Astro 101 Fall 2013 Lecture 9 Stars (continued) – Stellar evolution T. Howard. Spectral Classes. Strange lettering scheme is a historical accident. Spectral Class Surface Temperature Examples . 30,000 K 20,000 K 10,000 K 7000 K 6000 K 4000 K 3000 K.
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Stars (continued) – Stellar evolution
Strange lettering scheme is a historical accident.
Spectral Class Surface Temperature Examples
Further subdivision: BO - B9, GO - G9, etc. GO hotter than G9. Sun is a G2.
The Hertzsprung-Russell (H-R) Diagram
A star’s position in the H-R diagram depends on its mass and evolutionary state.
H-R Diagram of Nearby Stars
Note lines of constant radius!
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.
(< 8 Msun , focus on 1 Msun case)
- All H converted to He in core.
- Core too cool for He burning. Contracts. Heats up.
- H burns in hot, dense shell around core: "H-shell burning phase".
- Tremendous energy produced. Star must expand.
- Star now a "Red Giant". Diameter ~ 1 AU!
- Phase lasts ~ 109 years for 1 MSun star.
- Example: Arcturus
- Core shrinks and heats up to 108 K, helium can now burn into carbon.
4He + 4He -> 8Be + energy
8Be + 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 MSun star.
Horizontal branch star structure
He -> C
H -> He
- High rate of burning, star expands, luminosity way up.
- Called ''Red Supergiant'' (or Asymptotic Giant Branch) phase.
- Only ~106 years for 1 MSun star.
- Core continues to contract. Never hot enough for C fusion.
- He shell dense, fusion becomes unstable => “He shell flashes”.
- Whole star pulsates more and more violently.
- 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).
- Dead core of low-mass star after Planetary Nebula thrown off.
- Mass: few tenths of a MSun
- Radius: about REarth
Comparing with theory, can easily determine cluster age from H-R diagram.
Globular Cluster M80 and composite H-R diagram for similar-age clusters.
Globular clusters formed 12-14 billion years ago. Useful info for studying the history of the Milky Way Galaxy.
Schematic Picture of Cluster Evolution similar-age clusters.
Massive, hot, bright, blue, short-lived stars
Time 0. Cluster looks blue
Low-mass, cool, red, dim, long-lived stars
Time: few million years.
Time: 10 billion years.
Cluster looks red
Evolution of Stars > 12 M similar-age clusters.Sun
Low mass stars never got
past this structure:
Eventual state of > 12 MSun star
Higher mass stars fuse heavier elements.
Result is "onion" structure with many shells of fusion-produced elements. Heaviest element made is iron. Strong winds.
They evolve more rapidly. Example: 20 MSun star lives "only" ~107 years.
Fusion Reactions and Stellar Mass similar-age clusters.
In stars like the Sun or less massive, H -> He
most efficient through proton-proton chain.
In higher mass stars, "CNO cycle" more efficient. Same net result:
4 protons -> He nucleus
Carbon just a catalyst.
Need Tcenter > 16 million K for CNO cycle to be more efficient.
Endpoints of Massive Stars similar-age clusters.
Supernovae – extremely similar-age clusters.violent stellar explosions
Example supernova remnant—
Crabnebula in Taurus
Two major classifications of Supernovae similar-age clusters.
Supernova remnant in Vela
Light Curves of Supernovae Types similar-age clusters.
Neutron Stars similar-age clusters.
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 !
Rotation rate: few to many times per second!!!
Magnetic field: 1010 x typical bar magnet!
A neutron star over the Sandias?
Please read about observable neutron stars: pulsars.
Pulsars discovered 1967 by similar-age clusters.
Jocelyn Bell Burnell & Anthony
Pulsars – “Lighthouse” model similar-age clusters.
Crab nebula in X-rays (Chandra) similar-age clusters.
“Blinking” of the Crab pulsar
A brief digression -- Relativity similar-age clusters.
1905: Special Theory of Relativity similar-age clusters.
1915: General Theory of Relativity
Relativity stars with assigning “frames of reference” (coordinates) to
the Observer and the Event (or Thing) in question
Special Relativity covers situations where velocities may be
very high, but frames of reference are not
General Relativity as it says, the more general case: acceleration
between frames of reference is included
A (coordinates) tofundamental postulate of relativity:
The speed of light, c, in free (empty) space is a universal constant.
It does not get added to or subtracted from the frame of reference.
c = (approx.) 3 x 108 meters/sec
Note: the speed of light can be slower in solid, gaseous, or liquid media
(glass, water, air), but never faster than c.
This effect actually accounts for the bending of light rays in lenses,
when entering or leaving a body of water, etc.
Black Holes and General Relativity (coordinates) to
General Relativity: Einstein's (1915) description of gravity (extension of Newton's). It begins with:
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.
2. If you are in (coordinates) tofreefall, 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. (coordinates) toGravity 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.
Some Consequences of General Relativity: (coordinates) to
Mass “warps” space i.e., the amount of mass introduces
a “curvature” to what would otherwise be perfectly linear
This curvature of space is a different way of thinking about
gravity. It works, and explains a lot of things.
The curvature of space causes things to move in curved lines.
Even rays of light!
“Matter tells space how to curve. Space tells matter how to move.”
(Misner, Thorne, and Wheeler, 1973)
The “rubber sheet” analogy. (coordinates) to
Curvature of light Observed! In (coordinates) to1919 eclipse.
Sir Arthur Eddington
Gravitational lensing (coordinates) to. The gravity of a foreground cluster of galaxies distorts the images of background galaxies into arc shapes.
Other consequences of General Relativity: (coordinates) to
Gravitational Waves – caused by massive violent events
(e.g., coalescence of binary pulsars, formation of Bl. Holes)
– cause asymmetric warping of space
The “ripples” move at speed c thru the universe.
Might be detectable (but very weak).
We are searching for them now.
USA: LIGO Project
Elsewhere: Virgo, GEO, others
Proposed Space-based GW Observatory: LISA
LIGO Facility at Hanford, WA (2 (coordinates) tond facility is at Livingston, LA)
Proposed LISA mission (NASA-ESA) (coordinates) to
This is just a schematic diagram. The spacecraft will be
~ 5 million km apart.
Other consequences of General Relativity (coordinates) to
light received when elevator receding at some speed.
2. Gravitational Redshift
later, speed > 0
Consider accelerating elevator in free space (no gravity).
Received light has longer wavelength because of Doppler Shift ("redshift"). Gravity must have same effect! Verified in Pound-Rebka experiment.
time zero, speed=0
light emitted when elevator at rest.
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 (coordinates) to
Velocity needed to escape an object’s gravitational pull.
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, RS. RSM.
Black Holes (coordinates) to
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 MSun object is 9 km.
Event horizon: imaginary sphere around object, with radius RS .
Anything crossing the event horizon, including light, is trapped
Saturn-mass (coordinates) to
Black hole achieves this by severely (coordinates) tocurving space. According to General Relativity, all masses curve space. Gravity and space curvature are equivalent.
Like a rubber sheet, but in three dimensions, curvature dictates how all objects, including light, move when close to a mass.
Curvature at event horizon is so great that space “folds in on itself”.
Effects around Black Holes in on itself”.
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 in on itself”.
- Binary system: 30 MSun star with unseen companion.
- Binary orbit => companion > 7 MSun.
- X-rays => million degree gas falling into black hole.
Final States of a in on itself”.Star (simplified)
1. White Dwarf
If initial star mass < 8-12 Msun .
2. Neutron Star
If initial mass > 12 MSun and < 25 ? MSun .
3. Black Hole
If initial mass > 25 ? MSun .