Question: I have been interested in astronomy since I was a child. My question arose while I was studying the effects of Lagrangian points between multiple bodies.
Imagine a scenario of a binary (or multiple) neutron stars system. Provided the neutron stars are all very close to their critical mass and are running in a very compacted orbits around each other. Is it possible that the gravitational interaction between the neutron stars, similar to the principle of Lagrangian points, creates a spot of event horizon in the system?
I am deeply intrigued in this question, as I imagine the event horizon would be mobile and deforms corresponding to the motion of neutron stars. Also, with no singularity in the system, the event horizon would probably behave differently compare to those around a black hole. — Peterson
Answer: As you likely already know, Langrange Points are positions where the gravitational pull of two massive objects, such as the Earth and the Sun, precisely equals the centripetal force required for a small object to move with them. This is quite different than an event horizon, which is a boundary in space-time beyond which information cannot be transmitted. In other words, objects which are within an event horizon cannot be detected from an observer outside the event horizon. Event horizons are associated with very massive objects, such as black holes, which create gravitational fields that are so strong that they halt the travel of light propagating from the black hole. Neutron stars are not massive enough to produce event horizons. Also, Langrange Points do not naturally have the physical properties to create a massive object that can lead to an event horizon.
Question: Hi, my name is Kemp and I just finished watching a program from the Discovery Channel entitled “Understanding the Universe”. The program triggered an odd question, do blackholes create dark energy and/or dark matter relative to their size? — Kemp
Answer: No. The only connection between black holes and dark energy is the “darkness” of their names. Black holes are sources of intense gravity from which even light cannot escape. Dark energy is the theoretical entity that accounts for the majority of the energy content of the universe and is responsible for the overall acceleration of the expansion of the universe.
Question: What do black holes look like on radio telescope images? Specifically how does a visible light image of a black hole compare to a radio telescope image of the same black hole?
I’m curious whether gravitational time dilation and redshifting causes visible light to change into radio frequency EM energy, and therefore not be visible. What happens to visible light when it is redshifted too much? Can it turn back into visible light when it leaves the gravity field? Is there a frequency at which there is a required energy to quantize the photons back into visible light? — Ray
Answer: Let me first answer your first question about what black holes look like when measured with a radio telescope. In fact, there are no direct measurements that we can make that tell us what a black hole looks like. Black holes are always measured through indirect means, such as measuring objects in their gravity field that tell us about the black hole’s existence. For example, at radio wavelengths we can measure emission from the water molecule that traces the gas in the disk around a black hole. By measuring the velocity of this water emission we can infer the mass of the black hole.
As for your second question about gravitational time dilation and redshifting, the expansion of the universe does shift emission at shorter wavelengths down to longer wavelengths, such as radio waves. The redshifting of EM waves does not wrap around back to shorter wavelengths, though. The shifting to longer wavelengths monotonically approaches infinite wavelength.
Question: How do black holes form? — Harsh
Answer: I think that my colleagues at NASA have produced a very nice explanation of what black holes are and how they form. In a nutshell, when a massive object has not way to support itself against gravity, it collapses in upon itself. If that object is massive enough, then its gravitational pull can arrest even the propagation of light, making it appear “black” to us.
Question: A 10 solar mass object would have 10 solar masses enclosed in a radius of 30 km. The density of a spherical object scales as M/R3. Relative to the 10 solar mass black hole, how many times denser would a black hole the mass of the Earth be? How many times less dense would a black hole of 1 million solar masses be? — Paul
Answer: As my colleague Cole Miller points out in his description of the properties of black holes and neutron stars, unlike ordinary things (e.g., rocks), which have a size roughly proportional to the cube root of their mass, black holes have radii proportional to their mass. Taking the event horizon of a black hole as the definition of its outer boundary, the event horizon of a nonrotating black hole the mass of our Sun would have a radius of about 3 kilometers. This implies that the more massive the black hole is the denser it is, meaning that larger black holes are not very dense. For example, a one-billion solar mass black hole, which is the type of black hole that is thought to exist at the center of some galaxies (like ours), has an average density just twenty times the density of air.
Question: Thank you for answering! Is the supposed spinning the reason all the body’s caught in the pull stay in a synchronized belt around it? If so, where does a quasar emit? “North and south?” Or anywhere it pleases? I know very little known, but these questions are the reason I can’t sleep at night. — Nathan
Answer: Quasars emit their outflows which are suggested to coincide with the rotation axis of an embedded black hole. This direction is preferred as it represents the direction where the angular momentum is lowest, thus allowing the outflow material to propagate away from the black hole.
Question: Can you help with this new question could a man made neutron star be made to then collapse it into a black hole, but I am not sure if it is answerable. A neutron generator can generate to 108 neutrons per second. A 1 microampere ion beam accelerated at 200 kV to a titanium-tritium target can generate up to 108 neutrons per second. The neutron yield is mostly determined by the accelerating voltage and the ion current level. If you were going to attempt to create a man made neutron star using a neutron generator in space, because this is the only place where you could do this experiment. First using the neutron generator which can make 108 neutrons per second, you would start by putting all the neutrons into one place. As you generate, and put all the neutrons into one place you would probably have a microscopic sphere of neutron matter after a while. So here is the problem, I do not know the exact point in spherical mass where neutrons become stable in a neutron star, because its the pressure from gravity that is compacting them, and stopping them from decaying back to protons. If you knew the exact point in spherical mass where neutrons become stable with gravity, you could calculate the number of neutron generators you would need, with the time that the neutrons would decay back into protons. The other thing I do not know is does the exact point in spherical mass in a neutron star where neutrons are stable, would that exact point in a spherical mass of neutrons have the same strength gravity as a medium sized star to crush the neutrons into place, and keep the neutrons stable, as you were putting them together with the neutron generator. Making a sphere the size of a mile may not be enough because you would have to compact the neutrons together closely in the same way a medium sized star does to keep the neutrons stable from decaying. So does a mile, or more of a sphere of non-compacted neutrons be enough to create a gravitational field as strong as a medium sized star, or does the sphere of neutrons need to be compacted together more to create stronger gravity to hold the neutrons into place. So to put it as simple as possible is putting neutrons together in a sphere a mile or more in spherical mass going to create enough gravity to compact the neutrons into place to keep the neutrons stable from decaying into protons. Getting more material to complete this man neutron star to collapse it into a black hole would not really be a problem if you parked it next to a giant star, because the gravity would draw in the material from the giant star and add it to its own mass to make it larger, and I read somewhere the neutron star would not burn so close to a giants star, to absorb its material, and add it to its own mass. Can you help with any of the questions. — Nicholas
Answer: I think that your question is whether a neutron star can become a black hole. Neutron stars are thought to have masses between the Chandreskhar limit of 1.39 solar masses to about 3 solar masses. If a neutron star gathers more mass and gets to the point where its mass reaches about 10 solar masses, its mass will overcome the neutron degeneracy pressure that supports it against gravity and collapse to become a neutron star.
Question: Regarding galaxy rotation. Although the supermassive black hole at the center of a Galaxy probably isn’t strong enough to speed up the rotation of its more distant stars so as to make the velocity curve level, nevertheless the time-dilation effect of such a black hole will make the orbital rotation speeds of the inner stars appear slower than it actually is as seen from the point of view of an observer outside the galaxy in question. Couldn’t this be a more reasonable explanation rather than hypothesizing ‘dark matter’? — Tom
Answer: I don’t think that time dilation will affect the observed speed of stars near a black hole as observed from a point far from the black hole. Time dilation affects what local observers measure, which means that the time measured by an observer at a star near a black hole will see time running slower than the time measured by the far-away observer.
Question: Just to clarify, Hawking’s radiations takes place simultaneously to the black hole growing? and, then becomes dominant once the black hole stops growing? Further, having depleted with a final outburst/explosion leaving behind ” nothing ” – to what level is this “nothingness” defined? No mass,no energy,no waves, no “quantum fluctuations”?..no dark matter, no dark energy? It seems to be that it is becoming increasingly difficult to define ” nothing “?! — Simon
Answer: As you have said, “Hawking Radiation” is the theoretical process by which black holes that spin lose energy (and, therefore, mass). For black holes that lose more mass than they gain (i.e. those that do not collect a lot of mass due to accretion), in theory a black hole can lose all of its mass and “vanish”. This means that all matter associated with the black hole has been emitted as energy.
Question: Does a black hole move through spacetime or does it draw spacetime to “it” giving the illusion its moving? — Nigel
Answer: Black holes move through spacetime just like any other object in the universe. It is true, though, that black holes “warp” spacetime more than other objects due to their extremely intense gravitational field.