Galaxy part 1

B. Galaxy

B.1 Galaxy In Universe
       In a universe of thousands of galaxies spread with a very large distance and large respectively as well. Our galaxy, which is where our sun as a member, called the Milky Way. In English is called Milky Way. The diameter of the Milky Way galaxy 80,000 light years away (one light second = 300,000 km, which means that light propagating in one second and a distance of 300,000 km). The nearest galaxy with the Milky Way is the Magellan Clouds (Magellanic Clouds). The distance that the two galaxy light 160,000 years. In the universe there are thousands of galaxies spread. May the Lord step that the All-Knowing, how vast the universe.
       By mapping the haze of gas and bright main sequence stars (O stars and B) and by observing the direction and motion of a large number of O and B stars, as well as the distribution of rotational motion of the nebula and galaxies at various distances from its center, it can be concluded that the distribution of stars in we are a large disc-shaped galaxy that contains flattened about 100,000 light-years away and the sun is about 30,000 light years from the galactic center. Lamellar structure of this galaxy is caused by rotational motion and the sun moves around the center of our galaxy with a speed of 250 km / s and with a period 200 million years.
       The main part of the galaxy composed of stars that form a relatively flat disc with cembungan at its center called the nucleus of the galaxy. The stars in this galaxy's core distance is close to one another. Extends out from the core there is disk like a spiral galaxy that churned out from the galactic core called the spiral arms. Figure 1 shows a chart of the Milky Way galaxy 100,000 light-years in diameter and the sun is at one of the spiral arms in the position of 30 000 light years from the galactic center.Spiral arm is made up of cosmic clouds of gas and dust that many called the interstellar medium. Along with gas and dust clouds have a lot of young stars, some of which are very hot and very bright. In a cloud of interstellar gas and dust in spiral arms is still the formation of new stars.
           Besides, the galaxy contains numerous star clusters (star cluster), a group of stars that may have the same origin and age. There are two types of star clusters namely open clusters and globular clusters. The most common and the number of thousands is an open group or also with a group called galaktika, which usually consists of several hundred stars that are not so strongly bound together by gravity in space. Open cluster is located on the main disk galaxy and usually located in or near spiral arms. Globular clusters form a system of hundreds of speroidal layering this dish, and each cluster contains hundreds of thousands of stars. Besides that there are not any stars to be members of the group, but the number far exceeds the star cluster and the entire region is at the star cluster. Any stars and star clusters form a so-called halo galaktika. Hello This volume covers an area which exceeds several times larger than the main disk galaxy. Galaktika radio emission suggests that the halo contains gas, and the x-ray radiation from the gas. This indicates that the gas is very hot in the order of 10⁶ K. This hot gas extends only to the inside of this halo called the corona and galactic.
            The closer towards the galactic core, especially near the middle core cembungan disk galaxies, stars and star clusters of the halo is more tightly than in areas far from the galactic core. Halo has a thickness of at least 100 000 TC, and even in the direction of the galactic plane in diameter can be up to two or three hundred thousand light years, and extends far beyond the circumference of the main disk galaxy. among the stars of its members.

Figure 1. Part Milky Way with the sun at one point.

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Galaksi bag. 1

B. Galaksi

B.1  Galaksi Dalam Jagat Raya

       Dalam jagat raya tersebar ribuan galaksi dengan jarak yang sangat besar dan masing-masing berukuran besar pula. Galaksi kita, yaitu tempat matahari kita sebagai anggotanya, dinamakan Galaksi Bima Sakti. Dalam bahasa Inggrisnya dinamakan Milky Way. Diameter galaksi bima sakti 80.000 tahun cahaya (satu detik cahaya = 300.000 km, artinya dalam satu detik cahaya merambat dan menempuh jarak 300.000 km). Galaksi yang terdekat dengan Bima Sakti adalah Awan Magellan (Magellanic Clouds). Jarak kedua galaksi itu 160.000 tahun cahaya. Di dalam jagat raya terdapat ribuan galaksi yang tersebar. Kiranya Tuhan jualah yang Maha Mengetahui, betapa luas jagat raya ini.

       Dengan memetakan kabut gas dan bintang terang deret utama (bintang O dan B) dan dengan mengamati arah dan gerak sejumlah besar bintang O dan B, serta distribusi nebula dan gerak rotasi galaksi pada berbagai jarak dari pusatnya, maka dapat disimpulkan bahwa sebaran bintang-bintang dalam galaksi kita berbentuk piringan besar yang pipih berisikan sekitar 100.000 tahun cahaya dan matahari berada sekitar 30.000 tahun cahaya dari pusat galaksi. Bentuk pipih galaksi ini disebabkan oleh gerak rotasinya dan matahari kita bergerak mengitari pusat galaksi dengan laju 250 km/s dan dengan periode 200 juta tahun .

       Bagian utama dari galaksi tersusun dari bintang-bintang yang membentuk piringan yang relative pipih dengan cembungan pada bagian tengahnya yang dinamakan inti galaksi. Bintang-bintang pada inti galaksi ini jaraknya dekat satu dengan yang lainnya. Melebar keluar dari inti galaksi terdapat piringan seperti spiral yang melilit keluar dari inti yang galaksi yang dinamakan lengan spiral. Gambar 1 memperlihatkan bagan galaksi bima sakti dengan diameter 100.000 tahun cahaya dan matahari berada pada salah satu lengan spiral pada kedudukan 30.000 tahun cahaya dari pusat galaksi. Lengan spiral ini tersusun dari awan gas dan debu kosmik yang banyak sekali yang disebut medium antar bintang. Bersamaan dengan awan gas dan debu ini terdapat banyak sekali bintang muda, beberapa diantaranya sangat panas dan sangat terang. Pada awan gas dan debu antar bintang pada lengan spiral ini masih terjadi pembentukan bintang –bintang baru.

           Disamping itu galaksi berisikan banyak sekali gugus bintang (star cluster), suatu kelompok bintang yang mungkin memiliki asal dan usia yang sama. Ada dua jenis gugus bintang yaitu gugus terbuka dan gugus bola. Yang paling umum dan jumlahnya ribuan adalah gugus terbuka atau yang disebut pula dengan gugus galaktika, yang biasanya terdiri dari beberapa ratus bintang yang terikat tidak begitu kuat oleh grafitasi bersama dalam ruang. Gugus terbuka terletak pada piringan utama galaksi dan biasanya berada dalam atau dekat dengan lengan spiral. Gugus bola yang jumlahnya ratusan membentuk sistem speroidal melapis piringan ini, dan masing-masing gugus berisikan ratusan ribu anggota bintang. Disamping itu terdapat bintang sembarang yang bukan menjadi anggota gugus, tetapi jumlahnya jauh melebihi gugus bintang dan berada pada seluruh daerah bintang gugus tersebut. Bintang sembarang dan bintang gugus ini membentuk apa yang dinamakan halo galaktika. Halo ini meliputi wilayah yang volumenya melampaui beberapa kali lebih luas dari piringan utama galaksi. Emisi radio galaktika mengisyaratkan bahwa halo ini berisikan gas, dan adanya radiasi sinar-x dari gas tersebut. Hal ini menandakan bahwa gas itu sangat panas dalam orde 10⁶ K. gas panas ini hanya terbentang pada bagian dalam dari halo ini dan dinamakan korona galaktik.

            Makin dekat kearah inti galaksi terutama dekat cembungan inti ditengah piringan galaksi, bintang dan gugus bintang dari halo ini lebih rapat dibandingkan dengan pada daerah yang jauh dari inti galaksi. Halo memiliki ketebalan paling tidak 100.000 TC dan bahkan dalam arah bidang galaksi diameternya bisa sampai dua atau tiga ratus ribu tahun cahaya, dan terbentang jauh diluar lingkar piringan utama galaksi. di antara bintang-bintang anggotanya.

Gambar 1. Bagian Galaksi Bima Sakti

Di sadur dari buku astronomi 2009

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Kumpulan Foto 2 (Photo Collection 2)

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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 A^o (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 A^o 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)

with Cv and Cfot is constant (constant). Cfot constant Cv and this can be taken in such a way that
             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 A^o
            B = apparent magnitude in the blue area (mB) with λef = 4300 A^o
            V = apparent magnitude in the visual area (mV) by A^o 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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