goutal@dec-parrot.UUCP (03/02/86)
I'm not an expert, I'm asking for an expert answer on behalf of my six-year-old daughter (who's no expert either). We have access to independent facts in the form of her encyclopedia (at the least), but need someone to pull them together somewhat... The question she asks is, why don't the comets get gobbled up by black holes out beyond the planets? Or, how probable is it that any given comet might get thus, er, eaten? (Probably not accurate to say "destroyed".) Or, how many comets might might thus be eaten over a period of time? The answer I gave her was that, as far as anyone knows, black holes are not all that common, at least not ones that are big enough to do that kind of damage. We wondered if a black hole that was big enough (however you want to measure "big") to absorb comets might be big enough to be detectable in the orbits of the known planets, thus qualifying as "Planet X" or whatever, that was what Pluto was supposed to be but apparently isn't after all. Can anyone shed any light on this for us? -- Kenn Goutal ...decwrl!dec-rhea!dec-parrot!goutal
nather@utastro.UUCP (Ed Nather) (03/03/86)
In article <1457@decwrl.DEC.COM>, goutal@dec-parrot.UUCP writes: > The question she asks is, why don't the comets get gobbled up by > black holes out beyond the planets? > > Can anyone shed any light on this for us? Hidden in the question is (what appears to be) an assumption that is incorrect: that black holes are better at eating distant things than an equivalent amount of "luminous mass" would be. Not so: their ability to attract and eat things is proportional to their mass and inversely proportional to the SQUARE of the distance to the "eatee" just like anybody else. A massive star that becomes a black hole is no better at cosmic gastronomy as a black hole than it was as a star. I guess the basic reason cosmic things don't eat each other very often is the enormous distances between them. Despite their appearance on photographs as near-solid objects, a galaxy of stars is mostly empty space. If you shot through one, even edge-on, the chance of hitting a star (getting eaten) is very small; you'd get bored long before you'd get eaten. -- Ed Nather Astronomy Dept, U of Texas @ Austin {allegra,ihnp4}!{noao,ut-sally}!utastro!nather nather@astro.UTEXAS.EDU
ethan@utastro.UUCP (Ethan Vishniac) (03/03/86)
In article <457@utastro.UUCP>, nather@utastro.UUCP (Ed Nather) writes: > In article <1457@decwrl.DEC.COM>, goutal@dec-parrot.UUCP writes: > > The question she asks is, why don't the comets get gobbled up by > > black holes out beyond the planets? > > > > Can anyone shed any light on this for us? > > Hidden in the question is (what appears to be) an assumption that is > incorrect: that black holes are better at eating distant things than > an equivalent amount of "luminous mass" would be. Not so: their > ability to attract and eat things is proportional to their mass and > inversely proportional to the SQUARE of the distance to the "eatee" > just like anybody else. A massive star that becomes a black hole is no > better at cosmic gastronomy as a black hole than it was as a star. > In fact, it is arguable that black holes are worse at consuming passing objects since the cross section for a direct hit is miniscule. A black hole the size of the sun would have a radius of a few kilometers. On the other hand, the radius for tidal disruption would be the same. -- "Ma, I've been to another Ethan Vishniac planet!" {charm,ut-sally,ut-ngp,noao}!utastro!ethan ethan@astro.UTEXAS.EDU Department of Astronomy University of Texas
ethan@utastro.UUCP (Ethan Vishniac) (03/03/86)
In article <458@utastro.UUCP>, ethan@utastro.UUCP (Ethan Vishniac) writes: > objects since the cross section for a direct hit is miniscule. A black > hole the size of the sun would have a radius of a few kilometers. On ^^^^ Of course I meant to say "mass" -- "Ma, I've been to another Ethan Vishniac planet!" {charm,ut-sally,ut-ngp,noao}!utastro!ethan ethan@astro.UTEXAS.EDU Department of Astronomy University of Texas
steve@jplgodo.UUCP (Steve Schlaifer x3171 156/224) (03/04/86)
In article <1457@decwrl.DEC.COM>, goutal@dec-parrot.UUCP writes: > I'm not an expert, I'm asking for an expert answer on behalf of my > six-year-old daughter (who's no expert either). > > The question she asks is, why don't the comets get gobbled up by > black holes out beyond the planets? > Essentially, comets are very close to the sun, stars and black holes, which are just really big stars that have collapsed into themselves, are very far away. In the same way that the stars that we see at night don't have any discernable affect on the motions of the planets or the comets, black holes also have no affect on the planets or comets. They are just too far off for their gravity to have much effect. Anything that was both close enough and big enough to gobble comets, would also cause very large disturbances in the orbits of the planets which would be easily detected by modern (or even primitive) instruments. -- ...smeagol\ Steve Schlaifer ......wlbr->!jplgodo!steve Advance Projects Group, Jet Propulsion Labs ....group3/ 4800 Oak Grove Drive, M/S 156/204 Pasadena, California, 91109 +1 818 354 3171
notch@srcsip.UUCP (Michael k Notch) (03/13/86)
In article <1457@decwrl.DEC.COM> goutal@dec-parrot.UUCP writes: >The question she asks is, why don't the comets get gobbled up by >black holes out beyond the planets? Or, how probable is it that >any given comet might get thus, er, eaten? (Probably not accurate >to say "destroyed".) Or, how many comets might might thus be >eaten over a period of time? > I thought about your questions and even though I am not an expert, my answer is this: if planets like Earth and Jupiter do not get "sucked" into a Black Hole, that indicates that we may be too far away from a Black Hole. -- "But... What about Naomi?" USENET: ihnp4!umn-cs!srcsip!notch Michael k Notch philabs!srcsip!notch Honeywell SRC/SIP/MVT MN17-2349 --