Black Holes in Binary Systems

After collapse to the neutron star stage, stars with masses less than 2-3 solar masses should remain neutron stars, gradually radiating away their energy, because there is no known mechanism for further combination, and forces between neutrons prevent further collapse. But this neutron force is the last stand, and our best calculations indicate that this repulsion which prevents collapse cannot withstand the gravity force of masses greater than 2 to 3 solar masses. Such neutron stars would collapse toward zero spatial extent - toward a "singularity". Once they collapsed past a certain radius, the "event horizon", then even light could not escape: black hole.

Since black holes by their very definition cannot be directly observed, proving their existence is difficult. The indirect evidence for the black hole Cygnus X-1 is a good example of the search for black holes. Another excellent candidate in an object which was discovered in one of the Magellanic Clouds.

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Cygnus X-1

Doppler studies of this blue supergiant in Cygnus indicate a period of 5.6 days in orbit around an unseen companion. The mass of the companion is calculated to be 8-10 solar masses, much too large to be a neutron star.

1. An x-ray source was discovered in the constellation Cygnus in 1972 (Cygnus X-1). X-ray sources are candidates for black holes because matter streaming into black holes will be ionized and greatly accelerated, producing x-rays.

2. A blue supergiant star, about 15 times the mass of the sun, was found which is apparently orbiting about the x-ray source. So something massive but non-luminous is there (neutron star or black hole).

3. Doppler studies of the blue supergiant indicate a revolution period of 5.6 days about the dark object. The calculated mass of the dark object is 8-10 solar masses; much too massive to be a neutron star which has a limit of about 3 solar masses - hence black hole.

This is of course not a proof of a black hole -- but it convinces most astronomers.

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Magellanic Cloud Black Hole

An object discovered in one of the distant Magellanic Clouds in 1982 had all the earmarks of a black hole. How can "black hole" status be verified?

Steps of investigation

1. Observe the orbital period of the binary system
2. Observe the spectrum of the visual star, compute the orbital speed from Doppler effect, calculate orbit radius
3. Compute sum of masses from velocity and period
4. Estimate the mass of the visual from spectral type
5. Deduct mass of visual to find mass of dark object

The conventional criterion is that if the mass of the dark object is >3 solar masses, then it must be a black hole.


Result
41 hour period
Orbit radius 7 million miles
14-18 solar masses total
Visual about 6 solar masses
8-12 solar masses in dark object
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