A team led by students explores the mass-radius relationship of white dwarf stars, observing in their data evidence for quantum mechanics and Einstein’s theory of general relativity.
At the heart of every white dwarf star, the dense stellar object that remains after a star burns its gas supply as it nears the end of its life cycle, lies a quantum mystery: as white dwarfs add mass, they shrink in size until they become so small and tightly compacted that they cannot be maintained by collapsing in neutron star.
This puzzling relationship between the mass and size of a white dwarf, called the mass-radius ratio, was first theorized by Nobel laureate astrophysicist Subrahmanyan Chandrasekhar in the 1930s. Now a team of astrophysicists by John Hopkins has developed a method for observing the phenomenon itself, using astronomical data collected by the Sloan Digital Sky Survey and a recent set of data published by the Gaia Space Observatory. The combined datasets provided more than 3,000 white dwarfs for the team to study.
A report of their findings, led by senior Hedkins Vedant Chandra, is now published in The Astrophysical Journal.
“The mass-radius ratio is an impressive combination of quantum mechanics and gravity, but for us it’s the opposite – we think that as mass accumulates, it should get bigger,” said Nadia Zakamska, an associate professor in the Department of Physics and Astronomy. led the student researchers. “The theory has existed for a long time, but the remarkable thing is that the data set we use is of unprecedented size and unprecedented accuracy. These measurement methods, which in some cases were developed years ago, suddenly work much better, and these old theories can finally be explored. “
“The way I praised my grandfather is that you actually see quantum mechanics and Einstein’s theory of general relativity coming together to give that result. He was very excited when I said that. “- Vedant Chandra, student John Hopkins
The team obtained its results using a combination of measurements, including primarily the gravitational effect of redshift, which is the change in wavelengths of light from blue to red as light moves away from an object. This is a direct result of Einstein’s theory of general relativity.
“For me, the beauty of this job is that we all learn these theories about how light will be affected by gravity in school and textbooks, but now we actually see this connection in the stars themselves,” said fifth-year graduate student Hsian-Chih. Hwang, who proposed the study and first recognized the effect of gravitational redshift in the data.
The team also had to consider how the star’s motion in space could affect the perception of its gravitational redshift. Similar to the way in which the fire engine siren changes its height depending on its movement relative to the listener, the light frequencies also change depending on the movement of the light-emitting object relative to the observer. This is called the Doppler effect and is essentially a distracting “noise” that complicates measuring the effect of gravitational redshift, said study co-author Sihao Cheng, a fourth-year student.
To account for variations caused by the Doppler effect, the team classified white dwarfs in their sample set by radius. They then average the redshifts of stars of similar size, effectively determining that no matter where the star itself is or where it moves relative to Earth, an internal gravitational redshift of a certain value can be expected. Think of this as averaging all the steps of all fire trucks moving in an area at a time – you can expect any fire engine, no matter which direction it is moving, to have an internal slope of that average.
These internal gravitational redshifts can be used to study stars that are observed in future data sets. The researchers say the upcoming data sets, which are larger and more accurate, will allow their measurements to be further refined and that these data could contribute to future analysis of the chemical composition of white dwarfs.
They also say that their research represents an exciting advance from theory to observed phenomena.
“As the star gets smaller as it becomes more massive, the effect of gravitational redshift also increases with mass,” says Zakamska. “And it’s a little 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 and compact object. And that’s exactly what we saw in the data. “
The team even found captive audiences for their research at home – where they worked in the coronavirus pandemic.
“The way I praised it to my grandfather is that you actually see quantum mechanics and Einstein’s theory of general relativity coming together to give that result,” Chandra said. “He was very excited when I said it that way.”
Reference: “Gravitational Measurement of the Red Displacement of the White Dwarf-Radius Mass Connection” by Vedant Chandra, Xiang-Chi Hwang, Nadia L. Zakamska and Sihao Cheng, August 25, 2020, Astrophysical Journal.
DOI: 10.3847 / 1538-4357 / aba8a2