A blue straggler is a type of star that is more luminous and bluer than expected. Typically identified in a stellar cluster, they have a higher effective temperature than the main sequence turnoff point for the cluster, where ordinary stars begin to evolve towards the red giant branch. Blue stragglers were first discovered by Allan Sandage in 1953 while performing photometry of the stars in the globular cluster M3. [1] [2]
Standard theories of stellar evolution hold that the position of a star on the Hertzsprung–Russell diagram should be determined almost entirely by the initial mass of the star and its age. In a cluster, stars all formed at approximately the same time, and thus in an H–R diagram for a cluster, all stars should lie along a clearly defined curve set by the age of the cluster, with the positions of individual stars on that curve determined solely by their initial mass. With masses two to three times that of the rest of the main-sequence cluster stars, blue stragglers seem to be exceptions to this rule. [3] The resolution of this problem is likely related to interactions between two or more stars in the dense confines of the clusters in which blue stragglers are found. Blue stragglers are also found among field stars, although their detection is more difficult to disentangle from genuine massive main sequence stars. Field blue stragglers can however be identified in the Galactic halo, since all surviving main sequence stars are low mass. [4]
Several explanations have been put forth to explain the existence of blue stragglers. The simplest is that blue stragglers formed later than the rest of the stars in the cluster, but evidence for this is limited. [6] Another simple proposal is that blue stragglers are either field stars which are not actually members of the clusters to which they seem to belong, or are field stars which were captured by the cluster. This too seems unlikely, as blue stragglers often reside at the very center of the clusters to which they belong. The most likely explanation is that blue stragglers are the result of stars that come too close to another star or similar mass object and collide. [7] The newly formed star has thus a higher mass, and occupies a position on the HR diagram which would be populated by genuinely young stars.
The two most viable explanations put forth for the existence of blue stragglers both involve interactions between cluster members. One explanation is that they are current or former binary stars that are in the process of merging or have already done so. The merger of two stars would create a single more massive star, potentially with a mass larger than that of stars at the main-sequence turn-off point. While a star born with a mass larger than that of stars at the turn-off point would evolve quickly off the main sequence, the components forming a more massive star (via merger) would thereby delay such a change. There is evidence in favor of this view, notably that blue stragglers appear to be much more common in dense regions of clusters, especially in the cores of globular clusters. Since there are more stars per unit volume, collisions and close encounters are far more likely in clusters than among field stars and calculations of the expected number of collisions are consistent with the observed number of blue stragglers. [7]
One way to test this hypothesis is to study the pulsations of variable blue stragglers. The asteroseismological properties of merged stars may be measurably different from those of typical pulsating variables of similar mass and luminosity. However, the measurement of pulsations is very difficult, given the scarcity of variable blue stragglers, the small photometric amplitudes of their pulsations and the crowded fields in which these stars are often found. Some blue stragglers have been observed to rotate quickly, with one example in 47 Tucanae observed to rotate 75 times faster than the Sun, which is consistent with formation by collision. [9]
The other explanation relies on mass transfer between two stars born in a binary star system. The more massive of the two stars in the system will evolve first and as it expands, will overflow its Roche lobe. Mass will quickly transfer from the initially more massive companion onto the less massive; like the collision hypothesis, this would explain why there are main-sequence stars more massive than other stars in the cluster which have already evolved off the main sequence. [10] Observations of blue stragglers have found that some have significantly less carbon and oxygen in their photospheres than is typical, which is evidence of their outer material having been dredged up from the interior of a companion. [11] [12]
Overall, there is evidence in favor of both collisions and mass transfer between binary stars. [13] In M3, 47 Tucanae, and NGC 6752, both mechanisms seem to be operating, with collisional blue stragglers occupying the cluster cores and mass transfer blue stragglers at the outskirts. [14] The discovery of low-mass white dwarf companions around two blue stragglers in the Kepler field suggests these two blue stragglers gained mass via stable mass transfer. [15]
Blue stragglers are also found among field stars, as a result of close binary interaction. Since the fraction of close binaries increases with decreasing metallicity, blue stragglers are increasingly likely to be found across metal poor stellar populations. The identification of blue stragglers among field stars however is more difficult than in stellar clusters, because of the mix of stellar ages and metallicities among field stars. Field blue stragglers however can be identified among old stellar populations, like the Galactic halo, or dwarf galaxies. [4]
"Yellow stragglers" or "red stragglers" are stars with colors between that of the turnoff and the red-giant branch but brighter than the subgiant branch. Such stars have been identified in open and globular star clusters. These stars may be former blue straggler stars that are now evolving toward the giant branch. [16]
A globular cluster is a spheroidal conglomeration of stars that is bound together by gravity, with a higher concentration of stars towards its center. It can contain anywhere from tens of thousands to many millions of member stars, all orbiting in a stable, compact formation. Globular clusters are similar in form to dwarf spheroidal galaxies, and the distinction between the two is not always clear. Their name is derived from Latin globulus. Globular clusters are occasionally known simply as "globulars".
An open cluster is a type of star cluster made of tens to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, and many more are thought to exist. Each one is loosely bound by mutual gravitational attraction and becomes disrupted by close encounters with other clusters and clouds of gas as they orbit the Galactic Center. This can result in a loss of cluster members through internal close encounters and a dispersion into the main body of the galaxy. Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years. In contrast, the more massive globular clusters of stars exert a stronger gravitational attraction on their members, and can survive for longer. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring.
A star is a luminous spheroid of plasma held together by self-gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night; their immense distances from Earth make them appear as fixed points of light. The most prominent stars have been categorised into constellations and asterisms, and many of the brightest stars have proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. The observable universe contains an estimated 1022 to 1024 stars. Only about 4,000 of these stars are visible to the naked eye—all within the Milky Way galaxy.
Star clusters are large groups of stars held together by self-gravitation. Two main types of star clusters can be distinguished. Globular clusters are tight groups of ten thousand to millions of old stars which are gravitationally bound. Open clusters are more loosely clustered groups of stars, generally containing fewer than a few hundred members, that are often very young. As they move through the galaxy, over time, open clusters become disrupted by the gravitational influence of giant molecular clouds. Even though they are no longer gravitationally bound, they will continue to move in broadly the same direction through space and are then known as stellar associations, sometimes referred to as moving groups.
Tucana is a constellation in the southern sky, named after the toucan, a South American bird. It is one of twelve constellations conceived in the late sixteenth century by Petrus Plancius from the observations of Pieter Dirkszoon Keyser and Frederick de Houtman. Tucana first appeared on a 35-centimetre-diameter (14 in) celestial globe published in 1598 in Amsterdam by Plancius and Jodocus Hondius and was depicted in Johann Bayer's star atlas Uranometria of 1603. French explorer and astronomer Nicolas Louis de Lacaille gave its stars Bayer designations in 1756. The constellations Tucana, Grus, Phoenix and Pavo are collectively known as the "Southern Birds".
A variable star is a star whose brightness as seen from Earth changes systematically with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either:
A Cepheid variable is a type of variable star that pulsates radially, varying in both diameter and temperature. It changes in brightness, with a well-defined stable period and amplitude.
Messier 4 or M4 is a globular cluster in the constellation of Scorpius. It was discovered by Philippe Loys de Chéseaux in 1745 and catalogued by Charles Messier in 1764. It was the first globular cluster in which individual stars were resolved.
Messier 80 is a globular cluster in the constellation Scorpius. It was discovered by Charles Messier in 1781, being one of his first discoveries.
Messier 62 or M62, also known as NGC 6266 or the Flickering Globular Cluster, is a globular cluster of stars in the south of the equatorial constellation of Ophiuchus. It was discovered in 1771 by Charles Messier, then added to his catalogue eight years later.
47 Tucanae or 47 Tuc is a globular cluster located in the constellation Tucana. It is about 4.45 ± 0.01 kpc (15,000 ± 33 ly) from Earth, and 120 light years in diameter. 47 Tuc can be seen with the naked eye, with an apparent magnitude of 4.1. It appears about 44 arcminutes across including its far outreaches. Due to its far southern location, 18° from the south celestial pole, it was not catalogued by European astronomers until the 1750s, when the cluster was first identified by Nicolas-Louis de Lacaille from South Africa.
The red-giant branch (RGB), sometimes called the first giant branch, is the portion of the giant branch before helium ignition occurs in the course of stellar evolution. It is a stage that follows the main sequence for low- to intermediate-mass stars. Red-giant-branch stars have an inert helium core surrounded by a shell of hydrogen fusing via the CNO cycle. They are K- and M-class stars much larger and more luminous than main-sequence stars of the same temperature.
40 Cancri is a binary star system in the zodiac constellation of Cancer, located about 614 light years from the Sun in the Beehive Cluster. It is a challenge to view with the naked eye, having an apparent visual magnitude of 6.61. The system is moving further from the Earth with a heliocentric radial velocity of 34 km/s.
NGC 1818 is a young globular cluster in the north-west part of the Large Magellanic Cloud, about 3.2 kpc from the center. It was discovered by Scottish astronomer James Dunlop in 1826, and has since been well studied.
A stellar collision is the coming together of two stars caused by stellar dynamics within a star cluster, or by the orbital decay of a binary star due to stellar mass loss or gravitational radiation, or by other mechanisms not yet well understood.
SX Phoenicis is a variable star in the southern constellation Phoenix. With an apparent visual magnitude ranging around 7.33, it is too faint to be readily seen with the naked eye and requires binoculars. It is located 272 light years from the Sun, as determined from an annual parallax shift of 12 mas.
T Scorpii, or Nova Scorpii 1860, was a nova in the globular cluster Messier 80 (M80). It was discovered on 21 May 1860 by Arthur von Auwers at Koenigsberg Observatory and was independently discovered by Norman Pogson on May 28 at Hartwell observatory. It was at magnitude 7.5 at discovery, reaching a maximum of magnitude 6.8, outshining the whole cluster.
BL Boötis is a pulsating star in the constellation Boötes. It is the prototype of a class of anomalous Cepheids which is intermediate in the H-R diagram between the type I classical Cepheids and the type II Cepheids.
NGC 6388 is a globular cluster of stars located in the southern constellation of Scorpius. The cluster was discovered by Scottish astronomer James Dunlop on May 13, 1826 using a 20 cm (9 in) reflector telescope. It was later determined to be a globular cluster by English astronomer John Herschel, who was able to resolve it into individual stars. NGC 6388 is located at a distance of approximately 35,600 light-years (10.90 kpc) from the Sun. Due to its apparent visual magnitude of +6.8, binoculars or a small telescope are required to view it.
NGC 121 is a globular cluster of stars in the southern constellation of Tucana. It is the oldest globular cluster in the Small Magellanic Cloud (SMC), which is a dwarf satellite galaxy of the Milky Way. This cluster was first discovered by English astronomer John Herschel on September 20, 1835. The compiler of the New General Catalogue, Danish astronomer John Louis Emil Dreyer, described this object as "pretty bright, pretty small, little extended, very gradually brighter middle". The cluster is located at a distance of around 200,000 light-years (60 kpc) from the Sun.
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