A New Way to Measure Distances in the Universe

If we want to understand the Universe, we have to start with its size. Ancient people had no idea there was a Universe the way we understand it now, and no idea of its size. They thought there was the Earth, with everything else rotating around it. It was the only conclusion within reach for a long time.

As the science of astronomy got going, our understanding grew. And our understanding of the Universe’s size grew along with everything else. But along the way, there were many misunderstandings.

In 1929, Edwin Hubble’s work showed that the Universe was about 280 million light-years across. By the mid-1950s, improved science and telescopes showed that the Universe was 4 billion light-years across. The study of quasars throughout the 50s and 60s showed that the Universe was about 25 billion light-years across. By the 90s, the number grew to about 30 billion light-years, then to 94 billion light-years early in this century. The reality of a Universe that’s 94 billion light-years across is much different than a Universe of only 280 million light-years across.

The history of astronomy shows how our misunderstanding of the vast distances between objects in space has skewed our understanding of the Universe. Throughout history, astronomers have struggled to gauge distances accurately. In the early days, astronomers worked on the concept of parallax to determine distances. But it only works for shorter astronomical distances. Parallax is the first rung on the cosmic distance ladder.

The parallax method can’t help when it comes to measuring the distance to other galaxies. To measure those vast distances, astronomers rely on types of stars called standard candles. Astronomers know the absolute magnitude of standard candles, and by measuring their apparent magnitudes and comparing the two, they get a measure of an object’s distance. They can find a standard candle in another galaxy and find the distance to the galaxy.

The two most common standard candles are Cepheid variables and RR Lyrae stars. Now a new method of using RR Lyrae stars is delivering even more accurate distance measurements.

In new research published in the journal Nature Astronomy, a team of astronomers from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) proposed a new method of determining distances using a specific subset of RR Lyrae stars. Their research article is “The use of double-mode RR Lyrae stars as robust distance and metallicity indicators.” The lead author is Dr. CHEN Xiaodian.

The history of our understanding of the Universe is the history of increasingly refined methods of measuring it. The team of Chinese astronomers thinks they’re onto the next refinement, and it has to do with RR Lyrae stars.

RR Lyrae stars are named after a particular star called RR Lyrae, the type’s brightest example. They’re variable stars that are also periodic. They’re commonly found in globular clusters and were once called cluster-type variables. RR Lyrae stars are post-main sequence stars that have left the red giant stage of stellar evolution and are on what’s known as the horizontal branch. They’re also population II stars with low metallicity.

RR Lyrae stars are useful distance-measuring tools because of their predictable pulsations. Their pulsations and their brightness are governed by the period-luminosity relation. Their period-luminosity relation in the infrared links their pulsation with their luminosity. They’re also analyzed using a period-colour relationship, and these have helped modern astronomers find the distances between us and other objects and regions with increasing accuracy.

But there are some problems with using RR Lyrae stars as distance-measuring tools. Metallicity can cloud observations, and they can be extraordinarily faint in distant galaxies. Multiple RR Lyrae stars in a dense environment—in the core of a globular cluster, for example—can blend together in observations. That means what observers think is a single star is much brighter and leads to erroneous conclusions about its distance.

The team of Chinese researchers has found a way around those problems by focusing on a sub-set of RR Lyrae stars called double-period RR Lyrae stars. They’re stars that pulsate at two different periods simultaneously. They have a close relationship between their luminosity and pulsation period for the same elemental abundance. Astronomers think that they can use these stars to measure the distances to other galaxies within 1-2%.

Only about 5% of RR Lyrae stars are double-period stars, and they’re called RRd stars. They’re unique because their double periods are linked to their stellar properties, like elemental abundance and mass. This is critical in the new research because elemental abundance is more difficult to measure than a star’s period. The team’s method eliminates the need for spectroscopy.

“We find that the elemental abundance can be represented by two periods, and thus a period-luminosity relation independent of the elemental abundance was established,” said lead author Dr. CHEN Xiaodian.

“Our work provides a method by which distance measurements of nearby galaxies can be obtained from photometry alone, without relying on spectroscopic observations,” said Dr. DENG Licai, a senior researcher at NAOC and co-author of the study.

One of the prime roles played by RR Lyrae stars is to determine distances to other galaxies with precision. These precise measurements feed into our understanding of the Universe itself, how it evolved, what role dark matter plays, and other questions.

“This will increase the sample of galaxies with high-precision distance by a factor of 20 or more,” said Dr. DENG Licai.

Astronomers are constantly working on improved ways of measuring things in the Universe, with distance being one of the most important. All that work has created the cosmic distance ladder, a series of overlapping methods that astronomers use to measure the distances to increasingly more distant objects. If there are errors in any rung, then those errors are compounded in the rung above it. So the more accurate each rung of the ladder is, the more accurate the following rung is. RR Lyrae stars and Cepheid variables play an important role in determining the distances to other galaxies. So improving our RR Lyrae accuracy will increase the accuracy of our measurements of not only galaxies but things like galaxy clusters.

In the very near future, observatories like the Vera Rubin Observatory will find tens of thousands more RRd stars in galaxies near us. This new method means that astronomers will have a leg up on measuring the distances to all those galaxies.

This work improves the accuracy of our measurements of distances to all things in the Universe. Step by step, rung by rung, our understanding of the Universe is becoming clearer.

More:

• Press Release: Astronomers Propose Novel Method of Measuring Galaxy Distances
• Research: The use of double-mode RR Lyrae stars as robust distance and metallicity indicators
• Universe Today: Astronomers Improve Their Distance Scale for the Universe. Unfortunately, it Doesn’t Resolve the Crisis in Cosmology