Stellar Evolution is the group of theories that combine to form the idea that each star has a life cycle. Although the idea seems obvious, it is only recently that many of the theories could be tested and thus verified.

Important ideas that lead us to understanding Stellar Evolution

A. Nuclear Reactions - NOT the same as Chemical Reactions. Those involve the transfer or sharing of electron clouds without changing nuclei.

1. Two types - Fission and Fusion (our main concern)
2. Building bigger atoms
Atoms are made of P+, N, and e-
1836em, 1837em, and 1em are their respective masses
The proton-proton chain is a critical part of Thermonuclear Fusion


What do you get if you fuse 3 He?
It takes large amounts of energy to force the fusion, but the reaction itself gives off more energy than was put into it -- until Fe. It takes more energy to form an Fe nucleus than the reaction gives off. The same is true for all nuclei larger than Fe. So where do the larger atoms come from?

B. Luminosity (brightness)

Apparent vs. Absolute - although Absolute magnitude is used to make many determinations about stars, Apparent Magnitude is important for measuring distance to those same stars.
History - 130bc Hipparchus created a scale of (apparent) magnitudes, perhaps assuming that all stars were the same distance from us. His scale had exactly 6 magnitudes. To his eyes, the brightest star was roughly 100 times brighter than the dimmist. Each step was about 2.512 times dimmer than the previous number, with 6 being the dimmest (2.512^5 ≈ 100).
Absolute magnitude is the measure of how bright a star would be if it were 10 parsecs (about 32.616 light years, or 3 × 10^14 kilometres) away. The word parsec stands for "parallax of one second of arc", and one parsec is defined to be the distance from the Earth to a star that has a parallax of 1 arcsecond.
Star Name Const. Apparent Magnitued
Sirius -CMa -1.46
Canopus -Car -0.72
Rigel Kentaurus -Cen A -0.01
Arcturus -Boo -0.04
Vega -Lyr 0.03
Capella -Aur 0.08
Rigel -Ori 0.12
Procyon -Cmi 0.38
Achernar -Eri 0.46
Betelgeuse -Ori 0.50
Venus -4
Full Moon -12
Sun -27

C. Spectral Class - indicates color and temperature

  1. 1891 Edward C. Pickering (Harvard) - A-O classes based on strength of H lines
  2. Stable system by 1901 Sun is G2 (A0-A9).
Class Characteristic Color Temp (1000K) Example Notes
O He Blue 28 - 50 E Ori Rare, super-bright (1millionXsol)
B He, H Pale Blue 9.9 – 28 Rigel, Spica
A H White 7.4 – 9.9 Vega, Sirius
F Metals, H Creamy 6 – 7.4 Procyon
G Ca, Metals Yellow 4.9 – 6 Sol, aCen A
K Ca, Molecules Orange 3.5 – 4.9 61 Cygni
M TiO, others, CaI Red 2 – 3.5 Barnard's Star Most commom (80% of main sequence)
Extended classes (Failed stars)
R CN, C Orange, Dark 3.5 – 5.4
S ZrO, mild C mol Crimson 2 – 3.5 R Cyg
N C2 Red 1.9 – 3.5 R Lep
L Metal hydrides Burgundy 1.3 – 2.0
T Methane Brown 0.7 - 1.5 SIMP 0136 Similar to Venus temperature
Recent research which has attempted to count the number of proplyds (protoplanetary discs, clumps of gas in nebulae from which stars and solar systems are formed) suggests that the number of stars in the galaxy should be several orders of magnitude higher than previously estimated. These proplyds tend to form in groups and may be in a survival-race with each other. The first one to form will become a proto-star, a very violent object likely to disrupt other proplyds in the vicinity, stripping them of much of their accumulated material. The victim proplyds may still go on to become main sequence stars or brown dwarf stars of the L and T classes, but quite invisible to us. Since these dwarfs live so long, over time they should have accumulated throughout the galaxy.

D. Hertzsprung-Russell (H-R) Diagram

H-R diagram.svg

E. Hints to explain generations of Star Formation

  1. Start with a nebula that is the remnant of an exploded star (perhaps leftover from a star 15 X Sol)
  2. Add a newly exploded star (N49 erupted thousands of y. a. In the center exists a powerful pulsar.)
  3. The shockwaves expanding from the supernovae, trigger star-formation as they propagate through gas clouds between the stars.

How does a star's life go?

1. Birth is hard – Tau-Tauri's
Diffuse Nebulas (Kaler p393)
Angular Momentum may cause star to split during formation, and perhaps even again.
High-mass stars are much hotter and consume their fuel much more rapidly. All stages, including formation proceed at a greatly increased rate.
Upper limit of stellar mass is unknown, but >120Msun seems likely.
Lower limit seems to be 0.08Msun
Lifetime of a star =1/M^2.5
2. Life CAN be harder -- Variables
Cepheid Variables named after Delta Cephei (3.9-5.1)
Periods ~1 – 50days F5 - G5 ->Larger Magnitude Range
Mira Variables – long term and not as regular as Cepheids
Periods ~100 – 700days M, R, N, S Red Giants
Irregular Variables
Semi-regulars, Eruptive Variables, Interacting Binaries
Nova – Explosion from a main sequence star to form a White Dwarf
-8 to -10
Supernovae – Destruction of a star
-17 to -19
3. Death is like death, man -- Types of Supernovae
I – no Hydrogen lines in their spectra
Ia – No He. Very bright (at 10pc, 500 full moons, but decay very quickly
Ib – Strong He lines. (at 10pc, 100 full moons)
Result – White Dwarf
II – Have H lines. About as bright as Ib.
Decay rather slow – almost constant for a time, but generally irregular dimming
expand about half as fast as Ia. mass too low to form Fe core
4. Death stories (based on mass at birth) aka: Main Sequence divided into 3 parts
a. Lower – have ages approaching 17 billion years (K0)
b. Intermediate - <8Msun produce white dwarfs
(ex. Sun)
6.5by from now He core surrounded by H shell - still on main seq.
7.5by from now (moving up and right on the HRd) Red Giant 1000X cur. brightness – 100X cur. Radius
Helium Flash – as Carbon is formed, jumps left for 10% of its life
AGB Ascent – unstable, 2 shells turning on and off, larger, brighter, redder than first trip to red giant - loses mass at every hick-up, so cancel next ignition (Key: winds are key... removed too much mass)
~half of original mass (a 7M star has lost 80%)
core has shrunk to size of Earth, temp 10^9K
Burning shuts down, with Planetary Nebula moving out at 20km/s.
Cooling nebula dissipates after 50000years and corpse cools and dims for billions of years.
Heaviest white dwarf is 1.4Msun (Chandrasekhar limit)
c. Upper Main Sequence - >8Msun
>40M mass loss is high during main sequence lifetime
While lighter stars are stopping at C&O core, more massive star ignites Ne, Mg, Si, S, Fe each forming successively smaller shells
(An O7 ex.)
If it starts at 20M, takes 8my to use up H. He burning takes 10% of main sequence lifetime (1my). Carbon fusing stage is about 10% of He burn-time. Now >1M of O burns in only 20yrs, then Si becomes Fe in a week! We now have a star with an Fe core about 1.4M about 2000km across. Si is gone. Fe begins compression. (Iron is the most tightly bound of all atomic nuclei, so no energy can be gained by its fusion.)
Something monumental is about to happen!
Core is Fe. Cannot hold itself up... contracts at speeds up to 1/4c
In less than 0.1s, radius drops from 1000km to <50km. In a few more seconds, radius ~10km, releasing LOTS of energy. 10^46Nm (mostly neutrinos) are released in the time it takes to snap your fingers (our galaxy normally radiates 10^38Nm/s).
Density >10^12 g/mL (1,000,000Xwhite dwarf)
Protons and electrons merge into neutrons, which halt collapse.
Part of imploding core rebounds and produces shock wave...
rips outward and causes supernova to erupt. A star has died.
5. Results of Death (based on core mass just before death)
White Dwarf – (<=1.4M)
Neutron Star - (~1.4M) and a radius of about 10km. There should be 10^9 in Milky Way.
All Neutron Stars may be Pulsars.
Pulsar – rapidly spinning neutron star releasing radio, optical, and X-ray waves
First discovered -- 1.337011s per pulse and disappears for 2 months
Crab Nebula -- 0.0316s radius ~20km spinning over 30 times a second
Millisecond Pulsar – 0.00113s Weak radiators (old). Fast spin (young). Suggests a massive companion, feeding it mass and thus angular momentum. 400+ are known
Black Hole - (>3M)