Most calculations of stellar evolution reveal that for the majority of a star's life, the outwardly apparent changes in its size, temperature or luminosity are very gradual, taking many millions of years to become noticable. There are some phases, however, when the changes may be perceptible within a single human lifetime. During the birth of a star, a #T-Tauri# or #maser# - producing phase may ensue at which time a star varies erratically in brightness over a period of hours or years. Towards the end of its life as a red giant star, its distended outer layers may partake of a semi-rhythmic oscillation which also causes the brightness of the star to vary a hundred-fold in a few days or months. Finally, for some stars that are more than 8 times the mass of the sun, the star may end its life in a titanic explosion called a #supernova#. During this fleeting phase lasting a month or so, an otherwise insignificant star may outshine its entire host galaxy. There are also other structural changes that a star may experience during its life whose observation would tell us a great deal about these little- understood episodes. For example, we know of many red giant stars, and stars surrounded by #planetary nebulae#, but until recently, astronomers have never observed a single star as it reaches either of these stages.
SIRIUS-B is the famous white dwarf companion of the nearby bright star Sirius in the constellation #Canes Major#, 8.6 light years away. The color of Sirius was reported as 'red' several thousand years ago by the Greeks, Romans and even the Babylonians. Could it be possible that within the recent span of human civilization we may have witnessed a star change from a red giant to a #white dwarf#? As a red giant star, the combined light from both stars would have been as bright as Venus in the night sky, glowing with an angry red or deep crimson color. Such a rapid change from a red giant to a white dwarf cannot currently be satisfactorily explained by modern theories of #stellar evolution# especially since no astronomer has ever detected any debris left over from the red giant star. The same historical records suggest that #Algol# and #Procyon# may also have had different colors than they do today. At the present time, no definitive conclusion has been reached on the 'Red Sirius' puzzle or the veracity of the other aledged color changes.
Although astronomers know of thousands of #variable stars#, it has always been the case, until recently, that no variable star has ever been observed to cease its pulsations. The unspoken rule-of-thumb for earth bound observers has been 'Once a variable star, always a variable star'. But, theoreticians have predicted from their models that such a phase of variability may not last for more than a few thousand years. Although it would be rare, the possibility of someone observing a star passing through this phase cannot be entirely discounted, indeed, it may already have happened.
RU Camelopardalis is a 9th magnitude, red supergiant star discovered in 1907, and classified as an ordinary #Cephied#-type variable star with a regular, 22 day period of light variation. Then, inexplicably, in 1963, the difference between its maximum and minimum brightness began to diminish; since 1965 no detectable variation has been reported. No other example of this nature is known to astronomers.
One of the most remarkable devices of science is the spectroscope invented by Joseph Fraunhofer in 1807. With it came the birth of a whole new discipline devoted to the chemical analysis of luminous bodies by merely identifying the spectral fingerprints of the known elements by the light that they emitted. It does not matter that the body is a billion miles away, or even a billion light years for that matter. By employing very sensitive, modern versions of Fraunhofer's original design, the elements in distant stars could be cataloged, and their abundances and temperatures calculated with the help of 20th century #quantum mechanics#. It was quickly discovered by the early decades of this century that all stars visible to our instruments consisted of 3 parts hydrogen for every one part of helium. In addition, somewhat less than 0.1% of a star's mass was in the form of all the remaining elements in the #periodic table#. Yet, although most stars show such a uniform composition, there appear to be a handful that depart quite markedly from this otherwise universal recipe.
Przybylski's Star, also called HD 101065, is an 8th magnitude star in #Centaurus# and a member of a class known as #peculiar A- type stars#. Visually, you would not find it particularly striking compared to its thousands of neighbors in the sky, but, unique it is! When the light from this star is analyzed with a #spectroscope#, instead of finding the familiar #spectral line# fingerprints of hydrogen, calcium, iron or other 'light elements', a bizarre mixture is revealed. Lines from elements with unfamiliar names like holmium, samarium, dysprosium, and neodymium clutter the spectrum. These lines are so strong, in fact, that those from the more familiar elements cannot be discerned at all! Since its discovery in 1963, four other 'Lanthanide series' stars have been found. Why the peculiar lines? One possibility suggests that the powerful magnetic fields on the star's surface prevent the mixing of gases here. Under the influence of #radiation pressure#, the heavier elements float to the surface and are more easily detected from the earth. In reality, the actual element abundances may be quite normal, but through the action of radiation pressure, the elements constituting the normal minority become the most easily detected in the star's spectrum.
White dwarfs represent the last stage in the lives of stars not much different than our sun. They are approximately the size of the earth, yet contain much of the original mass of the star whose core it once was. Neutron stars, on the other hand, are only about 20 miles in diameter with nearly the same mass as the sun, are as dense as the nucleus of an atom. Their origins are not clearly understood, but their formation is generally thought to involve the implosion caused by a supernova in a star perhaps 8 - 10 times the mass of the sun. Finally, for stars more massive than this, the supernova explosion proceeds with such violence that the resulting core implosion can not be resisted even by the #nuclear force#. Gravity gains the upper hand and the core's collapse proceeds unabated until at a diameter of about 1-2 miles, the core becomes a black hole. This is the most collapsed state of matter known, and from it nothing, not even light, may escape.
Astronomers are delighted when they happen to discover binary systems composed of one or more of these collapsed stellar remnants, since by a variant of Kepler's Third Law, and from estimates of their orbital periods and separations, the masses of these cinders may be ascertained: a vital ingredient for testing theoretical predictions. In some cases, we also discover that, rather than quiescent dense bodies, these objects may be the foci of some of the most violent releases of energy to be found in a purely stellar environment. Consider, for example, the binary system 4U1820-30.
This is a star system in which one member is a white dwarf and the other is probably a neutron star. Located in the globular cluster #NGC-6624# (a rare find in itself!) some 20,000 light years away in the direction of the constellation #Sagittarius#, these two stars orbit one another once every 11 minutes and are only about 80,000 miles apart. No other double star system known has a separation this little and only their diminuitive sizes make this possible. The white dwarf whose size is less than the earth's has a mass of 70 times that of Jupiter. The companion neutron star is perhaps only 10 - 20 miles in diameter, but has a mass of 1.5 times that of the sun's. 4U1820-30 is a strong producer of #X-ray radiation# It is this property that permitted its first detection from earth. The history of the system is a mystery because as normal stars, each would have been millions of miles in diameter. For their dense cinders to be as close to one another as they are now, the original stars would have been within each others atmospheres during much of their lifetime. It is intriguing to think that we are witnessing the last vestige of two stars that absorbed one another, but whose dense cores never merged completely into a single object.
SS433 An important feature of many active binary systems in which one member is a normal star while the other is a collapsed object, is the presence of an #accretion disk#. An interesting case of such a phenomenon can apparently be found in an object called SS 433. This unusual object is located 15,000 light years away inside of what appears to be a supernova remnant called W-50 (not to be confused with the petrolium product WD-40!) in the constellation #Aquila#. SS 433's optical spectrum contains moving #emission lines# that are both red and blue shifted simultaneously. The #doppler motion# of the lines imply heated gas moving at 26% the speed of light. It is believed that SS 433 may consist of two stars. One of the stars is transfering material across space to its the companion, which may be a neutron star or even a black hole. The material flows into an circulating accretion disk orbiting the dark companion, and then falls onto the surface of the companion. This mass transfer happens so quickly that two jets of ionized gas are ejected along the axis of the disk in opposite directions at nearly the speed of light. A detailed study of the physics of such a phenomenon may have considerable repurcussions for our understanding of the even larger accretion disks believed to orbit much larger black holes in the nuclei of most galaxies. At a smaller scale, they may also have been the the pre-cursors of solar systems like our own.
Although astronomers have become increasingly facil in predicting the evolution of individual stars spanning a variety of masses and compositions, the origin of star clusters continues to be only partially understood. It is believed that a large cloud collapses and fragments into a miriad of smaller, denser cores, each continuing to collapse under its own self-gravity until it becomes an individual star or binary system. The gaseous remains of the parent cloud, under the combined radiation pressure of the stars in the nascient cluster, are quickly dissipated and become undetectable in time. This is a satisfying model so far as it goes, yet, there may be more to such a model than has hitherto been considered.
AM-4 is a #star cluster# located 700,000 light years away in the constellation #Hydra#, containing only a few dozen stars, none brighter than about the 21st magnitude. It is so distant that it is probably not associated with the Milky Way, and is either a runaway or one of four known examples of an indigeneous, extragalactic star cluster; origin unknown. No small clouds as small as those responsible for producing star clusters in our own Milky Way have ever been detected in inter-galactic space. How and where was this cluster born with, apparently, no host galaxy to provide its initial conditions?
The death of a star cluster is a subject that astronomers have pondered for several decades now, much of the advancement in this field has occured with the availability of fast computers. Beginning with a few thousand stars, the mutual gravitational forces between them may be expressed using #Newton's Law of Gravity#. A considerable amount of calculations later, one may determine the appearance of such a cluster by interrogating the computer model. What the theoretician discovers is that, like a leaky bag of gas, a star cluster evolves by a slow process of attrition of its original stellar population. The process takes millions of years. One by one nearly all of the stars are ejected from the cluster leaving behind a few straggelers. The star cluster #Upgren 1# appears to have followed the model predictions quite well indeed.
Upgren 1 is cluster of stars in the constellation #Canes Venatici# containing only five members with brightnesses between 7th and 10th magnitude at a distance of only 380 light years. It is one of the sparsest clusters known, having an estimated #cluster age# of 3 billion years. Upgren 1 may have had as many as 50 members or more, but over the eons its retinue of stars has slowly dwindled to the handful we can now observe. It is unique among all known galactic #open clusters# simply because we are able to observe it in its last stages. In a few million years, the stars will continue to tug and pull at one another through their mutual gravitational forces. In the end, only a few scattered binaries will remain, and whatever pictures we have to remember its fleeting existence by.