Astronomers distinguish between two classes of black holes: Stellar-mass and Supermassive (see above Hubble Space Telescope press release image). Stellar-mass black holes have masses from about one to 10 times the mass of the sun, and are the end products of the evolution of very massive stars.
White dwarfs represent the last stage in the lives of stars not much different than our sun. They are approximately the size of the earth, yet contain much of the original mass of the star whose core it once was. Neutron stars, on the other hand, are only about 20 miles in diameter with nearly the same mass as the sun, are as dense as the nucleus of an atom. Their origins are not clearly understood, but their formation is generally thought to involve the implosion caused by a supernova in a star perhaps 8 - 10 times the mass of the sun. Finally, for stars more massive than this, the supernova explosion proceeds with such violence that the resulting core implosion can not be resisted even by the nuclear force. Gravity gains the upper hand and the core's collapse proceeds unabated until at a diameter of about 1-2 miles, the core becomes a black hole. This is the most collapsed state of matter known, and from it nothing, not even light, may escape.
Over the last few decades, astronomers think they have found a handful of objects in the Milky Way that are very good candidates for stellar-mass black holes.
Cygnus X-1 is the brightest X-ray source in the constellation Cygnus and appears to coincide with a faint 9th magnitude, supergiant star identified as HD 226868, located 8,100 light years away. It is also known to be a binary star with an orbital period of 5.6 days. A careful study of the X-ray emission detected by the UHURU satellite revealed that Cyg X-1 is highly variable. In addition to the regular 5.6 day period that is seen optically, variations as short as 0.001 seconds have been recorded in the system's X-ray emission. The available data suggest that the optically visible star HD 226868 is orbited by a black hole which draws in some of the matter from the visible star, converting it into X-rays. The mass of the unseen companion is estimated to be at least 3.4 solar masses, a number much more than the mass of either stable white dwarfs or neutron stars, both of which would be optically invisible. In addition, the X- rays that such objects produce would be much less energetic than found in the Cygnus X-1 system. Considering this, together with the rapid, millisecond variability which suggests an X-ray source only 300 kilometers across, the evidence for a black hole orbiting HD 226868 is currently thought to be very convincing. Due to the mass of the optically visible companion star estimated to be about 6-8 times the sun's, most stellar evolution models predict that within a few million years such a star will end its life as a supernova leaving behind either a neutron star or a second black hole.
Since the discovery of quasars and other 'active' galaxies in the 1960's, astronomers have been forced to propose that supermassive black holes are the culprits responsible for the enormous outpourings of energy seen in these objects.
In a catalog of the most remarkable objects in the universe one would be terribly remiss if one did not include those which host some of the most titanic outpourings of matter and energy imaginable. In these 'active galaxies', great volumes of matter are being expelled from mysterious regions barely a light year in diameter located deep in the nuclear regions of the galaxies. The entire host galaxy reels from the explosion which triggers waves of star forming activity extending thousands of light years from the active, nuclear core region. Among the best studied examples of 'starburst' or 'mini-quasar' activity can be found in the great star systems called Cygnus-A and Centaurus-A.
Cygnus-A is a peculiar-looking, 15th magnitude galaxy located in the constellation Cygnus which would probably never have come under scrutiny were it not for the fact that it is the host for the strongest radio source in the entire sky, excluding the sun of course. Located 600 million light years away, this galaxy is among the giants of the universe with a mass estimated at 100 trillion times the sun's mass. It consists, apparently, of two nuclei separated by 5500 light years, embedded in a galaxy extending some 450,000 light years across. Just as for Arp 220, the two nuclei of Cygnus-A are probably all that remain of two separate galaxies that passed too close to each other and merged together. The total radio power from Cygnus-A exceeds the total electromagnetic output of over 2 trillion stars like our sun. This in itself is not surprising, since the galaxy contains enough mass to account for 100 trillion suns. What makes this output phenomenal is that the radio emission does not come from stars in the optical galaxy, instead, it originates in two optically invisible regions located 160,000 light years on either side of the optical galaxy! These clouds consist of a hot plasma of electrons moving in magnetic fields at nearly the speed of light. The clouds may have been produced and replenished over time by jets of relativistic plasma ejected from the galactic nucleus. Such double radio sources are not uncommon in the universe and jets connecting the optical nucleus with the distant radio lobes are often seen. What causes this peculiar ejection? Current thinking suggests that in the cores of these galaxies, gigantic billion-solar-mass black holes lurk. Surrounding them are swirling disks of gas and stars from which some material eventually falls into the black holes, releasing enormous quantities of energy, radiation, and jets of plasma and magnetic fields. The jets travel along a path of least resistance perpendicular to the disk plane and the plasma eventually ends up in dumbbell-shaped lobes to either side of the galaxy.
Centaurus-A, also called NGC 5128, is at a distance of 16 million light years making it the nearest active galaxy to the Milky Way and the third strongest radio source in the sky. It is an elliptical galaxy about 200,000 light years in size, and has a dark obscuring band crossing its central region. NGC 5128 has been the site of several titanic eruptions from its nuclear region over the last 100,000 years. At radio wavelengths, the emission originates in two lobes straddling the galaxy just as for Cygnus-A. The lobes extend 2 1/2 million light years and, as seen from the earth, subtend 20 times the diameter of the full moon from end to end. They would make a dramatic sight if they could be seen with the naked eye! The lobes are powered by relativistic electrons ejected from the galaxy's core 100 million years ago. Within the nucleus of NGC 5128, radio astronomers have also detected a second, dumbbell-shaped radio source which aligns with the larger lobes but measures only 30,000 light years across. Portions of this nuclear feature can be seen with X-rays and optically. Most of the X-rays come from a minuscule region perhaps only 1/100 of a light year across, and the X-ray brightness varies appreciably with time. Even more energetic gamma rays have been detected with 100 times the power of the entire gamma ray output of the rest of the galaxy.
There have also been observations by the Hubble Space Telescope which also point towards the existence of supermassive black holes. Further discussions of these discoveries can be found over at the HST public affairs archive:
Plus additional candidates for supermassive black holes in the cores of the galaxies M32, NGC 3115 and NGC 4594.
There are other, smaller black hole candidates that can be found described in the scientific literature:
A0620-00 3-4 solar masses Cygnus X-1 4- 8 solar masses Cal 87 ( LMC ) 5 - 8 solar masses Sco X-1 3 - 10 solar masses GS2000+25 3 - 10 solar masses GX339-4 3 - 10 solar masses 1E1740.7-2942 100 - 10000 solar masses Messier 106 36 million solar masses
So, are black holes real? The answer is that they are the least exotic explanation we can offer for some of the energetic things we are seeing in the universe. No other mechanism has proven to be as helpful in accounting for the great variety of data we have now accumulated. We will never be able to actually see a black hole, but by their handiworks we will come to know them well. We can never directly observe electrons or quarks either, but we know they are there nonetheless. In the court of Nature, sometimes circumstantial evidence is all we will ever have to judge our theories by.
Copyright 1997 Dr. Sten Odenwald
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