 Accretion disks also create jets of material flowing out from the center of the disk and perpendicular to it. This matter is orbiting a magnetic field that stretches out from the central mass to very great distances. We'll use M87's jet to illustrate how it works. The jet of material streaming out from the center indicates that the galaxy has an active galactic nucleus, AGN for short. That is, it has a supermassive black hole at its center that is accumulating large amounts of matter from an accretion disk. We've known about the jet of plasma shooting out from the core of M87 since 1918, when astronomer Herbert Curtis saw a ray of light connected to the galaxy center. 5,000 light years long and 2 light years wide. Several things stand out about this jet. It's blue, it's very bright, it consists of chunks or knots, and it terminates in a plume. You may have also noted that there is no counter jet going out the other way, like we've seen in other galaxies. The jet is understood to have been formed in a strong magnetic field created by the interaction between a spinning black hole and the rotating accretion disk. Then, at the point where matter from the accretion disk is crossing the event horizon into the black hole, a small percentage of the charged particles are swept into this magnetic field and ejected into the jet at the black hole's escape velocity, which is near the speed of light for objects as massive as a black hole. These escaping particles are forced into circular orbits around the strong magnetic field. The European Space Agency's integral gamma ray observatory has observed extremely hot matter just a few milliseconds before it would have crossed into the black hole. This lens support for the theory, but just how this is accomplished is not yet understood. These circularly accelerating ions create electromagnetic radiation across a wide spectrum, including radio, visible, and X-ray light. This is what we are seeing with our radio, optical, and X-ray telescopes. The two key jet features we observe directly are its apparent luminosity and its apparent motion across the sky. A study done by a team of astronomers using the European very long baseline interferometer radio telescope network analyzed the motion of one of the knots near the jet's origin at the black hole. They found that one of the components moved 30 milli arc seconds over two years. That's a very tiny amount, but when you multiply it by the large distance to M87 we find that the distance traveled was eight light years. To travel eight light years in just two years means its velocity is four times the speed of light. We call the apparent velocities greater than the speed of light superluminal motion. Here's how it works. Suppose we have an object at location A at time t1 that moves to location B at time t2, the travel time being delta t. D is the distance traveled. It will equally objects velocity times its travel time. We're observing this motion from a great distance at an angle theta from the object's line of motion. We see only the proper or transverse motion across the sky designated here as D prime. Our start time is the object's start time plus the time it takes the light to get from point A to point O where we are. Our end time is the object's end time plus the time it takes the light to get from point B to point O. With that we can calculate the observer's view of the object's velocity in terms of the object's view and vice versa. If we plug in the numbers we found for not C in the M87 jet, we find that the apparent velocity of 4 times the speed of light turns out to be 0.97 times the speed of light in the object's frame of reference. And the apparent elapsed time of 2 years turns out to have taken the object almost 67 years. It was not traveling faster than the speed of light. Note that this only happens when the velocity of the object is near the speed of light and in addition the viewing angle is small. Another relativistic effect at play here is called relativistic beaming. To illustrate consider an inertial reference frame moving to the right at relativistic speeds with respect to an aligned reference frame on the right. A particle emits a photon at an angle alpha from the line of motion. The angle measured in the frame on the right can be computed using the Lorentz transformation. Using M87C1's velocity as the velocity and 60 degrees as a sample angle, we see that the observed angle alpha prime is considerably smaller at only 8.2 degrees. A synchrotron radiating electron moving at speeds far smaller than the speed of light will emit radiation in all directions. Distant observers would see just the portion of the light radiated in their direction. As the speed of the electron increases, these light rays shift in the direction of the emitting object's motion. As the velocity of the emitting particle approaches the speed of light, the observed angle approaches zero. The light is beamed ahead of the emitter in the direction of the emitter's movement. This is the case no matter what the emitted angle is in its own frame of reference. For trillions of continually emitting particles, like the electrons in the M87 jet, this beaming effect increases the photon density in the direction of the movement, causing the jet's luminosity to increase. This explains why the jet looks so bright. And because the jet moving in the opposite direction will have their photons beamed away from the observer, the jet becomes invisible. This explains why we see only one jet in M87. Our last relevant effect is called relativistic Doppler shift. Due to space contraction, when we applied the Lorentz transformation against the frequency of a photon emitted in the same fashion as we just covered, we find that the frequency observed is greater than the frequency transmitted. This explains why the M87 jet is so blue.