Dasar - Dasar Spektroskopi Bintang

    Spektroskopi adalah suatu cabang ilmu dalam astronomi yang mempelajari spektrum benda langit. Dari spektrum suatu benda langit dapat kita peroleh informasi mengenai temperatur, kandungan/ komponen zat penyusunnya, kecepatan geraknya, dll. Oleh sebab itu, spektroskopi merupakan salah satu ilmu dasar dalam astronomi.

Salah satu landasan spektroskopi adalah Hukum Kirchoff (1859):
Bila suatu benda cair atau gas bertekanan tinggi dipijarkan, benda tadi akan memancarkan energi dengan spektrum pada semua panjang gelombang
Gas bertekanan rendah bila dipijarkan akan memancarkan energi hanya pada warna, atau panjang gelombang tertentu saja. Spektrum yang diperoleh berupa garis-garis terang yang disebut garis pancaran atau garis emisi. Letak setiap garis atau panjang gelombang garis tersebut merupakan ciri gas yang memancarkannya.
Bila seberkas cahaya putih dengan spektrum kontinu dilewatkan melalui gas yang dingin dan renggang (bertekanan rendah), gas tersebut tersebut akan menyerap cahaya tersebut pada warna atau panjang gelombang tertentu. Akibatnya akan diperoleh spektrum kontinu yang berasal dari cahaya putih yang dilewatkan diselang-seling garis gelap yang disebut garis serapan atau garis absorpsi.
Deret Balmer
Ilmuwan Swiss yang bernama Balmer merumuskan suatu persamaan deret untuk memprediksi panjang gelombang dari garis serapan yang dihasilkan gas hidrogen. Persamaan terebut dikenal dengan deret Balmer.

dengan : λ: panjang gelombang serapan (cm)
RH : tetapan Rydberg (= 109678)

Teori Kuantum Planck

Planck mempostulatkan bahwa cahaya diradiasikan dalam bentuk paket - paket energi kecil, yang disebut kuantum. Teori inilah yang mendasari terciptanya bidang baru dalam dunia fisika, yaitu fisika kuantum.

Planck mengatakan bahwa energi dari tiap foton
Eo = h. f = hc//λ
h : tetapan Planck (h = 6,63 x 10^-34 J.s)
f : frekuensi dari foton
c = kecepatan cahaya (= 3.10^5 km/s)
λ = panjang gelombang foton

Pembentukan spektrum Bintang
Pola spektrum bintang umumnya berbeda-beda, pada tahun 1863 seorang astronom bernama Angelo Secchi mengelompokan spektrum bintang dalam 4 golongan berdasarkan kemiripan susunan garis spektrumnya.

Miss A. Maury dari Harvard Observatory menemukan bahwa klasifikasi Secchi dapat diurutkan secara kesinambungan hingga spektrum suatu bintang dengan bintang urutan sebelumnya tidak berbeda banyak. Klasifikasi yang dibuat oleh Miss Maury selanjutnya diperbaiki kembali oleh Miss Annie J. Cannon. Hingga sekarang klasifikasi Miss Cannon ini digunakan.

Tabel 1 : Rangkuman klasifikasi bintang yang saat ini umum digunakan (sering digunakan ungkapan : Oh Be A Fine Girl (or Guy), Kiss Me) untuk mengingat urutan klasifikasi kelas spektrum bintang. (klik gambar untuk tampilan lebih jelas!).

Subkelas
Klasifikasi spektrum bintang O, B, A, F, G, K, M masih dibagi lagi dalam subkelas, yaitu
B0, B1, B2, B3, . . . . . . . . ., B9
A0, A1, A2, A3, . . . . . . . . ., A9
F0, F1, F2, F3, . . . . . . . . . ., F9

Semakin besar angka yang menyatakan menunjukkan suhu bintang semakin rendah pula. Pengunaan subkelas ini dimaksudkan agar pengklasifikasian spektrum bintang menjadi lebih spesifik sehingga lebih jelas dan tepat.
(untuk informasi lebih lanjut tentang kelas spektrum bintang di sini.)

M-K Kelas (Kelas Luminositas Bintang)
Bintang dalam kelas spektrum tertentu ternyata dapat mempunyai luminositas yang berbeda. Pada tahun 1913 Adam dan Kohlscutter di Observatorium Mount Wilson menunjukkan ketebalan beberapa garis spektrum dapat digunakan untuk menentukan luminositas bintang.
Berdasarkan kenyataan ini pada tahun 1943 Morgan dan Keenan dari Observatorium Yerkes membagi bintang dalam kelas luminositas, yaitu :

• Kelas 1b
  Maharaksasa yang kurang terang
• Kelas II
  Raksasa yang terang
  

• 
  

Nature of the Universe Chapter 2 Motions of Heavenly Bodies

In this chapter, we will discuss the apparent motion of the heavenly bodies. We will understand why the Sun, the Moon and the planets "move," but stars do not.

Motion of Stars
Everyone knows that the Sun rises from the east and sets in the west. Less well known is that almost everything on the sky, including the Moon, planets and most of the stars, also rises from the east and sets in the west. This is the major movement of objects on the sky and it is due to the rotation of the Earth.

We could imagine that the Earth is at the center of a large sphere, called the celestial sphere; and the Sun, stars, etc. are located on the sphere. Because the Earth is rotating from the west to the east, everything on the celestial sphere will apparently move from the east to the west. This is why the Sun rises from the east.

From the picture, we can see that those stars near the north celestial pole never set. We call them circumpolar stars. One of the circumpolar star, called Polaris, is special because it is very near the north celestial pole. Thus, it appears to be stationary.


Here are three simulations of what can be seen in the northern hemisphere. The first one is pointing to the north. (Note that Polaris does move a little bit.) The second and the third show the motions of other stars at south and the east respectively.

Motion of the Sun
Stars do not move on the celestial sphere. They are fixed. Thus, if we throw away the rotation of the Earth, stars are stationary. (Note: The truth is that some stars do move on the celestial sphere. We call this the proper motion of stars. Usually, proper motion of a star is very small and can only be detected if we observe the star for decades.)

There is one important exception however. The Sun is also a star, but the Sun does move on the celestial sphere because the Earth revolves around it. It moves from west to east, and completes a full circle in a year. The path that the Sun traces out on the celestial sphere is called the ecliptic and the twelve constellations that the Sun goes through are the zodiac. (Note: Ecliptic does also go through the constellation Ophiuchus, but due to historical reason, it is not included in the zodiac.) These are the origin of zodiac in astrology. Contrary to common belief, the Sun does not spend equal time on each ecliptic constellation.
Question: What is a day?
Answer: Most people will define a day as the time the Sun comes back to the same position relative to the ground, for example, from one midday to another. This is exactly how the sundial works. This defines the solar day. There is another less common definition. We call it a day if other stars come back to the same positions. This is the sidereal day. Due to the revolution of the Earth, a solar day is longer than a sidereal day. Approximately, a year has 365 solar days but 366 sidereal days. Do you know why?

Introduction to Astronomy

This material (including images) is copyrighted!. See my copyright notice for fair use practices. 

Hello, explorer! You are about to start a journey that will take you to the farthest reaches of space and the innermost depths of matter and from the earliest beginning of time to the future billions of years from now. Introductory astronomy classes have the daunting task of introducing students to the wonders of the entire universe in one short course, often just one semester or one quarter long. Though the places and events you will encounter will sometimes be mind-boggling, I hope you will find it such a fascinating experience that you will want to learn more about those places in another course or in your own free time in the library or in your backyard with binoculars or telescope (or even better, at star parties on a mountain far from the city lights with your local astronomy club). 

The first part of this chapter takes you on a tour of the universe in space and time to give you some context---``set the stage and introduce the characters'', some familiar and others quite obscure but still vital to the play. It is like a travel brochure you read before your vacation trip. One word of warning: a lot of numbers and facts are presented in the first section but do not try to memorize them. What is important is to get a sense of the relative scale of things. 

In grade school you probably memorized a lot of facts about the planets and stars and when you were older you wondered, ``but how do they know that?'' In the following chapters you will learn how astronomers measure the distances, sizes, and ages of these objects and determine what they are like and what makes them appear the way they do. This textbook emphasizes the techniques and process astronomers use to find out about the universe around us and the unifying principles operating ``behind the scenes''. Facts will be given as examples of what is found when those techniques are used or as examples of a particular effect of a physical principle in operation. 

The second part of the chapter presents a brief description of the philosophy and method of science and the role astronomy plays in our attempts to understand the universe scientifically. At the end is a discussion of the non-science often confused with astronomy called astrology. Vocabulary terms in the text are set in boldface to help you find them.