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The Deaths of Stars 恆星的衰亡

最低質量 ( < 0.4 M ) 恆星 ( 紅矮星 ), 結構上大規模對流離開主序時, 氫元素幾乎全部用罄, 不會演化成巨星星體冷卻, 成為黑矮星這種恆星壽命非常長, 比宇宙現在年齡還長, 意思是黑矮星都還沒死!

低質量 (0.4 to 8 M ) 恆星晚年 helium shell fusion Hydrogen shell fusion

低質量 (0.4 to 8 M ) 恆星晚年 II 低質量水平分支的巨星, 其核心溫度約 2 億 K, 不足以點燃需要 6 億 K 的碳與氧的融合反應所以剩下 carbon-oxygen core 氦殼層融合星體再次膨脹由於有兩層融合反應, 這次體積變得更大, 演化進入 asymptotic giant branch (AGB) 漸近巨星支 8 倍太陽質量的 AGB 星其半徑達火星軌道, 光度達 10 4 L After being an AGB, the star becomes a supergiant ( 超巨星 )

Luminosity (L ( ) 1. The star shines by shell hydrogen fusion: The inert core shrinks and the outer layers expand. 3. Core helium fusion begins with the helium flash (*). Horizontal branch Redgiant branch 2. Luminosity increases and surface temperature decreases, so the star moves up and to the right on the H-R diagram (along the red-giant branch). Surface temperature (K) Before the helium flash: A red-giant star

Luminosity (L ( ) 4. The star now shines by shell hydrogen fusion and core helium fusion: The core expands and the outer layers shrink. 6. Eventually all of the core helium is used up Horizontal branch 5. Luminosity decreases and surface temperature increases, so the star moves down and to the left on the H-R diagram (into the horizontal branch). Surface temperature (K) After the helium flash: A horizontal-branch star

7. The star now shines by shell hydrogen fusion and shell helium fusion: The core shrinks and the outer layers expand. Horizontal branch 9. Eventually the star sheds its outer layers to form a planetary nebula. 8. Luminosity increases and surface temperature decreases, so the star moves up and to the right on the H-R diagram (along the asymptotic giant branch). Surface temperature (K) After core helium fusion ends: An AGB star

低質量 (0.4 to 8 M ) 恆星晚年 III Triple-alpha process 對溫度很敏感 (e.g., PP chain rate T 4 ; triple-α T 40 ), 只要溫度升高一點, 融合速率便急遽加快一系列的 helium shell flash (cf 之前在核心的 helium flash) thermal pulses 最後超巨星外圍膨脹, 溫度下降, 電子與離子復合 (recombine), 放出光子, 加上殼層氦閃發出的光子, 光壓造成物質向外噴發, 冷卻而凝固成塵埃中心熾熱星體發光紫外線游離周圍氣體, 使其發光, 成為行星狀星雲 (planetary nebula) HR 圖上向左移動

Mira Variables ( 米拉變星 ) Mira ( wonderful in Latin) A pulsating red giant (M < 2 Msun) on the AGB P > 100 d, 1~3 mag variations size and temperature changes Significant mass loss (120 km/s), eventually becoming a planetary nebula Mira by HST UV observations by Galex showing the Mira bow shocks Loop Streamers Mira Bow shock

低質量恆星的外圍噴發出行星狀星雲, 核心則演化成白矮星白矮星以電子簡併壓力平衡萬有引力質量 ( 萬有引力 ) 的上限稱為 Chandrasekhar limit ( 錢氏極限 ) ~1.4 M 白矮星的密度達 10 9 kg/m 3 超過此極限, 連電子簡併壓力也無法抵擋引力, 核心會繼續塌縮 Subrahmanyan Chandrasekhar, 1990 Nobel Prize winner

天狼星 (Sirius) 的伴星是顆白矮星 Sirius A (11,000 K), Sirius B (30,000 K) An optical image of Sirius A and Sirius B taken by the HST. Sirius B is seen to the lower left. The diffraction spikes and concentric rings are instrumental effects. http://hubblesite.org/newscenter/archive/releases/2005/36/image/a/ An X-ray image by the Chandra Observatory. Sirius B is seen to the lower left. The diffraction spikes and concentric rings are instrumental effects. Sirius B is the brighter source.

Sol: (a) The spectral type of a star is determined by the stellar spectrum, by relative line strengths (type) and line width (luminosity class). (b) It is a temperature sequence, from the hottest O type stars to the coolest T. (c) The Sun is a G2 V star. (d) It is of luminosity class V, i.e., a main sequence star (also called dwarf ).

Absolute Magnitude Sol: (a) and (b) are as seen on the left. (c) The Sun is estimated to be about 5 billion years old, from the dating of materials on the Earth, from the Moon, and from the meteorites. The age is also consistent with our knowledge of stellar evolution. (d) The young Sun has more H than He in the core. With more particle numbers, there would be more thermal pressure to support, hence a lower energy output from, the core.

Sol: (a) The energy source of a main-sequence star is the thermonuclear reactions in the core. Such reactions produce, in addition to energy, also neutrinos, which are detected. (b) Near the end of the main-sequence stage, the core contracts to become degenerate. Because the pressure of the degenerate matter does not increase when heated, unlike the normal ideal gas, the He core, when heated to 100 million K, is ignited in a violent way, This releases lots of energy in a short time, hence termed helium flash. (c) The helium flash happens only in stars with masses in a certain range, because for a higher mass star, the core density is lower so it does not become degenerate when He burning starts, whereas for a lower mass star the core temperature never becomes hot enough to initiate the He fusion.

電子簡併壓力白矮星質量越大, 直徑越小

Nova( 新星 )is a close binary containing a white dwarf. The ordinary companion star fills its Roche lobe so deposits fresh H onto the WD.

Nova Herculis 1934 during explosion ( 3 mag) 2 months later (+12 mag)

Luminosity (L ( ) 7. Sun s age: about 50,000 years older than Stage 5 6.PN 5. Sun s age: 12.365 billion years 4. 2nd RG Age 12.32 Gyr 8. White Dwarf Age ~100,000 years older than Stage 7 Sun is horizontalbranch star Sun becomes a red giant 3. He Flash 12.23 Gyr 1. Today MS Sun s age: 4.56 billion years 2. Red Giant Age 12 Gyr Surface temperature (K)

中質量 (8 to 25 M ) 與大質量 (>25 M ) 恆星晚年核心消耗速度快, 收縮快, 以劇烈方式點燃下級核反應核心萬有引力強, 連擠壓原子的力量都撐不住原來貼靠在原子核外面的電子被擠進原子核, 結合成中子 ( 中子簡併狀態 ) 中子星 (neutron star) 劇烈收縮造成強力反彈, 把外層爆發開超新星爆發 (supernova explosion) When a supernova explodes, it may outshine the entire host galaxy.

例如 Betelgeuse 每年損失 1.7 x 10-7 M, 物質以 10 km/s 速率噴出, 環繞在星球周圍達 1/3 光年超巨星有強烈恆星風

中大質量恆星演化晚期結構外層成為超巨星, 直徑相當於木星軌道核心大小約如地球, 有層層核反應 (cf 洋蔥 )

下一級核融合反應越來越快矽核融合鐵元素鐵原子核當中的質子與中子已經縮得很緊, 進一步核融合無法再釋放能量鐵核塌縮 (D~3000 km, collapses in 0.1 s) 10 seconds

鐵核塌縮溫度達 5 billion K, 極高能量的 gamma rays 光子將鐵原子核光分解 (photodisintegration) 恆星花了數百萬年在主序上, 將氫氦融合成鐵, 然後在不到一秒內, 又將鐵元素分解成質子中子與電子! 核心密度急速增大, 在塌縮後 1/4 秒, 密度達 4 x 10 17 kg/m 3 ( 相當於原子核的密度 ), 電子與質子結合成中子, 放出大量微中子 (neutrinos) 中子簡併壓力抗拒塌縮核心反彈 (core bounce) 超新星爆發 (supernova explosion) 雲消霧散後, 成為超新星遺骸 (supernova remnant) 超新星不是星, 就好像流星也不是, 兩者都是現象

AstroNews

位於金牛座方向的蟹狀星雲 (Crab Nebula), 距離我們 6000 光年, 源於 AD1054 年超新星爆發 ( 當時中國天官記錄有詳細記載, 故稱中國超新星星雲本身大小約 7 光年 10 光年, 以每秒 1500 公里速率向外膨脹西元 1054 年七月 ( 宋仁宗至和元年五月 ) 金牛座超新星爆炸, 據記載最明亮時相當於太白 ( 金 ) 星的光芒, 長達 23 天在白天可見, 直到 1056 年四月 ( 宋嘉祐元年三月 ) 肉眼才看不見天關客星 http://www.phys.ncku.edu.tw/~astrolab/e_book/history_c/captions/guest_star.html

Gum Nebula 直徑超過 2300 光年, 跨越天空 60 度, 離我們最近的部分只有 300 光年 Gum Nebula 是全天空最大的超新星遺骸, 來自 11,000 年前的超新星爆發

http://twanight.org/newtwan/photos.asp?id=3004052

Cassiopeia A 超新星遺骸, 離我們 10,000 光年地球應該於 300 年偵測到爆發事件, 但歷史並無記載 X 射線影像電波影像

歷史上的超新星 (OB association in Scorpius-Centaurus 在 2 百萬年前距離地球 < 150 光年, 曾發生超新星爆發 ) 1054 AD 中國超新星 1572 Tycho supernova 1604 Kepler supernova 下一個? Chandra SN1604 Chandra SN1572

銀河系鄰近的星系 Large Magellanic Cloud 當中, 原來不起眼的某顆星, 於 1987 年 2 月在地球上看到其爆發, 耀眼異常, 稱為 SN1987A

Image and text copyright Akira Fujii 王為豪拍攝 http://www.astrographics.com/galleryprintsindex/gp1614.html

The Expanding Crab Nebula 1973 to 2001

超新星的種類單一大質量恆星衰亡後, 核心變成超新星 Type II supernovae 光譜裡有明顯氫線最亮時達絕對星等 -17 等, 光度變暗有急緩 Semi-detached 雙星系統中白矮星可能藉由吸積伴星物質, 造成核心碳融合, 而引發超新星爆發 Type Ia supernovae 光譜裡沒有氫線最亮達 M~ -19 等, 之後緩緩 (1 年 ) 變暗

Supernova Type Ia Type II 來源光譜接近 Chandrasekhar 極限的白矮星吸積老年恆星附近沒有氫氣, 有很多其他元素譜線大質量恆星演化的結果年輕恆星附近主要為氫氣極亮星等 19 to 20 17 全部爆發能量 5 x 10 43 joules 10 44 joules 噴發出的質量 0.5 M 5 M 噴發速率 10,000 km/s 5,000 km/s

超新星爆發的能量來源一個 1.4 M 的星體塌縮成 R~15 km ( 也就是中子星 ) 為例 E grav = GM 2 /R = 6.6 10 11 N m 2 /kg 2 (1.4 2 10 30 kg) 2 / 15 10 3 m = 4 10 46 joules So, the gravitational energy is sufficient to power an SN.

發現超新星比對星系影像超新星爆發早期光譜 (Filippenko 1997, ARAA 35, 309) Useful web reference http://www.arikah.com/encyclopedia/supernova

Ia 型超新星能量來自核融合, 爆發產生多種 radioactive isotopes, e.g., nickel 衰變成 cobalt 目前已經觀測到 ( 別的星系中 ) 超過數千顆超新星銀河系當中應該每 36 年有顆 Type Ia SN, 每 44 年有顆 Type II SN 那麼每個世紀應該看到約 5 顆超新星 Q: 為什麼沒有看到那麼多呢?

Ia 型超新星爆發為臨界現象, 一旦條件足夠發生, 便有相同的現象 Ia type supernovae may have very similar intrinsic luminosities. If so, they will serve as good cosmic standard candles that allow us to measure their distances (and the distances of their host galaxies) on cosmic scales.

MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft impacted the surface of Mercury, as predicted, at 3:26 p.m. EDT (3:34 p.m. ground time) on April 30, 2015. http://messenger.jhuapl.edu/