If we want to understand the Universe, we have to start with its size. The ancients had no idea that the Universe existed as we understand it today, and had no idea of its size. They thought there was an Earth, with everything else revolving around it. This is the only conclusion that can be reached for a long time.
As the science of astronomy advanced, our understanding grew. And our understanding of the size of the Universe has grown along with everything else. But along the way, there were many misunderstandings.
In 1929, work by Edwin Hubble showed that the Universe is 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 is about 25 billion light-years across. By the 90s, the number had grown to about 30 billion light-years, then to 94 billion light-years in the early part of this century. The reality of a Universe that is 94 billion light-years across is very different than a Universe that is only 280 million light-years across.
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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 measure distances accurately. In the early days, astronomers worked with the concept of parallax to determine distances. But this only works for shorter astronomical distances. Parallax is the first rung on the cosmic distance ladder.
The parallax method does not 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 magnitude 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 in the galaxy.
The two most common common candles are Cepheid variables and RR Lyrae stars. Now a new way of using the stars of RR Lyrae is delivering 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. star 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. A team of Chinese astronomers think they are onto the next refinement, and it has to do with the stars of RR Lyrae.
RR Lyrae stars are named after a particular star called RR Lyrae, the brightest example of the type. These are variable stars that are also seasonal. They are usually found in globular clusters and are sometimes called cluster-type variables. RR Lyrae stars are post-main sequence stars that have left the red giant stage of stellar evolution and are in the so-called horizontal branch. They are also population II stars with low metallicity.
RR Lyrae stars are useful distance measurement tools because of their predictable pulsations. Their pulses and their luminosity are governed by the period-luminosity relationship. Their period-luminosity relationship in the infrared relates their pulsation to their luminosity. They are also analyzed using the period-color relationship, and they have helped modern astronomers find the distances between us and other objects and regions with increasing precision.
But there are some problems with using the RR Lyrae stars as distance measuring tools. Metallicity can cloud observations, and they can be very faint in distant galaxies. Many RR Lyrae stars in a dense environment—for example, in the core of a globular cluster—can cluster together in observations. That means what observers think a single star is brighter and leads to false conclusions about its distance.
A team of Chinese researchers found a way around those problems by focusing on a sub-set of RR Lyrae stars called double-period RR Lyrae stars. They are stars that pulsate in two different periods at once. They have a close relationship between their brightness and pulsation period for both elemental abundances. Astronomers think 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 they are called RRd stars. They are unique because their double periods are linked to their stellar properties, such as elemental abundance and mass. This is critical to the new research because elemental abundance is more difficult to measure than the age of a star. The team’s method eliminates the need for spectroscopy.
“We found that the elemental abundance can be represented by two periods, thereby establishing a period-luminosity relationship that is independent of elemental abundance,” 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 main roles played by the stars of RR Lyrae is to determine the distances to other galaxies with precision. These precise measurements enable 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 distances by a factor of 20 or more,” said Dr. DENG Licai.
Astronomers continue to develop improved ways of measuring objects in the Universe, with distance being one of the most important. All that 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, those errors are merged into the rung above it. So the more accurate each rung of the ladder is, the more accurate the following rung. RR Lyrae stars and Cepheid variables play an important role in determining distances to other galaxies. So improving our accuracy of RR Lyrae will increase the accuracy of our measurements not just of galaxies but of objects like galaxy clusters.
In the 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 one step in measuring distances to all galaxies.
This work improves the accuracy of our measurements of distances to everything in the Universe. Step by step, step by step, our understanding of the Universe becomes clearer.
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