University of Ljubljana Faculty of Mathematics and Physics

Department of physics

Seminar – 4 th class

How stars become Type I and Type II supernovae

Author: Zdenka Serušnik Supervisor: dr. Tomaž Zwitter

Ljubljana, March 2011

Summary

In this seminar the basic conducts how stars become supernovae will be discussed. Type I supernovae come from less massive stars, and may be the incinerations of white dwarfs. When more massive stars, which contain more than about 8 solar masses, exhaust their hydrogen in the core, they become red giants. Later, heavier elements are built up in the core, and the stars become supergiants. After cores of heavy elements form, the stars explode as Type II supernovae. Only a few optical supernovae have been seen in our galaxy, and a few have been detected in the infrared. Supernova remnants can best be studied with radio and x-ray astronomy. The nearest, brightest supernova to be seen since 1604 erupted in 1987 and became slightly brighter than the3rd magnitude. It was in the Large Magellanic Cloud, and is now declining. Detection of neutrino bursts from the supernova was especially significant, and led not only to verification of our model of supernovae but also to new basic information about particle physics. Probably, high-energy particles and cosmic rays from space originate from supernovae. We shall discuss supernovae in general.

1

CONTENTS

1 INTRODUCTION……………………………………………………………….....3

2 RED SUPERGIANTS……………………………………………………………..3

3 TYPE OF SUPERNOVAE………………………………………………………....4 3.1 Current models.......................................................................................................5

3.1.1 Type Ia supernovae……………………………………………………....6

3.1.2 Core–collapse supernovae………………………………………….........7

3.2 Why the name ˝supernova˝ ?.................................................................................9

4 DETECTING SUPERNOVAE………………………………………………….....10

4.1 History of supernova observation.......................................................................10

5 SUPERNOVA REMNANTS……………………………………………………....12

6 CONCLUSION…………………………………………………………………...13

7 REFERENCES…………………………………………………………………....14

2

1 INTRODUCTION

Every 30 years or so, one of the massive star in our galaxy blows itself apart in a supernova explosion. Supernovae are one of the most violent events in the universe.The word ˝supernova˝ was coined by Swiss astrophysicist and astronomer Fritz Zwicky, and was first used in print in 1926. Several types of supernovae exist. [1] Supernovae (Figure 1) were originally classified on the basis of their optical properties. Type II supernovae show conspicuous evidence for hydrogen in the expanding debris ejected in the explosion; some Type I explosions do not. Recent research has led to a refinement of these types, and a classification in terms of the types of stars that give rise to supernovae. A Type II is produced by the catastrophic collapse of the core of a massive star. A Type I supernova is produced by a sudden thermonuclear explosion that disintegrates a white dwarf star. Type II supernovae occur in regions with lots of bright, young stars, such as the spiral arms of galaxi-es. They apparently do not occur in elliptical galaxies, which are dominated by old, low-mass stars. Since bright young stars are typically stars with masses greater than about 8 times the mass of the Sun, this and other evidence led to the conclusion that Type II supernovae are produced by massive stars. Some Type I supernovae show many of the characteristics of Type II supernovae. These supernovae apparently differ from Type II because they lost their outer hydrogen envelope prior to the explosion. The hydrogen envelope could have been lost by a vigorous outflow of matter prior to the explosion, or because it was pulled away by a companion star.The general picture for Type II supernovae goes something like this. When the nuclear power source at the center or core of a star is exhausted, the core collapses. In less than a second, a neutron star (or, if the star is extremely massive, a black hole) is formed. The formation of a neutron star releases an enormous amount of energy in the form of neutrinos and heat, which reverses the implosion. All but the central neutron star is blown away at speeds in excess of 50 million kilometers per hour as a thermonuclear shock wave races through the now expanding stellar debris, fusing, lighter elements into heavier ones and producing a brilliant visual outburst that can be as bright as several billion Suns.Some Type I supernovae, in contrast, are observed in all kinds of galaxies, and are produced by white dwarf stars, the condensed remnant of what used to be Sun-like stars. In recent years these Type I supernova have been used as a distance indicator which determined the rate of expansion of the universe. This research has led to the astounding discovery that the expansion of the universe is accelerating, possibly because the universe is filled with a mysterious substance called dark energy.

B A

Figure 1: The supernova (A) before and (B) after February 24, 1987. Supernova 1987 A (visual brightness +2,9 (mag, max) ) was the nearest observed supernova of 1604 since Kepler᾽s Supernova of 1604, which occured before the invention of the telescope. Su- pernova 1987A, peculiar and of Type II, was one of the most interesting objects for the astrophysicists in the 1980s (some even say of this century). It occured in the Large Magellanic Cloud (only 169 000 light years from the Earth). The site of the explosion was tra ced to the location of star called Sanduleak that had a mass estimated at approximately 20 Suns. [2]

2 RED SUPERGIANTS

Stars that are much more massive than the Sun use up their store of hydrogen very quickly. These prodigal stars whip through their main sequence lifetimes at a rapid pace. A star of 15 solar masses may take only a million years from the time it first reaches the main sequence until the time when it has exhausted all hydrogen in its

3

core. This is a lifetime a ten-thousand times shorter than that of the Sun. The star becomes a red giant, because the star exhausts the hydrogen in its core and its outer layers expand.For these massive stars, the core can then gradually heat up to 100 million degrees, and the triple–alpha process begins to transform helium into carbon. After its ignition, the helium then burns steadily, avoiding the helium flash which occurs in less massive stars. By the time helium burning is concluded, the outer layers have expanded even further, and the star has become much brighter than even a red giant. We call it a red supergiant (Figures 2,3).

Figure 2: A red supergiant. The massive star is very big and in its expanding stage. Massive stars have a mass larger than 3 times that of the Sun. Some are 50 times that of the Sun. They evolve in a similar way to a less massive stars until they reach their main sequence stage.The stars shine steadily until the hydrogen in their cores has fused to form helium (this takes only millions of years in a massive star). The massive star then becomes a red supergiant and starts to burn helium core which is surrounded by a shell of cooling, expanding gas. [3]

Figure 3: An H-R diagram showing the evolutionary tracks of 5- and 10- Solar-mass stars as they evolve from the main sequence to become red supergiants. For each star, the first dot represents the point where hydrogen burning starts, the second dot the point where helium burning starts, and the third dot the point where carbon burning starts. The luminosity is shown on the vertical axis. The horizontal axis is a measure of effective temperature. [4] Betelgeuse, the star that marks the shoulder of Orion, is the best-known example. Supergiants are inherently very luminous stars, with absolute magnitudes of up to -10, one million times brighter then the Sun. A supergiant΄s mass is spread out over such a tremendous volume, though, that its average density is less than one millionth that of the Sun.The carbon core of a supergiant contracts, heats up, and begins fusing into still heavier elements. Possibly, even iron builds up. The iron core is surrounded by layers of elements of different mass, with the lightest toward the periphery.

3 TYPES OF SUPERNOVAE

Supernova is a dying star, which explodes in a glorious burst. Supernovae are of two different types (Table 1). The best current models indicate that Type I supernovae represent the incineration of a white dwarf (which results from a star with a moderate mass). On the other hand - Type II supernovae are the end of very massive stars and show that stellar evolution can experience a run away and go out of control.The two types can be distinguished by their spectra and by the rate at which their brightnesses change. They have also different shapes of their light curves (Figure 4). Rudolph Minkovski pointed out the distinction in the 4

1940΄s, based on the observation that some supernovae – those of Type II – have prominent hydrogen lines when they are at maximum light, while hydrogen lines are absent in the others – those of Type I. Type I are 1,5 magnitudes (4 times) brighter than Type II supernovae and they take place in all types of galaxies. So they are more often seen. Type II supernovae are seen only in spiral galaxies. Indeed, they are usually found near the spiral arms, which endorses the idea that they come from massive stars. (Since the massive stars are short-lived, they are found in the spiral arms near where they were born.) Doppler–shifted absorption lines show that gas is expanding rapidly in both cases: on the order of 5,000 - 20,000 km/sec (or rougly 3 % of the speed of light) for Type I΄s and half that for Type II΄s.

Table 1: Two quite different situation apparently lead to supernovae. Type I supernovae represent the incineration of a white dwarf, which results from a star of moderate mass. Type II supernovae take place in very massive stars. They show that stellar evolution can experience a run away and go out of control. [5]

Type I Type IISource White dwarf in binary Massive starSpectrum No hydrogen lines Hydrogen linesPeak 1,5 magnitudes brighter than Type

II, Sharper Broader when graphed vs. time Light curve Rapid rise

Decay with several-week half-lifeAll approximately the same magnitude

Different magnitudes

Location All type galaxies Spiral galaxies onlyExpansion On the order of 5,000 – 20,000

km/sApproximately 5,000 km/sec

Radio radiation Absent Present

Figure 4: Supernovae are classified as Type I or Type II depending upon the shape of their light curves and the nature of their spectra. The graph shows a typical comparison of light intensity of Type I supernovae and that of Type II supernovae as a fun- ction of the number of days since peak. Type I supernovae (green) is much brighter, but decays much faster than Type II super- novae (blue). Curves from James B. Kaler; Scientific American Library, 1992. [6]

3.1 Current models

In any case, as part of attempt to understand supernovae, astronomers have classified them according to the absorption lines of different chemical elements that appear in their spectra. The first element for division is the presence or absence of line caused by hydrogen. If a supernova`s spectrum contains a line of hydrogen (known as the Balmer series in the visual portion of the spectrum) it is classified Type II; otherwise it is Type I. Among those type, there are subdivisions according to the presence of lines from other elements and the shape of light curve (Table 2).

5

Table 2: Supernova taxonomy. Among Type I and Type II supernovae, there are subdivisions according to the presence of lines from different chemical elements and the shape of the light curve. [7]

Type Characteristics

Type I

Type Ia Lacks hydrogen and presents a singly ionized silicon (Si II) line at 615.0 nm,near peak light.

Type Ib Non-ionized helium (He I) line at 587,6 nm and no strong silicon absorptionfeature near 615 nm.

Type Ic Weak or no helium lines and no strong silicon absorption feature near 615 nm.

Type II

Type IIP Reaches a ``plateau`` in its light curve.

Type IIL Displays a ``linear``decrease in its light curve (linear in magnitude versus time).

The supernovae of Type II can also be sub-divided based on their spectra.While most Type II supernova show very broad emission lines which indicate expansion velocities of many thousands of kilometres per second, some have relatively narrow features (these are called Type IIn, where the `n`stands for `narrow`). Supernovae that do not fit into normal classifications are designated peculiar, or `pec`. A few supernovae (such as SN 1987K and SN 1993J) appear to change types: they show lines of hydrogen at early times, but, over a period of weeks to months, become dominated by lines of helium. For describe the combination of features normally associated with Types II and Ib, they used the term ``Type IIb``.

3.1.1 Type Ia supernovae

A Type Ia supernovae, in contrast to Type II supernovae, apparently take place in a binary system containing a white dwarf when material from the second star (possibly also a white dwarf) falls onto the white dwarf. Type Ia supernovae thus take place in the late stages of medium-mass stars.In spite of Type Ia supernovae share a common underlying mechanism, there are several means by which a supernova of this type can form. If a carbon-oxygen white dwarf accreted enough matter to reach the Chandrasekhar limit of about 1,4 Solar masses (for a non-rotating star), it would no longer be able to support the quantity of its plasma through electron degeneracy pressure and so it would begin to collapse. (The value of 1,4 Solar masses is known the Chandrasekhar limit after the astronomer S. Chandrasekhar-known far and wide as `Chandra`. So, a star that has a final mass of less than 1,4 Solar masses will end its life as a white dwarf and, when it is in a binary system, as a supernova.) In any case, the current view is that this limit is not normally attained; increasing temperature and density inside the core ignite carbon fusion as the star appproaches the limit, before collapse is initiated. Within a few seconds, a fundamental fraction of the matter in the white dwarf goes through nuclear fusion, releasing enough energy to set free the star in a supernova explosion. An outwardly expanding shock wave is produced, with matter reaching velocities on the order of 5,000-20,000 km/s. There is also important increase in luminosity, reaching an absolute magnitude of 5 billion times brighter than the Sun (or -19,3), with little variation.A close binary star system is one model for the formation of this category of supernova. The larger of the two stars is the first to develop off the main sequence. It expands to form a red giant. The two stars now divide a common envelope, causing their reciprocal orbit to shrink. The giant star then pours over most of its envelope, losing mass until it can no longer keep on nuclear fusion. At this moment it becomes a white dwarf star, compiled primarily of carbon and oxygen. Accidentally the secondary star also developes off the main sequence to form red giant. The material from the giant is accreted by the white dwarf, causing the latter to increase in mass.The merger of two white dwarf stars, with the combined mass momentarily exceeding the value of 1,4 Solar masses, is another model for the formation of a Type Ia explosion. A white dwarf could also accrete the material from other types of companions, including a main sequence star, if the orbit is sufficiently close.After the explosion,Type Ia supernovae follow a characteristic light curve. This luminosity is produced by the radioactive decay of nickel-56 through cobalt-56 to iron-56. The fact that the peak luminosity of the light curve

6

was believed to be consistent across Type Ia supernovae, having a maximum absolute magnitude of about -19,3, this would allow them to be used as a secondary standard candle to measure the distance to their host galaxies. In any case, recent discoveries find that there is some evolution in the average lightcurve width, and thus in the real luminosity of supernovae, although important evolution is found only over a large red-shift baseline.

As described above, not all supernovae without hydrogen lines in their spectra (that is of Type I) come from the incineration of white dwarfs. Some of the Type I supernovae seem to come from supergiant stars after all, though from supergiants that have lost their hydrogen-rich outer layers before their collapse. Apart from the absence of their outer atmospheres, the collapse caused by the buildup of iron atoms proceeds as for Type II supernovae.

3.1.2 Core–Collapse supernovae

Types Ib, Ic and various Types II supernovae are collectively called Core Collapse supernovae. A principal difference between Type Ia and Core Collapse supernovae is the source of energy for the radiation emitted near the peak of the light curve. The progenitors of Core Collapse supernovae are stars with extended envelopes that can attain a degree of transparency with relatively little expansion. Most of the energy powering the emission at peak light is derived from the shock wave that heats and ejects the envelope. On the other hand, the progenitors of Type Ia supernovae are compact objects, much smaller but more massive than the Sun, that must expand - and therefore cool – enormously before becoming transparent. Heat from the explosion is dissipated in the expansion and is not available for light production. The radiation emitted by Type Ia supernovae is thus entirely attributable to the decay of radionuclides produced in the explosion; elementary nickel-56 and its product cobalt-56. During this nuclear decay, gamma rays are absorbed by the ejected material, heating it to incandescence.In this case also, as the material ejected by a Core Collapse supernova expands and cools, radioactive decay eventually takes over as the main energy source for light emission. While a bright Type Ia supernova may expel 0.5–1.0 Solar masses of nickel-56, a Core Collapse supernova likely ejects closer to 0.1 Solar mass of nickel-56.

Type Ib and Ic supernovae

These cases, like supernovae of Type II, are probably massive stars running out of fuel at their centers. However, the progenitors of Types Ib in Ic have lost most of their outer (hydrogen) envelopes because of strong stellar winds or else from interaction with a companion. Type Ib supernovae are thought to be the result of the collapse of massive Wolf-Rayet star. There is some proof that a few percent of the Type Ic supernovae may be the progenitors of gamma ray bursts. Although it also believed that any hydrogen-stripped, Type Ib or Ic supernova could be the progenitor of gamma ray bursts, dependent upon the geometry of the explosion.

Type II supernovae

In the current leading model for Type II supernovae, a substantial core of heavy elements has formed in a massive star (of around 8-12 Solar masses). The core begins to shrink and heat up. The heavy elements represent the ashes of the previous stages of nuclear burning. The stages of oxygen and magnesium ˝burning˝ to form heavier elements take less than 1000 years. At last, silicon and sulfur ˝burn˝ to iron in only a few days. [7]The conditions in the center of the star change so quickly that it becomes very difficult to model them satisfactorily in sets of equations or on computers, but the new supercomputers allow to study rapidly changing conditions; whether the following model ultimately will prove correct is uncertain. The temperature becomes high enough for iron to form and then to undergo nuclear reactions (in this model). Iron nuclei have a fundamentally different property from other nuclei when undergoing nuclear reactions, so the stage is now set for disaster. Unlike other nuclei, iron absorbs rather than produces energy in order to undergo either fission or fusion. As the core shrinks this time, processes that release energy no longer can build up heavier nuclei. Iron cores are formed in the stars more massive than 12 Solar masses by other processes and these more massive stars then join the iron-collapse scenario.

The temperature climbs beyond billions of kelvins, and the iron is broken up into alpha particles (helium nuclei), protons, and neutrons by the high-energy photons of radiation that are generated. Since these processes absorb energy, the pressure at the center of the star diminishes, and the core begins to collapse. The process goes out of 7

control. Within miliseconds – a fantastically short time for a star that has lived for millions and millions of years – the inner core collapses and heats up catastrophically. As the outer core falls in upon the collapsing inner core, electrons and protons combine to make neutrons and neutrinos. The core collapses and (calculations show) bounces outward, since its gas has a natural barrier at nuclear densities against being compressed too far.The collision of the rebounding inner core with supersonically collapsing outer core sends off shock waves that cause heavy elements to form and that throw off the outer layers. The shock wave leaves the core and starts the explosion in less than 0,01 second, so the process is called a `prompt explosion`. So within that massive, evolved star the onion-layered shells of elements undergo fusion, forming an iron core that reaches Chandrasekhar-mass and starts to collapse. The inner part of the core is compressed into neutrinos, causing infalling material to bounce and form an outward-propagating shock front. The shock starts to stall, but it is reinvigorated by a process that may include neutrino interaction. The surrounding material is blasted away, leaving only a degenerate remnant, and only the core may be left behind. The effect is called the `delayed explosion`. (Figure 5)

Figure 5: Within a massive, evolved star (a) the onion-layered shells of elements undergo fusion, forming an iron core (b) that reaches Chandrasekhar-mass and starts to collapse. The inner part of the core is compressed into neutrinos (c), causing infalling material to bounce (d) and form an outward – propagating shock front.The shock starts to stall (e), but it is reinvigorated by a process that may include neutrino interaction. The surrounding material is blasted away (f), leaving only a degenerate remnant. [8]

The heavy elements formed either near the center of the star or in the supernova explosion are spread out into space. There they enrich the interstellar gas. Most Type II supernovae come from stars containing between 8 and 12 solar masses, however, they don΄t supply as much of the heavy elements as Type II supernovae from still more massive stars do. In any case, the heavy elements are present, when a star forms out of gas enriched by supernovae of both types. When the progenitor star is below about 20 Solar masses (depending on the strength of the explosion and the amount of material that falls back), the degenerate remnant of a core collapse is a neutron star. So the pressure caused by neutron degeneracy balances the gravitational force that tends to collapse the core, and as a result the core reaches equilibrium as a neutron star. We can detect that object as a pulsar and a x-ray binary. Above this mass the remnant collapses to form a black hole. (This type of collapse is one of many candidate explanations for gamma ray bursts, possibly producing a large burst of gamma rays through a hypernova explosion.) The theoretical limiting mass for this type of core collapse scenario was estimated around 40-50 Solar masses.Why do we call it a black hole? We think of a black surface as a surface that reflects none of the light that hits it. Similarly, any radiation that hits the surface of a black hole continues into the black hole and is not reflected. In this sense, the object is perfectly black.Above 50 Solar masses stars were believed to collapse directly into a black hole without forming a supernova explosion, although uncertainties in models of supernova collapse make accurate calculation of these limits difficult. Over about 140 Solar masses stars may become pair-instability supernovae that do not leave behind a black hole remnant.Our understanding of element formation in Type II supernovae is circumstantial, since the outburst itself is hidden from our view by the outer layers of the massive star. We can only study the transformed material later on, when it has spread out considerably.The fact that spectroscopic studies of the star η (eta) Carinae (Figure 6) show that a gas condensation spewed out 150 years ago contains a relatively large abundance of nitrogen indicates that the star has processed material in the carbon-nitrogen cycle. The star brought it to the surface as the star΄s material mixed up, and ejected it. A

8

massive star will become a supernova soon - but ˝soon˝ could be next year or in 10,000 years. This is expected from a massive star.Type I and Type II supernovae - for some of which we still see remnants (Figure 7) - are the source of the heaviest elements in the universe.They provide many of the heavy elements, which are necessary for life to arise. Heavy atoms in each of us were formed during such a supernova explosion.

Figure 6: The Eta Carinae Nebula, a nebula around a massive star that may become a supernova quite soon. Eta Carinae is 9000 light years away. A supernova there would be too far away to affect us, but would appear brighter than Venus. The star η Carinae exploded in 1843; it is in the brightest part of the nebula seen here. The nebulae around it are expanding outwards since the explosion. [9]

Figure 7: A small part of the Vela supernova remnant (in optical), which covers a region 6˚ square in the southern sky. This is an expanding envelope a supernova that exploded 12,000 years ago. Studies of reddening due to interstellar dust which blocks in the blue light of some stars on the right side of the image, show that the nebula is about 1500 light years away. (Photograph made from plates taken with the UK Schmidt Telescope.) [10]

3.2 Why the name ˝supernova˝ ?

The name ˝supernova˝ persists from a time when these events were thought to be merely unusually bright novae. The supernova in the Andromeda Galaxy in 1885 (known by the variable-star name of S Andromedae), since it was assumed to have the same brightness as a nova, gave a value for the distance to that galaxy that misled astronomers for years. They thought the Great Nebula in Andromeda was to close to be an independent galaxy. But Walter Baade and Fritz Zwicky realized the distinction between novae and supernovae in the 1930΄s. A nova uses up only a small fraction of a stellar mass and can recur, while supernova explosion represents the death of a star and the scattering of most of its material. Still there is some similarity between the two events: both novae and Type I supernovae are phenomena occuring in a binary system in which one member is a white dwarf star.

9

4 DETECTING SUPERNOVAE

In the weeks following the explosion, the amount of radiation emitted by the supernova can equal that emitted by the rest of its entire galaxy. The star may brighten by over 20 magnitudes during the explosion, a factor of 10 8 in luminosity. Doppler-shift measurements show high velocities, about 5,000 to 10,000 km/sec, that prove that an explosion has taken place.[3]

4.1 History of supernova observation

Hipparchus´ interest in the fixed stars may have been inspired by the observation of a supernova (according to Pliny). The earliest recorded supernova, SN 185, was viewed by Chinese astronomers in 185 AD. The sightings of 393, 1006 and 1181 AD are now also thought to have probably been supernovae. The 1006 supernova, in southern constellation Lupus, remained visible, even in northern locations, for at least two years. It was apparently the brightest ever, probably apparent magnitude -26. It was described in detail by Chinese and Islamic astronomers. Cronicles in China, Japan, and Korea recorded the appearance of ˝guest star˝ in the sky that was sufficiently bright that it could be seen in the daytime. It was in A.D. 1054. A reference to a sighting of the supernova in Constantinople has recently been discovered. Certain cave and rock paintings made by Indians in the American southwest may show this supernova, though this interpretation is controversial. They may depict the supernova explosion 900 years ago that led to the Crab Nebula (Figure 9). It had been wondered why only Oriental astronomers reported the supernova, so searches have been made in other parts of the world for additional observations. The light curve was not well enough observed even to determine whether it was a Type I or a Type II supernova. It may even be an example of some rare additional type.

Figure 8: The Crab Nebula, the result of a supernova seen in 1054 AD, is filled with mysterious filaments. The above image, taken by the Hubble Space Telescope, is presented in the three colours chosen for scientific interest taken through visible-light filters. The Crab Nebula spans about 10 light-years. In the nebula΄s very center lies a pulsar: a neutron star as massive as the Sun but with only the size of a small town. The Crab Pulsar rotates about 30 times each second. [11]

Supernovae SN 1572 and SN 1604, the latest to be observed with the naked eye in the Milky Way galaxy, had notable effects on the development of astronomy in Europe because they were used to argue against the Aristotelian idea that the universe beyond the Moon and planets was immutable. Johannes Kepler began observing SN 1604 on October 17, 1604. It was the second supernova to be observed in a generation (after SN 1572 seen by Tycho Brahe in Cassiopeia). Kepler΄s supernova (Figure 9) was slightly farther away from earth than Tycho΄s (Figure 10), though in another direction. One actually exploded about 10,000 years after the other, but they were seen only 32 years apart. They were both probably of Type I, because the light curves measured then were accurate enough for us to judge their type. Only faint remnants of them can be seen now.

10

Figure 9: NASA΄s three Great Observatories – the Hubble Space Telescope, the Spitzer Space Telescope, and the Chandra X-ray Observatory – joined forces to probe the expanding remains of a supernova. Now known as Kepler΄s supernova rem- nant, this object was first seen 400 years ago by sky watchers, including the famous astronomer Johannes Kepler. [12]

Figure 10: Tycho΄s supernova Remnant in x-ray. In fact, in 1572, the revered, Danish astronomer, Tycho Brahe, witnessed one of the last to be seen. The remnant of this explosion is still visible today as the shockwave it generated continues to ex- pand into the gas and dust between the stars. It΄s an image of x-rays emitted by its shock wave made by telescope onboard the ROSAT spacecraft. The nebula is known as the Tycho΄s Supernova Remnant. [13]

Not a single supernova has been immediately noticed in our galaxy since the invention of the telescope. Nor do we usually find supernovae in other galaxies before they have passed their peak. We would obviously like to detect a supernova right away, so that we could bring to bear all our modern telescopes in various parts of the spectrum. Since the development of the telescope the field of supernova discovery has extended to other galaxies, starting with the 1885 observation of supernova S Andromedae in the Andromeda galaxy. Supernovae provide important information on cosmological distances. During the twentieth century successful models for each type of supernova were developed. The scientists` comprehension of the role of supernovae in the star formation process is growing. Beginning in 1941, American astronomers Rudolph Minkowski and Fritz Zwicky developed the modern supernova classification scheme. In the 1960s astronomers found that the maximum intensities of supernova explosions could be used as standard candles, hence indicators of astronomical distances. Because some of the most distant supernovae recently observed appeared dimmer than expected, this supports the view that expansion of the universe is accelerating. Techniques were developed for reconstructing supernova explosions that have no written records of being observed. The age of supernova remnant RX J0852.0-4622 was estimated from temperature measurements and the gamma ray emissions from decay of titaniun-44, while the date of the Cassiopeia A supernova event was determined from light echoes off nebulae. In 2009 nitrates were discovered in Antarctic ice deposits that matched the times of past supernova events. However, now we can find supernovae automatically.

11

5 SUPERNOVA REMNANTS

Optical astronomers have photographed two dozen of supernova remnants (the stellar shreds that are left behind), in our galaxy alone, and many others in nearby galaxies (Figures 11,12). Strong x-radiation and radio radiation can also be detected. We know over 100 radio supernova remnants.The comparison of optical, infrared, radio, and x-ray observations of supernova remnants tell us about supernova itself and about its interaction with interstellar matter. Most supernova remnants have been discovered in the radio part of the spectrum; few supernova remnants are known in the visible.

We find the most famous remnant in the location where Chinese astronomers reported their ˝guest star˝ in 1054 AD. When we look in the constellation Taurus (this is the reported position in the sky), we see the Crab Nebula (Figure 8) that clearly looks as though it is a star torn to shreds. The Crab appears to be about 1/5 of distance from the Sun to the center of our galaxy. We find its age by noting the rate at which the filaments in the Crab Nebula are expanding. Tracing the filaments back in time shows that they were a single point approximately 900 years ago. The discovery that the filaments are rich in helium and are expanding at thousands of km/sec as from an explosion further confirms that the Crab is a supernova remnant and helium is a product of stellar evolution.

Some citations about Crab Nebula by Andrej Čadež and the others:

᾽᾽Spectroscopy of the Crab Nebula along different slit directions reveals the three-dimensional structure of the optical nebula. On the basis of the linear radial expansion result first discovered by V.Trimble in 1968, we make a three-dimensional model of the optical emission. Results from a limited of slit directions suggest the optical lines originate from a complicated array of wisps that are located in a rather thin shell pierced by a jet. The jet is certainly not prominent in optical emission lines, but the direction of the piercing is consistent with the direction of X-ray and radio jet. The shell᾽s effective radius is M 79᾽᾽, its thickness is about a third of the radius, and it is moving out with an average velocity of 1160 km s-1.᾽᾽ [14]

(Spectroscopy and Three-Dimensional Imaging of the Crab Nebula; Authors: Čadež , Andrej; Carramiṅana, Alberto; Vidrih, Simon; The Astrophysical Journal; Submitted on 5 Mar 2004)

᾽᾽Almost all astronomical instruments detect and analyze the first order spatial and/or temporal coherence properties of the photon stream coming from celestial sources. Additional information might be hidden in the second and higher order coherence terms, as shown long ago by Hanbury-Brown and Twiss with the Narrabri Intensity Interferometer. The future Extremely Large Telescopes and in particular (the 42 m telescope of the European Southern Observatory (ESO) could provide the high photon flux needed to extract this additional information. To put these expectations (which we had already developed at the conceptual level in the QuantEYE study for the 100 m OverWhelmingly Large Telescope to experimental test in the real astronomical environment, we realized a small prototype (Aqueye) for the Asiago 182 cm telescope. This instrument is the fastest photon counting photometer ever built. It has 4 parallel channels operating simultaneously, feeding 4 single Photon-Avalanche Diodes (SPADs), with the obility to push the time tagging capabilities below the nano-second region for hours of continuous operation. Aqueye has been extensively used to acquire photons from a variety of variable stars, in particular from the pulsar in the Crab Nebula. Following this successful realization, a larger version, named Iqueye, has been built for the 3,5 m New Technology Telescope (NTT) of ESO.Iqueye follows the same optical solution of dividing the telescope pupil in 4 sub-pupils, imaged on new generation than 50 picoseconds and dark-count noise less than 50 counts /second. The spectral efficiency of the system peaks in the visible region of the spectrum. Iqueye operated very successfully at the NTT in January 2009. The present paper describes the main features of the two photometers and present some of the astronomical results already obtained.᾽᾽[15]

(NASA ADS: Very fast photon counting photometers for astronomical applications: IquEYE for the ESO 3,5m New Technology Telescope ; Authors: Čadež, Andrej and the others)

12

From a study of the rate at which supernovae appear in distant galaxies, we estimate that supernovae should appear in our Galaxy about once every 30 years. Interstellar matter obscures much of the Galaxy from our optical view. Still, one wonders why we haven΄t seen one since the time of Kepler. It΄s about time. Radiation from hundreds of distant extragalactic supernovae is on its way to us now. The light from a supernova in our galaxy may be on its way too. Maybe it will get here tonight!

Figures 11 and 12: Filaments of the supernova remnant known as the Veil Nebula, NGC 6695, in the Cygnus Loop. It is only 2500 light years from us. It is a remnant of a star that exploded between 5,000 and 8,000 years ago. The filaments are formed by a shock wave meeting an inhomogeneous interstellar medium (the gas between the stars). [16,17]

6 CONCLUSION

Stellar evolution is the process by which a star undergoes a sequence of radial changes during its lifetime. This lifetime is considerably longer than the age of the universe. It ranges from only a few million years for the most massive to trillions of years for the least massive. Of course the mass of the star determinates its fates.All stars are born from nebulae or molecular clouds ( clouds of gas and dust). Nuclear fusion powers a star for most of its life. The less massive stars, such as the Sun, gradually grow in size until they reach a red giant phase, after which the core collapses into a dense white dwarf and the outer layers are expelled as a planetary nebula. Some of these white dwarfs have companion stars, and become novae or supernovae. More massive stars more regularly come to explosive ends. Larger stars can explode in a supernova as their cores collapse intoan extremely dense neutron star or black hole. It is not clear how red dwarfs die because of their extremely long life spans, but they probably experience a gradual death in which their outer layers are expelled over time.

Now astrophysicists come to understand how stars evolve by observing numerous stars at various points in their lifetime, and by simulating stellar structure using computer models.

13

7 REFERENCES

[1] Supernova, http://sl.wikipedia.org/wiki/supernova [Accessed 4 March 2011]

[2] Figure 1: http://thaiastro.nectec.or.th/library/sn1987a/sn1987a.html [Accessed 2 March 2011]

[3] Figure 2: http:// www.astro.keele.ac.uk/workx/starlife/Starpage 5-26.html [Accessed 4 March 2011][4] Figure 3: Jay M. Pasachoff, Astronomy:From the Earth to the Universe, Fourth Edition, 1993 Version, ISBN 0-03-094660-3.[5] Jay M. Pasachoff, Astronomy:From the Earth to the Universe, Fourth Edition, 1993 Version, ISBN 0-03 -094660-3.[6] Figure 4: http://hacastronoimy.com/sn/intro_to_sn.htm [Accessed 27 March 2011][7] Supernova, http://en.wikipedia.org/wiki/Supernova [Accessed 27 March 2011]

[8] Figure 5: http://en.wikipedia.org/wiki/supernova [Accessed 10 March 2011]

[9] Figure 6: http://www.atlasoftheuniverse.com/nebulae/nge3372.html [Accessed 10 March 2011]

[10] Figure 7: http:// apod.nasa.gov/apod/image/vela_roe.gif [Accessed 11 March 2011][11] Figure 8: http://apod.nasa.gov/apod/image/0802/crabmosaic_hst_big.jpg [Accessed 13 March 2011]

[12] Figure 9: http://chandra.harvard.edu/photo/2004/kepler [Accessed 12 March 2011]

[13] Figure 10: http://apod.nasa.gov/apod/ap960623.html [Accessed 12 March 2011]

[14] Spectroscopy and 3D imaging of the Crab Nebula,http://arxiv.org/abs/astroph/0403153 [Accessed 11

January 2012]

[15] NASA ADS: Very fast photon counting photometers for asronomical applications: IquEYE for the ESO 3,5 m New Technology Telescope, http://adsabs.harvard.edu/abs/2009SPIE.7355E..15B [Accessed 11 2012][16] Figure 11: http://astroimages.de/en/gallery/NGC6992wf.html [Accessed 14 March 2011][17] Figure 12: http://www.davidarling.info/encyclopedia/v/Veil_Nebula.html [Accessed 14 March 2011]

14