Globular clusters supposedly all formed at the same time long ago, but some look young. How do astronomers rescue the belief that they are ancient groupings of stars?
Live Science (echoed on Space.com) introduced, once again (10/05/2003, 7/12/2008), the problem of “blue straggler stars” (BSS) – blue supergiant stars in globulars that appear too young to inhabit such old clusters (the hot blue stars are thought to form and explode in too short a time to exist in ancient clusters that supposedly formed soon after the big bang). Reporter Mike Wall personified them thus:
While such star clusters are many billions of years old, some of them manage to stay young at heart while others speed along toward decrepitude, astronomers found.
Given this range of incompatible ages, how can astronomers explain it? One way is to tweak the “evolutionary rate” dial:
“By studying the distribution of a type of blue star that exists in the clusters, we found that some clusters had indeed evolved much faster over their lifetimes, and we developed a way to measure the rate of aging,” lead author Francesco Ferraro, of the University of Bologna in Italy, said in a statement.
The astronomers used an analogy of sediment sinking in a tank to classify clusters with blue stragglers concentrated near the center of some clusters, while others appear more homogeneous. It’s not clear, though, that astronomers can simply tweak a dial to keep theory intact. Why would some clusters evolve faster than others?
A few clusters had blue stragglers distributed throughout, making them appear young. Some seemed old, with the stragglers already clumped in the center. And others were somewhere in between.
“Since these clusters all formed at roughly the same time, this reveals big differences in the speed of evolution from cluster to cluster,” said co-author Barbara Lanzoni, also of the University of Bologna. “In the case of fast-aging clusters, we think that the sedimentation process can be complete within a few hundred million years, while for the slowest it would take several times the current age of the universe.”
Apparently their response boils down to, that’s just the way things are.
Did the original paper in Nature by Ferraro et al. come up with any better explanation, perhaps some law of nature that tweaks the evolutionary rate dial in a predictable way?
Globular star clusters that formed at the same cosmic time may have evolved rather differently from the dynamical point of view (because that evolution depends on the internal environment) through a variety of processes that tend progressively to segregate stars more massive than the average towards the cluster centre. Therefore clusters with the same chronological age may have reached quite different stages of their dynamical history (that is, they may have different ‘dynamical ages’). Blue straggler stars have masses greater than those at the turn-off point on the main sequence and therefore must be the result of either a collision or a mass-transfer event.
The fixed parameter in this story is the supposed ancient date of cluster formation. So far, the variables are undefined: a “variety of processes”. However, that word “process” never appears in the rest of the paper. The authors referred to “the long-term effect of dynamical friction acting on the cluster binary population (and its progeny) since the early stages of cluster evolution,” but did not explain why that fraction should vary all over the map from cluster to cluster. With “crude approximations” they claimed some success for a few clusters.
It’s also not clear why the “dynamical age” should differ from the absolute age. A cluster should only have a true age. Despite their hubris, if the “dynamical age” appears young, what are they saying?
The signature of the parent cluster’s dynamical evolution encoded in the BSS population has now been finally deciphered: the shape of the radial distribution of BSSs is a powerful indicator of dynamical age. A flat radial distribution of BSSs … indicates that dynamical friction has not yet had a major effect even in the innermost regions, and the cluster is still dynamically young.
The authors still don’t seem to understand the issue. Why should dynamical friction (if it is a law of nature) work differently in different clusters? Isn’t this begging the question of dynamical age? Calling a cluster “dynamically young” because friction hasn’t yet pulled the BSS stars into the center is theory-laden – dependent on the notion that all globular clusters formed around the same time after the big bang. The authors did attempt independent confirmation:
The quantization into distinct age-families is of course an oversimplification: in nature a continuous behaviour is expected and the position of rmin should vary with continuity as a sort of clock hand. This allows us to push our analysis further and define the first empirical clock able to measure the dynamical age of a stellar system from pure observational quantities (the ‘dynamical clock’): in the same way as the engine of a chronometer advances a clock hand to measure the flow of time, in a similar way dynamical friction moves rmin within the cluster, measuring its dynamical age. [Note: rmin refers to the radius of the portion of the distribution of blue stragglers beyond the cluster core].
Their “empirical clock,” however, is calibrated to two other theory-laden models on how long a cluster should take to reach its relaxation state, based on the theory that globulars are ancient: i.e., “two theoretical estimates commonly used to measure the dynamical evolution timescales of a cluster, namely the central and the half-mass relaxation times”. But those models are said to be less precise than the new proposed model: “In fact, the radial distribution of BSSs simultaneously probes all distances from the cluster centre, providing a measure of the overall dynamical evolution and a much finer ranking of dynamical ages.”
In the end, then, their taxonomy of “dynamical ages” looks like special pleading to maintain big-bang and stellar-evolution theory, without independent support for believing it represents “the way things are,” other than the anomaly can be made to fit current thinking.
This episode serves as a demonstration of how normal science is conducted in a post-Kuhnian world. To see why, you need to come at the article and the paper as a fair-minded skeptic. Don’t just take their word for it that clusters are ancient but have different dynamical histories: say, “Show me.” Are you convinced by their hand-waving attempts to rescue their theory?
To a member of the guild working within the paradigm, everything makes perfect sense. Guild members are not asking whether the paradigm is true. Instead, the name of their game is keeping the paradigm going. An anomaly shows up: blue stragglers that shouldn’t be there. Morever, these BSS intruders are distributed all over the map from cluster to cluster: some concentrated at the center, some homogeneous with BSS at the edges. The “way things are” is not “the way things should be” if the paradigm were true. No problem; we’ll just invent a new parameter called “dynamical age” that is calibrated to the assumed age of the clusters; the more concentrated the BSS, the older the dynamical age compared to the cluster age. Very useful, isn’t it? It serves as a way to classify the clusters within the paradigm. Theory rescued.
If this sounds like self-serving poppycock, you have not yet been hoodwinked by “normal science.” You were thinking that science is supposed to be an honest search for the truth about the universe. How naive. If that were the goal of science, reporters would certainly not have regurgitated the oracles of the guild uncritically.