Neutron stars and black holes

[kalexia]

This is more of a curiosity than a thread topic.

Pulsars are rotating neutron stars that have magnetic fields that are in a different direction to their magnetic poles, but why are some neutron stars not pulsars? Is it because the fields are aligned?

Also, how can micro-black holes exist? I thought the main properties of a black hole were the combination of 'zero' size and high mass, so how can a 'hole' of about 20 nano kg have the high gravity that is associated with a black hole? :hyper:

[Jay-qu]

Pulsars are rotating, and thats what causes the pulsating. So a neutron star not rotating wouldnt appear to pulsate.

 

Small black holes can be made just like large ones - compress a mass within its schwatzchild radius. For a low mass this radius would be incredibly tiny and so would require very extreme conditions to produce a black hole. Such black holes are sometimes called primordial black holes because it is believed that only the extreme conditions of the early universe could have made them.

[kalexia]

Pulsars are rotating, and thats what causes the pulsating. So a neutron star not rotating wouldnt appear to pulsate.

 

Small black holes can be made just like large ones - compress a mass within its schwatzchild radius. For a low mass this radius would be incredibly tiny and so would require very extreme conditions to produce a black hole. Such black holes are sometimes called primordial black holes because it is believed that only the extreme conditions of the early universe could have made them.

 

That makes sense. I forgot about the whole radius thing. However, I thought that all neutron stars rotated as a result of their spin velocity increasing due to its compression to a smaller size (after supernova)? Why do certain NS not rotate?

[Jay-qu]

Im sure they all rotate to some degree, but pulsars just rotate faster which allows us to observe the periodic pulsation.

[CraigD]

Also, how can micro-black holes exist? I thought the main properties of a black hole were the combination of 'zero' size and high mass, so how can a 'hole' of about 20 nano kg have the high gravity that is associated with a black hole?

Small black holes can be made just like large ones - compress a mass within its schwatzchild radius. For a low mass this radius would be incredibly tiny and so would require very extreme conditions to produce a black hole. Such black holes are sometimes called primordial black holes because it is believed that only the extreme conditions of the early universe could have made them.

Jay-qu is, I think, correct, though I’d add that that very small black holes are predicted to be created by conditions such as those produced by an hydrogen fusion bomb.

 

Due to Hawking radiation, it’s believed (but by no means with certainty, or by all physicists) that very small black holes can’t exist long enough for to be very significant.

 

According to the theory describing Hawking radiation, the time required for a black hole to “evaporate” all of its mass away is given by:

 

[math]t = \frac{5120 \pi G^2M_0^3}{\hbar c^4}[/math]

 

Supplying the constants and manipulating this equation gives required mass ([math]M[/math]) of about 7216 kg for a black hole with a lifespan of 1 second, or, alternately, that a black hole of that mass must consume a like mass every second to avoid evaporation. A 20 nano kg (2*10^-8 kg) black hole would evaporate in 10^-47 seconds – so little time that it practically never existed.

Pulsars are rotating neutron stars that have magnetic fields that are in a different direction to their magnetic poles, but why are some neutron stars not pulsars? Is it because the fields are aligned?

Pulsars are rotating, and thats what causes the pulsating. So a neutron star not rotating wouldnt appear to pulsate.

Current theory predicts that a non-rotating neutron star could only be formed by a non-rotating star. AFAIK, no such stars have been observed, and, according to present best theory of how stars form, it’s unlikely any exist.

 

My understanding is that practically all neutron stars rotate, have surrounding clouds of radiation-producing matter, and magnetic and rotational axes misaligned enough that practically are Rotation-powered pulsars, though of varying intensity and spectra. Because the “lighthouse beam” of this radiation is narrow, however, the likelihood of a particular pulsar being both close enough and aligned so that the beam intersects Earth, is small, so we have observed only a small fraction of the number of pulsars that actually exist.

[Guest Lambus]

very cool hawking radiation equation!