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The Question

(Submitted April 15, 1997)

I just finished reading Kip Thorne's book "Black Holes and Time Warps". I seem to recall an article in the last 6 months or so, I thought from the NY Times Science pages, about a new theory on collapsing stars that allowed for black holes to form from stars much smaller than previously thought. The mechanism as I recall was for a short burst of radiation release such that the neutron pressure that normally keeps the star from collapsing is released. I recall that one of the authors was in his 80's and the other in his 90's. Could you give me a reference to this work?

The Answer

I believe you are referring to a paper by Gerald Brown with Hans Bethe called "A Scenario for a Large Number of Low-Mass Black Holes in the Galaxy". The paper was published in the Astrophysical Journal, volume 423, page 659, in March 1994. The mechanism they propose "softens" the equation governing the collapse of a neutron star so that stars with lower mass that previously thought can produce black holes after a supernova explosion. Hans Bethe turned 90 years old this past July, and was presented by the APS with a prize that will be set up in his name. The prize will be awarded annually to an astronomer or nuclear physicist (or someone who is both!). Hans considers himself both an astronomer and a nuclear physicist, and is the person who explained the generation of energy in the Sun by the fusion of hydrogen to helium. He won the Nobel prize for this research in 1967. He is a national treasure, and as you will see at http://www.nobel.se/physics/laureates/1967/bethe-bio.html he is still a vibrant and happy member of the physics community. In fact, he is still publishing. His latest paper in the "Ap. J" was in December, on Supernova Shocks. I'm sorry I don't know the age of the other author of the paper.

Here's the blurb about the project that appeared in Physics News Update:

August 25, 1993

WHY DON'T MORE SUPERNOVAS LEAVE BEHIND PULSARS? The venerable Hans Bethe, pioneer in the study of stellar nuclear physics for decades, and Gerald Brown of Stony Brook, propose that some collapsing stars---with masses as low as 18 solar masses---refrain temporarily from vanishing into a black hole and instead pause at the neutron-star level of compactification long enough to trigger a supernova explosion. The residual core would thereafter shrink without a trace into a black hole. Bethe and Brown invoke a hypothesis about neutron stars introduced in the last few years by Brown and others. According to this model, as the density at the core of a collapsing star exceeds three times the density of ordinary matter, electrons might not necessarily combine with protons to form neutrons, as was thought, but might instead spawn K mesons and neutrinos. In place of the traditional pure neutron matter, the collapsing core would then be a more compressible mixture of protons and neutrons, buoyed up for a time by the restless motions of the neutrinos. When they depart the collapse would continue, leaving behind either a "nucleon star" or a black hole. (Science, 13 August 1993.)

Regards,
Padi Boyd,
for the Ask an Astrophysicist team

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