The oldest stars in the universe are shrouded in darkness. Their redshift is so high that we can only wonder about them. The James Webb Space Telescope will be our most effective telescope for observing the very early universe and should be observing up to z = 15. But even it has limitations.
In order to observe the first stars of the universe, we need a larger telescope. The big telescope in the end.
The Ultimate Large Telescope (ULT) is currently just a concept. No doubt the James Webb space telescope will show us a lot about the early universe, but we all know that we want an even more powerful telescope in the future. This is how we expand our knowledge.
The desire to build the ULT is driven by our insatiable curiosity. We want to know more about the early days of the universe and the stars that lived there. In an article, a group of scientists outlines what the ULT will be, why it is necessary, and how it will work.
The title of the new paper is “The Ultimately Large Telescope – What Type of Setup Do We Need to Recognize Population III Stars?” The main author is Anna Schauer, a research assistant at the Department of Astronomy at the University of Texas, Austin. The paper is available on the prepress page arxiv.org.
Stars are divided into three populations: Population 3, Population 2, and Population 1. Pop 3 stars are the oldest, the first stars that formed, and consisted only of elements of the Big Bang: hydrogen, a little helium, and tiny amounts of Lithium and beryllium. Because they are the first stars, they have a very low metallicity. (In astrophysics, anything heavier than hydrogen and helium is considered metal.)
Next came Pop 2 stars. As the Pop-1 stars lived and died, they merged heavier elements and spread them across the universe. Pop 2 stars are formed from part of this matter, so they have a higher metallicity than Pop 1 stars.
Pop 1 stars are the “third generation” of stars. As third generation stars, they have an even higher metallicity than Pop 2 stars. Each generation of stars creates more metals that can be picked up by the subsequent generation.
Our own sun is a pop 1 star. It has a high metal content for a star of around 1.4 percent. Although astronomers speak of high metallicity in stars, the overwhelming mass of a star is still hydrogen.
Pop 3 stars hold many secrets about the early universe, and astronomers want access to those secrets.
Infrared telescopes can see the distant past. The James Webb observes the red-shifted light of the earliest galaxies in the near infrared (NIR). But it can’t break the barrier to look further back if there were no galaxies, just stars.
For this we need the ULT.
Since the first stars had almost no metals, they formed and developed differently. Researchers believe that they were more massive than later populations, hundreds of times more massive than the sun. They also quickly burned their fuel and lived a shorter life.
Scientists also theorize that most of these Pop 3 stars exploded as pair instability supernovae. In this terminology, “pair” does not mean a binary pair. It describes the production of pairs of electrons and positrons in the star that lead to the star’s collapse and then to the thermonuclear explosion of the star that gets out of control when it goes into the supernova.
But much of it is theory based on our observations and understanding of the other populations of stars. Nobody has ever seen a Pop 3 star. Hence the ULT.
The ULT has to be strong enough to find the “mini halos” in which Pop 3 stars have formed. The current theory states that large galaxies form and exist in halos of dark matter. The theory predicts that the Pop 3 stars of the universe also formed in halos, albeit much smaller. Finding the first stars means finding their halos.
When the authors went into the limitations of the JWST to find these halos, they wrote: “But also with this new facility
This brings us to the ULT itself. The ULT should have a 100 meter mirror. This is absolutely enormous when you consider that the largest optical telescope currently under construction – the European Extremely Large Telescope (EELT) – will have a 39.3-meter mirror that consists of 798 separate segments. The EELT is already reaching the limits of our technology, so a 100 meter mirror may still be out of the reach of our technical technologies for a while. In their work, the authors write: “While 100 m is obviously a challenge, it lies in the range of possibilities for mid-century technology.”
But the size may not be the most interesting thing about this potential telescope. According to the authors, this monstrosity should be built on the moon.
The moon is the only really realizable place for the ULT. There’s no way to take it out into space and park it in a LaGrangian Halo orbit like the JWST. And it cannot be built on Earth where the atmosphere would interfere with your observations.
Referring to previous considerations in a Moonscope, the authors say that the ULT is best built in a lunar crater that has permanent shadows. Even then, it would still have to be cryogenically cooled. The ULT would be at the moon pole and would always point to the zenith, and it would have no hinge bracket. While earlier considerations of a Moonscope talked about a liquid level, the new paper doesn’t mention this.
Its target area would be limited by the moon’s precession, and exposure could take up to several days. Adding some kind of tracking function like a moveable prime focus platform could increase this exposure time.
There is no plan to build this telescope yet, so the paper is not going to be very detailed. In fact, the paper itself is more concerned with what astronomers will use it for and with models of how it should be used.
But if it is ever built – and we bet it will eventually – it probably won’t be alone on the moon. In fact, the moon appears to be an ideal location for certain types of telescopes, especially radio telescopes.
NASA recently announced funding for a study of a radio telescope on the other side of the moon. It is tentatively referred to as the Lunar Crater Radio Telescope (LCRT). As the name suggests, it would be built in the naturally concave shape of a suitable crater.
The idea of the moon telescope goes back to the Apollo period. During this time, an Apollo project scientist named Richard Vondrak introduced himself to building radio telescopes in lunar craters.
One of the Apollo missions actually put a telescope on the moon. When Apollo 16 landed on the moon on April 21, 1972, he was carrying an ultraviolet telescope that took 178 pictures of the universe.
China has also recently brought telescopes to the moon. The Chang’e-3 lander brought a remote-controlled telescope to the moon in 2013, and in 2019 its Chang’e-4 lander brought a small radio telescope to the moon.
Although these small telescopes are interesting and the Apollo 16 telescope is part of space history, they are hardly part of the same discussion as the ULT.
If the ULT is ever built and has a 100-meter primary mirror, it could be one of the ultimate technological achievements of mankind. And by allowing observation of the first stars of the universe, it could finally answer some of the questions that stand in the way of our understanding of the universe.