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Milky Way Galaxy

The Massive Stars Orbiting Sagittarius A*

The massive black hole at the center of the Galaxy, Sagittarius A*, has an unexpected feature (or perhaps a bug if you're a theorist): it is orbited by numerous young, massive stars. So inexplicable are these stars that their existence has been dubbed the “paradox of youth” by some astronomers.

Many bright stars are visible at infrared wavelengths within 4″ (0.15 parsecs at 7.62 kpc distance) of Sgr A*. They have the spectra and luminosities that are characteristic of B main-sequence stars, which makes them similar to the local stars Regulus (α Leo) and Spica (α Vir). Unlike their counterparts in the Galactic disk, they are the most massive stars found around the central black hole. Elsewhere in the Galaxy, the largest hydrogen-burning stars are the OB giants, which are more massive and shorter-lived than the B main-sequence stars. Examples of OB giants are the bright blue stars of the Orion Nebula. In contrast to the absent OB giants, the B main-sequence stars are overrepresented relative to the less-massive main-sequence stars as compared to the stellar populations found in the Galactic disk. The physical processes that place stars close to the black hole are therefore biased towards the B main-sequence stars.

The orbits of half-a-dozen B main-sequence stars are well determined from over a decade of observation. One star, called S2, has been observed through a complete 15 year orbit. All orbits are highly elliptical and randomly oriented. They are a diagnostic for the mass and distance from Earth of Sgr A*.

The B main-sequence stars are between 3 and 15 solar masses and live anywhere from 10 million years to 100 million years on the main-sequence.[1] This short lifetime is the problem: theorists had believed that gravitational tidal forces would prevent stars from forming close to a massive black hole, and they had thought the time required for a massive star to sink close to the black hole from farther out, where stars can form, is much longer than the lifetimes of these stars, of order 1 billion years. Which of these two ideas is mistaken? The firm identity of the bright stars as B main-sequence stars and the random orientation of their orbits are clues.

So, other than that “youth is wasted on the young,” what can be said about the “paradox of youth?” The observations enable us to reject many theories, and they force the research on other theories to focus on one problem: how do the stars at the galactic center redistribute energy and momentum among themselves?

The first class of theory that can be rejected immediately asserts that each of these bright stars is actually a neutron star, degenerate dwarf, or black hole enshrouded in an envelope of gas.[2] These theories predate the measurements of stellar spectra that show the stars to be B main-sequence stars, so initially they only had to match the brilliance of the stars. These theories resolved the “paradox of youth” by giving us old stellar remnants that are rejuvenated when they acquired their shells of gas. The problem then shifted to how the compact objects orbiting the central black hole acquire their shells—whether through accretion of interstellar gas, through collision with other stars, or through some other event—and whether these shells are acquired at a sufficiently rapid rate to create the stars we see. Now, with spectra measured for the the bright stars, the shrouded compact objects must now mimic B main-sequence stars, which is implausible. Experience from stellar evolution suggests that a shrouded compact object resembles a red giant or a supergiant star. After all, a star in the red giant phase is a degenerate dwarf inside shells of helium and hydrogen. A shrouded neutron star should also be a giant. A shrouded black hole suffer from the deeper problem of a short life, since the black hole should consume the shell surrounding it. Under these circumstances, we are better served by accepting that the bright stars are B main-sequence stars, and not mimics.

This leaves us with only to resolutions to the “paradox of youth”: either the stars now within 4″ of Sgr A* were born elsewhere, and their orbits changed rapidly to place them close to the central black hole; or the stars were born near Sgr A*, and some mechanism ensured that the orbits of these stars were random. The first of these two directions is by far the more reasonable, given what is known about the stars and their orbits.

The problem in creating the stars in close orbit around Sgr A* is that they would be born in an orbital plane, much like the stars in the Galactic disk or the planets in the Solar System. The reason is that for a gas cloud to be dense enough to precipitate into stars near Sgr A*, it would need to be a disk orbiting the black hole. The theory is that stars would precipitate from the outer edge of a transient disk. This would leave behind stars with orbits lying the the plane of disk. The stars we see do not orbit in the same plane, so for the theory to work, one needs a source of entropy, some mechanisms that destroys the order of a disk of stars.

The black hole itself can be a source of entropy if it is spinning. In a mechanism called the Lense-Thirring effect, the orbits of objects around a black hole can precess if the black hole is rotating. For this precession to occur, the axis of the orbit must be tilted relative to the black hole's rotation axis. If both axes were aligned, they would remain aligned. If the orbital axis were perpendicular to the rotation axis, the orbital axis would precess around the rotation axis in the equatorial plane of the black hole. In between these extremes, the orbital axis would precess on a cone around the axis of the black hole, so that the angle between the orbital axis and the rotation axis is fixed. For a set of stars initially orbiting a spinning black hole in a common plane, the Lense-Thirring effect tilts the orbit of each star out of that plane, with the rate of precession dependent on the size of the orbit, so that over time each star orbits in its own unique plane. This multiplicity of orbital planes can give the appearance of randomness, but in fact each orbital plane is related to every other, because the axis of each orbital plane is separated from the black hole's rotation axis by the same angle. If one plotted the direction of each orbital axis on the sky, one would get a circle on the sky, with the center of the circle defining the black hole's rotation axis.

The stars we see near Sgr A* have no order to their orbits. Not only does each star lie in its own unique orbital plane, but the orbital axes of these stars as a group are oriented randomly on the sky. The order expected for stars born in the same orbital plane and then precessed to different orbital planes is not present in the stars around Sgr A*. This implies that the stars were not born near the black hole unless some other mechanism is at work to randomize the stellar orbits close to the black hole.

From this one sees that the “paradox of youth” is not a problem of where the stars were born, but of how the stellar orbits around Sgr A* change so dramatically in less than 10 million years. Since this change does not occur through an interaction with the central black hole, it must occur through interactions with other stars.

[1]Eisenhauer, F., et al. “Sinfoni in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month.” The Astrophysical Journal 628 (20 July 2005): 246–259.

[2] Morris, Mark. “Massive Star Formation Near the Galactic Center and the Fate of Stellar Remnants.” The Astrophysical Journal 408 (10 May 1993): 496–506.

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