Just A Short Note Series #3
START:
- Hydrogen fusion (Simplified: H or D to He)
- Lifespan: millions to billions of years
EVENT:
+ Hydrogen fuel is depleted and fails to support further nuclear fusion
+ Stellar temperature drops and fails to keep star stable
+ Stellar core contracts under its own gravity
+ Stellar core temperature and pressure increase
+ Helium fusion takes over (Simplified: He to C)
+ Outer stellar layer pushed away from core by this novel energy release, cools down, becomes red in color
RESULT:
RED GIANT
- Helium fusion
- Lifespan: millions of years
EVENT:
+ Helium fuel is eventually depleted and fails to support further nuclear fusion
RESULT:
[1] Stellar mass > 5 times solar mass: HIGH MASS STELLAR DEATH
[2] Stellar mass <= 5 times solar mass: LOW MASS STELLAR DEATH
EVENT:
[1] LOW MASS STELLAR DEATH
+ Stellar core contracts once more
+ But only sufficient to ignite fusion of remaining Helium in the shell around stellar core
+ Resultant stellar instability with pulsating surface activity
RESULT:
- Helium fusion
- Constantly changing in brightness and size
- [1A] Outer stellar layers drift away into space in pulses: "PLANETARY" NEBULA
- [1B] Inner stellar core eventually exposed: WHITE DWARF
EVENT:
[1BI] For a solitary white dwarf, nuclear fusion ceases
[1BII] In a binary star system, stellar material funnels from the red giant to the white dwarf; this newly accreted material provides energy to reignite a "runaway" nuclear fusion
RESULT:
[1BI] N/A
[1BII] SUPERNOVA (TYPE IA)*
- Carbon fusion (Simplified: C to O, N, etc)
- Lifespan: weeks to months
Figure 1
EVENT:
[2] HIGH MASS STELLAR DEATH
+ Stellar core contracts once more
+ But has sufficient pressure to ignite Carbon fusion (Simplified: C to O, N, Ne; which fuse to Na, Mg, Si, S; which fuse to Fe, Ni, Co etc)
+ Iron, which is very stable and resists further fusion, accumulates in stellar core
+ When Iron accumulation reaches a critical mass (1.5 times solar mass) after millions of years, electrons are forced out of orbit by gravity
+ These electrons then merge with Iron nuclei, instantaneously reducing stellar volume
+ Extreme potential energy is released in shockwaves colliding with outer stellar materials
RESULT:
- Fusion of heavier elements producing all known natural elements in the universe
- Lifespan: weeks to months
- [2A] SUPERNOVA REMNANT[2B] STELLAR REMNANT
Figure 2
GAMMA RAY BURST
- GRB is the brightest electromagnetic emission event observed in the universe
- It is followed by less energetic emission of longer wavelengths: AFTERGLOW
- Lifespan: milliseconds to minutes
Figure 3- Compact gaseous nebula
- Glows brilliantly due to radioactive decay of fusion products
- Expelled at great velocity, expands into space and eventually fades
- May collide with molecular clouds during expansion, producing "COMETARY" KNOT
- As noted above, stellar remnant mass is always > 1.5 times solar mass
- The Tolman-Oppenheimer-Volkoff Limit is around 3.5-4 times solar mass
EVENT:
[2BI] Stellar remnant < TOV Limit: neutron-degenerate matter remains stable
[2BII] Stellar remnant near TOV Limit: individual neutrons break down due to gravity
[2BIII] Stellar remnant > TOV Limit: neutron-degenerate pressure is exceeded: implosion
RESULT:
[2BI] NEUTRON STAR
- Very compact star 15-20km in diameter
- PULSAR: a strongly magnetic, spinning neutron start that emits rhythmic bursts of radio waves
- MAGNETAR: a neutron star with an even stronger magnetic field; may emit GRB
Figure 4
[2BII] QUARK STAR (STRANGE STAR)
- A hypothetical ultra-dense star heavier but smaller than a neutron star
- Neutrons break down under immense gravity into constituent up- and down-quarks
- Some may change to strange-quarks, forming a phase of strange-matter
Figure 5
[2BIII] BLACK HOLE
- A region of deformed spacetime within which gravity prevents anything, including electromagnetic radiation (e.g. light) from escaping beyond a "point of no return" called the Event Horizon, thereby blocked from the external observation
- It has only three independent physical properties:
- Mass, Charge, Angular momentum (rotational or non-rotational)
- Rotating black holes are surrounded by a region of spacetime in which it is impossible to stand still, called the Ergosphere, but still possible to escape
- Matter falls into the accretion disc of a Black Hole so that it grows continuously
- This process is heated by friction and emits radiation at Event Horizon
- At the center of a Black Hole is a region where spacetime curvature becomes infinite: GRAVITATIONAL SINGULARITY
Figure 6
* N.B. Supernovae are classified according to their light curves and the absorption lines of different elements in their spectra.
Figure 1: Supernova remnant RCW86 is the result of a supernova explosion in AD 185. Expending shockwave at x-ray energy heats interstellar gas (blue and green), while cooler infrared energy radiates from interstellar dust (yellow and red). Abundance of iron but lack of a neutron star suggest that a white dwarf had been destroyed in a binary star system.
Figure 2: Crab nebula M1 is the result of a supernova explosion in AD 1054. In this event the stellar remnant is the Crab pulsar.
Figure 3: Wolf-Rayet star WR124 and its surrounding nebula. These extremely hot and massive stars have shed most of their hydrogen. They are candidates for future GRB.
Figure 4: Supernova remnant Cassiopeia A with a rapidly cooling neutron star having a core of frictionless neutron superfluid.
Figure 5: Neutron star RX J1856.5-3754 in the constellation Corona Australis had been previously suggested to be a quark star. Other candidates remain.
Figure 6: Binary Black Hole in 3C75 (in galaxy cluster Abell 400) co-orbiting at the core of two merging galaxies and blasting out jets of intense radio wave emission. Besides supernovae, this is another mechanism from which Black Holes can form.
