STARS THAT SHOULDN’T EXIST

At first glance, blue straggler stars cannot possibly exist. They are apparently young stars in ancient stellar environments. As Stuart Clark finds out, the resolution to this astrophysical conundrum potentially opens the door to a richer diversity of stars than we previously imagined.

Dr Stuart Clark

Ask any astronomer what the colour blue means in a star and they will tell you: stellar youthfulness. The reason is simple, the colour of a star tells you the temperature of its surface, which is determined by the star’s mass. The more massive the star, the hotter it burns and the faster it uses up its nuclear fuel.

Hence, whereas our Sun is a pretty average yellow star that will shine steadily for around 9 billion years, the most massive stars shine blue and live for only a few million to a few hundred million years at most.

Next, ask the astronomer how old a globular star cluster is and they will tell you: ancient. Globular clusters are spheres of ancient stars that formed during the initial collapse of their host galaxy, roughly 10–12 billion years ago.

With this one-and-done approach to star formation, it means that the only stars now surviving in globular clusters are similarly ancient. Hence, these particular stars shine with the yellow light of less massive, longer lived stars. The bluer, more massive stars have all long since died. Or have they?

As observations of the globular clusters became more precise, so astronomers noticed a small population of youthful blue stars among the stellar methuselahs. They are stars that simply should not exist – yet they do and, as modern astronomy has increasingly unlocked the mystery of their existence, we now realise that it could force a re-evaluation of everything we thought we knew about how half the stars in the Galaxy evolved. This is the story of the blue straggler stars.

A puzzle from the 1950s The mystery began in 1953, when American astronomer Allan Sandage, who would go on to become one of the greatest observational cosmologists of his age, was studying globular cluster M3. To determine the age of the cluster, he was plotting the stars on a graph: surface temperature, which gives the star its colour, versus brightness, which is measured in magnitudes. Hence, astronomers call these plots, colour-magnitude diagrams.

All the stars in the Universe will sit somewhere on such a graph, the most famous version of which is known as the Hertzsprung–Russell diagram. Its most obvious feature is a curving band of stars known as the main sequence. This is where the majority of stars sit, it is populated by stars that are in their stable ‘adulthood’, comfortably fusing hydrogen into helium.

As these stars run out of hydrogen in their cores, they move off the main sequence and head towards the upper right-hand quadrant of the graph, passing through the sub giant region, to become red giants. This means that plotting a globular cluster’s stars on a colour-magnitude diagram can reveal the cluster’s age by looking for where the main sequence ends.

It works because the stars in a globular cluster all formed at more-or-less the same time. So, the first stars to leave the main sequence are the massive, short-lived blue stars. Hence, for a globular cluster of 10 billion years old, the only stars on the main sequence will be those yellow and red stars with life spans of more than 10 billion years.

The trouble for Sandage was that there were blue stars still on the main sequence – not many to be sure, just a handful of stragglers. Yet the number of them made no difference, they simply should not have been there in the first place. They shouted stellar youth in a celestial environment where that was thought impossible. As more people studied them, the evidence slowly mounted. In a typical globular cluster of around 100,000 stars, a few hundred would be blue stragglers. There were only two solutions, either there was a fundamental error in our understanding of stellar evolution, or the stars themselves had undergone some kind of transformation that allowed them to masquerade as younger objects.

Cosmic collisions?

Decades passed until the 1990s arrived. By then, computer power was changing the way astronomers studied stars, allowing them to simulate the internal stellar processes better than ever.

Alison Sills, now at McMaster University, Ontario, Canada, was a graduate student at the time. She was looking for a PhD topic at Yale University and heard about the blue stragglers. An astronomer at Yale by the name of Charles Balen had a crazy idea but he needed someone versed in computer modelling to help him investigate.

“His idea was that you could get stellar collisions happening, two stars actually running into each other. I was like, wow, that is cool,” Alison tells Astronomy Now .

…computer power was changing the way astronomers studied stars, allowing them to simulate the internal stellar processes better than ever.

It was a natural hypothesis when one considers the crowded environment of a typical globular cluster. The nearest star to the Sun is Proxima Centauri, which is about 4.25 light years, or about 1.3 parsecs, away. So, in round numbers, the density of stars in our corner of the Universe is roughly two stars per cubic parsec. In the centre of a globular cluster, there could be as many as 10,000 stars in that same volume.

Instead of a night sky showing constellation patterns, a planet in the core of a globular cluster would see an almost permanent daytime sky, thanks to all the starlight.

So the basic idea was simple, in such a crowded environment, collisions are not only possible but common on cosmological timescales. When two low-mass stars collide and merge, the resulting object acquires fresh hydrogen and becomes more massive, effectively emerging as a rejuvenated star. It burns hotter and bluer and therefore appears younger. Blue stragglers, therefore are the products of stellar collisions. But as Sills discovered, they are not particularly spectacular.

“Collisions sound dramatic but they’re actually not that energetic, at least not the ones that would happen in globular clusters,” she says. Instead the stars are moving quite sedately relative to one another. “It’s messy in the sense that you’ve got gas going all over the place butit’s not energetically exciting. And it only lasts for months to a year – maybe even less – so it doesn’t do anything spectacular, and it doesn’t last very long. That makes it hard to see,” she says.

But what we can see are the stars that result from those gentle collisions and, by the end of her investigation, Sills was convinced that this was how blue stragglers are formed. Her computer models of collided stars confirmed it. But…

“About six months after my PhD had said, ‘Yes they’re collisions’, there was incontrovertible evidence that they had to be binary mergers,” she says.

Cosmic cannibals?

The new work came from Bob Mathieu, University of Wisconsin–Madison, who had been tracking the orbits of binary stars in star clusters. Binary stars are pairs of stars that are held in each other’s gravitational fields. Mathieu showed that in two of the older clusters he was studying, almost all of the blue stragglers were still in binary systems.

“You don’t get that if they all come from collisions,” says Sills, who teamed up with Mathieu to investigate further. Their secret weapon was the recently launched Hubble Space Telescope. It was able to peer into the hearts of these clusters in great detail. (see Box: The Hubble factor.)

A crucial difference between their individual studies was that while Sills was looking at globular clusters, Mathieu was looking at open clusters. These are found within the Galaxy and are nests of stellar siblings that all formed at the same time. Crucially, the stars are not as densely packed as in the core of a globular cluster, and so binary stars are more common.

The problem is that it is unfeasible for the binary system to survive the collision of one of its members with a third, interloping star. So there must be something else happening to rejuvenate one of the stars into a blue straggler.

In Mathieu’s scenario, the transfer of matter from one star to another becomes almost inevitable as the more massive star in the system begins to evolve. As it leaves the main sequence by expanding into a red giant, it can grow larger than its own gravitational field, crossing into the sphere of influence of its smaller companion, which then starts enrobing itself with material pulled from its swollen companion.

Fattened on its partner’s outer layers, the once-smaller star becomes more massive and hotter, reviving itself as a blue straggler while its companion is reduced to being a faint white dwarf. Therefore, blue stragglers are created by the careful, sustained feeding from a companion star, making them cosmic cannibals. So which is it really, collisions or cannibalism?

Fattened on its partner’s outer layers, the once-smaller star becomes more massive and hotter, reviving itself as a blue straggler, while its companion is reduced to being a faint white dwarf.

The solution finally came not from a sustained research programme of investigation but from a newly minted post-doctoral researcher at the European Space Agency who was trying to learn some new software.

Two pathways revealed

In 2009, Giacomo Beccari, who now works at the European Southern Observatory, Garching, was attempting to understand a piece of software that extracted the brightnesses of stars from a set of data. He saw in the archives that there were a lot of data from the globular cluster M30. So, manual in one hand, keyboard under the fingers of his other he set about identifying M30’s blue stragglers and got more than he expected.

There were two distinct sequences of blue stragglers in its colour-magnitude diagram. One sequence lay brighter and bluer; the other slightly redder. Convinced he had made a mistake with the software, Beccari ran the analysis again and again and again. Each time he got the same answer. It had to be real.

The simplest explanation was that the sequences represented two different populations: one formed by collisions, the other by mass-transfer in binaries. The finding suggested that both processes were not only operating in the same cluster but had done so in a way that left visible signatures on the cluster’s stellar demographics.

“M30 is critical because it was the first time that we saw clearly that there were these two channels of formation,” Beccari tells Astronomy Now . Or as Sills puts it: “The story, of course, is more complicated than a or b, it’s a and b. Both things can happen.”

So is the mystery of the blue stragglers now closed? Hardly. According to Emily Leiner, Illinois Institute of Technology, the details of their formation process, especially involving the binary star pathway, appears to be much more nuanced than previously thought.

The uncertainty arises from the fact that astronomers have yet to develop sufficiently detailed computer models to describe how binary stars interact.

“Most models assume that the entire envelope of the giant star ends up on the accreting star. That’s almost certainly not true but that’s how we model it,” says Leiner, who points out that, once a star begins to lose material, it will also carry away rotational energy and current computer models are just not up to the task of fully simulating this.

“It opens up this whole can of worms that’s really difficult to model. We don’t even necessarily know when that mass transfer process is going to be stable,” she says.

This is important because, if it is stable, then the companion will accrete a lot of material and end up a blue straggler. When it is unstable, the entire system could become cloaked in gas, the stars’ orbits decay and they end up merging. That too might lead to a blue straggler but from a different pathway. “It opens up this whole bunch of theoretical uncertainties,” says Leiner.

This is significant because it calls into question almost everything we thought we knew about the evolution of binary and multiple stars. When you consider that such systems make up at least half the stars in the Galaxy, that represents an enormous level of misunderstanding about how stars evolve.

Hubble Space Telescope and Blue Stragglers

“The Hubble Space Telescope was the revolution in the study of blue stragglers,” says Giacomo Beccari, European Southern Observatory, Garching. That is because its position above the atmosphere allows it to see much finer details than ground-based telescopes. Blue stragglers are most numerous in the dense cores of globular clusters, where crowding severely limits what can be resolved from the ground, making it difficult to identify individual stars near cluster centres or to place them accurately on colour-magnitude diagrams.

Hubble allowed astronomers to resolve individual stars deep into cluster cores for the first time. This enabled the reliable identification of blue stragglers, precise brightness measurements and measurements of their spatial distribution relative to other stellar populations.

In addition to this, ultraviolet observations with Hubble also proved valuable. Blue stragglers are hotter than most cluster stars and therefore stand out more clearly at shorter wavelengths. This reduces any confusion with cooler red giants and ordinary main-sequence stars. In some clusters, such as M30, Hubble data also revealed distinct blue-straggler sequences, providing evidence for multiple formation channels operating within the same system (see main story).

“Stellar evolution is one of the great accomplishments of 20th century astrophysics. We have excellent models, so if you have the mass of a star, you can pretty much predict how it’s going to evolve and end its life with a really high degree of accuracy. With binary stars, that’s just not the case,” says Leiner.

Yet we know that they create some of the most interesting objects in the Universe. For example, the type 1a supernovae that we use to measure the expansion of the Universe, occur exclusively in binary systems. There are other kinds of exploding objects and the gravitational waves sources we have been detecting for the past decade all come from binary mergers.

“I think there’s been a really big renaissance of interest in binary evolution because of these things,” says Leiner.

Yellow stragglers

Although they have found a way to roll back the clock, not even blue stragglers can last forever. Once rejuvenated, they remain on the main sequence longer than their unaltered siblings. Eventually, however, time will catch up with them and they will run out of nuclear fuel, evolve into subgiants, then giants and finally white dwarfs. Yet here again, they could well be distinctive if Leiner is correct.

During Leiner’s PhD thesis, she studied the open cluster M67, identifying a number of sub giant stars that appeared to be a little more massive than expected. Because of their position on the colour-magnitude diagram, she termed them yellow stragglers. She now believes that these are older blue stragglers that are finally ageing.

The blue stragglers may have been the most obvious stellar curiosities that astronomers found but they serve as an object lesson that as our instruments and capabilities continue to improve, we are likely to find other, more subtle oddities that need explaining. Stellar evolution is nowhere near as straightforward as we once thought. Subtle but systematic differences in brightness or chemical composition could be the smoking guns of interesting stellar lifetimes, ones in which their environment and interactions have altered their appearances and status. Especially for stars in binary systems where they can potentially interact, feed off each other, collide and merge.

Much more than simple oddities, the blue stragglers tell us that the Galaxy’s stellar population is a lot more varied and complex than ever we had imagined.

…it calls into question almost everything we thought we knew about the evolution of binary and multiple stars.

Dr Stuart Clark is Astronomy Now ’s Editor.

Hubble reveals how environment shapes blue straggler formation

A recent study using the NASA/ESA Hubble Space Telescope demonstrated a clear connection between a globular cluster’s environment and the way its blue straggler stars form and survive.

Contrary to earlier expectations, that the highest numbers of blue stragglers would occur in the densest environments, where stellar collisions are more frequent, the Hubble results show the opposite trend. Low-density clusters host significantly more blue stragglers than their dense counterparts. This pattern arises because the dominant formation channel for blue stragglers in these systems is binary evolution: stars in tight pairs transfer mass from one to the other, effectively rejuvenating the accreting star and making it hotter and brighter.

In high-density cluster cores, frequent close encounters with other stars disrupts these binary systems before mass transfer can occur. The fragility of binaries under such conditions reduces the number of stars that can evolve into blue stragglers.