Blue Stragglers: A Study of Stellar Longevity

Charles Danforth,
The Johns Hopkins University, Baltimore, MD 21218
January 19, 1998


Though Blue Straggler Stars (hence BSSs) only became a ``household'' term in astronomical circles recently, they have been known about for quite some time. In 1953, Sandage noted that a few stars in M3 appeared blue-ward and with greater luminosity than those at the Main Sequence Turn Off (MSTO), yet coincided more or less with the extended cluster main sequence (Stryker 93). He noted this ``blue extension'' as peculiar because any stars of high luminosity on the Main Sequence (hence MS) should have long since evolved.

Since both stellar lifetime on the main sequence and luminosity are proportional to a positive power of mass, in a large enough sample of stars, the main sequence will be unpopulated above a certain luminosity. All of the stars with tex2html_wrap_inline28 (and hence tex2html_wrap_inline30) will have evolved off the main sequence. Blue stragglers are notable because they are apparently normal MS stars of luminosity and mass greater than those currently evolving toward the red giant phase (Figure 1). It is as if something was keeping these stars young longer than their peers.

Over the years, BSSs were found in other clusters by Sandage and others. At this point (at least by 1995) there have been over 700 BSSs detected in the more than 30 globular clusters and a few galactic clusters searched. Every cluster which has been studied in enough detail has produced at least one BSS and most have a dozen or more (Bailyn 95). Ferraro et al (1997) counted 171 in the core of M3 with another 30 or so existing in the outer fringes.

Field star contamination?

The first and most logical assumption is that the sample of stars has been polluted with other stars from the field which are not members in the cluster and happen to either lie along our line of site, or have been somehow captured by the cluster. However the tendency for BSSs to concentrate in the cores of clusters as well as their consistent radial velocities with other cluster members belies this theory. They are definitely part of the clusters.

Origin Theories

Clearly these stars are either being kept young, are intruders from a different population, or are anomalous in their composition, causing them to burn more quickly. There are many theories as to explain the odd behavior of BSSs.

Horizontal Branch confusion?

An original origin theory put forward by Sargent (1968) was that BSSs were horizontal branch (HB) stars coincidentally crossing ZAMS. His measurements showed lower surface gravities than were to be expected from MS stars implying the expanded atmospheres of evolved stars. However, later work showed that, evolved stars do have lower surface gravities, initially high-mass stars evolve to the blue end of the HB while most BSSs lie on the red side of the MS (Stryker 93). Furthermore, most BSSs are substantially fainter than HB stars.

Recent star formation?

Perhaps BSSs are not being kept in a state of youth, but are, in fact, members of a younger generation forming some period of time after the first cycle of star birth. In this case, the BSSs represent a smaller, group with it's own turn off. While a number of blue stragglers appear to the red of the MS, almost as if they were evolving through a secondary turn-off, they show no such coordinated movement as is visible with the main cluster population. In any case, a secondary period of star formation is unlikely in globular clusters because of the extreme lack of star forming materials.

Another piece of evidence against the recent formation theory was uncovered by Leonard (1996) in his studies of spectroscopic binary BSSs in M67. The fraction of binary BSSs in M67 which display periods of less than 10tex2html_wrap_inline32 days is 0.6tex2html_wrap_inline340.24. However, in the galaxy as a whole, the fraction has been established as 0.200tex2html_wrap_inline340.034. The fact that BSSs appear in short-period binaries more often than normal stars implies that their binary nature might be significant in their uniqueness.


By far the preferred theories today fall under the class of stellar mergers and mass transfer. This presents an attractive way of increasing the mass of a star at a time later than the formation time. It appears that BSSs may arise via several of these pathways.

Mass transfer:

Leonard (1996) shows that many BSSs are members of short-period binaries (as are many normal stars). If these are symbiotic binaries, they may share mass between stars. For instance, if the more massive of the pair has recently exhausted its core hydrogen it may fill its Roche lobe and dump mass onto the lower-mass companion. With increasing mass, the core increases in size, expanding into regions with less processed material,expanding the core hydrogen burning lifetime of the star.

The BSS D266 in NGC2354 appears to have been formed in this way (Lapasset and Ahumada 96). Photometric measurements show that this very close binary system has a period of 0.6 days and is probably very nearly a contact system. A large temperature ratio suggests that the partner star has evolved already and has lost mass onto the smaller-mass companion, enriching it and forming a BSS.

Coalescence of primordial binaries:

Coalescence appears to be one of the primary BSS creation pathways. A close binary pair may loose angular momentum through a variety of means and spiral closer, eventually merging into a common envelope. Perhaps angular momentum is carried off by a third companion star or a close encounter with another cluster star. Mass loss via stellar wind could also cause the orbits to collapse leading to a coalescence.

Models by Sandquist, Bolte and Hernquist (1997, hence SBH) and by others before them show that a binary behaving in this manner will proceed from a classic contact binary sharing a common photosphere to a bar-shaped object with touching cores. Finally, the two cores will merge and the pair becomes a flattened single star with very rapid rotation.

Shara, Saffer and Livio (1997) have observed tex2html_wrap_inline38 (>100 times that of the sun) in BSS19 in 47Tuc. This 1.7 solar mass BSS could well have been formed by coalescence, though other formation mechanisms have not been ruled out. Other observations (Lapasset et al 96) have shown that BSSs often have high rotational velocities giving the coalescence theory more plausibility.

Furthermore, in the SBH models, most of the two progenitor's stock of He is kept in the core of the merger star (figure 2). Observations have shown that the ``Blue Straggler Main Sequence'' lies a little to the red of the ZAMS on a CMD and diverges as luminosity increases (figure 3). This is suggestive of stars born with He-enriched cores but unenriched outer layers (Mandushev et al 97).


Along the same lines as the coalescence origin theory is the whole realm of collisions, devised by Hills and Day (1976). The hydrodynamics of such stellar collision were most recently modeled by Lombardi, Rasio and Shapiro (1998). They found that, as in the case of primordial binary coalescence, there is only negligible mixing of He and heavier elements into the outer envelope.

As to the probability of such an event happening; the density of inner regions of a globular cluster virtually guarantees encounters to be frequent. The effective cross-section of multiple star systems is truly enormous, so collisions between binary and higher systems must be more efficient than those of single stars. Predictions have been made that 85 The hydrodynamics of such stellar collision were recently modeled by Lombardi, Rasio and Shapiro (1998). They found that, as in the case of primordial binary coalescence, there is only negligible mixing of He into the outer envelope.

One thing that both the collision and coalescence theories both predict is that no BSS should have more than twice the mass of a current star at MSTO. Unfortunately, masses have been directly measured for only a few BSSs thusfar, (Shara, Saffer and Livio, 97), and these are of no help disproving either of the above theories. Conceivably, a very massive BSS could be constructed from the collision or coalescence of three or more stars or the collision of a BSS with another star, but the likelihood of such an event within the lifetime of a BSS is quite small (Leonard 96).

Recent observations of M3 (by far the best-studied group of BSSs to date) by Ferraro et al (1997) suggest that several of these formation theories may be correct. Recent HST observations allowed individual stars down to the very core of M3 to be resolved and thus a very deep CMD to be found. The 171 BSS candidates were found to have a curious radial distribution with a strong peak at the center and a secondary peak some 320 arcseconds out (Figure 5). This suggests two processes at work. The central batch of stars in the densest region of the cluster have the highest probability of stellar collisions, thus the high number of BSSs in the core region. As radius increases, encounters become less likely and thus primordial binaries are likely to last long enough to coalesce. On the other hand, increasing radius and time between encounters also limits the rate at which angular momentum can be lost to unbound third-parties.


While BSSs undoubtedly exist all throughout the galaxy, it is in globular clusters that they are most easily studied. These clusters are home to hundreds of thousands of stars each, each of which must be photometrically measured and classified. Only recently have we had the ability to resolve individual stars all the way to the core of these star fields (Ferraro et al 97, Mandushev et al 97) to obtain a sample of almost all the stars in the system. Furthermore, only recently were algorithyms developed to accurately separate the overlapping images of stars (such as IRAF's DAOPHOT). These two developments are one reason why the study of BSSs has taken so long to really mature!

But our problems are far from over. There is still the ambiguity between BSSs and normal MS stars at the MSTO and below. It is entirely likely that there exist a whole population of ``red stragglers'' with tex2html_wrap_inline40. Only with an unfeasibly long time base are these distinctions made. Often this problem is punted by setting a magnitude cutoff and examining only a subset of a cluster BSS population (Ferraro et al 97), or by setting a color cutoff (Mandushev et al 97). Furthermore, evolved BSSs will follow different tracks from the currently evolving non-stragglers. It is conceivable that BSSs both pre- and post-evolution account for some of the spread inherent in CMDs.

There is the problem of optical blends. The light received from a close binary or line-of-sight double cannot be resolved into separate stars. The integrated light of the pair biases the sample considerably. If BSSs are indeed mergers between two or more stars, what happens if one of the stars has already evolved? How does this effect the product straggler?


The study of BSSs has bloomed in the last decade with the advent of powerful new hardware and software. With this comes a new interest in what BSSs can tell us about cluster dynamics. Assuming the current theories as to formation remain in favor, BSSs provide a new probe into stellar structure and evolution.

A better understanding of BSSs both in clusters and in the galaxy as a whole will help tremendously with the study of other galaxies. Since individual stars are often unresolvable, it is only the integrated light from a galaxy that is available to study. Populations of blue stragglers will effect the overall color of galaxies.


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