Chinese astronomers established a high-precision ranging method for galaxies in batches

On June 20, the international academic journal Nature Astronomy published an important achievement led by Chen Xiaodian, associate researcher of the National Astronomical Observatory of the Chinese Academy of Sciences. The research team found that the two-period RR Lyrae variable star is the best standard candlelight, using its two periods to measure galaxy distances and no longer require elemental abundance information, which enables high-precision ranging of galaxies in batches.

A hundred years ago, American astronomer Edwin Hubble measured the distance of the first extragalactic Andromeda Nebula, thereby determining the existence of extragalactic galaxies, pioneering the study of galactic astronomy. With the development of technology, astronomers have been able to measure distances to distant galaxies tens of billions of light-years away, leading to the realization that the Milky Way is just a speck of stardust in the vast universe. Currently, astronomers are concerned with how to more accurately obtain the distance of a star, a galaxy, or even the entire universe.

According to researchers, measuring the distance of stars usually requires the use of “measuring scale”, that is, standard candles. A standard candlelight is like a lamp of known power, its intrinsic brightness is consistent, and the farther away from it, the darker it will feel. It has been observed that the brightness of a standard candlelight decreases with the square of the distance. There are two standard candles commonly used in stars: young (tens of millions of years) Cepheids and old (tens of billions of years) RR Lyra. Their intrinsic brightness is tens of thousands and a hundred times brighter than the sun, respectively.

So, how do people know the intrinsic brightness of these two types of stars? The brightness of such stars varies periodically with time, and there is a linear periodic relationship between periodicity and intrinsic brightness. Using the week-light relationship, the intrinsic brightness of these two types of stars can be obtained, and then the distance can be calculated by comparing the intrinsic brightness with the observed brightness.

Using this method, an object distance with an error of 5%-10% can be obtained, and if you want to get a more accurate distance, you need to determine whether the standard candlelight is enough standard. Astronomers have found that the intrinsic brightness of a star is affected by the abundance of elements, that is, stars with different heavy elements have different intrinsic brightness.

Therefore, astronomers who want to continue to reduce the error of celestial distances need to measure the elemental abundance of these standard candles. Elemental abundance is expensive to measure and relies on spectroscopic measurements. China’s Guo Shoujing telescope has obtained tens of millions of spectra, which is one of the largest spectral libraries in the world. However, objects with spectral measurements are still just the tip of the iceberg. Currently, fewer than five extragalactic objects have a distance error of less than 2 per cent.

Chen Xiaodian’s research team used data such as Guo Shoujing’s telescope in China to discover for the first time the linear relationship between multiple periods and metal abundance of the biperiodic RR Lyra variable star, and then established the periodic-luminous relationship of the biperiodic RR Lyrae variable star. Based on this week-light relationship, the distance error of galaxies can be optimized to 1%-2%.

China’s space station survey telescope will be launched in the next two years, and it will be able to discover biperiodic RR Lyrae variable stars in nearly 100 neighboring galaxies. Using this method, the sample of galaxies with high distance accuracy will be enlarged by a factor of 20. Scientists can expect to see a detailed three-dimensional visual image of the Local Group and get a Hubble constant with an error of 1%. (Source: China Science News Gan Xiao)

Standard candlelight art (Courtesy of National Astronomical Observatory)

Three-dimensional art of the Local Group (Courtesy of the National Astronomical Observatory)

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