Untitled

Continuation of Stars page

Let us take a worthy diversion to learn some of the characteristics of a single typical Main Sequence star: our Sun. You can start with an overview by running through the U. of Oregon Solar Astronomy page. Marshall Space Flight Center hosts a Solar Physics site where you can pick up much useful information by clicking on topics in the left margin. These sites supplement the treatment here.

The importance of our Sun, underlying the reasons for the attention given it below, is its internal density and temperature profile, as seen in this diagram. It indicates that in the core the temperature will have risen to levels that permit (require) fusion of hydrogen:

Density and temperature profiles for the Sun's interior.

The Sun, whose alternate name is Sol, is a Spectral Type G star on the M.S. and, like most stars of similar or lower mass, began as a T-Tauri protostar, which started out as a distended cloud of dense dust and molecular gas that took 10 - 100 m.y. to contract into the hydrogen-burning stage. The Sun is an average star on the Main Sequence. Its size is approximately 1,392,000 km (865,000 miles) in diameter (109 Earth diameters) (it is 130,000 times the volume of Earth). Temperatures in its central core are estimated to reach about 15,000,000° K; at the solar surface (the photosphere) the temperature has decreased to about 5600° C. (5770° K; 10,400 ° F). It rotates on an average of 35 earth days at its equator. Here is a diagram showing the principal features associated with the Sun (and by implication, most Main Sequence stars):

The principal interior and exterior features of the Sun.

From Astronomica.com

A slightly different version below also identifies the Sun's main features:

Another diagram showing the Sun's main components and features.

The solar "atmosphere", largely a plasma of ionized gases, extends outward from the photosphere, through the chromosphere, a transitions zone, and then to its outer atmosphere, or corona. The corona is very thin (much lower density than Earth's atmosphere), very hot (up to 4,000,000° K), and made up mainly of hydrogen and helium ions moving at high velocities (hence, the high temperatures). Both shock waves and magnetic lines of force impel the particles. The corona is hard to see under ordinary viewing conditions. But, it stands out when photographed during solar eclipses (left, below) or through an instrument called the coronagraph (right). The heights reached by the corona limit vary considerably over time, related in part to solar storms.

The solar corona, seen from Earth during a solar eclipse

The solar corona as imaged by a coronagraph that can visualize both the Sun's surface and atmosphere under the normal non-eclipse condition.

Between the corona and the photosphere is a gaseous atmosphere called the chromosphere (brighter inner zone) in which temperatures vary around 500,000°K. The next image, made by the SOHO satellite launched in 1996, shows the photospheric surface, with its distinctive convective patches known as granules and sunspots. Superimposed around this image is a telephoto of the Sun's chromosphere taken during the solar eclipse of February 1998 and part of the lower corona, composed of tenuous gaseous envelope in which kinetic temperatures of fast-moving molecules range from 1,100,000 to 1,670,000 °C.

SOHO image of the surface of the Sun with an image of it's corona superimposed around it.

SOHO's EIT instrument has produced an even more detailed image of the roiling surface of the Sun, caused by convective transfer of hot gases:

SOHO EIT image of the Sun's surface (the small square pattern is tied to the individual CCD detectors in the instrument).

This is a more detailed look at the chromosphere taken through a ground helioscope, using a green filter and blocking out the Sun's surface. At a height of its upper limit, from 10000 to 16000 km (6000 to 10000 miles), temperatures may be as high as 1,000,000° K. but can be as low as 20000° K next to the photosphere.

The chromosphere of the Sun, imaged through a green filter.

As early as 1610, Galileo and others first noted dark areas on the Sun's bright photosphere, its apparent surface. These were named, aptly, sunspots and refer to large (often several times the Earth's diameter) individual or clustered features on the Sun's surface that are several thousand degrees cooler than their surroundings. Here is a helioscope image of sunspots in March, 2001.

This black and white image also shows great detail within a sunspot, which can be as much as twice the diameter of Earth:

Closeup of a sunspot; the dark central area is called the umbra.

Sunspots are cooler than their surroundings (around 3400° C compared with the 5600° C for the Sun's average surface temperature) They tend to cluster around a band extending from the solar equator. They are caused by churning up of hydrogen gas by the strong solar bipolar magnetic field. That is shown in this diagram:

Flow of gases below a sunspot, controlled in part by the Sun's magnetic field.

On average, sunspots come and go on an 11 year cycle (min-max-min) but that cycle is further extended to 22 years because of a polarity change (north-south-north) during the full interval. Sunspots are associated with increases in expulsion of charged particles in the solar wind and thus can produce interference in radio broadcast signals on Earth. Below it is a plot of sunspot frequency measured over the last 400 years.

Seen in a close-up, sunspots have a distinctive appearance as seen in this solar telescope photo; the surrounding surface contains small, irregular bright areas called granules which are the upwelled part of numerous local convection currents that carry hot hydrogen gas to the photosphere (dark areas represent the inward return paths of this circulating gas).

Surface of the Sun showing a sunspot and surrounding granules.

A similar view, made through a Swedish solar telescope, creates the impression of "bumps" or irregularities on the Sun's surface - a convincing depiction that this surface is not strictly smooth.

Departures from a smooth surface on the Sun.

Various turbulent flow patterns (given descriptive non-technical names) develop in and around sunspots, as depicted in these ground-based telescope pictures:

Close-up views of features associated with sunspots.

The closest view yet of a sunspot and its neighborhood has been made by the Swedish Solar Telescope.

A sunspot with a border marked by , as imaged by the Swedish Solar Telescope.

Our Sun (and by extrapolation, stars in general) undergoes considerable variations in exterior activities, along with some changes in energy and solar wind output, over time periods of months to a few years. The term "Sun storm" is applied to one type of such phenomena. The magnetic field generated by the Sun varies in this instance. On Earth, these variations can notably affect radio and TV signal transmissions (satellites, and even astronauts in space, are also influenced). The Sun goes through different cycles of varying activity; these show characteristic periodicity. Here are three solar images showing the appearance of the Sun from a half cycle that goes from a solar minimum to a maximum in 3 1/2 years (part of the seven year cycle):

Variations in the appearance of the Sun over a 3 1/2 year time span.

Solar flares extending well beyond the photosphere occur during Sun storms (and, by inference, this is likely to happen also to many, perhaps most, stars). Here is a TRACE image acquired in the summer of 2000 that shows some spectacular solar flaring.

Solar flares stemming from the Sun, as imaged by TRACE.

The base of a solar flare, imaged by SOHO, might remind you of an active lava surface in a volcanic caldera, except in the example below it is very much hotter and incandescent gases rise much higher.

Site of a solar flare during a period when it has not reached a considerable height.

SOHO is capable of time-lapse imagery so that the history or sequence of a flare (prominence) can be displayed as a series. This image shows the succession of growth and dissipation of a prominence as observed with the LASCO sensor on SOHO. The instrument also provides temperature data, giving an average of 107,000 °K for the gases that make up the feature.

Development of a solar prominence on July 1, 2002 as imaged by SOHO.

The largest and strongest solar flare recorded in history occurred in early November of 2003, during which several other prominent flares were observed. Much disruption of communications was inflicted. This is a SOHO image of this strongest flare on November 4:

Using its Extreme Ultraviolet Photometer System (XPS) instrument, SORCE (Solar Radiation and Cloud Experiment), a new solar observing satellite launched in early 2003, caught the series of these strong flares during this October-November, 2003 period. Rapid monitoring of flare activity is imporntant on Earth since these can cause severe radio disturbances in the terrestrial atmosphere and can damage orbiting satellites. A SOHO Extreme UV (EIT) image of the Sun's face is shown in this diagram along with the plot of solar flares during this span of intense activity:

SOHO UV image of the Sun and plot of solar flare activity made by SORCE's XPS.

A somewhat different picture of the Sun's activity is gained by converting x-radiation into an image; the one shown here was obtained by the Japanese Yohkoh satellite:

Yohkoh satellite x-ray image of the Sun.

SOHO is equipped to image the Sun at various wavelengths in the Ultraviolet. Images at 1710, 1950, and 840 nanometers (blue, green, red respectively) are combined in this false color composite which emphasizes UV emission variations on the Sun's surface.

False color composite of the Sun's surface developed from three UV wavelength images.

In the Extreme Ultraviolet wavelength region, here is a view taken from the above image that is colored blue to accentuate one of the invisible wavelengths in the UV. Plumes of very hot gases enter the chromosphere from discrete locations out of the Sun.

One type of plume, discovered only 30 years ago, is called Coronal Mass Ejection (CME), in which electrified hydrogen gas is expelled considerable distances from the Sun's surface, following helical pathways outward; this SOHO captured this effect:

A solar Coronal Mass Ejection (mainly, the white teardrop-shaped plume; SOHO image.

A CME expels a huge number of particles over a short period. Here is a four panel time sequence of the CME that occurred on March 20, 2000:

The Coronal Mass Ejection event of March 20, 2002 measured by SOHO's LASCO instrument over a two hour period.

Another feature above the corona-photosphere boundary, associated with loops and flares, has been dubbed "moss" because of its spongy light-dark appearance owing to variations in temperature within the 10000 to 1000000° K range. Here it is imaged by TRACE (Transition Region and Coronal Explorer) in ultraviolet light:

It should be no surprise that the Sun (and by inference all stars) undergoes various physical movements and displacements in and near the photosphere. Some of these result from violent events that send seismic-like waves within the surficial gases. A whole field - helioseismology - has evolved to study these phenomena. A broad review of this subject is found at this Stanford University site. The subject deals mainly with systematic oscillations detectable at the surface and waves that can be used to probe the Sun's interior structure and dynamics.

Here is a generalized map of the Sun's interior in terms of sonic velocities that are sensitive to temperature differences. Red refers to hotter zones in which acoustic speeds are slowed; blue to faster - hence cooler - zones.

Cutaway diagram of the Sun's interior as defined by variations in acoustic wave speed.

Surficial disturbances, including explosions, also produce acoustic waves (largely at low frequencies). The illustration below shows a sound wave associated with the emergence of a solar flare which radiates outward in the gas much as a ripple moves when a stone is cast in water.

The progression of a disturbance analogous to a sound wave in the photosphere, initiated by emergence of a solar flare; a sequence of SOHO images.

One of the consequences of these techniques for penetrating the solar interior, making it as though "transparent", is the ability to see some broad features on the surface of the Sun not facing the observing instrument. The Michelson Doppler Imager on SOHO used sonic waves to create this pair of images, the top showing sunspots on the visible solar face and the bottom detecting features on the opposite face.

Several satellites have been placed in space to get more information on the heliosphere - which pertains mainly to the solar environment beyond the Sun's corona. Our understanding of this huge feature - it consists of the Sun, the planets, and the solar cavity through which flows the solar wind - is summarized in this diagram:

Diagram showing the nature and extent of the heliosphere.

The heliosphere's magnetic field is responsible for deflecting extrasolar particles, such as makes ups part of cosmic rays, from neighboring stellar sources.

There is continuous emission of charged particles, mainly hydrogen ions in a plasma, from the corona outward into the heliosphere and beyond. This is the solar wind which through particle expulsion at high velocities causes these to escape the Sun's gravitational field and travel great distances outward through the solar system. These particles along with galactic cosmic rays (stellar winds, some part of which is derived from stars that expel gamma rays and energetic ions of many atomic species) continuously bombard the Earth; the Van Allen Belts provide major protection from the solar and cosmic influx that otherwise would affect (or even prevented the inception of) life on our planet. Solar flares and magnetic storms occur randomly to periodically on the Sun during which the wind is intensified. Here is a view which shows particles being pushed beyond the Sun's gaseous envelope.

Part of the solar wind field being expelled from the Sun.

Ulysses, a joint ESA-NASA program managed out of JPL, was launched on October 6, 1990 to monitor several aspects of the heliosphere environment. A good review of this project is found at the Ulysses Home Page.

Traveling far into the outer reaches of the solar system, Ulysses makes a complete orbit around the Sun once every 6 years. Here is a plot of the data on the Solar Wind achieved during two cycles, one during a solar minimum and the other during a maximum (these occur during magnetic pole reversals over an eleven year period) :

Ulysses plot of solar wind variations, using an instrument called SWOOPS, from 1990 into 2003.

Another Ulysses instrument, called DUST, provides data on the density and motions of the myriads of interplanetary dust (also called "stardust") moving around the Sun. This stardust is mostly very small particles in sizes of less than a millimeter. The distribution appears to be controlled in part by solar wind maxima and minima, as seen in this diagram.

Ulysses DUST-determined distribution of interstellar dust (lower concentrations in blues and greens; higher in yellows and reds.

During the minimum the highest dust concentration collects below the plane of the ecliptic (defined by planetary orbits) but reverts to that plane during a maximum.

Moving at an average speed of 26 km/sec, the transit time for a given dust particle to orbit around the Sun has been determined to be about 20 years. Using these data and extrapolating from other information, this diagram depicts the location of major dust concentrations around the Sun and extending to the local stars in our part of the galaxy:

Artist depiction of dust clouds in the local stellar environment around the Sun.

P.C. Frisch, University of Chicago

Knowledge of stardust distribution and behavior is relevant to a better understanding of the history of planetary bodies in dust clouds around stars, as discussed on page 20-11.

The strong magnetic field associated with the Sun is believed to form around stars in general, particularly those still burning their elemental fuel. Sometimes the materials ejected from stars will follow magnetic field lines that organize the escaping gases and accompanying non-gaseous elements into prominences and loops. This is beautifully displayed in the Hourglass planetary nebula (see below) that has developed around a Red Giant (see below)that is evolving into an eventual White Dwarf. At the Red Giant stage, the star can have a magnetic field between 50 and 500 Gauss, strong enough to induce the Zeeman effect. This effect, associated with strong magnetic fields, alters the spectrum of excited molecules; in the Hourglass case, water is the substance giving off the light associated with the loops.

The Hourglass Nebula, formed around a Red Giant (similar to the type shown in the inset), in which conspicuous loops are guided by magnetic fields; the loops are made evident by Zeeman excitation of water molecules.

With this in-depth examination of one star, the Sun, let us return to the evolution of stars in general, as this diagram suggests, stars undergo a series of changes as they form and then pass through their fuel burning cycle. They can be further categorized in terms of their end products at completion of the burning; the type of star that evolves depends on the initial mass of H gas as it reaches its early stage of burning. Stars can also be classed according to their relative ages into Population I and Population II types. Type I stars are generally younger, and continue to form even today. In Spiral galaxies, they are most common in the arms. They contain a larger proportion of the heavier elements (which, as shown later, are largely produced during late stages in an earlier large star's history which ended in its explosive destruction that spewed out these elements into the gas-dust nebular debris from which present type I stars have formed). Type II stars are older, having burned much of their fuel, and generally reside near the galactic core in Spirals or are the dominant stars in Elliptical galaxies. Type II's are deficient in heavier elements which means that they developed when the Universe was younger from raw materials that had not yet accumulated these elements; many are small enough to have lived long lives. These chemical differences are evident in the spectra characteristic of each type: this pair of spectral strips shows the upper one, from a Population II star in the Milky Way, to display almost exclusively hydrogen lines whereas a Population I star (the Sun) in the same galaxy (lower strip) has those lines plus many others representing different excited elements beyond helium in atomic number.

Star formation is a continuous process in galaxies, even those that formed early in Universe time. Older galaxies tend to have used up much of their hydrogen gas fuel dispersed in intergalactic space, so that the rate and number of new stars will be less (in general, diminish with time). Some galaxies (uncommon) as observed today show large numbers of new stars formed over intervals of hundreds of millions of years. These so-called "starburst galaxies" display numerous areas of light blue-white representing many stars that formed during a narrow span of time. NGC3310, seen below, contains several hundred clots of young stars, each clot containing up to a million stars (estimated):

The Galaxy NGC3310, characterized by numerous starburst clots.

A more obvious and dramatic example of localized starbursts is in the NGC604 nebula in the M33 galaxy, some 2.7 million light years away. The image below, which shows at least 200 young blue-white stars, was made by combining Hubble observations over several years. The nebula itself, 1300 l.y. wide, is a small part of the full galaxy.

Blue-white stars forming in NGC604, part of the M33 galaxy.

The H-R and Evolution diagrams above show several classes of stars whose initial mass lies below that of the Sun. Of particular interest are the Brown and Red Dwarfs (the Brown, in particular, whose masses can be as low as 0.08 that of the Sun [arbitrarily set at 1; Black Dwarfs have even less mass]). (Another class of small stars, the White Dwarf, is the end product of stages of expansion or explosion of large stars.) A recent HST infrared image (right) of the Orion Nebula (shown on the left below in visible light) has revealed that Brown Dwarfs (the brownish-orange spots) are widespread throughout the region here imaged.

The Orion Nebula imaged in visible and infrared light by HST

This next image shows more than 30 Brown Dwarfs (the most ever observed in one small region) in the vicinity of the rho Ophiuchi Cloud (some 570 l.y. away). These are each only a small fraction of the Sun's mass and are less than 1 million years old. In the image, made in the infrared by ESA's Infrared Space Explorer, are several bright, very massive stars.

Brown Dwarfs around the Ophiuchi Cloud; ESA image.

The Brown Dwarfs do produce internal energy from limited fusion of deuterium but never reach the hydrogen fusion stage of larger stars (for this reason they have been called "failed stars). Their surface temperatures fall below 2600° C, insuffient to fuse lithium in the outer layers (Li spectral lines become an indicator of this type of dwarf). These have low luminosities and are hard to detect even in our galaxy, the Milky Way. Yet they may be very abundant within galaxies (one estimate holds them to approach luminous stars in number), accounting for a considerable fraction of the total mass of stellar bodies. Since the Dwarfs burn their hydrogen at very low rates, they will be long-lived. The smallest of the Dwarfs are not much larger than some giant planets, into which they grade (a planet does not produce a significant output of radiation through nuclear processes). In October 2000, astronomers reported spherical objects smaller than Brown Dwarfs, ranging in mass from 5 to 15 times that of Jupiter, that appear to "free float" (do not orbit stars). They are not hot enough to initiate any nuclear burning. They may be incipient "dwarfs" that could grow larger into eventual stars. What we presently know about brown dwarfs is neatly summarized in this diagram:

The relation of brown dwarfs to Sun-sized stars and Jupiter-sized planets.

Source: Scientific American

Not shown on the H-R plot are the Red Dwarfs. These are M type stars that plot on the Main Sequence near the lower right of the H-R diagram. They have masses ranging from about 0.1 to 0.4 that of the Sun; their surface brightness is less than 1/2000th of the Sun. Their surface temperatures are around 3000° K, producing light that is distinctly red. They do burn their hydrogen fuel but are too small to develop a helium core; the helium as produced is redistributed throughout the star by convection. Because of very slow fusion rates, the Red Dwarfs are capable of very long lifetimes (up to 100 billion years). They are fairly common within galaxies but do not contribute much to the total galactic mass. Barnard's star, which is the second closest to the Sun (5.9 light years) is a Red Dwarf. Below is GL623, a Red Dwarf with an even smaller companion (possible White Dwarf - see below):

A Red Dwarf star; the apparent separating material is an artifact produced in this HST image.

Both Brown and Red Dwarfs are primary, that is, they are not the final product of a multi-step stellar evolution. They are simply clots of gas that did not accrue enough mass as hydrogen to burn efficiently until forced to leave the Main Sequence by inflationary or explosive means.

Near the top left of the H-R diagram, above the Main Sequence, is a region containing an extreme opposite of the Dwarfs (but not named in the plot): the Blue Giants and Supergiants. These stars simply grew into masses that carried them beyond the upper limits assigned to the Main Sequence. Typically, a Blue Giant is more than 40000 times more massive than the Sun, has a diameter at least 8 times greater, and has surface temperatures exceeding 20000° K. It tends to have a short life span (~100 million years) but can go into a Red Giant phase (see below). One of the brightest stars in the sky is Rigel, in the Constellation Orion, a Blue Supergiant (B type), as seen here:

Among the brightest of the Main Sequence stars are the B types with surface temperatures in excess of 11000° K. Perhaps the best known are a small cluster of blue-white stars known as the Pleiades (the Seven Sisters) which lie in the Milky Way only about 375 light years from Earth.

The Pleiades cluster: bright, massive Main Sequence stars of Type B.

The HST has found a star, in the Pistol Nebula, that is currently the brightest known in the Milky Way, being ten million times more luminous than the Sun, and 100 times more massive. The star, only 25000 l.y. away, is estimated to have begun its hydrogen burning only about 1-3 million years ago. In this view, the red "clouds" around the central star may, according to one interpretation, be hydrogen gas and other material shedding from the star perhaps as it enters a destructive phase, or, less likely, are still involved in continuing collapse onto that star. This view is in the infrared; in visible light the star is shrouded with opaque dust.

The Pistol Star, the brightest yet found in the Universe.

The Wolf-Rayet star is one type of very massive O star which has a surface temperature of around 50000° K. It is short-lived after reaching the Main Sequence and before destroying itself it sheds much of its mass by expulsion of hydrogen driven away by its stellar winds. This next image shows a Wolf-Rayet (WR) star (arrow) (NGC2359) imaged in the visible; below it is WR124 (in the constellation Sagittarius) imaged in the near infrared. Both show the extent to which the gases are expelled even as the parent star remains intact (this separates the WR types from planetary nebulae in which the central star has exploded.

Wolf-Rayet star 2359, in visible light; star at tip of black arrow.

Wolf-Rayet star 124, seen in infrared light

WR stars are rare. Less than 200 have been detected in the Milky Way Galaxy over the last 150 years

The Sun, now about 5 b.y. old, has a life expectancy of another 5 b.y. or so until it converts (following some arm up and to the right on the H-R diagram) first into a Red Giant (hot contracted core but cooler outer envelope of greatly expanded [up to 100x the normal star diameter] diffuse gases emitting surface radiation in the visible red). A feel for just how large a Giant is relative to a typical G star (e.g., our Sun) is given by this scale diagram - the large Red Giant is Arcturus:

True to scale size difference between the Sun and the Red Giant Arcturus.

Red Giants develop as the hydrogen in the deep interior (core) is finally depleted and the helium derived from it tries to fuse (burn) to carbon. The core shrinks even as fusion continues in the outer regions of the star. Energy is rapidly lost, so that hydrostatic equilibrium is disturbed, allowing for expansion driven by stellar winds. The energy density of the star's surface, being lower, shifts emerging light wavelengths from bluish towards red. A Red Giant, sometimes described as a "bloated" star, can exist up to 500,000 million years. (Note: our Sun is scheduled to become a Red Giant in about 5 billion years when its final fuel material will expand as a hot gas out to roughly the limits of the present Earth orbit). Below is a typical Red Giant, Betelguese (actually classed as a Red Supergiant; it is present in the constellation Betelguese and can easily be seen by a small telescope) as seen by the HST; among other well known Red Giants are Arcturus and Aldabaran (of which no good images were found by an Internet Search).

HST image of a typical Red Giant in the constellation Betelguese.

This next view is a radio telescope image of Betelguese; on the right is an analysis of the temperature profile in its outer shell:

Another Red Giant in the Mira star group shows a pronounced asymmetry of its outer envelope, as imaged in the UV:

This next HST image shows the globular cluster M10. It is notable for the large number of Red Giants and some Blue Giants, besides smaller stars on the Main Sequence.

Red and Blue Giants in Globular Cluster M10

What happens to a star after hydrogen and helium fuel is consumed depends on its size. Smaller stars (Spectral types A through M) end up as the surviving cores of Red Giants which are greatly reduced in size to the White Dwarf stage. Larger stars (O and B) undergo a different process that involves explosive shedding of nearly all their remaining gaseous matter and synthesized elements in a supernova (see next page). Type O stars (8-10 solar masses) follow a sequence that involves a small supernova which ends with a White Dwarf. More massive stars which explode leave a small stellar body known as a Neutron star (next page).

After the bulk of the mass is shed from the outer envelope of a Red Giant whose initial mass was < 10 solar masses, it will have lost nearly all its remaining nuclear fuel, shrinking rather abruptly (over a few thousand years) to a radius much less than Sun size (some as small as the Earth) and ending up as a dense, hot core (~1.4 solar masses) that becomes a very hot, luminous White Dwarf (surface temperatures as high as 170,000° K). A White Dwarf is, as the name suggests, small (but it differs from the Brown Dwarf group described above by having started out with a mass greater than 1): A star not much larger than the Sun, shrinks to a size comparable to Earth but with a density of about 1,000,000 g/cubic cm. Its core mass is said to consist of degenerate matter, i.e., owing to quantum effects its pressure no longer depends on temperature, i.e., can vary independently, - in this case close-packed electrons are degenerate (a state in very dense matter in which the pressure in a very hot gas or plasma depends on density but is independent of temperature) but not protons or neutrons. The White Dwarf nevertheless is still hot and bright. The ultimate fate of a White Dwarf is to cool and fade away. Both White and Brown Dwarfs can eventually lose any fusionable fuel and become Black Dwarves (star "cinders") that are no longer luminous but continue to radiate heat away.

Although White Dwarf stars are small, they still shine early in their history. The HST has succeeded in detecting these stellar "midgets" amidst nearby stars. The image below show seven tiny bright dots which are actually White Dwarfs:

White Dwarf stars (circled; right) in a Globular Cluster; HST.

One or more rings or shells often represent the shedding of matter in the final ejection phase around a Red Giant as it becomes a White Dwarf star. This stage is comparatively rapid, taking 10000 - 20000 years for the rings to disperse. These rings, and their shape variants, are also referred to as planetary nebulae (a misnomer in that planets are not the end product; early observers (the first being Wm. Herschel, discoverer of Uranus) once thought that, at the then poorer spatial resolution of their telescopes, the objects they saw with a torus- or disk-like appearance resembled an early stage of a planet's formation). The scale of expansion places the edge of these gaseous envelopes at diameters around 1000 times that of our solar system.

The important thing to remember about planetary nebulas (or, in Latin, nebulae) is that they are the atmospheres of dying stars that are shed as these stars use up their fuel, expand into the Red Giant phase, and finally throw off the remaining gas envelope at high speeds into surrounding space. We see these nebulae after they have pushed out huge distances from their parent star = which usually becomes a White Dwarf (some larger stars that end up as Neutron stars may also shed their remaining gases). Stars of 8 solar masses and smaller end their lives in spurts of the ejected material making up planetary nebulae, spread over thousands of years, in contrast to supernovae (next page), which are the death throes of stars of mass greater than 8 suns that explode just once, almost instantaneously, and disperse their debris at high speeds over shorter time periods.

A classic example of a planetary nebula is M57, the Ring Nebula, as seen by HST (red tones represent excited hydrogen; green is associated with ionized oxygen).

HST image of M57, a typical ring (torus) nebula.

However, the prototype of a planetary nebula is also the youngest yet found, the Stingray Nebula, which first appeared about 20 years ago. It shows both a central ring and opposing lobes, the hallmarks of these nebulae, formed as explained below. It is near the Earth; all (or most) planetary nebulae observed in the detail shown in the following images are formed within our Milky Way.

The Stingray Nebula; blue = Hydrogen excitation; green = Oxygen; red = Nitrogen.

At first (when telescope acuity was limited), the planetary nebulae were thought to be normally spherical, resulting when strong winds blew out the remaining gases equally in all directions. Now, only about 10% remain that way as expansion continues. Some nebulae, such as Abell 39, are still nearly spherical having possibly not yet broken up into streamers:

HST image of the planetary nebula Abell 39,

Another apparently spherical nebula is nicknamed the Owl Nebula (NGC 3587) for its obvious resemblance to an owl's face. Located in the Milky Way ~2000 l.y. from Earth, this nebula contains three distinct layers: a faint dark blue outer ring consisting of now dispersed gases expelled in the early stages; a medium blue middle ring driven by superwinds, and an inner light blue ring, plus a purplish central filling that represents material that has migrated inward:

The Owl Nebula, photographed by two terrestrial telescopes, with their images combined.

A typical elliptical form is associated with IC418, which shows a delicate lacing of gas streamers within:

The more common expression of a planetary nebula shows the gases (which appeared colored in the images below because of excitation into ionic states from UV radiation emanating from the star remnant) to distribute in a wide array of shapes. Rings (torus), ellipsoids, bi-polar lobes, and streamers are shape components of the nebulae. One to several (combination) factors account for this diversity: winds of different speeds coming off the surviving star remnant; accretion tails, binary stars (one star not exploded influences the dispersal of the nebula), and complex twisting of magnetic lines of force all are likely to play roles.

In the first stages of development, the gas expulsions have been called proto-nebulae. A famed example is known whimsically as "Gomez' Hamburger, named after its discover, Arturo Gomez using a telescope at the Cerro Toledo Observatory in Chile. The "buns" are gas clouds that glow her in visible reflected light; the "meat" appears to be thick obscuring dust:

The Gomez "Hamburger" proto-nebula; HST image.

Another example is this early stage of proto-nebula development around an exploding Red Giant, as seen by the HST WFC:

The proto-nebular phase of a disrupting Red Giant.

The most striking views of planetary nebulae in later development stages are taken as ultraviolet images because the gases expelled from a dying star are excited by ultraviolet radiation during the process. Other HST images of planetary nebula are instructive: beneath is NGC7027 in which the explosion of gases is in an early stage.

In the next image, the Cats-Eye nebula (NGC6543), some 3000 l.y. away, appears to be a later stage of the outward propulsion of gases around a Red Giant (possibly one of a binary pair, with the second star a possible dwarf) in which several rings are made luminous through excitation by expelled particles. This is the color scheme most often used for the nebula.

HST image of the Cats-Eye Nebula (NGC6543).

A second color scheme is also chosen to present the "beauty" of this nebula:

Different color rendition of the Cats-Eye nebula.

A recent observation made using the Nordic Optical Telescope (ground-based) has revealed that the Cats-Eye nebula is surrounded by a halo ring and filaments extending beyond that. This stunning image shows the full Cats-Eye complex in a false color rendition in which the red is due to excited nitrogen and greens and blues to oxygen:

The Cats-Eye nebula seen in its full extent by the Nordic telescope.

The HST has caught a Red Giant in the act of finishing its existence in this state as it transposes into the Twin Jet nebula. As shown in this next image, some of its outer, less dense mass is being ejected from the main body of the star:

The Twin Jet nebula, M2-9, representing bipolar expulsion of gases from a dying Red Giant.

The Red Spider nebula (NGC6537), below, shows a common feature noted in many explosive stars, namely, lobes (usually in a pair) of gas being driven outward at high speeds (1 million km/hr) by stellar winds (in this case moving at even higher velocity). The lobes result from shock that compresses the gas expelled as the nebula develops. Note the ripples in the lobes. These lobe consist of the now cast-off gases that once formed the bulk of the star whose core has survived after it has passed into the White Dwarf phase.

HST image of the Red Spider Nebula, with its two shock-compressed gas lobes.

Similar to this is what is whimsically called the "Garden Sprinkler" nebula. Its curved lobes of ejected gases are traveling well beyond the dying parent star.

The Sprinkler, made from gas lobes that extend in opposite directions from a star in the process of dying; this is not yet a typical planetary nebula.

The next example shows the Crescent nebula. In the lower right of the image below is a black and white ground telescope view of the elliptical (16 by 24 light years) gas cloud being propelled by strong stellar winds outward from the dying WR 136, a Wolf-Rayet star (derived from a super red giant, and extremely hot, such that outer mass loss has exposed inner shells of helium, or even nitrogen or carbon (see page 20-7). The color portion is an HST view of the outer edge of part of the nebula; the different hues relate to compositional variations.

WR 136 and its elliptical nebula being driven outward by shock waves and stellar winds; the color portion is an HST view.

Still another example shows details of the wispy strands of gases and particles in a supernova found in the Vela group in the Milky Way, as imaged by the Schmidt telescope at the Anglo-Australian Observatory in New South Wales:

Strands of material ejected from a supernova in the Vela Group.

As the nebula expands over time and debris organizes into these wispy strands, the entity takes on a filamentous structure as shown here in the Veil Nebula, some 2500 l.y away in the Milky Way:

This next image shows a violent explosion around NGC6302, a very hot (surface temperature of 250000°C or 450000°F) star 4000 l.y. away. Now almost gone, the destroyed star is invisible in the Hubble image below. The nebular material has much dust but also water ice crystals around particles as the expelled vapor condenses in outer space:

One of the most peculiar nebulae is the Red Rectangle, whose shaped is defined by two pairs of jets emanating from a center that is a binary star pair, HD44179. The resulting X-shapes seem to have "ladder bars" joining them. Much of the red color is due to excitation of dust.

The Red Rectangle nebula; a Hubble ST Wide Field Camera image.

The Hubble Space Telescope has now gathered hundreds of images showing "dying" stars, i.e., those in their last stages of fuel burning that are shedding matter explosively. The variety and complexity of a star's final activities has proved to be much more diverse than known from the era of conventional telescopic observations. A recent NASA press release documents work done by astronomers at the University of Washington and elsewhere that illustrates different observed end stages, shown in a panel of six images typifying this diversity of gaseous envelopes (these are typical planetary nebulae):

These brief descriptions define each observation: Top Left: A round planetary nebula with a bright inner shell and fainter outer envelope; this uniform expansion is the mode predicted for the final phase of the Sun's demise; Top Center: A hot remnant star surrounded by a green (color assigned) oval in which older gas is pushed ahead to form a bright interior rim; more gas further out shows hot spots (red); Top Right: A spherical outer envelope and an elongated inner "balloon" shell, both inflated by a fast wind from the interior star; Bottom Left: A "butterfly" or bipolar (two-lobed) nebula; Bottom Center: A bright central star at the center of a dark cavity bounded by a football-shaped rim of dense, blue and red gas; the star's former outer layers is shown in green; note long greenish jets; Bottom Right: A planetary nebula with a pinwheel or spiral structure with blobs of gas ejected from the central star.

The lobed Ant Nebula (or Menzel 3), has been the subject of a recent explanation for some of the unusual shapes associated with this class of stellar objects:

According to Dr. Adam Frank and colleagues at the University of Rochester, as stars age and begin to shed materials, they appear to slow their rotation. But as that material leaves the parent star, the star's core begins to rotate more rapidly. With increased rotation, the associated magnetic field becomes stronger and influences the patterns or shapes of the escaping material.

The image on the left below shows NGC6751, in the constellation Aquila, but actually located some 6500 l.y. from Earth. It exploded less than 5000 years ago and the outer shell of the star has now moved out from the white hot central core (light yellow in center) to produce a near spherical shell about 0.8 l.y. in diameter. The outer gases (mostly hydrogen, in orange) are cooler than the (bluish) inner gases. Notice the radial streaks of gas marking the trajectories of these streamers. The colors given to the various gas components are computer modifications of colors perceived by HST owing to excitation by UV radiation. On the right is the Eskimo nebula.

A planetary nebula formed by the ejecta of exploding star NGC6751, as imaged by HST. The Eskimo nebula.

The HST has now obtained a good image and temperature data for what is called the Boomerang nebula. Despite the obvious incandescence that makes visible the two extensions, the temperatures measured for in these lobes were as low as -272° K, just above absolute zero and possibly lower than the general cosmic background radiation, making this feature the coldest region of visual mass around a central star yet found in the Universe:

One of the most "delicate" appearing nebulae is Barnard's Merope Nebula, just 380 light years from Earth, and found in the celestial sphere within the space defined by the Pleides. In this long-exposure ground telescope image, the material is mainly dust reflecting light from a nearby star (just beyond the upper right corner):

Reflected light from dust close to a nearby star (beyond the scene) that causes a raylike pattern in part of the scene.

The appearance of these planetary nebulae can be misleading. Orientation of a nebula relative to our vantage point from Earth, may present shapes that are distorted from their reality. For example, a ring may actually be the edge-on view of a cylinder (looking down axis). Most planetary nebulae are non-spherical gas-dust ejections showing usually axisymmetric morphology. We see this gas because it is excited (and thus glows) by ultraviolet radiation from the surviving White Dwarf as the Red Giant phase nears its end. The gas envelope shapes (jets, interlocking rings, "rectangles", etc.) are the consequence of strong stellar winds overtaking, at this stage, earlier, slower particle winds involved in Red Giant growth. The shapes assumed (such as the Butterfly type) indicate some degree of asymmetric wind release. Another factor is the likelihood of these nebulae being influenced or associated with companion binary stars (or a star and a large orbiting planet).

More massive stars than those discussed above proceed through their final stages of fuel consumption by different processes. These result in events called novae and supernovae, of sufficient importance and complexity to warrant treatment on the next page (20-6). The end result can be a white dwarf star, a neutron star, or a Black Hole, depending on the star's mass.

Another way in which a star can be destroyed is by being torn apart as it approaches a Black Hole. Although extremely luminous conditions called quasars (page 20-6) are thought to be light given off as stars fall into a Black Hole, individual star destruction is seldom observed. However, one such event has been recorded by X-ray telescopes around a galaxy that is 700 million light years away. What takes place is shown in these next two figures, identified parts of which are actual observations but the rest are artist's rendidtions of the detected changes and a reconstruction of the sequence of events in the past and the future - read the captions for details:

The upper panel is an artist's version of what is happening to an individual star being sucked into the Black Hole; the lower left is a Chandra X-ray image of the glowing Black Hole; the lower right shows an optical telescope viewof the present conditions.

A reconstruction of this star's destruction: Upper left - star not yet affected by circular black hole to its right; Upper right - star during approach is elongated by black hole's gravity; Lower left - star is pulled apart, with most material continuing in motion around the black hole but 1-2% being carried into the b.h.; Lower right - star totally destroyed with its material orbiting around and past the black hole.

Having now talked about star types and their histories, let's summarize what is known or extrapolated about the estimates of the percentages of each star type in the Universe. This is difficult for all but the nearest galaxies because most stars cannot be resolved into individuals. A better inventory is available for the Milky Way. Here it is: Red/brown dwarfs = 70%; Main Sequence (F,G,K) = 20%; White Dwarfs = 8%; Main Sequence (O, B, A) = 2%. A more tenuous extrapolation to the Universe; M stars = 30%; LT (red/brown dwarfs) = 30%; White Dwarfs = 20%; Evolved Supergiants = 10%; OBA stars = 5%; FGK stars = 5%. Of note is the relative rarity of Sun-like G stars (perhaps as infrequent as 2%).

We close this page with a discussion about star formation in the early Universe - the saga of the first stars. (A good summary of this topic is found in Science News. Evidence is building that stars began to form about 100 to 250 million years after the Big Bang. Hydrogen gas that had been concentrating in protogalactic clumps or clouds was at that time hotter than in later gas clouds as the Universe matured. One reason for these is that there was little more than hydrogen and helium as the heavier elements had not yet been synthesized and dispersed (by supernovae; see next page); such elements lower gas cloud temperatures. Thus, in the first nebulae the stars that form were mainly massive - tens to several hundred times a solar mass. The early galaxies thus contain many more huge blue stars (O and B) relative to F, G and M stars that make up the bulk of the star populations seen in the developed galaxies we observe today. These Giants burned rapidly, typically after 3 to 5 million years following their compaction into hydrogen-burners, and were thus short-lived. They were quite luminous and had surface temperatures in the 100000 °K range. Those with less than 250 solar masses destroyed themselves as supernovae; greater than 250 solar mass stars ended up as black holes.

Of course, these stars (which fall in the category of Population III, the group consisting of just hydrogen and some primordial helium as fuel at their start) were short-lived. At present none of these have been observed but are being looked for in the early Universe. Because of their size, they burn out and explode very rapidly. Thus, they would have existed mainly in the first billion (or significantly less) years. The HST cannot see star objects that for back in time but the James Webb Space Telescope scheduled to launch in 2010 may be able to detect evidence of their existence. But, enough is known, or seems probable, about the astrophysics of star formation under conditions that likely prevailed in the first millions of years after the Big Bang to be able to model their inception and subsequent history during early times.

One such computer-driven model (using ENZO, a cosmological hydrodynamics code) has been developed by Tom Abel (Pennsylvania State University ) and colleagues (Gregory Brant, Oxford U. and Michael Norman, UC-San Diego) over the last 7 years. The model divides regions of an opaque, dark matter-rich Universe into cells of varying dimensions. When the program is run, the primordial hydrogen filling this still dark space begins to clot and gradually warm as it seeks to condense. The events leading to the First Stars can be examined by the model over time and at successively higher magnifications (cell sizes cover smaller volumes). Here is a series of computer-generated images (each panel involving the growth of the hydrogen cloud [first row] as it proceeds into a star; the second and third rows representing later, more detailed looks at the stages involved) that give rise to the initial star, which formed near the center of each hydrogen gas cloud. See this figure's caption (click on lower right) for description of the information presented.

The Penn State computer model output for the formation and rapid destruction of the first stars; Top row: hydrogen gas clots and begins to warm up; Center row: at higher magnification, the gas has heated up into a warm, then hot contracting nebula which eventually collapse into a large (100 solar masses) single star; Bottom row: This star rapidly contracts, heating up to temperatures that synthesize elements heavier than Lithium, and then explodes as a Supernova.

The clouds of cold dark matter (CDM), which also contained the hydrogen that separates to make these first stars, typically contained enough star material to produce 100000 sunlike stars. However, the actual star population coming out of a cloud forms fewer, more massive stars. These in a few million years reached an end-stage where they exploded as supernovae (see next page), at a rate (frequency) much greater than later stars in evolved galaxies. In so doing, driven by supernova winds, they dispersed small amounts of heavier elements into space to mix with the pervasive hydrogen/helium. Thereafter, various protogalaxies began to form along lines described at the bottom of page 20-2. This process may have been aided by the Black Holes, which themselves might coalesce, left behind after the supernovae had cleared out much of the star population

These first stars may have been numerous enough to provide radiation that helped to dissociate hydrogen into a proton and an electron, which is the mechanism that produces re-ionization, after which the early Universe becomes transparent to electromagnetic radiation (including visible light photons), and the Universe lit up with the first stellar bodies that may have existed as star clusters.

The Abel-Barnes-Norman First Star model is neatly summarized in the article "The Real Big Bang", in the December 2002 issue of Discover Magazine. You can also learn more on the Penn State Eberly College of Science Internet site. The Astronomy Department at PSU is among the largest in the U.S.

As might be expected, other models for early stars take a different position. One espoused by Kenneth Lanzetta of Princeton University also believes that first stars, almost devoid of heavier elements, formed rapidly and early in cosmic time. But these stars, he proposes, did organize into larger numbers sufficient to exist in actual protogalaxies.

In related models, these first stars that began to "precipitate" out of the hot hydrogen gases were created within filamentous stringers (especially at crossing nodes), as shown below as an artist's depiction, which were destined to break up into protogalaxies. These, in turn, were then the gravitational attractors for more gas that helped the protogalaxies to develop into spiral, elliptical, globular, and irregular galaxies that began to proliferate after about a billion years, and then to dominate, after 2 billion years or so, the expanding Universe.

Artist's conception of the filamentous clots of hydrogen gas that led to the first, generally massive stars that seeded the protogalaxies.

A group at the University of Chicago, led by Jason Tumlinson, has called attention to the likelihood that the second generation of stars may have contained a higher percentage of numerous small (low mass) stars than previously thought that contained significant amounts of the first generation of heavy elements, as shown in this diagram:

A model of the earlly Universe showing that a significantly larger number of stars form during the second generation after the Big Bang than during the first generation.

Note the use of the term "Dark Ages" for the time during recombination into reionization during which we have yet to be able to see the stars and galaxies by sampling EM radiation. The second generation occurred near the end of reionization, at a redshift (see page 20-9) around 7 (at present the oldest stars detected display redshifts just greater than 6).

The process of new galaxy formation has slowed with time, so that today fewer new ones are being organized. As time went on the temperature reduction in gas clouds that happens as heavy "metals" are dispersed from supernovae has caused increasing proportions of smaller stars so that the population of galaxies has experienced overall increases in numbers of individuals. In this model, the maximum numbers of stars has occurred about 5-7 billion years after the Big Bang. As this is happening the number of Giants has decreased proportinately, as the early ones ended their lives and fewer massive stars were produced. Since this peak, the total number of stars has decreased relatively since the available hydrogen in the galaxies (including their halos) has been dropping in quantity (no new hydrogen is created in large amounts). In the future, the majority of remaining stars will be small ones that have long lives.

This population history is summarized in the next diagram, with the dashed white line indicating the above model. There is, however, a recently reported competing model which is based on arguments favoring an intense period in early cosmic time of stars of all sizes, with these numbers then decreasing as bigger ones are destroyed and few newer ones are created:

Model for star formation early in Universe history.

In time the bulk of the galaxy that evolved from the clouds of original stars was enriched in hydrogen H_II, but surviving molecular hydrogen was still available for further star formation. Protogalaxies in the early Universe were more close-spaced and tended to collide to start the growth of the galaxies extant today. As time progressed, the early massive stars exploded in large numbers, much of the debris, containing the heavier elements, were expelled into intergalactic space to mix with hydrogen and here and there clump into new clouds that evolved into more galaxies. (Gradual enrichment of elements with atomic numbers higher than helium is the norm, since supernovae continue to occur beyond the early days of the Universe.) As we have seen above, the tendency since then has been to gather groups of galaxies into clusters that comprise present cosmic structure.

At the (recent) time this chart was made, the actual post-Big Bang time when these first stars began to form was believed to be less than 0.5 billion years. Some argued for a first production starting about 300 million years after the B.B. Reliable results from the Wilkenson Microwave Anisotropy Probe (WMAP; discussed on page 20-9) have moved the inception of star formation back in time to about 200 million years post-B.B.

As described 9 paragraphs earlier, in the beginning, there were probably a larger number of stars with masses >250 solar mass than in later times (the Type III stars mentioned above). Since these are one source of black holes, their abundance may have controlled the number of galaxies that formed thereafter. We know that black holes (discussed on the next page) are believed to lie within a galaxy's central core (some have already been affirmed or inferred from on-going studies). They could function as a gravitational nucleus that activates galaxy formation. However, some of these core black holes may develop after a galaxy has developed its initial structure.

Stars are responsible for nearly all the visible light in the Universe. In its early eons, this visible light averaged, from all kinds of star types, wavelengths that fell in the blue region of the spectrum. Today, that average visible light radiation spread has shifted to longer wavelengths, producing a blue-green (similar to turquois) light. Of course, individual stars of a range of colors are not of that shade (aren't blue-green), but taken together their numerically weighted sums of all visible wavelengths would be represented by this blue-green value. In time, the average color will continue to shift towards the red and, tens of billions of years from now this will, in fact, be associated with the then dominant star type, the Red Dwarf.

The bottom line to the gist of this page is that stars appear to be the most obvious and dominant type of large single body making up the Universe. But as we shall see on pages 20-9 and 20-10, stars actually comprise less than one percent of the mass of the Universe. But, because of their luminosity, they give the impression that they are the "top dogs" of the Cosmos. They do have importance beyond their seeming low ranking in the mass inventory because they are necessary partners in planet formation - and as far as we are concerned, one of their kind has been the controlling "parent" of our (insignificant) planet.

Navigation

Primary Author: Nicholas M. Short, Sr. email: mailto:%20nmshort@ptd.net