Posts Tagged ‘black hole’

White Dwarf Explosions and Medium Black Holes

Tuesday, September 9th, 2008 by Bellatrix

Here on Earth death usually means the end but in space, stars can have quite the interesting afterlives, and stellar corpses can even interact. The fact that stellar remnants interact is nothing new, however a new theory based on observations and computer simulations may explain a new type of supernova and help end a debate about black holes. First, let me lay out a little background.

A white dwarf is the stellar remnant of a low mass star. A star of about 2 or less solar masses will die in what is called planetary nebulae and leave behind a white dwarf. They are small dense objects about the size of earth with the mass of the sun that have an inert carbon core and are no longer do nuclear burning. For higher mass stars they die in what is called a supernova, a massive explosion where the star blows off its outer layers and leaves behind a neutron star, or if massive enough a black hole. Now there are two different kinds of supernova explosions. One is what I just mentioned, when a high mass star explodes. The other kind is when a white dwarf has a companion star. The white dwarf collects, or accretes, matter from its companion star. Once it reaches high enough mass the surface of the white dwarf reignites nuclear burning eventually then exploding in a supernova. Each one of these kinds of supernovae has a very different light signature, or spectrum.

A new paper was published describing a new way of igniting a white dwarf and a new type of supernova. In this new process a white dwarf wanders too close to a black hole. The strong gravity of the black hole causes tidal disruption in the white dwarf, it pulls and flattens the white dwarf into a pancake shake, and this compresses the star’s material reigniting nuclear burning. As each section of the star is squeezed through a point of maximum compression, the extreme pressure causes a sharp increase in temperatures, which triggers explosive burning. The explosion ejects half the material from the star while the rest falls into the black hole. This in-falling material heats up and gives off x-rays. So this supernova should have a different spectrum and be followed up by a glow of x-rays.

Now the interesting thing is this process would only be possible with a black hole of a particular mass, neither too big nor too small. It would have to be between 500 to 1000 solar masses. Theoretically and observationally we only know of small black holes, several solar masses, or super massive ones on the order of millions of solar masses. So proof of this process would mean there are intermediate mass black holes, which would beg the question of where do these black holes come from?

These types of supernova are thought to be 100 times less frequent than the other types of supernova. The Synoptic Survey Telescope, planned for 2013, will be observing hundreds of supernova per year. So far this new process between white dwarfs and black holes has been successfully modeled with computer simulations, but hopefully with this new telescope we will be able to observe the spectrum of these supernova. This would provide proof for this theory, answer some questions, and lead to some new ones.

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Black Hole Catapult

Thursday, September 4th, 2008 by Bellatrix

It is now generally accepted in the astrophysics community that at the center of nearly all galaxies lays a super massive black hole. However, there exists a theory about black holes that could change that. The theory, derived from Einstein’s General Theory of Relativity, says that if two black holes were to merge, gravitational waves would fly out in one direction, kicking the black hole in the opposite direction like a recoil. The theory sounded interesting but no one had ever observed two black holes merging, let alone a black hole recoiling. The possibility of two massive black holes merging was also just theory, as you can imagine it’s hard to observe two invisible things crashing into each other.

Well a team from the Max Planck Institute for Extraterrestrial Physics (MPE) has, for the first time, witnessed these events. The team witnessed a recoiling black hole moving at a very high speed of 2650 kilometers per second! Because of the tremendous power of this recoiling effect the black hole was catapulted from its parent galaxy. The astronomers were able to track it by the gas, or accretion disk, moving with the black hole; and also by the excited gas that was left behind.

This new observation has many implications. It brings this process from the world of just theory to the world of actuality. It not only tells us that these “super kicks” happen but also that black holes do merge, and lends support to the theory of gravitational waves, which have yet to be directly observed. This also means that there are galaxies out there without a super massive black hole at the center. That fact raises questions about the role or dependence on the central black hole and galaxy formation. Is the black hole there at the start of the galaxy? Is it needed for the galaxy to evolve normally? And what effect does it have on the host galaxy when it looses its black hole?

Astrophysicists, observers and theorists alike, are invigorated to start trying to answer some of these questions. Both earth and space based telescopes will be set to try and detect more of these events, and work is being done to get gravitational wave detectors working. Theorists will also be getting going on more details of these types of events with the help of computer simulations. Weird to think there might be these super massive black holes just floating around out in between the galaxies, lurking there, possibly waiting for a future spacecraft to fly right in.

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White dwarfs and Neutron Stars: Maybe Not So Different

Tuesday, September 2nd, 2008 by Bellatrix

For a long time know we’ve thought that we had certain basics of stellar evolution figured out, but as always it seems in science once we think we know something a wrench gets thrown into the works. When a star dies one of three outcomes can happen. If the star is around or below about 8 solar masses, the star will die in a planetary nebula where it puffs off its outer layers and leaves behind a white dwarf star. If it is more massive than that about 8 solar masses the star will go supernova and leave behind a neutron star, or if massive enough a black hole. While both white dwarfs and neutron stars are both stellar remnants they are considered to be quite different objects.

A white dwarf is an inert stellar core left hot from the burning of the star but it is thought to gradually just cool off and after billions of years would become cold and dark, or a black dwarf. They are dense small objects, about the size of earth with the mass of the sun. But white dwarfs have never been particularly strange and bizarre, unlike neutron stars. Neutron stars are also small dense objects with masses of around 2 solar masses but packed inside the size of a city. Neutron stars are incredibly hot, made entirely of neutrons and supported against further collapse by the Pauli exclusion principle; which says no two neutrons can occupy the same quantum state. Neutron stars have very strong magnetic fields, and spin incredibly fast usually spinning several times to several hundred times per second. These strong magnetic fields combined with their very short period make them “pulse” due to particle acceleration near the magnetic poles. These pulsars produce streams of radio emission making them like beacons we are able to detect and even listen to.

New observations from a joint observatory between NASA and JAXA (Japanese Aeronautics Exploration Agency) have detected a white dwarf pulsar. AE Aquarii emits pulses of high-energy X-rays as it whirls around on its axis, and this is the first time pulsar like activity has been detected in a white dwarf. The hard x-rays match the star’s spin period of once every 33 seconds. The hard X-ray pulsations are very similar to those of the pulsar, or neutron star, in the center of the Crab Nebula. In both objects, the pulses appear to be radiated like a lighthouse beam, and a rotating magnetic field is thought to be controlling the beam. Astronomers think that the extremely powerful magnetic fields are trapping charged particles and then flinging them outward at near-light speed. When the particles interact with the magnetic field, they radiate X-rays.

No doubt after this discovery we may begin pointing our X-ray telescopes at more white dwarfs, or maybe even questioning some of the pulsars we’ve already identified as neutron stars. This may also have us questioning what we thought we knew about the death of low mass stars, as it doesn’t appear as peaceful a progression as previously thought. This discovery shows there may be more or different processes happening with white dwarfs that may need to be investigated.

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