A New Technique to Figure Out How Old Stars Are

Our understanding of the universe, and of the Milky Way, is built on an edifice of individual pieces of knowledge, all related to each other. But each of those pieces is only so accurate. The more accurate we can make one of the pieces of knowledge, the more accurate our understanding of the whole thing is.

The age of stars is one such piece. For years, astronomers have used a method of determining the age of stars that has a 10% to 20% margin of error. Now, a team of scientists from Embry-Riddle Aeronautical University has developed a new technique to determine the age of stars with a margin of error of only 3% to 5%.

Current star-dating techniques rely on observing stars on the main sequence, which is kind of like adult-hood for stars. The technique looks at stars that have begun to ‘die’, which in this case means they’re exhausting their hydrogen. Furthermore, scientists usually can only tell the age of a star by figuring out the age of the population they’re a part of. They know the age of some individual stars, but mostly we know the age of star clusters rather than the individual stars themselves.

The reasons for that are fairly complex, but our star-dating techniques have led to some strange, rather obviously impossible conclusions, like finding star clusters in the Milky Way that are older than the Milky Way itself.

The technique developed by a team at Embry-Riddle, led by Physics and Astronomy Professor Dr. Ted von Hippel, relies on measurements of white dwarfs, rather than on main sequence stars. White dwarfs are remnants of stars that have left the main sequence after running out of fuel. Our own Sun will end its life as a white dwarf.

The new technique measure the mass, surface temperature, and whether its atmosphere has hydrogen or helium.

“…knowing whether there is hydrogen or helium at the surface is important because helium radiates heat away from the star more readily than hydrogen.”

Dr. Ted von Hippel, Physics and Astronomy Professor, Embry-Riddle University.

“The star’s mass matters because objects with greater mass have more energy and take longer to cool,” said von Hippel, director of Embry-Riddle’s Physical Sciences Department Observatory and 1.0-meter Ritchey-Chretien telescope. “This is why a cup of coffee stays hot longer than a teaspoon of coffee. Surface temperature, like spent coals in a campfire that’s gone out, offer clues to how long ago the fire died. Finally, knowing whether there is hydrogen or helium at the surface is important because helium radiates heat away from the star more readily than hydrogen.”

A star’s mass is still key to determining its age, and it’s still difficult, especially for large populations of white dwarfs. But thanks to the Gaia Satellite, that’s getting easier.

Professor von Hippel’s new method takes advantage of the data provided by the European Space Agency’ Gaia mission. Gaia is making a 3D map of the Milky Way by measuring the positional and radial velocity of about 1 billion stars in the Milky Way and in the Local Group. Gaia measures star distances with extreme accuracy, and that’s what von Hippel’s team took advantage of.

Gaia was able to measure star distances with great accuracy, and von Hippel and his team used that accuracy to determine the radius of stars based on their brightness. From there, they used existing information on the star’s mass-to-radius ratio to determine the mass, a missing ingredient in determining a star’s age.

The final touch, which helps give the new technique its precision, is to figure out the star’s metallicity. Metallicity refers to the abundance of different chemical elements in the star. This information allows them to refine the age of the star.

At the recent American Astronomical Society meeting, members of von Hippel’s team presented two posters on their work. The first focused on a binary pair of stars with one white dwarf and one main sequence star. The second focused on a binary pair of white dwarfs.

“The next level of study will be to determine as many of the elements in the periodic table as possible for the main sequence star within these pairs.”

Dr. Ted von Hippel, Physics and Astronomy Professor, Embry-Riddle University.

“The next level of study will be to determine as many of the elements in the periodic table as possible for the main sequence star within these pairs,” von Hippel said. “That would tell us more about Galactic chemical evolution, based on how different elements built up over time as stars formed in our galaxy, the Milky Way.”

Von Hippel says that the method is still being developed, and can still be considered in its preliminary phase. But it holds a lot of promise, and the team hopes that eventually they will learn the ages of all of the white dwarfs in the Gaia dataset. “That could allow researchers to significantly advance our understanding of star-formation within the Milky Way,” von Hippel said.

Von Hippel made note of a comparison between the field of archaeology and the field of astrophysics. In archaeology, we use carbon-dating to determine the age of all kinds of objects: tools, structures, fossils, Stone-Age sites. The ages of things give us an understanding of the timeline of events on Earth. The same is true for the Universe.

“For today’s astronomers, without knowing the age of different components of our galaxy, we don’t have context. We’ve had techniques for dating celestial objects, but not precisely.”