You’ve probably heard the saying, “We will sell no wine before its time.” While most of us rarely think about the topic when buying retail wines, the more adventurous might go for age-worthy bottles – immature wines which can taste quite better at their peak. How long do you wait? Is there a mathematical model for that?
Imagine finding a bottle of wine labeled with a date before even the winnery was founded or before the birth of its founder.
So, how do you tell how old something is? Have you ever played the game “How old is it?” with a group of people? Or in a classroom? Are some better than others at estimating? Ever watch the TV show Antiques Roadshow?
Astronomers and physicists talk about things that are billions of years old and many many light years away.
There’s this thing about cosmological ages. Distance ladders and methods which use so-called standard candles for gauging distances. Stellar physics (evolutionary) models. Observational classification systems using surface temperature (color) and brightness and mass, as in HR diagrams for star clusters. And Big Bang models.
So, put all these techniques and models together, find an interesting star, and ask how old it is. What could go wrong, eh. Sources of uncertainty.
Which brings us to this Space.com article: How Can a Star Be Older Than the Universe? by David Crookes, All About Space magazine (October 16, 2019)
Space Mysteries: If the universe is 13.8 billion years old, how can a star be more than 14 billion years old?
For more than 100 years, astronomers have been observing a curious star located some 190 light years away from Earth in the constellation Libra. … HD 140283 — or Methuselah as it’s commonly known — is … one of the universe’s oldest known stars.
In 2000, scientists sought to date the star using observations via the European Space Agency’s (ESA) Hipparcos satellite, which estimated an age of 16 billion years old. Such a figure was rather mind-blowing and also pretty baffling. As astronomer Howard Bond of Pennsylvania State University pointed out, the age of the universe — determined from observations of the cosmic microwave background — is 13.8 billion years old. “It was a serious discrepancy,” he said.
“One of the uncertainties with the age of HD 140283 was the precise distance of the star,” Bond told All About Space. “It was important to get this right because we can better determine its luminosity, and from that its age — the brighter the intrinsic luminosity, the younger the star. …”
There were also uncertainties in the theoretical modelling of the stars, such as the exact rates of nuclear reactions in the core and the importance of elements diffusing downwards in the outer layers, …
“Another factor that was important was, of all things, the amount of oxygen in the star,” Bond said. HD 140283 had a higher than predicted oxygen-to-iron ratio and, since oxygen was not abundant in the universe for a few million years, it pointed again to a lower age for the star.
A 2014 follow-up study updated the star’s age to 14.27 billion years. “The conclusion reached was that the age is about 14 billion years and, again, if one includes all sources of uncertainty — both in the observational measurements and the theoretical modelling — the error is about 700 or 800 million years, so there is no conflict because 13.8 billion years lies within the star’s error bar,” Bond said.
Physicist Robert Matthews of Aston University believes the answers lie in greater cosmological refinement. “I suspect that the observational cosmologists have missed something that creates this paradox, rather than the stellar astrophysicists,” he said, pointing to the measurements of the stars being perhaps more accurate. “That’s not because the cosmologists are in any way sloppier, but because age determination of the universe is subject to more and arguably trickier observational and theoretical uncertainties than that of stars.”
“The most likely explanations for the paradox are some overlooked observational effect and/or something big missing from our understanding of the dynamics of the cosmic expansion,” Matthews said. Precisely what that “something” is, is sure to keep astronomers challenged for some time.
Universe Today (2009) >
Essentially, astronomers determine the age of stars by observing their spectrum, luminosity and motion through space. They use this information to get a star’s profile, and then they compare the star to models that show what stars should look like at various points of their evolution. … This method of determining star age … relies on the accuracy of the models.
There’s a new technique that was recently developed called gyrochronology, and it’s based on the rotational speed of a star. The speed that a star rotates is steadily changing throughout its life, and it’s dependent on the star’s age and color. If you know a star’s color and rotational speed, you can calculate its age to within an uncertainty of 15%.
Scientific American > How do scientists determine the ages of stars? Is the technique really accurate enough to use it to verify the age of the universe? by Stephen A. Naftilan, professor of physics in the Joint Science Department of the Claremont Colleges and Peter B. Stetson, senior research officer at the Dominion Astrophysical Observatory in Victoria, British Columbia (October 21, 1999)
The essential feature of a star cluster that lets us estimate its age is that each cluster contains stars with a range of masses.
In the case of a single star, its brightness and temperature don’t tell us much. Because these properties stay fairly constant for 90 percent of its lifetime, the star could be fairly young or fairly old, and we wouldn’t be able to tell the difference. In a star cluster, we have the advantage that stars of all masses formed at about the same time. So all we have to do is look at the cluster and determine how hot and how massive is the hottest, bluest, most massive star that has not yet entered the late, unstable period of its life. The star’s mass tells us how much fuel the star had when it was born, and the star’s brightness tells us how fast it is burning that fuel. We know that the star is just about to start becoming unstable – after all, the stars that are more massive have already started to become unstable. We also know that its fuel is just about exhausted. The ratio of how much fuel the star had in the beginning to how fast it has been burning that fuel tells us how long the star has been alive. (By analogy, if we know how much kerosene our hurricane lamp contained when we lit it and how fast it consumes the kerosene, and if the lamp is just now starting to go out, then we can deduce how long it has been lit.) Because all the stars in the cluster are the same age, the age of that one star tells us the age of the entire cluster.
The basic physics of how hydrogen is converted to helium in the centers of stars and the amount of energy generated by this process is comparatively simple and well understood. For much of the 20th century, the main limitation to our knowledge of stellar ages has been due to the difficulty of measuring the distances to the clusters – especially the distances to the oldest clusters, the globulars, which are comparatively far away. (We know how bright a star looks, but to know how bright it really is, you have to know how far away it is: is it like a headlight a mile away or an airport beacon 10 miles away? In the dark of the nighttime sky with no reference points, it’s pretty hard to tell.)
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Forbes > “This Is How Astronomers Know The Age Of The Universe (And You Can, Too)” by Ethan Siegel, Senior Contributor (Dec 10, 2019).
Here’s Ethan Siegel’s take on estimating the age of the universe. Redshift-distance relation. Hubble constant. Standard candles vs. standard rulers. Cosmic matter-energy composition. And why there’s confidence in the value 13.8 billion years.
“You cannot simply change the age of the Universe by changing the Hubble constant.” You must take the Universe’s composition into account (% normal matter, % dark matter, % dark energy) and use a mathematical model consistent with observations.
Seigel’s article contains several images with useful captions.
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