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Bars form when a spiral galaxy’s stellar disk becomes unstable to a large-scale pattern that lines up stars’ elongated orbits across the center. Whether that instability grows depends on the disk’s motion and self-gravity, the distribution of mass in the galaxy’s center, and how the surrounding dark matter halo responds. Gas and encounters can affect the process, but none is a universal switch that determines whether a galaxy will have a bar.
What a galaxy bar is—and how it grows
A stellar bar is an elongated arrangement of stars that crosses a galaxy’s center. It is not a rigid object placed inside the disk; it is a collective structure that emerges from the stars’ gravity and motion.
A useful way to picture its growth is to imagine that some stellar orbits become slightly elongated rather than nearly circular. If those elongations line up, their gravity can reinforce the alignment, drawing more stars into the pattern. NASA’s account of a Hubble study quotes team member Bruce Elmegreen: “The tiny elongations in the stars’ orbits grow and they get locked into place, making a bar.” In dynamical terms, the disk supports a growing, global non-axisymmetric instability—a pattern that breaks the disk’s approximate circular symmetry.
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Disk motion and self-gravity
A dynamically cool stellar disk, whose stars have relatively orderly motions, can respond strongly to disturbances. When the disk’s own gravity is important enough, small changes in stellar orbits can reinforce one another rather than simply disperse. These conditions make a bar more likely to grow, but they do not guarantee one: the galaxy’s full mass distribution and dynamical history also matter.
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The inner mass distribution
The amount and arrangement of mass near the center can alter whether a bar instability takes hold. In collisionless simulations, compact classical bulges prevented bars from growing for at least 4 billion years, even in models with a maximal stellar disk and low Toomre Q. That is a result for those modeled systems, not evidence that every unbarred galaxy has a compact bulge.
A 2024 analysis of the TNG50 cosmological simulation found that its barred galaxies had systematically higher central stellar mass relative to dark matter before bar formation. Taken together, these results point to the importance of central structure without establishing one universal recipe for barred or unbarred galaxies.
Does dark matter prevent bars?
Not as a general rule. Early models found that a massive, stabilizing halo could make a disk less prone to bar formation. But a halo that responds dynamically is different from a fixed background: a live halo can absorb angular momentum from a bar and help the bar grow. Halo mass and halo response therefore cannot be reduced to “more dark matter means fewer bars.”
Do gas or encounters trigger—or prevent—bars?
Gas content
Gas can influence a bar’s formation and later evolution, but gas-rich galaxies are not automatically barred-free. An analysis of the local S4G survey found bars across a broad range of atomic gas fractions and colors. Any relationship between gas and bar frequency depends on the sample and how bars are measured.
Companions and mergers
A passing companion or merger can perturb a disk and affect its path toward a bar. Such an encounter is not required in every formation history, however. In its TNG50 sample, researchers found no clear link between mergers and the disk instabilities leading to bars. Interactions may promote, delay, create, or destroy bars under different conditions; their effects depend on timing and on the state of the galaxy.
Can a galaxy develop a bar later?
Yes. Bar formation is part of galaxy evolution, not a permanent label set at a galaxy’s birth. As a disk’s structure and dynamics change, its susceptibility can change too. Once present, a bar can redistribute angular momentum and drive gas inward, contributing to central star formation and the buildup of central structures. Bar-driven inflow has also been proposed as a way to fuel active galactic nuclei, but observational confirmation of that connection has been elusive; it should not be treated as an inevitable result of having a bar.
What observations reveal—and why reported bar fractions differ
Bar fractions are measurements of particular samples, not universal constants. They vary with cosmic epoch, galaxy-mass selection, bar definition, observing wavelength, and image resolution.
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|---|---|---|
| Hubble/COSMOS study, as reported by NASA in 2008 | More than 2,000 spiral galaxies; about 20% of the distant sample had bars, compared with nearly 70% of modern counterparts. | A study-specific comparison across cosmic time, not a fixed fraction for all galaxies today. |
| S4G local-universe analysis, 2018 | Bar frequency reached approximately 0.70 near a stellar mass of 109.7 solar masses. | A local-sample result with mass-dependent trends. The analysis found differences from some SDSS results and showed that resolution thresholds can reproduce some apparent survey trends. |
These findings are not directly contradictory: one compares distant galaxies with modern counterparts, while the other examines local galaxies and trends with stellar mass. Selection and measurement choices must be considered before comparing the percentages.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.The short answer
A bar grows when a galaxy’s stellar disk can sustain and amplify a coherent, elongated pattern of stellar orbits. Disk temperature and self-gravity, central mass structure, the response of the dark matter halo, gas, and interactions all shape that possibility. No single factor explains every galaxy; bars emerge from coupled dynamics and histories that can change over time.
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