Hubble Explores the Formation and Evolution of Star Clusters in the Large Magellanic Cloud
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NGC 1466, one
of the 5 clusters in the LMC in which the level of dynamical evolution (or
"dynamical age") was measured. Credit: ESA/Hubble & NASA.
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Just as people of the same age can vary greatly in appearance and shape, so do collections of stars or stellar aggregates. New observations from the NASA/ESA Hubble Space Telescope suggest that chronological age alone does not tell the complete story when it comes to the evolution of star clusters.
Previous research on the formation and evolution of star clusters has suggested that these systems tend to be compact and dense when they form, before expanding with time to become clusters of both small and large sizes. New Hubble observations in the Large Magellanic Cloud (LMC) galaxy have increased our understanding of how the size of star clusters in the LMC changes with time[1].
Star clusters are aggregates of many (up to one million) stars. They are active systems in which the mutual gravitational interactions among the stars change their structure over time (known to astronomers as "dynamical evolution"). Because of such interactions, heavy stars tend to progressively sink towards the central region of a star cluster, while low-mass stars can escape from the system. This causes a progressive contraction of the cluster core over different timescales and means that star clusters with the same chronological age can vary greatly in appearance and shape because of their different “dynamical ages”.
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This image
from the NASA/ESA Hubble Space Telescope reveals an ancient, glimmering ball of
stars called NGC 1466. It is a globular cluster — a gathering of stars all held
together by gravity — that is slowly moving through space on the outskirts of
the Large Magellanic Cloud, one of our closest galactic neighbours. NGC 1466
certainly is one for extremes. It has a mass equivalent to roughly 140 000 Suns
and an age of around 13.1 billion years, making it almost as old as the
Universe itself. This fossil-like relic from the early Universe lies some 160
000 light-years away from us. NGC 1466 is one of the 5 clusters in the LMC in
which the level of dynamical evolution (or "dynamical age") was
measured. Credit:
ESA/Hubble & NASA.
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Located nearly 160 000 light-years from Earth, the LMC is a satellite galaxy of the Milky Way which hosts star clusters covering a wide range of ages. This differs from our own Milky Way galaxy which primarily contains older star clusters. The distribution of sizes as a function of age observed for star clusters in the LMC is very puzzling, as the young clusters are all compact, while the oldest systems have both small and large sizes.
All star clusters, including those in the LMC, have been found to host a special type of re-invigorated stars called blue stragglers [2]. Under certain circumstances, stars receive extra fuel that bulks them up and substantially brightens them. This can happen if one star pulls matter off a neighbour, or if they collide.
As a result of dynamical aging, heavier stars sink towards the centre of a cluster as the cluster ages, in a process similar to sedimentation, called “central segregation”. Blue stragglers are bright, making them relatively easy to observe, and they have high masses, which means that they are affected by central segregation and can be used to estimate the dynamical age of a star cluster [3].
Francesco Ferraro of the University of Bologna in Italy and his team used the Hubble Space Telescope to observe blue stragglers in five (coeval) old LMC star clusters with different sizes and succeeded in ranking them in terms of their dynamical age.
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This
illustration demonstrates the two ways that blue stragglers in star clusters
form. The illustration on the left shows
the collision model where two low-mass stars in an overcrowded environment
experience a head-on collision, combining their fuel and mass to form a single
hot star. The illustration on the right
depicts the "vampire" model consisting of a pair of stars that
undergo a transformation, with the lower-mass star draining its larger-mass
companion of the hydrogen that fuels its rebirth. Credit: NASA/ESA.
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“We demonstrated that different structures of star clusters are due to different levels of dynamical ageing: they are in different physical shape despite the fact that they were born at the same cosmic time. This is the first time that the effect of dynamical ageing has been measured in the LMC clusters” says Ferraro.
“These findings present intriguing areas for further research, since they reveal a novel and valuable way of reading the observed patterns of LMC star clusters, providing new hints about the cluster formation history in the LMC galaxy,” adds co-author Barbara Lanzoni.
More information: F. R. Ferraro et al. Size diversity of old Large Magellanic Cloud clusters as determined by internal dynamical evolution, Nature Astronomy (2019). DOI: 10.1038/s41550-019-0865-1
This animation
(artist’s impression) shows the movement of blue stragglers in a star cluster
over time. Blue straggler stars are blue, bright stars, with a higher mass than
the average for a cluster, and they are expected to sink towards the centre of
a star cluster over time. Those closest to the cluster core are the first to
migrate inwards, with more distant blue stragglers progressively moving inwards
over time. Credit:
ESA/Hubble & NASA, L. Calçada.
Notes:
[1] The observations were achieved from a set of long exposures acquired with the NASA/ESA Hubble Space Telescope’s Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS) for five old star clusters in the Large Magellanic Cloud galaxy, secured under proposal 14164 (PI: Sarajedini).
[2] Blue stragglers are so called because of their blue colour, and the fact that their evolution lags behind that of their neighbours.
[3] Blue stragglers combine being relatively bright and having high mass by the standards of globular cluster stars, but they are not the only stars within these clusters that are either bright or massive.
Source: ESA/Hubble Information Centre,
