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Black Holes and Neutron Stars

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How do Neutron stars form? What happens if you get sucked in by a black hole?
Why don't neutron stars and black holes give off light? How do scientists know there are such things as Neutron Stars and Black Holes?
How does Einstein's relativity explain gravity?
Do galaxies have black holes at their centers?
Can you describe the 'annihilation' when matter and anti-matter meet?
Since black holes bend light rays, would a star or galaxy that is behind the black hole appear in two or more different positions?
When a star supernovas, what is the time differential between the flash of light Earth astronomers see, and the x-rays, neutrons, or what-have you reaching Earth?
What's the difference between pulsars and neutron stars?
What is the life cycle of a star?
How do gravitons escape from a black hole?
How much "basic" astronomy and math is required to learn about deep-space objects such as black holes and neutron stars?
What is the temperature of a black hole? Do they spin in one direction, do they have tops and bottoms? Do they move in a direction?

Josh Gannon writes, "How do the Neutron stars form? What do Black Holes do, and if you get sucked in by one what happens?"

Neutron stars form when stars die. While stars are burning, the heat in the star pushes out and balances the force of gravity. When the star's fuel is spent, and it stops burning, there is no heat left to counteract the force of gravity. Whatever material is left over collapses in on itself. Stars that have about 3 times the mass of the Sun compact into neutron stars. A supernovae explosion is usually associated with the formation of neutron stars. 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 the left over energy released as radiation. This process continues converting the 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:

Josh Gannon writes, "I was wondering how come neutron stars and black holes don't give off light? Also I wanted to know if Scientists really are sure if there is such things as Neutron Stars and Black Holes, and if they are sure how to they know?"

Neutron stars and black holes don't give off light because they are not burning. Neutron stars and black holes are the result of stars that have used up all their fuel and can no longer sustain a nuclear reaction. That is why they collapse.

Scientists are reasonably sure that neutron stars and black holes exist because both theory and observations support their existence. But scientists long ago claimed that the Sun circled the Earth, and theory and observation confirmed that notion. It is certainly possible that the theories are wrong and the observations are of something different.

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Jon Clark writes, "After reading about general relativity I find I am more at a loss to understand what gravity is than when I thought Newton had it right. If, like Einstein said, gravity can be considered a warp in the fabric of spacetime, then how am I to understand what causes the mutual attraction of two masses in space which are distant enough to not be affected by the warp? For example, two galaxies which are billions of light years away from each other apparently have a gravitational effect on each other. How and why?"

If two masses have a mutual attraction, then they are close enough to be affected by each other's warp in space-time. Both Newton and Einstein deal with the same thing--just in different ways. Newton's laws quantify the mechanics behind a force that one body seems to have on another (or more accurately, that bodies have on each other). Einstein disposes of the notion of a force and explains the effect as curvature of space-time. Newtonian thinking says that galaxies that are billions of light years apart affect each other because of gravity; Einstein says they affect each other because of how their masses curve space-time. Newton's laws are descriptive--they tell what happens, where Einstein's theories attempt to tell how.

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Dave Garrison writes, "Is it safe to assume now that all (or at least the vast majority of) galaxies have black holes at their centers? (And if so, how did the writers of Star Trek V think they could get away with 'The Great Barrier' deal at the center of the Milky Way?)"

The high concentration of mass measured at the center of galaxies suggests that black holes reside there. Because our own Galaxy's center is largely obscured from view by interstellar dust, it's difficult for astronomers to see what is there. Radio and infrared observations show stars and gas moving very fast near the center. But as you get closer to the center, you would expect stars to slow down because there is less mass to exert gravitational force--most of the matter is located farther out. Also, the high rotational speeds observed suggest a huge mass to keep the gas molecules from flying away. Scientists figure an object with a mass of several million times that of the Sun resides at the center of our galaxy, and that object is most likely a black hole (see http://www.sciencenews.org/sn_arch/10_5_96/fob1.htm for more information). Observations of other galaxies suggest a similar mass distribution.

A renegade Vulcan commandeers the Enterprise and takes her to Galactic center in Star Trek: The Final Frontier. According to Lawrence M. Krauss, the trip would take about 15 years to go the 25,000 light years from here to there at Warp 9 (about 1500 times the speed of light). Obviously, the Enterprise made the trip in less time or we would still be watching the movie. Science fiction writers need to take certain liberties to make their stories exciting. For example, if you and I accelerated to even near-light speeds, like they do in Star Trek, the g-forces would tear us apart. The "Great Barrier" at Galactic center serves plot development, builds audience tension, and provides a framework for special effects, just as the notion of traveling at faster-than-light speeds does. (Source: Krauss, Lawrence. The Physics of Star Trek. New York: HarperCollins, 1995.)

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Dave Garrison writes, "Everyone speaks about 'annihilation' when matter and anti-matter meet. Can you describe this process? If it is solely a release of energy . . . what type? ie x-rays, gamma rays, etc. Does the resultant mass just go to zero?"

Antimatter is made of atoms with oppositely charged particles--negatively charged nuclei and positively charged electrons. The electron's antiparticle, for example, is a positron. Scientists can produce antimatter particles in particle accelerators, but matter and antimatter particles can also appear in "empty" space because of fluctuations in electromagnetic and gravitational fields. These "virtual" (mass-less) particles exist only briefly--they appear, move apart, and then back together. They annihilate each other and the result is pure radiation. The nature of the radiation depends on what the particles are. If positrons and electrons annihilate, they produce gamma radiation at 511 kilo-electron volts; protons and antiprotons produce a spike at around 1 Giga-electron volt. (Source: Krauss, Lawrence. The Physics of Star Trek. New York: HarperCollins, 1995.)

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Phil McDonald writes, "I understand that there is a horizon at which light is bent by a black holes gravity (neither sucked in nor unmolested). If this is so, would it not be possible (in theory) to view the same star or galaxy in two or more different positions in the night sky. In extereme cases, would the light not even give different red-shift readings due to the extra distance traveled? If light that we view has been bent by a black hole, would it not appear to wobble slightly, due to the fact that the black hole would not have its mass perfectly evenly distributed as it rotated ?"

Yes, it is possible to see an object in two or more positions due to the gravitational lensing effect. Gravitational lensing was first discovered in 1980 when astronomers Robert Carswell and Ray Weynmann noticed 2 quasars (0957+561A and 0957+561B) that were very close together with the same redshift.

They found that the two quasars were actually the same, and an intervening galaxy was acting like a lens to produce a double image. (Source: Zeilik, Michael., and John Gaustad. Astronomy: The Cosmic Perspective. New York: John Wiley & Sons, Inc., 1990.)

Black holes and other dark matter produce gravity lensing events in much the same way. But gravitational lenses don't always produce a double image. Depending on alignment and mass distribution, an object can appear as arcs or double, triple, and quadruple images. Eric Ostrander found a quadruple gravitational lensed object while processing Hubble Space telecope images.

This image was created with support to Space Telescope Science Institute, operated by the Association of Universities for Research in Astronomy, Inc., from NASA contract NAS5-26555 and/or Grant [number], and is reproduced with permission from AURA/STScI.

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Suzanne Lennartz writes, "If a star supernovaed and then collapsed to form a neutron star, what is the time differential between the flash of light Earth astronomers would see, and the x-rays, neutrons, or what-have you reaching Earth? Can you name any recently supernovaed neutron stars, and if and when any emissions from them might reach earth? "

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.

When a star collapses to form a neutron star, the tremendous pressure from the gravity causes something called inverse beta decay. Inverse beta decay occurs when an electron and a proton combine to make a neutron and a neutrino. So the atoms are ionized (stripped of their electrons) and the protons in the nucleus are converted to neutrons. As the pressure continues to increase, the atomic nuclei fall apart to form a gas of mostly neutrons (hence, the name neutron star), and particles called neutrinos are released. Then, the increasing pressure causes the neutron gas to pack into a crystalline solid. The neutrinos (mass-less particles that zip around at the speed of light) carry away most of the energy. Neutrinos rarely interact with solid matter; neutrinos produced by the Sun constantly pass through you and me and the Earth.

Energy not carried away by the neutrinos is released by the supernova explosion in the form of electromagnetic radiation that observers see as a brightening of the star. The expanding shock wave heats up surrounding interstellar gas and dust, which glows and emits x-rays. The expanding shell of glowing dust and gas looks like a big smoke ring and is called a supernova remnant. Heating up the gas and dust slows and weakens the shock wave.

How does a supernova affect you, the observer? The neutrinos are released as the neutron core forms (before the supernova explosion) so they will reach you first. (Scientists here on Earth can detect neutrinos using special techniques that record the rare interactions with solid matter.) Then you will see light from the explosion as the brightening of a star, and later you will see the glowing of the supernova remnant and, with the right equipment, record the accompanying x-rays. If you are anywhere but near the supernova explosion (for example on a planet or spaceship orbiting it), you are in no danger from the shock wave.

Human beings have only observed for sure six supernovae explosions in our Galaxy. The earliest record is from AD 185, and the most recent was in AD 1604. A supernova explosion was observed in the Large Magellanic Cloud (a companion galaxy to ours) in 1987. Although the Large Magellanic Cloud is nearby in astronomical terms, the light from the supernova explosion took 170,000 years to reach the earth.

(Source: Zeilik, Michael., and John Gaustad. Astronomy: The Cosmic Perspective. New York: John Wiley & Sons, Inc., 1990.)

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Eric Muhlethaler writes, "What really is the difference between pulsars and neutron stars? Or are they the same thing? If so then why don't they just call everything neutron stars?"

Pulsars are neutron stars that seem to emit radio waves in rapid bursts. Pulsars don't really turn radio waves on and off--it just appears that way to observers on Earth because the star is spinning. What happens in that the radio waves only escape from the North and South magnetic poles of the neutron star. If the spin axis is tilted with respect to the magnetic poles, the escaping radio waves sweep around like the light beam from a lighthouse. Far away on Earth, radio astronomers pick up the radio waves only when the beam sweeps across the Earth.

It's possible that all neutron stars emit radio waves. But unless the beam of escaping radio waves sweep across our field of "vision," we do not see them pulsing.

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Steve Seibel writes, "I'm a high school freshman and am researching the life cycle of stars and was wondering if you could send me any info you have. It would also be helpful if you could tell me where the sun is in it's life cycle"

The very fact that we can see stars tells us that they radiate energy and, therefore, are changing over time. An average star like the Sun has a life sort of like this:

The gravitational collapse of a protostar from a cloud of hydrogen a state of equilibrium takes about 100,000 years. Then the material slowly contracts and heats due to gravitational energy for around 20 million years. When the material gets hot enough, the nuclear fusion reaction begins converting hydrogen to helium. The fusion reaction makes the star give of radiation in the form of heat and light, and the star shines for 10 billion years or so. This is the present state of our Sun. Larger stars have more gravitational energy, so they burn hotter and use their fuel up more quickly. When most of the hydrogen fuel is spent, the star expands to become a red giant, fusing helium into heavier elements for another half billion years (give or take). Then all nuclear fusion stops, and the star blows off its outer material, revealing its hot, glowing core. It is now a white dwarf. The white dwarf takes a billion years or so to slowly cool and die. (Source: Zeilik, Michael., and John Gaustad. Astronomy: The Cosmic Perspective. New York: John Wiley & Sons, Inc., 1990.)

The Sun has been shining (using nuclear fusion to convert hydrogen to helium) for about 5 billion years, which leaves another 5 billion to go before it engulfs Mercury's orbit as a red giant.

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M. Yaman writes, "We know that any information, including light waves, cannot escape from a black hole. The mass of a black hole is information, right? So how do gravitons escape and bring us how massive a black hole is? Doesn't this fact make any trouble? I mean, do physicsts say gravitons have little or no mass--let's not bother with them ?"

There are two ways of looking at the problem you describe. It depends on how you define gravity. If you are talking in Newtonian terms, the mutual attraction between two objects is a force that requires gravitons to carry it over a distance to the other object. But Einstein's relativity does away with gravitons and explains the effect as a curvature of space-time. The amount of curvature depends upon how much mass an object has. Thus, the relativistic answer to your question is that the information we receive about the black hole's mass is from the effects that the curvature of space-time has on objects around the black hole.

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Michael Ullman writes, "I have a burning (no pun intended!) interest in the larger, more spectacular aspects of astronomy i.e. black holes, nuetron stars, the first moments of the universe, etc. I am not particularly interested in beginning, "plain vanilla" astronomy such as the visible stars, constellations and the like. How would one go about exploring (intellectually) these things without "basic" astronomy, and what, or how much "basic" astronomy and math would be required to pursue these interests?"

I think you can pursue your "burning" interest in astronomy with almost any background. There are many sources--books, magazines, video, software, observatories--that can explain many aspects of cosmology and astrophysics in terms that "regular" people can understand. Sky and Telescope is, in my opinion, a good place to start, although you may have to research a bit. For example, to understand an article about cepheid variables, you may have to crack open an encyclopedia. (But that's the best way to learn.)

Math (or lack of math) shouldn't get in your way. Many popular books are written for the masses, with hardly any math. Of course, some basic algebra will help you understand the more technical stuff. But most people can understand a lot of astronomy without a whole lot of math. And certainly you can learn about things like stars and black holes without having to go outside and learn the constellations.

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Lehan Marcus writes, "What is the temperature of a black hole? Do they spin in one direction, do they have tops and bottoms? Do they move in a direction?"

If a star that is rotating collapses and forms a neutron star or black hole, its rotation (angular momentum) doesn't just disappear, it must go somewhere. That's called Conservation of Angular Momentum. Conservation of Angular Momentum is why a neutron star spins so fast--the regular rotation of a star suddenly compressed into a smaller space means bigger rotation. So it only makes sense that when a black hole forms, angular momentum is conserved.

According to Stephen Hawking, black holes not only rotate, but they rotate on an axis (which gives them a "top" and a "bottom"). A rotating black hole black hole bulges out at the equator (like the Earth does), and the faster it is going, the more it bulges out. Thus, the size of a black hole depends upon its mass, and its shape is determined by its angular momentum. (source: Hawking, Stephen W., A Brief History of Time: From the Big Bang to Black Holes. New York: Bantam Books, 1988. pp. 91-92.)

Similarly, if a star was moving (with reference to an observer) in a certain direction before it collapsed into a black hole, the black hole would continue on the same course.

You can find information abour the temperature of a black hole at http://129.85.13.105/www_erec/Thermo2.html.

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Black Holes and Neutron Stars

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