It would take about a million years, traveling at the speed of light, to cross our galaxy, the Milky Way. And there are hundreds of billions or even trillions of galaxies in the observable universe. How can we possibly measure the distances to these stars and galaxies from the third rock orbiting a middling star in a small spiral arm of an average spiral galaxy?
The only way astronomers can measure distances to cosmic objects directly is with a technique called parallax. Parallax essentially measures how much an object appears to wiggle over time against background stars. Launched in 2013, the European Space Agency (ESA) Gaia mission was the ultimate parallax calculator, a sky-mapping satellite tasked with building a detailed 3D map of the Milky Way, plotting the positions and distances of nearly two billion stars in the process.
Gaia provided parallax distances with unprecedented precision. But even Gaia had its limits. An accuracy of 0.001% for the nearest stars dropped to 20% for stars near the Galactic Center 30,000 light-years away. If astronomers want to measure more distant objects—the vast majority in the universe—they have to get creative.
Framing most of this creativity is a simple idea: If we imagine that all stars have identical luminosities, meaning they all emit the same amount of light from their surfaces, then their apparent brightness, i.e., how bright the stars appear to a detector here on Earth, solely depends on their distance. Because a star emits light in all directions, the light spreads like an expanding ball, and the ball’s radius is the distance between the star and Earth. High school math in the form of the Inverse Square Law of Light then immediately gives us the distance to the star.
If only it were so simple. Stars come in a range of luminosities, making this simple way of calculating their distance impossible. But in the early 20th Century, US astronomer Henrietta Swan Leavitt made a significant breakthrough that would overcome this seemingly insurmountable problem—discovering the first standard candle, an astronomical object with a known luminosity.
Leavitt was studying a type of star called a Cepheid whose brightness rapidly increases and then slowly dims repeatedly. Observing Cepheids in the Small Magellanic Cloud, Leavitt uncovered a strong relation between these stars’ apparent brightness and the timescales over which their brightening and dimming pattern repeated: The brighter the star, the slower it blinked. With all the stars in the Small Magellanic Cloud presumably roughly the same distance from Earth, it could be inferred that each Cepheid’s average luminosity must be related to its period. The period–luminosity relationship, Leavitt’s Law, was born.
Circled in the center of each of these pixelated images is a variable Cepheid star used as a milepost marker for measuring the rate of expansion of the universe. Photo credit: NASA, ESA, CSA, STScI, Adam G. Riess (JHU, STScI).
The publication of Leavitt’s results in 1912 opened the door to a new way of measuring cosmic distances. “If you have a nearby Cepheid, for which you have the parallax distance, and you have a distant Cepheid, and they both have the same period, then you know that they both have the same luminosity,” explains University of Portsmouth Associate Professor of Astrophysics Or Graur. “The more distant one is fainter, but only because of its distance, not because of something intrinsic to the physics of the star.”
In 1925, Edwin Hubble wielded Leavitt’s Law to determine that the Milky Way is not the extent of the universe, as was thought by many, but just one of many galaxies that make up the universe—a revelatory discovery. He measured distances to Cepheids he had spotted in mysterious M31 and M33. Though crude by today’s standards, Hubble’s calculations placed these systems at immense distances, proving they were bright, distant galaxies (Andromeda and the Triangulum Galaxy) filled with billions of stars, and not relatively faint, small, and nearby nebulae (clouds of dust and gas in space where relatively small groupings of stars form).
Hubble later used Leavitt’s Law to show that galaxies are generally receding from us because the universe is expanding—another paradigm-shifting discovery. From this, Hubble even provided the first estimate of the rate at which the universe is expanding, a parameter known as the Hubble constant (more on this later).
In Leavitt and Hubble’s time, little was known about the nature of Cepheids. Astronomers now know Cepheids are five to 20 times the mass of the sun, and up to 10,000 times as luminous. Their pulses are driven by an internal process known as the κ–mechanism, wherein heat builds up and traps light in a layer of the star that expands like a balloon, before cooling, contracting, and releasing that light, after which the process starts all over again.
Their luminosity and cyclical nature are why Cepheids have been central to most measurements of cosmic distances since Leavitt’s revelation. In fact, they have become a vital rung in the “cosmic distance ladder,” wherein each distance measurement technique relies on calibrated shorter-range methods. In other words, we can only get to the next rung of the cosmic distance ladder having stepped on the previous rungs.
Parallax is usually the first rung of the ladder, used to calibrate the second rung, which is often Cepheids. Though Gaia has taken away some of this work, as Lou Strolger of the Space Telescope Science Institute, explains: “Everyone thinks we’re still in this wobbly era where we’re still trying to bootstrap to get to distances of the nearest Cepheids, but Gaia really solved that problem for us,” he says. “Now we can directly measure some of the most nearby Cepheids in our own galaxy and calibrate Leavitt’s law to target extra-galactic distances.”
Even with the most advanced telescopes, Cepheids are only good yardsticks up to a few tens of millions of light years. Beyond this, astronomers need to step onto the next rung of the cosmic distance ladder. This rung is often type Ia supernovae.
Supernovae are powerful stellar explosions that come at the end of a star’s life. Type Ia supernovae specifically are thought to be the explosions of white dwarfs—long-dead remnants of stars that had been slightly more massive than the sun in their heyday—and they have two wonderful things going for them, says Strolger. “They’re extremely luminous, which means they can be seen through most of the age of the universe, and they have a very similar light curve shape which is calibratable—no other supernovae have that characteristic.”
An all-sky view of the Milky Way and neighboring galaxies, showing the parallaxes of 96 million stars measured by ESA’s Gaia satellite. Photo credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO
Graur, who literally wrote the book (Supernova) on these stellar explosions, goes into more detail, explaining that the width of type Ia supernovae light curves, meaning the time it takes to reach peak brightness and then to decline, is correlated with intrinsic luminosity: “More luminous Ias have broader light curves than less luminous Ias, and there is a mathematical relation between the two properties.” Hence, type Ia supernovae are referred to as standardizable, not standard, candles.
The parallax-Cepheids-type Ia supernovae ladder is the most popular technique used in cosmology because it is generally regarded as being the most accurate. This is why it has been wielded heavily in attempts to measure the venerable Hubble constant. The Hubble constant has turned out to be fundamentally important to our understanding of the universe, setting its size and age scales, and even being used to check if our picture of how the universe began and evolved holds true.
In the 1990s, astronomer Wendy Freedman and her colleagues employed the world’s first sophisticated optical observatory in space—NASA’s Hubble Space Telescope—to take a new measurement of the Hubble constant. Freedman’s team measured the distance to Cepheids and then used these to calibrate distances calculated from various other longer-range methods, including type Ia supernovae. Measurements from the different methods largely agreed, concluding that the universe was expanding roughly seven times slower than Hubble initially thought. More recent and precise measurements using Cepheids and type Ia supernovae, including from the James Webb Space Telescope (JWST), have backed up Freedman’s measurements.
While all of this was going on, ESA’s Planck satellite launched in 2009. Europe’s mission to study the relic radiation from the Big Bang—the cosmic microwave background—managed to extract a value for the universe’s expansion rate when it was just 380,000 years old. This differed significantly, but the discrepancy was to be expected as the Hubble constant is only constant in space, not over time. However, when physicists simulated the universe’s evolution from the time of the cosmic microwave background to the present day, they calculated that it should be expanding slower than the rate that standard candles had provided.
Though the Hubble constant values derived by these different methods were fairly similar, discrepancies were too large to be explained simply by flaws in measurement or observational technique. Either there had been some fundamental mistake in the values derived from today’s universe or there was a problem understanding how the universe began and evolved. This mystery, which remains unresolved, is known as the Hubble tension.
Might the problem be that standard candles are not so standard after all? Already, astronomers know that type Ia supernovae are standardizable, not standard, candles. Moreover, the underlying physics of type Ia supernovae remains somewhat unclear. This is a source of uneasiness in the community, as Strolger illuminates. “It’s the classic hammer problem, where you have a beautiful tool that allows you to do a wonderful thing, but you don’t know why it works,” he explains. “You don’t know how reliable it is in different environments and, more importantly, over cosmic time.”
Perhaps even more worryingly, Cepheids are not really standard candles either. They are standardizable candles, just like type Ia supernovae. Deeper understanding of the nature of Cepheids has uncovered that the period–luminosity relation, Leavitt’s Law, also depends on metallicity, the abundance of elements heavier than hydrogen and helium, like iron, in the star. “For Cepheids, we don’t know their metallicity, and have no way of measuring it for them,” says Freedman.
“The other issue is that Cepheids are found in the disk of galaxies, so dust and crowding of images become serious problems,” Freedman continues. “You can only statistically correct for crowding, not for an individual Cepheid, so if you get the crowding wrong, the dust correction wrong, you’re going to get the metallicity wrong.”
A type Ia supernova at a distance of approximately 11.5 million light-years from Earth. Photo credit: NASA, ESA, A. Goobar (Stockholm University), and the Hubble Heritage Team (STScI/AURA).<span
For these reasons, Freedman has been exploring alternatives to Cepheids. The past decade has seen tip of the red giant branch (TRGB) stars take center stage in a lot of her work. TRGBs are low-mass red giant stars, of one- to two solar masses. At a point near the end of their life, they begin to fuse helium in their core, causing a flash in luminosity before settling down to a lower luminosity state. “We can measure that peak luminosity really accurately, and that is our standard candle,” says Freedman. “We don’t have to standardize—it’s standard in the I-band [near-infrared; 806 nm].”
Moreover, TRGBs are the best understood standard candles theoretically, meaning their distance measurements are based on solid foundations, and many of them reside in the halo of galaxies where there is little dust obscuring observations.
Very recently, Freedman and colleagues used TRGBs instead of Cepheids to calibrate type Ia supernovae and take a measurement of the Hubble constant, combining archive Hubble Space Telescope and recent JWST data. Surprisingly, the value they derived was more consistent with the value derived from the cosmic microwave background than values derived from standard candle measurements.
Yet despite this suggesting a resolution to the Hubble tension, Freedman is cautious. “I just don’t think right now we have the accuracy to say for certain whether or not there’s a Hubble tension.” In fact, measuring the value of the Hubble constant with an accuracy of 1% or less in order to get to the bottom of the Hubble tension is widely regarded as one of the most important challenges in modern astrophysics.
This is why Freedman’s current work aims to add another independent rung to the cosmic distance ladder: Carbon stars, known in the literature by the far less catchy name of J-region asymptotic giant branch (JAGB) stars.
Like the Goldilocks fairytale, these stars have masses that are “just right” to have carbon in their atmospheres: not too massive to burn the carbon before it gets to the surface, and not too slight to be unable to dredge up the material in the first place. Their narrow mass range corresponds to a constant average luminosity in the J-band (near-infrared; 1,220 nm), making them another ideal standard candle.
Already, Freedman’s team has attempted to measure the Hubble constant using carbon stars instead of Cepheids, with results in line with those derived from her TRGB star measurements. Ultimately, she would like to use JWST to measure Cepheids, TRGB, and JAGB stars in the same galaxy to calibrate and improve the accuracy of all three methods.
Freedman’s efforts to find new, more reliable standard candles are reflective of a wider endeavor to add new rungs to the cosmic distance ladder, and even create completely independent distance ladders, in order to check existing results and calibrate existing methods—all towards the goal of reaching that long-sought 1% accuracy benchmark.
JWST will continue to be crucial in this endeavor, providing images of galaxies containing standard and standardizable candles. And with data from ESA’s Euclid mission (launched in 2023) and NASA’s Nancy Grace Roman Space Telescope (scheduled for launch in 2026/27) soon to be available, astronomers will have powerful new tools to dissect, discover, and measure distances to standard candles. Whether these new space-based observations will be enough to resolve astronomy’s most pernicious current problem—the Hubble tension—is unknown. What is certain is that standard candles will continue to play a big part in the journey towards understanding how the universe began and has evolved into the cosmos we enjoy today.
Benjamin Skuse is a science and technology writer with a passion for physics and mathematics whose work has appeared in major popular science outlets.
