagai contoh, magnitudo fotografi bintang Rigel mfot = -0,03 dan magnitudo visualnya adalah mv = 0,14, jadi indeks warna bintang Rigel adalah mfot - mv = -0,17. Sebaliknya untuk bintang yang lebih merah dari Vega, indeks warnanya negatif. Sebagai contoh, bintang Betelgeuse yang mempunyai magnitudo fotografi mfot = 2,14 dan mv = 0,70, sehingga indeks warnanya adalah mfot - mv = 1,44.
Pada awalnya, pelat fotografi hanya peka pada cahaya biru–ungu. Dengan makin berkembangnya fotografi, maka dapat dibuat pelat potret yang peka pada daerah panjang gelombang lainnya, seperti daerah kuning, merah, bahkan inframerah. Kemudian, dengan mengkombinasikan pelat potret dengan filter, dapat diperoleh berbagai sistem magnitudo.
Pada tahun 1951, H.L. Johnson dan W.W. Morgan memperkenalkan sistem magnitudo yang disebut sistem UBV, yaitu :
U = magnitudo semu dalam daerah ungu (mU) dengan λef = 3600 Ao
B = magnitudo semu dalam daerah biru (mB) dengan λef = 4300 Ao
V = magnitudo semu dalam daerah visual (mV) dengan λef = 5500 Ao
Gambar 1. Daerah Kepekaan Pengukuran Magnitudo
Dalam sistem ini, indeks warnanya adalah U – B dan B – V. untuk bintang standar Vega, U = B = V atau U – B = B – V = 0. Dengan demikian, bintang yang mempunyai harga (B – V) > 0, maka bintang tersebut labih merah dari Vega, sebaliknya, bintang dengan (B – V) < 0, akan lebih biru dari Vega. Gambar 1. Menunjukkan fungsi kepekaan untuk sistem magnitudo UBV.
Dewasa ini pengamatan fotometri tidak lagi menggunakan pelat film, tetapi dilakukan dengan menggunakan kamera CCD (digital), sehingga untuk menentukan bermacam-macam sistem magnitudo hanya ditentukan oleh filter yang digunakannya saja. Selain sistem magnitudo yang dibicarakan diatas, masih ada lagi sistem magnitudo lainnya seperti yang diperlihatkan dalam Tabel. 2.
Tabel. 2 Berbagai Sistem Magnitudo
yang sering digunakan dalam penelitian Astronomi
Sistem Magnitudo | Magnitudo | Warna | λ Efektif (Ao) | Lebar Pita (Ao) |
Sistem UGR dari Becker | U | Ultraviolet | 3.690 | 500 – 700 |
G | Hijau | 4.680 | ||
R | Merah | 6.380 | ||
Sistem UBV dari Johnson dan Morgan | U | Ultraviolet | 3.500 | 800 – 1000 |
B | Biru | 4.350 | ||
V | Kuning | 5.550 | ||
Sistem Stromgren (Sistem ubvy) | u | Ultraviolet | 3.500 | ~ 200 |
v | Violet | 4.100 | ||
b | Biru | 4.670 | ||
y | Hijau | 5.470 | ||
Sistem Stebbins dan Withford | U | Ultraviolet | 3.550 | 600 - 1500 |
V | Violet | 4.200 | ||
B | Biru | 4.900 | ||
G | Hijau | 5.700 | ||
R | Merah | 7.200 | ||
I | Inframerah | 10.300 |
Di sadur dari buku astronomi 2009.
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