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.
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:
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 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.
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).
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 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.
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.
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.
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 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.
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:
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 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. Hypernovae are tied to the destruction of stars with masses at least 20 times that of the Sun. 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:
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:
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:
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:
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: