A barred spiral galaxy is a spiral galaxy with a straight, elongated structure of stars cutting across its center. Instead of spiral arms winding outward from a compact core, the arms extend from the tips of this central bar, giving the galaxy a distinctive shape. It’s not a rare type. Nearly 70 percent of spiral galaxies in the local universe have bars, making this the most common variety of spiral galaxy, and our own Milky Way is one of them.
How the Bar Changes the Shape
In a standard spiral galaxy, the arms curl outward from the galaxy’s central bulge. In a barred spiral, a rigid-looking band of stars stretches across the core, and the spiral arms branch off from the ends of that bar rather than from the center itself. The bar is not just a visual feature. It acts as a major structural component that influences how gas, dust, and stars move throughout the galaxy.
Bars come in different sizes and strengths. Some are short and subtle, barely distinguishable from an elongated bulge. Others are long and dramatic, dominating the galaxy’s inner region. NGC 1300, one of the most photographed barred spirals, sits about 69 million light-years from Earth and shows this structure clearly: a bold bar of older, orange-tinted stars flanked by dark dust lanes, with blue star-forming arms sweeping away from each end. Interestingly, galaxies with especially large bars can develop a “spiral within a spiral,” a tight swirl of gas and dust nested inside the bar itself.
How Astronomers Classify Them
Barred spirals fit into the Hubble classification system, the framework astronomers have used since the 1930s to sort galaxies by shape. They receive a “B” in their designation. An SBa galaxy is a barred spiral with tightly wound arms and a large central bulge. An SBc galaxy has loosely wound arms and a smaller bulge. SBb falls in between. The tighter the arms are wound, the larger the bulge tends to be.
This lettering system runs parallel to the one for unbarred spirals (Sa, Sb, Sc), and the two branches form the famous “tuning fork” diagram that Edwin Hubble originally proposed. The classification is based on visual appearance, not on how old or evolved a galaxy is.
Why Bars Form
Bars develop because of gravitational instabilities in a galaxy’s rotating disk of stars. As the disk spins, small density variations can grow and reinforce each other, pulling stars into elongated orbits that collectively trace out a bar shape. The key process is the redistribution of angular momentum: the spinning energy of the disk gets shuffled between the stars and the surrounding halo of dark matter, and under the right conditions, this exchange allows a bar to emerge.
Not every spiral galaxy develops a bar, and the conditions matter. A galaxy with a very dense central bulge can resist bar formation even if its disk is otherwise unstable. The surrounding dark matter halo plays a role too. Halos that rotate in the same direction as the disk, but not too quickly, tend to favor bar growth. If the halo spins too fast, it can suppress the instability. The interplay between visible matter and dark matter is central to whether a bar takes hold.
Bars also appear to be a relatively modern phenomenon. A Hubble Space Telescope survey of more than 2,000 spiral galaxies found that only 20 percent of spirals had bars 7 billion years ago, compared to nearly 70 percent today. This suggests that bars tend to form in galaxies that have had time to settle into stable, well-organized disks.
Bars Are Not Permanent
Despite their solid appearance, galactic bars are not necessarily permanent structures. In gas-rich spiral galaxies, bars may be transient features with lifetimes of roughly 1 to 2 billion years. The bar gradually funnels gas inward, building up mass at the galaxy’s center. That growing central mass concentration weakens the gravitational conditions that sustain the bar, and it eventually dissolves. A new bar can later reform once the disk becomes unstable again, creating a repeating cycle.
This means the high fraction of barred spirals we see today doesn’t require each bar to be ancient. Galaxies may cycle through barred and unbarred phases over cosmic time.
What the Bar Does to Its Galaxy
The bar is the single most important driver of slow, internal change in a disk galaxy. As it rotates, it exerts a gravitational torque on nearby gas and dust, stripping angular momentum from material in the inner regions. Gas that loses angular momentum falls inward. Large bars can push gas from the outer disk all the way into the central kiloparsec (roughly the innermost 3,000 light-years) of the galaxy.
This inward flow of gas has two major consequences. First, it fuels bursts of star formation near the galaxy’s center. Surveys show that about 50 percent of actively star-forming galaxies have bars, compared to only 29 percent of inactive galaxies. Second, it feeds the supermassive black hole lurking at the core. Barred galaxies tend to have more massive central black holes than their unbarred counterparts. Simulations show the peak of the black hole mass distribution in barred galaxies is roughly twice that of unbarred galaxies, a difference of about 40 million solar masses that accumulates through higher accretion rates over billions of years.
Once gas reaches the central region, smaller-scale instabilities can develop, funneling material even closer to the black hole. This cascading process, large bar to central region to nuclear instabilities to black hole, helps explain why bars and active galactic nuclei are often found together.
The Milky Way’s Bar
Our galaxy is a barred spiral. The bar stretches roughly 3.1 to 3.5 kiloparsecs across (about 10,000 to 11,400 light-years in diameter), with the near end angled toward us at roughly 27 degrees from the line between the Sun and the galactic center. Some studies suggest a longer, thinner component extending 4 to 4.5 kiloparsecs, oriented at about 45 degrees.
You can’t see the Milky Way’s bar by looking at the night sky. It’s buried behind dense clouds of gas and dust in the direction of the constellation Sagittarius. Astronomers mapped it using infrared observations, which can penetrate dust, and by tracking the distribution of certain types of red giant stars across the galaxy’s inner region. The bar’s shape has axial ratios of about 1:0.4:0.3, meaning it’s roughly 2.5 times longer than it is wide and slightly flattened from top to bottom.
The Milky Way’s bar also shows signs of what astronomers call a “pseudobulge,” a peanut-shaped or boxy central structure that forms through the bar’s own gravitational influence rather than through galaxy mergers. This is a hallmark of secular evolution, the slow internal reshaping of a galaxy driven by its own bar.