Astronomy

Why do type Ia supernovas produce more iron than type II

Why do type Ia supernovas produce more iron than type II


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My course book on astronomy states the following.

Older stars seem have higher oxygen abundances than iron. Explanation is that back in the days when these older stars were being formed type II supernova's were common, while type Ia weren't. So later, when the type Ia was becoming more common, the - now younger - stars were formed with higher iron abundances.

Why are type Ia supernova's better for iron enrichment than type II, and were these type II supernova's in any way better for higher oxygen abundances - or just less good at producing iron (and why)?


Context

Iron has the highest nuclear binding energy of all the elements (not completely true, but sufficiently accurate in an astronomical context). So, fusion of light elements into iron or something lighter is an exothermic process - you gain energy doing it, allowing the star to function. This is what happens in the last stages of a type II supernova. The core of a massive star in its last moments of life is hot and dense enough to fuse silicon into iron. Just before the supernova explosion, there is an iron ball of about 1.4 solar masses at the centre.

The progenitor of a supernova type Ia is a binary system where a "normal" star loses mass to a compact stellar remnant (a white dwarf). Once the white dwarf Contexthas accreted enough mass to be above a limit of 1.4 solar massses, fusion starts again, completely disintegrating the compact object.

Explosion

A SN Ia completely destroys the white dwarf progenitor in a runaway fusion process.

In a SN II, the pressure on the central iron ball exceeds the degeneracy pressure exerted by the electrons in the iron atoms' electron shell. The Fermi principle in quantum mechanics states that no Fermion (such as an electron) may occupy the same quantum mechanical state as another. The pressure exerted here is so large that the electrons of the iron atoms can no longer obey it and are forced into the nucleus, where they react with the protons to form neutrons.

Iron abundance

Why do SN Ia enrich their environment with more iron than SN II? This is not so much a matter of iron production, but about how much of that iron ends up in interstellar space where it can be part of a new generation of stars. In a SN Ia, the progenitor is completely destroyed, scattering all its constituent atoms into its host galaxy. A SN II forms a compact remnant, either a neutron star or a black hole. A lot of the later, heavier fusion products end up not being carried outward in the supernova explosion but become part of the compact remnant.

Note that a lot of the heavy elements scattered from a supernova of the "exploding massive star" result from an abundance of neutrinos escaping the central explosion and reacting with the outer shell of lighter elements that is blasted away.


Type Ia Supernovae: Properties, Models, and Theories of Their Progenitor Systems

Introduction
Supernovae are magnificent phenomena in the night sky, and have always been a marvel to human beings. A supernova is a stellar explosion that emits a burst of radiation resulting in an extremely luminous object that may outshine its entire host galaxy before fading from view over several weeks or months. One class of supernovae, known as Type Ia Supernovae (SNe Ia), is characterized by the absence of hydrogen emission lines in spectra and the presence of a prominent silicon Si II absorption line near maximum light (1). With uniform light curves and spectral evolution, SNe Ia have been increasingly used as reliable indicators of distance in measuring important cosmological constants (2). This usage has led to a need for a more intensive study of the nature of SNe Ia.

A SN Ia explosion is often the result of thermonuclear disruption of a carbon-oxygen white dwarf that accretes mass from its companion in a binary system, and thereby reaches the Chandrasekhar limit of 1.4 M⊙ (solar mass) (3). However, there is no simple means of identifying the immediate progenitor of a SNe Ia, nor of deriving information about its properties from observations (1). One way to determine the progenitors of SNe Ia is to eliminate the unlikely candidates from a pool of possible systems if they show any significant contradiction with the physical principles or observational data. Since there is not yet a best candidate whose properties agree with all the theoretical or observational criteria, identification of progenitor systems of SNe Ia remains difficult.

Properties of SNe Ia Progenitor Systems
The spectroscopic properties of SNe Ia give some indication of the composition of their progenitors. The absence of hydrogen emission lines indicates that the star contains little (less than 0.1 M⊙) to no hydrogen before explosion the presence of a silicon Si II absorption line near maximum light suggests that nuclear fusion from pre-explosion matter into intermediate-mass elements like silicon takes place in the explosion (1). The observed velocity (mean v = 5000 km/s and peak v > 20000 km/s) of SNe Ia explosion ejecta agrees with the calculation result of about 1 M⊙ of C and O fusing into iron-group elements or intermediate-mass elements. This fact implies that the progenitor star is composed of mostly carbon and oxygen (1).

According to observational data, most SNe Ia share very similar peak luminosities, light curves, and spectra. This strongly indicates that a unique class of progenitor systems exists. Upon closer study of these properties, Chandrasekhar-mass (1.4 M⊙) white dwarfs are suggested to be the best-fitting model (1). Since 85% of observed white dwarfs have masses of no more than 0.8 M⊙ and large-mass white dwarfs are extremely rare, the only way for a SN Ia progenitor white dwarf to reach the Chandrasekhar limit is to be in a close binary system where it can accrete mass from the companion star (3).

The results of a radio observation program lasting over two decades at the Very Large Array, a radio observatory located in New Mexico, USA, imply a very low density for any possible circumstellar material established by the progenitor before explosion. This conclusion would rule out the possibility of white dwarf mass-accretion via stellar wind from a massive binary companion. Hence, the progenitor system could only be a white dwarf that accretes mass from a low mass companion by Roche lobe flow due to gravity, as suggested in single-degenerate models, or the merger of two white dwarfs, as suggested in double-degenerate models (4).

The Origin of Diversity of SNe Ia Luminosity
The above discussions all point to the currently favored model for SNe Ia progenitors: a relatively homogeneous class of C+O white dwarfs accreting mass from their companions in binary systems. However, SNe Ia also have many observed differences, among which the most important is the diversity of luminosity. Since SNe Ia are used as standard distance indicators in cosmology, this diversity and its origin requires an answer. Below are possible explanations based on various explosion models.

I. C/O ratio of white dwarf progenitors:
The brightness of a SN Ia is determined by the mass of 56Ni synthesized during the explosion, which ranges from 0.4 – 0.8 M⊙ for most SNe Ia (5). It is postulated that as the C/O ratio increases in the progenitors, the mass of 56Ni will increase, and this consequently causes a greater luminosity (2).

II. The age of progenitor systems:
As suggested by Nomoto et al (2003), in an older binary system, the mass of the companion star of the white dwarf is smaller, and the mass which can be transferred from the companion to the white dwarf is smaller. This implies that the original total mass of carbon and oxygen of the white dwarf is larger as the white dwarf reaches Chandrasekhar mass. By calculation, the explosion of a larger portion of carbon and oxygen will lead to smaller luminosity. Therefore older progenitor systems produce dimmer SNe Ia (2).

III. Morphology of the Host Galaxy:
It is observed that the most luminous SNe Ia occur only in spiral galaxies. Both spiral and elliptical galaxies can have dimmer SNe Ia (6). This may due to the different ages of the companion stars. As suggested above, SNe Ia that occur in older progenitor systems have smaller luminosities. In elliptical galaxies, star formation has long since ceased, and most of the progenitor systems are too old to produce very luminous SNe Ia. However, in spiral galaxies, star formation continues to occur, and so these galaxies can have both old and young progenitor systems. They can host luminous SNe Ia as well as dim ones (2).

Models of Pre-Supernova Evolution
Two ways by which white dwarfs in binary systems can accrete mass toward Chandrasekhar mass and cause SNe Ia have been proposed: single-degenerate and double-degenerate. Models for both scenarios have some elements that explain the observed data, and some that do not.

Double-degenerate models
I. Mechanism:
Two C+O white dwarfs in a close binary system are brought together by the emission of gravitational radiation. When the lighter white dwarf with the larger radius fills its Roche lobe, the Roche lobe is dissipated within a few orbital periods and forms a massive and hot disk configuration around the heavier white dwarf. Then the two merge into one, reaching Chandrasekhar mass and giving rise to SN Ia explosion (7).

II. The Weaknesses of Double-Degenerate Models:
(i) When the lighter white dwarf forms a disk configuration around the primary white dwarf, the disk is rotationally supported, and so carbon ignition cannot happen immediately. The most likely result of this scenario is off-center carbon ignition if the mass-accretion rate is higher than 2.7×10-6 M⊙/year. This reaction will convert the composition of the white dwarf from C-O to O-Ne-Mg. The consequence, however, is more likely to be an accretion-induced collapse to a neutron star rather than a SN Ia explosion (7).
(ii) Galactic chemical evolution results do not agree with the double-degenerate models. In particular, Kobayashi et al. (1998) performed the chemical evolution calculations for both double-degenerate and single-degenerate models and argued that the early heavy element production of double-degenerate models, which is formulated as O/Fe as a function of Fe/H, is inconsistent with the observations (7,8).
(iii) The observed SNe Ia have a similar amount of 56Ni as a production of explosion. The merging of two white dwarfs of different mass, composition, and angular momentum with different impact parameters will lead to very different burning conditions with a different amount of 56Ni produced, which disagrees with the observations (1).

III. The Strengths of Double-Degenerate Models:
(i) The absence of hydrogen lines in SNe Ia spectra can be naturally explained by double-degenerate models since only C+O white dwarfs with little or no hydrogen are involved in this scenario (1).
(ii) Merging white dwarfs can reach Chandrasekhar mass easily, while in single-degenerate models, achieving a sufficient mass-accretion rate is a major difficulty.
(iii) Many binary systems with two white dwarfs are identified. Among the eight known systems with orbital periods of less than half a day, there is one system [KPD 0422+5421 (9)] whose mass could exceed Chandrasekhar mass. Population synthesis predicts that there could be more sufficiently massive merger candidates found in short-period white dwarf binary systems (1).

Single-degenerate models
I. Mechanism:
A C+O low mass white dwarf in a binary system accumulates hydrogen-rich or helium-rich matter from the companion star by mass overflow, reaches a critical mass near the Chandrasekhar mass and explodes due to thermonuclear disruption. Another model, known as the sub-Chandrasekhar model, suggests an alternative road of evolution: before the white dwarf reaches a critical mass limit, a layer of helium forms on top of C+O core and ignites the C+O fuel (1).

II. The Weaknesses of Single-Degenerate Models:
(i) According to single-degenerate models, since a great portion of the mass that the white dwarf accumulates is hydrogen, hydrogen lines should be seen in the spectra of SNe Ia. However, hydrogen has not yet been found in any SNe Ia. The failure to detect hydrogen in SNe Ia is a factor that may rule out single-degenerates as appropriate candidates for SNe Ia progenitor systems (8).
(ii) Theoretically, few mass-accretion rates can lead to thermonuclear explosion. For low accretion rates below 10-8 M⊙/year, repeated nova outbursts will occur before the white dwarf reaches Chandrasekhar mass, and in these eruptions, more mass will be lost than accreted between eruptions. On this track the white dwarf will never reach Chandrasekhar mass (8). For higher rates (10-8 – a few ×10-7 M⊙/yr), the white dwarf will lose mass due to helium shell flashes (1). For even higher rates of accretion above a few ×10-7 M⊙/year, a hydrogen-rich red-giant envelope will form outside the white dwarf and mass will be lost due to winds. Moreover, no observation has given evidence to the existence of the debris of this red-giant envelope in SNe Ia explosion (1).

III. The Strengths of Single Degenerate Models
(i) A class of binary systems, namely the Supersoft X-ray Sources, has been identified. In this system hydrogen-rich matter is being transferred from the companion star at so a high a rate that hydrogen burns steadily outside the C+O core of the white dwarf (10). If the accreted mass can be retained, the mass of the white dwarf can actually increase toward Chandrasekhar mass. These systems may serve as strong candidates for SNe Ia progenitors in the single-degenerate scenario (1).
(ii) There are other good candidates that exist, such as symbiotic systems or recurrent novae (8).

Summary and Conclusions
Based on current knowledge of observational evidence and physical principles, it can be confidently concluded that the progenitors of SNe Ia are a homogeneous class of compact white dwarfs composed of carbon and oxygen that accrete mass from binary companion stars.

The luminosity of a SN Ia can offer some indications about its progenitor. Generally, the progenitor white dwarf of a brighter SN Ia has higher C/O ratio and a younger age, and appears in a spiral galaxy the progenitor white dwarf of a dimmer SN Ia has lower C/O ratio and an older age, and appears in either a spiral or an elliptical galaxy.

Two kinds of models, double-degenerate and single-degenerate, are proposed to explain pre-supernova evolution. As discussed, there are observational and theoretical arguments that support and contradict each. However, double-degenerate models have more significant conflicts with theories, and with the discovery of Supersoft X-ray Sources, single-degenerate models are favored today.

As new observation technologies in x-ray, radio, and high resolution optical spectroscopy are being developed, more information concerning the properties of SNe Ia progenitor systems will be obtained. In particular, an unambiguous choice between single-degenerate and double-degenerate models can be made if the absence or presence of hydrogen in SNe Ia is determined conclusively by observations.

References
1. W. Hillebrandt and J. Niemeyer, Annual Review of Astronomy and Astrophysics 38, 191 (2000).
2. K. Nomoto, et al., in From Twilight to Highlight: The Physics of Supernovae, W. Hillebrandt & B. Leibundgut, Eds., ESO/Springer Series “ESO Astrophysics Symposia” (Springer, Berlin, 2003).
3. M. Partharsarathy, D. Branch, D. Jeffery, & E. Baron, New Astronomy Reviews 51, 524 (2007).
4. N. Panagia, et al., in American Institute of Physics Conference Proceedings, Cefalu’, Italy, 11-24 June 2006, (American Institute of Physics, Melville, NY, 2006).
5. P. Mazzali and L. Lucy, Monthly Notice of the Royal Astronomical Society, 295, 428 (1998).
6. M. Hamuy, M. M. Phillips, R. Schommer, and N. B. Suntzeff, The Astronomical Journal, 112, 2391 (1996).
7. C. Kobayashi, T. Tsujimato, K. Nomoto, I. Hachisu, and M. Kato, Astrophysical Journal, 503, L155 (1998).
8. M. Livio, Type Ia Supernovae: Theory and Cosmology, J. Niemeyer, & J. Truran, Ed. (Cambridge Univ. Press, Cambridge, 1999).
9. C. Koen, J. Orosz, and R. Wade, Monthly Notice of the Royal Astronomical Society, 200, 695 (1998).
10. P. Kahabka, E. Van Den Heuvel, Astronomy and Astrophysics, 35, 69 (1997).


Supernova explosions

Iron cannot release energy by fusion because it requires a larger input of energy than it releases. So the iron core continues to be subjected to gravity, which pushes the electrons closer to the nuclei than the quantum limit allows, and they disappear by combining with protons to form neutrons, giving off neutrinos in the process. Once this process starts, in a fraction of a second, an iron core the size of the earth and with a mass like our Sun, collapses into a ball of neutrons a few kilometers across. This gravitational collapse releases an enormous amount of energy, more than 100 times what our Sun will radiate over its entire 10 billion year lifetime. This energy blows the outer layers of the star off into space in a giant explosion called a supernova (plural: supernovae.) The ball of neutrons left behind is called a neutron star and is incredibly dense. In some cases the remaining mass is large enough that gravity continues to collapse the core until it becomes a black hole.

The explosion sends a shock wave of the star's former surface zooming out at a speed of 10,000 km/s, and heating it so it shines brilliantly for about a week. This shock wave compresses the material it passes through and is the only place where many elements such as zinc, silver, tin, gold, mercury, lead and uranium are produced. Over several months the gases cool and fade in brightness and join the debris of interstellar space. This debris has in it all of the elements that were created in the star's core. Millions or billions of years later, this debris may be incorporated into new stars. The fact that the Earth contains elements that are produced only in supernovae is evidence that our solar system, planet and bodies contain material that was produced long ago by a supernova.

The crab nebula is a remnant from a supernova that went off in 1054 A.D. When Betelgeuse explodes as a supernova it will be more than 10 times brighter than the full moon in our sky. It is only 640 light years away, and could have already become a supernova, but the light from it just hasn't reached us yet.

Supernovae occur in stars with at least 8 solar masses.

What is a Type Ia Supernova?

Dr. Melissa Graham Describes The Different Types Of Supernovae

Just as there are different types of stars, there are different types of supernovae. They are classified empirically based on the elements identified in their spectrum. The core collapse supernovae described above are called Type II if they display hydrogen, Type Ib if they show helium, and Type Ic if neither hydrogen nor helium are present (these are arbitrary choices of representative letters). Although these categories were initially defined based on observational evidence, astronomers now understand the physical differences of the progenitor stars and their explosions that give rise to these classifications. As described above, a massive star becomes like an onion with the heaviest element, iron, fused in the center, and concentric shells of lighter elements out to helium and hydrogen. Since Type Ib do not show hydrogen but do show helium, this indicates that at the time of core collapse, the star did not have a hydrogen shell. Similarly, Type Ic have neither a hydrogen nor a helium shell, and their spectra show heavy elements such as iron from the core. How could this be? In massive stars that burn hotly and brightly, radiation pressures are large enough to blow the outer layers off the star. In more massive stars, more mass is lost from the outer shells - thus it is expected that stars of 8 to 20 solar masses become Type II, and more massive stars become Type Ib and Ic. This hypothesis has been confirmed for some of the nearest such supernovae, when the massive star visible in pre-explosion images has disappeared. There is one more empirical classification of supernovae called Type Ia. As with the Type Ic, the Type Ia do not show hydrogen or helium, but they do have remarkably strong silicon absorption lines, and also show iron. All Type Ia are very bright, and have similar intrinsic luminosities - this means they all release the same amount of energy, and a lot of it. These characteristics indicate they are not caused by a star's core collapse, but are thermonuclear explosions of 1.4 solar mass carbon-oxygen white dwarf (COWD) stars. A star which is initially 2-8 solar masses is not hot enough to fuse elements heavier than carbon and oxygen. At this stage the star cools, shrinks, loses most of its mass during a planetary nebula phase, and becomes a COWD star. These stars are very dense - the mass of the sun but the size of Earth - and only stable when less than 1.4 solar masses. However, if a COWD has a binary companion it may accrete matter, and grow. At the critical mass, a thermonuclear runaway reaction fuses most of the material to radioactive nickel in a matter of seconds, which then decays to iron. The remaining material is burned into lighter elements like silicon. Although COWD stars are too faint for direct confirmation as the progenitor, they are the only known physical scenario which simultaneously explains the brightness, similarity, and spectra of Type Ia supernovae.


Cosmology

Dark energy is often characterized by its ratio of pressure to density, that is, its equation of state, w=P/ρc 2 (Fig. 2). If a is an arbitrary length scale in the universe, then the density of some component of the universe evolves as ρ ∝ a −3(1+w) . Normal matter has w=0 it dilutes with volume as space expands. If dark energy has an equation of state w=−1, corresponding to Einstein's cosmological constant, Λ, then it has the strange property that the energy density does not dilute as the universe expands it must be a property of the vacuum. Other possibilities exist: if w<−1 the dark energy density is ever growing, and will ultimately result in a 'big rip,' destroying galaxies and even subatomic particles. However, w can also be a scalar field with w>−1 (generally known as quintessence), and the value of w can evolve over time.

The top two panels show the remarkable improvements in w made using SNe Ia over the past decade (assuming a flat universe), the bottom two show the importance of improving systematics in the early years of the next decade 111 . (a) ΩMw statistical-only constraints circa 1998 112 . (b) By 2008, Kowalski et al., combining many data sets, showed that systematic uncertainties are significant 113 . (c) Expected constraints for the year 5 results of SNLS, assuming additional low-z SNe, and double the number of z>1 SNe from HST, assuming there is no improvement in systematic uncertainties from the third-year result. (d) Assumes the low-z data are on the Sloan photometric system, and a factor of 2 improvement in measurements of fundamental flux standards. Judged by the area of the inner 68.3% contour, the improvement from the 1998 results is a factor of 3, 5, and 10, including systematics.

Measurements of 〈w〉 or w(z) are obtained by building a map of the history of the expansion of the universe using SNe Ia as standardized candles. SNe Ia can show a factor of ten or more difference in peak luminosity, but the luminosity is correlated with the time it takes the supernova to rise and fall in brightness 17 . Therefore, the width of the lightcurve is measured and used to correct the peak luminosity. Common parametrizations include the 'stretch factor 8 ', s, proportional to lightcurve width, or Δm15(B), the number of magnitudes the B-band lightcurve falls in 15 days after peak brightness, inversely proportional to lightcurve width 17 . A correction must also be made for colour, since redder SNe are dimmer, both intrinsically, and due to dust 18,19 . Various techniques are used to determine these parameters by fitting lightcurve models to the data, but leading fitters include MLCS2k2 (ref. 20), SALT2 (ref. 21), and SiFTO 22 .

To determine cosmological parameters, an observed Hubble diagram (distance versus redshift) is constructed, and cosmological parameters are varied in a model that is fitted to the data. If we express distances as magnitudes, as a distance estimator, we can use 23 :

where is the peak magnitude of the SN in the B-band (the blue filter where the SN is brightest), s is the lightcurve stretch, c is the colour of the SN (a linear combination of UB and BV, relative to some reference colour 22,23 ), α is the slope of the stretch-luminosity relation, β is the slope of the colour-luminosity relation, and M is a measure of the absolute magnitude of the SN combined with the Hubble constant. Note that since relative magnitudes are used, neither the absolute magnitude of the SN, nor the value of the Hubble constant must be known. , s, and c are measured from a fit to each SN lightcurve, while M, α, and β are constants determined from the overall cosmological fit.


What Causes Supernovae Explosions?

On the evening of November 11, 1572, twenty-six-year-old astronomer Tycho Brahe was about to make a discovery that would change his life and consequentially boost the scientific revolution significantly. While casually staring at the night sky, he suddenly noticed a very bright unfamiliar star in the Cassiopeia con­stellation. The star, which was as bright as Venus, was located in the inner parts of the famous W-shaped constellation, which was well known to many common people, let alone astronomers. What Tycho saw looked like the appearance of a new star (nova stella). He was so astonished that he sought the confirmation of others to assure himself that he was not hallucinating.

Unknown to Tycho, such new stars had appeared during the previous centuries (“guest stars” in Chinese records), with a much brighter star reported in 1006. While these events were very important to astrologers, they had no lasting effect on astronomical thinking at the time. Tycho, however, realized that such an event was revolutionary. By accurately and repeatedly measuring the position of the “nova,” Tycho showed that it was much further than the moon. In one night, Tycho managed to scientifically falsify the millennia-old Aristotelian belief that anything beyond the sphere of the moon cannot change. This convinced Tycho that the “known” cosmology was wrong and motivated him to devote his life to performing measurements of stars and planets to study the “true” cosmology. His hard, lifelong work paid off. His careful measurements of the positions of the planets enabled the discovery of the law of gravity by Johannes Kepler and Isaac Newton. Kepler would later say that if Tycho’s star did nothing else, it produced a great astro­nomer. Yet, even Tycho and Kepler could not have appreciated that what seemed like a new star was actually an explosion of unimaginable power and that such explosions are crucial for our existence.

Tycho’s “nova” faded from sight about eighteen months after its discovery. However, modern instruments reveal a spectacular sight at the position recorded by Tycho. Observations of the same spot in the sky by NASA’s Chandra X-ray observatory, nearly five centuries later, show material expanding with velocities of thousands of kilometers per second with a mass of a few billion billion billion tons, comparable to that of the Sun. Tycho had evidenced a powerful explosion, which we now term “supernova.” The huge amount of energy released in a few months is equivalent to the output of a few billion billion billion H-bombs. At the current rate of emission, it would take our Sun several billion years, of order of its lifetime, to release the same amount of energy. This phenomenon turned out to be very rare, occurring roughly once per century within a galaxy like our own. While super­novae have not been detected in the Milky Way during the last three centuries, modern technology has allowed thousands of supernovae to be detected in other galaxies. Supernovae are not just beautiful cosmic fireworks—many of the elements that are essential for life, such as calcium and iron, are believed to have been produced by supernovae.

But what causes these explosions? There are currently two explanations involving theoretical mechanisms that are related to the two ways in which stars die. Stars cannot shine forever for the simple reason that their energy supply—nuclear burning—is finite. What happens to stars once they exhaust their nuclear fuel (mainly hydrogen) is believed to depend crucially on their mass. One of the most important theoretical discoveries in astrophysics is that a critical mass exists above which stars cannot sustain themselves against their own gravitational pull without a continuous supply of energy. The two types of star endings depend on whether their mass is above or below this critical mass, which is called the Chandrasekhar mass limit, named after Subrahmanyan Chandrasekhar (Member, 1941, 1976), one of its discoverers. If, by the time a star exhausts its fuel, it has a mass greater than this limit, the core of the star cannot sustain itself and collapses. A huge amount of energy is released when the core collapses to the tiny size of a few kilometers, becoming a black hole or a neutron star. While most of this energy is emitted in invisible neutrinos, a small fraction of this energy ejects the outer parts of the star, creating an explosion sufficient to produce a supernova. Such a theoretical event is called a core-collapse supernova.

Stars that are less massive than the Chandrasekhar mass limit, or lose enough mass during their life to become so, are able to resist gravity once their nuclear fuel is exhausted. Gravity does manage to shrink them considerably, however, and they settle at a radius of a few thousand kilometers. Such dense stars, with a mass comparable to that of the Sun (one million times the mass of Earth) and a size comparable to that of Earth, are called white dwarfs. These stars are abundant and have been extensively studied observationally and theoret­ically. Without their previous nuclear energy supply, white dwarfs slowly become dimmer and eventually become unobservable. While white dwarfs are stable, their exceptional high density make them very powerful thermo­nuclear bombs. If properly ignited, white dwarfs are capable of powerful thermonuclear explosions with a sufficient energy release to account for a supernova. Such theoretical events are called thermonuclear supernovae. An appealing aspect of theories involving nuclear energy is that they naturally explain why the energy-per-unit mass released by supernovae is comparable to that of H-bombs and our Sun. Yet neither core collapse in massive stars nor thermonuclear explosions of white dwarfs have been theoretically established to account for the supernovae that we see. While core collapse is likely inevitable, it has not been shown that the outer parts can be ejected successfully. While white dwarfs can explode if ignited, a robust ignition mechanism has not been identified. Yet the two explosion mechanisms are widely believed to occur, based on several successes for explaining supernovae observations and given that we simply do not have better ideas.

Perhaps the best clues for identifying an explosion mechanism have come from a few nearby supernovae where a star, located in pre-explosion images at the precise position of the supernovae, had disappeared in images taken after the supernova had faded away. In all of these cases, the exploding star was massive and at a late stage of its life in accordance with the core-collapse theory. This scenario was confirmed for the famous supernova that exploded in the Large Magellanic Cloud galaxy in 1987—the closest among these supernovae—in which the predicted neutrino burst was detected.

However, core collapse of massive stars cannot account for all of the supernovae. A significant fraction of supernovae occur in elliptical galaxies where massive stars simply do not exist. These massive stars have lifetimes of only millions of years and have long died in elliptical galaxies where stars have ceased forming for billions of years. Interestingly, it turns out that all supernovae that occur in such old stellar environments seem to share many distinctive emission properties, belonging to a separable class called type Ia. These supernovae are distinct by not showing any trace of hydrogen and helium, the most abundant elements in the universe, but exhibiting significant abundance of much heavier ­elements such as silicon, nickel, and iron. These features are naturally explained if these supernovae are the thermonuclear explosions of white dwarfs, which are abundant in all stellar ­environments, including elliptical galaxies, and are predominantly made of carbon and oxygen rather than hydrogen and helium. Perhaps the strongest piece of evidence in this direction is the huge amount—a considerable fraction of the ejected materials—of the unstable isotope Ni 56 detected in these supernovae, which is an expected outcome of a thermonuclear ­explosion. In fact, given the short lifetime of a few days, this isotope must have formed in the explosion itself, favoring a nuclear origin. While type Ia are the only supernovae known to occur in elliptical galaxies, they are observed to occur in all stellar environments, including spiral galaxies like our own. In particular, the supernova observed by Tycho, which occurred in our own galaxy, was a type Ia supernova. Although observations of type Ia supernovae are compatible with explosions of white dwarfs, they do not provide any hint as to why some white dwarfs suddenly explode.

Before we address the mystery behind the sudden explosion of white dwarfs, it is worth mentioning the importance of the resulting type Ia supernovae. The large amounts of Ni 56 that are observed in these supernovae decay to Co56 and then to iron. Type Ia supernovae are the most efficient known production sites of iron. A significant amount of the iron in the universe, including a large fraction of Earth’s mass and the iron in your blood, was produced in such supernovae. In addition, type Ia supernovae have turned out to be very useful tools for measuring fundamental properties of the observable universe. Among the wide variety of supernovae, type Ia are the brightest and comprise a relatively homogeneous group. Their high brightness allows them to be detected clearly across the observable universe. Their homogeneity allows a calibration of their intrinsic luminosity, which enables their distance to be measured. An object with a given intrinsic luminosity appears dimmer when it is more distant and brighter when it is closer, allowing a determination of its distance based on the measurement of its apparent brightness. Astrophysical objects with known intrinsic luminosity are rare and precious, and type Ia supernovae are one of the few such “standard candles” that can be seen from cosmological distances. In recent years, distances across the observable universe that were measured using type Ia supernovae led to the surprising conclusion that the expansion of the universe is accelerating, prompting fundamental modifications to the Big Bang cosmological model. The 2011 Nobel Prize in Physics was awarded to three astronomers due to this application of type Ia supernovae. The growing importance of the use of type Ia as accurate rulers for cosmological distance has strengthened the need for a firm theoretical understanding of these explosions.

This brings us back to the pressing issue of why some white dwarfs explode (about 1 percent of white dwarfs account for type Ia supernovae). During the last several decades, the standard lore has centered on increasing the mass of these white dwarfs toward the unstable Chandrasekhar limit where an explosion is assumed to occur. In the most popular scenarios, the mass of some white dwarfs grows with time, due to material deposited by a nearby companion. Despite its popularity, this scenario has never been convincingly shown to lead to an explosion, and there is observational evidence that there are not enough relevant systems to account for the rate of type Ia supernovae even if they were to explode. Also lacking an explanation is the amount of Ni 56 , which varies by about an order of magnitude between the faintest and brightest type Ia supernovae. While the shortcomings of this Chandrasekhar model have been presented in many recent papers and reviews, it remained popular mostly due to the fact that there were no other good alternative suggestions.

We now believe we have found the solution to this problem: type Ia supernovae occur when two white dwarfs directly collide with each other. The explosion is triggered by the shock waves that result from the collision, which occurs at velocities of thousands of kilometers per second due to the gravitational pull of the white dwarfs. Such collisions lead to successful explosions over a wide range of white dwarf masses and produce the right range of Ni 56 masses seen in type Ia supernovae. They have nothing to do with the Chandrasekhar mass. While it was known that such collisions could lead to explosions, it was never regarded as a possible explanation for type Ia supernovae since it was believed that such collisions were very rare. A very small rate was thought to occur in dense stellar systems, with less than 1 percent of all type Ia supernovae possibly resulting from such collisions.

In a paper published last year by Members Boaz Katz and Subo Dong, it was argued that the rate of collisions actually may be as high as the total rate of type Ia supernovae. The argument was based on the fact that most stars are observed to start their life in small groups of three or more. It was shown that collisions are likely in triple systems, consisting of two white dwarfs orbited by a third star, due to the three-body gravitational interaction between them. The rate of such collisions can reach the rate of Ia supernovae if a few tens of percent of stars remain in triple systems when they become white dwarfs, a quite reasonable assumption. That type Ia supernovae are the result of common collisions in triple systems seems like a highly unlikely explanation at first thought. Given the small size of white dwarfs, the chance of a collision at any given passage is roughly one in a million. But numerical calculations by Katz and Dong showed that a few percent of these systems lead to a collision. The key is that the systems have billions of years to spare, during which the white dwarfs experience millions of passages. Due to the subtle yet persistent pulls from the distant third star, each passage of the two white dwarfs occurs at a slightly different configuration and, after a million passages, the chance for a collision becomes significant.

In a combined effort led by Member Doron Kushnir with Katz, Dong, Member Rodrigo Fernandez, and Eli Livne of Hebrew University, the detailed process of white dwarf collisions was calculated and shown to reproduce several features of the broad distribution of type Ia supernovae. For the first time since their discovery by Tycho more than four centuries ago, a detailed scenario for a significant fraction of supernovae was studied in which the explosions could be numerically calculated from first principles based on the physically well understood processes of Newtonian dynamics, hydrodynamics, and thermo­nuclear burning. In a paper published by our group, the model was shown to successfully pass three independent and robust observational tests, including the successful recovery of the wide distribution of Ni 56 masses. It is thus very likely that what Tycho thought was the sudden birth of a new star was actually the violent death of two. He might have been gratified to learn that the two stars collided due to the application of the same law of gravity that would later be deduced based on his dedicated observations.

Boaz Katz, John N. Bahcall Fellow and Member (2012–13) in the School of Natural Sciences, works on various problems in high-energy astrophysics and few-body dynamics. Subo Dong, Ralph E. and Doris M. Hansmann Member (2012–13) in the School, works on extrasolar planets, dynamics, microlensing, and time-domain astronomy. Member (2012–13) Doron Kushnir works on various problems in high-energy astrophysics, in particular, type Ia supernova explosions.


Simulations uncover why some supernova explosions produce so much manganese and nickel

Researchers have found white dwarf stars with masses close to the maximum stable mass (called the Chandrasekhar mass) are likely to produce large amounts of manganese, iron, and nickel after it orbits another star and explodes as Type Ia supernovae.

A Type Ia supernova is a thermonuclear explosion of a carbon-oxygen white dwarf star with a companion star orbiting one another, also known as a binary system. In the Universe, Type Ia supernovae are the main production sites for iron-peak elements, including manganese, iron, and nickel, and some intermediate mass elements including silicon and sulfur.

However, researchers today cannot agree on what kind of binary systems triggers a white dwarf to explode. Moreover, recent extensive observations have revealed a large diversity of nucleosynthesis products, the creation of new atomic nuclei from the existing nuclei in the star by nuclear fusion, of Type Ia supernovae and their remnants, in particular, the amount of manganese, stable nickel, and radioactive isotopes of 56-nickel and 57-nickel.

To uncover the origin of such diversities, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) Project Researcher Shing-Chi Leung and Senior Scientist Ken'ichi Nomoto carried out simulations using the most accurate scheme to date for multi-dimensional hydrodynamics of Type Ia supernova models. They examined how chemical abundance patterns and the creation of new atomic nuclei from existing nucleons depend on white dwarf properties and their progenitors.

"The most important and unique part of this study is that this is so far the largest parameter survey in the parameter space for the Type Ia supernova yield using the Chandrasekhar mass white dwarf," said Leung.

A particularly interesting case was the supernova remnant 3C 397. 3C 397 is located in the Galaxy about 5.5 kpc from the center on the galactic disk. Its abundance ratios of stable manganese/iron and nickel/iron were found to be two and four times that of the Sun respectively. Leung and Nomoto found the abundance ratios among manganese, iron and nickel are sensitive to white dwarf mass and metallicity (how abundant it is in elements heavier than hydrogen and helium). The measured values of 3C 397 can be explained if the white dwarf has a mass as high as the Chandrasekhar mass and high metallicity.

The results suggest remnant 3C 397 could not be the result of an explosion of a white dwarf with relatively low mass (a sub-Chandrasekhar mass). Moreover, the white dwarf should have a metallicity higher than the Sun's metallicity, in contrast to the neighboring stars which have a typically lower metallicity.

It provides important clues to the controversial discussion of whether the mass of the white dwarf is close to the Chandrasekhar mass, or sub-Chandrasekhar mass, when it explodes as a Type Ia supernova.

The results will be useful in future studies of chemical evolution of galaxies for a wide range of metallicities, and encourage researchers to include super-solar metallicity models as a complete set of stellar models.

Leung says the next step of this study would involve further testing their model with more observational data, and to extend it to another subclass of Type Ia supernovae.


Question: ) Why Is Iron Is The Heaviest Element That Can Be Produced By Nuclear Fusion In A Star? A) Iron Is The Heaviest Element That Does Not Radioactively Decay. B) Iron Is The Element With The Lowest Mass Per Nuclear Particle. C) Neutron Degenerate Pressure Prevents Iron Fusion. D) It Is Not, Carbon Is The Heaviest Element That Can Be Produced. E) A&B 9) .

) Why is iron is the heaviest element that can be produced by nuclear fusion in a star?

A) Iron is the heaviest element that does not radioactively decay.

B) Iron is the element with the lowest mass per nuclear particle.

C) Neutron degenerate pressure prevents iron fusion.

D) It is not, carbon is the heaviest element that can be produced.

9) Which type of object is the leftover of a star’s core after a core-collapse supernova?

10) What experimental evidence do we have that black holes exist and are real.

A) We can observe the gravitational influence of some black holes on nearby stars.

B) We can detect light emitted by the accretion disks of some black holes.

C) We can observe radiation emitted from the event horizons of black holes.

11) If the event-horizon of a hypothetical black hole is 4 km in diameter, what would its mass be?

D) 13,000 times the Sun’s mass

E) The mass of a black hole is a nonsensical concept, the size of the event horizon has no relation to mass.

12) Following from the correct answer to the question above, could such a black hole of this mass be created at the end of a star’s life?

A) No, it would be a neutron star.

B) No, it would be a white dwarf.

C) No, it is more massive than what is possible even for the progenitor star.

D) No, and this is too small to be even a star.

13) The Hyades is a star cluster located 153 lightyears from Earth. You measure the flux of light from the Hyades and from

another star cluster called the Macky cluster. You find that the flux of light from main sequence stars of the same temperature in the Hyades is 43 times that of the stars in the Macky cluster. Roughly how far away is the Macky cluster from Earth?

E) The same distance (153 light-years)

14) Measuring only the total flux of light from one of the objects below provides an estimate of its distance which one?

A) The accretion disk of a black hole

C) A Type Ia (white dwarf) supernova

D) A Type II (core-collapse) supernova

15) You discover a redshifted galaxy moving at a speed of 1600 km/s. Assuming a Hubble constant of 76 km/s/Mpc, how far away is the galaxy?

16) Which force could prevent further gravitational collapse of an inert stellar core with a mass over 1.4 MSun.

B) Proton degeneracy pressure

C) Neutron degeneracy pressure

D) Electron degeneracy pressure

17) Population I stars have higher metallicities than Population II stars because

A) The interstellar medium has become increasingly enriched in heavy elements over time.

B) The Population I stars are more massive, so they produce iron and other metals in their cores.

C) Population II stars are mainly brown dwarfs, so they are inert and do not produce metals at all.

D) Population II stars are only found in elliptical galaxies.

E) Population I stars are nearly entirely composed of metals since they are white dwarfs.

18) After the end of its main sequence life and giant phases, could the Sun produce a nova or a Type Ia supernova?

A) Yes, it should since it is a Population I star.

B) Yes, but we do not know for sure whether it will.

C) No, it is not part of a stellar binary system.

E) No, it should produce a Type II (core-collapse) supernova.

19) Which of the following statements are true about white dwarf supernova and regular novae.

A) In a nova, fusion is triggered in surrounding material accumulated by the white dwarf.

B) In a white dwarf supernova, all the degenerate carbon in the white dwarf fuses instantaneously.

C) White dwarf supernovae are significantly more energetic than novae.

D) Both white dwarf novae and supernovae require a binary companion to occur.

20) Why do radio signals from pulsars often appear with extremely short intervals?

A) Because of turbulent flow in the accretion disks of black holes.

B) Because of turbulent flow in the accretion disks of white dwarfs.

C) Because neutron stars can spin rapidly due to conservation of momentum in core-collapse supernovae.

D) Because white dwarfs can spin rapidly due to conservation of momentum in core-collapse supernovae.

E) Because brown dwarfs can spin rapidly due to conservation of momentum in core-collapse supernovae.

E) There is no force than can prevent further gravitational collapse, above 1.4 MSun the core will collapse completely,


Most Of Our Galaxy's Antimatter Comes From Supernovae

For more than 40 years, scientists have known that the center of the Milky Way is rich in antimatter, but they have disagreed on its origins. Now, a paper in Nature Astronomy claims to have the answer.

The center of our galaxy produces an astonishing dose of gamma-rays. These are attributed to matter and antimatter particles annihilating each other, converting their mass to energy in the form of high-frequency photons. Where then did the antimatter come from? There is still plenty of head-scratching among cosmologists as to why the universe contains so much more matter than antimatter, but given that it does, the existence of any substantial amount of antimatter takes some explaining.

The supermassive black hole at the center of the galaxy was an early favored culprit, and dark matter has also been blamed. However, according to Dr Roland Crocker of the Australian National University, these can be ruled out. Instead, the source is a subgroup of Type Ia supernovae, occurring when two white dwarf stars collide.

The collisions produce the titanium isotope 44. Titanium-44 decays to scandium and then to calcium, emitting positrons – the antimatter equivalent of electrons – in the second decay. The half-life of these two processes is barely 60 years, so we might expect the number of gamma-rays to surge after supernovae and drop off quickly thereafter.

However, Crocker told IFLScience that the positrons hang around for roughly a million years before they collide with ordinary matter in the interstellar medium, causing their annihilation. Consequently, we're still seeing plenty of the resulting gamma-rays, even though there has not been a supernova of this (or any) type in our galaxy for centuries.

The paper rejects some alternative theories. Dark matter would be expected to produce higher energy positrons than we witness. Crocker told IFLScience: “There have been attempts to model dark matter that avoid this.” However, he thinks these fail Occam's razor, relying on overly complex explanations. Type II supernovae produce enormous amounts of nickel-56, much of which release positrons during decay, but according to Crocker this happens so early during explosions that the positrons become trapped, never reaching the interstellar medium and dispersing their gamma-rays through the galaxy.

Not all Type Ia supernova do the trick, however. Titanium production requires “an unusual amount of high density helium,” Crocker told IFLScience, something that only occurs when a binary system contains two white dwarf stars, with masses between 1.4 and 2.0 times that of the Sun. These gradually coalesce and explode, but only after the larger star has captured the smaller one's helium. Despite these specific requirements, such events, known as SN 1991bg-like supernova, are common enough to account for the astonishing 10 43 positron annihilations that occur every second in our galaxy.


How stars die: the shocking truth supernova theories come of age.

Roughly once a second, a star somewhere in the universe explodes. Some of these stars are blown to smithereens, strewing ashes through space. Others lose only their outer layers, leaving behind an unimaginably dense core.

Both kinds of explosions, known as supernovas, represent the most powerful events in the cosmos and have some of the most far-reaching astronomical consequences.

Although nature has no trouble making stars explode, researchers do. For years they couldn't find a model to account for the fireworks. Instead of producing a titanic blowup, many of their efforts just bombed out.

That's because available computer power limited scientists to simplistic, one-dimensional simulations of a complex, multidimensional problem. Now, thanks to supercomputers and improved software that enable astronomers to explore more realistic models, scientists say they have discovered how stars break up.

"This is a real breakthrough," comments Alexei V. Filippenko of the University of California, Berkeley. "For decades, people have been trying to get stars to blow up, but nothing panned out. Here, with the increase in computing power . . . astronomers are successfully explaining physics that could not be modeled in previous calculations."

Astronomers detailed their insights into two classes of supernovas last month at a meeting of the American Astronomical Society (AAS) in Tucson.

Type II supernovas, the explosion of stars at least eight times the mass of the sun, leave behind a dense core. Type Ia supernovas typically involve stars about 1.4 times as massive as the sun and burn completely.

The new view of Ia supernovas promises to narrow the gap between teams of researchers who ascribe widely different values to the Hubble constant, a measure of the expansion rate, age, and size of the universe (SN: 10/8/94, p.232).

It takes about 10 million years for a massive star to mature. For most of that time, it battles successfully against collapse by burning nuclear fuel, which generates heat and an outward pressure sufficient to counter gravity. During this time, it fuses hydrogen, helium, and other light nuclei, forming heavier elements (SN: 2/4/95, p.70). The star must continue to build increasingly heavy nuclei in order to maintain its source of energy.

But once such a star begins making nuclei as heavy as iron and nickel, it has signed its death warrant. Forming any heavier nuclei would take away energy rather than release it. Its fuel depleted, the star can no longer resist gravity's tug and collapses in a matter of hours to days.

Just before it collapses, the core of such a star may have a diameter of 3,000 kilometers, a temperature of a few billion kelvins, and a density of 10 billion grams per cubic centimeter. Afterwards, the core shrivels to a diameter of 30 km, the temperature climbs to 200 billion kelvins, and the density increases 10,000-fold. Protons and electrons squeeze together, and the compact core soon becomes a tiny, rapidly whirling ball of neutrons -- a neutron star.

Soon after the implosion, material from outside the core begins raining down. At the same time, the core rebounds, sending out a shock wave. The speeding wave rapidly loses energy, stalling some 100 kilometers beyond the core. Moreover, the infalling gas acts as the lid on a pot, containing the wave.

Will this massive star ever explode?

The answer, according to earlier, independent work by James R. Wilson of Lawrence Livermore (Calif.) National Laboratory and Hans A. Bethe of Cornell University, lies in subatomic particles called neutrinos.

In addition to generating a shock wave, the neutron star emits a fireball of neutrinos equivalent to the radiation that would be produced if 50,000 bodies with the mass of Earth were suddenly converted into energy. These neutrinos carry heat from the star's core to the outlying layers of gas.

It seems that this heat should give the shock wave the extra oomph it needs to blow the lid off the star. But in the one-dimensional model that

astrophysicists had relied on, matter has the same restrictions as beads on a string -- it can't push aside material directly in front of it. Thus, the neutrino-heated gas just above the core can't rise and energize the shock wave stalled above it.

Instead, the neutrinos heat only the thin layer of gas directly above the core. Unable to cool by rising and expanding, this gas lowers its temperature by emitting neutrinos of its own. Thus the core stays hot, the gas raining down stays cold, and the shock wave goes nowhere.

In this scenario, the star has only a slim chance of going bang.

Theorists got a dose of reality 8 years ago with the dazzling debut of 1987A, the first type II supernova visible to the naked eye since the time of Johannes Kepler. In the first few hundred days after they observed the outburst, researchers found clear signs that 1987A wasn't the spherically symmetrical explosion that theory predicted.

X rays and gamma rays from deep within the exploded star appeared sooner than expected, indicating that the inner and outer parts of the star had mixed thoroughly in the outburst. Some material from the deepest layers was found in the first one-third of the ejected debris. And some material from the star's outer layers was observed only later in the explosion.

"It was more like scrambled eggs than sunny-side up," recalls Willy Benz of the University of Arizona in Tucson. Further analysis suggested that the star had ejected more material in some directions than in others.

Prompted by these findings, several research groups developed multidimensional models to account for the lopsided explosion. But the models couldn't explain the mixing. So Benz, in collaboration with Marc Herant and Stirling A. Colgate of Los Alamos (N.M.) National Laboratory, began work on a two-dimensional model that would. Another team, which includes Adam S. Burrows and John C. Hayes of the University of Arizona and Bruce A. Fryxell of NASA's Goddard Space Flight Center in Greenbelt, Md., developed a similar model. They described their simulations at last month's AAS meeting.

In the new models, colder gas from outside the core still rains down on the core and meets an increasingly higher concentration of neutrinos. But in two dimensions the gas is free to rotate like a ferris wheel.

This swirling motion has a profound effect. As the infalling gas absorbs neutrinos and grows hotter, it floats upward in huge bubbles, like giant hot-air balloons. The heated gas imparts its energy to a much larger percentage of the star than it did in the one-dimensional model. By converting heat into motion, the neutrinos aid the shock wave expanding from the collapsed core.

The wave still stalls, but it does so farther from the star's center, in a considerably less dense region, notes Burrows. This time, the shock wave stops only momentarily. In a few seconds, the wave gathers enough speed to explode as a type II supernova.

If the bubbles of heated gas are large enough, they rocket the neutron star core into space. This may explain the high velocity of neutron stars, Burrows says.

Although the broad outline of their studies seems to match observations, Burrows and Benz both emphasize that they need to extend their work to three dimensions. In both models, the exploded stars produce far too much yttrium, thorium, and strontium.

The two teams differ in their exact interpretation of why massive stars explode as type II supernovas. Burrows, for example, eschews the pot lid analogy, arguing that the neutrino-driven transfer of heat by rising gas bubbles holds the key. Nonetheless, notes Benz, "we've moved from having models that failed to arguing about the interpretation of models that are successful."

Although they have less mass, Ia supernovas flash even more brightly than type II supernovas because they produce 10 times as much radioactive nickel. Astronomers believe that Ia supernovas form a set of "standard candles," meaning that they all have the same intrinsic brightness, like lightbulbs of a particular wattage (SN: 10/8/94, p.232).

Standard candles enable researchers to measure the distance from the Milky Way to various galaxies. If a galaxy lies far enough away, astronomers can use that measure to calculate the Hubble constant. The premiere standard candle remains a type of pulsating star known as a Cepheid variable. But astronomers can detect Ia supernovas in galaxies 10 to 100 times farther away than the Cepheids, potentially improving measurements of the Hubble constant.

Allan R. Sandage of the Carnegie Observatories in Pasadena, Calif., and his colleagues have used Ia supernovas to calculate a Hubble constant of about 50, a number that implies the universe is about 20 billion years old. That makes many theorists happy, because it doesn't conflict with the ages of some of the oldest known groupings of stars in the cosmos, estimated to be 16 billion years old.

However, his team's calculations conflict with those of many other research groups, who get a higher value for the Hubble constant and a correspondingly younger age for the cosmos.

Results of a survey of Ia supernovas suggest a way to reconcile the disparity. In analyzing half of the 50 type Ia supernovas discovered during the past 4 years at the Cerro Tololo Inter-American Observatory (CTIO) in La Serena, Chile, researchers found that not all of these exploded stars have the same intrinsic brightness.

In particular, Ia supernovas that have the longest peak brightness are in fact more luminous than those that fade faster. CTIO astronomers Mario Hamuy, Mark Phillips, and their colleagues also announced at the Tucson meeting that the brightest Ia supernovas typically reside in either spiral galaxies or galaxies with many bright, young stars.

Phillips and his colleagues note that, in order to calibrate the distance to supernovas in galaxies farther from the Milky Way, Sandage's team relied on two nearby Ia supernovas. One of these supernovas exploded in 1972, the other in 1937, but both faded slowly.

The slow decline indicates that each of these reference supernovas was slightly more luminous than Sandage's team had assumed, Phillips says. The Chile-based astronomers assert that when the true luminosity of these supernovas is taken into account, their survey will yield a Hubble constant of between 60 and 70, a value more in line with recently reported results that give the universe an age of 8 to 12 billion years.

But what makes the intrinsic brightness of Ia supernovas vary in the first place? Philip A. Pinto of the University of Arizona suggests that the answer lies in the properties of the stars that end their lives in these explosions.

Astronomers believe that Ia supernovas occur when a kind of dense star known as a white dwarf gravitationally grabs a critical amount of matter from a companion star, sparking a thermonuclear explosion.

Standard theory holds that all white dwarfs that give rise to Ia supernovas have the same mass -- about 1.4 times that of the sun. But Pinto proposes that some white dwarfs steal more matter than others, resulting in explosions that can vary slightly in power and luminosity.

Other researchers have other explanations, and thinking remains unsettled. Even though computer simulations of type Ia events are more advanced than those of the type II phenomenon, "we still don't understand how these stars explode," Pinto says. The CTIO findings may well spark a minor revolution in the way astrophysicists think about these explosions, he adds.

"Every kid loves an explosion," notes Colgate. "This is just the biggest one you can play with -- at least in your head."


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