spectral classes strange lettering scheme is a historical accident. spectral class surface...
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Spectral Classes
Strange lettering scheme is a historical accident.
Spectral Class Surface Temperature Examples
OBAFGKM
30,000 K20,000 K10,000 K7000 K6000 K4000 K3000 K
RigelVega, Sirius
Sun
Betelgeuse
Further subdivision: BO - B9, GO - G9, etc. GO hotter than G9. Sun is a G2.
Main Sequence
White Dwarfs
Red Giants
Red Supergiants
Increasing Mass, Radius
on Main Sequence
The Hertzsprung-Russell (H-R) Diagram
Sun
A star’s position in the H-R diagram depends on its mass and evolutionary state.
Stellar Evolution:Evolution off the Main Sequence
Main Sequence Lifetimes
Most massive (O and B stars): millions of years
Stars like the Sun (G stars): billions of years
Low mass stars (K and M stars): a trillion years!
While on Main Sequence, stellar core has H -> He fusion, by p-p chain in stars like Sun or less massive. In more massive stars, “CNO cycle” becomes more important.
Evolution of a Low-Mass Star(< 8 M
sun , focus on 1 M
sun case)
- All H converted to He in core.
- Core too cool for He burning. Contracts. Heats up.
Red Giant
- Tremendous energy produced. Star must expand.
- Star now a "Red Giant". Diameter ~ 1 AU!
- Phase lasts ~ 109 years for 1 MSun
star.
- Example: Arcturus
- H burns in hot, dense shell around core: "H-shell burning phase".
Eventually: Core Helium Fusion
- Core shrinks and heats up to 108 K, helium can now burn into carbon.
"Triple-alpha process"
4He + 4He -> 8Be + energy8Be + 4He -> 12C + energy
- Core very dense. Fusion first occurs in a runaway process: "the helium flash". Energy from fusion goes into re-expanding and cooling the core. Takes only a few seconds! This slows fusion, so star gets dimmer again.
- Then stable He -> C burning. Still have H -> He shell burning surrounding it.
- Now star on "Horizontal Branch" of H-R diagram. Lasts ~108 years for 1 M
Sun star.
Helium Runs out in Core-- All He -> C. Not hot enough-for C fusion.
-- Core shrinks and heats up, as-does H-burning shell.
- Get new helium burning shell (inside H burning shell).
Red Supergiant
- High rate of burning, star expands, luminosity way up.
- Called ''Red Supergiant'' (or Asymptotic Giant Branch) phase.
- Only ~106 years for 1 MSun
star.
"Planetary Nebulae"
- Core continues to contract. Never hot enough for C fusion.
- He shell dense, fusion becomes unstable => “He shell flashes”.
- Eventually, shells thrown off star altogether! 0.1 - 0.2 MSun
ejected.
- Shells appear as a nebula around star, called “Planetary Nebula” (awful, historical name, nothing to do with planets).
- Whole star pulsates more and more violently.
White Dwarfs
- Dead core of low-mass star after Planetary Nebula thrown off.
- Mass: few tenths of a MSun
- Radius: about REarth
-- Density: 106 g/cm3! (a cubic cm of it would weigh a ton on Earth).
-- Composition: C, O.
- White dwarfs slowly cool to oblivion. No fusion.
Evolution of Stars > 12 MSun
Higher mass stars fuse heavier elements.
Result is "onion" structure with many shells of fusion-produced elements. Heaviest element made is iron. Strong winds.
Eventual state of > 12 MSun
starLow mass stars never gotpast this structure:
They evolve more rapidly. Example: 20 M
Sun star lives
"only" ~107 years.
Star Clusters
Comparing with theory, can easily determine cluster age from H-R diagram.
Open Cluster
Globular Cluster
Following the evolution of a cluster on the H-R diagram
100 LSun
Temperature
Lu
min
osit
y
LSun
LSun
LSun
LSun
LSun
Globular clusters formed 12-14 billion years ago. Useful info for studying the history of the Milky Way Galaxy.
Globular Cluster M80 and composite H-R diagram for similar-age clusters.
Schematic Picture of Cluster Evolution
Time 0. Cluster looks blue
Time: few million years.Cluster redder
Time: 10 billion years.Cluster looks red
Massive, hot, bright, blue, short-lived stars
Low-mass, cool, red, dim, long-lived stars
Fusion Reactions and Stellar Mass
In stars like the Sun or less massive, H -> Hemost efficient through proton-proton chain.
In higher mass stars, "CNO cycle" more efficient. Same net result: 4 protons -> He nucleusCarbon just a catalyst.
Need Tcenter
> 16 million K for CNO cycle to be
more efficient.
(mass) ->
Sun
Neutron Stars
If star has mass 12-25 MSun , remnant of supernova expected to be a tightly packed ball of neutrons.
Diameter: 10 km only!
Mass: 1.4 - 3(?) MSun
Density: 1014 g / cm3 !
A neutron star over the Sandias?
Please read about observable neutron stars: pulsars.
Rotation rate: few to many times per second!!!
Magnetic field: 1010 x typical bar magnet!
Black Holes and General Relativity
The Equivalence Principle
Here’s a series of thought experiments and arguments:
1) Imagine you are far from any source of gravity, in free space, weightless. If you shine a light or throw a ball, it will move in a straight line.
General Relativity: Einstein's (1915) description of gravity (extension of Newton's). It begins with:
2. If you are in freefall, you are also weightless. Einstein says these are equivalent. So in freefall, light and ball also travel in straight lines.
3. Now imagine two people in freefall on Earth, passing a ball back and forth. From their perspective, they pass it in a straight line. From a stationary perspective, it follows a curved path. So will a flashlight beam, but curvature of light path small because light is fast (but not infinitely so).
The different perspectives are called frames of reference.
4. Gravity and acceleration are equivalent. An apple falling in Earth's gravity is the same as one falling in an elevator accelerating upwards, in free space.
5. All effects you would observe by being in an accelerated frame of reference you would also observe when under the influence of gravity.
Examples:
1) Bending of light. If light travels in straight lines in free space, then gravity causes light to follow curved paths.
Gravitational lensing. The gravity of a foreground cluster of galaxies distorts the images of background galaxies into arc shapes.
2. Gravitational Redshift
Consider accelerating elevator in free space (no gravity).
time zero, speed=0
later, speed > 0light received when elevator receding at some speed.
light emitted when elevator at rest.
Received light has longer wavelength because of Doppler Shift ("redshift"). Gravity must have same effect! Verified in Pound-Rebka experiment.
3. Gravitational Time Dilation
Direct consequence of the redshift. Observers disagree on rate of time passage, depending on strength of gravity they’re in.
Escape Velocity
Velocity needed to escape an object’s gravitational pull.
vesc
= 2GM R
Earth's surface: vesc = 11 km/sec.
If Earth shrunk to R=1 cm, then vesc = c, the speed of light! Then nothing, including light, could escape Earth.
This special radius, for a particular object, is called the Schwarzschild Radius, R
S. R
S M.
Black Holes
If core with about 3 MSun
or more collapses, not even neutron
pressure can stop it (total mass of star about 25 MSun
?).
Core collapses to a point, a "singularity".
Gravity is so strong that not even light can escape.
RS for a 3 M
Sun object is 9 km.
Event horizon: imaginary sphere around object, with radius RS .
Event horizonR
S
Anything crossing the event horizon, including light, is trapped
Like a rubber sheet, but in three dimensions, curvature dictates how all objects, including light, move when close to a mass.
Black hole achieves this by severely curving space. According to General Relativity, all masses curve space. Gravity and space curvature are equivalent.
Effects around Black Holes
1) Enormous tidal forces.
2) Gravitational redshift. Example, blue light emitted just outside event horizon may appear red to distant observer.
3) Time dilation. Clock just outside event horizon appears to run slow to a distant observer. At event horizon, clock appears to stop.
Do Black Holes Really Exist? Good Candidate: Cygnus X-1
- Binary system: 30 MSun
star with unseen companion.
- Binary orbit => companion > 7 MSun
.
- X-rays => million degree gas falling into black hole.