Part VI - The Alchemy of the Heavens
Stellar evolution and the age of stars
Errata In Part V - Stellar Populations I erroneously stated that Walter Baade noted the low metallicity of Population II stars. It was Joseph W. Chamberlain and Lawrence H. Aller who first published research identifying low metallicity stars in 1951.
My vacation put a crimp in these things, but I hope to get back on track with them now.
In 1906, Karl Schwartzchild published a paper modeling a star as an incandescent, stable ball of gas in considerable detail, using basic principles of physics. This model showed that the density and temperature of the gas increased towards the core, and described how energy could be transported from the center of a star outwards by either convection (like water boiling in a pot) or radiation (like heat escaping a fireplace). Sir Arthur Eddington built on this to describe details of conditions in the center of a star. His model showed that a star would contract while its centers heated up until it reached about 20 million degrees, at which time it would stop contracting and become a stable star.
In 1939, astronomers Hans Bethe and Carl von Weizsacker developed study of nuclear fusion, showing how atoms could be transformed into other atoms by fusing with protons, releasing enough energy to power a star. This model, known as the carbon-nitrogen-oxygen, or CNO cycle, worked well for stars like our own. For less massive stars, however, with their lower core temperatures, the energy released by the CNO cycle could not predict the luminosity of the stars. Bethe determined that for these stars, free protons could bind to other free protons to create helium in quantities sufficient to explain the stars, in a cycle known as the proton-proton chain.
The first stellar models were all developed by hand, using desk calculators. This led to trade-offs between examining a crude model of the interior of a star over a long period of time, or using a detailed model but only being able to examine a brief amount of time. As the electronic computer became available, these were refined into detailed models that showed how the hydrogen was consumed in the cores of stars, leading from the main sequence to the so-called red giant phase.
In 1955, R. Harm and Martin Schwartzchild (son of Karl Schwartzchild - astronomy runs strong in that family!) published 15 models, some laboriously calculated by hand over the course of a year, some using electronic computers, of stellar evolution. The models presented the interior of the stars in three zones: the core, the outer envelope, and the intermediate zone between them. These models, though, simple, presented clear views of how stars change over time, and represented the first comprehensive attempt to understand how stars evolve.
This work ushered in a new era in astronomy where electronic computers were used to model conditions in stars, developing more and more detailed views into the lives of the denizens of the galaxy. Built up by gravity's inexorably attraction, and powered by the intense conditions that this has produced in their cores, stars can be regarded as finely tuned furnaces. The radiation and heat produced by fusion in the cores of the stars counters the inward pulling gravity, and each star settles (for the most part) into a stable state known as hydrostatic equilibrium, where the two forces balance out.
Stars of different mass perform their balancing acts in different ways. Low mass stars, under about 0.8 solar masses, are entirely convective: Gamma rays produced by their nuclear reactions are immediately absorbed by stellar material, heating it. This stellar material rises from the hot cores to the surface, cools, and then sinks back into the core. Because of this, fresh hydrogen is always available to feed the thermonuclear reactor at the core throughout their lives as stars. And their lives are long indeed - much longer than the universe has been in existence. Every low-mass star that the Milky Way has ever spawned is still around today.
More massive stars, ranging from about 0.8 up to 8 solar masses, differ from their less massive brethren in how the energy they produce gets transported to the surface. Because of the intensity of the nuclear reactions at their core, the stellar material absorbs the gamma rays from the core and reradiates it as lower energy X-rays. These are in turn absorbed by the next higher layer above the core, which warms and re-ratiates at an even lower ultraviolet energies, and so on up towards the surface of the star. The photons careen off of the charged particles around them in a sort of drunkards walk that has only a very slight outward tendency, compelled by the decreasing temperature as they move away from the core. Once the radiant energy is about two thirds of the way to the surface, roiling convection takes over, transporting heated gas to the surface where it cools and sinks back down again.
For stars on the more massive end of this range, the CNO process drives energy generation in their cores. Trace amounts of carbon, nitrogen, and oxygen catalyze the fusing of hydrogen into helium. This reaction requires higher central temperature and pressure. The payoff is a much greater reaction rate which drives convection in the lower layers. For stars like this, a deep convection zone surrounds the core, while above this zone, radiative processes transport the energy the rest of the way to the surface.
Life gets more dramatic for stars with even greater masses. The crushing weight of these stars compresses their cores to incredible pressures and temperatures, driving thermonuclear reactions into overdrive. At these rates, the sheer pressure of photons on the stellar material is enough to push on the overlying layers and destabilize them.
The greater reaction rates in high mass stars not only consumes hydrogen at a greater rate than in low mass stars, it also limits the amount of hydrogen available to the star. Convection can carry stellar material down deeper into the star, and in low mass stars this makes nearly all the hydrogen in the star available for fusion. Material doesn't really move around in the radiative zones, however. Fresh hydrogen from the surface of a star like the sun will never get to the core, reducing the amount of hydrogen available to it over it's life. Once hydrogen in the thermonuclear core has been completely converted into helium, energy production slows and gravity causes the star to contract. This raises the pressure around the core enough to let hydrogen fusion occur in a shell around the core, driving an overall inflation of the star. A Sun-like star will fluff up until it is the equivelant size of Mercury's orbit. The star is now known as a red giant, growing brighter but cooler and leaving the main sequence in a characteristic branch on the HR diagram.
Eventually the core contracts enough for helium tobegin fusing into carbon and oxygen. This is carried out via a reaction that requires higher temperatures and yields much less energy per reaction. However, the rate of this reaction is so high that it more tha makes up for the lower yield. The star is now it it's horizontal branch phase, where it will remain burning helium for about 10 percent of its former life on the main sequence.
Sometime during this period, the star will undergo oscillations similar to those of Cepheid variables, but at much lower luminosities. Is has become an RR Lyrae variable, whose nearly constant luminosity provide helpful "standard candles" for gauging distances within the Milky Way. All too soon, the stars supply of helium is exhausted in the core, and it again contracts, driving helium burning in the shell. All this shell burning again boosts the stars luminosity, and the star enters the so-called asymptotic giant branch. A Sun-like star at this stage would be a distended giant as big as the orbit of Mars.
Instabilities within the star at this point lead to cycles of alternating helium and hydrogen fusion in different layers in the star. The resulting thermal pulses create tremendous winds, blowing stellar material off the surface of the star, exposing the hot core and creating planetary nebula.
For more massive stars, pressures and temperatures are high enough to continue the process of fusing ever heavier elements in their cores, and sustaining multiple shells of fusing element. As this happens, the density in the core decreases, and gravity compresses the star to compensate. This increases the rate of thermonuclear reactions, hastening the demise of the star. A 25 solar mass star, for instance, consumes the hydrogen in its core in about 7 million years. Helium fusion consumes the helium in the core in only 500 thousand years. Oxygen fusion, producing silicon, consumes the oxygen in the core in only 6 months, while the resulting silicon is fused into iron in a single day.
During these various power plays, the massive star's surface temperature swings wildly from being red-cool to blue-hot and back, while it's total luminosity creeps ever upward. At times, it enters periods of instability and it undergoes regular pulsations as a Cepheid variable. All too soon, though, it hits the end of the line. Heavy element fusion in the star is driven by high temperatures and pressures, but releases much less energy. Iron, the result of silicon fusion, obstinantly refuses to fuse into anything greater. The temperatures and pressures required to fuse iron demand energies greater than those that are released by iron fusion, and nuclear fusion stops. The newly dormant iron core, unable to maintain its thermal pressure, implodes in less than a second.
The resulting release of gravitational energy is stupendous. A shock wave ripples through the star, detonating a supernova explosion of titanic proportions. For the next weeks or months, the supernova can outshine the rest of the galaxy in which it resides. Much of this afterglow is the result of elements heavier than iron that were forged in the supernovas intense fires, as nickel and other heavy elements decay into lighter isotopes.
Two-fer plus one! Shapley 1 in real life and Elite
The Fine Ring Nebula, Shapley 1, was discovered in 1936 by Harlow Shapley, and it is the first destination of our tour where real life and Elite Dangerous really diverge. A beautiful planetary nebula blown off from a Sun-like star at the end of it's life, Shapley 1 provides an example of the fate of our own Sun at the end of its life.
The nebula in Elite is visually very similar to real life images, but differs in distance and in the star at its core. Current estimates place Shapley 1 at about 4900 light years from Earth, while the nebula lies a mere 1000 light years away in Elite. Strangely, while the real life Shapley 1 surrounds a white dwarf - the cooling core of the Sun-like star that created it. In Elite, however, the nebula surrounds a Wolf-Rayet star - a massive blue giant nearing the "iron flash" that will result in its supernova. Some Wolf-Rayet stars do produce planetary nebula as they blow off their outer layers, so we can forgive Frontier this.
Extremely hot compared to the Sun (5780 degrees Kelvin), or even the main sequence O type star that bred it (30,000 degrees Kelvin), Wolf-Rayet stars at the center of planetary nebula typically have surface temperatures of 80,000 degrees Kelvin or more. These stars are actively losing material from their surface, and will supernova in relatively short times (though still in the hundreds of thousands of years).
While there are closer Wolf-Rayet stars in Elite, Shapley 1 is particularly beautiful, and well worth the trip.
Omicron Eridani B - the Shapley 1 that should be
While Shapley 1 bears a Wolf-Rayet star in its core in the Elite galaxy, the real life nebula surrounds a white dwarf. In lieu of the Shapley 1 white dwarf, let's take a trip to the first
white dwarf to be discovered, Omicron Eridani B.
As Sun-like stars age, they burn helium in their core and then a shell surrounding their core, producing sufficient energy to blow off the outer portions of the star. Known as "envelope ejection", this produces distinctive planetary nebula, and exposes the brilliantly hot (but relatively dim) core of the star. This core gradually cools, but remains distinctively white. On the HR diagram, white dwarfs make up about 10 percent of the stars in the Galaxy. When Hertzsprung began his work, however, white dwarfs were unknown, because they are so dim that none are visible to the naked eye. When white dwarfs were first discovered telescopically, they were originally thought to be type M stars, red dwarfs. While Omicron Eridani B was discovered in 1783 by William Herschel, it wasn't until 1910 that it was discovered by Henry Norris Russel, Edward Pickering, and Williamina Fleming that the star was white, and not red.
As the core of a former sun, Omicron Eridani B is remarkably dense, packing half a solar mass into a sphere with a radius of only 1.4 percent of the Sun. While the star is brilliantly hot (16,500 degrees Kelvin), it is dim, with a total luminosity of only 1.3 percent of the Sun.
Next: We live in a spiral - mapping the arms of the Milky Way.