Questions and Answers

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  1. How do larger-than-the-sun masses form?
  2. Why does a black hole's mass have to be compressed into a point? Why does time stop at that point?
  3. Where can I find more information on black holes, especially video clips, pictures, and other electronic media?
  4. Where does all the stuff that gets sucked in go and does a black hole have the same amount of gravity as it did when it was a star?
  5. What do dark asteroids, dark matter and black holes have in common?
  6. Do you have any ideas that I would be able to incorporate into a visual presentation?
  7. How does particle-antiparticle theory relate to black holes?
  8. Do you have more information on neutron stars?
  9. Considering Black Holes have no volume how could they have a size?
  10. If light has no mass, then how does a black hole 'suck' in light?
  11. When a supernova explodes, what collapses and why does it form a neutron star?
Q Joe Fruscione, a student at the University of Delaware asks: "How is it that larger-than-the-sun masses actually form?"

A Stars form from clouds of dust, mostly hydrogen, that float around in space. The particles in the cloud become attracted to each other with their own gravity and start to move towards each other. This is called gravitational collapse. As the cloud collapses the particles rub together and heat up. Pretty soon, the particles are bunched together enough to stop the collapse. Heat keeps building up and, when it gets hot enough, it begins to burn hydrogen in a nuclear reaction called fusion. It puts out heat and light and is now a star. The outward pressure of the heat and light counteracts the inward pull of gravity, and the star remains stable.

There are limits on how much mass a star can have. If the mass is more than about 100 solar masses, the outward pressure is greater than gravity, and the star blows apart. If the mass is less than about 0.08 solar mass, the star will fail to light. If Jupiter were 84 times more massive, it would be a star.

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Q Kevin Hanson writes, "My understanding of black holes is that it must be dense enough that light can not escape from it. Your statement that it is a pin point doesn't make sense to me. It would seem that it could be 100 miles in radius and still be a black hole. Also why would time stop. I don't see a reason for this."

A When a star is burning, it holds its shape because the outward force of its radiation (light, heat, etc.) balances the attractive force of its gravity. When very massive stars stop burning, the force of gravity is so strong that not even the densely-packed mass can support itself and

. . . the star quickly shrinks in size until finally it is crushed, presumably to the size of a pinhead, then to the size of a microbe, and finally to a realm of size smaller than ever measured by humans. At this point, according to theory, there is infinite density. This point is called the black hole singularity. (Hewitt, et al. Conceptual Physical Science. New York: HarperCollins, 1994. pp 713-14)
Hewitt, et al. describe a black hole singularity as having "infinite density." How can this be? Density is mass per unit volume, defined by the relationship:

Density=Mass/Volume

When a star's mass is collapsing into a singularity, its volume decreases towards zero. As the volume decreases towards zero, the density, which is inversely proportional to the volume, rises towards infinity. So a singularity, because it has no volume, has infinite density.

According to Stephen Hawking, as a black hole singularity forms, "it would be an end of time for the collapsing body" (Hawking, Stephen. A Brief History of Time: From the Big Bang to Black Holes. New York: Bantam Books, 1988. p 88). This is because space-time (the 3 physical dimensions and the 4th dimension of time) does not exist at the singularity. A black hole singularity is analogous to what the Universe was, in theory, before the Big Bang when all matter was compressed into a very tiny point. Time, for us, began when that singularity expanded, creating 4 dimensional space-time.

Beyond a certain distance from the black hole singularity, however, its mass affects other things (stars, planets, Federation starships) gravitationally, just as the Sun affects the Earth. Objects orbit black holes-- that's one of the ways astronomers search for them. But within a certain distance of a black hole singularity, the gravitational pull is so strong that nothing--not even light--can escape. This distance is called the event horizon. The event horizon is not a physical boundary, but a gravitational one. It is a point of no return for anything that crosses it. The structure of a black hole is something like this:


Since light cannot escape from the event horizon of a black hole singularity, it radiates no light and thus appears black. The radius of the area defined by the event horizon is proportional to the mass of the black hole--roughly three kilometers for each solar mass.

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Q Paulino Leon writes, "I'm a student at ITESM system in Ciudad Juarez, Mexico. I am doing research on "blackholes," and I would like to ask you if you could be so kind to send me information concerning this topic or some locations on the internet where I could be able to find this type of information."

Winfield Peterson writes, "I am an eighth grade student doing research on black holes. I recently visited your World Wide Web site during my researching and I thought you might be able to help me find more information. I would appreciate any video clips, pictures, or other electronic media as well as information. I am especially interested in the following more specific information on black Holes:

  1. How black holes form.
  2. Any pictures or video clips of black holes.
  3. Locations of black holes.
Thank you very much for your time."

A One of the best places to find more information about black holes (or anything else) is at your local library. Encyclopedias, astronomy and physics textbooks, and magazines like Sky and Telescope are great places to find stuff about black holes.

The Internet also has a lot of information. You can find many Web links on my black hole page or use Web search engines to find what you want. NASA has all kinds of stuff, including pictures from the Hubble telescope. I found some cool gifs and mpeg animations of black holes at these sites:


Astronomers are finding black holes all the time. Many astronomers think that spiral galaxies (like our own galaxy, The Milky Way) rotate around gigantic black holes. The following is a list of some black holes, but more are being found all the time:

Cygnus X-1
Circinus X-1
V404 in Cygni
V861 Sco in Scorpius
LMC X-3 in the Large Magellanic Cloud
You can probably find these black holes in an Astronomy textbook or magazine. Your librarian may be able to help you search for science magazines with articles about other black hole discoveries. When astronomers find a black hole, they usually write up an article and publish it in a magazine like Nature, so others can learn about what they found. Some of the stuff they write is pretty technical and hard to read, but you can get locations and dates. Plus, using a variety of sources helps make an impressive bibliography for your report.

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Q Larry Lane asks, "I have 2 questions. 1, where does all the stuff that gets sucked in go? and 2, do black holes have the same amount of gravity as it did when it was a star?"

A I'll start with your 2nd question first. As I discussed above, all the mass of a black hole gets crushed by gravity into a single point called a black hole singularity (see Kevin Hanson's question). Surrounding the singularity is an area called the event horizon . Inside the event horizon, the gravitational pull from the singularity is so strong that not even light can escape (making it black). But outside of the event horizon, planets or stars are affected just as if the mass in the singularity was the mass of a burning star. The amount of mass is what's important, not its arrangement.

If the Sun suddenly collapsed into a black hole (although it does not have enough mass to do so), all its mass would be crushed into a single point with the event horizon spanning about 6 kilometers across. The Earth would continue to follow the same orbit it is in now. That's because the force of gravity between 2 things is dependent upon their masses and their distance. If there is no change in mass, there is no change in distance.

When stuff does cross the event horizon of a black hole, it is pulled into the singularity, and its mass grows. Doesn't that mean that a black hole will keep on growing until it consumes all the matter in the Universe? Luckily, no. Stephen Hawking discovered that when virtual particles (undetectable quantum particles that carry gravity and light) enter the event horizon and get sucked into the singularity, they use up more energy than they have and, thus, contribute negative energy to the black hole (Hawking, Stephen. A Brief History of Time: From the Big Bang to Black Holes. New York: Bantam Books, 1988. pp106-7). Because of this, the mass of the singularity decreases and the black hole eventually "evaporates" away. Lucky for us, huh?

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Q Ann O. Heberlein asks, "I would like to know what dark asteroids, dark matter and black holes have in common. My main topic of research is dark asteroids. As there is little written on this subject, I've taken to gathering information on dark matter. (Stuff that doesn't reflect light.)

I recently finished reading a book by Paul Hapern entitled, COSMIC WORMHOLES. . . . He referred to exotic matter as some thing in a black hole. What is exotic matter?"

A Asteroids (also called "minor planets") are large chunks of debris that orbit the Sun, mostly in the asteroid belt. The asteroid belt, situated between the orbits of Mars and Jupiter, contains more than 100,000 asteroids (Zeilik and Gaustad. Astronomy: The Cosmic Perspective. New York: John Wiley & Sons, 1990. p 331). The largest asteroid is Ceres, which measures 940 kilometers in diameter, followed by Pallas (588 km) and Vesta (576 km).

Asteroids are classified by how much sunlight they reflect. And the amount of light an asteroid reflects depends on what it is made of. S-type asteroids, for example, contain silicate materials and reflect about 15% of the light that hits them. Dark asteroids, or C-type asteroids, contain carbon and magnetite and only reflect about 2 to 5 percent of sunlight. M-type asteroids contain other metals and reflect about 10% sunlight (Zeilik and Gaustad. Astronomy: The Cosmic Perspective. New York: John Wiley & Sons, 1990. pp 332-334). About 3/4 of all asteroids are dark asteroids, which hang out in the outer fringes of the asteroid belt, closer to Jupiter.

Dark matter is stuff that we know is present in galaxies, but we cannot see it. You can use different methods to measure the mass of a galaxy. You can measure the amount of light--the more light there is, the more stars the galaxy contains, and the more mass it has. You can also see how much gravity there is by measuring how fast a galaxy is spinning. The faster it spins the more gravity it has. The more gravity it has, the more massive it is. But when you compare the measurements you got from these two techniques, there is a discrepancy; the measurements do not agree. Astronomers find that 90 to 99% of the mass in all galaxies, and thus of the Universe, is in a form that we cannot see. This "missing mass" is called dark matter.

Scientists do not agree on what they think dark matter actually is. Some think it is big stuff--black holes, neutron stars, brown dwarfs, and other hard-to-see matter like dark asteroids. Other scientists, usually particle physicists, say that the missing mass consists of exotic matter--sub-atomic particles that cannot be seen. Examples of exotic particles include Higgs bosons, gauginos, sleptons, squarks, W' and Z' bosons, X and Y bosons, Majorons, familons, axions, paraleptons, ortholeptons, technipions, B' hadrons, magnetic monopoles, and excited leptons. (Fun to list, but otherwise kind of useless information for "regular" people.) Exotic particles are not "ordinary" matter--like you and me and everything we see. They are predicted by theory and have yet to be detected (Austern, Matt. "The Particle Zoo" online in Sci.physics FAQ part 4, 1996). For a more detailed discussion of dark matter and its detection, see my paper Cosmic Hide and Seek: the Search for the Missing Mass .

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Q TooSence writes, "I am a student in the eighth grade and I am doing a project on black holes for the talented and gifted program that I am in. I have done much research on the topic and have written my report, but I would like to do a visual presentation. Do you have any ideas that I would be able to incorporate into a visual?"

A Using visuals for black hole projects is difficult because you cannot see a black hole. You can photograph objects moving around black holes, and you can animate black hole sequences, but there are no pictures of black hole singularities.

Here are some ideas that you can use for your presentation:

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Q Michael McCaffery writes, "I am doing a research project for my high school and I have found many things regarding the formation, function, and other things about black holes. I recently attended a Stephen Hawking lecture and he talked about particle- antiparticle theory. I think this theory relates in some way to black holes but I am not quite sure how."

A Particles and antiparticles that appear in empty space adjacent to black holes greatly affect the black holes. "Empty" space isn't all that empty. Fluctuations of electromagnetic and gravitational fields produce pairs of light - or gravity-carrying particles (these are called "virtual" particles because we cannot detect them). These particle pairs consist of a particle and an antiparticle which exist only briefly. They appear, move apart and then together, annihilating each other.

It might seem that these particles are created from nothing, which is a violation of conservation of energy laws. but one particle has positive energy and the other has negative energy. Added together, their combined energies = 0.

What do short-lived pairs of virtual particles have to do with black holes? When particle pairs appear near a black hole event horizon, one of the particles may fall into the black hole, leaving the other particle free. This has two effects:

  1. Without its partner, the escaping particle becomes a real particle, and appears, to us on earth, as radiation coming from the black hole (called "Hawking radiation"). Actually, Hawking radiation radiates from just outside the black hole.

  2. The particles with negative energy that fall into the black hole contribute negative energy to the black hole. This decreases its mass, and the black hole eventually "evaporates" away.

(Source: Hawking, Stephen. A Brief History of Time: From the Big Bang to Black Holes. New York: Bantam Books, 1988. pp 105-107)

Because we are living, thinking beings, we can appreciate the irony of this situation. Black holes--giants so powerful that nothing can escape their grasp--are ultimately defeated by tiny particles smaller than an atom.

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Q Keith J. Wheeler writes, "I'm doing a small research paper for a Earth Science Solar System course. If you had more information on neutron stars I would greatly appreciate all the help I can get. If you can, please keep it as "formula free" as possible. Thank you."

A Most of the stuff in astronomy textbooks regarding neutron stars is pretty technical. But I enjoy trying to imagine what they look like.

In my Black Holes and Neutron Stars page, I say that neutron stars spin very fast. How do we know? Because some of them are pulsars. Astronomers can measure the rotation rate of a neutron star in the Crab Nebula, for example, because it appears to pulse on and off about 30 times a second. Remember, the pulse is really a flashlight- like beam of light (or x-rays, etc.) that sweep around as the neutron star rotates. A rotation rate of 30 times a second means that the neutron star spins around completely 30 times every second. If you were standing on the surface of a neutron star spinning that fast, you would be going about 1,890,000 meters per second. That's pretty fast!

Why do neutron stars spin so fast? The answer is something called conservation of momentum. We can see conservation of momentum here on earth when we watch an ice skater spinning on the ice. When she pulls in her arms and leg, she spins faster. That's because the speed of the spin is dependent on the distribution of the mass, that is, how spread out something is. When the skater pulls her mass in closer to the axis of the spin, there is more energy available and her body spins faster. That's conservation of momentum.

The same thing happens to a star. Stars, including our Sun, spin on their axis like the Earth does. When a spinning star collapses and forms a neutron star, its mass is not as spread out. But it conserves its momentum by spinning faster, just as a skater does.

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Q Peter Harkess writes, "In Stephen Hawkings book, A Brief History of Time, he refers to Black Holes as having a "size". Considering Black Holes have no volume how could they have a size? Does "size" refer to the mass of the Black Hole or the distance between the Event Horizon and the singularity? If the latter is true, does the distance between the Event Horizon and the singularity vary and what determines this?"

A When Hawking refers to a black hole's size, he is talking about the size of the event horizon. The size of the event horizon depends upon how much mass the singularity contains. If you fell into a black hole, the event horizon would grow a bit because the contribution of your mass to the singularity would increase its gravitational strength. This mass-size relationship is a direct one--the radius of the event horizon grows 3 km for each solar mass contained in the singularity.

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Q Nicholas Clifford writes, "If light has no mass, then how does a black hole 'suck' in light?"

A Since light has energy (a unit of light energy is called a photon) it has an equivalent mass, defined by Einstein's famous relationship E=mc^2. Light is a unique thing--sometimes it acts like moving energy and other times like a particle.

To understand how gravity can bend and trap light, you must think in terms of Einstein's Theory of General Relativity, which deals with gravitation. Before Einstein, the model of gravity was Newton's: gravity as a force that acts on matter with an intensity depending upon mass. But Einstein's model of gravity is one of curved space- time, with curvature dependent upon mass.

Light takes the shortest distance (and thus shortest time) between two points. In "flat" space, we see this as a straight line. But in curved space-time, the light appears to bend. I say "appears," because that's how it looks to an observer at a distance. When you're talking about relativistic stuff, you have to consider different viewpoints. If you were the light ray, you would see yourself moving in a straight line because space would appear to be "flat."

So a black hole doesn't really suck in light--it's intense gravitational field curves space-time so much that a straight line away from it leads straight back to it.

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Q Ben Sosinski writes, "I was wondering that if a supernova explodes, what collapses and why does it collapse, to form a neutron star?"

A To understand what explodes and what collapses, we need to talk about what happens during a supernovae explosion.

Young stars are hydrogen, and the nuclear reaction converts hydrogen to helium with energy left over. The left over energy is the star's radiation--heat and light. When most of the hydrogen has been converted to helium, a new nuclear reaction begins that converts the helium to carbon, with energy left over. This process continues from carbon to oxygen to silicon to iron. Nuclear fusion stops at iron. If you could slice a very old star in half, it may look (sort of) like this:


The star has layers of different elements. The outer layers of hydrogen, helium, carbon, and silicon are still burning around the iron core, building it up. Eventually, the massive iron core succumbs to gravity and it collapses to form a neutron star. The outer layers of the star fall in and bounce off the neutron core which creates a shock wave that blows the outer layer outward. This is the supernovae explosion.

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