Supernovas
After a Supernova........and before. Supernovae are divided into two basic physical types: Ia. These result in some binary star systems between a red giant and a white dwarf. In such a system, mass flows from the red giant to the white dwarf. Eventually, so much mass piles up on the white dwarf that it can no longer support itself and it collapses. II. These supernovae occur at the end of a massive star's lifetime, when its nuclear fuel is exhausted and it is no longer supported by the release of nuclear energy. If the star's iron core is massive enough then it will collapse and become a supernova. However, these types of supernovae were originally classified based on the existence of hydrogen spectral lines: Type Ia do not show hydrogen lines, while Type II do. In general this observational classification agrees with the physical classification outlined above, because massive stars have atmospheres (made of mostly hydrogen) while white dwarf stars are bare. However, if the original star was so massive that its strong stellar wind had already blown off the hydrogen from its atmosphere by the time of the explosion, then it too will not show hydrogen spectral lines. These supernovae are often called Type Ib supernovae, despite really being part of the Type II class of supernovae.  What Causes a Star to Blow Up? Gravity gives the supernova its energy. For either type of supernova, mass flows into the core, from a companion star (I) or by the continued making of iron from nuclear fusion (II). Once the core has gained so much mass that it cannot withstand its own weight, the core implodes. This implosion can usually be brought to a halt by neutrons, the only things in nature that can stop such a gravitational collapse. Even neutrons sometimes fail depending on the mass of the star's core. When the collapse is abruptly stopped by the neutrons, a bounce occurs, thus turning the implosion into an explosion. ka-BOOM!!! When the core is lighter than about 5 solar masses, it is believed that the neutrons are successful in halting the collapse of the star creating a neutron star. Neutron stars can sometimes be observed as pulsars or X-ray Binaries. When the core is heaver (Mcore > ~ 5 solar masses), nothing in the known universe is able to stop the core collapse, so the core completely falls into itself, creating a black hole, an object so dense that even light cannot escape its gravitational grasp. The core is only the very small centre of an extremely large star that for many millions of years had been making many (but not all) of the elements that we find here on Earth. When a star's core collapses, an enormous blast wave is created with the energy of about 1028 mega-tons. This blast wave ploughs the star's atmosphere into interstellar space, propelling the elements outward as the star becomes a supernova remnant. In addition to making elements, supernovae scatter the elements (made by both the star and supernova) out in to the interstellar medium. These are the elements that make up stars, planets and everything on Earth including ourselves. Although many supernovae have been seen in nearby galaxies, supernova explosions are relatively rare events in our own Galaxy, happening once a century or so on average. The last nearby supernova explosion occurred in 1680, It was thought to be just a normal star at the time, but it caused a discrepancy in the observer's star catalogue which historians finally resolved 300 years later, after the supernova remnant (Cassiopeia A) was discovered and its age estimated. In 1987 there was a supernova explosion in a companion galaxy to the Milky Way. Supernova 1987A, which is shown at the top of the page, is close enough to continuously observe as it changes over time thus greatly expanding astronomers' understanding of this fascinating phenomenon. A supernova remnant (SNR) is the remains of a supernova explosion. SNRs are extremely important for understanding our Galaxy. They heat up the interstellar medium, distribute heavy elements throughout the Galaxy, and accelerate cosmic rays. Supernova can affect us in some important ways. First and foremost, we and much of the Earth are made of the material supernovae created. According to current theories about the formation of the Universe, all of the original material in the Universe was hydrogen and helium, with very slight traces of some other materials. All the stuff we, and the Earth around us, are made of, like iron and oxygen and carbon, has come from that initial material being fused to form heavier elements in the cores of stars. But the heaviest elements, like iron, are only formed in the massive stars which end their lives in supernovae. Our blood has iron in the haemoglobin which is vital to our ability to breath. So without supernovae, most forms of life on Earth, including us, would not be possible. And much of the material the Earth is made of would not exist. Supernovae also create shock waves through the interstellar medium (the stuff between stars), compressing material there. Astronomers believe that these shock waves are vital to the process of star formation, causing large clouds of gas to collapse and form new stars. No supernovae, no new stars. Supernovae throw much of the material from their parent star back out into the interstellar medium, changing its chemical composition. This adds many elements to the interstellar medium which were not present before, or were only present in trace amounts. Other less massive stars also enrich the interstellar medium, but lack many of the heavier elements. The gradual enrichment of the interstellar medium with heavier elements has made subtle changes to how stars burn: the fusion process in our own Sun is moderated by the presence of carbon. The first stars in the Universe had much less carbon and their lives were somewhat different from modern stars. Stars which will be formed in the future will have even more of these heavier elements and will have somewhat different life cycles. So supernovae play a very important part in this chemical evolution of the Universe. A supernova has to be within 10 parsecs (30 light years) or so to be dangerous to life on Earth. This is because the atmosphere shields us from most dangerous radiations. Astronauts in orbit may be in danger if a supernova is within 1000 parsecs or so. No stars currently within 20 parsecs will go supernova within the next few million years. There are some indirect effects, though, which are harder to evaluate: the possible effects on the Earth ozone layer is listed in the article above. Additionally, according to one calculation, the neutrino flux from a nearby supernova might heat up the Sun.
Supernova Type	Thermonuclear (Type Ia)	Core Collapse (Type II)
Maximum Luminosity	3 x 109 Suns (MB = -19.5)	3 x 108 Suns (MB = -18.5 +/- 1)
Spectrum	No hydrogen lines. Lines of many heavy elements.	Hydrogen lines. Continuum
Where found	Among old star systems (globular clusters, galactic bulge, elliptical galaxies).	Among young star systems (young star clusters, star-forming regions in disk galaxies).
Parent Star	White dwarf in binary system.	Massive star (usually a red supergiant).
Trigger mechanism	Mass transfer from companion.	Collapse of iron core.
Explosion mechanism	Thermonuclear explosion of carbon/oxygen core --> iron.	Rebound shock from neutron star surface: neutrino pressure.
Left behind	Nothing	Neutron star.
Debris	Mostly iron.	All kinds of elements.

 

Supernova Remnants

Shell-type remnants: The Cygnus Loop (above left) is an example of a shell-type remnant. As the shock wave from the supernova explosion ploughs through space, it heats and stirs up any interstellar material it encounters, thus producing a big shell of hot material in space. We see a ring-like structure in this type of SNR because when we look at the edge of the shell, there is more hot gas in our line of sight than when we look through the middle. Astronomers call this phenomenon limb brightening.

Crab-like remnants: These remnants (also called plerions) resemble the Crab Nebula (above right). These SNRs are similar to shell-type remnants, except that they contain a pulsar in the middle that blows out jets of very fast-moving material. These remnants look more like a "blob" than a "ring."

Composite Remnants: These remnants are a cross between the shell-type remnants and crab-like remnants. They appear shell-like, crab-like or both depending on what part of the electromagnetic spectrum one is observing them in. There are two kinds of composite remnants -- thermal and plerionic. Thermal composites: These SNRs appear shell-type in the radio waveband (synchrotron radiation). In X-rays, however, they appear crab-like, but unlike the true crab-like remnants their X-ray spectra have spectral lines, indicative of hot gas. Plerionic composites: These SNRs appear crab-like in both radio and X-ray wavebands; however, they also have shells. Their X-ray spectra in the centre do not show spectral lines, but the X-ray spectra near the shell do have spectral lines.

Naturally, if the supernova explosion was recorded in history, as is the case of many SNRs less than a few thousand years old, we know the age of the corresponding SNR. However, sometimes historians are not certain if a recorded guest star was a supernova or was the same supernova as a corresponding remnant. It is therefore important to be able to estimate the age of SNRs. An easy way to guess the age of a SNR is to measure the temperature of the hot gas using X-ray spectroscopy. From this observation we can estimate the velocity of the shock wave, and then infer the age from the shock velocity. This is easy to do, but not very accurate, because there are a number of complicated processes that can heat up or cool down the gas which are independent of shock velocity. Supernova remnants greatly impact the ecology of the Milky Way. If it were not for SNRs there world be no Sun or Earth. Every element found in nature, except for hydrogen and helium, was made in either a star or a supernova explosion. So, how did these elements come to be on Earth? Through the action of supernova remnants. The gas that fills the disk of the Milky Way is called the interstellar medium ( ISM). In the parts of the galaxy where the ISM is most dense (for example in the Galaxy's spiral arms), the ISM gas can collapse into clumps. Clumps that are above a critical mass (somewhere between the mass of Jupiter and the Sun) will ignite nuclear fusion when the clumps gravitationally collapse, thus forming stars. Therefore, the chemical composition of the ISM becomes the chemical composition of the next generation of stars. Because, supernova remnants mix supernova ejecta (including the newly formed elements) into the ISM, if it were not for supernova remnants our solar system, with its rocky planets, could never have formed. In addition to enriching our Galaxy with heavy elements, supernova remnants add a great deal of energy to the ISM (1028 mega-tons per supernova). As the shock wave moves outward it sweeps across a large volume of the ISM, impacting the ISM in two primary ways: The shock wave heats the gas it encounters, not only raising the overall temperature of the ISM, but making some parts of the Galaxy hotter than others. These temperature differences helps to keep the Milky Way a dynamic and interesting place. The shock wave accelerates electrons, protons, and ions to velocities very close to the speed of light. This phenomenon is very important, because the origin of the cosmic rays is one of great outstanding problems in astrophysics. Most astronomers believe that most cosmic rays in our Galaxy used to be part of the gas in the ISM, until they got caught in a supernova shock wave. By rattling back and forth across the shock wave, these particles gain energy and become cosmic rays.

 

Dwarfs

A black dwarf is a white dwarf that has cooled down enough that it no longer emits light. See the Imagine the Universe science pages for the differences between white dwarfs, neutron stars, and black holes.  A white dwarf is formed when a star has burned all of its original hydrogen and helium fuel to elements such as carbon, nitrogen and oxygen. If the star doesn't have enough mass, the pressure at its centre is too low to burn these elements further, and so it no longer produces heat. It is, however, still hot from the earlier burning stages, so it still glows for a while until it cools down. It takes tens to hundreds of billions of years for it to cool down entirely, and the Universe hasn't been around that long--the oldest stars are between 10 and 20 billion years old. Therefore there are no black dwarfs yet, but there will be in the future.

Brown dwarfs are small size objects, believed to result from condensations of fragments of molecular clouds. A brown dwarf has a small mass, too low to ignite nuclear fusion. Such a star, would be small, not much larger than Jupiter, and warm from its contraction. It would emit copious infrared radiation, thus the name brown dwarf.

Accretion disks arise when material (usually gas) is being transferred from one celestial object to another. "accretion" means collecting of additional material. Two major places where astronomers see accretion disks are in binary star systems (two stars orbiting each other) and active galactic nuclei.  I will discuss an accretion disk in a binary star system, but the basic ideas are the same for all cases. If one star in a binary system is a compact object such as a very dense white dwarf star and the other star is a normal star like the sun, the white dwarf can pull gas off the normal star and accrete it onto itself. Since the stars are revolving around each other and since angular momentum must be conserved, this gas cannot fall directly onto the white dwarf, but instead spirals in to the white dwarf much like water spirals down a bathtub drain. Thus material flowing from the normal star to the white dwarf piles up in a dense spinning accretion disk orbiting the white dwarf. The gas in the disk becomes very hot due to friction and being tugged on by the white dwarf and eventually loses angular momentum and falls onto the white dwarf. Since this hot gas is being accelerated it radiates energy, usually in x rays which astronomers detect and use to identify and study accretion disks.