This Star is an Odd One

The Cosmic Companion
Apr 30 · 3 min read

One star in our own Milky Way, J1124+4535, is quite an oddball, featuring a chemical composition unlike that of any other star in the galaxy. Astronomers using the the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) found the star had low amounts of magnesium, but high concentrations of europium and other metals heavier than iron. This is the first time a star within our galaxy has been found to have such an unusual chemical composition.

Stars form in groups from vast clouds of gas, so examining the chemical composition of stars can provide clues as to their birthplace. Stellar bodies within dwarf galaxies surrounding the Milky Way have been detected with a similar chemical makeup to J1124+4535, suggesting this unusual red giant star may have joined our galaxy when the Milky Way merged with a small dwarf galaxy.

Computer simulations show that spiral galaxies, like the Milky Way, can form halos from the accretion of smaller dwarf galaxies. Image credit: Takayuki Saito/Takaaki Takeda/Sorahiko Nukatani/4D2U Project, NAOJ

“Initial observations showed that J1124+4535, located in the constellation Ursa Major (Big Dipper), had low abundances of certain elements, such as magnesium. Follow-up observations with the High Dispersion Spectrograph on the Subaru Telescope confirmed the low levels of magnesium but found comparatively high levels of europium. This is the first time an element ratio like this has been observed in a star in the Milky Way,” the National Astronomical Observatory of Japan (NAOJ) reports.

The Subaru Telescope observatory, seen as the International Space Station (ISS) passes overhead. Image credit: NAOJ

You are What You Eat

Surrounding the Milky Way is a halo of older stars, containing bodies which formed in the earliest era of the formation of our galaxy. These are joined by other stars, born in other clusters of stars, which fell into orbit around the Milky Way. As small clusters of stars joined the much-larger Milky Way, their structures would have fallen apart. However, the orbital motion and chemical makeup of these stars would be preserved, providing astronomers a means of determining their origin.

Magnesium, calcium, and oxygen are the produced in supernova explosions of massive, short-lived stars. Because of this, these materials build up in the early phases of galactic evolution. Iron, on the other hand, is largely produced in binary star eruptions called type-1a supernovas. Concentrations of that metal collect more slowly over time, producing a unique chemical makeup of the surrounding galaxy.

The Subaru Telescope High Dispersion Spectrograph, used to determine the chemical makeup of J1124+4535. Image credit: NAOJ

It’s Like a Rainbow…

By separating the light coming from a star into its component colors in a process known as spectroscopy, astronomers are able to see bright lines known as emission spectra from elements producing light. They can also see dark lines known as absorption spectra from materials which absorb light, allowing researchers to determine which elements are in the star. Utilizing spectrographs obtained from LAMOST, astronomers at the Subaru Telescope determined the unusual chemical makeup of this wayward star.

“Stars similar to J1124+4535 are very rare and have only been found in dwarf spheroidal galaxies. The discovery of J1124+4535 could be the start of chemical identification of stars accreted from dwarf spheroidal galaxies,” researchers explained in Nature Astronomy.

Computer simulations of the growth of galaxies suggest that galaxies such as our own Milky Way grow by merging with smaller galaxies, including dwarf galaxies like the one that J1124+4535 once called home.

During the 13.6 billion years since the formation of the Milky Way, our galaxy has merged with many smaller galaxies, and in 2.4 billion years, the brightest satellite galaxy of the Milky Way, the Large Magellanic Cloud, will collide with our own family of stars. In roughly four billion years, we will collide with a much larger galaxy than our own, Andromeda.

The Cosmic Companion

Exploring the wonders of the Cosmos, one mystery at a time

The Cosmic Companion

Written by

James Maynard is the author of two books, and thousands of articles about space and science. E-mail:

The Cosmic Companion

Exploring the wonders of the Cosmos, one mystery at a time

Welcome to a place where words matter. On Medium, smart voices and original ideas take center stage - with no ads in sight. Watch
Follow all the topics you care about, and we’ll deliver the best stories for you to your homepage and inbox. Explore
Get unlimited access to the best stories on Medium — and support writers while you’re at it. Just $5/month. Upgrade