Observing the earliest stars is one of the holy Grails of astronomy. Now, a team at the University of Hong Kong led by astronomer Jane Lixin Dai is proposing a new method for detecting them. If it works, the approach promises to open a window on the origin of the cosmos itself.
The earliest stars in the Universe formed very soon after the Big Bang. Astronomers call them “Population III” (or Pop III) stars. They’re different from the Sun and other stars in the modern cosmos for a variety of reasons. They formed mainly from the hydrogen and helium in the newborn cosmos. From there, they grew to outrageous sizes and masses very quickly. That growth had a price. Those stars had very short lives because they blew through their core fuels very quickly. However, fusion at their cores and the circumstances of their deaths created the first elements heavier than hydrogen and helium. Those new elements seeded the next generations of stars.
Population III stars were the Universe’s first stars. They were extremely massive, luminous stars, and many of them exploded as supernovae. Image Credit: DALL-ESo, why can’t we detect these early stellar behemoths? For one thing, they existed too far away, too early in history, and their light is very faint. That’s not to say they are undetectable. Astronomers just need advanced methods and technology to spot them.
How to “See” the First StarsProfessor Dai’s team just published a study that suggests a connection between these first stars and nearby black holes. In short, they looked at what happens when a Pop III star interacts with a black hole. Essentially, it gets torn to shreds and gobbled up. For example, the supermassive one at the heart of our Milky Way Galaxy—called Sagittarius A*— does this. It has a regular habit of ripping apart stars that wander too close. When such a tidal disruption event (TDE) happens, it releases huge amounts of radiation. If the same thing happens in another galaxy—no matter how far away—the light from the event is detectable. As it turns out these tidal disruption event flares have interesting and unique properties used to infer the existence of the ancient Pop III stars.
The alien star S0-6 is spiraling toward Sagittarius A*, the Milky Way’s central supermassive black hole. S0-6 likely came from another galaxy and it may get gobbled up or torn up by interactions with the supermassive black hole. Courtesy: Miyagi University of Education/NAOJ.“As the energetic photons travel from a very faraway distance, the timescale of the flare will be stretched due to the expansion of the Universe. These TDE flares will rise and decay over a very long period of time, which sets them apart from the TDEs of solar-type stars in the nearby Universe,” said Dai.
In addition, the expansion of the Universe stretches the wavelengths of light from the flares, according to Dai’s colleague, Rudrani Kar Chowdhury. “The optical and ultraviolet light emitted by the TDE will be transferred to infrared emissions when reaching the Earth,” Chowdhury said. Those emissions are exactly the kind of light new generations of telescopes are built to observe.
Searching for First Stars with Advanced TelescopesThis detection method is right up the alley of the JWST and the upcoming Nancy Grace Roman telescopes. Both are optimized to sense dim, distant objects via infrared wavelengths. They should be able to search out the stretched light from those long-gone Pop III stars unfortunate enough to encounter a black hole. In particular, the Roman telescope will use its wide-field instrument to gather the faint infrared light from stars born at the earliest epochs of cosmic time.
Artist’s impression of the Nancy Grace Roman space telescope (formerly WFIRST). Credit: NASA/GSFCAstronomers generally accept that these first stars formed perhaps as early as a hundred million years after the Big Bang. That’s when overly dense regions filled with hydrogen and helium began to experience gravitational collapse. The stars that formed in those first birth crèches were purely hydrogen and helium—in other words, they were “metal-free”. They lived perhaps a few million years before exploding as cataclysmic supernovae. (By comparison, the Sun has existed for some 4.5 billion years and has another few billion years left before it becomes a red giant and then a white dwarf.) The heavier elements created inside those first stars got blasted out to space, enriching the nearby molecular clouds with infusions of carbon, oxygen, nitrogen, and other elements. Some of the largest first stars could have collapsed directly to form black holes.
Finding these first stars and their emitted light (particularly from possible interactions with early black holes) will give astronomers amazing insight into conditions in the early Universe. Even though those stars are long gone, JWST, Roman, and other telescopes can look back in time and see their dim, infrared light. If Dai’s method works, those telescopes could be responsible for the discovery of tens of Pop III stars each year.
For More InformationHKU Astrophysicists Discover a Novel Method for Hunting the First Stars
Detecting Population III Stars through Tidal Disruption Events in the Era of JWST and RomanNancy Grace Roman Space Telescope
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There are plenty of crazy ideas for missions in the space exploration community. Some are just better funded than others. One of the early pathways to funding the crazy ideas is NASA’s Institute for Advanced Concepts. In 2017 and again in 2021, it funded a mission study of what most space enthusiasts would consider only a modestly ambitious goal but what those outside the community might consider outlandish—landing on Pluto.
Two major questions stand out in the mission design: How would a probe arriving at Pluto slow down, and what kind of lander would be useful on Pluto itself? The answer to the first is one that is becoming increasingly common on planetary exploration missions: aerobraking.
Pluto has an atmosphere, albeit sparse, as confirmed by the New Horizons mission that whizzed past in 2015. One advantage of the minor planet’s relatively weak gravity is that its low-density atmosphere is almost eight times larger than Earth’s, providing a much bigger target for a fast incoming aerobraking craft to aim for.
Fraser discusses future missions to Pluto.Much of the NIAC Phase I project was focused on the details of that aerobraking system, called the Enveloping Aerodynamic Decelerator (EAD). Combined with a lander, that system makes up the “Entrycraft” that the mission is designed around. Ostensibly, it could alternatively contain an orbiter, and there are plenty of other missions discussing how to insert an orbiter around Pluto. Hence, the main thrust of this paper is to focus on a lander.
After aerobraking and slowing down to a few tens of meters a second, from 14 km/s during its interplanetary cruise phase, the mission would drop its lander payload, then rest on the surface, only to rise again under its own power. The answer to the second question of what kind of lander would be useful on Pluto is – a hopper.
Hoppers have become increasingly popular as an exploration tool everywhere, from the Moon to asteroids. Some apparent advantages would include visiting a wide array of interesting scientific sites and not having to navigate tricky land-based obstacles. Ingenuity, the helicopter that accompanied Perseverance paved the way for the idea, but in other words, the atmosphere isn’t dense enough to support a helicopter. So why not use the current favorite method of almost all spacecraft – rockets?
Fraser discusses the results from New Horizons.A hopper would fire its onboard thrusters to reach the area on Pluto’s surface and then land elsewhere. It could then do some science at its new locale before taking off and doing so again somewhere else. The NIAC Phase I Final Report describes five main scientific objectives of the mission, including understanding the surface geomorphology and running some in-situ chemical analysis. A hopper structure would enable those goals much better than a traditional rover at a relatively low weight cost since Pluto’s gravity is so weak.
Other objectives of the report include mathematical calculations of the trajectory, including the aerobraking itself and the stress and strain it would have on the materials used in the system. The authors, who primarily work for Global Aerospace Corporation and ILC Dover, two private companies, also updated the atmospheric models of Pluto with new New Horizons data, which they then fed into the aerobraking model they used. Designing the lander/hopper, integrating all the scientific and navigation components, and estimating their weights were also part of Phase I.
The original launch window for the mission was planned as 2029 back in 2018, though now, despite receiving a Phase II NIAC grant in 2021, that launch window seems wildly optimistic. Since the mission would require a gravity assist from Jupiter, the next potential launch window would be 2042, with a lander finally reaching the surface of Pluto in the 2050s. That later launch window is likely the only feasible one for the mission, so we might have to wait almost 30 years to see if it will come to fruition. Sometimes crazy ideas take patience – we’ll see if the mission team has enough of that to push it onto the surface of one of the most interesting minor planets in the solar system.
Learn More:
B. Goldman – Pluto Hop, Skip, and Jump
UT – NASA is Now Considering a Pluto Orbiter Mission
UT – Should We Send Humans to Pluto?
UT – New Horizons Team Pieces Together the Best Images They Have of Pluto’s Far Side
Lead Image:
Artist’s depiction of the Pluto Lander mission design.
Credit- B. Goldman / Global Aerospace Corporation
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