Age Is Just a Number: Radioactive Dating in Stars

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Title: Uranium Abundances and Ages of R-process Enhanced Stars with Novel U II Lines
Authors: Shivani P. Shah et al.
First Author’s Institution: University of Florida
Status: Published in ApJ

As someone who has recently left their 20’s, I think a lot about how age shows up in my body. I can look up my birth date on the calendar, even count all the minutes of my existence, but I don’t need to go through all that work. Something inside of me just feels… older. While the self realization of the unyielding passage of time on my mortal form may be daunting, I find solace in the fact that I’m no different than the stars — they also carry around their own clocks. Today’s article is about an interesting technique to determine how old a star is by looking at how much uranium is “ticking” in its atmosphere.

The Smallest Hand of the Clock

The technique is based on radioactive dating, a tool used in a variety of ways but most famously in carbon dating. Living things on Earth have carbon atoms in their bodies, some of which are carbon-14 (C14). C14 is a radioactive isotope of carbon, meaning it has a slightly different mass than other carbon atoms, and it decays over time. Even though it decays, C14 is regularly replaced while an organism is alive, and so the ratio between regular carbon and C14 within living things stays mostly steady. Once the flow of C14 stops (aka something dies), the ratio between carbon and C14 changes as the latter decays. By measuring the remaining ratio and comparing it to the normal ratio, we can calculate how much time has passed for the correct amount to decay.

Radioactive dating isn’t just for living things. C14 decays at such a rate that you can use it to measure the ages of objects up to around 50,000 years old, but the same technique works for any element with a radioactive isotope. Our best estimate for the age of our planet and solar system comes from radioactive dating using different elements that have radioactive isotopes that decay at slower rates.

Uranium is an element that is perfect for this, as its radioactive isotope takes billions of years to decay. Today’s article is all about trying a new way to measure the abundance of uranium in stars to find an age estimate for stars with radioactive dating. It’s known as nucleocosmochronometry (a word that spans one and a half Scrabble boards).

Tiny Atoms in Massive Stars

plot of an example uranium feature in the spectrum of a star

Figure 1: An example uranium feature in a star’s spectrum. The black points are the data. The red line is the best model for the spectrum that includes uranium. The authors also fit a model that doesn’t include uranium, which is the blue dashed line. By measuring the difference in the two models, the authors estimated an abundance of uranium responsible for absorbing the missing light. [Adapted from Shah et al. 2023]

It might seem incredibly difficult to detect atoms in a massive star that is light-years away, but it’s actually one of astronomy’s oldest tricks (astrobites has a whole guide about it!). Each element and molecule has its own characteristic fingerprint — its spectral signature — in the form of the specific wavelengths of light that it absorbs (called spectral lines). These can be measured in a lab, and then by looking at the light coming from a star and seeing which wavelengths are being absorbed, we can tell what’s in the star’s atmosphere.

This works great until elements and molecules have lines very close to one another, which is a major challenge with uranium. As it turns out, the typical line used to measure uranium abundance is blended with both an iron line and a cyanide feature (Figure 1). It’s still possible to get a measurement, but today’s authors wanted to use two new uranium lines to measure abundances and see how well they agreed with the single-line method. Even though these new lines are also blended, three measurements allow the authors to do a better job of describing the certainty of the measurement by using statistics to compare the abundances measured between the three lines.

plot of age estimates for four stars

Figure 2: The age estimate of the four stars (names on the x axis) from this work (colored squares) compared to other work (white squares). The accepted age of the universe is included as a black dashed line. The age estimate is not a ratio of uranium to a uranium isotope since only total uranium was measured, but uranium against europium, which does not have any radioactive isotopes so should be a constant and appropriate comparison. [Adapted from Shah et al. 2023]

Do You Have the Time?

The authors measured the abundances of uranium for four stars and compared the results from a fit using just a single-line measurement in each of the stars to one using all of the uranium lines. They found that the abundances from both methods were within a reasonable range of one another.

When the authors used the abundances to measure an age (Figure 2), they found ages that were similar to those calculated with just the single measurement. You might notice that the age estimates have big error bars — some stretch to an age older than the universe! There are clearly still some challenges with the method in general, in part because it’s hard to know how much uranium was in the star to begin with. The authors chose these four stars for the study because they are examples of stars that should have had more uranium. Regardless, the production rates of uranium remain a big question mark.

The Clocks Keep Spinning

It’s worth mentioning that the uranium that makes these stellar clocks tick formed in merging neutron stars, the dramatic burst of atomic creation when two “dead” stars collide. A star’s clock, and even its very existence, is due in part to the stars that came before them. Makes me think about how even if my body feels old, my time being alive has been traced through thousands of lifetimes similar to my own. 30(+) be damned, I’m going for a walk.

Original astrobite edited by Jessie Thwaites.

About the author, Mark Popinchalk:

I’m a PhD Candidate at CUNY/Hunter College based at the American Museum of Natural History. I study the age of stars by measuring how quickly they rotate. I enjoy ultimate frisbee, baking bread, and all kinds of games. My favorite color is sky-blue-pink.