The Stars that Time Forgot

Our Milky Way galaxy is surrounded by a tenuous halo of gas and stars, stretching 300,000 light years from the Galactic Centre. Containing less than a percent of Galactic stars, this halo was built through the accretion of smaller galaxies over billions of years as the Milky Way grew.

Inhabiting the halo are about 150 globular clusters, each a dense ball of around a million stars held together by their mutual gravity. The stars in these clusters are old, possibly highlighting the first stars formed when a new galaxy is born, with some forged in the halo of the Milky Way, whilst others were wrenched from smaller galaxies as they were torn apart by the Milky Way’s tidal forces.

Given their age, astronomers are keen to understand the role of globular clusters in galaxy evolution, and many have been intensely studied. But they realised that globular stars are not the oldest stars in the universe, something revealed by their chemical composition.

When a star is on the main sequence it is burning hydrogen in its core to produce energy - that is, through thermonuclear processes the star is fusing hydrogen atoms together to create the next heaviest element, Helium. Our Sun is currently doing this right now, about half-way through its main sequence lifetime. It’s converting hydrogen to helium in its core at a rate of approximately 600 million tonnes per second. 

Progressively, as stars continue to evolve, they will then commence burning helium and fusing this into the next set of elements like carbon, oxygen and neon. The process continues across a number of different elements until it reaches iron, where no more fusion can occur (as such energy is lost from the system). Elements heavier than iron are produced through other processes, such as cosmic ray fission or merging high-mass objects like white dwarfs and neutron stars. 

This synthesis of different elements is where all the matter that surrounds us comes from - the carbon in our bodies, the iron in the buildings around us, the oxygen that we breath - all were formed inside the core of a star that eventually became our Sun, and the solar nebula (from which the Earth and other bodies in the Solar System formed).

After the Big Bang, the elemental make-up of the universe was extremely simple, being roughly 75% hydrogen and 25% helium. Once this gas cooled and collapsed, the first stars formed out of this chemically simple material. These are the first stars to have formed in the Universe. 

But within these first stars, the light elements were forged into heavier elements which then polluted surrounding gas as these stars shed their materials as they died. The subsequent generations of stars, formed from this polluted material, became more and more chemically enriched. 

Rather confusingly, astronomers refer to this chemical enrichment of elements more massive than helium as metallicity. For example, a star that is found to have a chemical signature high in oxygen, carbon, neon and nitrogen is considered to be “metal-rich” even though none of these elements are considered metals as we know them here on Earth.

Astronomers studying globular clusters found that all are enriched above a certain amount, indicating that they were formed from non-pristine gas. The presence of this metallicity floor seems to key in understanding where and when the globular clusters formed.

With a new observational study, presented in the journal Nature by University of Sydney PhD student, Zhen Wan, the situation has now changed. The focus is the Southern Stellar Stream Spectroscopic Survey (S5 for short), which has observed a stream of stars in the constellation of Phoenix. The characteristics of this stream tells us it is a disrupting globular cluster, but the team noticed something strange about its chemical abundance; it sits well below the metallicity floor and is so unlike all the others. 

The S5 Project, which began in 2018, is a spectroscopic survey that uses the 3.9m Anglo-Australian Telescope located at the Siding Springs Observatory, in central north-west NSW. Mounted onto the telescope is the 2-degree-field (2dF) system - a robot which places 392 small optical fibres on a plate with up to 0.3 arcseconds of accuracy, collecting the light from distant objects before sending it 50m below the telescope where it is analysed by the AAOmega spectrograph.

The four different surveys that the S5 project encapsulates are:

• S5 Streams: The main survey of the collaboration with goals to measure the radial velocity and metallicities of stream members

• S5 Halo: Surveying the Milky Way halo for interesting objects like Hyper-Velocity Stars, extremely metal-poor stars, RR Lyraes and White Dwarfs

• S5 Lowz: Observing and documenting low-redshift galaxies to increase the database of these nearby faint galaxies, so that researchers can better train algorithms to build up a larger sample of these objects

• S5 Hires: Using the MIKE instrument located at the Las Campanas Observatory in Chile, this survey aims to capture high-resolution spectra of bright stream members to derive stellar parameters and precise elemental abundances, helping determine the chemical evolution of stars

The Phoenix globular cluster must have been formed at a location, or time, when the chemical enrichment was significantly different to the remainder of the cluster population. The astronomers are trying to understand where this might be, but the observations suggest that Phoenix might be the last of a previous population that inhabited galaxies in their most embryonic state. In the past, there would have been more of them, but over time the gravity of the Milky Way has ground down the population, leaving only remnants and traces behind.

Clearly, the Phoenix globular cluster is telling us something important about the evolution of galaxies, but we have yet to work out what this is. The hunt is on for the signs of Phoenix’s siblings, opening a new window on galaxy formation. 

Video Credit: The University of Sydney