The first stars of the Cosmos remain elusive, but astronomers are getting closer

The first stars of the universe, one of astronomy’s holy grails of observation, may still be as elusive today as they were a quarter of a century ago. But astronomers are getting closer to these first points of light. While it’s true that these so-called Population III (Pop III) stars have yet to be detected, theorists are making progress in understanding how they evolved, became active, and perhaps even died.

To these first stars we owe everything in our field of vision. Neither the few thousand points of light that we can see with the naked eye, nor the planets of our solar system would exist if it were not for these long-awaited stellar progenitors.

Even without detecting them, the researchers place better constraints on their initial masses and lifetimes. An upcoming article in The Astrophysical Journal presents new evolutionary models that predict that these early stars would have had initial masses ranging from 100 to 1,000 times that of our Sun.

Our models calculate Pop III stars with masses a thousand times that of the Sun, but we don’t know if such massive stars actually existed, said Guglielmo Volpato, a doctoral student in astronomy at the University of Padua in Italy and lead author of the paper. to me. There are good arguments that, in the absence of metals, the fragments of gas clouds that collapse to form stars can be much heavier than in the presence of metals, he says. These arguments inspired us to explore these higher mass ranges, he says.

When did these stars first form?

Following the “standard model” of Pop III star formation, they are predicted to begin forming at a redshift of about 20 to 30, Volpato says. This corresponds to about 100 to 200 million years after the Big Bang, he says. In contrast, the first known galaxies recently detected by the Webb Space Telescope are at a redshift of only 13.

How did they end their lives?

For massive and very massive stars, if the star goes through all the major phases of core burning, then it has an iron core surrounded by a structure called onion skin, Volpato says. These types of stars could end their lives by exploding as a supernova or collapsing into a black hole, he says.

Understanding how these Pop III stars evolve and die is not just a matter of mere curiosity. Such research has implications for understanding how primordial stellar black holes may provide the seeds for supermassive black hole assembly, the authors note.

How long did they stay in the main hydrogen burning sequence?

In our article, we consider stars with an initial mass between 100 and 1,000 times the mass of our Sun, Volpato says. For these stars, their hydrogen-burning lifetimes range from 1.6 to 2.6 million years; increasing as the initial mass of the star decreases, he says.

That’s about half the main life sequence of the largest stars in our solar neighborhood. And it’s only a fraction of the lifetime of our Sun burning hydrogen that will stay on the main sequence for about 10 billion years.

In contrast to our Sun, which will end its life as an expanding red giant, Volpato says that Pop III stars of about 300 times the mass of the Sun or more should end their lives by collapsing directly into very massive black holes.

These so-called ‘collapses’ are stars that collapse under their own gravity to form a white dwarf, neutron star or black hole. When looking for signatures of collapsing Pop III stars, astronomers will look for pulsed gamma-ray bursts (GRBs).

The collapse scenario is the most accepted model for long-lived GRB progenitors, says Volpato. Following the collapse of the stellar core and the formation of a black hole with an accretion disk, a large amount of energy is deposited along the polar directions that the GRBs emit, he says.

The emission spectrum of the glow produced by a GRB Pop III should be detectable by current space-based gamma-ray observatories, Volpato says. This glow comes from the interaction of gamma-ray radiation with the environment surrounding the progenitor star, he says,

As for the detection of Pop III stars in the optical spectrum?

“These stars are very difficult to observe mainly because of the enormous distance and their very dim luminosity,” Volpato said.

Volpato says that future data from next-generation ground-based gravitational wave detectors, such as the Einstein telescope and Cosmic Explorer, may be able to detect gravity waves from hitherto undetected black holes created by Pop III stars.

As for Pop III stargazing with the Webb Space Telescope?

Even with the Webb telescope, observing a single star would require gravitational lensing with a magnification factor comparable to those inferred for the most distant known objects, Volpato says.

If the target were a Pop III star cluster or galaxy, then it would be possible to use Webb without the need for gravitational lensing, he says.