How far can a telescope see? There is no farthest, only farther...

"How far can your telescope see?"

This is a question that is often asked when working at an observatory, but the answers given are often not satisfactory to the questioner...

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The origin of human concept of space and time must be associated with the starry sky above our heads, and then extended to the universe - all around, up and down, from ancient times to the present. The continuity, periodicity and guidance presented by the starry sky attract human beings to gaze and contemplate for a long time.

01

As far as the eye can see

On a clear, moonless, light-pollution-free night, the dimmest star you can see with the naked eye is 6th magnitude (apparent magnitude), and the farthest planets, stars, and galaxies you can see are: Uranus, Eta Carinae, and the Andromeda/Triangulum Galaxy.

〇Uranus

This light blue-green planet is an average of 2.5 billion kilometers away from Earth, has a light transit time of 2 hours and 40 minutes, an apparent magnitude of about 6, and can reach a maximum brightness of 5.5 during opposition (occurring once every 1 year and 4-5 days).

η Carinae

It is a star in the southern sky and one of the stars discovered by humans so far that may explode in a supernova at any time. It is 7,500 light-years away from the Earth and its current apparent magnitude is 4.3.

○V762 Cas Cassiopeia

It was once thought to be the farthest star visible to the naked eye (estimated to be about 14,000 light-years away), but according to the latest Gaia EDR3 results, its distance is only about 2,500 light-years.

In addition, there are some stars about 10,000 light-years away from the Earth, whose apparent magnitude is at the edge of what is visible to the naked eye. It is not very meaningful to talk about which one is the farthest. Most of them are variable stars with very unstable brightness, and the data on their apparent magnitude may not be accurate enough.

〇Andromeda Galaxy M31/Triangulum Galaxy M33

They are 2.54 million light-years and 3 million light-years away from the Earth respectively, with apparent magnitudes of 3.44 and 5.72 respectively. They are about 15 degrees apart in the sky and are both on their way to the Milky Way.

Of course, there are also transient astronomical phenomena such as supernovae and gamma-ray bursts, which can extend what humans can see to billions of light years in a short period of time.

02

"Time Machine" and "Horizon"

Using time to express distance has been around since ancient times. In astronomy, "light years" have been used to express distance for nearly 200 years. Einstein's theory of relativity is based on the experimental fact that the speed of light is constant, which closely links time and space. When humans look up at the stars, they are actually looking back in time and space. The farther the distance, the earlier the time and the older it is.

The invention of the telescope has extended human vision, like a magical "time machine", giving us the opportunity to look at the "horizon" deep in the universe and see through the past of the universe. This is probably one of the reasons why astronomy is so attractive. As Edwin Hubble said: The history of astronomy is the history of the ever-receding "horizon".

The history of astronomy is a history of receding horizons.

——Edwin Powell Hubble (1936)

"The Realm of the Nebulae"

03

The most distant ones

Back to the question at the beginning, "How far can you see?" In fact, there is no definite answer. After a telescope is built, astronomers can tell you clearly how faint the celestial body is that can be seen, not how far it is.

Take the Hubble telescope, for example. How far can it see? The answer is constantly changing as astronomers race to find new objects to top the list. Hubble has been constantly breaking its own records, extending the edge of humanity's ability to look back into the universe with ever-changing discoveries.

The Hubble Space Telescope (HST) has made a revolutionary contribution to the study of the early universe. | Image source: NASA/ESA

Before the launch of the Hubble Space Telescope, ground-based optical telescopes could only observe celestial bodies with a redshift of no more than 1, which is about half the age of the universe since the Big Bang. Hubble's latest record has reached a redshift of 11.1, which is about 3% of the age of the universe. Its successor, the Webb Space Telescope (JWST), will be able to detect an earlier and more distant universe.

Now let’s take a ride on the “time machine” and count the most distant objects that have been confirmed to have been detected so far.

The most distant celestial body confirmed to date | Image source: see annotation

The first exoplanet candidate, M51-ULS-1b, discovered by the Chandra X-ray Telescope in the spiral galaxy M51 28 million light-years away in October 2021, remains to be further confirmed.

In early April 2022, an international research team discovered a new candidate for the most distant galaxy, HD1, with a redshift of 13.27, 100 million years younger than GN z11 and about 1.2 billion light-years farther away. However, we still need to be cautiously optimistic about this, because the only spectral line they used to confirm the redshift value is only a tentative result of 4σ, and it cannot be said to be "confirmed by spectral lines."

Schematic diagram of the evolution of the universe. The circles indicate the most distant galaxies confirmed and candidate galaxies, GN z11 and HD1. | Image source: Harikane et al., NASA, EST and P. Oesch/Yale

04

What expands is space, what redshifts is light

Astronomers probe the depth of the universe and study its history by measuring the extent to which the wavelength of photons emitted by distant celestial bodies is stretched by the expansion of space, the so-called redshift z. The larger the redshift value, the greater the distance in time and space. The cosmic microwave background radiation (CMB) corresponds to a redshift of about 1100, when the universe had evolved for 380,000 years. The precise determination of redshift is achieved through a series of spectral certifications, that is, splitting the electromagnetic radiation of celestial bodies, just like using a prism to make a rainbow, and identifying the offset of the spectral lines of known atoms, ions or molecules at specific wavelengths, so as to accurately determine the redshift.

Schematic diagram of the redshift of light and the change of spatial distance in the expanding universe | Source: Rob Knop

But the light from more distant galaxies is so faint that direct spectroscopic measurements are sometimes not feasible, so astronomers have instead come up with some clever rough spectroscopic techniques to give redshift estimates.

One of them is called the "Lyman break method", which is to obtain a wider spectral shape by covering a variety of different filters from optical to near-infrared bands, and look for a characteristic sharp "step" in it - the "Lyman break", which is caused by the strong ultraviolet light with a wavelength less than 91.2 nanometers emitted by stars and galaxies in the first billion years of the universe being completely absorbed by the diffuse neutral hydrogen gas. The position of the "break" on the spectrum depends on the distance of the galaxy, thus giving an estimate of the galaxy's redshift.

This method can screen out galaxies in the early universe in batches, but their identity confirmation still requires the use of more sophisticated spectral measurements to determine the redshift value. Before that, they can only be called candidates.

In his 1936 book "The Nebular World", Edwin Hubble described the experience of exploring the early universe: "With increasing distance our knowledge of the universe rapidly exhausts. Eventually, we reach the dim edge - the limit of detection of the telescope. There, we measure shadows and look for almost no more substantial landmarks amidst ghostly measurement errors." It still makes sense to read it today.

With increasing distance, our knowledge fades, and fades rapidly. Eventually, we reach the dim boundary—the utmost limits of our telescopes. There, we measure shadows, and we search among ghostly errors of measurement for landmarks that are scarcely more substantial.

——Edwin Powell Hubble (1936)

"The Realm of the Nebulae"

If we compare the current universe to a centenarian, we have seen a fragment of him when he was 3 years old through the "time machine", which is almost the limit of Hubble's operation. The new "time machine" Webb (JWST) is ready in space and is about to blow the horn to explore the earlier universe, constantly breaking new records, discovering thousands of the first generation of luminous celestial bodies (stars, galaxies, etc.) in the universe, and looking back to the universe in its infancy.

05

Those out of reach

"There is no farthest, only farther..." can be used as a slogan for human exploration of the universe, but in fact, there is still a farthest. The existence time of the universe (about 13.8 billion years) and the speed of light are both limited, so the "observable universe" is also limited, with a radius of about 46.5 billion light years. The reason why this radius is much larger than 13.8 billion light years is because the universe is constantly expanding.

Finally, let's go back to the question at the beginning. In fact, even in the solar system, it is not that simple to answer the question "how far can we see?" Astronomers have never stopped exploring the edge of the solar system while constantly pushing the horizon of the universe to the origin of time. Will the next one be called "Farfarfarout"? Although it is uncreative, it does not affect our expectations at all.

Comparison of the distance between the asteroid Farfarout and some solar system objects (AU is about 150 million kilometers) | Source: R. Candanosa, S. Sheppard and B. Bays

While we are looking forward to Webb's great performance, let's not forget Voyager 1, which is carrying human information and moving forward alone. It is the human spacecraft that has flown the farthest so far. Even though it knows that it will take at least 10,000 years, it will still strive to break out of the solar system and witness the scenes beyond the edge of the solar system for mankind.

Voyager 1: Currently 23.27 billion kilometers from Earth | Source: NASA

Rotating Editor-in-Chief: Du Fujun

Editor: Wang Kechao