Cosmology
Astronomy - Astrobiology - Earth Sciences
Origins, Evolution, Metamorphosis, Extinction




Nova, Supernova; Neutron Stars, Pulsars,
Colliding Stars.

Novae and Supernovae

Periodically, a bright object appears in a galaxy and remains that way for days to months. It is referred to, erroneously, as a new star or Nova (Latin for "new"; plural, Novae). It is, in fact, a star that for more than one reason experiences a major flare-up that later dies down leaving the star intact but with loss of material. Star V838 in the Monoceras constellation underwent such a flare-up of a blue star, releasing considerable material as it reached a luminosity of ~600000 times that of the Sun.

Nova developing in star V838.

Soon after its discovery, this Nova has been examined in close detail by the HST, yielding this dramatic sequence of images:

<Sequential images, made by the HST, of clouds of dust and gas moving outward from the V838 Nova.

Recently, a new hypothesis about its effects on possible planets orbiting it contends that the flare-up consumed these planets. There is, of course, no direct evidence that there were any planets around this star. But the spectra obtained for the Nova phase of V838 show a strong enrichment in Li, Al, Mg and other elements that could have been concentrated in planetary bodies that were caught up in and destroyed by the expanding shell of gases in the flare-up. This is the predicted fate for most planetary systems as the parent star expands its gaseous envelope. Our Solar System will likely be destroyed by such a process in about 5 billion years hence (see page 20-11).

Novae events involve release of huge amounts of Gamma ray and X-ray radiation. The XMM-Newton spacecraft sensors on September 22, 2006 captured this X-ray image of a Nova near the center of the Milky Way:

X-ray image of a galactic Nova.

Novae are common events in individual galaxies. One way in which these occur is as follows: What in fact is being observed is a binary star system, one member of which is a White Dwarf and the other a Red Dwarf or even more massive Main Sequence or Red Giant stars. The process involves stripping off of Hydrogen from the larger companion star which streams toward and is added as an accretion disk to the White Dwarf, whose gravity controls the activity. This artist's concept in the right illustration below shows what happens (in reality, the material being removed is often not be luminous which is why the actual process is not observed around such star pairs). On the left is an actual case, in which Chandra has produced an X-ray image of star Mira B, a White Dwarf, receiving material pulled from its companion, Mira A, a Red Giant.:

Artist's conception (right) of gaseous Hydrogen and other materials being removed from a Red Giant onto a White Dwarf; an actual example (left) is the streaming of material from Star Mira A to Star Mira B.

This accretion process causes a buildup of Hydrogen gas around the White Dwarf until compression under the strong gravity raises the temperature to 107 °K, at which condition nuclear fusion occurs. This causes a sudden brightness of the White Dwarf and a rapid consumption of the accreted Hydrogen some of which also may be expelled. The process repeats through a number of cycles, at time scales of 1000s to 10000s of years per flare-up. Novae are therefore recurring events, without star destruction at each occurrence, in distinction to the Supernovae described below. The illustration below shows a Nova near its peak brightness; the specks around it are ejected Hydrogen (the star near the bottom may [?] be the source of the accreted material):

Nova in the galaxy T Pyxidis

This star, normally a small ordinary type in the Milky Way about 20000 light years from Earth, is an eruptive variable in which the Hydrogen-burning undergoes a significant flare-up, enlarging the star somewhat but not directly shredding off or expelling significant mass. At its peak, the star had about 600,000 times the energy output as the Sun but in time settled back to its prior state.

As hinted at above, the hallmark of a Nova event is that it RECURS (happens more than once; repeats), because while explosions shed some stellar material each time the star brightens, the process of Hydrogen-burning continues until all outer material is removed. This has been the case for star RS Ophiuchi, some 5000 light years away, which has become a Nova 5 times in the last 108 years. Now a white dwarf, this star erupted in February, 2006 and became bright enough to be found by the naked eye. It is also a strong source of X-rays. It has shedded its outer atmosphere in a near-perfect ring, as shown here:

The Nova RS Ophiuchi, seen in a telescope in March of 2006.

More massive stars, originally with 8 to 50+ solar masses, burn their gaseous fuel (in the plasma state [atoms are ionized]) much more rapidly until nuclear processes force the gases away at high velocities from the core in an explosion whose early stage may be seen from Earth for a few years as a hugely luminous event called a Supernova. Red Giants are the most common star type that is destroyed in this way. One such very bright event was imaged by HST on April 28, 1998 in the spiral galaxy NGC3982; the Supernova is the large blue-white object in an arm off the galactic center. Supernovae occur, on average, about once every 30-50 years in a galaxy.

1998 Supernova in galaxy NGC3982.
Credit: H. Dahle

The hallmark of a Supernova event might be verbalized by the magician's dictum: "Now you see it - now you don't". This refers to the short span of time in which a Supernova shows its distinct characteristics - from a few years to a thousand or more years. Consider these Hubble views of five Supernovae:

Five Supernovae events.

In each case, the Supernova - a very bright spot - appears for a short time (weeks to months), then disappears.

The rapid rise and decrease of luminosity during a Supernova event (labelled "Transect") was captured visually through a telescope looking at GRB 011121 (a Gamma-ray-burst source; see below):

Time-sequenced observations of the Supernova associate with a GRB event.

In February, 1987 the brightest Supernova in nearly 500 years, SN1987A (located in the Large Magellanic Cloud), was first discovered in the southern hemisphere skies by an Observatory in Chile. Here is a before-after image made by a telescope at the Anglo-Australian University:

Optical telescope image of stars in the Large Magellanic Cloud (left), one of which (arrow) became a large, brilliant Supernova in 1987.

Since 1987, it is being continuously monitored both from Earth and from the HST, providing a "stellar" example of the self-destruction of a star by catastrophic explosion. It appears to still be in a declining luminosity phase going into the 21st Century.

Sequential observations of SN1987A in which variations in luminosity have been density-sliced to showing its gradual decline in brilliance; in this rendition blue has been assigned to highest luminosity and red to lowest.

This next image is one of the most spectacular views of 1987A yet acquired by the HST. The single large bright light is a star beyond the Supernova environs. Around the central Supernova is a single ring but associated with the expansion of expelled gases are also a pair of rings further away that stand out when imaged at a wavelength that screens out much of this bright light.

A pair of distant rings around SN1987A; a brighter continuous ring closer to the star's remnants is discussed below.

Visual changes around the star remnant and its surroundings have since been observed over the last 15+ years, as shown in this sequence made with the Wide Field Camera on the HST.

Sequence of WFC HST images of the inner right around SN1987A

This Supernova is also expressive as a concentrated source of X-ray, UV radiation, and Radio waves. Here is the first Chandra X-ray image of SN1987A:

Chandra image of SN1987A

The next figure shows SN1987A seen in the optical range by HST (upper left), by an Australian Radio Telescope (upper right), and by Chandra on two dates (lower left: Oct 1999; lower right: Jan 2000):

Images of SN1987A, as described in the above paragraph.

The SN event began about 167,000 years ago, based on distance measurements but its light burst is only now arriving at Earth. The star 20000 years earlier first cast off an envelope of gases as it expanded to a Red Giant. As its core collapsed, it finally exploded violently in seconds, pushing away exterior gases driven by shock waves, and releasing a huge burst of neutrinos as the core protons and electrons were squeezed into neutrons. It is heating of the gases in the ring by these shock waves that has now been producing first a few, then more, of the bright light spots in the ring. In time, it is predicted that the spots will merge and the whole ring will become bright.

As will be demonstrated by subsequent Supernovae images shown on this page, a SN is one of the most photogenic phenomena observed in the Cosmos. This is supported by this image - one of the most beautiful ever acquired - of SN W49B made from a Chandra X-ray image rendered blue and two Palomar 100 inch ground telescope images registered in the green (Visible) and red (Near IR):

Supernova W49B.

Supernovae in our galaxy and others nearby can appear as very bright light sources sometimes visible to the naked eye. One of the most famed in history is known familiarly as Supernova Kepler, named after the great astronomer who first observed it on August 8, 1604. In celebration of the 400th anniversary of its sudden appearance as a "new star" brighter than any of the planets, astronomers have produced this composite of Chandra, Hubble, and Spitzer Space Telescope images (the one on the right is recording mainly [in red] the dust in the Supernova):

Supernova Kepler.

Another prime example of a bright, long-lasting Supernova is this Palomar telescope view of the Crab Nebula (left), with an HST Wide Field Camera view of the volume within the square shown on the right).

Image pair of the Crab Nebula showing an image from the Palomar telescope and an enlarged section of the nebula taken by the Hubble Space Telescope.

Lets take a closer look at this nebula - perhaps the most studied by astronomers to date - by first showing a ground telescope view made by the CFHT and then an HST view. Both show remarkable detail, once again proving that for closer astronomical objects ground telescopes can compete with the excellence achieved by the Hubble Space Telescope.

CFHT telescope image of the Crab Nebula.

HST image of the Crab Nebula.

The Crab Nebula is famous in history. It was first observed on July 4, 1054 A.D. by Chinese astronomers as a suddenly appearing bright light, seemingly within the Taurus constellation, which remained intense enough so that for a few years it could be seen even during the day. Modern telescope views show that filaments are streaming from the explosion center at speeds up to half that of light. This Supernova is, like others in general, an extremely energetic event, radiating from short wavelengths (Gamma rays) through the visible and into the long wave Radio region. A Pulsar-Neutron star (see below), rotating 30 times a second, has been detected in its central region.

The Crab Nebula has a notably different shape when imaged with X-ray radiation by the Chandra Telescope. We show this X-ray image combined with a Visible light image made by the HST. A ring structure emerges and a jetlike protuberance extends roughly perpendicular to the ring.

A composite image of the Crab Nebula made by combining the above image with a visible light image made by the HST.

Recently, the HST returned images of the Crab nebula that show the details of the excited gaseous filaments now extending far out into space from the neutron star core. The principal element in many of these filaments is identified by its (process-determined) color: Hydrogen = orange; Nitrogen = red; Sulfur = pink; Oxygen = greenish.

Gaseous filaments within the Crab Nebula in an excited state rendering them luminous as seen in this HST image.

This next image demonstrates how long exposure times can bring out much more detail in a distant astronomical object. Chandra has looked at Cassiopeia A, with an exposure time of 11.5 days. X-ray wavelengths bring out the distribution of Fe and Si, along with other elements. Of special note are the red jets emanating from the still expanding gaseous matter. The surviving Neutron star is not evident in the image.

Long-time exposure of x-radiation around Cassiopeia A, imaged by Chandra.

An HST image shows the filamentous structure of the Cassiopeia A Supernova. The star that blew up in this constellation was about 10000 light years away. Thus, the event took place at that star around 10000 years ago. Historical records note a bright star made a first appearance in the late 1600s in the sky location of Cassiopeia A; what we see today is the dispersal of material after about 400 years. In this rendition, Oxygen-rich clouds of gas/particles are blue; Sulphur is red.

The Cassiopeia A Supernova.

The Spitzer Space Telescope has examined Cassiopeia A in the near IR. This image of the gaseous matter around the burst star looks much like a fireball as we would see an aerial bomb burst on Earth:

Near IR Spitzer image of the Cassiopeia A Supernova.

However, a surprise greeted investigators when this burst was imaged using a combination of IR bands. Observe this image.

Longer wave Infrared image of Cassiopeia A, obtained through the Spitzer telescope.

One of the bands used to make the image is centered on 24 µm. The excited gaseous material making up the glowing filaments was determined to be moving near the speed of light, so that pictures taken a year apart show distinct positional differences. This does not fit a simple growth of the explosion nebula that began 324 years ago. The tentative interpretation: the Neutron star that remains after the Supernova can produce "echoes" by repeating blasts that re-energize the gaseous material. Calculations indicate a Neutron star event about 50 years ago that sent a blast wave through the outwardly progressing gas. A mechanism to cause this Neutron star activity is still speculative.

From the preceding images, it should be obvious that Supernovae are the "spectacular fireworks show" that delights both astronomers and the public alike when the resulting images are widely displayed. In recent years, astronomers have become quite adept at spotting a Supernova soon after it explodes and then training a variety of sensors - both ground- and spaceborne - to preserve the high moments of the event's expansion. Here is still another "sensation", SN49, in the Large Magellanic Cloud:

SN49, with its dramatic filamentous structure; this is a composite made with images from both the Chandra and Spitzer telescopes.

Also in the Large Magellanic Cloud is Supernova N312D. It is 163000 light years away. The image below, made as a composite of HST and Chandra images, shows the extent to which it has expanded and dispersed after 3000 years. By this time the excited gases are diffuse so that the visual stage of the Supernova is no longer dominant. :

Supernova N132D; pink denotes Hydrogen excitation; purple is from Oxygen excitation; blue associates with high temperature X-ray excitation.

Another Supernova example is Eta Carinae, in the 19th Century the second-brightest star in the sky (southern hemisphere) but today too faint to be seen with the naked eye. When processed using a combination of red and UV filter images from HST, the central part appears as an apparent "cloud" of matter which is actually mainly a light burst from this Supernova, now some 10 billion miles across, that resulted from the explosion of a star 150x more massive than our Sun.

Enlargement of the central gaseous dumbell shaped envelope around Eta Carinae, as imaged through red and ultraviolet filters on the HST camera.

The Red Giant, TTCygni (in the constellation Cygnus), is a carbon-rich star which as it explodes expels carbon monoxide (CO) in a discrete ring that has now advanced to about 0.25 light years from the central Giant.

A ring of excited CO gas around the late stage Red Giant TTCygni.

A variant of the gas distribution around a Supernova is sometimes referred to as "a stellar geode", (the term "geode" is an analogy to rocks which contain cavities, usually lined with crystals). N44G, in the Large Magellanic Cloud (160000 l.y. away), is a star which acts like a Supernova to drive surrounding gases into a "bubble" using stellar wind and UV radiation. This HST view uses a red filter to detect Hydrogen and a blue filter to respond to sulphur excitation. The cavity is presently about 35 l.y. in diameter. More than one explosion is suspected (perhaps several Supernovae). This cavitation process is relatively rare.

A massive bubble created by stellar wind/UV radiation, possibly powered by a Supernova explosion.

Once a Supernova is spotted, its rather short history can be monitored in terms of changes in luminosity over time. The graphs below plot brightness variations for several Supernovae of recent vintage and for older Supernova whose remnants are still visible.

Changes in Supernovae brightness over time (age in years back from the present, plotted on a log scale.

Astronomers have distinguished between two general types of Supernovae, separated by the intensity of the luminosity and by the pattern of decreasing light output over time. These are simply labeled: Type I and Type II Supernova. The basis for each type is 1) a Type I Supernova has no Hydrogen in its spectra, and 2) Type II shows Hydrogen in the spectra. Type I is further subdivided into Ia, which results from a thermonuclear explosion of a White Dwarf star, and Ib and Ic, which are caused by collapse of layered massive stars (with iron cores) which then blow up as shock waves (powered in part by neutrinos) expel the layers in huge explosions, leaving Neutron stars if the initial mass was 8 or above or Black Holes if mass was much higher. Type II stars are responsible for dispersion of heavier elements (made in the layers by fusion of initial Hydrogen, then higher atomic number elements, with increasing T and P with depth) into intergalactic space (page 20-7). Type II Supernovae are characterized by asymmetric Type II has proved particularly useful as another "standard candle" - any class of stellar or galactic objects whose (known) intrinsic luminosity (total power output) remains fairly constant at a specific time in their evolutionary history - in the quest to determine distances to far away stars/galaxies and to relate these to rates of expansion. The two types are shown here in this generalized plot:

Changes in luminosity with time for the two general types of Supernovae.

A classic example of a Type Ia thermonuclear explosion is Tycho's Supernova, first observed by the astronomer Tycho Brahe in 1572. Seen below, its gaseous and particulate constituents consist mainly of Silicon, Iron, Nickel, and other heavy constituents. Two prime examples of a Type II core-collapse Supernova are 1987A and the Crab Nebula, shown above. Here is another Type II Supernova, Puppis A, which shows the remnant neutron star:

Rosat X-ray image of Puppis A, with its remnant neutron star.

The Ia type Supernova has come center stage in the recent recognition that the Universe is now accelerating rather than slowing down. Type 1a results when a White Dwarf has grabbed so much matter from a neighboring star (with which it is paired; see top of this page) that it undergoes an implosion followed by a sudden explosion. This event is accompanied by a characteristic spectrum. Type 1a's are less common than the Types I and II; a 1a occurs on average about once every three years in a galaxy.

A star close to the Sun that explodes as a Supernova (or hyperNova; see below) can send shock waves and high-speed particles to distances that could envelop the Earth. This is very unlikely at any given time, such as NOW. But, statistically it is finitely possible, and could be one cause of mass extinctions of life on our planet. A group of astronomers have pointed out that a large number of O and B stars occur in a nearby cluster positioned in the sky near the meeting of the Scorpio and Centaurus constellations. Some ones in this cluster may have passed through Supernovae stages. That Earth may have been affected is implicated by evidence of a deficiency of interstellar matter (including gas) in the so-called "Local Bubble" within which the Sun lies. A consequence of this is that there is less material in our neighborhood that absorbs or impedes light from more distant parts of the Universe; this improves viewing conditions of those cosmic sources. There may be geologic evidence for Supernovae material having reached the Earth: marine deposits dated at 2 and 5 million years are enriched in an iron isotope that would be expelled during a Supernova explosion.

In December, 1997, astronomers observed a localized event in deep space which released more Gamma ray energy at that point than has been calculated to emanate from the entire Universe under a normal state. Because of their similarity to the short-lived, bright Supernovae, such events have been termed Hypernovae, which produce at least several orders of magnitude more energy (1053 -1054 ergs) than associated with a Supernova (~1051 ergs), but they seemingly form by a different mechanism. The initial flare-up may take only a few seconds to actuate but the effects can last for weeks to months. Some HyperNovae seem related to Gamma Ray bursts, described below. HyperNovae were most common in the early Universe when very massive stars underwent rapid burning of their Hydrogen fuel to heavier elements and finally exploded with fusionable fuel was expended. This is an example of a Hypernova - visually it looks like a Supernova but the measured energy release is much larger:

A Hypernova first observed on August 18, 2005.

In November of 2004, a group of British scientists announced the results of a sophisticated study of a Supernova that exploded about 1000 years ago which is shedding new insight into the ubiquitous cosmic radiation that permeates space. This event is still growing in the region near our Sun so that its effects have been now carefully documented. The detection system is known as H.E.S.S., for High Energy Stereoscopic System. As presently configured, four Chernkov telescopes located in the mountains of Namibia are tied together in an array. Together, these provide high definition of a form of blue-colored radiation (the Cherenkov effect) caused by the cosmic rays interacting with atoms in the upper atmosphere. This Supernova has proved to be a major source of very energetic cosmic rays (short wave Gamma rays). This strongly suggests the one predominant mechanism for production of such high speed particles is part of the Supernova explosion process. Here is a plot of the large area (about twice the diameter of the Moon, but, of course, invisible to the eye) of the expanded radiation field as picked up by the H.E.S.S. observation system:

Plot of Cherenkov radiation (a by-product of cosmic radiation exciting gaseous atoms in the Earth's upper atmosphere) detected by the H.E.S.S. array that correlates with the remnants of an expanding Supernova.

A star is in the Aries constellation, about 440 million light years away, experienced massive shedding of material as first seen on February 18, 2006. Its Gamma ray burst phase was especially long - more than 2000 seconds - and powerful - about 25x greater than a typical GRB. Many astronomers believe this event is a precursor to a huge Supernova explosion; telescopes will follow its history for weeks thereafter. Here is a preburst image of the star and then a few days later as the SWIFT satellite detected a ring of radiation flung off the now brighter star:

GRB060218, a possible early stage of a massive Supernova.

The Supernova process has certainly gone on constantly throughout cosmic time. Logically, one would expect this phenomenon to have acted regularly as far back as the first stars. One of the oldest observed Supernovae occurred at least 10 billion years ago:

Infrared image of a Supernova that formed about 10 billion years ago; it is the bright dot in the lower right image.

In May of 2008, announcement was made of a supernova within the Milky Way that occurred about 140 years ago, in the 1870s. It was not seen optically because it took place within a thick, shrouing cloud of dust near the galactic center. It was discovered using X-rays from Chandra. Here it is:

A young supernova near the Milky Way's center.

At almost the same time, another announcement was made of the discovery, quite by chance since the astronomer was looking at another earlier discovered supernova in the same galaxy 90 million light years away, of the first moments of a supernova. Using the Swift space observatory, the beginning consisted of a burst of X-rays. This image shows the galaxy, NGC2770, and the location of supernova 2008d and the earlier SN2007, and the X-ray burst of SN2008 that was detected on January 9, 2008:

Galaxy NGC2770 in UV light; the X-ray image of SN2008d as imaged by Swift.

Neutron Stars and Pulsars

The end product of a Supernova event associated with stars greater than about 8-10 solar masses is a Neutron star. Such stars develop from strong internal pressures that create neutrons by intense squeezing together of protons and electrons (remember: p + e ---> n); these neutrons are also degenerate. (Degenerate matter describes a condition in which the pressures exerted by the mass [as in a gaseous state] no longer depend on temperature but only on the [high] density reached at this stage; the matter is said to no longer obey the classical laws of physics). During the formation of a Neutron star, the prior state star (which may have a core as heavy as iron) develops a degeneration pressure that rises until it is capable of instigating a gravity-driven collapse down to a remarkably tiny size.

This class of stars winds up as small objects only a few kilometers wide but containing matter equivalent to 4-5 solar masses. Their densities can exceed 1014 gm/cc (or 107 denser than White Dwarfs). (A feel for this extreme density is gained from this comparison: A volume equivalent to a lump of sugar would contain 100 million metric tons [as measured on Earth] of neutron star matter.) These stars can be detected by telescopes that gather Gamma-ray, X-ray, and Radio radiation. Obviously, being of such small size Neutron stars are very hard to find by optical telescope, even though they can glow with intense radiance, unless they are very near to Earth within the M.W. galaxy. The HST has now provided the first-ever image in Visible light, shown below, coming a Neutron star. It is shining just in front of a nebular dust mass whose distance is just 400 light years away. (The light is produced by processes involving photon escape from a surface whose temperature exceeds 10000°K; the surface area is quite small in keeping with the miniscule size of the star.) The size of this object has been estimated to be only 28 km (16.8 miles) making it the second smallest intrinsically radiating object beyond our Solar System discovered to date by visual means.

A tiny neutron star (arrow), the first ever detected by the HST.

The XMM-Newton satellite has produced a strong Gamma ray image of a Neutron star just 500 light years beyond the Solar System, as shown here:

Geminga, a nearby Neutron star that is the second brightest source of Gamma rays in the sky.

This smallest imaged star, Geminga, is about 20 km (12 miles) in diameter and has a mass 1.5 times that of the Sun. It rotates at a speed of 4 revolutions per second. Its hot surface exudes strong X-rays and Gamma rays, some extending out as filaments (tails), being driven by its huge magnetic field. Electrons and positrons are also a part of the filament, the result of the electric field built by the rotation of the star within its magnetic field. The electrons accelerate outward but some evidence shows that the positrons are coaxed back to the star to settle into hot spots.

Some Neutron stars, called Pulsars, are known to have intense magnetic fields and to emit directional beams of strong pulses, best observed by Radio astronomy but also very evident in the X-ray region, in extremely regular intervals (with periods from about 1/1000th of a second to several seconds) whose cyclical nature is related to their (often rapid) rotation; the Earth must lie within the beam's solid angle in order to detect this Pulsar action (the pulses therefore are bursts of radiation from a constant beam detected intermittently from Earth. That is much like a searchlight's beam which, while sweeping continuously, appears to the viewer only when aligned momentarily as it passes through its cycle). This is illustrated by this diagram:

Pulsar diagram.

Pulsars are formed by the Neutron star's immense gravity pulling gas from Supernova debris (most Pulsars seem associated with Type II Supernovae), such that this gas is accelerated to a half or more of the speed of light (thus approaching relativistic speeds [those near light speed]); this gas "detonates" when it strikes the Neutron star surface. The magnetic field tends to funnel the fast-moving gas and particles onto narrow parts of the Neutron star's surface which become "hot spots. This releases great quantities of energy extending in the spectrum from Radio to X-ray regions. There are thousands of bursts of energy that rise from the surface many times each second giving rise to the periodicity detected by Radio telescopes and by X-ray observatories such as Chandra.

One of the Pulsars that has been extensively studied lies in the heart of the Crab Nebula which we have examined earlier. It shows the development of a pair of short jets of very hot gases that radiate strongly in the X-ray region of the spectrum.

Optical (right) and X-ray (left) images of the Crab Pulsar.

There is a very strong Pulsar in the Vega constellation. Here is a Chandra view of this feature, expanded in an inset:

A Pulsar (bright yellow ball) in X-ray excited gases in the Vega constellation

The jet (about 0.5 light years in length) around the Vega Pulsar continually shifts its position and shape, as monitored over many months by Chandra. This bespeaks of a significant variation in the configuration of its driving magnetic field.

Four views on different dates of one of the Pulsar jets around the Vega Pulsar; Chandra image.

Pulsars can be irregular in shape. This asymmetry is mainly the result of unequal distribution of X-ray excited gas around the central Neutron star. This is evident in PSR B1509-38.

A Pulsar with irregular surrounding gas and a less well developed pair of directed beams.

Some Pulsars leave a distinctive trail of excited particles behind them, resembling patterns seen in wind tunnel experiments. These are, in fact, dubbed "Wind Pulsars". The best documented example so far is the Mouse Pulsar, moving at a speed of 2 million km/hr (1.25 million mph) through space. This image combines data from Chandra and a Radio telescope:/p>

The Mouse Pulsar.

Most Neutron stars have very strong magnetic fields up to 1012 gauss (a normal star's field has a strength of around 100 gauss). A rare subclass of Neutron stars is called a Magnetar, or more formally, an AXP (Anomalous X-ray Pulsar). Only 15 have been found so far but they are probably much more common (it is estimated that about 1 in 10 pulsars are also magnetars). An AXP has a magnetic field measuring around 1014 Gauss (the current record holder, at 1015 Gauss, is SGR 1806-20, about 1000 times greater than a typical Neutron star and a million billion times that of the Sun's 5 Gauss). A Magnetar is similar to an SGR (Soft Gamma-ray Repeaters), another Neutron star variant that undergoes periodic variations in energy output. Both AXPs and SGRs are detected by their pronounced X-ray signals. The Rossi Explorer satellite is well-suited to study Neutron stars. The Magnetar N 39 has been examined by the HST; while not directly seeable, its presence is evident in the Visible as a collection of filamentous strands formed from shock waves released when a giant star exploded some thousands of years ago leaving a Magnetar behind.

A Magnetar Pulsar, N 39, a strong X-ray and Gamma-ray emitting Neutron star (8 second spin rate), with its nebular material arranged in strands; shown here is the HST visible-near IR image.

One magnetar, SGR 1900+14, has been found with a ring 7 light years across. This is how it appears in a Spitzer IR image (the ring appears to be heated dust):

A magnetar with a glowing ring.

Colliding Stars

At least some of the GRBs and X-ray bursts may stem from collisions or mergers of (usually two) star. As recently as the 1970s astronomers considered collisions to be rare stellar events. Although an actual collision has as yet not be observed by HST or other astronomical satellites and ground telescopes, phenomena associated with certain stellar configurations have now been postulated (attributed) to either head-on or glancing encounters between stars.

As we have seen, stars in the arms of spiral galaxies or the fringes of elliptical galaxies are very widely spaced and hence the probability of collision is low. But star distributions in central cores of these two types show much closer spacing (denser).

To appreciate the significant increase in density, if one counts the stars that are about 25 light years from our Sun, the number would be about 100 but if the same 25 l.y. volume is set around the center of a globular cluster, that star number rises to an order of about 1,000,000. This crowding means that those stars are very closely packed and hence capable of numerous collisions. Three processes make collisions much more likely: 1) a process called "evaporation", in which stars approach others and some are then flinged out of the grouping which contracts to such densities as to make collisions inevitable; 2) gravitational focusing, in which approaching stars have their pathways deflected so that two stars now follow a collision course; 3) tidal capture, in which neutron stars or Black Holes latch onto nearby stars and in time draw these into the high gravity

Theoreticians have developed computer models to simulate pictorially different modes of collision. Shown here is the sequence of change as two Sunlike stars are merged:

Simulated sequence showing the stages of collision of two stars of similar size.

The end result of a collision depends on several factors: 1) whether there is a direct hit or a glancing encounter; 2) the relative size (mass) difference between colliding bodies; 3) the terminal speed of each body. The process can be as brief as an hour; or as long as days to years (this rapid time for completion is one reason while such events have yet to be observed in "real time"). Any two of the 7 density types shown near the top of page 20-5 can experience a collision. In some combinations, such as a White Dwarf striking a Red Giant, the end result is two White Dwarfs (one being the incoming member; the other [Red Giant] dispersing and losing so much of its gas by the interaction that only its core remain which quickly evolves into the new White Dwarf, Or, one star remains relatively intact as the second star is incorporated within it.

In this last case, the result is that the now coalesced star pair has gained considerable mass. This means that it now appears to be a bigger star, and since the total mass determines the rate of Hydrogen fuel consumption, the new, brighter star would appear as though it will burn its Hydrogen mass much faster and thus appears to have a shorter lifetime - hence seems younger. A specific case: if two stars, each with a mass of the Sun (5 billion years old) that has a total burn out time of 10 billion years, collide and form a single star with twice the mass, the now more luminous composite star would have a life expectancy of 800 million years. This seems to be the best explanation of "Blue Straggler" stars - much brighter than the majority of stars in a globular cluster. This is evident in this HST image of NGC 6397:

Large blue stars in globular cluster NGC6397.

Primary Author: N. M. Short, Sr.