天文学概论7

原星到恒星
Star emerges from the enshrouding dust cocoon
Ignition of H He fusion
processes
恒星形成的证据
Optical
Young, very massive star
Smaller, sunlike stars, probably formed under the influence of the massive
聚合成更重的元素
Fusion into heavier elements than C, O:
requires very high temperatures; occurs only in very massive stars (more than
8 solar masses).
The Life “Clock” of a Massive Star (> 8 Msun)
core.
He-core gets denser and hotter until the next stage
of nuclear burning can begin in the core:
He fusion through the
“Triple-Alpha Process”
4He + 4He 8Be + g
8Be + 4He 12C + g
氦聚合
He nuclei can fuse to build heavier elements:
When pressure and temperature in the He core become high enough,
红巨星演化 (5M）
C, O
Inactive He
mass or less.
Temperature, density and pressure decreasing
流体静力学平衡
Imagine a star’s interior composed of individual
shells.
Within each shell, two forces
have to be in equilibrium with
Let’s compress a massive star’s life into one day…
H He
Life on the Main Sequence
11 12 1
10
2
+ Expansion to Red Giant: 22 h, 9
3
24 min.
8
4
H burning
765
H He
He C, O
钱德拉塞卡极限
白矮星得质量越大其体积越小。
压力变大,直到电子简并压力再也无法对抗重力。
WDs with more than ~ 1.4 solar masses can not exist!
质量在双星中的转移
在一个双星系统,每个恒星星控制一个有限的空间区域, 洛西瓣（洛西表面）
Lagrange points = points of stability, where matter can remain without being pulled towards one of the stars.
This condition uniquely determines the interiLeabharlann Baidur
structure of the star.
This is why we find stable stars on such a narrow strip (Main Sequence) in the Hertzsprung-
WR 124
类太阳恒星的遗迹-白矮星
类太阳恒星建立一个 碳-氧(C、O)的核心, 它并不点燃碳聚变。
氦燃烧壳不断倾销C和O 到核心。C、O核心坍缩
和物质堕向核心。
形成白矮星
白矮星
残留物(C、O核心)
密度超高: 1 汤匙白矮星: mass ≈ 16 tons!!!
沙滩球大小的WD物质将超过一个远洋班轮!
http://v.ku6.com/show/_sPbiIdn6fBiiOhu.html
恒星生命的终结
当恒星中得所有核燃料用完后, 重力会战胜压力，恒星死亡。 大质量恒星会在一个巨大的爆炸中
死去——产生超新星。
稍小点质量恒 星会死在一个 不那么引人注 目的事件,称为
一个新星
红矮星
Stars with less than ~ 0.4 solar
Si burning: 0.03 ms
The final 0.03 msec!!
后主序星演化的终结
Fusion proceeds; formation of Fe core.
M > 8 Msun Fusion stops
at formation of C,O core.
Supernova
Evolution of 4 - 8 Msun stars is still uncertain.
第五章 恒星的起源和演化
恒星起源和演化学说
• 有关恒星起源和演化学说的发展，大体 上可分为三个阶段：
第一阶段，1850~1920年 第二阶段，1925~1960年 第三阶段，1960年至今
天体的演化阶段
• 引力收缩阶段 • 主星序阶段 • 红巨星阶段 • 爆发阶段 • 临终阶段
http://www.56.com/u55/v_OTE2MDE1MzI.html
类太阳恒星
Sunlike stars (~ 0.4 – 4 solar masses) develop a helium core.
Expansion to red giant during H burning shell phase Ignition of He burning in the He core Formation of a degenerate C,O core
Russell diagram.
能量传输结构
主序星膨胀成红巨星
5.3 红巨星阶段
恒星离开主星序阶段，开始向红巨 星演化，质量特别大的恒星则向红超巨 星演化。大部分脉动变星都是处于红巨 星演化后期的恒星。
红巨星演化
4 H → He He
H-burning shell keeps dumping He onto the
物质可以从一个恒球到另一个通过内心的拉格朗日点L1流到另 一颗恒星。
循环的恒星演化
双星系 统中物质的 转移可以显 著改变了恒 星的质量并 影响他们的 恒星演化。
双星系统中的白矮星
吸积盘
X-ray emission T ~ 106 K 白矮星从同伴中吸积物质 Binary consisting of WD + MS or Red Giant star 角动量守恒=> accreted matter forms a disk, called accretion disk. 吸积盘中的物质可以被加热到~ 1 million K => X-ray emission => “X-ray binary”.
generation mechanism than the PP chain takes over:
The CNO Cycle.
能量传输
Energy generated in the star’s center must be transported to the surface.
Inner layers:
each other:
Gravity, i.e. the
weight from all
Outward pressure
layers above
from the interior
Outward pressure force must exactly balance the weight of all layers above everywhere in the star.
白矮星密度高达1,000,000 g/cm3（地球密度为5.5g/cm3）
White Dwarfs: Mass ~ Msun
Temp. ~ 25,000 K Luminosity ~ 0.01 Lsun
Low luminosity; high temperature => White dwarfs are found in the lower left corner of the Hertzsprung-Russell diagram.
Ne burning: 6 ms
23:59:59.996
H He He C, O
H He He C, O
C Ne, Na, Mg, O Ne O, Mg O Si, S, P
O burning:
23:59:59.99997
3.97 ms
C Ne, Na, Mg, O Ne O, Mg
O Si, S, P Si Fe, Co, Ni
masses are completely convective.
Hydrogen and helium remain well mixed throughout the entire star.
No phase of shell “burning” with expansion to giant. Star not hot enough to ignite He burning.
质量损失和星风
Stars like our sun are constantly losing mass in a stellar wind ( solar wind).
The more massive the star, the stronger its stellar wind. Farinfrared
Flow of energy
恒星结构
Energy transport via convection
Energy transport via radiation
Energy generation via nuclear fusion
Sun
Basically the same structure for all stars with approx. 1 solar
In the sun, this happens primarily through the proton-proton (PP) chain
The CNO Cycle
In stars slightly more massive than
the sun, a more powerful energy
5.1引力收缩阶段
快收缩阶段：从星云过渡到恒星的 阶段。
慢收缩阶段：当内部压力与引力几乎 相等时，原恒星处于准流体力学平衡状
态，便开始慢收缩过程。
超新星
汇聚引起加热
原星盘
Conservation of angular momentum leads to the formation of protostellar disks birth place of planets and moons
Mass loss in stellar winds may reduce them all to < 4 Msun stars.
M < 4 Msun
M < 0.4 Msun
Red dwarfs: He burning never ignites
5.4 爆发阶段
有行星状星云的核心爆发，新星爆发 和超新星爆发。
Radiative energy transport
g-rays
Gas particles of solar interior
Outer layers (including photosphere):
Convection
Cool gas sinking down
Bubbles of hot gas rising up
He burning: (Red Giant Phase) 1
h, 35 min, 53 s
11 12 1
10
2
9
3
8
4
765
H He
H He He C, O
He C, O C Ne, Na, Mg, O
C burning: 6.99 s
11 12 1
10
2
9
3
8
4
765
C Ne, Na, Mg, O Ne O, Mg
star
Infrared
The Cone Nebula
Bok Globules:
~ 10 to 1000 solar masses;
Contracting to form protostars
球体
5.2 主星序阶段
质量越大，光度越大，能量消耗越 快，恒星停留于主星序的时间越短
恒星的能力来源
Recall from our discussion of the sun: Stars produce energy by nuclear fusion of hydrogen into helium.