Stellar Evolution: After the Main Sequence

Sifat-sifat dasar bintang Perhatikan langkah eksperimen berikut: Carilah sekeompok bintang yang diperkirakan memiliki jarak yang sama terhadap bumi. Ukurlah magnitudo semu dan warna bintangbya Gambarkan hubungan antara kedua besaran tersebut ( diagram Hertzsprung-Russell ) Apakah yang anda harapkan????? Meskipun agak susah,anda akan melihat kebanyakan bintang berada pada garis tunggal pada diagram tersebut. Semua bintang memiliki komposisi yang sama yakni 75% H, 24%He, 1% unsur lain Sejak lahir, bintang lupa tentang lingkungan yang melahirkan mereka n Jadi....hanya masa dan masalah umur.

Klasifikasi Bintang O = Oh B = Be A = A F = Fine G = Girl K = Know M = me

Bintang O, B dan A masuk kelas awal Bintang K dan M masuk kelas lanjut Penggolongan bintang berdasarkan atas temperatur permukaannya

Pembagian sub kelas spektrum bintang Dari Oo sampai 09 Bo sampai B9 dan seterusnya

Klasifikasi Luminositas Bintang Kelas 1 a = maha raksasa yang sangat terang Kelas 1 b = maha raksasa yang kurang terang Kelas II = bintang raksasa yang terang Kelas III = raksasa Kelas IV = sub raksasa Kelas V = deret utama

Definisi evolusi bintang Bintang dilahirkan, berkembang, padam, dan tak bersinar lagi Evolusi bintang butuh waktu yang sangat panjang Dibutuhkan kemampuan manusia untuk menempatkan bintang dalam urutan umur yang benar

Energi bintang Energi sedetik yang dihasilkan matahari sama dengan energi yang dibangkitkan semua pembangkit di bumi selama beberapa juta tahun. Hidrogen merupakan bahan bakar utama bintang Umur matahari dapat diperkirakan dengan mengukur umur fosil alga

Energi yang hilang di matahari Berdasarkan persamaan einstein tentang perubahan massa menjadi energi dapat dihitung massa matahari yang hilang 4 H → He +2 P + E ( ada 0,7% massa yang hilang) Berdasarkan persamaan luminositas dapat ditemukan bahwa per detik ada 4 juta ton massa matahari yang hilang Selama 5 milyar tahun hanya 0,03% massa matahari yang hilang

Pembentukan bintang Bintang terbentuk dari kabut antarbintang (nebula) yang menggumpal dan mengerut kr gaya gravitasinya. Saat gumpalan gas mengerut, temperaturnya naik sehingga temperatur dipusatnya cukup untuk berlangsungnya energi nuklir

Ruang diantara bintang berisi gas dan debu (materi antar bintang) dengan kerapatan kecil namun volumenya sangat besar Akibat ledakan, di suatu tempat materi antar bintang menjadi lebih mampat dibanding sekitarnya, bagian luar akan ditarik oleh gravitasi bagian dalam sehingga akan mengerut dan semakin mampat (kondensasi)

Bintang deret utama Stabil dalam waktu lama Cocok untuk perkembangan kehidupan bumi Setimbang hidrostatis: energi dari rekasi nuklir menyebabkan tekanan di dalam bintang mampu menahan pengerutan bintang dan bintang menjadi mantap.

Berdasarkan hubungan luminositas dan massa disimpulkan bahwa Bintang bermassa besar lebih boros menguras hidrogen Berdasarkan hubungan massa dan umur bintang dapat disimpulkan bahwa Bintang bermassa besar makin singkat berada di deret utama

A star’s lifetime on the main sequence is proportional to its mass divided by its luminosity The duration of a star’s main sequence lifetime depends on the amount of hydrogen in the star’s core and the rate at which the hydrogen is consumed The more massive a star, the shorter is its main-sequence lifetime

The Sun has been a main-sequence star for about 4 The Sun has been a main-sequence star for about 4.56 billion years and should remain one for about another 7 billion years

During a star’s main-sequence lifetime, the star expands somewhat and undergoes a modest increase in luminosity

When core hydrogen fusion ceases, a main-sequence star becomes a red giant

Red Giants Core hydrogen fusion ceases when the hydrogen has been exhausted in the core of a main-sequence star This leaves a core of nearly pure helium surrounded by a shell through which hydrogen fusion works its way outward in the star The core shrinks and becomes hotter, while the star’s outer layers expand and cool The result is a red giant star

As stars age and become giant stars, they expand tremendously and shed matter into space

Fusion of helium into carbon and oxygen begins at the center of a red giant When the central temperature of a red giant reaches about 100 million K, helium fusion begins in the core This process, also called the triple alpha process, converts helium to carbon and oxygen

In a more massive red giant, helium fusion begins gradually In a less massive red giant, it begins suddenly, in a process called the helium flash

After the helium flash, a low-mass star moves quickly from the red-giant region of the H-R diagram to the horizontal branch

H-R diagrams and observations of star clusters reveal how red giants evolve The age of a star cluster can be estimated by plotting its stars on an H-R diagram

The cluster’s age is equal to the age of the main-sequence stars at the turnoff point (the upper end of the remaining main sequence)

As a cluster ages, the main sequence is “eaten away” from the upper left as stars of progressively smaller mass evolve into red giants

Stellar evolution has produced two distinct populations of stars Relatively young Population I stars are metal rich; ancient Population II stars are metal poor The metals (heavy elements) in Population I stars were manufactured by thermonuclear reactions in an earlier generation of Population II stars, then ejected into space and incorporated into a later stellar generation

Many mature stars pulsate When a star’s evolutionary track carries it through a region in the H-R diagram called the instability strip, the star becomes unstable and begins to pulsate

Cepheid variables are high-mass pulsating variables RR Lyrae variables are low-mass, metal-poor pulsating variables with short periods Long-period variable stars also pulsate but in a fashion that is less well understood

There is a direct relationship between Cepheid periods of pulsation and their luminosities

Mass transfer can affect the evolution of close binary star systems Mass transfer in a close binary system occurs when one star in a close binary overflows its Roche lobe

Gas flowing from one star to the other passes across the inner Lagrangian point

This mass transfer can affect the evolutionary history of the stars that make up the binary system

Key Words alpha particle Cepheid variable close binary color-magnitude diagram contact binary core helium fusion core hydrogen fusion degeneracy degenerate-electron pressure detached binary globular cluster helium flash helium fusion horizontal-branch star ideal gas inner Lagrangian point instability strip long-period variable main-sequence lifetime mass loss mass transfer metal-poor star metal-rich star overcontact binary Pauli exclusion principle period-luminosity relation Population I and Population II stars pulsating variable star red giant Roche lobe RR Lyrae variable semidetached binary shell hydrogen fusion triple alpha process turnoff point Type I and Type II Cepheids zero-age main sequence (ZAMS) zero-age main-sequence star