Ketika kita menatap bintang di langit malam, kita dihadapkan pada kubah raksasa yang disebut bola langit. Orang yunani kuno membagi bola langit ini ke dalam daerah-daerah yang disebut rasi. Sampai saat ini diketahui ada 88 rasi bintang. Nama-nama rasi ini kebanyakan bersumber dari mitologi Yunani seperti Canis Major, Ursa Minor, Scorpio, dan Orion.
Ada banyak cara dalam penamaan bintang di antaranya dengan memberi nama dari bahasa Yunani (Scorpio, Crux, Ophiucus, Aquarius, Orion), penamaan berdasarkan rasi tempat bidang tersebut berada (contoh : Alpha Centauri berarti bintang paling terang pada konstelasi Centauri, bintang kedua paling terang disebut Beta, dan seterusnya), dan penamaan berdasarkan nomor urutnya dalam katalog atau cara modern (contoh : NGC 6205).
Salah satu cara pengklasifikasian bintang adalah berdasarkan suhunya dan kemiripan susunan garis spektrumnya. Ada beberapa versi pengklasifikasian bintang, berikut pengklasifikasian bintang menurut Angelo Secchi (1863):
1. Kelas spektra O
Berwarna biru, temperatur > 30.000 K, garis-garis He terionisasi, garis N terionisasi 2x, garis Si terionisasi 3x, garis H tampak tapi lemah.
Contoh bintang : Alnitak, Bintang 10 Lacerta.
2. Kelas spektra B
Berwarna biru, temperatur 11.000 – 30.000 K, garis He netral, garis Si terionisasi 1 atau 2 x, garis O terionisasi, garis H tampak lebih jelas ketimbang kelas O.
Contoh bintang : Rigel, Spica.
3. Kelas spektra A
Berwarna biru, temperatur 7.500 – 11.000 K, garis H sangat kuat, garis Mg, Si, Fe, dan Ca terionisasi 1x, garis logam netral tampak lemah.
Contoh bintang : Sirius, Vega.
4. Kelas spectra F
Berwarna biru keputih-putihan, temperatur 6.000 – 7.500 K, garis H lebih lemah dari kelas A, garis Ca, Fe, Cr terionisasi 1x, garis Fe dan Cr netral.
Contoh bintang : Canopus, Procyon.
5. Kelas spectra G
Berwarna putih kekuning-kuningan, temperatur 5.000 – 6.000 K, garis H lebih lemah, garis Ca terionisasi, pita molekul G-Band sangat kuat.
Contoh bintang : Capella, Matahari.
6. Kelas spectra K
Berwarna jingga kemerah-merahan, temperatur 3.500 – 5.000 K, garis H sangat lemah, garis logam netral mendominasi, Pita Titanium Oksida tampak.
Contoh bintang : Arcturus, Aldebaran.
7. Kelas spectra M
Berwarna merah, temperature 2.500 – 3.000 K, pita molekul Titanium Oksida sangat mendominasi, garis logam netral tampak dengan jelas..
Contoh bintang : Betelgeuse, Antares.
Selain penggolongan kelas spectra O-B-A-F-G-K-M, ada juga yang mengklasifikasikan ke dalam kelas W-O-B-A-F-G-K-M-R-N-S. Untuk mudah mengingatnya, bisa menggunakan jembatan keledai Wow-Oh-Be-A-Fine-Girl-Kiss-Me-Right-Now-Sweetie. Dari situ terlihat bahwa bintang yang paling panas warnanya justru biru, bukan merah. Semakin merah suatu bintang, maka semakin dingin suhunya.
sources: http://www.forumsains.com
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