Harvard-Smithsonian Confirms Black Hole Existence after 40 Years | Hacker News Reader

Harvard-Smithsonian Confirms Black Hole Existence after 40 Years

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45 pointschermor10 comments

Wait, it's only 5 million years old? That seems ludicrously young given astronomical timescales.

I'm also curious how such a thing came to be. How did we get a binary system with a much smaller, and presumably much older star (since it is now a black hole), accompanied by a much larger, younger star?

Actually, the stars were very likely created together. The star that made the black hole was likely much more massive. More massive stars have shorter lifetimes than less massive ones (because the luminosity and consequently the rate of fuel burning goes as something like M^3.5), so it died before the companion.

Original article from PhysicsWorld: http://physicsworld.com/cws/article/news/46362

From the article:

  "Black holes, if you recall, are created when stars run out of fuel and die. In dying, they collapse into a much smaller mass with such enormous gravity that it sucks in just about anything that comes light years near it."

I may be wrong, but doesn't the mass of the black hole roughly equal the mass of the star? I know some is lost during the explosion, but I thought the mass stayed roughly the same and just the volume significantly decreased.

Essentially correct. The dying star does not collapse into a "smaller mass", the amount of mass stays the same (difference between the stars original mass and the cast off from the preceding supernova). It just gets crushed into a very tiny volume.

Well, not really. The mass of the black hole created equals the amount of mass that falls into it, of course, and to first order the mass of the BH will be equal the mass of the iron core of the star. That mass is however quite a bit smaller than the mass of the star, and very much smaller than the initial mass of the star. (A star has to start out with a mass > 8* the mass of the Sun to go supernova, and that will only guarantee that it forms a neutron star whose mass will be of order 1 Solar mass. The rest is lost first during the life of the star, as massive stars are so luminous they drive their own matter off from the surface, and then ejected in the explosion itself.

Edit: The article has a number off, as it states that the black hole has a mass of 15 Solar masses and that the companion is 19 times larger. The original article says the companion is 19 Solar masses, which makes more sense.

I know some is lost during the explosion, but I thought the mass stayed roughly the same and just the volume significantly decreased.

The first part answers the second. A lot is lost in the explosion.

I believe the author means to use "small" as a descriptor of volume rather than of measure, and "mass" as a generic noun rather than a quantity. In fact, interpreting "a small mass" as "something light" would be incorrect without a constant density.

It's not a great sentence, because it's easy to get confused like that, but it is correct.

Interesting that Kip Thorne wanted a year of porn and Hawking wanted four years of political satire, although the whole thing was obviously a joke.

Even for normal low standards of science journalism, this article is awful. "...such enormous gravity that it sucks in just about anything that comes light years [sic] near it." An object passing light-years away would follow the same hyperbolic trajectory as if interacting with a 15 solar mass peanut butter sandwich.

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