One question we’re asked at BINTEL all the time is “how far can I see?”
The answer is sometimes surprising!
In the Solar System the most distant object we can see is the planet Saturn. This appears as a bright “star”, especially around this time when it comes closest to the Earth when our respective orbits around the Sun line up.
To see details like the rings of Saturn or the bands on Jupiter and its moons will require a telescope.
The two outer giant planets, Uranus and Neptune, will need telescopes to see and they can’t be spotted just with your eyes. (There is some debate about whether you can see Uranus with your eyes under the right conditions if you know where to look. If it can be seen this way, it certainly wasn’t recorded as a wandering planet by any ancient culture.)
Forget about even thinking about trying to see Pluto with your eyes. It’s way too faint. Even with mid to large-sized amateur telescopes, if you manage to find it, it will only appear as a dim star in the eyepiece.
There’s some astro objects beyond our home galaxy which are easy to spot.
The two Magellanic Clouds are satellite galaxies of our Milky Way which are easily seen as large, misty patches the far southern part of the sky when you’re away from city lights on a Moonless night. The Large Magellanic Cloud or LMC is some 163,000 light years away. The Small Magellanic Cloud or SMC is a bit more distant at around 206,000 light years.
The Large and Small Magellanic Clouds Image via ESO/S. Brunier – ESO
Tricky to see from the southern half of Australia due to its position in the northern sky, the Andromeda Galaxy or M31is generally considered to be the most distant astro object you can see with just your eyes. It’s about 2.5 million light-years away.*
Once you wheel out a telescope the furthest distance how far you can see rises dramatically. Even in small telescope you’ll be able to easily see elliptical galaxies like “The Hamburger Galaxy” or Centaurus A (NGC 5128) which is 11–13 million light-years away or super-giant galaxy, Messier 87 (M 87) that contains trillions of stars that lies over 53 million light-years away. (And there’s lots more too.)
But what about individual stars?
In the Milky Way, a good candidate for the most distance star visible to your eyes is V762** in the constellation of Cassiopeia. This is a variable star around 16,308 light years away, give or take a few inches. This is about halfway to the centre of the Milky Way.
V762 is a not typical of most of the stars you can see overhead at night. The Milky Way is vast, and stars spread so thinly throughout it that most of the stars you can see with your eyes are within about 1000 light years of Earth. This tiny section of our galaxy. Even a small telescope will show many more, fainter stars that are further away and can’t be see with just your eyes.
The most star we know of in the Universe is called Earendel.
Found in 2022, we see this star as it was at 12.9 billion years ago or about 7 percent of the current age of the Universe. It’s some 28 billion light-years away.
Distant star Earendel as seen by the Hubble Space Telescope in 2022
(To answer the question “How can this be 28 billion light-years away when the Universe is only 13.5 billion years old???” Astronomers measure distance in the cosmic realm by using terms like comoving distance. This takes into account stretches in the space between two points due to the expansion of the Universe. If you were to journey from Earth to where Earendel is located, it would take much longer than 28 billion years as seen from Earth as space would have expanded even further during your journey. )
Even with the advanced and highly tuned optics of the Hubble Space Telescope Earendel was only spotted after its image was magnified between one thousand and forty thousand times by gravitational lensing of the massive galaxy cluster, WHL0137-08 between us and the star. (We touched on gravitational lensing in this blog article. First predicted by Albert Einstein, it’s where massive objects like stars or even galaxies bend light travelling near them.)
“We almost didn’t believe it at first, it was so much farther than the previous most-distant, highest redshift star,” said astronomer Brian Welch of the Johns Hopkins University in Baltimore. “Normally at these distances, entire galaxies look like small smudges, with the light from millions of stars blending together,” said Welch. “The galaxy hosting this star has been magnified and distorted by gravitational lensing into a long crescent that we named the Sunrise Arc.” he continued.
One of these “dots” along the Sunrise Arc was discovered to be star.
Now the more powerful James Webb Space Telescope (JWST) has taken a further look at Earendel.
Earendel as seen by the JWST
The new, close-up view of Earendel still shows it in the bent, arc of light caused by gravitational lensing. But JWST has revealed Earendel to be a massive star about a million times brighter then Sun. Not only have the new images helped us learn the type of star that Earendel is, but also strongly hinted that it has a companion star.
The alignment of Earendel and the galaxy between us will eventually drift away over a long period meaning the gravitational lensing that lets us spot this distant star will also disappear. There’s still plenty of time left for JWTST and its successors to learn more about these distant stellar remnants from the early days of the Universe.
* There’s some debate about whether another member of our local group of galaxies, the Triangulum Galaxy at 2.7 million light-years can be see with good vision under right conditions.
**Again, there is some debate about whether V762 Cassiopeiae is the furthest star that can be see with your eye alone.