Pune (Maharashtra) [India], August 31 (ANI): A groundbreaking study led by Professor Anupam Bhardwaj from the Inter-University Centre for Astronomy and Astrophysics (IUCAA) has used 40 oxygen-rich Mira variable stars located in 18 stellar clusters of our galaxy, the Ministry of Education said in a release on Sunday.
The research team monitored these Mira stars over an extended period, establishing their mean luminosities and pulsation periods. The European Space Agency’s Gaia mission played a crucial role by providing precise geometric distances to these star clusters, which are located between 13,000 and 55,000 light-years from Earth. This enabled an absolute calibration of the stellar luminosities of the Mira variables, providing an unprecedented level of precision.
The resulting “absolute” period-luminosity relationship for these Mira variables provides an independent calibration of the supernovae used in the cosmic distance ladder, without relying on Cepheid variables. This achievement allowed the team to determine the Hubble constant with a remarkable 3.7 percent precision. The study was recently published in the Astrophysical Journal.
“We used Miras in our galaxy as anchors for the first time to determine the most precise cosmic expansion rate based on these cool stars,” said Prof. Bhardwaj, the study’s lead author. “Like Cepheid variables, the Mira variables in our own galaxy allowed us to establish a three-anchor baseline calibration of the extragalactic distance ladder, with additional Mira variables from two external galaxies. This work highlights that metal abundance affects Mira luminosity three times less than Cepheids, making Miras a promising alternative tool for Hubble constant determination.”
Nobel Laureate Adam Riess, of the Space Telescope Science Institute and Johns Hopkins University, is a co-author of the study. He noted the significance of the findings, saying: “The consistency between Cepheid and Mira anchored Hubble constant values further suggests the Hubble tension is unlikely due to measurement errors, and points to a more fundamental cause, including the possibility of new physics.”
Dr. Marina Rejkuba, another co-author and staff astronomer at the European Southern Observatory, emphasized the long-term impact of the research. “This study combines the fields of stellar astrophysics and cosmology. I would expect it to have a long-term impact as it ensures our understanding of the potential of Mira variable stars as a new, well-calibrated anchor for Hubble constant determination,” she said.
While the calibration of Miras at the first step of the distance ladder now matches the precision of Cepheids, the overall uncertainty in the Mira-based Hubble constant measurement is limited by the small number of galaxies with known Miras. Currently, only two supernova host galaxies are known to contain Miras. However, scientists expect the Rubin Observatory to discover a large number of Miras in such galaxies, opening new opportunities to map the age and size of the universe with greater precision.
Mira, also known as Omicron Ceti, is a star that dramatically changes its brightness over time in a regular pattern. First observed in the 17th century, Mira was the earliest known example of a “variable star” — one that does not shine with constant brightness. The name Mira means “the wonderful” in Latin, and it became the prototype for an entire class of stars known as Mira variables.
Mira variables are giant stars that undergo regular cycles of expansion and contraction, causing their brightness to vary predictably over periods ranging from 100 to 1,000 days. These stars are relatively cool, with surface temperatures around 3,000 Kelvin — roughly half the temperature of the Sun’s surface — and are in the late stages of their life cycle.
One of the most important features of Mira variables is the strong relationship between their brightness and the length of their pulsation cycles. This allows astronomers to use them as “standard candles.” Standard candles are celestial objects with known true brightness, enabling scientists to calculate their distances by comparing their intrinsic brightness with how bright they appear from Earth. This technique is a critical step in the “extragalactic distance ladder,” which astronomers use to measure distances across the universe and study its expansion.
The rate at which the universe is expanding today is known as the Hubble constant. This value is central to cosmology because it helps determine the size and age of the universe. However, a major puzzle known as the “Hubble tension” has emerged. When astronomers measure the Hubble constant using nearby stars such as Cepheid variables and exploding stars called Type Ia supernovae, they obtain a higher value than when they calculate it based on observations of the early universe using cosmic microwave background data and other indirect methods.
This discrepancy has led to vigorous debate, as it suggests the universe may be expanding faster today than current models predict. Scientists are working to resolve whether the difference arises from unknown physics or the need to revise existing cosmological models. Discoveries like those involving Mira and other variable stars are expected to play a pivotal role in resolving this cosmic mystery. (ANI)
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