Dark star (Newtonian mechanics)

This article is about astronomical objects described by Newtonian mechanics. For the modern concept based on dark matter, see dark star (dark matter). For the modern concept based on dark energy, see dark energy star.

A dark star is a theoretical object compatible with Newtonian mechanics that, due to its large mass, has a surface escape velocity that equals or exceeds the speed of light. How light is affected by gravity under Newtonian mechanics is questionable but if it were accelerated the same way as projectiles, any light emitted at the surface of a dark star would be trapped by the star’s gravity rendering it dark, hence the name.

Einstein’s general theory of relativity has yielded more insight into the nature of objects of extraordinary mass. Such objects by modern understanding would be described in more modern terms as black holes.

Dark star history

John Michell and dark stars

During 1783 geologist John Michell wrote a long letter to Henry Cavendish outlining the expected properties of dark stars, published by The Royal Society in their 1784 volume. Michell calculated that when the escape velocity at the surface of a star was equal to or greater than lightspeed, the generated light would be gravitationally trapped, so that the star would not be visible to a distant astronomer.

If the semi-diameter of a sphere of the same density as the Sun were to exceed that of the Sun in the proportion of 500 to 1, a body falling from an infinite height towards it would have acquired at its surface greater velocity than that of light, and consequently supposing light to be attracted by the same force in proportion to its vis inertiae, with other bodies, all light emitted from such a body would be made to return towards it by its own proper gravity. This assumes that light is influenced by gravity in the same way as massive objects.

Michell’s idea for calculating the number of such "invisible" stars anticipated 20th century astronomers' work: he suggested that since a certain proportion of double-star systems might be expected to contain at least one "dark" star, we could search for and catalogue as many double-star systems as possible, and identify cases where only a single circling star was visible. This would then provide some sort of statistical baseline for calculating the amount of other unseen stellar matter that might exist in addition to the visible stars.

Dark stars and gravitational shifts

Michell also suggested that future astronomers might be able to identify the surface gravity of a distant star by seeing how far the star’s light was shifted to the weaker end of the spectrum, a precursor of Einstein’s 1911 gravity-shift argument. However, Michell cited Newton as saying that blue light was less energetic than red (Newton thought that more massive particles were associated with bigger wavelengths), so Michell’s predicted spectral shifts were in the wrong direction. It is difficult to tell whether Michell’s careful citing of Newton’s position on this may have reflected a lack of conviction on Michell’s part over whether Newton was correct, or whether it was just academic thoroughness.

Laplace and dark stars

In 1796, the mathematician Pierre-Simon Laplace promoted the same idea in the first and second editions of his book Exposition du système du Monde, apparently independently of Michell (it was removed from later editions). Unlike a modern black hole, the object behind the horizon is assumed to be stable against collapse.

Wave theory

Later Laplace, and most researchers during the 19th century, generally ignored the idea of "dark stars", since light was then thought to be a massless wave and therefore not influenced by gravity.

Indirect radiation

Dark stars and black holes both have a surface escape velocity equal or greater than lightspeed, and a critical radius of r ≤ 2M.

However, the dark star is capable of emitting indirect radiation – outward-aimed light and matter can leave the r = 2M surface briefly before being recaptured, and whilst outside the critical surface, can interact with other matter, or be accelerated free from the star by a chance encounter with other matter. A dark star therefore has a rarefied atmosphere of “visiting particles”, and this ghostly halo of matter and light can radiate, albeit weakly.

Comparisons with black holes

Radiation effects

A dark star may emit indirect radiation as described above. Black holes as described by current theories about quantum mechanics emit radiation through a different process, Hawking radiation, first postulated in 1975. The radiation emitted by a dark star depends on its composition and structure; Hawking radiation, by the no-hair theorem is generally thought of as depending only on the black hole's mass, charge, and angular momentum, although the black hole information paradox makes this controversial.

Light-bending effects

Although "historical" Newtonian arguments may lead to the gravitational deflection of light (Newton, Cavendish, Soldner), general relativity predicts twice as much deflection in a lightbeam skimming the Sun. This difference can be explained by the additional contribution of the curvature of space under modern theory: while Newtonian gravitation is analogous to the space-time components of general relativity's Riemann curvature tensor, the curvature tensor also contains purely spatial components, and both forms of curvature contribute to the total deflection.

See also

Star portal

• Magnetospheric eternally collapsing object
• Black hole
• Q star

References

• Spolyar, Douglas; Freese, Katherine; Gondolo, Paolo (2008). "Dark Matter and the First Stars: A New Phase of Stellar Evolution". Physical Review Letters 100 (5): 051101. Bibcode 2008PhRvL.100e1101S. doi:10.1103/PhysRevLett.100.051101. 
• Katherine Freese, Paolo Gondolo, and Douglas Spolyar "The Effect of Dark Matter on the First Stars: A New Phase of Stellar Evolution", Proceedings of First Stars III, Santa Fe, New Mexico, 16–20 July (2007).
• John Michell "On the means of discovering the distance, magnitude etc. of the fixed stars" Philosophical Transactions of the Royal Society (1784) 35–57, & Tab III
• Simon Schaffer "John Michell and black holes", Journal for the History of Astronomy 10 42–43 (1979)
• Gary Gibbons, "The man who invented black holes [his work emerges out of the dark after two centuries]", New Scientist, 28 June pp. 1101 (1979)
• J Eisenstaedt, "De L'influence de la gravitation sur la propagation de la lumière en théorie Newtonienne. L'archéologie des trous noirs", Arch. Hist. Exact Sci. 42 315–386 (1991)
• Werner Israel, "Dark stars: The evolution of an idea", pages 199–276 of Hawking and Israel (eds) Three hundred years of gravitation (1987)
• Thorne, Kip, Black Holes and Time Warps: Einstein's Outrageous Legacy, W. W. Norton & Company; Reprint edition, January 1, 1995, ISBN 0-393-31276-3.

Especially Chapter 3 "Black holes discovered and rejected".

• Maggie McKee, "Universe's first stars may have been dark", New Scientist, 3 December (2007)

Notes