Yet it seems clear that Hubble's contributions justify his name being attached to the expansion of the universe. Although Shapley had pioneered the use of Cepheid variable stars to establish distances, he failed to carry his campaign into intergalactic space-- in part no doubt because he was disinclined to think there were any stars beyond the Milky Way. (According to one undocumented story, Humason once showed Shapley evidence of Cepheids in a photograph of a spiral galaxy, but Shapley, convinced that the spirals were whirlpools of gas, wiped off Humason's identifying marks,explaining that there could be no Cepheids there.)...
For convenience, cosmologists these days combine the Hubble constant and the deceleration parameter into the quantity omega, symbolized by the Greek letter {}. Omega directly indexes the cosmic mass density. As mentioned in the preface, if omega is less than one the universe is destined to expand forever, and if omega is more than one it is destined to collapse. If omega is exactly one, the universe is at critical density and will expand forever, at a rate that approaches but never quite reaches zero.
{Our galaxy belongs to the Virgo Supercluster.}
Cepheid variables are young giant stars that have entered an unstable stage of evolution. The specifics of why they pulsate provide an example of the elegance one can find in something as simple as a star. As a Cepheid contracts, it gets hotter. Heat flowing into the outer portions of the star (its "atmosphere") energizes atoms of singly ionized helium. ("Singly ionized" means that these atoms are missing one of the electrons they would normally have.) The added energy knocks another electron off the helium atoms, making them doubly ionized. Doubly ionized atoms tend to absorb light. As a result, the atmosphere of the star becomes opaque. An opaque atmosphere retains heat, like a blanket; therefore it grows hotter. As it gets hotter it expnads. As it expands it cools--naturally enough, since it now is spreading all its energy over a greater area. As helium atoms cool, they return to their singly ionized state. The atmosphere, transparent once again, beings to collapse, and the cycle begins anew. Each cycle typically takes a few weeks.
There are two types of supernovae, designated Type I and Type II.
Type I supernovae are thought to arise in binary systems. The supernova candidate is a dwarf star whose orbit carries it close enough to its larger and less dense companion star so that it can, by virtue of its gravitational force, strip gas from the blowsy atmosphere of the companion. As time passes, the dwarf keeps gaining weight in this fashion, until eventually its mass surpasses the Chandrasekhar limit (named for the Indian astrophysicist Subrahmanyan Chandrasekhar, who discovered the phenomenon theoretically). At theis point, equal to 1.44 times the mass of the sun, the dwarf weighs so much that it collapses even further. Dwarfs are already so dense that normal atoms cannot survive inside them: Their protons, neutrons, and electrons, crushed together cheek to jowl, are kept from merging further by quantum medhanical forces acting principally amoung the electrons. (This state, called degenerate matter, is extremely dense by the terrestrial standards: A spoonful of dwarf-star matter set down on EArth would weigh as much as a Rolls Royce limousine.) Yet once a binary dwarf star exceeds the Chandrasekhar limit and collapses further, the weight of matter bearing down on the core smashes its imposing degeneracy structure, and there ensues a titanic nuclear explosion that vaporizes the star. The advantage of Type I--specifically, the subgroup called Type Ia--supernovae to cosmologists is that they all have similar absolute magnitudes. This makes them useful as standard candles. Moreover they are the brightest form of supernvae in the wavelengths of visible light, making them conspicuous to astronomers searching the skies. Preliminary measurements of Type Ia supernovae indicates a value for the Hubble constant of about 50, yielding an age for the cosmos comfortably greater than that of the oldest stars.
While Type I supernovae are dwarfs, Type II supernovae are giants. They collapse not because they have gained mass, but because they have run out of nuclear fuel at the core. As they reun out of fuel they become unstable--there is no longer enough radiative pressure pushing outward to balance the inward pull of gravity-- and then they deflate. Since giant stars burn furiously and consequently die young, Type II supernovae are usually found in the arms of spiral galaxies, where the stars originated and from which location they have not had time to venture very far. Type II's are seldom found in elliptical galaxies, where few new stars form, while Type Is may turn up anywhere there are binary stars, which is to say in all sorts of galaxies. Type IIs are more powerful than Type I supernovae, but they look dimmer--a full magnitude, meaning 2.5 times, dimmer--because they release 99 percent of their energy not as light but in the form of neutrinos....
It is known as the "Geminga" pulsar. Estimates of the pulsar's trajectory and the rate at which its spin is slowing down suggest that the pulsar has traveled as much as 1,000 light years since originating in a supernova that occurred about three hundred thousand years ago, probably within a few hundred light-years of Earth. That's close enough for our distant ancestors to have beheld the supernova as a "new" star in Orion, bright as the full moon, dominating the sky for about two years. The supernova's shock wave, arriving at the solar system roughtly ten thousand years later, would have been deflected by the solar wind before it reached Earth. But it could have left traces farther out, on the pristine surfaces of Saturn's and Neptune's moons. If telltale evidence of the supernova is found there by future space probes, we will have another instance of how cosmic evolution can influence the local environment.