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Space Phenomenon
Stellar Evolution, the Birth and Death of the Star.
The evolution of a star in the current Galactic configuration is comparatively well documented. There is debate on early stellar populations prior to development of the populations seen today. The discussion will concentrate on post Population III stars; the theory of Population III is discussed elsewhere, here we will concentrate on stars in a typical stellar nursery, for instance the trailing edge of spiral arms.
As discussed in the Galaxy Section the material in the spiral arms represents not necessarily bright young stars but simply a higher density of dust. There are two types of gas that interest us in these regions, H I and H II regions. H I regions also known as Ho are areas of electrically neutral hydrogen. These are cold areas that do not emit light. H II regions are areas where the gas has been ionised. That is the electrons have been liberated from their shells leaving a plasma behind. This makes the hydrogen atom positive in charge and so sometimes H II regions are called H+ regions. The most well known form of hydrogen is the 21cm line that allowed the mapping of the Galaxy. Further evidence of the matter between the stars is found in the stationary absorption lines in the spectra of certain spectroscopic binary stars. These lines specifically calcium and sodium are from the interstellar medium.
The H II regions are liberated because they coincide with areas of stellar birth. The stars may not yet be emitting light in visible wavelengths but they are shinning in the UV wavelengths with sufficient energy to promote electrons.
Genzel and Stützki 1989, conclude the findings of a large-scale mapping project conducted on the Orion Molecular Cloud using long wavelength reconnaissance. At about 450 parsecs distance the Orion Molecular Cloud is the nearest region showing recent OB formation. That is newly formed bright stars.
Genzel and Stützki describe the clumping of the interstellar medium into clouds on the scales of around 10-3 parsecs to several parsecs. Their work involves the interpretation of stellar nurseries as the product of pressure waves from supernova, stellar winds channelled via radiation and magnetic fields. This explains the highly dynamic evolution of the cloud on a time scale of 106 years. Their examination of the coagulating dust on millimetre and submillimetre observations are convincing for the birth of stars. The most important part of their work focuses on the post stellar formation of the accretionary disk. In previous rotating cloud-collapse hypothesis, the angular momentum has been too great to allow coagulation of planet sized bodies. However using UV and near IR observations Genzel and Stützki find high velocity flows moving from the central disk akin to quasar like ejections. The ejecta material has temperatures between 1000 K and 3000 K and manifests its self in two separate entities, high and low velocity jet flow. High velocity flows move with velocities in excess of 250 kms-1 where as slow flows move with velocities near or less than 35 kms-1 with inferred mass loss of 10-3±0.3 solar masses. They argue that the discovery of such jets allows the loss of angular momentum from the system in allowing the settling and accretion of matter. Their work offers an alternative to the multiple body hypothesis for separating momentum by splitting the central mass into separate protostars, a theory not in line with what we observe in our solar system.
Cameron,1988, discusses matter in the accretionary disk, the term for this form of nebula being a primitive solar nebula. However, due to the angular momentum Cameron finds that the average interstellar cloud is not unstable enough to collapse on its own. The cloud would expand indefinitely were it not for the presence of a boundary pressure. Cameron cites the presence of a surrounding gas of higher temperature and lower density as being a valid boundary. For a cloud to collapse, Cameron argues the need for external forces increasing the compressive pressure or for an internal temperature fall.
Cameron’s work concentrates on evidence of short-lived radioactives in the early solar system material. These elements are clearly the products of stellar nucleosynthesis and must have been formed a short time before the formation of the solar system. They would be transported into the primitive nebula in a time shorter than 3*106 years. In early work they were attributed to the remnants of a supernova explosion and were used to add credence to the supernova trigger hypothesis. For this theory to hold the cloud would need to exist close to an O or B star which would ionize the lower density matter raising ambient pressure and compressing it. The radioactives formed in the explosion would be injected into nebula at that point.
The paper offers a more refined examination of that theory explaining that stars would not form exactly in that manner. Cameron uses data, circa 1988, obtained from the IRAS to complete his theory. Detailed studies of dense molecular clouds have shown that spread throughout them with some tendency for local clusters are a large number of cores. These cores have a density at least 30 times nebula density and with masses of one solar mass or slightly less and radius of slightly under 0.1 parsecs. An absolute radius measure is difficult to obtain, as it is difficult to assess the diffuse boundaries of the cores. Cameron proposes the radioactives be transported via field lines into the nebula as oppose to the injection via supernovas. Cameron believes that early field lines in a stellar nursery channels material to form the protostars.
Whereas previous workers modelled the collapse of clouds surrounding protostars by assuming spherical symmetrical infall of material and dynamics that included no interference from other stars. That is cloud collapse is unaided by stellar objects even if large bodies are responsible for its initial formation, Woolfson 1998. O’Dell and Beckwitz find the probability that post cloud formation interactions could send gravitational perturbations through the nebula material allowing the initial condensation of material to begin the cycle of events. Their theory overcomes the need for removal of angular momentum by invoking stellar perturbations.
Similar to the subject matter discussed by Cameron 1988 and Genzel and Stützki 1989, they argue a supernova trigger mechanism for collapse. This theory is supported by the presence of radioactives that could only be formed from nucleosynthesis but on a time scale too short for the usual distribution of supernova remnant clouds. However, similar to Cameron their theory refines the trigger theory. O’Dell and Beckwitz assume a sequential perturbation whereby one nebula collapse would cause "nearby" nebula to begin to coalesce. This would speed the condensation stage of the accretionary theory many fold and would overcome the unlikely-hood of star formation. That is if a single supernova had only a slight chance of causing nebula collapse the amount of stars forming in the galaxy would be far fewer than observed. O’Dell and Beckwitz overcome this disadvantage.
Further work by Bo Reipurth, John Bally, Robert A. Fesen & David Devine in 1998 they found the accretion requires the release of angular momentum, they believe that an important mechanism for achieving this seems to be the production of jets along the polar axes of the young stars. But the presence of massive, luminous stars within the same star-forming region can affect the forming stars by stripping away their circumstellar envelopes with ultraviolet radiation, thereby removing the reservoir of gas from which the stars are built up and exposing the disks to photoerosion. They present observations of four highly collimated jets from young stars that appear to have been stripped of their circumstellar molecular cloud cores in this way. The production of jets seems to have been largely unaffected. If these jets are also photoionized, their mass loss rates can be determined from observations with much greater accuracy than for normal shock-excited jets.
Nature 396, 343 - 345 (1998) © Macmillan Publishers Ltd.
In early 1999 the Hubble Space Telescope returned images of disks of dust encircling young stars. Though similiar images have been resolved many times previously the recent ones offer astronomers a better view of what may be the early formative stages of planetary systems. The pictures obviously do not show planets however, the edge-on disks seen by Hubble provide some of the clearest views to date of potential planetary construction zones. Although more than a dozen possible extrasolar planets have been discovered/theorised over the past few years, as the discussion above shows, there is a lack of unequivocable information on the development of the planetessimals in the early nebula. Even in nearby star-forming regions, circumstellar disks are hard to see largely because the glare of the central star overpowers the feeble reflected light from the disk. An exception occurs when the disk is close to edge-on, eclipsing the infant sun.
"While the existence of these disks has been known from prior infrared and radio observations, the Hubble images reveal important new details such as a disk's size, shape, thickness, and orientation," said Deborah Padgett of Caltech's Infrared Processing and Analysis Center.
"(The Hubble) images show dark clumps and bright streamers above and below the dust lanes, suggesting that raw material is still falling into these disks and driving outflowing jets of gas from the forming stars," Padgett said. Padgett's results are reported in a paper to appear in the March 1999 issue of The Astronomical Journal.
NASA homepage recent developments February 1999
The Evolution of the Solar System Post or Syn Stellar Collapse.
The body of material published today still reflects the three classic lines of thought on the theory of the origin of the solar system, that is encounter, cogenetic/nebular or accretion. Cogenetic was first proposed by Immanuel Kant and in a different form by Pierre Laplace in the eighteenth century. (Nicolson 1998). It states that a star and its planets would form at the same time from as rotating body of gas. Though this theory has been modified slightly few authors argue that the planets formed before at least a protostar was in existence.
Encounter theories date back to 1745 when G.L.L Buffon suggested a comet, stellar collision caused the planets, comets being thought to be much larger in the past. (Nicolson 1988). A.W. Bickerton enhanced these theories further in 1878 when he suggested that stellar collisions might produce the planets. The final stage was by T.C. Chamberlain and F.R. Moulton who proposed ejecta from a stellar encounter rather than impact would create the planets. (Gribben 1996 and Nicolson 1988). M. Woolfson in 1978 argued that cool low density protostar interactions formed the planets. The advantage of Woolfson’s work is that it relieves the problem of angular momentum from the system. Gough ,1997, argues the loss of angular momentum from our sun to the solar wind via its magnetic field. This slows the rotation of the outer convective layers. This is a similar though less intense version of Genzel and Stützki’s findings and adds credence to their theory above Woolfson’s. Papaloizou and Lin, 1995, find that splitting of a central body could cancel angular momentum, however this clearly is not the case for our Sun and therefore we must look for other mechanisms for the formation of planets.
The disadvantage of Woolfson’s system is that it makes planets rare but large and close to their sun (Science July 1996). The other disadvantage is that it does not correlate with the IRAS data that suggests that accretion can take place within a rapidly rotating frame of reference. The indication given by Woolfson’s programme to model particle flow was that it would show how encounter theory could work, thereby starting with a conclusion and working to fit data to it. One queries the logical positivism of such a theory. When queried on his stance on globular cores, (Cameron 1988), Woolfson seemed unaware of their existence. Woolfson also argues that the "observed" extra solar planets support his theory. Unfortunately, from examination of the literature, (Schilly 1996), the presence of extra solar planets might not be convincing. The star 51 Pegasi has a planet with orbital radius of 0.05 AU and a period of four days. Tau Bootes has a planet with orbital radius of 0.0047 AU and Upsilon Andromeda has a planet of period of 0.054. (Science July 1996). Planets can neither rotate this rapidly or at such proximity to the sun without severe disruption. They would accrete matter in a stream to the star as can be seen in certain binary star systems.
Many of the problems with the papers on the subject are that the science is moving ahead rapidly and old data does not yield accurate enough information. Pre-Hubble work is now questionable and the Arcachon meeting in France, March 1998, approved the ESA Hubble successor for a 2007 launch. (Physics World April 1998.) Other research by a group led by Robert Angel have proposed a space based interferometer with aperture of 80 metres given by four satellites that would suppress Hubble. (Watson 1998.). Current knowledge of the star formation activity is based mainly on UV work. However dust grains can obscure the observations and remit in far-infrared wavelengths. (Cimatti et al. 1998). ROSAT an X-ray telescope has detected X-ray emissions from protostars. These show the collapse of an object within a nebula before it enters the visual wavelengths. (Neuhäuser 1997). This offers unparalleled views of early accretion but calls cogenetic theories into doubt, as it seems stars condense slightly before planets.
During condensation, Rothery quotes gravitational attraction as the force holding the condensing grains although electrostatic forces are more likely. (Hewman and Herbey 1996). He also states pock marks in the dust grains from evaporated ice. Close to the sun where the terrestrial planets were formed, the material is continually cooling towards zero therefore ice would not freeze then evaporate because locally temperatures never dropped low enough within time. Taylor et al. 1998 have been monitoring micrometeor accretion onto the Earth at the South Pole. These represent the Earth moving through the orbital plane of small nebula debris. Further examination might lead to a history that would corroborate Rothery but current work puts doubt on gravitational attraction and frictional pockmarked surfaces in the early nebula.
In their paper Hewman and Herbey 1996, neglect the fact that chondrules of very different compositions exist and any estimates of the lost volatiles depends on assumptions about the precursors. This immediately introduces errors into their argument. Assumptions are also made in the paper by Wänke and Dreibus. In their two component system component B would have been the main constituent of Jupiter yet the Galileo Space Probe found very little water present. (NASA Web Pages Galileo Homepage) They also do not offer an explanation of how components A and B were produced. Wänke and Dreibus do not account for Cl and I concentrations in SNC as Antarctic contaminants. Their work is almost biased in that their presentation of data neglects chlorine and sulphur because of lack of correlation between SNC and Martian soil.
Most recently, September 1999, a pair of researchers from University College Dublin, have come forward to offer an explanation of chondrule formation that states that all chondrules were formed within a period of a matter of minutes 4.5billion years ago when a Gamma Ray burst irradiated the solar system. Brian McBreen and Lorraine Hanlon believe that a gamma ray burst from within 300 light years would have imparted to the disc enough energy to fuse up to 100 Earth masses of dust material into matter that solidified as chondrules. The newly formed dense chondrules would settle into a plane about the sun and condense quickly, thereby overcoming the paradox of removing the angular momentum from the system. Unfortunately were this theory proven correct it would appear that the solar system would be a rarity in space. (September 11 1999 New Scientist)