The oldest known naked eye star in our Milky Way Galaxy is not some flashy red supergiant on the verge of going supernova, like Betelgeuse in the constellation of Orion. Nor is it part of an ancient globular cluster in the halo of our Milky Way.
Instead, Mu Cassiopeia A is a simple field star — a yellow dwarf of the same spectral type as our own sun, but one that clocks in at an astonishing 12.7 billion years old. It’s part of a visual binary (double star) system known as Marfak, from the Arabic ‘Al Marfiq’ (the elbow), located in the Northern constellation of Cassiopeia.
Located only some 25 light years away, Mu Cas A is headed towards us along our line of sight at a clip of some 97 km per second. In fact, by 5200 A.D., the star will have moved into the constellation of Perseus.
Most of the stars in our galaxy aren’t as old as Mu Cas A & B, Astronomer Gerard van Belle, director of science at Lowell Observatory in Flagstaff, Ariz., told me via email. This binary star tells us about what the earliest generation of stars were like from the birth of the Milky Way galaxy, which was expected to have happened only 800 million years after the Big Bang, he says.
On any given clear night in the Northern Hemisphere, those who can find a dark enough spot can see it with the naked eye. And with a decent pair of binoculars, you are guaranteed success, even if you live in a suburban area with light pollution.
Here’s how to find Mu Cas A on the sky.
From the giant W in Cassiopeia, follow the two outer arms of the ‘W’ to where (if extended below the W), those two arms would intersect, says van Belle. That’s the location of Mu Cas A, he says.
The primary star, mu Cas A (sometimes noted as ‘Aa’) is a G5V main sequence star – like our sun, just a touch redder, says van Belle. For its color, it’s cooler than expected, because it’s old and therefore has low ‘metal’ content, he says. But unlike our sun, it has very little elements heavier than hydrogen or helium, says van Belle.
Incredibly, Mu Cas was likely formed in a dwarf galaxy that became part of our Milky Way even before the disk of our own galaxy formed. Mu Cas and other stars like it in our galaxy are fascinating because they offer stellar theorists a view onto the earliest epochs of our galaxy.
Mu Cas A lies in the general field of the Milky Way and might have belonged to a cluster at one time, but if so, the cluster has dissipated, Howard Bond, an astronomer at Penn State University and former at The Space Telescope Science Institute, told me via phone.
The star probably belonged to a couple of small galaxies that collided together to make the Milky Way, says Bond.
A couple of decades ago, stars like Mu Cas A were the source of controversy since for a while it seemed that our galaxy’s oldest stars might be older than the cosmos itself.
The so-called Methuselah star, known in scientific circles by its catalog number of HD140283 is a case in point.
A well-known reference object in stellar evolution, its peculiar chemical composition, proximity and absence of reddening makes it an interesting case-study of so-called Population II stars (old metal-poor halo stars), note the authors of a 2024 paper appearing in the journal Astronomy & Astrophysics.
Earlier estimates from observations dating back to the year 2000 placed the star at some 16 billion years old, says NASA. But new Hubble Space Telescope age estimates have reduced the range of measurement uncertainty, so that the star’s age overlaps with the universe’s age, says NASA.
A Long Fast Orbit
Lying some 190 light years away, HD140283’s orbit carries it down through the plane of our galaxy from the ancient halo of stars that encircle the Milky Way, says NASA.
The halo stars are among the first inhabitants of our galaxy and collectively represent an older population from the stars, like our sun, that formed later in the disk, NASA notes.
HD140283 is still one of the oldest stars for which an age has been determined, says Bond. I would not claim it is the oldest known, but it still is one of the best ones for measuring an age, because it is so nearby and relatively bright, he says.
Viewable with binoculars, the Methuselah star is now thought to have an age of some 12.3 billion years, at least a billion and a half years younger than the current age of our cosmos.
As for Mu Cas A?
Now at the end of its life, Mu Cas A is expected to remain on the main sequence as a hydrogen-burning star for roughly another billion years. When Mu Cas A does begin its red giant phase, it will have lived about a third longer than our own sun, which is expected to veer off the main sequence in another 6 billion years or so.
