A team of researchers, including scientists from MIT and the Carnegie Institution of Science, has analyzed the chemical composition of stars in the fossil galaxy known as Segue 1. The dwarf galaxy, containing roughly 1,000 stars, sits 75,000 light years away from Earth, and is host to a set of unusual features that are allowing astronomers to observe the composition of stars from the early universe.
What makes the fossil galaxy so interesting is not its age, its relatively stunted size, or the fact that it sits on the edge of the known universe as one of the faintest galaxies ever detected. Segue 1 is of unique interest because it represents an example of a galaxy that has simply failed to evolve. It is widely accepted that dwarf galaxies form the building blocks for larger, more evolved galaxies such as our own. Galaxies make this transition via the natural evolution of their component stars progressing through their reproductive cycle and birthing a multitude of new stars, in the process altering the galaxy's chemical make-up.
Segue 1 never made this transition. At some point in the ancient past, the tiny galaxy stalled and failed to produce the next generation of stars, leaving it at an evolutionary stand-still. Because of this, the fossil galaxy is very small, comprised of only 1,000 stars rather than the 1 million stars contained in the average galaxy. The stars present in Segue 1 are also very young, which gives astronomers the chance to observe the chemical composition of stars from the early universe, very soon after the Big Bang.
At this point clarification is needed as to how we define a young star. We have discovered innumerable instances of stellar nurseries birthing new stars in our own galaxy, however in evolutionary terms whilst these stars are new, they are not young. The gasses that coalesced to create these new stars are the product of generations of older stars forming, and subsequently going supernova, thus creating new clouds of stellar material. Therefore, the material used in the creation process for the new generation has already been re-used many times, with each supernova adding more varied and denser elements to the gas clouds, which will then go on to form new stellar nurseries. Thus the composition of the stellar material from which the stars are made is actually very old.
This is not the case for the stars in Segue 1, where stellar evolution halted at an early stage of development, leaving its stars chemically immature. "It tells us how galaxies get started," states Anna Frebel, assistant professor of Physics at MIT. "It’s really adding another dimension to stellar archaeology, where we look back in time to study the era of the first star and first galaxy formation."
The team analyzed Segue 1 by taking readings from six of its red giant stars with the Magellan telescopes, located in Chile, supplemented by data from the Keck Observatory, in Hawaii. The stars were chosen for being the brightest in a very faint galaxy, with the stellar behemoths yielding some interesting observations.
It was found that stars in Segue 1 contained only a tiny amount of metallic elements. There were strong indications that all elements heavier than helium had come from only one or two supernovas that occurred fairly soon after the galaxy's formation. Soon after these explosions, the galaxy shut down, starved of gas and ceasing to produce new stars. The team also discovered a lack of "neutron-capture elements," the materials chiefly responsible for the creation of intermediate-mass stars, thus further limiting any chance Segue 1 had of evolutionary advancement.
Looking to the future, the team hopes to spend more time analyzing the chemical composition of Segue 1, as well as searching for other fossil galaxies, in an attempt to illuminate whether the evolutionary standstill of Segue 1 was a rare failure, or one of many fossils from the early universe.
The paper detailing the team's findings on Segue 1 has been published in the Astrophysical Journal.