Compact objects fall under two categories - neutron stars or black holes. Neutron stars are the ultra-dense, compact remnant cores of massive stars. They are made almost entire of neutrons and have densities comparable to the density of an atomic nucleus. These neutrons are held together and kept from transmuting back into normal matter by the neutron star’s intense gravity which arises from its extraordinary compactness. A teaspoon of neutron star material would contain a mass of roughly a billion tons. The minimum and maximum mass possible for any neutron star is between ~0.1 and ~3 times the Sun’s mass. Below the minimum mass, the neutron star’s gravity is too weak to hold the star together and the star “decompresses” into normal matter. Above the maximum mass, the neutron star’s gravity becomes sufficiently strong to crush it into a black hole.
Figure 1: Artist’s impression of a neutron star whose intense gravity is lensing light from the background.
Nevertheless, the physics of matter at ultra-high densities remains poorly understood. Bahcall, Lynn & Selipsky (1990) propose that the same type of matter found in a neutron star could be stably confined by an alternative means other than gravity. Such a form of matter, though still considered ultra-dense, would have densities far below what is found in a neutron star. The outcome is that a compact object made of such a form of matter could exceed 3 times the Sun’s mass and would not collapse into a black hole under its own gravity since it is not as compact as a neutron star. These objects are termed “Q-stars”.
Theoretical models by Miller, Shahbaz & Nolan (1997) show Q-stars can be up to several times the Sun’s mass, far above the maximum mass for neutron stars. Furthermore, Q-stars that are several times the Sun’s mass can have radii less than 1.5 times the event horizon radius of a black hole of corresponding mass. Basically, a black hole’s event horizon is a non-physical boundary around a black hole, and within it, gravity is strong enough to keep even light from escaping. Since a black hole does not have a true surface, its event horizon could be regarded as its “surface”.
Figure 2: Radius of a Q-star plotted as a function of its mass. Miller, Shahbaz & Nolan (1997).
A non-rotating Q-star with 12 times the Sun’s mass can have a radius as small as ~52 km. In comparison, a black hole of the same mass would have an event horizon radius of 36 km. This difference is less than a factor of 1.5 and shows that a Q-star can be comparable in size to the event horizon of a black hole of corresponding mass. As a consequence, it may be difficult to observationally determine whether a high-mass compact object with several times the Sun’s mass is a black hole or a Q-star.
One possible method to distinguish a black hole from a Q-star would be to observe the accretion of material by the high-mass compact object. If the object were a Q-star, the accretion flow would eventually intersect the surface. If the accretion flow extends further inwards, closer than what would otherwise be the surface of the Q-star, it would be good evidence that the high-mass compact object is a black hole rather than a Q-star. An example of a known high-mass compact object that could turn out to be a Q-star is V404 Cygni - an object currently thought to be a black hole with ~12 times the Sun’s mass. Even so, one should be mindful that Q-stars are purely theoretical constructs and they may not exist at all.
- Bahcall, Lynn & Selipsky, “New Models for Neutron Stars”, ApJ (1990) 362, 251.
- Miller, Shahbaz & Nolan, “Are Q-stars a serious threat for stellar-mass black hole candidates”, MNRAS (1990) 294: L25-L29.