Take the 2-minute tour ×
Astronomy Stack Exchange is a question and answer site for astronomers and astrophysicists. It's 100% free, no registration required.

Following this article, it is stated that:

As a star drifts too close to a supermassive black hole, intense tidal stresses rip the star to shreds. As this happens, the shredded material will be dragged into the black hole’s accretion disk — a hot disk of gas that is gradually pulled into the black hole’s event horizon, bulking up the black hole’s mass, or blasted as energetic jets from its poles.

Questions are,

  1. What is the energy type that escapes from the poles?
  2. What is the magnitude of the power?
  3. Can this energy be potentially harnessed?
  4. How far into space do these energy jets blow?

Also, if you feel like discussing this: if mass m goes into the black hole and comes out as energy E from the poles, assuming the speed of light c has no play in this transaction of going in or out, what happened to c^2 from E=m*c^2? Could a black hole be hypothetically faster than light? Or maybe reverse direction of light? (I even got confused from this thought...)

So, if those jets are not pure energy, but are energized matter,

  1. What type/kind of matter it would be? (gas? solid? liquid?)
  2. Is all the matter that comes in, also goes out? I am thinking if the black hole works like a car's engine - does it have conversion efficiency?
  3. Can this matter be cultivated, harnessed, or converted back to what it was or something new all together?


This question is presented here after it was asked on the space stackexchange. I want to leave it there because it has some relevant discussion to this question overall.

share|improve this question
Cross-posted from Space.SE. Please note that cross-posting on multiple Stack Exchange sites just to bypass qualitative requirements (described in How to Ask) or because the question was closed with request to clarify or narrow it down is dissolved. Please instead work towards editing original question to be answerable. Thanks! –  TildalWave Aug 19 at 20:07
This question applies in both areas, and it already has a good discussion there, but no answer. –  KingsInnerSoul Aug 19 at 20:08

1 Answer 1

up vote 4 down vote accepted

The article you've read is not quite accurate/correct. A more correct pictue is as follows:

  1. A star may approach a super-massive black hole (SMBH) so closely that the tidal forces of the SMBH tear it appart. The distance to the SMBH at which this happens is often referred to as the tidal radius. For a (non-rotating) SMBH with a mass in excess of about $10^8$M$_\odot$, the tidal radius is within the event horizon and hence this process is invisible.

  2. About half (depending on the initial orbit of the star) of the stellar material is caught by the SMBH onto a elliptic orbits (the other half escapes on hyperbolic orbits), but due to gas interactions and dissipation (shocks transfer orbital energy into heat which is then radiated away) it soon circularises to form a (or contribute to an existing) accretion disc. (Note that feeding from stars is not thought to be the main mechanism for the formation of accretion discs; a more likely process is the direct infall of gas clouds).

  3. Material in the accretion disc orbits the BH on nearly circular orbits. It cannot simply fall into the SMBH because its angular momentum is conserved. However, gas at adjacent radii orbits at different velocities (differential rotation), i.e. the disc is shearing. Viscosity in the gas then causes angular momentum to be transported outwards, mass being transported inwards, and the material to heat up. The material in the disc is a hot (presumably) magnetized plasma, and the viscosity is thought to be provided by the so-called magneto rotational instability. The time scale for this process is, however, very long and only becomes efficient ($\sim10^6$yr) rather close ($\sim100$AU) to the SMBH.

  4. The temperature of the disc material is extremely high, such that it radiates in the X-ray and UV, visible as quasars. This radiation is the main energy output from the accreting BH and constitutes about 10% of the gravitational energy gain $mc^2$ ($m$ being the mass accreted). This radiation interacts with any surrounding material (including the outer accretion disc itself, in particular if it is warped), generating an outflowing wind (by its radiation pressure) and possibly preventing further accretion (this corresponds to the Eddington limit).

  5. The rotating magnetized disc may also generate collimated bi-polar outflows, commonly referred to as jets. However, IMHO, only a small fraction of the energy can be emitted in these jets, already because the jets have low entropy (they are dominated by ordered motion), while the hot disc has high entropy.

share|improve this answer
care to post this on the question on the Space Exploration SE? If you can? –  HDE 226868 Aug 20 at 17:06
@HDE226868 Cannot, for that question has been put on hold (no answers possible). –  Walter Aug 21 at 8:00

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.