A student-led team is studying the mass-radius relationship of white dwarf Stars observing evidence of quantum mechanics and Einstein’s general theory of relativity in their data.
At the heart of every white dwarf star – the dense stellar object that remains after a star has burned its fuel reserve of gases as it nears the end of its life cycle – lies a quantum puzzle: when white dwarfs add mass, they shrink one size until they are so small and densely compressed that they can no longer support themselves and become one Neutron star.
This enigmatic relationship between the mass and size of a white dwarf, known as the mass-radius relationship, was first theorized in the 1930s by Nobel Prize-winning astrophysicist Subrahmanyan Chandrasekhar. Now a team of Johns Hopkins astrophysicists have developed a method to observe the phenomenon for themselves. It used astronomical data collected from the Sloan Digital Sky Survey and a data set recently published by the Gaia Space Observatory. The combined datasets yielded more than 3,000 white dwarfs for the team to study.
A report of their findings, led by Hopkins Senior Vedant Chandra, is now published in The astrophysical journal.
“The mass-radius relationship is a spectacular combination of quantum mechanics and gravity, but not intuitive for us. We think that an object should get bigger the bigger it is, ”says Nadia Zakamska, Associate Professor in the Department of Physics and Astronomy who supervised the student researchers. “The theory has been around for a long time, but the notable thing is that the dataset we’ve used is unprecedented in size and size accuracy. These measurement methods, which in some cases were developed years ago, suddenly work much better and these ancient theories can finally be explored. “
“I praised my grandfather that basically quantum mechanics and Einstein’s general theory of relativity come together to achieve this result. He was very excited when I put it that way. “- – Vedant Chandra, Johns Hopkins student
The team obtained their results using a combination of measurements, including primarily the gravitational redshift effect, which is the change in wavelengths of light from blue to red as the light moves away from an object. It is a direct result of Einstein’s general theory of relativity.
“For me, the nice thing about this work is that we all learn these theories about how light is influenced by gravity in schools and textbooks, but now we actually see this relationship in the stars themselves,” says the PhD student in the fifth Jahr, Hsiang -Chih Hwang, who proposed the study and first saw the gravitational redshift effect in the data.
The team also had to consider how a star’s movement through space might affect the perception of its gravitational redshift. Similar to how a fire engine siren changes pitch according to its movement in relation to the hearing person, the frequencies of light also change depending on the movement of the light emitting object in relation to the viewer. This is known as the Doppler effect and is essentially a disruptive “noise” that makes it difficult to measure the gravitational redshift effect, says research associate Sihao Cheng, a fourth-year PhD student.
To account for the deviations caused by the Doppler effect, the team classified white dwarfs in their sample by radius. They then averaged the redshifts of stars of similar size and determined so effectively that regardless of where a star itself is or where it moves in relation to the Earth, an intrinsic redshift in gravity of a certain value can be expected. Imagine taking an average measurement of all parking spaces of all fire trucks moving in a given area at any given time. You can assume that any fire truck, regardless of which direction it is heading, has an intrinsic slope of this average value.
These intrinsic gravity redshift values can be used to study stars that will be observed in future data sets. The researchers say that upcoming datasets, which are larger and more accurate, will allow their measurements to be further fine-tuned, and that this data could contribute to future analysis of the white dwarf’s chemical composition.
They also say their study marks an exciting advance from the theory of observed phenomena.
“Since the star becomes smaller with increasing mass, the gravitational redshift effect also increases with mass,” says Zakamska. “And that’s a bit easier to understand – it’s easier to get out of a less dense, larger object than it is to get out of a more massive, compact object. And that’s exactly what we saw in the data. “
The team even finds a captive audience for their research at home – where they did their work amid the coronavirus pandemic.
“The way I extolled my grandfather is that quantum mechanics and Einstein’s general theory of relativity come together to get that result,” says Chandra. “He was very excited when I put it that way.”
Reference: “A redshift measurement of the gravitational-red-mass-radius relationship” by Vedant Chandra, Hsiang-Chih Hwang, Nadia L. Zakamska and Sihao Cheng, August 25, 2020, Astrophysical Journal.
DOI: 10.3847 / 1538-4357 / aba8a2