Stellar nucleosynthesis. Stellar nucleosynthesis is the process by which the natural abundances of the chemical elements assemble in the cores of stars. Stars are said to evolve (age) with changes in the abundances of the elements within. Stars lose most of their mass when it is ejected late in their stellar lifetimes, thereby increasing the abundance of elements heavier than helium in the interstellar medium.
The term supernova nucleosynthesis is used to describe the creation of elements during the explosion of a star. The primary stimulus to the development of the theory of nucleosynthesis was the variations in the abundances of elements found in the universe. A second stimulus to understanding the processes of stellar nucleosynthesis occurred during the 20th century, when it was realized that the energy released from nuclear fusion reactions accounted for the longevity of the Sun as a source[1] of heat and light.
History[edit] [edit] Cross section of a red giant showing nucleosynthesis and elements formed. Triple-alpha process. Overview of the triple-alpha process. The triple-alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei (alpha particles) are transformed into carbon.[1][2] Older stars start to accumulate helium produced by the proton–proton chain reaction and the carbon–nitrogen–oxygen cycle in their cores. The products of further nuclear fusion reactions of helium with hydrogen or another helium nucleus produce lithium-5 and beryllium-8 respectively, both of which are highly unstable and decay almost instantly back into smaller nuclei.[3] When the star starts to run out of hydrogen to fuse, the core of the star begins to collapse until the central temperature rises to 108 K (8.6 keV).
At this point helium nuclei are fusing together faster than their product, beryllium-8, decays back into two helium nuclei. Once beryllium-8 is produced a little faster than it decays, the number of beryllium-8 nuclei in the stellar core increases to a large number. Discovery[edit] Supernova. A supernova (abbreviated SN, plural SNe after "supernovae") is a stellar explosion that is more energetic than a nova. It is pronounced /ˌsuːpəˈnoʊvə/ with the plural supernovae /ˌsuːpəˈnoʊviː/ or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months. During this interval a supernova can radiate as much energy as the Sun is expected to emit over its entire life span.[1] The explosion expels much or all of a star's material[2] at a velocity of up to 30,000 km/s (10% of the speed of light), driving a shock wave[3] into the surrounding interstellar medium.
This shock wave sweeps up an expanding shell of gas and dust called a supernova remnant. Nova means "new" in Latin, referring to what appears to be a very bright new star shining in the celestial sphere; the prefix "super-" distinguishes supernovae from ordinary novae which are far less luminous. Discovery[edit] Quark-nova. A quark-nova is the theorized violent explosion resulting from the conversion of a neutron star to a quark star. Analogous to a supernova heralding the birth of a neutron star, a quark nova signals the creation of a quark star. The concept of quark-novae was suggested by Dr. Rachid Ouyed[1] (University of Calgary, Canada) and Drs. Dey and Dey (Calcutta University, India).[2] When a neutron star spins down, it may convert to a quark star through a process known as quark deconfinement. The resultant star would have quark matter in its interior.
Rapidly spinning neutron stars with masses between 1.5 and 1.8 solar masses are theoretically the best candidates for conversion due to spin down of the star within a Hubble time. Theoretically, quark stars would be radio-quiet, so radio-quiet neutron stars may be quark stars. Direct evidence for quark-novae is scant; however, recent observations of supernovae SN 2006gy, SN 2005gj and SN 2005ap may point to their existence.[5][6] See also[edit] Quark star. Creation[edit] It is theorized that when the neutron-degenerate matter, which makes up neutron stars, is put under sufficient pressure from the star's own gravity or the initial supernova creating it, the individual neutrons break down into their constituent quarks (up quarks and down quarks), forming what is known as quark matter. This conversion might be confined to the neutron star's center or it might transform the entire star, depending on the physical circumstances.[1] Such a star is known as a quark star.
Ordinary quark matter consisting of up and down quarks - also referred to as u and d quarks -, has a very high Fermi energy compared to ordinary atomic matter and is only stable under extreme temperatures and/or pressures. This suggests that only quark stars comprising neutron stars with a quark matter core will be stable, while quark stars consisting entirely of ordinary quark matter, will be highly unstable and dissolve spontaneously.[2][3] Strange stars[edit] Strange stars[edit] Not a Quirk But a Quark ... a Quark Star! Astronomers recently announced that they have found a novel explanation for a rare type of super-luminous stellar explosion that may have produced a new type of object known as a quark star. Three exceptionally luminous supernovae explosions have been observed in recent years. One of them was first observed using a robotic telescope at the California Institute of Technology's (Caltech) Palomar Observatory.
Data collected with Palomar's Samuel Oschin Telescope was transmitted from the remote mountain site in southern California to astronomers via the High-Performance Wireless Research and Education Network (HPWREN), funded by the National Science Foundation (NSF). The Nearby Supernova Factory research group at the Lawrence Berkeley Laboratory reported the co-discovery of the supernova, known as SN2005gj. Researchers in Canada have analyzed this, along with two other supernovae, and believe that they each may be the signature of the explosive conversion of a neutron star into a quark star.
Hypernova. Eta Carinae, in the constellation of Carina, one of the nearer candidates for a future hypernova A hypernova (pl. hypernovae) is a type of supernova explosion with an energy substantially higher than that of standard supernovae. An alternative term for most hypernovae is "superluminous supernovae" (SLSNe). Such explosions are believed to be the origin of long-duration gamma-ray bursts.[1] Just like supernovae in general, hypernovae are produced by several different types of stellar explosion: some well modelled and observed in recent years, some still tentatively suggested for observed hypernovae, and some entirely theoretical.
Numerous hypernovae have been observed corresponding to supernovae type Ic and type IIn, and possibly also at least one of type IIb.[2] The word collapsar, short for collapsed star, was formerly used to refer to the end product of stellar gravitational collapse, a stellar-mass black hole. History of the term[edit] Gamma-ray bursts[edit] Causes of hypernovae[edit]
Type II supernova. A Type II supernova (plural: supernovae) results from the rapid collapse and violent explosion of a massive star. A star must have at least 8 times, and no more than 40–50 times, the mass of the Sun for this type of explosion.[1] It is distinguished from other types of supernovae by the presence of hydrogen in its spectrum. Type II supernovae are mainly observed in the spiral arms of galaxies and in H II regions, but not in elliptical galaxies. Stars generate energy by the nuclear fusion of elements. Unlike the Sun, massive stars possess the mass needed to fuse elements that have an atomic mass greater than hydrogen and helium, albeit at increasingly higher temperatures and pressures, causing increasingly shorter stellar life spans. There exist several categories of Type II supernova explosions, which are categorized based on the resulting light curve—a graph of luminosity versus time—following the explosion.
Formation[edit] Core collapse[edit] Theoretical models[edit] Type Ib and Ic supernovae. The Type Ib supernova Supernova 2008D[1][2] in galaxy NGC 2770, shown in X-ray (left) and visible light (right), at the corresponding positions of the images. NASA image.[3] Types Ib and Ic supernovae are categories of stellar explosions that are caused by the core collapse of massive stars. These stars have shed (or been stripped of) their outer envelope of hydrogen, and, when compared to the spectrum of Type Ia supernovae, they lack the absorption line of silicon.
Compared to Type Ib, Type Ic supernovae are hypothesized to have lost more of their initial envelope, including most of their helium. Spectra[edit] When a supernova is observed, it can be categorized in the Minkowski–Zwicky supernova classification scheme based upon the absorption lines that appear in its spectrum.[4] A supernova is first categorized as either a Type I or Type II, then sub-categorized based on more specific traits. Formation[edit] The onion-like layers of an evolved, massive star (not to scale). See also[edit] Possible new class of supernovae puts calcium in your bones. In the past decade, robotic telescopes have turned astronomers' attention to scads of strange exploding stars, one-offs that may or may not point to new and unusual physics. But supernova (SN) 2005E, discovered five years ago by the University of California, Berkeley's Katzman Automatic Imaging Telescope (KAIT), is one of eight known "calcium-rich supernovae" that seem to stand out as horses of a different color.
"With the sheer numbers of supernovae we're detecting, we're discovering weird ones that may represent different physical mechanisms compared with the two well-known types, or may just be variations on the standard themes," said Alex Filippenko, KAIT director and UC Berkeley professor of astronomy. "But SN 2005E was a different kind of 'bang.' It and the other calcium-rich supernovae may be a true suborder, not just one of a kind. " "It's a confusing, muddy situation now," said Filippenko. Astronomers have so far found only one example of this beast, however.
Wolf-Rayet star. Hubble Space Telescope image of nebula M1-67 around Wolf–Rayet star WR 124. Wolf–Rayet stars (often referred to as WR stars) are evolved, massive stars (over 20 solar masses when they were on the main sequence) which are losing mass rapidly by means of a very strong stellar wind, with speeds up to 2000 km/s. They typically lose 10−5 solar masses a year, a billion times higher than the sun.[1] Wolf–Rayet stars are extremely hot, with surface temperatures in the range of 30,000 K to around 200,000 K.[2] They are also highly luminous, from tens of thousands to several million times the bolometric luminosity of the Sun, although not exceptionally bright visually since most of their output is in far ultraviolet and even soft X-rays.
The naked eye stars Gamma Velorum and Theta Muscae, and R136a1 in 30 Doradus, the most massive and luminous star known, are all Wolf–Rayet stars. Observation history[edit] M1-67 is the youngest wind-nebula around a Wolf-Rayet star, called WR124, in our Galaxy.