Determining the age of a star can be one of the trickiest measurements an astronomer can make. Stars don’t obviate their age like humans do — with wrinkles around the eyes, grey hair, or brittle bones. Instead stars give hints at their age through things like their motion, rotation rate, chemistry, and placement in the Galaxy. Finding “extrema” on the age spectrum for stars — the oldest and youngest — is much easier than trying to find middle aged objects like our Sun. In this article, we reveal four fast facts about the oldest stars in the Galaxy.
Jackie Faherty, astrophysicist, Senior Scientist with the American Museum of Natural History and Editor at dailygalaxy.com. Jackie was formerly a NASA Hubble Fellow at the Carnegie Institution for Science.
Fast fact #1: When We Say “Old” We Might Mean Different Things
When we say “Old” it can mean a variety of things. Sometimes what Astronomers are referring to is that an object is relatively old. Meaning when comparing a sample of objects we can tell that one source is older relative to the sample being studied. For instance, when looking at stars in the Pleiades, one might try to figure out what the “oldest” star is amongst that collection of co-moving, co-evolving, 120 million year old objects. From an absolute sense, our current understanding of the Universe is that the Big Bang occurred ~14 billion years ago. Stars took a while to form — probably a few hundred million years — and the first stars were massive, extremely unstable, and didn’t last very long (or so we think). The Sun is ~4.5 billion years old which is relatively young in relation to the age of the entirety of the Universe. Astronomers start referring to stars as “old” in an absolute sense when we infer they are > 10 billion years or so. That isn’t written in stone. Some might refer to an 8-9 billion year old star as “old” as well. Since we struggle to really be able to say if something is 8 billion years or 9 billion years or 10 billion years old, just know that the closer you get to the age of the Universe, the more you fall in the “elderly” category for stars.
Fast fact #2: Older Stars Tend To Be Metal Poor
The oldest stars that we know about show their age by a lack of “metals” in their light fingerprints. The way astronomers characterize stars is by taking their light, passing it through a prism and seeing what kind of chemistry is present. The fingerprint (or the spectrum) of the Sun for instance shows rich signatures of Calcium, Hydrogen, Helium, Sodium, Oxygen, and many other elements and molecules. But older stars are limited in what they have in their spectra. When they formed there was far less available in the Universe. Just after the Big Bang, there was Hydrogen, Helium, and traces of Lithium (the first three elements of the periodic table). Beryllium and Boron (the fourth and fifth elements on the periodic table) required cosmic ray fission to form and everything else required those first stars to coalescence and then explode. So those very first stars which we refer to as Population III stars, would have been mostly Hydrogen and Helium and would have given way to the next generation — what we call Population II stars — which would have slightly more of the heavier elements. Hence if you find something that is > 10 billion years old, you will not find much Oxygen, Carbon, Nitrogen, Iron, etc and that object will be referred to as a “metal poor star”.
Fast fact #3: Older Stars Usually Move VERY Fast
All stars are in motion. With the rarest of exceptions (e.g. hypervelocity stars being ejected from the Galaxy), all stars in the Milky Way are pulled into an orbit around its center. As stars age, they have more and more celestial encounters which can “kick” them around. Consequently older stars tend to be “kinematically heated” or sped up by lots of encounters with other stars and giant molecular clouds. Stars rarely will collide with one another but they often inflict gravitational energy on each other as they pass. Depending on where in the Galaxy an object is orbiting (the center where it is dense vs. the outer arms where there are less objects) the impact of the “kicks” will vary. While young stars tend to be identified by their relatively small velocity (they’ve had very few encounters and stay close to their natal environment), older stars tend to zip through the Galaxy. We measure velocities in speeds of kilometers per second. The average speed of a run of the mill star in the Galaxy is < 100 km/s. For the oldest stars we find that they move several hundred kilometers per second which brings them close to the escape velocity of the Galaxy (~500km/s depending on where the object is located).
Fast Fact #4 The Oldest Star On Record Is….
Given the difficulty in precisely age-dating a star it is hard to say which object Astronomers have cataloged that holds the record for oldest stellar object ever observed. One which is often noted as the oldest in a small radius around the Sun is called HD 140283 or Methuselah star. This star is a very bright object in the constellation Libra and has been known since the early 1900’s to be particularly peculiar. Looking at its spectrum, we find that it has more than 200 times LESS Iron than the Sun does. That means this source would be categorized as a Population II object or a star that probably formed shortly after the Big Bang occured. It has a well measured distance and is roughly 200 light years away making it very easy to obtain follow-up data. Based on it’s position on astrophysical diagnostic diagrams such as the Hertzsprung Russell figure, this source is estimated to have an age very close to ~14 billion years. The uncertainty on the age from all diagnostics leaves it within what is allowed from our understanding of when the Big Bang likely occurred. There are several other objects at an extreme age that also exceed the metal-poor condition of HD 140283. For instance, SMSS J160540.18-144323.1, HE0107-5240, and 2MASS J18082002−5104378 are all ultra metal poor, and have ages likely > 13 billion years old.
The star-filled image at the top of the page, taken with the NASA/ESA Hubble Space Telescope’s Wide Field Camera 3 (WFC3), shows star cluster NGC 1866 found at the very edges of the Large Magellanic Cloud, a small galaxy located near the Milky Way. The cluster was discovered in 1826 by Scottish astronomer James Dunlop, who cataloged thousands of stars and deep-sky objects during his career. It’s possible, says NASA, that the stars within globular clusters are so old that they were actually some of the very first to form after the big bang.
Editor, Jackie Faherty, astrophysicist, Senior Scientist with AMNH. Jackie was formerly a NASA Hubble Fellow at the Carnegie Institution for Science. Aside from a love of scientific research, she is a passionate educator and can often be found giving public lectures in the Hayden Planetarium. Her research team has won multiple grants from NASA, NSF, and the Heising Simons foundation to support projects focused on characterising planet-like objects. She has also co-founded the popular citizen science project entitled Backyard Worlds: Planet 9 which invites the general public to help scan the solar neighbourhood for previously missed cold worlds. A Google Scholar, Faherty has over 100 peer reviewed articles in astrophysical journals and has been an invited speaker at universities and conferences across the globe. Jackie received the 2020 Vera Rubin Early Career Prize from the American Astronomical Society, an award that recognises scientists who have made an impact in the field of dynamical astronomy and the 2021 Robert H Goddard Award for science accomplishments.