28 billion light years! Hubble discovers the most distant star in the universe, whose light took 12.9 billion years to reach Earth

The Hubble Space Telescope has made an important discovery, according to a new study published in the journal Nature [1]. It has observed the most distant known star, currently 28 billion light-years from Earth. The light from this star took 12.9 billion years to reach Earth.

Einstein's special theory of relativity tells us that the speed of light is the fastest in the universe. So, how did this star travel 28 billion light years in 12.9 billion years? Isn't that much faster than the speed of light? How did Hubble discover such a distant star?

Stars are arguably the fundamental building blocks of the universe. They illuminate the dark universe, and our Sun is an ordinary star. Under gravity, hundreds of millions of stars come together to form a structure - a galaxy - that spans from tens to hundreds of thousands of light years. Galaxies further form a vast network of cosmic galaxies.

Stars are plasma spheres composed of large amounts of hydrogen and helium. Inside, nuclear fusion reactions are taking place, constantly emitting energy to the outside and generating radiation pressure to resist their gravitational collapse. The lifetime of a star depends directly on its mass. The more massive the star, the more intense the fusion reactions, the faster the fuel is consumed, and the shorter the lifetime.

For stars with more than 100 times the mass of the Sun, the lifetime is only a few million to tens of millions of years. For stars with masses several times that of the Sun, their lifetimes are usually a few hundred million years. A star like the Sun can live 10 billion years. Red dwarfs are the smallest type of stars in the universe, with less than half the mass of the Sun, but can live hundreds of billions or even trillions of years.

Massive stars are crucial to the evolution of other stars and even life. Back when BIGBANG was born 13.8 billion years ago, only large amounts of hydrogen and helium were produced during the initial nucleosynthesis process, and other elements were almost non-existent. It was only after the birth of massive stars that various heavy elements were synthesized through nuclear fusion and supernova explosions. Many of the elements that make up the human body originally originated in massive stars, which is why "we are all stardust".

Due to the short lifespan of massive stars, the universe has been in existence for 13.8 billion years and there are no more massive stars in the early universe. To study those unique stars, we have to look deeper into the universe because the speed of light is finite. If we look far enough, we can see the universe early enough to see massive stars in the early universe.

But compared to galaxies, stars are much smaller and fainter. By now, the light from stars in the early universe is so faint that even the powerful Hubble Space Telescope cannot see such faint starlight. In fact, with Hubble's resolving power, it can only distinguish a star as far as 100 million light years away. So, how did Hubble observe such a distant star this time?

This is due to a phenomenon predicted by Einstein's general theory of relativity - the gravitational lensing effect. According to general relativity, objects with mass distort the surrounding space-time, and the greater the mass and density, the stronger the distortion effect on the surrounding space-time.

A huge cluster of galaxies will strongly distort the nearby space. Distant objects that would otherwise be behind a galaxy cluster are distorted by the curved space when their light reaches the vicinity of the cluster. The galaxy cluster distorts and magnifies the image of the background objects like a magnifying glass, which is the gravitational lensing effect.

In Hubble's latest observation of the gravitational lensing effect, the foreground object WHL0137-08 is a giant cluster of galaxies currently 6.9 billion light-years from Earth (at a redshift of 0.566). In the distant universe behind it, a galaxy is perfectly aligned with the cluster and the Earth, and the image of the distant background galaxy is distorted by the cluster's strong gravitational pull, forming a long arc.

In the distorted image, a star (possibly a binary system) can be identified from the background galaxy, numbered WHL0137-LS. Astronomers have also named this star Grendel, which means "rising star or morning star" in Old English.

After analysis, it took 12.9 billion years for the light from Elendil to reach Earth, which means it lived in a universe only 900 million years old. At that time the star was located in a galaxy about 4 billion light-years away from the then proto-galaxy. But the light it emits will not reach the Milky Way in 4 billion years because the universe is expanding.

Hubble's law suggests that the entire universe is expanding. For galaxies that are farther apart, the more the space between them expands per unit time, it appears that the galaxies are receding from each other faster. Eventually, the light from Elendil took 12.9 billion years to complete its journey to the Milky Way, instead of 4 billion years.

Due to the constant expansion of space over the past tens of billions of years and the accelerated expansion that began 5 billion years ago under the influence of dark energy, the galaxy in which Elendil is located has now retreated 28 billion light years and 24 billion light years in 12.9 billion years.

Although this galaxy is receding at a rate of 1.86 times the speed of light, well beyond the speed of light, this is due to the effects of space expansion, not its speed relative to background space, which is not inconsistent with special relativity. Galaxies themselves do not move very fast in the universe. For example, the Andromeda galaxy is moving at about 110 km/s relative to the Milky Way.

The radius of the current Hubble volume has expanded to 46.5 billion light years, and this galaxy is still far from the end of the universe. We don't even know if there is an end to the universe, because no one knows how big the volume of Hubble is 46.5 billion light years away.

Astronomers have estimated through analysis that Elendil is at least 50 times the mass of the Sun, and may even reach a maximum of 500 times that of the Sun. It emits enormous amounts of energy outward and is millions of times more luminous than the Sun. Although this massive star is very bright, the starlight decays extremely after a distance of tens of billions of light years, making it impossible to be directly observed by Hubble.

However, the brightness of Elendil is magnified thousands of times by the galaxy cluster's powerful gravitational lensing, which allows Hubble to observe it directly. Elendil became the most distant star ever discovered. But to understand the nature of this star, we need the help of the more powerful James Webb Space Telescope (JWST).

The most powerful space telescope in human history, Webb is still in the process of being commissioned. For now, it is only testing and observing stars near our solar system. In a few months, when Webb is fully commissioned, it will be used to observe Elendil, allowing astronomers to delve into the nature of stars in the early universe.