Visions of the Cosmos

Astrophysics

STARS

Superstars and Supernovae

The stars that bejewel the firmament are of many different types and sizes as we saw in the previous page of the web-site 'Types of Star'.   The vast majority are actually quite small in comparison with the Sun and are classified as 'Red Dwarfs'.  The 'Sun like' stars, which make up the majority of the rest of the stars, are classified by their spectra as belonging to the 'K' and 'G' ranges.  They are sometimes referred to, rather strangely, as orange and yellow dwarfs.   Like our Sun they continue in the main sequence stage for around eight to fifteen billion years.   Much rarer in the present Universe are the  supersuns which are over 10 solar masses and even rarer are those of over 100 solar masses.  Stars of under 8 solar masses end their lives like our Sun buy collapsing into white dwarfs.   When a massive star of over about 8 solar masses reaches the end of its main sequence life it explodes as a supernova or collapses into a black hole.  During the end of its main sequence life such a star undergoes first the triple alpha reaction in which carbon nuclei are synthesized from helium-4, then the production of oxygen nuclei from helium-4 and carbon-12 and then the synthesis of all the isotopes of the chemical elements up to and including iron and nickel.  When it reaches this stage known as the iron peak is undergoes a massive explosion called a supernova.  Most supernovas follow the process described in detail in the next  web-page entitled 'The Magic Furnace'.  Very massive stars may collapse into black holes and very very massive stars may undergo a special kind of supernova explosion called a pair-instability supernova'.

Thermonuclear Reactions

We have seen in the section on our star the Sun how atomic nuclei fuse together with the liberation of energy (mostly in the form of gamma rays) to give more massive nuclei in a processes called 'thermonuclear burning'.  Thus hydrogen nuclei (protons) are converted to helium-4 nuclei which at higher temperatures burn to carbon and oxygen nuclei.  This process continues to iron in more massive stars and this is connected with the binding energies of protons and neutrons (collectively called nucleons) within atomic nuclei.

It may seem strange that protons which are positively charged can form stable complex particles with each other and with neutrons to form atomic nuclei.  One would think that the powerful repulsive electrostatic forces (Coulomb forces) between the protons would prevent this from happening.  The reason is that there are even more powerful forces of attraction between both protons and neutrons which operate at very short distances ( about 2x 10-15 metre) and encourage the formation of atomic nuclei.  These are referred to as 'the strong forces' and will be discussed more extensively later in the web-site section on particle physics.  The mass of an atomic nucleus is always less than the sum of the masses of all the component nucleons (protons and neutrons).  The loss of mass when nucleons form a nucleus is called 'the nuclear binding energy'.    It is this energy which is given out mostly as gamma radiation but also as heat and neutrinos when  a 'lower nucleus' such as helium is converted to a 'higher nucleus' such as carbon.  Similarly the nucleus of oxygen-16 has a 'higher binding energy' than the combined 'binding energies' of carbon-12 and helium-4 so that in the conversion of carbon-12 + helium-4 to oxygen-16 energy is given out. As shown in the diagrams below, the binding energy per nucleon increases until iron-56  is reached.  Up to this point it is possible to extract energy by thermonuclear reactions.  The conversion of protons into helium-4 liberates a great deal of energy.  The energy released by the conversion of helium-4 to carbon-12 in the triple alpha reaction is considerably less.  The amounts of energy released gets progressively smaller until iron-56 is reached.  At this point no more energy can be extracted and a supernova explosion occurs.   Elements higher up the periodic table is achieved during the explosion process by the addition of energy largely in the form of high energy neutrons.

Put very simply the synthesis of all the elements up to iron-56 are exergonic (exothermic using the older word).  They are often referred to as 'burning processes'.   Reactions in which elements above iron are formed are endergonic (endothermic using the older word).  It is noteworthy that all the elements below and including iron and nickel are the most abundant elements in the Universe

During these vast explosions the interstellar medium is enriched with all the stable (and very long lived radioactive) elements.   After vast periods of time these elements are incorporated into new stars and into planets and one day into plants, animals and people.  Hence the saying we are all made of star-dust.

Credits

Left hand illustration Credit Professor David J. Raymond New Mexico Tech Socorro,

http://www.physics.nmt.edu/~raymond/classes/ph13xbook/node216.html

Right hand illustration http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin.html

Supermassive Stars

Supermassive Stars of around  a hundred solar masses and over are very rare in the universe today but it is thought that the very first stars were far more massive from 100 up to 1000 solar masses.   This is thought to be due to the fact that the earliest stars were entirely composed of hydrogen and helium  and traces of lithium only.   Present day stars contain varying amounts of the other elements of the periodic table which seem to inhibit the growth of a star  so that only very few stars grow to gigantic sizes that they did when the universe was young and contained no metal other than traces of lithium.  

The word metal as used in astrophysics must be understood.  In chemistry the word metal has a specific meaning relating to the chemical behaviour of the element.  Thus elements such as iron, copper, sodium, potassium, calcium, magnesium, calcium and aluminium are clearly metals whilst carbon, oxygen, sulphur and phosphorus are clearly non-metals..  The word metal has a totally different meaning in astrophysics.  It means all elements above hydrogen and helium in the periodic table.   Thus carbon, oxygen, sulphur and phosphorus are metals in the astrophysical vocabulary.    They are of course still non-metals in planetary sciences.   We talk about the metallicity of a star to indicate how rich it is in elements other than hydrogen and helium.

There is a rapid growth in the branch of astrophysics relating to massive stars and to supernovae.   Every week some exciting new discovery appears in the scientific journals.   The month of May 2007 saw three exciting observations

1) the discovery of a large Earth-like planet about three times the mass of the Earth and one and a half times the diameter.  A special section of the web-site is devoted to this discovery under the Web-page 'Extra-solar Planets'.

2) the discovery of two massive stars in a binary system in the Large Magellanic Cloud which is dealt with below

3) the detection of a new type of violent supernova - in which gamma rays are producing matter/anti-matter pairs - pair-instability supernova.  This is also dealt with in the section.

Massive Binary Pair

NASA Publication 28 May 2007

Using NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) satellite and ground-based telescopes, astronomers have determined, for the first time, the properties of a rare, extremely massive, and young binary star system.  FUSE project scientist George Sonneborn of NASA Goddard Space Flight Center, Greenbelt, Md., is presenting these results today in a poster at the spring 2007 American Astronomical Society meeting in Honolulu, Hawaii.

The picture on the left is a false-color image from the Curtis Schmidt Telescope in Chile shows a large star-forming region in the Large Magellanic Cloud. The binary system LH54-425 is arrowed. It is located in the star cluster LH54. Credit: Chris Smith and the University of Michigan Curtis Schmidt Telescope at CTIO.

An artist depicts the binary system LH54-425, which consists of two very massive stars. The larger stars powerful stellar wind overpowers the smaller star's wind, creating a region of hot gas where the outflows collide. LH54-425 is in the Large Magellanic Cloud, a satellite galaxy of the Milky Way about 165,000 light-years from Earth. Credit: NASA illustration by Casey Reed.

The binary system, known as LH54-425, is located in the Large Magellanic Cloud, a satellite galaxy of our Milky Way. The binary consists of two O-stars, the most massive and luminous types of stars in the Universe.

Spectra obtained by Georgia State University astronomer Stephen Williams at the 1.5-meter (4.9 foot) telescope at the Cerro Tololo Inter-American Observatory in Chile show that the two stars contain about 62 and 37 times the mass of our Sun. "The stars are so close to each other -- about one-sixth the average Earth-Sun distance -- that they orbit around a common center of mass every 2.25 days," says Williams' colleague Douglas Gies of Georgia State University, Atlanta. With a combined mass of about 100 suns, the system is one the most extreme binaries known. The stars are probably less than 3 million years old.

Each star blows off a powerful stellar wind, and observations have provided the first details of what happens when the two supersonic stellar winds collide. The wind collision zone wraps around the smaller star and produces a curved surface of superheated gases that emit X-rays and far-ultraviolet radiation.

As the stars age and swell in size, they will begin to transfer substantial amounts of mass to each other. This process could begin in a million years. The stars are orbiting so close to each other that they are likely to merge as they evolve, producing a single extremely massive star like the more massive member of the Eta Carinae binary system. Eta Carinae is one of the most massive and luminous stars in the Milky Way Galaxy, with perhaps 100 solar masses. "The merger of two massive stars to make a single super star of over 80 suns could lead to an object like Eta Carinae, which might have looked like LH54-425 one million years ago," says Sonneborn. "Finding stars this massive so early in their life is very rare. These results expand our understanding of the nature of very massive binaries, which was not well understood. The system will eventually produce a very energetic supernova

Launched in 1999, FUSE is a NASA Explorer mission developed in cooperation with the French and Canadian space agencies by Johns Hopkins University, University of Colorado, and University of California, Berkeley. NASA's Goddard Space Flight Center, Greenbelt, Md., manages the program.

The Classification of Supernovae

Please note that the correct plural of Supernova is NOT Supernovas but Supernovae

The original classification of supernovae is still used although in the light of modern knowledge it is decidedly misleading.  

Supernovae are classified based on  their optical spectra. They are divided into 4 types, the naming convention of which only makes sense in a historical context.

In 1941 Rudolph Minkowski  recognized two types of supernovae Type 1 and Type 2

Type 1 showed NO hydrogen lines in their spectra. ---- Type 11 showed hydrogen lines in their spectra

In the mid-1980s further observations caused a more elaborate subdivision to be made

Type 1 supernovas were subdivided into three types   - -  Type 1a showed no hydrogen spectra but an obvious Si absorption line - -Type 1b showed no Si and no hydrogen spectra and Type 1c showed no Si, no hydrogen and no helium.

It is now known that Type 1b, Type 1c are really similar to Type 11.   They all three result from the core collapse of massive stars and it is only Type 1a that is fundamentally differerent.

Type 1a supernovae arise from the thermonuclear explosions of white dwarf stars.  These events occur in binary systems where material from the companion of a white dwarf (which might be well below the Chandrasekhar limit of 1.4 solar masses at the time of its formation) is continually added to the white dwarf until the very moment that the mass goes just over the limit and the white dwarf collapses to a neutron star and undergoes a supernova explosion.  Since (theoretically at least) all Type 1a supernovae are derived from the same mass of material they should all have the same luminosity.  Thus they are used as standard candles in the calculation of the distances of very far off galaxies and this has played a vital role in calculating the velocity of the recession of the galaxies and in the discovery of 'the dark energy effect' which will be discussed in the cosmology section of this web-site.

Thus we have a more modern classification of supernovae.

Type 1a arise from a thermonuclear explosion caused by the action of added mass onto a white dwarf which is derived from a companion main sequence star in a binary system.

Types 1b, 1c and Type 11 and another newly discovered and very violent 'Pair Instability Type of Supernova' are all derived by the core collapse of massive stars.  This subject is discussed below.   Core collapse can also result in the formation of a 'Black Hole'.

Mega Supernovae

Some very massive stars are believed to collapse into black holes if the  limit goes above that for the formation of a neutron star.  As previously mentioned it is widely thought that the first stars were very massive - this did present a problem in that much if not all the precious material formed in the star during the production of the higher chemical elements would not be used to enrich the interstellar medium and thus futures star systems like our own solar system but would be lost in the formation of a black hole.   Surprisingly another scenario presented itself and led to the theory of 'a Pair Instability Supernova in which a massive explosion blew the star to smithereens and prevented the formation of a black hole. On 17 May 2007 a team of astronomers at the Berkely Campus of the unof the university of California reported the observation of just such a supernova.

On 18 September 2006 in the Galaxy NCG 1260 Robert Quimby,   a graduate student at the university of Texas reported the observation of the most gigantic supernova ever seen.  It is now thought that it was an example of a type of supernova very rarely occurring in the present day Universe but probably common in the time of the first stars - in fact 'A Pair Instability Supernova'.  Unlike typical supernovae which reach their peak rapidly and then die away into obscurity after a few months this one, code named SN2006gy, took 70 days to reached full brightness and remained as bright as any previously observed supernova for more than 3 months.   It was about 50 m times as luminous as any supernova previously observed and it outshone all the stars in its Galaxy eight months after its initial appearance.  Remember a typical supernova is brighter than all the stars in its galaxy and burns as much in a few moments as the Sun will have done in the whole of its ten billion year lifetime.   The picture below shows an artists impression of what this immense supernovae may have looked like

This artist's illustration shows what the brightest supernova ever recorded, known as SN 2006gy, may have looked like. The fireworks-like material (white) shows the explosive death of an extremely massive star. Before it exploded, the star expelled the lobes of cool gas (red). As the material from the explosion crashes into the lobes, it heats the gas in a shock front (green, blue and yellow) and pushes it backward. Credit: NASA/CXC/M.Weiss
 

Hyperlinks

Stars

Types of Stars

The Magic Furnace