Generations of stars continue to recycle the same galactic matter through their cores. There is a gradual overall increase in the abundance of elements made by fusion. 70% Hydrogen 28% Helium 2% “heavy” elements CHEMICAL ENRICHMENT is the increase of “heavy” elements due to on-going star birth and star death. The INTERSTELLAR MEDIUM slows down and stops the matter ejected from stars – otherwise it would get lost to intergalactic space. Before stars from, gases must cool – clouds of atomic hydrogen become clouds of molecular hydrogen as cooling takes place. Matter added to interstellar space by Stellar winds Star “death” events (supernovas, planetary nebulas, etc) Low mass stars like the Sun have stellar winds which grow a lot stronger when the star inflates to a red giant.
The Sun will return ½ of its mass to interstellar space. The other ½ will comprise the remaining white dwarf. We believe this to be true for all low mass stars. High mass stars will return mass to interstellar space, as well, and the remaining mass will comprise the core – a neutron star. This can become a black hole if the conditions are right. High mass stars are dynamic and explosive! During a supernova event, a BUBBLE of hot, ionized gas will expand rapidly around the star. Bubbles actually make up between 20-50% of the disk of the Milky Way. Bubbles are not easy to detect. Shock waves are waves of pressure that move faster than the speed of sound in the gas. a A SUPERNOVA REMNANT can be seen as a leading edge or wall of fast moving gas. Supernova remnants can generate COSMIC RAYS, which are electrons, protons and neutrons travelling at the speed of light. They can penetrate the Earth’s atmosphere and reach the surface. You are struck by about one per second, on the average. While flying in an airplane, you are struck by more. Cosmic rays tend to channel along the magnetic field lines at the North and South Poles. Flights are restricted along paths over the Earth’s poles because of this.
The bubble from a single supernova event can be 100 light-years (30pc) across. It is common to find enormous SUPERBUBBLES. These occur because stars generally form in CLUSTERS. In this case, the stars are about the same age (cosmically speaking) – they form at about the same time and will each die at about the same time (within a few tens of thousands of years of each other) Each star generates its own bubble. Coalescing bubbles form superbubbles, which can be thousands of light years across! Some superbubbles break out of the disk of the MWG – this is called a BLOWOUT, similar to a gigantic volcanic eruption. The superbubble wil shoot out into the HALO of the MWG. Gravity will pull the bubble back down to the disk. The gas cloud will begin to cool and eventually become available for new star birth. The rising and falling of the gas bubble is described as a GALACTIC FOUNTAIN If you were to take an interstellar voyage across the MWG, the majority of your time would be spent cruising through regions of warm, atomic hydrogen interspersed with bubbles of hot, ionized gas. Every thousand of light years or so you would encounter a cooler, denser cloud of atomic hydrogen. In these cooler regions, the density is 100 times greater (than in the warm regions) and there is a stronger magnetic field.
Matter remains in the warm atomic stage of the star-gas-star cycle for millions of years. Gravity slowly draws blobs of this gas together into tighter clumps, which radiate energy more efficiently as they grow denser. This process of shrinkage and coagulation takes a much longer time than any of the other steps in the journey from star death to star birth. This accounts for the large amount of gaseous matter in the atomic hydrogen stage. Some of the heavy elements in the region of atomic hydrogen is in the form of DUST GRAINS, flecks of carbon and silicon that resemble particles of smoke and form in the winds of red giant stars. Dust grains remain n the interstellar medium unless they are destroyed by a passing shock wave or incorporated into a PROTOSTAR. Although dust grains make up about 1% of the mass of atomic hydrogen clouds, they are responsible for the absorption of visible light that prevents us from seeing through the disk of the galaxy. As temperatures drop further in the center of a cool cloud of atomic hydrogen, hydrogen atoms combine into molecules forming MOLECULAR CLOUDS, the coldest and densest collections of gas in the interstellar medium. Throughout much of this molecular gas, the temperatures hover just above ABSOLUTE ZERO.
Molecular hydrogen (H sub-2) is the most common gas, by far, in these clouds. Molecular hydrogen is too cold to produce an emission line, so much of what we know about molecular clouds is based on the emission lines of the other compounds in these clouds – Carbon Monoxide, Water, Ammonia and Alcohol. There are about 80 different molecules in the cold, dense clouds – these are the most common. Gravitational forces in molecular clouds gather molecules into the compact cores that eventually become protostars. Protostars often have violent jets of gas spurting outward. This turbulence will stir up nearby regions of the molecular cloud, probably preventing other stars from forming in the vicinity of the growing protostar.
Once a few stars form in a cluster, their radiation begins to erode the surrounding as in the molecular cloud. UV photons from young, high mass stars heat and ionize the gas, and winds and radiation pressure push the ionized gas away. This prevents much of the gas in a molecular cloud from turning into stars. The Eagle Nebula (p.551) is a classic example of molecular cloud erosion in an area of star birth. Sometimes the UV radiation will sear the surface of the molecular cloud. This will strip the electrons away thus destroying the molecules. This matter that evaporates from the molecular cloud will join the hotter ionized gas. Despite the recycling of matter from one generation of stars to the next, the star- gas-star cycle cannot go on forever. With each new generation, some of the galaxy’s gas becomes permanently locked away in stellar corpses (white dwarfs, neutron stars, black holes) that never retuen material to space. The interstellar medium is slowly running out of gas, and the rate of star formation will gradually taper off over the next 50 million years or so. Eventually, star formation will cease.