If we want to understand the universe, we have to start with its size. Ancient people had no idea that there was a universe as we understand it now, and they had no idea of its size. They believed that the Earth existed and everything else revolved around it. This was the only conclusion attainable for a long time.
As the science of astronomy developed, our understanding grew. And our understanding of the size of the universe is growing along with everything else. But there were many misunderstandings along the way.
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 in the 1950s and 1960s showed that the universe is about 25 billion light years across. By the 1990s, the number had grown to about 30 billion light years, then to 94 billion light years at the beginning of this century. The reality of a universe 94 billion light years across is very different from a universe only 280 million light years across.
The history of astronomy shows how our misunderstanding of the vast distances between objects in space has distorted our understanding of the universe. Throughout history, astronomers have struggled to accurately estimate distances. 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 step of cosmic distance ladder.
The parallax method cannot help when it comes to measuring the distance to other galaxies. To measure these vast distances, astronomers rely on types of stars called standard spark plugs. Astronomers know the absolute magnitude of standard candles, and by measuring their apparent magnitude and comparing the two, they get a measure of the object’s distance. They can find a standard candle in another galaxy and find the distance to the galaxy.
The two most common standard spark plugs are Cepheid variables and RR Lira stars. Now a new method using the RR Lyrae stars provides 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 for determining distances using a specific subset of RR Lyrae stars. Their research paper is “The use of dual-mode RR Lyrae stars as robust indicators of distance and metallicity.” The lead author is Dr. CHEN Xiaodian.
The history of our understanding of the universe is a history of increasingly sophisticated methods of measuring it. The team of Chinese astronomers believe they are on their way to the next clarification, and it has to do with the stars RR Lyrae.
RR Lyrae stars are named after a particular star called RR Lyrae, the brightest example of this type. They are variable stars that are also periodic. They usually occur in globular clusters and were once called cluster variables. RR Lyrae stars are post-main sequence stars that have left the red giant stage of stellar evolution and are at what is known as horizontal branch. They are also low-metallicity Population II stars.
The RR Lyrae variable stars fall within a defined region of a Hertzsprung-Russell diagram color vs. brightness. Image credit: by Rursus – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=3142076
RR Lyrae stars are useful distance measuring tools because of their predictable pulsations. Their pulsations and brightness are controlled by period-luminosity relation. Their period-luminosity relationship in the infrared relates their pulsation to their luminosity. They are also analyzed using a period-color relationship, and this has helped modern astronomers find the distances between us and other objects and regions with increasing accuracy.
But there are some problems with using the RR Lyrae stars as distance measuring tools. Metallicity can cloud observations, and they can be extremely faint in distant galaxies. Multiple RR Lyrae stars in a dense environment—in the core of a globular cluster, for example—can merge together in observations. This means that what observers think is a star is much brighter, leading to erroneous conclusions about its distance.
The team of Chinese researchers found a way around these problems by focusing on a subset of RR Lyrae stars called double-period RR Lyrae stars. They are stars that pulsate in two different periods at the same time. They have a close relationship between their brightness and pulsation period for the same elemental abundance. Astronomers believe they can use these stars to measure distances to other galaxies to within 1-2%.
Only about 5% of RR Lyrae stars are double-period stars and are called RRd stars. They are unique because their double periods are related to their stellar properties, such as elemental abundance and mass. This is critical in the new study because elemental abundances are more difficult to measure than the period of a star. The team’s method eliminates the need for spectroscopy.
“We find that elemental abundance can be represented by two periods, and thus a period-luminosity relationship independent of elemental abundance is established,” said lead author Dr. CHEN Xiaodian.
“Our work provides a method by which distance measurements to nearby galaxies can be obtained from photometry alone, without relying on spectroscopic observations,” said Dr. DENG Likai, senior researcher at NAOC and co-author of the study.
One of the main roles of the RR Lyrae stars is to precisely determine distances to other galaxies. These precise measurements fit into our understanding of the universe itself, how it evolved, what role dark matter plays, and other questions.
“This will increase the sample of high-precision distance galaxies by a factor of 20 or more,” said Dr. Deng Likai.
Astronomers are constantly working on improved ways to measure things in the universe, distance being one of the most important. All this work created the Cosmic Distance Ladder, a series of overlapping methods that astronomers use to measure distances to increasingly 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 next rung. RR Lyrae stars and Cepheid variables play an important role in determining distances to other galaxies. So improving our accuracy on RR Lyrae will increase the accuracy of our measurements not only of galaxies, but also of things like galaxy clusters.
Astronomers use the Cosmic Distance Scale to determine the distances to distant objects. Increasing the accuracy of each rung in the ladder increases the accuracy of those above it. Image credit: ESOIn 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 astronomers will have a leg up on measuring the distances to all these galaxies.
This work improves the accuracy of our measurements of the distances to all things in the universe. Step by step, step by step, our understanding of the universe becomes clearer.