Novae, Supernovae; Neutron Stars and Pulsars; Quasars and Black Holes; Gamma Ray Bursts; and Star Collisions.
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.
Soon after its discovery, this Nova has been examined in close detail by the HST, yielding this dramatic sequence of images:
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:
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.:
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):
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:
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.
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". Consider these Hubble views of five Supernovae:
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):
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 later before-after image made by a telescope at the Anglo-Australian University:
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.
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.
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.
The most recent image (lower right) is here enlarged to show detail.
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:
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):
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):
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):
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).
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.
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. (Recall that the Crab nebula was imaged in four spectral regions, as
displayed on page I-3 in the Introduction).
The Crab Nebula has a notably different shape when imaged with X-ray radiation by the Chandra Telescope. We show that below, and beneath it is a striking image made by combining this X-ray image with a Visible light image made by the HST. A ring structure emerges and a jetlike protuberance extends roughly perpendicular to the ring.
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.
Supernovae are also quite impressive when rendered as images using X-radiation. Witness this Chandra view of Cassiopeia A:
This next image demonstrates how long exposure times can bring out much more detail in a distant astronomical object. Again, Chandra has looked at Cassiopeia A, but 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.
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. The event took place at that star around 10000 years ago. Historical records note a bright star appearance in the 1600s. In this rendition, Oxygen-rich clouds of gas/particles are blue; sulphur is red.
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:
However, a surprise greeted investigators when this burst was imaged using a combination of IR bands. Observe this image.
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.
Individual filaments have been examined in detail. Here is a segment of the Cygnus loop, part of a Supernova in the Cygnus constellation, seen in Visible (top) and in an image produced from Gamma radiation:
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; the elements associated with particular colors are identified in the caption:
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. :
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.
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 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.
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.
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:
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:
The Ia type Supernova has come center stage in the recent recognition that the Universe is now accelerating rather than slowing down (see page 20-10 where the behavior of this type is considered in detail). 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 somewhat different mode of destruction of a massive star involves violent,
chaotic expansion of red-glowing (in the visible) gases from a star type known
as a Wolf-Rayet star, as shown below, here seen in an early stage of expansion
around the still-intact central Giant (about 40-50 solar masses).
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:
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:
The most recent major star experiencing shedding of material occurred on February 18, 2006. The star is in the Aries constellation and is about 440 million light years away. 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; telescope 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:
Most recent implies that the Supernova process goes on constantly. Logically, one would expect this phenomenon to have acted regularly as far back as the first stars. One of the oldest observed Supernova is at least 10 billion years old:
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 (see also page 20-2a). 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.
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:
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:
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.
This Pulsar is shown as a close-up enlargement:
There is a very strong Pulsar in the Vega constellation. Here is a Chandra view of this feature, expanded in an inset:
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.
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.
Here is an X-ray image of the Pulsar 3C58 as observed by the Chandra Observatory. It's initiating Supernova event was first observed in 1158 A.D. The Neutron star at the center of the Pulsar source is rotating at 15 times a second. Its radiation excites the cloud of particles surrounding it.
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>
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 (only 10 have been found so far) is called a Magnetar, or more formally, an AXP (Anomalous X-ray Pulsar). 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. One magnetar, N 39, has been imaged by the HST and appears in the Visible as a collection of filamentous strands formed from shock waves released when a giant star exploded some thousands of years ago.
Black Holes and Quasars
As seen in the star evolution diagram on page 20-2, when gases and other matter in stars having solar masses much larger than the Sun gravitationally contract into small, compact bodies,
the result is a Black Hole (B.H.), so called because the gravity associated
with its extremely dense mass (if the mass in the Sun were to collapse into a Black Hole, this would yield a density of about 1022 grams per cubic meter; larger stars would produce densities several orders of magnitude greater). This supermass prevents all detectable internal radiation originating from within from escaping
beyond its event horizon (sphere of influence). The distance from a B.H.'s center to the horizon is known as the Schwartzchild
radius . Since the B.H. is itself invisible (black), its existence must usually be inferred from its gravitational effect on surrounding stars and interstellar matter. Before their observational discovery, Black Holes were predicated to exist from General Relativity considerations. Black Holes indeed have such strong gravitational influence that they notably warp the fabric of Einstein's spacetime dimensionality.
Black Holes can vary in dimensions, the smallest in the general class being
much less than a kilometer in diameter but packing dense mass equivalent to about
3 solar masses. (Theory indicates that mini-Black Holes can be as small as a few centimeters or even microscopic in size; so far, none this small have been detected.) Humongous B.H.s can contain masses derived from billions of
infalling stars and galactic matter, attaining sizes exceeding that of our Solar System. Massive to Supermassive Black Holes may be the customary state at the center of spiral
and other galaxy types, having built up from millions stars and other matter converging inward as though moving to a drain. The HST view of NGC7742, a Seyfert type 2 active galaxy, shows a large glowing central region, within which a supermassive Black Hole is postulated. Its bright center probably represents a quiescent Quasar state, there are periodic flare-ups resulting from energy release when stars spiral past the B.H. horizon into its interior; note the ring of bright hot, largely younger stars beyond and the faint spiral arms further out.
Many Black Holes are the end product of Supernovae explosions of Red Giant stars, as these burn up their fuel and reach the stage where Fe becomes the dominant element (which does not further ignite by fusion; see page 20-7). Smaller stars end up as Neutron stars which in principle can coalesce into B.H.'s. The larger B.H's have masses from millions to billions greater than the Sun. At the other extreme small B.H's may have only a fraction of a solar mass, perhaps up to a billion tons occupying a tiny volume such that the density of just a teaspoon-full of this compact matter is still enormous. This extraordinary density is possible because under the great pressure that formed the B.H. electrons and the atoms themselves become very closely compacted. Smaller B.H's may be ubiquitous - millions of remnants from earlier explosions within the Milky Way and galaxies in general; they may even exist within the Solar System but are too small to affect its spatial fabric and perturb planetary orbits.
A Black Hole generally is so small - yet so massive - that its spacetime expression produces a curvature so pronounced that all internal energy and radiation is seemingly trapped beneath the B.H. (within its horizon). An exception may be Hawking radiation (named after Stephen Hawking who devised
the theory) consisting of particles created by quantum
processes and driven by the gravitational energy within and around the Hole. The mechanism by which this process takes place is an excellent example of "quantum weirdness". In the 'empty' space just outside the B.H.'s event horizon, virtual particles and antiparticles are constantly created (as happens in general in this environment throughout the Universe). Under the strong gravity field around the B.H., one of these particles, the one with positive energy, is likely to be propelled away while the other is captured and dragged into the B.H. Antiparticles have negative energy and those brought into the B.H. react with B.H. particles to reduce the mass of the Hole and thereby lower its gravitational field. This B.H. gravitational field, in turn, loses the energy it provided to make the virtual pair. The escaping particles constitute the Hawking radiation, which is too "faint" to be detected from Earth but nevertheless causes the B.H to slowly "evaporate".
This escaping (emitted) radiation is most effective for tiny Black Holes and provides a means by which they
can dissipate over extremely long times through this evaporation. While based on sound theoretical reasoning, Hawking radiation has not yet been directly detected. But if it is proved to exist, it provides a mechanism by which countless numbers of small primordial B.H.'s that formed at the outset of the Universe, because gravity was so intense then, have since vanished. At the present time, astrophysicists are learning more about B.H.'s by computer modeling and simulating their behavior.
Black Holes are also capable of ejecting matter in jets or streams of particles moving in beams almost at the speed of light. (Jets also occur during star formation and during late stages of star death). Here is an HST view of the well-known galaxy, M87, in which its billions of stars are not resolved so as to appear as a yellow glow. The central "star" is actually light emitted from the exterior around a B.H., probably as a Quasar.
Below are three more views of the jet streamer from M87; the top is imaged by Chandra in the X-ray region; the center is visible light; and the bottom from Radio waves. The origin of such streamers, found also associated with other galaxies, is still imperfectly known. But, the Black Hole(s) causing this ejection of gas and particles are the source of strong, directional electromagnetic fields. The gases may be excited by synchrotron radiation, causing photons whose energy levels extend over most of the Electromagnetic Spectrum.
Still another example of a jet associated with the presumed central Black Hole in a galaxy is Centaurus A (NGC5128) located some 11 million l.y. from Earth. On one side the jet is obvious but it has a faint companion on the other side. This jet pair lines up with the axis of rotation of the galaxy. The image, made by Chandra, is converted to a visible view using data sensed in the X-ray region of the spectrum:
This jet, which follows magnetic lines, is even more splendidly displayed in an HST image that is combined with a Chandra image, as shown below, with the strongest X-ray signals shown in blue:
Probably the best image to date of a paired jet associated with a supermassive Black Hole is that recently captured by HST as it trained on Quasar 3C120; the jet, composed of X-rays and electrons, follows strong magnetic lines:
A jet emanating from Quasar 3C273, which is powered by a Super Black Hole, extends out 100000 light years, as seen in this color composite made from a Chandra X-ray image (blue), a HST Visible image (green), and a Spitzer IR image (red):
These jets may be the same phenomena commonly detected by Radio astronomy, as illustrated near the bottom of page 20-3.
Galaxies are thought to have multiple Black Holes, most of which are relatively small. Evidence is growing that most spiral galaxies, at least, have one or more large Black Holes in their central regions. Such B.H.'s eem to serve as a stabilizing influence on the maintenance and evolution of a galaxy, causing stars and stellar gas and dust to migrate inward and be dragged into the Hole. The first case in which two supermassive B.H.'s occur in the central core of a galaxy has been found in NGC6240. This irregular galaxy is shown in visible light in the left HST image below; the Chandra image on the right indicates a pair of Black Holes, which create a strong X-ray signal (in blue; weaker X-rays in red and yellow) as infalling material is heated to very high temperatures. Astronomers predict that these B.H.'s will eventually merge by collision.
The amount of gas and dust surrounding a central Black Hole can be much greater than farther out in a galaxy. In NGC 1068 (also known as Messier 77), the HST image on the left of this next figure shows reflected light from the dust in blue, ionized Oxygen gas in yellow, and ionized Hydrogen gas in red; on the right is a Chandra image of its center in which the orange-red corresponds to highly energized material around the immediate B.H. that is emitting X-rays.
The image below shows not the invisible Black Hole itself, but the radiation emitted from excitation of gases and other matter drawn into the B.H. During this continuous exposure over 164 hours, the glowing gases underwent periodic flare-ups that combine in this composite pattern:
Chandra and Radio telescopy have now established that the central region of the Milky Way galaxy has a supermassive Black Hole (located in the celestial hemisphere at a point close to Sagittarius A); perhaps there is more than one in this inner part. Proof of the presence of a large B.H. in the M.W. was hard to come by, because the central region is shrouded by dust. As Infrared images of this region accrued, the stars within the dust region were imaged. This allowed determination of their orbits as time lapse views permitted calculation of their movements. Many stars showed just the pathways expected from theory that would occur if a B.H. were sucking in these bodies. Later surveys of the central region using Gamma ray and X-ray radiation to image the behavior of the B.H.-seeking stars confirmed the presence of a very large B.H. at the centerpoint of the Milky Way. However, as this Chandra image discloses, the strongly radiating dust and gas at X-ray wavelengths does not single out Sagittarius A or any other manifestation of the B.H.
The size of the M.W.'s central Black Hole has been hard to determine because of this masking matter. It mass has been estimated to be about 2.6 to 4 million solar masses. Early estimates placed the diameter of a sphere to its event horizon at about 1.5 to 23 million kilometers (1 to 14 million miles). Recently, studies done by penetrating Radio waves, using Radio telescopes, has shed light on its dimensions, so that the upper value is considered close. This Radio wave image shows Sagittarius A, a bright object that may be the glow of excited radiation around the B.H.:
In principle, Black Holes should sometimes collide (but the consequences are not yet defined explicitly), especially when two galaxies collide with the B.H's at their centers then interacting. Evidence for this is sparse. However, such an event is postulated for the observations by the Wide Field Camera on HST of NGC326. In the main view below is the pattern of jet lobes from that galaxy seen a few years ago. In the offset second image is a more recent observation in which the orientation of the principal jets has now shifted more than 90°. The favored explanation is that two Black Holes have now interacted causing the spin axis of one to shift notably.
As implied above, Black Holes play a large role in the life of a galaxy. Recent UV observations by the Galaxy Evolution Explorer (Galex) finds a broad relationship between B.H. size and galaxy size (the larger the first, the bigger the second; the study was confined to elliptical galaxies but probably holds true for spiral ones as well). It was also found that for large central B.H. galaxies, the production of new/young stars in the inner regions became distinctly sparse. This has been attributed to B.H.-controlled heating of Hydrogen gas to temperatures too high to form stars and/or to expulsion of the gas from the inner galaxy.
A Black Hole's incredible gravity pulls in particles from outside the event
horizon until their velocities are accelerated to nearly the speed of light. Matter is literally torn apart upon entering the Black Hole. As these particles close
in, monstrous energy releases produce continuous bursts of energy outside the
horizon, a process believed responsible for most Quasars (a contractive
term for "quasi-stellar" to describe a star-like appearance even though
the observed feature is not a single star). Quasars are extremely bright objects
(very high luminosity, comparable or even exceeding that of an entire typical galaxy), being considered by most astronomers to be the glow of radiation bursts ("hot spots"
of Gamma radiation, X-rays and Visible light) from both stellar and interstellar matter continuously infalling into the central regions of active galaxies, whose cores are probably supermassive Black Holes. While the majority of Quasars are located at or near a galaxy center, some occur in the spiral galaxy arms or in the regions beyond an elliptical galaxy's core. They were initially discovered as intense Radio wave sources detected by Radio telescopes. Now it is known that most Quasars are not accompanied by Radio waves (less than 2% are dominantly Radio sources, in which that wavelength region marks energy developed by synchrotron radiation) but are instead sources of more intense, shorter wavelength radiation. Here is an HST optical image of one (and possibly several) Quasar(s):
And here is a very bright central core of a Seyfert Galaxy, NGC 3516, with a Quasar producing a huge light emission (but probably being "intensified" by gravitational lensing) associated with infall to a massive Black Hole:
A Quasar in M1000, as imaged from variations in X-radiation monitored by the Chandra X-ray Observatory,
appears thusly:
Quasar HE 1013-2136 at a distance of 10 billion l.y., imaged by an ESO telescope on a Chilean mountaintop, seems to be drawing gases from a galaxy to the left:
This pair of images shows a Quasar in Visible (bright in the blue) and Infrared light.
The powerful Quasar qso 1 Zw 1, as seen in the Infrared, is also a strong Radio source (contours superimposed).
Most Quasars are so far away (but some more recent ones are nearby) that light arriving
at Earth left the Quasar source when the young Universe was only about 1/4 to 1/6 its present
size. Thus, most (estimates in excess of 75%) Quasars formed early in Universe history and many, particularly the larger ones, have since become either greatly diminished ("dormant", with occasional flare-ups) or are now extinguished in today's time frame. This generalized (smooth) plot of Quasar history, both in terms of time since the Big Bang and when the numbers of galaxies relative to the expansion size of the Universe are normalized to 1 (maximum), illustrates these points:
Since Black Holes can still form in young cosmological time, i.e., recently, throughout the Universe, conceivably they are giving rise (usually after only millions of years) to new Quasars. Quasars are made visible because of emission of light resulting from energy conversion as stars and interstellar gases are gravitationally sucked into supermassive Black Holes. HST has observed such events, which may be the case in this image of the elliptical galaxy NGC4261, in which the ring seemingly surrounds such a Black Hole.
Perhaps as much as 50% of the EM radiation in the Universe is related to Quasars around Black Holes. The Quasars result from material being pulled off nearby star(s), transferred as stellar winds along magnetic lines from the stars, and accumulating in a disk around the Black Hole. A study of Chandra data for J1655 leads to this pictorial interpretation:
This may be the most common mechanism for Quasar production.
Black Holes that occur outside galaxies, or in a star-sparse region within a galaxy, do not attract enough material to become readily visible by virtue of the excitation of incoming matter. But their presence is often suspected where an X-ray or gammma-ray source is observed without a corresponding visible body.
Recently, Black Holes have been detected in Globular Clusters by analyzing the patterns of movement and velocities of stars that can be resolved in the assemblages making up the clusters. These B.H.'s have estimated masses intermediate between the small isolated ones mentioned above and the Supermassive ones described in the previous paragraph. Although the numbers of points in the following plot relating B.H. mass to stellar assemblage mass are still few, a general trend that fits size to a straight line is evident:
In the early years after first postulated and then discovered, Black Holes were treated almost as a curiosity, without any special importance in the initial phases of the Universe's history. But, with the discovery that most (if not all) galaxies have B.H's in their core, there is a growing belief among astronomers that they are the necessary starting point in the formation of a galaxy, serving as the nucleus or core that attracts the matter that eventually organizes into a galaxy. Recent reports of both observational and theoretical studies now offer two important ideas: 1) both Black Holes and Neutron stars are more abundant in the inner or central part of a galaxy - a fact related to the idea that massive stars tend to form more readily in the core region; and 2) in early cosmological time Black Holes had a definite symbiotic relation to the processes that form and develop galaxies, i.e., massive B.H.'s can serve either as a nucleus for a growing galaxy or at the least aid in gathering matter into organized gas clumps that evolve into primitive galaxies.
In some respects, the smallest Black Holes are
an approximation to the supersingularity postulated as the starting point of
the Big Bang except that they have finite dimensions of meters to several kilometers
and even much larger for those in galactic centers depending on their amounts
of mass (can be equivalent to the cumulate mass of hundreds of millions to billions
of Suns). One theoretical class of Black Holes consists of concentrations of extreme densities collected
in "points" as small as 10-15 meters.
Some Black Holes are thought to be the sole surviving remnants of galaxies
that have been completely swept into them. Other Black Holes may have formed
during the first seconds of the Big Bang. There are increasing indications that supermassive Black Holes were in existence within the first billion years of the Universe. Many of these are either relics of the B.B. or remnants of early Supernovae.
Speculatively, one future outcome
for the Universe (depending on the ultimate mode of expansion [see page 20-8]), after 50 b.y. or so, could be a collection of billions of Black Holes that
eventually converge upon themselves to coalesce into a single ultra-dense Black
Hole that ultimately would become the singularity for the next Universe (in
this model, any number of successive Universes, exploding and contracting cyclically,
is feasible). Such a concept of repeating Universes (treated in more detail
on page 20-10) is referred to as the "Big Crunch", or even more colloquially,
as the "Bounce" in reference to the repetition of an explosion after total collapse
to the B.H. singularity.
Gamma Ray Bursts
Black Holes almost certainly play a role in what are called Gamma Ray Bursts (GRB). These are the most intense and copious releases of energy observed in the Universe - less than that of the Big Bang itself but much more than given out by Supernovae or Quasars. GRBs can at their outset release enough energy to give them a luminosity calculated to be 1019 greater than that of the Sun. They are characterized by extreme outputs over very brief periods, measured in seconds to minutes at their peak. At least one GRB is observed each day somewhere in the Universe, so they are rather common events, albeit less frequent than Supernovae.
Despite being the largest rapid release high energy events in the Cosmos, GRBs were unknown (sometimes mistaken for ordinary Supernovae) before 1967. The manner in which they were discovered is an interesting example of serendipity: Nuclear explosions on Earth release large quantities of Gamma ray energy. In the 1960s, the U.S. was seeking ways to detect Soviet nuclear tests, so it built and orbited Gamma ray, X-ray, and neutron detectors on military satellites. In the U.S. Air Force Vela program, the Vela-4 satellite detected many Gamma ray events, all at times that failed to correlate with any known nuclear blasts on Earth. These Gamma ray events were all proved (eventually) to emanate from well beyond Earth. Here is a plot of one of the first records:
GRBs give off tremendous amounts of energy extending through all wavelengths of the EM spectrum. The diagnostic signature of the GRB that separates it from Supernovae is the predominance of high energy Gamma rays over very short time periods. GBRs can be subdivided into two types: short burst (around 2 seconds) and long burst (more 2 seconds; initial emissions on the order of 20-30 seconds, with a few extending up to an hour). This time spike has been observed in GRBs detected by more sophisticated sensors that monitored such events. Thus, this example:
These GRBs puzzled astrophysicists. They were first thought to be in the Milky Way. And in fact some were actually located in our galaxy, where they occur on average about once in 10000 years. Afterglow radiation from one such event was observed on February 28, 1997 in the M.W. itself by an Italian X-ray satellite called BeppoSAX:
But, the frequency of occurrence, which as more observations were confirmed indicates at least one GRB every day, suggested that the vast majority of GRBs were located in galaxies well beyond the Milky Way. As more records of these events accumulated, it became evident that GRBs are not concentrated in specific regions of the sky but are distributed at random (isotropic) over the entire sky. GRB's are also randomly distributed in time - occurring anywhere in the Universe (thus over the full extent of time since the first galaxies). A large number seem to be distant, near the outer part of the observed Universe, and hence were most common in the early history of the Universe. Here is a map of the sky showing many of the larger GRBs, as detected by the Compton Gamma Ray Observer and BeppoSAX.
The BATSE (Burst and Transient Source Experiment) instrument on the Compton Gamma Ray Observatory (CGRO; see page 20-4) was particularly suited to detecting GRBs. Here is one image of an event that occurred several billion light years away:
These GRB events should generate radiation at wavelengths longer than those of Gamma rays. As studies of them expanded, traces of individual events were sought by other satellites that monitor at different wavelengths. The problem is that evidence of a GRB diminishes rapidly at shorter wavelengths. However, in time such events were picked up at various wavelengths when alerts were given and the sky locations established. Now, with experience this is the time frame for durations of GRBs over a range of wavelengths:
These signs of lower levels of energy at longer wavelengths persisting around a GRB are grouped under the general term "afterglow". X-rays proved useful as GRB signatures provided the searching satellite(s) could check out the source region within a few days. The X-ray emissions persist over periods of hours to days. This is one X-ray image of a presumptive GRB that was located in a galaxy nearby (some has classified this as a hyperNova):
Images acquired by BeppoSAX were especially helpful in the sky survey for GRBs. The top illustration consists of two intensity contoured images typical of X-ray renditions; note the reduction in intensity in just four days between February 28 and March 3. Below it is a pair of BeppoSAX images taken first on December 15, showing the GRB as a bright dot and then on December 16 as the afterglow had faded away.
Special attention was given to finding GRBs at visible (optical) wavelengths, since these are especially capable of measuring red shifts by which approximate distance to the source can be estimated. About half the GRBs give off light in the Visible for durations of a week or more. The HST and the Keck Observatory in Hawaii were pointed at targets reported by other observing satellites. Here is the HST image of event GRB 000301c.
A ground telescope imaging of another GRB shows the burst as seen in visible light (here the print is a negative) at 21 hours (left) and 8 days (right) after first detection. The rapid fading of the galaxy-sized feature is evident (note arrows)
Although not used a lot for this purpose, Radio telescopes have detected and imaged GRBs. Here is one made by the VLA group:
One very important GRB event led to some intriguing information that indicates that this phenomenon occurred much more often early in cosmic time (but continues til the present) and helps to confirm the huge amounts of energy involved. Its magnitude is equivalent to 100 million billion solar radiances. On December 14, 1997 the CGRO registered this event. Word was sent to BeppoSAX operators and to the HST and Keck telescopes to look for it as rapidly as possible. All succeeded. This is how the event was imaged by the HST:
The image on the right was taken on January 23, 1999 during its maximum. When a redshift distance measurement was made on the GRB, it was found to be some 12 billion light years from Earth, proving the surmise that GRBs have probably been part of the Universe's history since soon after the Big Bang. It was also the brightest object yet found at that far distance from Earth.
Thus, the pattern found for most GRB events is rapid emission of Gamma rays followed, as they fade, by the dominant radiation passing through X-ray, Visible, and Radio wavelengths, with the whole sequence being over in less than a few months.
GRBs are of such high interest that another dedicated satellite has been placed in orbit to look for these and similar events. This is HETE-2, the High Energy Transient Explorer, launched October 9, 2000 (the first HETE failed to separate from its third stage rocket). It is described at this MIT site.
The cause(s) of GRBs continues to be uncertain and tantalizing. As an aid to the following discussion, use this Fireball Model to provide a framework for the starting energy, the expansion of the GRB, and the time involved in reaching the afterglow phase:
The early idea of the explosion of material sucked in and around a neutron star (see top illustration on this page for a similar example) has been challenged. But, a variant postulates a role for a binary pair of Neutron stars which, if they should collide, should produce a huge release of energy. Still others attribute the GRBs to some involvement with Quasars. One school holds them to be the outcome of giant Supernovae (Hypernovae) which generate very powerful short-time energy release levels. A recent hypothesis takes still a new tack - the GRBs are associated with large clusters of galaxies which together have such a strong gravitational pull that they accelerate matter both within and around the galaxies to high speeds that, upon colliding with intergalactic matter, release energy at the Gamma-ray level.
Another hypothesis, known as the Paczynski Model (also known as a Collapsar event) and now the most favored explanation, starts with a supermassive (type O) rotating star that collapses to form a Black Hole that continues to draw more material around it until a critical state is reached that requires an intense Supernova-like explosion producing the GRB fireball. Essentially, all the mass involved is suddenly converted to energy in obeyance to the Einstein E = mc2 relation. There are indications that this energy release may be directed, something like the beam associated with a Pulsar. Calculations show that if such a beam generated from a GRB destruction of a neighboring massive star in the Milky Way were to strike the Earth, the intensity would destroy everything at and above the surface - oceans, vegetation, atmosphere, life (fortunately, the probability of this happening, both in terms of star sizes and of directionality of the beam, is considered quite low).
In fact, until the release of information in June 2004 about a GRB only 35000 l.y. away - either in or just outside the M.W. - no nearby events had ever been confirmed. The image below shows W49B as a color composite made from Chandra X-ray data (blue), and Palomar telescope images taken in the IR (green and red). The estimated initial release of energy over a 1 minute time span is 1013 greater than that of the Sun in that timeframe. The image represent the GRB status soon after the burst; the colors indicate enrichment in iron. The postulated beam associated with collapsar events was not oriented straight at the Earth and hence is not visible here.
Some of the above information has been extracted from an article in the December 2002 edition of Scientific American, entitled "The Brightest Explosions in the Universe", by N. Gehrels, L. Piro, and P. Leonard. The article contains this illustration that summarizes the authors' ideas on the formation of GRBs:
From Scientific American, December 2002
In their model, similar to some others proposed, GRBs are definitely associated explosive processes that will end up forming Black Holes. In one common mechanism, a massive star collapses and explodes as a Hypernova, leading to a disk of matter/energy surrounding a Black Hole; this is a fast process in the sense that at a critical time, the Hypernova ensues without anything discernible obviously leading up to it. Alternatively, over a long span of time (millions of years, the same end result could occur as two neutron stars mutually orbiting finally crash into each other. The wedge to the right of the 'Central Engine' conforms to a jet that carries the photons released in the GRB outward at near light-speed. This material moves outward as "blobs" that catch-up and coalesce forming internal shock waves that generate the Gamma bursts. With expansion over time, the high energy photons are replaced by those of progressively lower energies represented by X-rays, Visible light, and Radio waves as the emissions encounter the galactic/intergalactic medium. The final result is an afterglow that fades over time.
On March 29, 2003, HETE-2 captured a GRB (HETE Burst H2652 is also listed as GRB 030329 and SN2003dh) in a galaxy 2.6 billion light years distant and sent the occurrence of this event back to Earth so quickly that many observatories were alerted quick enough to train their telescopes on it within minutes. Thus, for the first time the earliest stages of a GRB could be monitored. This event proved one of the brightest ever observed. This plot of HETE data shows how brief was the main phase of the event.
A Radio telescope image of this GRB taken on April 22 shows a progressive distribution of decreasing energy moving outward. This seems to confirm the "fireball" model for ejection of matter (an alternate explanation, that material is ejected in huge blobs [the "cannonball" model], is apparently not valid for this observation). The expelled material moves at nearly the speed of light.
Spectrographic data showed that the initial burst of H2652 was rich in excited Silicon and Iron. These elements would be produced in a star whose mass is at least 30x that of the Sun, which would give rise to temperatures and pressures that generate nuclear reactions that fuse nuclei into Si and Fe. These are the conditions that favor a "super-Supernova", another way of referring to hyperNovae. Astronomers believe this is convincing evidence for that mode of generation of many (perhaps most) GRBs.
As knowledge of the roles of the predominant dark matter and dark energy (see pages 20-9 and 20-10) is increasing, another explanation (Louis Clavelli) - somewhat conjectural - has emerged. This holds that dark energy under the right conditions acts upon ordinary matter to convert it to dark matter. From theory, this should give off intense bursts of energy as the conversion proceeds, witnessed by us as GRBs.
The latest space telescope dedicated to GRB detection is NASA's Swift, launched on November 20, 2004. This spacecraft has three detection systems that can be activated within minutes of the one always on that picks up the Gamma-radiation. Swift is capable of finding and monitoring as many as 2 GRBs per week, far more than previous instruments, and will follow the changes to the stage when Black Holes as the end product is reached. Swift also can "see" back to the early days of the Universe out to almost 14 billion light years away. The spacecraft and its three telescopes are shown here:
In January, 2005 9 GRB events were detected by Swift. The first detection was in December, 2004, as shown below as an energy plot and the actual image derived from the data:
Swift also peered at the Cassiopeia A (circa 1680 BPE) Supernova to test its ability to measure afterglow using the X-ray Telescope;
As mentioned above, GRBs are all short-lived, even in human terms, lasting from milliseconds to hundreds of seconds. One very short duration type, which releases much less energy, is known as Gamma rays flashes. These have been observed by Swift and by earth-based telescopes. Their cause(s) may be Neutron star pair interactions but other mechanisms have not been ruled out. Here are several that occurred simultaneously:
Problems with explaining GRBs are compounded by observations (using
EGRET, NASA's orbiting Energetic Gamma Ray Experiment Telescope, part of the Compton X-ray Observatory) of about 170 sources of continuously emitting high energy Gamma rays. Thus, these do not display short-lived bursts. This class was first discovered by the Compton Gamma Ray Observatory (page 20-3). They may be associated with clumps of supersymmetric particles (page 20-1) including a type called the neutralino.
Needless to say, GRBs continue to fascinate cosmologists since they represent the largest and fastest explosive events beyond that of the Big Bang itself. As they are better understood, they may reveal the action of physical processes only now being speculated upon, and suggested by particle physics experiments. The next big step in studying GRBs and the continuous types will be the launch of GLAST (Gamma-ray Large Area Space Telescope), perhaps as early as 2007.
Colliding Stars
At least some of the GRBs and X-ray bursts may stem from collisions or mergers of (usually two) stars (check back at the bottom of page 20-5 for the earlier review of galaxy collisions). 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:
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:
An excellent summary of collision processes and consequences is offered in the November 2002 edition of Scientific American under the title "When Stars Collide", by Michael Sharma.
Primary Author: Nicholas M. Short, Sr.
