This image shows a detailed, thousand-color image of the Sculptor Galaxy captured with the MUSE instrument at ESO’s Very Large Telescope (VLT). Regions of pink light are spread throughout this whole galactic snapshot, which come from ionised hydrogen in star-forming regions. These areas have been overlaid on a map of already formed stars in Sculptor to create the mix of pinks and blues seen here. (Credit: ESO/E. Congiu et al.)

In a nutshell Scientists identified over 500 planetary nebulae in the Sculptor galaxy, 20 times more than previous surveys, using the MUSE instrument, but the distance estimate they produced was significantly larger than expected.

The commonly used planetary nebula luminosity function (PNLF) method suggested the galaxy is about 600,000 light-years farther away than other reliable techniques like the tip of the red giant branch (TRGB), likely due to interference from dust.

This finding reveals that dust in starburst galaxies can skew distance measurements, which may impact how astronomers calculate the expansion rate of the universe and interpret other cosmic structures.

SANTIAGO, Chile — The universe just got 600,000 light-years bigger, at least according to a new international study that’s making astronomers nervous. Astronomers have used dying stars to map distances across the universe for decades. Using the largest sample of cosmic distance markers ever collected from a single galaxy, scientists have discovered that something is making them appear much farther away than they should be.

NGC 253, nicknamed the Sculptor galaxy, sits about 11 million light-years from Earth, close enough that astronomers can study individual stars and gas clouds within it. It’s what scientists call a “starburst galaxy,” meaning it’s churning out new stars at a furious pace while simultaneously blasting material into space through powerful galactic winds. This is the galaxy that this research, accepted for publication in Astronomy and Astrophysics, focuses on.

Using the European Southern Observatory’s MUSE instrument, researchers created the largest and most detailed map ever made of this galaxy’s glowing gas. When they used a tried-and-true method to measure the galaxy’s distance, they got an answer that’s dramatically different from what other reliable techniques have consistently shown.

Conflicting Measurements

The discrepancy is huge by astronomical standards. The new measurements suggest the galaxy is about 600,000 light-years farther away than previously thought.

The ESO Very Large Telescope (VLT) in action during observations. (Credit: ESO/S. Brunier)

Planetary nebulae are beautiful, glowing shells of gas expelled by dying stars. Despite their name, these objects have nothing to do with planets. When a star like our Sun reaches the end of its life, it puffs off its outer layers, creating these colorful cosmic bubbles that can be seen across vast distances.

Planetary nebulae have long served as objects with known brightness that help measure distances to far-off galaxies. The technique, called the Planetary Nebula Luminosity Function (PNLF), relies on the fact that the brightest planetary nebulae in any galaxy always seem to have roughly the same maximum brightness, no matter where you look in the universe.

MUSE observations identified about 500 planetary nebulae in the Sculptor galaxy, roughly 20 times more than previous studies had found. With this massive sample size, the team expected to get the most accurate distance measurement yet. Instead, they got a result that disagrees with nearly every other method used to measure the galaxy’s distance.

Most distance measurements to the Sculptor galaxy, including recent studies using the tip of the red giant branch method, another reliable cosmic ruler, consistently place it about 11 million light-years away. But the new planetary nebulae analysis suggests it’s closer to 13.4 million light-years away.

Accurate distance measurements are crucial for understanding how fast the universe is expanding, the nature of dark energy, and the structure of cosmic space. When reliable methods start disagreeing, it signals that something important is happening that scientists don’t yet understand.

The researchers found that planetary nebulae in different parts of the galaxy give different distance estimates. Those near the galaxy’s dusty, star-forming center suggest an even greater distance, while those in the outer regions align more closely with the overall measurement.

Dusty Dimming

The culprit appears to be dust, tiny particles scattered throughout the galaxy that absorb and scatter light. The Sculptor galaxy is viewed nearly edge-on from Earth, meaning we’re looking through its entire dusty disk. This dust makes objects appear dimmer than they actually are, which in turn makes them seem farther away.

Using a technique that compares the brightness of different hydrogen emission lines, the researchers mapped dust throughout the galaxy. They saw that light from the galaxy was being heavily obscured by dust, with some regions showing more than six magnitudes of dimming. To put that in perspective, six magnitudes means an object appears about 250 times dimmer than it would without dust.

Computer models created by the team suggest that dust distributed throughout the galaxy’s disk can indeed bias planetary nebulae measurements toward greater distances. The effect is particularly strong when dust extends high above the galaxy’s central plane, exactly what might happen in a starburst galaxy where powerful stellar winds are stirring up material.

What it looks like when the Laser Guide Star (LGS) is launched from the VLT’s 8.2-metre Yepun Telescope and aims at the center of our galaxy, in the heart of the brightest part of the Milky Way. (Credit: G. Hüdepohl (atacamaphoto.com)/ESO)

The Sculptor galaxy’s vigorous star formation may be the key to understanding these discrepancies. About one-third of its star formation happens in a compact ring around the galaxy’s center. This intense stellar activity drives powerful outflows that can lift dust high above the galaxy’s disk.

Unlike quieter galaxies where dust settles into a thin layer, the Sculptor galaxy’s turbulent environment appears to have created a much thicker dust distribution. This means dust absorption affects planetary nebulae throughout the galaxy, not just those in the central plane.

“How such small processes can have such a big impact on a galaxy whose entire size is thousands of times bigger is still a mystery,” says study author Enrico Congiu, a European Southern Observatory researcher, in a statement.

Scientists are grappling with the “Hubble tension,” a persistent disagreement between different measurements of how fast the universe is expanding. Some measurements suggest the universe is expanding faster than theoretical models predict, while others align with predictions.

If other starburst galaxies suffer from similar dust-related effects, it could help explain some of the discrepancies in cosmic distance measurements. But the study also shows that the planetary nebula method still works well in areas with little dust. When scientists looked at the outer parts of the Sculptor galaxy, where there’s less dust in the way, their distance measurements matched up much more closely with other methods.

Advanced space telescopes like the James Webb Space Telescope, which observes in infrared light that penetrates dust more easily, may help resolve these issues.

Dust that seemed insignificant has turned out to be a major player in how we see the universe. By understanding how dust affects different measurement methods, astronomers can develop better strategies and more powerful telescopes for mapping the universe’s true scale.