Black Holes and Compact Objects

The pulsar at the heart of the Crab Nebula is a compact object. (Credit: NASA.)

When astronomers refer to “compact objects,” they are generally referring to objects significantly more dense than a typical star or a planet. For example, white dwarfs and neutron stars are extremely dense objects that result when their progenitors stars—Sun-like stars or smaller in the case of white dwarfs, and giants in the case of neutron stars—have run out of fuel for fusion. The stars are no longer able to produce a sufficient amount of radiation pressure in their cores to prevent their outer layers from collapsing. 

When the biggest stars collapse, they can trigger the formation of black holes—regions of space in which gravity is so strong that even light cannot escape.

Black Holes—Unseen Neighbors

Black holes with masses comparable to that of the Sun are scattered through the Milky Way and neighboring galaxies. Scientists also have found strong evidence of massive black holes—a million or more times more massive than the Sun—in the centers of many galaxies. In fact, one of these supermassive black holes sits at the heart of our very own Milky Way.

Detecting Black Holes

Compact objects are difficult to observe directly. Fortunately any ordinary matter falling toward them—or disappearing entirely into a black hole—tends to heat up and produce X-rays and gamma rays in the process, which are redirected into great streams, or “jets,” of matter and energy that surge into space at velocities close to the speed of light. But because Earth's atmosphere absorbs most of this kind of radiation, observations must be made from space. Measurements have revealed that these phenomena are responsible for the highest energies yet detected in the Universe.

Compact objects are subjects of intense research at KIPAC, whose scientists them using data from the Fermi Gamma-ray Space Telescope. The gamma-ray emissions from pulsars allow researchers to study how their intense, pulsed radiation is produced. Researchers now believe that the type of neutron star called pulsars behave like powerful magnets in which the poles are not aligned with the axis of the star's rotation. The energetic particles emanating from a pulsar form a beacon—similar to the beam of the lighthouse—which periodically sweeps across our line of sight once or twice per revolution. By taking an accurate count of pulsars, KIPAC scientists have been able to estimate how often stellar collapses take place in the Milky Way.

Developing Complex Models

In addition to neutron stars, the Fermi telescope has detected hundreds of black holes with powerful jets. To get a better idea of the structure of these jets and home in on the nature of the radiating particles, KIPAC scientists are using the Fermi data as well as observations in the radio, visible, and X-ray bands to build computer models. The models are complex but KIPAC scientists have been among the very first to accurately describe the twisted magnetic field that occurs when matter falls into a black hole and deduce how the field determines the precise alignment of the jet.

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