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Name: Oscar
Status: other
Grade: 9-12
Location: HI
Country: USA
Date: Winter 2012-2013

Concerning the newly discovered NGC 1277 black hole. Scientists say its size its 4 light days radius, but what does that mean, really? I thought the volume of a black hole infinitesimally small.

When giving the "diameter" of a black hole, astronomers mean the diameter of its event horizon, the boundary within which light cannot escape.

Richard E. Barrans Jr., Ph.D., M.Ed. Department of Physics and Astronomy University of Wyoming


The "size" that scientists refer to is the distance from the center to the "event horizon" - the point at which entering light can no longer escape. While the actual "object" that creates the black hole is infinitesimally small as you pointed out, its incredible mass and consequent gravitational effect extends very far. The "blackness" extends all the way to the event horizon.

Greg (Roberto Gregorius) Canisius College

First, let us get rid of the last part. Black holes are not infinitesimally small. Many are quite large. In the case of the "super black hole NGC 1277" the size is larger than any black hole discovered so far. The meaning of a "light day" is the distance light travels in a day. That is a big number! The speed of light is 299 792 458 (meters / second in vacuum). This is a "defined" quantity so it is "exact". Other physical constants are "adjusted" so that this number does not change. The other physical constants are "changed" so that the speed of light always has this defined value. Now, there are:

(60 sec /min)x(60 min /hr)x( 24 hr /day) = (86 400 sec /day). So in a day light travels 299 792 458 (meters / sec)x(86 400 sec/day) = 2.59x10^13 meters. So 4 of them is about 1 x 10^14 meters. That is a BIG number.

Vince Calder

Hi Oscar,

Thanks for the question. A "light day" is a measure of length. Specifically a "light day" is how far light will travel in one day. A black hole does have mass and its mass is concentrated into a very small (but not zero) volume which results in a large gravitational field. Since the volume of the black hole is not zero, it cannot be infinitesimally small.

I hope this helps. Please let me know if you have any additional questions.

Thanks Jeff Grell

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