# How fast can an object spin

1. Jul 12, 2013

### MathJakob

I'm a bit confused about quasars and neutron stars but is it the case that quasars are much much denser than our star and spin many times per second?

I think the sun completes one rotation in about 25 days and it's spinning at about 4467mph. I'm pretty sure that density will effect the speed that something can spin? So if a quasar has a mass a million times that of the sun, but it's only the size of new york, how can we work out of it's spinning faster in terms of it's size?

Obviously it is smaller but because it is denser would it be spinning faster?

I guess what I'm trying to ask is how fast to quasars spin and is there a limit as to how fast they can spin? If they spin too fast would gravity just rip them apart?

2. Jul 12, 2013

### SteamKing

Staff Emeritus
I think you are confusing the properties of neutron stars (sometimes referred to as pulsars) and quasars.

Quasars are very distant objects (on the order of billions of light years from earth) which emit copious amounts of electromagnetic radiation. Due to the sustained duration of this emission, it is thought that the EM radiation results from continuous infall of matter into a massive black hole. Because of their extreme distance from the earth, it is not possible to gauge whether these objects are rotating.

Pulsars, on the other hand, are much closer objects, on the order of a few thousand LY from earth. By using radio telescopes, one is able to study the EM emissions of a neutron star more closely. Since the size of these objects is also known with some accuracy, the angular velocity of a rotating neutron star can also be accurately determined. The fastest pulsars are the so-called 'millisecond' pulsars, which refers to the period of the emissions emanating into space. As the pulsars age and accrete additional matter, the period of rotation gradually increases.

Pulsars acquire such high rotation rates as a consequence of the conservation of angular momentum. If a large star has a nominal amount of initial angular momentum and rotates once is say a few days time, like the sun does, then when the star comes to the end of its life and collapses in a supernova explosion, the outer layers of stellar material are ejected and the collapsed core forms a neutron star. Because the neutron star remnant is less massive than the original star, it must speed up its rotation to have the same angular momentum as the star did before the supernova.

3. Jul 12, 2013

### MathJakob

Sorry if I missed the point but you're saying that pulsars are the fastest spinning objects? Is there a limit as to how fast they can spin?

4. Jul 12, 2013

### SteamKing

Staff Emeritus
No, while pulsars spin very fast (the fastest observed pulsar spins 642 times per second), smaller objects can spin much faster. A dentist's drill, for example, can spin up to 300,000 revolutions per minute (or 5000 times per second). Small gas turbines can spin as fast as 500,000 rpm.

To return to pulsars, however, the fastest ones have a surface velocity of about 20% of the speed of light. This is about as fast as a pulsar can spin without tearing itself apart. Only the tremendous gravity of a pulsar allows it to spin this fast. A typical pulsar contains as much mass as 1.5 suns in a body no more than 20 km in diameter.

5. Jul 12, 2013

### MathJakob

Thanks. I understand drills and such spin at very fast rpm but they don't weigh much, so to think that a pulsar with the density of a million solar masses can spin 500 times a second and reach a surface speed of 20% the speed of light I think is amazing. They must contain SO much energy.

6. Jul 12, 2013

### SteamKing

Staff Emeritus
The upper limit on the mass of a pulsar is about 3.2 times the mass of the sun (Msol). There are no pulsars with 'a density of a million solar masses', whatever that means. Any pulsars much bigger than 3.2Msol collapse into black holes.

7. Aug 13, 2013

### Astrofan

Recent studies have actually measured (using various methods) many Supermassive Black Hole spins, but for Quasars, being the most distant objects, have not been measured.

To clear up some confusion,

As far as general pulsars (that includes millisecond pulsars) are concerned, they actually "spin-down" by radiating by magnetic dipole radiation.

Millisecond pulsars are also called "recycled pulsars", and we think that they have gained angular momentum by accretion from their binary companions and are a distinct class of objects. But millisecond pulsars have already eaten up all their companions.

Another class of pulsars are in binaries, and they are currently "eating" their companions.

8. Aug 13, 2013

### Chronos

The upper limit on surface spin velocity is the speed of light. It is further limited by gravity. An object that spins more rapidly than its gravitational attraction force can constrain will become 'unbound', which is not observed in nature. The obvious conclusion is this does not happen.

9. Aug 13, 2013

### stevebd1

One way to look at the spin of objects is to compare the gravitational radius (M) and the spin parameter (a) where M=Gm/c2 and a=J/mc where G is the gravitational constant, c is the speed of light, m is the mass of the object and J is the angular momentum. M and a are also referred to as geometric units. By dividing a by M, you get a number between 0 and 1, the higher the number, the greater the spin with 1 being considered maximal (Note: This only appears to work for objects with at least the mass of 1 sol and not with other objects such as planets). Using this method, the sun has a spin parameter of a=0.185M while a neutron star can have up to approx. a=0.5M before shredding takes place and a black hole is predicted to go up to a=0.998M (there's already some evidence that some black holes spin at a=0.9M+).

10. Oct 28, 2013

### Pds3.14

Well, and object certainly can't spin with the surface at C.
If gravity is irrelevant to the object, and it is made of graphene, it could reach around 15 km/s or so.
Quasars, being black holes, can really spin at anything from 0 to C.

11. Oct 28, 2013

### Hornbein

The limit on the spin of a neutron star is that above a certain speed the star becomes asymmetrical and radiates gravitational waves. The fastest pulsars are at that limit.