Objek Subbintang

Objek Subbintang (Inggris:Substellar Object) adalah objek astronomi yang memiliki massa lebih kecil dari massa terkecil di mana fusi hidrogen dapat dipertahankan (sekitar 0,08 massa matahari). Definisi ini mencakup katai coklat dan bekas bintang yang mirip dengan EF Eridani B, dan juga dapat mencakup objek bermassa planet, terlepas dari mekanisme pembentukannya dan apakah mereka terkait dengan bintang primer atau tidak.

Dengan asumsi bahwa objek subbintang memiliki komposisi yang mirip dengan Matahari dan setidaknya bermassa Jupiter (kira-kira 10−3 massa Matahari), jari-jarinya kira-kira sebanding dengan Jupiter (kira-kira 0,1 jari-jari matahari) terlepas dari massa objek subbintang (katai coklat memiliki massa kurang dari 75 massa Jupiter). Hal ini karena inti benda subbintang tersebut berada pada rentang massa teratas (tepat di bawah batas pembakaran hidrogen) yang cukup berdegenerasi, dengan massa jenis ≈103 g/cm3, tetapi degenerasi ini berkurang dengan berkurangnya massa hingga hanya bermassa Jupiter. benda di bawah permukaan memiliki massa jenis kurang dari 10 g/cm3. Penurunan massa jenis ini menyeimbangkan penurunan massa, menjaga jari-jari tetap konstan.

Objek subbintang seperti katai coklat tidak memiliki massa yang cukup untuk meleburkan hidrogen dan helium, oleh karena itu Katai coklat tidak mengalami evolusi bintang biasa yang membatasi umur bintang.

Objek subbintang dengan massa tepat di bawah batas fusi hidrogen dapat memicu fusi hidrogen sementara di pusatnya. Meskipun ini akan memberikan energi, itu tidak akan cukup untuk mengatasi kontraksi gravitasi objek itu sendiri yang sedang berlangsung. Demikian juga, meskipun sebuah objek dengan massa di atas sekitar 0.013 massa matahari akan dapat melakukan fusi deuterium untuk sementara waktu, sumber energi ini akan habis dalam sekitar 106 – 108 tahun. Terlepas dari sumber energi ini, radiasi objek subbintang yang terisolasi hanya berasal dari pelepasan energi potensial gravitasinya, yang menyebabkannya mendingin dan menyusut secara bertahap. Objek subbintang di orbit suatu bintang akan menyusut lebih lambat karena suhunya yang stabil dan hangat dari bintang lain, berkembang menuju keadaan keseimbangan di mana ia memancarkan energi sebanyak yang diterimanya dari bintang.

Benda-benda dengan massa lebih rendah cukup dingin untuk memiliki uap air di atmosfernya. Spektroskopi inframerah dapat mendeteksi perbedaan warna air pada objek subbintanh berukuran raksasa gas, meskipun objek tersebut tidak mengorbit di sekitar bintang.

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