Astrophysical Directions by Michael Erlewine

 

 

In Astrophysical Directions
Introduction Coordinate Systems The Solar System The Solar Neighborhood The Galaxy Galactic Objects The Fixed Stars Star Clusters & Nebulae Non-Visual Astronomy External Galaxies Finder-Lists, etc.
Galactic Objects

 

  Stellar Evolution: The Lives of Stars

  Hertzsprung-Russell Diagram

  Double Stars

  Visual Binaries

  Spectroscopic Binaries

  Eclipsing Binaries

  Variable Stars (Types)

  Two Types of Variables: Instrinsic / Extrinsic

  Periodic Variables

  Cepheid Variables

  RR Lyrae Stars

  W. Virginis & RV Tauri Stars

  Long Period & Semi-Regular Variables

  Irregular Variables

  Flare Stars

  Magnetic Stars

  Cataclysmic Variables

  Dwarf Novae, Recurrent Novae

  Supernovae

  Supernovae Remnants

  Novae

  White Dwarfs

  Wolf-Rayet Stars

 

Stellar Evolution: The Lives of Stars

An investigation of our universe becomes the story of the stars, for aside from dust and gas, space contains: stars. Even such exotic objects as pulsars, neutron stars, and black holes are only the remains of stars. Almost all of the information assembled through the various branches of astronomical observation: visual, infrared, ultraviolet, x-ray and gamma-ray may best be examined in terms of the following question: What stage in the life history of a star do they describe?

Therefore, a grasp of the basic stages in the life history of a star provides an essential framework for astrophysical inquiry. It is difficult, perhaps impossible, to consider the various stages of life (and the sequence of these stages) in a star's history without being moved at the resemblance to our own life story. Here it seems is our own life story acted out in a macrocosmic drama before our eyes. It is now considered fact that the birthplaces of stars are the vast nebular clouds of dust and gas distributed throughout space (see nebulae, interstellar dust, T Tauri stars).

In these relatively cool and dark clouds, protostars form through a process of gravitational condensation or contraction. It is imagined that perhaps some outside force in the form of gravitational energy from a passing stellar object perhaps causes a dust cloud to begin the contraction process. These huge clouds are known to be non-homogeneous. They contain spots where the gas and dust is somewhat denser than in the surrounding regions of the cloud. Perhaps stimulated in some way, these denser areas attract still more material toward themselves until a huge amount of matter (many times the size of our solar system) is formed.

 

Star Diagram

 

See Figure A, where several protostars are forming in a vast cloud complex. The contraction process becomes acute, for nothing within the protostar can hold up the crushing weight of gas and dust that continues to accumulate. A crisis is reached. Through a friction-like process, the ever increasing pressure and density inside the protostar causes the temperature to rise in the protostar's center or core until 10 million degrees initiates a thermonuclear reaction. This reaction releases enormous radiant energy that, pushing out from within, stops the contraction process and: a star is born!

From this point forward, the life story of a particular star is dependent upon the size or mass of the original protester. The collapse of the proto-stellar material takes a relatively short amount of time in the star's life and, once the thermonuclear ignition takes place, the star's surface temperature rises rapidly and the star settles down to about ten billion years worth of being a star, in the common sense of the word. It is important, at this point, to examine the struggle going on within the stellar interior.

Once born, the star must live and die. The death of stars is inevitable and the life process is often conceived as one of thwarting or putting off this inevitable death, and thus prolonging life. The most fascinating aspect of the star's life is the intense struggle between the forces of gravity and contraction on one hand (the so-called outer forces) and the internal forces of radiation pressure pushing outward. As long as there is radiation coming from within, the forces of gravitational contraction are balanced, and stellar life as we observe it continues. In fact, the entire life of the star has to be conceived in terms of a continuous conversion process. These two archetypal forces create the stellar shell, which is well below the actual surface of the star itself. The thickness of this shell as well as its position near to or far from the stellar core suffers continual change and adjustment throughout the life of the star. The incredible weight of the many layers of gas first initiated continues to maintain and contain the radiant process -- a cosmic crucible. This pressure, and the inevitable collapse that must occur, is forestalled and put off by an incredible series of adjustments and changes going on within the core of the star. First of all, hydrogen burning (initiated at the birth of the star) continues for around ten billion years and this constitutes a healthy chunk of the stellar lifetime.

Our Sun is about half way through this stage at present and we can expect the Sun to continue as it is today for another five billion years or so. The exhaustion of hydrogen signals the onset of drastic changes in the life of the star and the next stage of that life. The radiant pressure of burning hydrogen has been all that has held back the initial contraction of the protester and when this is gone the stars core continues to contract. It has no material strong enough to stop such contraction and the core again shrinks, causing increased pressure, density, and temperature.

When the temperature at the center of the star reaches 100 million degrees, the nuclei of helium atoms (by-products of the hydrogen burning stage) are violently fused together to form carbon. The ignition of this helium burning at the stellar core again produces a furious outpouring of radiant energy and this energy release inside the star's core (as the star contracts) pushes the surface of the star far out into space in all directions. The sudden expansion creates an enormous star with a diameter of a quarter of a billion miles and a low surface temperature of between 3,000-4,000 degrees K -- a red Giant. In about five billion years, the core of our Sun will collapse while its surface expands, and this expansion will swallow the Earth. Our planet will vanish in a puff of smoke. Red stars like Antares and Arcturus are examples of this stage and kind of star. This Helium burning stage of the red giant continues for several hundred million years before exhaustion. With the Helium gone, the contraction process again resumes, and still greater temperatures, densities, and pressures result. At this point, the size or mass of the star begins to dictate the final course of the life. For very massive stars, the ignition of such thermonuclear reactions as carbon burning, oxygen burning, and silicon burning may take place, creating all of the heavier elements. These later stages in stellar evolution produce stars that are very unstable. These stars can vary and pulsate in size and luminosity leading in cases to a total stellar detonation -- a supernova.

Like ourselves, a star may end its life In one of several ways. When all the possible nuclear fuels have been exhausted (all conversions or adjustments made), the inexorable force of gravity (the grave) asserts itself and the remaining stellar material is confined in a sphere called a white dwarf. As the star continues to contract (having no internal-radiation pressure left), the pressures and densities reach such strength that the very atoms are torn to pieces and the result is a sea of electrons in which are scattered atomic nuclei. This mass of electrons is squeezed until there is no possible room for electron change and the very force of this impossibility withstands further contraction. The resulting white dwarf begins the long process of cooling off out in space. Becoming a white dwarf is only possible for stars with a mass of less than 14 solar masses. If the dying star has a mass that is greater than this limit, the electron pressure cannot withstand the gravitational pressure and the contraction continues. This critical limit of 14 solar masses is termed the Chandrasekhar Limit.

To avoid this further contraction, it is believed that many stars unload or blow off enough excess mass to get within the Chandrasekhar limit. The nova process is an example of an attempt of this kind. In recent years, it has become clear that not all stars are successful in discarding their excess weight and for them a very different state results than is found in the white dwarf. We have seen that the electron pressure is not strong enough to halt the contraction process and the star gets smaller and tighter. The pressure and densities increase until the electrons are squeezed into the nuclei of the atoms out of which the star is made. At this point, the negatively charged electrons combine with the positively charged protons to produce abundant neutrons. The resulting neutron force is strong enough to once again halt the contraction process and we have another type of stellar corpse: a neutron star. For a more detailed account of the neutron star, see Pulsars.

We have one further kind of 'dead' star. There is a limit to the size of star that can become a neutron star. Beyond a limit in mass of 21 solar masses, the degenerate neutron pressure can not withstand the forces of gravity. If the dying star is not able to eject enough matter through a nova or supernova explosion, and the remaining stellar core contains more than three solar masses, it cannot become a white dwarf or a neutron star. In this case there are no forces strong enough to hold up the star, and the stellar core continues to shrink infinitely! The gravitational field surrounding the star gets so strong that space-time begins to warp. When the star has collapsed to only a few miles in diameter (yet still has the same mass!), space-time folds in upon itself and the star vanishes from the physical universe. What remains is termed a black hole. (see section on Black Holes)

It should be clear at this point that all of the many kinds of stars and objects in space could be ordered in terms of the evolutionary stage they represent in the life of the stars. Just as each of us face what has been called the "personal equation" in our lives, so each star's life is made possible by the opposing internal and external forces. In the end, it appears that the forces of gravity dominate the internal process of adjustment and conversion that is taking place, just as in our own lives the failure of our personal bodies is a fact. And yet fresh stars are forming and being born, even now. The process of star life is somehow larger than the physical ends to the personal life of a star or a man, and our larger life is a whole, continuum, and continuing process that we are just beginning to appreciate. Some of the ideas that are emerging in regard to the black hole phenomenon are most profound, and perhaps are the closest indicators we have of how the eternal process of our life, in fact, functions.

 

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Hertzsprung-Russell Diagram

The Hertzsprung-Russell diagram is said to be the most important in all astronomy. It is a graph obtained by plotting the luminosities versus the temperatures of stars. As shown in the diagram, the luminosities are measured along the vertical axis and the surface temperature or spectral type along the horizontal axis. Every star for which the luminosity and temperature are known can be represented in this graph. Notice that the stars are not scattered in a random fashion over the diagram, but are grouped in three main regions.

 

The Life Story of our Sun

 

This tells us that there is a precise relationship between the temperature and luminosity of stars. Most stars are located along the main sequence that runs diagonally from the hot & bright stars in the upper left to the cool & dim stars in the lower right. Our Sun is near the middle of the main sequence. There is a second major grouping of stars in the upper right-hand corner of the diagram. These stars are bright & cool and called red giants. Betelgeuse, Antares, and Aldebaran are red giants. A third group of stars, white-dwarfs, can be found in the lower left-hand corner, These stars are hot and dim.

 

Stellar Evolution on the Hertzsprung-Russell diagram

 

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Double Stars

Not all stars stand alone as solitary beacons in space. A surprising number of stars are double, made up of two separate components, which may be either perfect twins or decidedly unequal in size and luminosity. Double stars, whose components are intrinsically associated and are in motion round a common center of gravity, are known as binaries. Optical doubles, in which the appearance is due to a chance alignment with one star almost behind the other, are much less common.

For a long time astronomers thought all double stars were a result of such chance alignment. The first discovery of a double star with a telescope occurred in 1650, although both Arabs and North American Indians have used the double stars Mizar and Alcor as a test of keen eyesight for centuries.

The optical doubles are often divided into two types: equal doubles and unequal doubles. In the equal doubles, both components are of similar magnitude (example: Gamma Aries), while in the unequal doubles, the magnitudes as well as the colors of the two stars will differ. (example: Beta Cygni). Again: optical doubles result when two stars appear close to one another through chance alignment rather than gravitational interdependence. One star may be at a much greater distance from the Earth than the other.

 

The Earth's view on various kinds of double and binaries

 

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Visual Binaries

Around 1767 it was recognized that most close pairs are not all optical (chance alignment), but in fact many double stars are physically associated with one another. These we called Physical Doubles or Binary Stars. Figure A. shows an unequal binary system in orbit around a common center of gravity. Perhaps the most famous binary system is that of Sirius found in 1834. The bright star Sirius displayed a 'weaving' sort of proper motion through space and it was deduced that this perturbed motion was caused by the presence of an invisible companion. The companion was discovered in 1862 and is only one ten-thousandth of the luminosity of its primary (Sirius) and is now known to be a dense star called a white-dwarf. (See Figures B and C) Most binary systems can only be resolved into two distinct components through the use of a large telescope and a device called a filar micrometer.

 

Physical doubles or binaries

 

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Spectroscopic Binaries

Spectroscopic binary stars are double stars that appear single in even the largest of telescopes, but whose double-ness becomes apparent from periodic changes in their spectra. The brighter component of the double star Mizar (Zeta Ursa Major) was the first star to be recognized as a spectroscopic binary. Some spectroscopic binaries have orbits oriented to our perspective such that they pass in front of one another or eclipse each other. These are termed eclipsing binaries and are illustrated in Figure D. Algol or Beta Persei, which undergoes eclipse every few days, was the first known eclipsing binary. Astrometric Binaries are a group of double stars in which the presence of the unseen companion is determined by its gravitational action on the motion of the visible primary, much like Neptune was discovered through the perturbations of Uranus. Both Sirius and Procyon are examples of astrometric binary systems.

 

Binary System

Figure A (above): Binary System. The two components revolve around the center of gravity of the system. The intersecting lines represent the center of gravity of an unequal system. The more massive component has the smaller orbit (a,b.c ... h), while the less massive component has the larger orbit (A,B,C ... H). If these were equal components, the center of gravity would be midway between the stars.

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Eclipsing Binaries

Spectroscopic binary stars are double stars that appear single in even the largest of telescopes, but whose double-ness becomes apparent from periodic changes in their spectra. The brighter component of the double star Mizar (Zeta Ursa Major) was the first star to be recognized as a spectroscopic binary. Some spectroscopic binaries have orbits oriented to our perspective such that they pass in front of one another or eclipse each other. These are termed eclipsing binaries and are illustrated in Figure D. Algol or Beta Persei, which undergoes eclipse every few days, was the first known eclipsing binary. Astrometric Binaries are a group of double stars in which the presence of the unseen companion is determined by its gravitational action on the motion of the visible primary, much like Neptune was discovered through the perturbations of Uranus. Both Sirius and Procyon are examples of astrometric binary systems.

 

Binary System

 

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Variable Stars (Types)

The term 'Variable Star' originally referred to those stars that vary in their brightness. All stars vary in one way or another. There are about 20,000 recognized variable stars listed in the well-known Catalogue of Variable Stars (See bibliography). The major types of variable stars are listed below.

 

Pulsating VariablesNumberExplosive VariablesNumber
C Classical Cepheids 696 N Novae 203
I (L) Irregular Variables 1687 Ne Nova-lie Variables
M Mira Ceti 4600 SN Supernovae 7
SR Semi-regular Variables 4423 RCB R Cr Borealis Stars 31
RR RR Lyrae Variables 4423 RW (I) RW Aur, T Tauri Stars 1005
RV RV Tauri Stars 100 UG U Geminorum Stars 210
C Cephei Stars 14 UV UV Ceti (flare) Stars 100
SC Scuti Stars 12 Z Z Camelopardalis Stars 19
CV CVn Stars 28

 

Eclipsing variables of all kinds total 4018. The more recent designations in the above table are in parenthesis. Individual variable stars within each constellation are named by letters and numbers that indicate their order of discovery. The first variable found within any constellation has the letter R assigned to it (example: R Coronae Borealis). Subsequently discovered variables take the letters S, T ... to Z; then RR, RS, RT ... SS, ST, and so on through ZZ.

After ZZ, variable stars are named starting from the beginning of the alphabet with AA, AB, AC ... through AZ, then BB, BC through BZ, and son on through QZ.

The preceding will take care of 344 stars in each of the constellations. If there are still more variables, they receive numbers from 335 on, preceded by the letter 'V' and followed by the constellation. An example would be V 335 Cygni, being the 335th variable star discovered in the constellation Cygnus.

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The Two Types of Variables

Extrinsic Variables

Extrinsic are not "true" variable stars. Their variations are not caused by something happening within them, but by the intervention of some external action or by changes in aspect, as when an ellipsoidal star revolves or rotates. The eclipsing variables are extrinsics that change brightness when two stars eclipse one another, total or partially, and sometimes stars appear to have variance due to obscuring matter drifting in front of them. These often show physical interaction with interstellar matter.

Intrinsic Variables

These are the true variable stars, for something within them is happening to cause the variation in their appearance. The rest of this article will describe some of the basic attributes of the various major types of intrinsic variable stars. A more thorough introduction to this category of fixed stars may be found in the bibliography at the end of this seroes of articles. Some intrinsic variables have a more or less regular rhythm or period and are termed Periodic Variables, while others are only periodic in a rough fashion and ire termed Semi-regular or Cyclic Variables.

These semi-regular variables may be seen to dissolve, in a step-by-step manner, into those stars whose variations show no obvious pattern, the Irregular Variables. The most spectacular of all the variables are the Novae, Supernovae, and other Cataclysmic or Exploding Variables.

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Periodic Variables

The periodic variables are giant and super-giant stars with periodic variations ranging from an hour or so to three years in length. In spectral class, the stars range from A to M and N (see Stellar Spectra). Those with the longest period have spectra of the latest type and those with the shortest periods tend to be A (or B) stars. Those of longest period are called Long-Period Variables and are red-giant stars. Stars with periods between a day and fifty days or more, the Cepheid Variables, are super-giant stars with spectral types near F or G at maximum. Stars with periods less than a day, called the RR Lyrae Variables, have spectral types between A and F, and absolute magnitudes near zero. Between the long-period variables and the RR Lyrae stars occurs a less well defined series of periodic variables, with giant or super-giant luminosities and spectral types between F And K.

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Cepheid Variables

The Cepheid Variables, so named because the naked-eye star delta Cephei is a typical example and first discovered, are all giants or super-giant stars. The pole star Alpha Ursa Minor is a Cepheid variable. The Cepheids are pulsating stars with periods ranging from a few days to several months and spectral types from F or G (at maximum light). All are reddest at minimum light, but never of spectrum later than K. The Cepheid variables are most important because they are bright enough to be observed in other galaxies besides our own, such as Andromeda. Since we know that the Ionger the period of a Cepheid, the more luminous it is, these variables have served as the 'standard candles' with which to explore external galaxies such as the Magellanic Clouds. Over 600 Cepheids have been discovered in other galaxies, as well as our own and this group of pulsating stars are often called Classical Cepheids. Cepheids are associated with dust filled regions of space.

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RR Lyrae Stars

RR Lyrae stars were first known as Cluster-type Variables, since they were discovered (in large numbers) in the high velocity globular clusters. RR Lyrae stars have absolute magnitudes near zero and spectral types near class A5. These stars are pulsating stars of very short period (usually less than one day) and only a slight dependence exists between period and luminosity, unlike the Cepheids. RR Lyrae stars (named after the star RR Lyrae, one of the brightest in this group) are very numerous in our galaxy, but are too faint to be seen in any but the nearest of the external galaxies. Some have been observed in the dwarf system in Sculptor and the Magellanic Clouds. RR Lyrae stars posses a high velocity motion that associates them with objects in the nucleus of our galaxy and other dust-free regions of space. As mentioned, they were first discovered in large numbers in globular clusters, which are very old and relatively free of interstellar dust.

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W. Virginis & RV Tauri Stars

W Virginis and RV Tauri stars have periods of more than a day to over a hundred days. This group spans the range between the RR Lyrae stars and the long-period variables. These stars all occur in globular clusters (as do the RR Lyrae stars) and are often called Type II Cepheids. They have a spectral class of F to 6, but display strong bright lines of hydrogen. The group of shortest period is called the RV Tauri stars, after a typical specimen while those having periods between ten and thirty days are W Virginis stars. Whereas all the classical Cepheids are found in the galactic plane (within the layer of dust and gas), the type II Cepheids occur at large distances from the plane of the galaxy and form a more nearly spherical system, like that filled by the globular clusters.

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Long Period & Semi-Regular Variables

Long-Period Variables

The Long-period variables, as their name suggests, have periods that range from ninety days up to six or seven hundred. They have spectra of Classes M, S, R, and N, -- the coolest stars -- and large light ranges (from between three to six magnitudes). They are very common in our galaxy, which contain 100,000 such stars. Perhaps the most famous of the long-period variables is Mira Ceti, "the Wonderful", which has been known for centuries. These stars are rare or never found in globular clusters.

Semi-Regular Variables

The semi-regular red variables form a group that grades into the long period variables. They have similar periods and spectra, but their ranges of brightness and their spectral class are much smaller. These stars may be considered cyclic rather than periodic. The lengths of individual cycles and the forms of individual light variations are much more irregular than for the long-period variables, which in turn are less regular than the Cepheids. The semi-regular red variables may be on the verge of becoming long-period variables. This group contains a number of super-giant M stars, such as Betelgeuse and Antares, with absolute visual magnitudes near 4 and a large enough angular diameter to be measured with the interferometer.

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Irregular Variabless

Still other variables may be termed Irregular, for they suffer brightness changes in abrupt and unpredictable fashion. These stars may continue at a constant brightness or, after small fluctuations for years or months, drop suddenly in brightness by six magnitudes in days or weeks, and return to maximum brightness over a period of years. The most famous irregular variable is R Coronae Borealis.

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Flare Stars

Flare stars are main-sequence stars that abruptly brighten by several magnitudes for a very sort time, then quickly revert to their usual brightness. These flare-ups are erratic, with no detectable periodicity. There is also a large increase in the ultraviolet (UV) in many cases. Typical flare stars such as AD Leonis and YX Canoris exhibit small microflares as often as several times each night, while larger spectacular flares are observed with frequencies of once a week to once a month.

Many astronomers believe that the flare phenomenon represents a localized release of energy within the star atmosphere, similar to flares on the Sun. From this point of view, the Sun is a flare star, but the brightness of a flare star so small compared with the Sun's total brightness, that the solar luminosity is not appreciably increase during a flare. One of the Sun's nearest neighbors, Proxima Centauri, is a flare star.

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Magnetic Stars

The existence of strong magnetic fields in certain variable stars has been known since 1946, but the observation of the longitudinal Zeeman-effect in the spectra of such stars. The magnetic field strength often shows strong fluctuations of an irregular type and often also a reversal of polarity.

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Cataclysmic Variables

The Explosive or Cataclysmic Variable, in which the star undergoes some sort of explosion, has fascinated mankind for centuries. An otherwise apparently normal looking star will suddenly brighten, reach a maximum, and then fade away in a more gradual manner. In general, the cataclysmic variable stars are divided into three groups on the basis of the intensity of the explosion or outburst: Dwarf Novae, Novae and Recurrent Novae, and the Supernovae.

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Dwarf Novae, Recurrent Novae

The brightness or Luminosity of an ordinary nova may increase by a factor of 100,000 (12.5 magnitudes) within only a day or two. This sudden brightening of the star into a nova results from a sudden swelling of the photosphere -- an explosion. Although both the temperature and radius of the star undergo change through the nova process, the rapid expansion of the star's radius is by far the more important of the two. The temperature even grows cooler as the star nears maximum.

Examination of the spectra (see Spectral Types) of novae bears out the idea that an explosion is taking place. Bright lines appear and become progressively more prominent. This is an indication that matter has left the surface of the star and surrounded it with kind of an extensive chromosphere. As the nova process continues, spectral analysis gives evidence of decreasing density. Bright lines, characteristic of most diffuse nebulae, appear and it is therefore understood that the envelope of the star is expanding into the surrounding space. A portion of the star is ejected.

Dwarf Novae

There are different kinds of novae. The Dwarf Novae (also called SS Cygni or U Geminorum stars) are repeating variable novae with a range up to six magnitudes. These dwarf novae repeat their outbursts at quasi-periodic intervals of a few weeks or months and are faint at minimum and around zero magnitude at maximum.

Recurrent Novae

The Recurrent Novae, much larger than the dwarf novae (they range in light between 8 and 10 magnitudes, undergo outbursts at irregular intervals of several decades. The fact that a star can undergo the nova process more than once is thought provoking. As violent as the nova process may appear, it seems to be but a pasting incident in the life of the star, which returns to very much the same condition that existed before the outburst. As we shall see later in the text, this is not true for the supernovae. The variable star T Coronae Borealis is a classic recurrent nova with outbursts in 1866 and in 1946.

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Supernovae

The nature of the Supernova explosion is another story entirely from that of the relatively tranquil nova process. Unlike the novae, whose essential condition remains unaltered after the blow-up, the supernova may suffer a complete collapse of its stellar core resulting in a super-dense star or even a black hole. (see Pulsars, Black Holes, Neutron Stars).

Supernovae are exploding stars reaching extreme luminosity (-18 Absolute Magnitude maximum), and a supeenova may outshine the combined luminosity of the entire galaxy in which it appears! Supernovae are also quite rare. There have been but a handful of supernovae within our galaxy in recorded history. Most supernovae are found in external systems or galaxies, and to date more than 400 such supernovae have been discovered. One of the best known appeared in M.31 (the Andromeda Galaxy) in 1885. There are at least two types of supernovae: SN I and SN II. Type I SN are powerful and brilliant, while Type II SR are faint and much less energetic. It is now believed that Type I SN are formed by the members of double-star systems. The cause of a supernova outburst is still the subject of intensive investigation (even controversy), but it is agreed that the onset of the explosion is ultimately related to instabilities in the structure of the star that arise when the supply of nuclear fuel in the central parts of the star is exhausted (see section on Evolution of Stars for more detail).

These instabilities occur only in stars whose mass is greater than about 1 1/2 times that of our Sun. Less massive stars, including the Sun, begin to contract when their nuclear fuel is consumed. In time, the pull of gravity is balanced by the pressure of degenerate electrons, an incompressible electron fluid that finally emerges because no two electrons can occupy the same energy state. When this stable configuration is reached, the star is called a white dwarf (which see), and gradually dies "not with a bang, but a whimper" as scientists delight in quoting. With stars of 1.5 solar masses, the density and temperature in the central core exceed the critical values beyond which stability is possible. The star collapses under the influence of gravity and an explosion occurs. In supernovae the outer shells of the collapsing star are ejected at ultrahigh velocity. In some, if not all, cases, a dense relic is left behind -- rotating neutron star or perhaps even a black hole! The resulting magnetic field on the surface of a neutron star can be more than a thousand billion times stronger than the average magnetic field on the Sun.

 

Historical Supernovae within our Galaxy

 

We have not observed any Supernovae.in our galaxy in over 300 years. Tycho Beahe wrote in De Stella Nova in 1573 about the supernovae that appeared in 1572: "...it was brighter than any other fixed star, including Sirius and Vega. It was even brighter than Jupite and maintained approximately its luminosity for almost the whole of November. On a clear day it could be seen ... even at noon."

The list below (with one exception) shows some of the major bright supernovae discovered in external galaxies. Figure A. shows some of the remnants of supernovae that have been discovered in our own galaxy. Many of these remnants are listed in the sections on Radio and x-ray sources. Supernovae remnants are often strong emitters of energy in the radio and x-ray frequencies Supernovae release gravitational energy in several forms. There is the radiant energy emitted in the early phases of the explosion. The matter simultaneously ejected carries away translational kinetic energy. The neutron star that survives is endowed with an enormous amount of rotational kinetic energy. As mentioned, it is believed that Type l supernovae are members of double-star systems. Their early evolution is similar to that of a single massive star. When they reach the white-dwarf stage, however, matter is transferred suddenly from the companion star, adding matter to the white dwarf and pushing the mass beyond the critical limit of 1.44 solar masses. At that point the core of the white-dwarf collapses violently, releasing energy as a supernova, leaving behind a binary system of an ordinary giant star and an x-ray source.

For a star much more massive than the Sun, the supernova evolution is different. The star also fuses hydrogen into helium in its core for a few hundred million years and, when the hydrogen is almost exhausted, the core contracts, the outer layers of the star expand, and the star becomes a red giant. Hydrogen continues to be burned in a shell around the core, as the core itself contracts until it heats up enough to fuse helium into carbon. When the helium is nearly exhausted, the core begins to burn the carbon. At that point, one of two conditions can occur. The ignition of the carbon could induce instabilities that would detonate the star as a SN II, leaving behind nothing but an expanding gaseous remnant. Or, if the carbon is safely ignited, the extraordinarily high temperatures in the core could generate neutrinos at an ever-increasing rate sapping the stars energy, causing the core to plunge to a total collapse. In this event, a final burst of neutrinos might carry away so much of the red giant's rotational momentum, that it would blow off the entire outer envelope of the star. An explosion of this kind would leave behind a gaseous remnant, in the center of which would be a pulsating pulsar a rapidly rotating neutron star) or a black hole.

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Supernovae Remnants

In our Galaxy of about 100 billion stars, a supernova occurs, on the average, once in every 100 years. For this reason, much of the research in supernovae has been done in galaxies external to our own. It is possible we may experience a supernova within our galaxy in the course of our lifetimes. Until that time, we must content ourselves with a search for the remains of previous supernovae. When a star does supernova, it radiates more energy than a billion Suns and ejects matter at close to the velocity of light for a period of about two weeks!

The expanding shell of debris creates a nebula that for hundreds, even thousands of years radiates vigorously in both the x-ray and radio regions of the spectrum. About 2 dozen of these remains of past supernovae or supernovae remnants have been discovered in our galaxy. Four of the remnants have been identified with supernovae whose sudden appearance in the sky can be found in historical records: A.D. 1006, 1054 Crab Nebula), 1572 (Tycho's Nova) and 1604 (Kepler's Nova) -- all prior to the telescope. Some of the most intense discrete radio and x-ray sources are associatled with supernovae remnants. As pointed out earlier in this article, many supernovae remnants contain a rapidly rotating super-dense neutron star called a pulsar. For more information, and the positions of galactic remnants, see Radio, X-ray, Pulsar sections of this series. (Also see the diagram of the supernovae list).

 

Historical Supernovae within our Galaxy

 

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Novae

The normal novae are brighter still than either the dwarf or the recurrent novae, and more common. More than 100 novae have been observed in our galaxy in the last one hundred years. It is estimated that about 25 novae brighter than 9th magnitude occur in our galaxy each year; although all are not visible due to either their intrinsic faintness or daytime skies.

Novae are designated by constellation and year of appearance. Novae Aquilae 1918 was the brightest seen this century. Most novae have an average range of 13 magnitudes or, in other words, an increase in brightness by a factor of 160,000 within a period of several days. The absolute visual or photographic luminosity at maximum can range as high as a million times that of the Sun. Novae may decline rapidly from peak luminosity or fade much more slowly. The novae that show rapid decline are several magnitudes brighter than those which decline more gradually.

As mentioned, it is believed that the nova explosion is confined to a relatively thin layer of the star's envelope, which expands and makes the star appear to swell. After this bubble has been blown off, the star appears little altered in either brightness or color. The bubble or expanding envelope becomes transparent as the expanding shell evolves into a nebula. The total Amount of stellar material lost is small, perhaps one thousandth of the star's mass or less.

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White Dwarfs

White dwarfs are sub-luminous and super-dense stars. A white-dwarf results when the thermonuclear reactions are exhausted in the stellar core. Not all stars become white-dwarfs (see Stellar Evolution). Only stars of less than 14 solar masses go the route of the white-dwarf. A typical white dwarf would have shed a large fraction of its mass into space perhaps in the nova or supernova process. With the exhaustion of the thermonuclear radiation, the gravitational forces cause the star to contract until the atoms have been stripped of their orbital electrons, due to the high internal pressure. The electrons, themselves, still exert an outward pressure and the star resists further, and a stable state results.

Weak nuclear reactions and the gravitational energy of contraction continue to furnish energy to keep the white dwarf feebly shining. White dwarfs have been known to astronomers for many years and are so common that it was believed that all dying stars somehow manage to eject enough material to become white dwarfs. White-dwarfs cool off and become, in time, black dwarfs., It is hard enough to see the dim white-dwarfs and no black-dwarfs have ever been found. White dwarfs occur in the lower left-hand corner of the Hertzspring-Russell Diagram (which see) or are, in other words, hot and dim.

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Wolf-Rayet Stars

The Wolf-Rayet stars or W stars are very blue and very hot. Their spectra display wide emission lines. About 25% of them are spectroscopic binaries and some are eclipsing binaries. They are enormously concentrated toward the galactic equator. Forty percent are within one degree of the equator, 70% within two degrees, and 95% within 5 degrees. Wolf-Rayet stars are highly luminous and very short-lived. They are of great value (potentially) in locating and tracing the spiral-arm pattern of our galaxy, since they do not have time to move far from the spiral arm where they originate. Several of these stars have been discovered as the central exciting star in planetary nebulae.

The Wolf-Rayet stars seem to comprise two separate spectral sequences, carbon and nitrogen stars, whose special characteristics indicate different chemical compositions. It has not been decided just where these stars fit into the Hertzsprung-Russel diagram. Their high temperatures and high luminosities indicate that they should come before the O's, while their tenuous atmospheric shells or envelopes suggest a relationship with the giant M's or the symbiotic objects.

 

 

© Copyright © 1997 Michael Erlewine

 

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