Milky Way’s Galactic Core, Stellar “Paradox of Youth”
The subject of this thread is what is known as the “paradox of youth” problem which is how to explain the recent discover of very young, massive, short lived stars that are located very, very, close to the galactic core. The “paradox of youth” problem is that the stars in question have a lifetime that is at most 100 MM years and it is very difficult to create a gas cloud with sufficient density to enable that number of very young stars to form so close to the galactic core. Half of stars in questions are located in three tight clusters of stars that are estimated to be 0.5 MM years old. There is no evidence of current star formation in the locations where the young stars in question are found which is puzzling, but perhaps not unexpected as simulations indicate that it is very difficult to get gas clouds to form that close to the Milky Way’s core massive compact object.
http://arxiv.org/abs/astro-ph/0508106v1
The following are excerpts from the above review paper “Stellar Processes Near the Massive Black Hole in the Galactic Center” by Tal Alexander which summarizes the observations and different hypotheses that have been developed to try to explain this paradox.
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About half of the young stars in the region are found today in three particularly massive, young (.5 Myr) clusters: The Quintuplet (∼30 pc from the center in projection, M ∼10^4M⊙…, The Arches (∼30 pc from the center in projection, M &10^4M⊙, R∼0.2 pc, …, and the central cluster around the Milky Way's massive black hole (Figer 2003). The three clusters contain in total hundreds of MS O-stars, tens of Wolf-Rayet (WR) stars and a few luminous blue variable stars (§2.2), which are ∼10% of all the massive stars (initial mass>20M⊙) in the entire Milky Way Galaxy.
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It has proved difficult to find a satisfactory explanation of how they could have formed so close to the massive black hole, or alternatively, of how they could have migrated inward from farther away in the course of their short lifespans. This is the so-called “paradox of youth”. The question applies to any of the young stars in the inner parsec, but particularly so to the central cluster of the “S-stars”, which exist a mere few hundredths of a parsec from the massive black hole. The problem of the young stars has become one of the major outstanding issues in galactic core research.
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The young population in the inner ∼1 pc is often loosely described as the “OB-stars”. This general designation can be misleading, as it fails to convey the significant systematic differences that exist in the population. An important open question is the nature of the connection, if any, between the S-stars inside ∼0.04 pc and the luminous emission line stars further out, on the 0.04–0.4 pc scale. While it is plausible to assume that these are different components of the same parent population, it should be noted the two groups have distinct locations, kinematics and stellar properties. The stars detected so far in the two young star disks at p∼1”–10” are luminous OB supergiants, giants and WR stars of various types (Genzel et al. 2003b; Paumard et al. 2001; Paumard et al. 2004).
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The solutions proposed so far for the riddle of the young stars (see reviews by Genzel et al. 2003b; Ghez et al. 2005) fall into three main categories: unusual modes of star formation near the Milky Way's massive black hole; rejuvenation of old stars from the local population; and dynamic migration or capture from farther out, where stars can form. While each has some attractive features, none is quite satisfactory. The paradox of youth remains unsolved at this time.
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Comment:
The overall universe is estimated to be 13.7 billion years old. Our galaxy is one of the oldest galaxies in the universe and is estimated to be 13.6 billion years old. An interesting question is how to explain why the mass of the compact massive object at the centre of the Milky Way SgrA* is only 3.6x10^6 solar masses.
(The abbrievation “SgrA*” is short for Sagittarius A* is used to designate the Milky Way’s black hole. The Milky Way’s black hole (BH) is located in the constellation Sagittarius and is designated as “Sagittarius A*”.)
The Milky Way galaxy formed during the crowded early times in the universe and would therefore have opportunities for multiple mergers with other galaxies, in addition to time to accrete its own very massive compact objects. The current theory of galaxy formation hypothesizes that all galaxies that have a central massive object at their core have gone through a quasar phase. It is therefore natural to compare the Milky Way to galaxies that going through their “quasar phase’. Quasars at z=2, for example when the universe has only 3.3 billion years old, have an average black hole mass in the order of 10^8 to 10^9 solar masses, a hundred to a thousand times larger than the Milky Way’s compact object.