Ask An Astronomer 

Here are the answers to selected questions submitted between . Questions may have been edited for clarity or brevity. Click on a link to move directly to the answer.

• Gamma Ray Bursts
• Rings around Planets
• Virtual Particles and Dark Matter
• Learning about Dark Matter through Gravitational Lensing
• Different Types of Gravity Waves

1. What are Gamma-Ray Bursts?

   Gamma-Ray Bursts (GRBs) are short flashes of high-energy photons. They were first detected in 1967 by the Vela satellites, which were launched by the US to monitor the nuclear weapons tests. Since then GRBs have been one of the biggest mysteries in astronomy. GRBs show diverse of durations and intensities as a function of time. Their durations vary from milliseconds to several thousand seconds. Every GRB is unique. No two bursts have the same time profile. The BATSE instrument on NASA's Compton Gamma-Ray Observatory detected about one burst per day during nine years of operation. The data from the Compton Gamma-Ray Observatory also show that GRBs come form every direction in the sky.

   One of the first questions about GRBs explored by scientists was their location. We know for sure that they are not in the solar system. Do they happen in our own Galaxy or in galaxies millions of light years away? Today there is no doubt that they occur far away from our Galaxy. Some bursts came from about 10 billion light years away when the Universe was still very young. However, before 1990s, most astronomers believed that GRBs occur in our Galaxy because their if they occurred in distant galaxies, then they must be unbelievably energetic.

   GRBs are often said to be the most energetic events in the Universe or the biggest bangs after the Big Bang. A typical GRB can be one hundred thousand times brighter than a whole galaxy. Recent observations imply that the energy of GRBs is focused into a narrow jet which is a few degrees wide. This means the total energy released by a GRB might be not too much more than a supernova. This also means that we can only see a small fraction (about one in five hundred) of GRBs because they can only be seen when we, the observer, are within the jet of GRBs.

   One of the most important discoveries in recent years is that many bursts have optical afterglows following the gamma-rays. Afterglow emissions can last for months. The origin of afterglows is understood very well, though the source of the gamma-rays still a puzzle. After the gamma-ray flash of a GRB, the GRB jet sweeps up materials and slows down. The interaction between the jet and the external medium will radiate electromagnetic energy ranging from X-ray to radio.

   Another very interesting and important discovery is the link between GRBs and supernovae. On April 25, 1998, a GRB and a supernova were both found at the same time in the same place. The chance of coincidence was only one in ten thousand. We know that supernovae (more precisely, core collapse supernovae) are made from the death of massive stars. Many astronomers believe the connection between them is real and at least some GRBs are related to the death of massive stars.

   In the last few years, a lot of progress has been made on how to make the gamma-rays in GRBs. Though the central engine of GRBs remains unknown, it is widely believed that the central engine puts a huge amount of energy into a small amount of material and produces ultra-relativistic jets which are moving at 99.999% of the speed of light. Different shells of the jets are moving at slightly different speed. Some fast shells are initially behind slow shells and can catch up slow shells. The collisions between those shells will convert the energy into gamma-rays.

   The central engine of GRBs has remained enigmatic since the discovery in 1967. There used to be many theoretical models on the central engine. Among these speculations, only a few are still viable. However, there are a couple of best-bet models. One favored model is the Collapsar/Hypernova model. In this model, a rotating massive star, say fifty times of the mass of the sun, will collapse after it burns out all its nuclear fuel. Its core will collapse into a black hole and an accretion disk surrounding the hole. Relativistic jets can be made and penetrate the star. The jets will convert its energy into gamma-rays by internal collisions mentioned above. Neutrinos and anti-neutrinos can annihilate to provide power for the relativistic jets. The rotating energy of the black hole and the disk can also be extracted to power the jets. A supernova can happen at about the same time and accompany the GRB. Another favored model is the Supranova model, which is also related to the death of massive stars. In this model, instead of making a black hole, the core collapses into a neutron star and makes a supernova. After a delay of several hours or days, the new born neutron will collapse into a black hole and make relativistic jets. There is yet another model: double neutron mergers. Two neutron stars in a binary system will coalesce as they lose energy due to gravitational waves. Again, black holes, accretion disks and relativistic jets are made to power GRBs.

   GRBs are detected by satellites. Currently, the HETE-2 satellite is in operation. NASA will launch the SWIFT on December 5, 2003 to hunt for GRBs. The GLAST is scheduled for September of 2006. These programs will continue to improve our knowledge of GRBs as their ancestors did.

2. Is Saturn the only planet that has rings?

   Saturn is not the only planet that has a ring system, but it is by far the most prominent. Uranus has a system of five rings (here's a picture) and Jupiter also has two rings (here's another). However, while Saturn's rings are composed of dust particles, the rings around Uranus and Jupiter are primarily composed of an electromagnetic plasma (hot ionized gas!). There are several competing theories for the formation of Saturn's rings. The first is that Saturn's rings are a remnant left over from when the planet was formed. As gas and dust collapses to form a planet it forms a disk that orbits the young planet. Material in this disk gradually falls onto the planet or diffuses away. The rings could be what is left of that initial proto-planetary disk.

   The second possibility is that the dust in the rings is composed of small bits of material that was knocked off during collisions with with moons, meteoroids, asteroids, or comets. The dust continues to orbit the central planet in an orbit similar to its parent body. It is this collection of dust that appears as rings. The rings around Saturn have an extra advantage; there are a series of moons orbiting the planet at small orbital radii that help to shepherd the dust into rings. These moons are known as the Shepherd moons, for obvious reasons. The Shepherd moons keep the dust from slowly diffusing away and they allow Saturn's rings to achieve significantly higher densities then the rings around Jupiter and Uranus. Because of this higher concentration of material, the rings around Saturn are the only rings that you can see from Earth with a pair of binoculars. In the future the question of the origin of Saturn's rings may be resolved by a measurement of the age of the rings. If the ring material is several billion years old it must have formed in concert with the planet. However, younger material would point toward moons and asteroids as the source of the dust.

3. Can dark matter be made up of virtual particles?

   The short answer is no. The truth, however, is that currently no one in the entire world knows what exactly dark matter is. Sure, theories of particle physics provide us with many potential candidates for dark matter, but we've never directly observed one, and there are many competing theories with different predictions. So how can I be sure then that dark matter is not made up of virtual particles? Perhaps it'll help if I briefly explain what both dark matter and virtual particles are.

   1. Dark matter
      
      Astronomers have suspected since the 1930's that the universe is made up of more "stuff" than we can directly observe. Studying the dynamics of galaxies within galaxy clusters and later of stars within individual galaxies, astronomers since then have shown over and over again that these enormous objects must contain substantial quantities of dark matter. The galaxies and stars, respectively, are observed to be moving so fast that they would escape from their hosts if these weren't quite a lot more massive than what we infer from their observed light. Today we even have evidence of dark matter from the study of the universe as a whole. On the one hand we have a very good understanding of how much ordinary matter exists in the universe from predictions of successful theories about the early universe. But on the other hand we know from completely independent measurements of the cosmic microwave background, the "afterglow of the Big Bang", that the total amount of gravitational mass is about 5 times larger. The difference between these two is generally attributed to dark matter. The important point here is that dark matter is not observed directly, but indirectly through the gravitational effects it has on other objects, such as the stars within a galaxy, galaxies within clusters, or even the universe as a whole.
      
      
   2. Virtual particles
      
      First off a disclaimer: I'm not a particle physicist, and although I have a basic understanding of Quantum Field Theory, I can by no means claim to be an expert on virtual particles. Here's what I know about them.
      
      Virtual particles are a convenient way for physicists to describe interactions between "real" particles. When an electron passes close to a proton, for example, the negatively charged electron will feel an electromagnetic Coulomb force from the positively charged proton, and vice versa. One way of describing this force is as an exchange of a virtual photon between the electron and the proton. This virtual photon acts as the carrier of the electromagnetic force. In the same way virtual W and Z bosons transmit the weak force, virtual gluons the strong force, and one can even think of virtual gravitons as mediators of the gravitational attraction between two massive bodies. Virtual particles usually (always?) exist for very small amounts of time. In fact their lifespan is usually so small that they can leverage the Heisenberg uncertainty principle and behave in quite unconventional and unexpected ways. Virtual particles routinely violate energy conservation and even causality. The reason they can get away with this is their extremely short lifespan. As long as the non-causality or energy non-conservation occurs on timescale small enough, they will be protected by Heisenberg's uncertainty principle, since the effects can never be observed. Averaged over longer times their existence vanishes. Pairs of virtual particles continuously pop in and out of existence from the vacuum and we never notice. The important points here are that virtual particles are not very tangible, don't exist for any significant amount of time, and don't contribute to the gravitational mass of an object.

   Now can you see why dark matter can't be made up of virtual particles? The existence of dark matter is inferred from the motions of very macroscopic objects (stars, galaxies, the universe) on very large timescales (millions to billions of years) caused by the gravitational effects of the dark matter. Virtual particles, on the other hand, exist only fleetingly and don't enhance an object's gravitational mass.

4. What have we learned more about dark matter from the affects of gravitational lensing?

   We have learned quite a bit about dark matter from the affects of lensing. Dark matter is odd, but not so odd that it doesn't contribute to gravity. An example of what astronomers can learn from gravitational lensing is the total mass of the object doing the lensing (typically a galaxy or cluster of galaxies -- some idea of the morphology of the lens can also be learned). Based in part on lensing experiments, astronomers believe galaxies are as much as 90% dark matter. That is, all the regular matter we see around us -- stars, planets, us -- may make up only one tenth of the actual matter in the galaxy, and all the rest of it is invisible.

   For more information on lensing, I suggest this site .

5. I've heard that there are gravity waves in the Earth's atmosphere. Are they the same as the gravity waves that come from merging neutron stars or black holes?

   No, the two phenomena are not the same. It's just that their names are similar

   Gravity waves in the Earth's atmosphere are called that because gravity is the restoring force that causes the waves to move. For example, when you speak, your vocal cords make little regions of slightly compressed air. The gas pressure in these compressed regions is then slightly higher than the surrounding air, so the compressed regions push against the surrounding air and cause it to move. Ta-da! A sound wave is born. Sometimes sound waves are called pressure waves because the gas pressure is what's causing the sound wave to move through the air. Now, gravity waves in the atmosphere are the same thing except that gas pressure is weak compared to the gravity that the gas feels. Imagine I could grab on to a piece of the atmosphere and lift it up. What would happen? It would fall back down, just like anything else I might lift up. It turns out that when it falls and hits the ground, it will in turn lift up adjacent pieces of the atmosphere. Ta-Da! It's a gravity wave. Just like a sound wave, except that gravity, not pressure, is the important force. Now, the gravity waves that come from merging black holes are completely different. These are sometimes called gravitational waves or gravitational radiation. The easiest way to think of these waves is as "ripples" in space itself. These ripples are extremely small, extremely hard to detect (in fact they have not been directly detected yet), and have nothing to do with the gravity waves in the Earth's atmosphere described above.

Thanks to Alex McDaniel, David Lai, Shawfeng Dong, Gabe Prochter, Ian Dobbs-Dixon, Jay Strader, Justin Harker, Karrie Gilbert, Kyle Lanclos, Laura Langland-Shula, Lynne Raschke, Marla Geha, Michael Kuhlen, Nick Konidaris and Scott Seagroves