The Week in Space and Physics: The First Stars
On the search for the first stars, the death of a planet, Starship and the shape of the Milky Way
The outbreak of World War II proved an unexpected boon for astronomy. Across Europe and Asia cities dimmed their lights, a move intended to hinder the attacks of enemy warships and planes. In California, where Japanese warships prowled off the coast and aircraft threatened aerial strikes, authorities sought to hide the presence of American ships by imposing blackouts along the coast.
At the Mount Wilson Observatory, close to Los Angeles, these darkened skies left the stars clearer than they had been in decades. Walter Baade, an astronomer based there, took advantage of the moment by turning its telescopes towards Andromeda, the closest large galaxy to Earth. Within it, he realised, he could see individual stars - the first such observations ever made of Andromeda.
Those stars fell into two groups, he found. The first, which he called Population I stars, looked more or less like our Sun. Later he would conclude these stars were younger and rich in metallic elements (which, in astronomy, refers to any element other than hydrogen or helium). The other, which he called Population II, were far older stars, and contained a lower proportion of metals.
This division is, of course, rather broad. Yet it reveals something important about how the universe is changing over time. As stars have come and gone over the past fourteen billion years, they have gradually increased the proportion of metals in the cosmos. Over time new stars have thus tended to contain increasing amounts of metal and have become, in astronomical parlance, less “pure”.
In the 1970s, however, astronomers realised something odd. Population II stars are purer than Population I stars, but there is a limit on that purity. Yet the universe, at the moment the first stars came into being, was almost perfectly pure. It was made, almost entirely, of hydrogen and helium, with very few heavier atoms. Somewhere, then, should have been another population of stars - Population III, if you will - that were extremely pure, with very little in the way of metal.
Yet stars like this had never been seen. Population III stars have long been entirely hypothetical. Astronomers don’t even know what they would look like - though they suspect they would have been vast, up to a thousand times larger than the Sun. If so, they would have lived and died fast, exploding into supernovae a few million years after they formed. Population III stars, then, may have only briefly appeared on the cosmic stage, vanishing long before we turned up.
Even so, traces of their presence may linger. Those early stars would have burned hot and given out strong ultraviolet radiation. That, in turn, would have ionised the clouds of hydrogen and helium then filling the universe, creating a detectable signature. Researchers, indeed, recently claimed to have spotted this signature in data from James Webb telescope, though whether they really did is rather questionable.
Others have focused on the debris those stars would have left behind them. One recent team claims to have spotted signs of this - observing clouds in ancient galaxies that appear poor in iron but rich in carbon and oxygen. If so, these could be the remains of the first stars, and the later birthplace of the Population II stars.
Still, this is not quite the same as a true discovery of the earliest stars. Many questions still remain about them and how they lit up the young cosmos. Though telescopes like the James Webb allow us to penetrate further into the universe’s history than ever before, we still haven’t seen far enough to see the moment the first stars came into being.
Watching a Planet Die
One day, billions of years from now, the Sun will begin to die. Like other stars of its kind, it will gradually swell as it cools; growing in size as it transforms into a red giant. Exactly how big the Sun will get when this happens is a matter of debate, but it is likely to expand by a hundred times or more.
That would envelop the planets of Mercury and Venus, and perhaps Earth as well, sucking them into the outer layers of our star. Such an event will, of course, be devastating for these planets. Yet it may not be fatal. Astronomers have spotted planets which may have survived such close brushes with their stars, going on to live hellish afterlife around a dying sun.
Still, most planets will not be so lucky. Fragments of long dead planets have already been spotted around some white dwarf stars, hinting at a violent past. Now, for the first time, astronomers have caught such an event as it happens - watching as a distant star engulfed and destroyed one of its planets.
The first hint of something unusual came from the Zwicky Transient Facility, an instrument designed to spot fast changes in the night sky. Over a period of ten days the facility saw a star suddenly brighten before fading away over several months. Such events are sometimes known as “red nova”, and occur when one star devours another, releasing a burst of energy.
In this case, however, the event was not bright enough for that explanation to work. Instead the watching astronomers concluded that the star had engulfed a planet roughly the size of Jupiter or Neptune. That would have made it a “hot Jupiter”, a commonly found type of gas giant that orbits extremely close to its star.
Having detected an event of this kind, astronomers will now be on the lookout for others. Calculations suggest that around one planet is devoured every year in our galaxy, so they should be relatively common events. Indeed, one day in the distant future, astronomers on some faraway planet may see a similar flash of light coming from our own star.
Starship Legal Woes
When will Starship fly again? After last month’s test flight, Elon Musk said SpaceX could be ready to try again within two or three months. From an engineering perspective that may be doable. SpaceX has plenty of hardware that can be quickly readied for flight and has already started work on repairing and improving the launch pad.
Yet engineering challenges are not the only thing holding back the development of Starship. More significant are the legal and regulatory issues around launching rockets. Indeed, without these regulatory concerns, Starship could have flown months earlier than it actually did and may have flown several more times already.
The most immediate issue is an FAA investigation into the launch failure. Such investigations occur after all launch mishaps with the aim of ensuring that any issues identified do not pose a risk to human safety. SpaceX may, for example, be asked to fix the flight termination system - which failed to destroy the rocket promptly when commanded - or to implement time consuming upgrades to the launch pad.
But SpaceX is also facing a legal challenge. Activists have sued the FAA, arguing that the agency failed to carry out proper environmental checks before granting SpaceX a launch licence. At the heart of things is the enormous dust cloud kicked out by Starship’s engines. Debris subsequently rained down across a vast area, far exceeding a worst-case analysis submitted by SpaceX before the launch.
These two challenges are likely to be the real limit on how quickly Starship can fly again. Even if they both resolve in SpaceX’s favour, they will certainly take longer than two months to do so.
The Shape of the Galaxy
Does the Milky Way galaxy have four spiral arms, or two? Radio observations have long suggested it has four main arms, with several smaller arms or spurs between them. Yet that, according to a recent study, would make our galaxy something of an outlier.
Most spiral galaxies, the study notes, have two main spiral arms extending outwards from their cores. In some cases these split up, forming multiple spiral arms in their outer regions. Galaxies with four arms extending out from the centre are rare - which means the Milky Way is either exceptional, or we have simply got its shape wrong.
To support their argument, the study authors used data from the Gaia telescope to map out the positions of thousands of young stars. They then found that the Milky Way is probably a barred spiral galaxy, with two arms extending outwards from a central bar before breaking apart into multiple arms further from the core.
As to Starship, I imagine we will get only one more launch before year's end, for the reasons you cited. Continued lawsuits and regulator roadblocks could very well see China returning to the Moon before the US.