How Many Types of Black Holes Are There?
It may be because of the difficulty in describing black holes that we hold such a fascination with them. They are objects with zero volume and infinite mass, which defy all our conventional ideas about everyday life. Yet perhaps as equally intriguing as their description are the different types of black holes that exist.
Stellar-Mass Black Holes
These are the smallest type of black holes known currently and most form from what is known as a supernova, or the violent explosive death of a star. Currently, two types of supernova are thought to result with a black hole.
A Type II supernova occurs with what we call a massive star, whose mass exceeds 8 solar masses and does not exceed 50 solar masses (a solar mass being the mass of the sun). In the Type II scenario, this massive star has fused so much of its fuel (initially hydrogen but slowly progressing through the heavier elements) through nuclear fusion that it has an iron core, which cannot undergo fusion. Because of this lack of fusion, degeneracy pressure (an upward force that arises from electron motion during fusion) decreases. Normally, degeneracy pressure and the force of gravity balance out, allowing a star to exist. Gravity pulls in while the pressure pushes outward. Once an iron core increases to what we call the Chandrasekhar Limit (about 1.44 solar masses), it no longer has sufficient degeneracy pressure to counteract gravity and begins to condense. The iron core cannot be fused, and it is compacted until it blows. This explosion destroys the star and in its wake will be a neutron star if between 8-25 solar masses and a black hole if greater than 25 (Seeds 200, 217).
A Type Ib supernova is essentially the same as the Type II, but with a few subtle differences. In this case, the massive star has a companion star that strips away at the outer hydrogen layer. The massive star will still go supernova because of a loss of degeneracy pressure from the iron core and create a black hole given that it has 25 or more solar masses (217).
A key structure of all black holes is the Schwarzschild radius, or the closest you can get to a black hole before you reach a point of no return and are sucked into it. Nothing, not even light, can escape from its grasp. So how can we know of stellar-mass black holes if they emit no light for us to see? Turns out, the best way to find one is to look for x-ray emissions coming from a binary system, or a pair of objects orbiting a common center of gravity. Usually this involves a companion star whose outer layer gets sucked into the black hole and forms an accretion disk that spins around the black hole. As it falls closer and closer to the Schwarzschild radius, the material gets spun to such energetic levels that it emits x-rays. If such emissions are found in a binary system, then the companion object to the star is most likely a black hole.
These systems are known as ultra luminous X-ray sources, or ULXs. Most theories say that when the companion object is a black hole it should be young but recent work by the Chandra Space Telescope shows that some may be very old. When looking at a ULX in galaxy M83 it noticed that the source preceding the flare was red, indicating an older star. Since most models show that the star and black hole form together then the black hole must be old too, for most red stars are older than blue stars (NASA).
To find the mass of all black holes, we look at how long it and its companion object take to complete a full orbit. Using what we know of the mass of the companion object based off its luminosity and composition, Kepler’s Third Law (period of one orbit squared equals the average distance from the orbiting point cubed), and equating the force of gravity to the force of circular motion, we can find the mass of the black hole.
Recently, a black hole birth was seen. The Swift Observatory witnessed a gamma ray burst (GRB), a high energy event associated with a supernova. The GRB took place 3 billion light years away and lasted for about 50 milliseconds. Since most GRB last about 10 seconds, scientists suspect this one was the result of a collision between neutron stars. Regardless of the source of the GRB, the result is a black hole (Stone 14).
Though we cannot confirm this yet, it is possible that no black hole is ever fully developed. Because of the high gravity associated with black holes, time slows down as a consequence of relativity. Therefore, time at the center of the singularity may stop, therefore preventing a black hole from fully forming (Berman 30).
Intermediate-Mass Black Holes
Until recently, these were a hypothetical class of black holes whose mass is 100’s of solar masses. But observations from the Whirlpool Galaxy led to some speculative evidence for their existence. Typically, black holes that have a companion object form an accretion disk that can reach up to 10’s of millions of degrees. However, confirmed black holes in the whirlpool have accretion discs that are less than 4 million degrees Celsius. This could mean that a bigger cloud of gas and dust is surrounding the more massive black hole, spreading it out and thus lowering its temperature. These intermediate black holes (IMBH) could have formed from smaller black hole mergers or from supernova of extra-massive stars. (Kunzig 40). The first confirmed IMBH is HLX-1, found in 2009 and weighing in at 500 solar masses.
Not long after that, another one was found in galaxy M82. Named M82 X-1 (it being the first X-ray object seen), it is 12 million light years and has 400 times the mass of the sun. It was only found after Dheerraj Pasham (from the University of Maryland) looked at 6 years of X-ray data, but as far as how it formed remains a mystery. Perhaps even more intriguing is the possibility of IMBH's being a stepping stone from stellar-mass black holes and supermassive black holes. Chandra and VLBI looked at object NGC 2276-3c, 100 million light years away, in the X-ray and radio spectrums. They found that 3c is about 50,000 solar masses and has jets similar to supermassive black holes which also inhibit stellar growth (Scoles, Chandra).
It was not until HXL-1 was found that a new theory for where these black holes came from developed. According to a March 1st Astronomical Journal study, this object is a hyper luminous x-ray source on the perimeter of ESO 243-49, a galaxy 290 million light years away. Near it is a young blue star, hinting at a recent formation (for these die fast). Yet black holes are by nature older objects, forming typically after a massive stars burns through its lower elements. Mathiew Servillal (from the Harvard-Smithsonian Center for Astrophysics in Cambridge) thinks that HXL is actually from a dwarf galaxy that collided with ESO. In fact, he feels HXL was that dwarf galaxy's central black hole. As the collision occurred, gases around HXL would be compressed, causing star formation and thus a possible young blue star to be in proximity to it. Based on the age of that companion, such a collision likely occurred about 200 million years ago. And because the discovery of HXL relied on data from the companion, maybe more IMBHs can be found using this technique (Andrews).
Another promising candidate is CO-0.40-0.22*, which is located in the molecular cloud its named after near the center of the galaxy. Signals from ALMA and XMM-Newton found by a team led by Tomoharu Oka (Keio University) were similar to other supermassive black holes but the brightness was off and implied 0.22* was 500 times less massive, clocking in at roughly 100,000 solar masses. Another good piece of evidence was the speed of objects inside the cloud, with many reaching near-relativistic speeds based on the Doppler shifts the particles underwent. This can only be achieved if a high-gravity object resided in the cloud to accelerate the objects. If 0.22* is indeed an intermediate black hole, it likely did not form in the gas cloud but was inside a dwarf galaxy that the Milky Way ate long ago, based off models that indicate a black hole is 0.1 percent the size of its host galaxy (Klesman, Timmer).
Supermassive Black Holes
They are the driving-force behind a galaxy. Using similar techniques in our analysis of stellar-mass black holes, we look at how objects orbit the center of the galaxy and have found the central object to be millions to billions of solar masses. It is thought that supermassive black holes and their spin result in many of the formations we witness with galaxies as they consume material that surrounds them at a furious pace. They seem to have formed during a galaxy’s own formation. One theory states that as matter accumulates in the center of a galaxy, it forms a bulge, with a high concentration of matter. So much, in fact, that it has a high level of gravity and thus condenses the matter to create a supermassive black hole. Another theory postulates that supermassive black holes are the result of numerous black hole mergers.
A more recent theory states that supermassive black holes may have formed first, before the galaxy, a complete reversal on current theory. When looking at quasars (distant galaxies with active centers) from just a few billion years after the Big Bang, scientists witnessed supermassive black holes in them. According to cosmological theories, these black holes are not supposed to be there because the quasars have not existed long enough to form them. Stuart Shapero, an astrophysist at University of Illinois at Urbana Champaign, has a possible solution. He thinks that the 1st generation of stars formed from “primordial clouds of hydrogen and helium” which would also exist when the first black holes formed. They would have had plenty to munch on and would also merge with one another to form supermassive black holes. Their formation would then result in sufficient gravity to accumulate matter around them and thus galaxies would be born (Kruglinski 67).
Another place to look for proof of supermassive black holes impacting galactic behavior is in modern galaxies. According to Avi Loeb, an astrophysist at Harvard University, most modern galaxies have a central supermassive black hole “whose masses seem to correlate closely with the properties of their host galaxies.” This correlation seems to be related to the hot gas that surrounds the supermassive black hole which could impact the behavior and environment of the galaxy including its growth and the number of stars that form (67). In fact recent simulations show that supermassive black holes get most of the material that helps them grow from those small blobs of gas around it. The conventional thought was that they would grow mostly from a galaxy merger but based off the simulations and further observations it seems that the small amount of matter that constantly falls in is what is key to their growth (Wall).
Regardless of how they form, these objects are great at matter-energy conversion, for after ripping matter apart, heating it up, and forcing collisions between the atoms that only a few can get sufficiently energetic to escape before encountering the event horizon. Interestingly, 90% of material that falls into black holes never actually gets eaten by it. As the material spins around, friction is generated and things heat up. Through this energy buildup, particles can escape before falling into the event horizon, leaving the vicinity of the black hole at speeds approaching the speed of light. That being said, supermassive black holes do go through ebbs and flows for their activity is dependent on matter being near it. Only 1/10 of galaxies actually have an actively-eating supermassive black hole. This may be because of gravitational interactions or the UV/X-rays emitted during active phases pushes matter away (Scharf 34, 36; Finkel 101-2).
They mystery deepened when an inverse correlation was discovered when scientists compared a galaxies star formation to the activity of the supermassive black hole. When the activity is low, star formation is high but when star formation is low the black hole is feeding. Star formation is also an indication of age and as a galaxy becomes older the rate of new stars being produced decreases. The reason for this relation eludes scientists, but it is thought that an active supermassive black hole will eat too much material and create too much radiation for stars to condense. If a supermassive black hole is not too massive then it may be possible for stars to overcome this and form, robbing the black hole of matter to consume (37-9).
Interestingly, even though supermassive black holes are a key component of a galaxy which possibly contains a vast multitude of life, they can also be destructive to such life. According to Anthony Stark of the Harvard-Smithsonian Center for Astrophysics, within the next 10 million years any organic life near the center of the galaxy will be destroyed because of the supermassive black hole. Much material gathers around it, similar to stellar-mass black holes. Eventually, about 30 million solar masses worth will have accumulated and be sucked in at once, which the supermassive black hole cannot handle. Much material will be cast out of the accretion disk and become compressed, causing a starburst of short-lived massive stars that go supernova and flood the region with radiation. Thankfully, we are safe from this destruction since we are about 25,000 light years from where the action will take place (Forte 9, Scharf 39).
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© 2013 Leonard Kelley