 One of the ways for a black hole to form is for a neutron star to accumulate enough mass to collapse. Here's an illustration with the neutron star surrounded by an accretion disk supplied by gas from the stellar winds of a nearby blue giant star. Driven by inward gravitational forces and outward centripetal forces, in-falling matter into massive objects like stars, neutron stars, and black holes always form accretion disks. There are a wide variety of these complex structures, but they all have two basic characteristics. They are thin in that the disk's radius is much, much larger than its depth, and they are thick enough to ensure that photons created inside the disk will interact with matter inside the disk at least once before escaping. The movement of the matter in the disk is controlled by the central object's gravity. It is said to be Keplerian because it follows Kepler's laws. Here we have marked the outer most orbit and the inner most orbit. Gas in the outer ring has the velocity to remain in orbit. In order to spiral inward, something has to slow it down. In an accretion disk, that would be friction with adjacent matter. Note that a disk of dark matter could never play a part of star or black hole formation via accretion disks, because dark matter does not interact with anything and therefore there is no friction to slow it down. In addition to slowing the gas down, this friction causes the gas to heat up. The more massive the central object, the hotter the inflowing matter becomes. For neutron stars and black holes, the temperatures reach millions of degrees Kelvin. At these temperatures, the gas emits detectable amounts of x-rays in all directions. When the gas reaches the innermost circular orbit, it crashes onto the surface of the neutron star. Over time, the material builds up on the surface and ignites in a thermonuclear explosion. Such an event produces a bright flash of x-ray emissions called a type 1 x-ray burst. Such bursts from low-mass neutron binary star systems are very common. Thousands have been observed to date from over a hundred accreting neutron stars. But, once the mass of the neutron star grows to the point that the gravitational inward pressure exceeds the neutron outward pressure, the star collapses. In a matter of seconds, all its mass recedes beyond the event horizon and it disappears. As a gas cloud moves from the innermost stable circular orbit to the event horizon, time dilation reduces the frequency of the x-rays emitted by the hot gas, as viewed by a distant observer. In this example, the observer sees that the time between peaks at the start are exactly correct for x-rays. The closer the gas gets to the event horizon, the longer the peak-to-peak interval. But even at the innermost stable circular orbit, the increase is small. But, as the gas approaches the event horizon, the interval quickly reaches hours, days, years. At the horizon itself, the time between an observed peak and the next one takes longer than the age of the universe. Any light emitted by the in-falling matter becomes infinitely red-shifted as the object passes over the horizon. You never see the gas enter the black hole, it just fades away. So, for a neutron star, there's a surface for the in-falling matter to crash into, accumulate and explode. But once it collapses into a black hole, the gas just disappears and the thermonuclear explosions cease. We can conclude that an x-ray binary system without thermonuclear x-ray bursts contains a black hole. Arthur Eddington, the astronomer who first captured the bending of light by the sun that supported Einstein's special relativity theory, proposed that, for any object in the depths of space, there is a maximum luminosity beyond which radiation pressure will overcome gravity, and material outside the object will be forced away from it, rather than falling inwards. This maximum is now called the Eddington Luminosity, and it puts a limit on how fast matter can flow into a black hole, now called the Eddington Limit. This is very useful for analyzing quasars. A quasar is an extremely active galactic nuclei thought to be created when vast amounts of matter are continually flowing into a supermassive black hole. With Eddington, a quasar's luminosity gives us the mass of the central black hole, and with the mass and luminosity, we can calculate the maximum rate for increasing the black hole's mass. For example, a black hole with a measured luminosity of 10 to the 40th watts has the mass of 700 million suns. With this mass, its maximum rate of increase is approximately 3 solar masses per year. It will take 233 million years to double its mass.