The most distant object visible to the naked eye

When we look up at the sky full of stars, we will always be fascinated by the vastness of the universe, but what you may not know is that the stars we see are not as far away as we think most of the celestial bodies visible to the naked eye in the night sky are not more than 1000 light years away from the Earth.

For example, among the familiar stars of the Big Dipper, the farthest star, the "Star of the Sky", is only 123 light-years away from the Earth; the brightest star in the night sky, Sirius, is only 8.6 light-years away from the Earth; and the distance between Polaris and us is at most is only 434 light years.

However, the range of 1000 light years only applies to the more common stars, if an object is particularly bright, it is possible to see it directly even if it is very far away from us, so the question arises, how many light years away from Earth is the farthest object visible to our eyes?

In astronomy, there is a concept called "apparent magnitude", which simply describes the brightness of a celestial body that we can observe directly with our naked eyes. The concept was first introduced by the ancient Greek astronomer Hipparchus, who divided the many stars in his catalog into six classes according to their brightness, with the 1st class being the brightest and the 6th class the faintest.

Modern astronomy has quantified the "apparent magnitude", which is defined as 2.512 times the difference in brightness between two adjacent magnitudes, and in addition to this, it has significantly broadened the range of values of the "apparent magnitude" and introduced negative numbers, such as The "apparent magnitude" of the Sun is -26.7, and the "apparent magnitude" of the full Moon is -12.8.

"The higher the apparent magnitude, the lower the brightness, and it is generally believed that an object with an apparent magnitude of 6 is the farthest object visible to the naked eye, so we can assume that the farthest object visible to the naked eye is the farthest object from us and The "apparent magnitude" is less than or equal to 6.

Among the known stars, the one that best meets this condition is Huagai III in Cassiopeia, also known as V762 Cassiopeiae (V762 Cas), a red supergiant with an "apparent magnitude "V762 Cas, a red supergiant with an apparent magnitude of 5.86, is about 14,818 light-years from Earth.

In addition to stars, there are also extragalactic galaxies visible to the naked eye in the night sky, the most distant of which is the Triangulum Galaxy (M33), which has an apparent magnitude of 5.72 and is about 3 million light-years away from Earth, with a This galaxy has an apparent magnitude of 5.72, is about 3 million light-years from Earth, and has a diameter of about 61,000 light-years, making it the third largest galaxy in the group (the first and second are Andromeda and the Milky Way, respectively).

So is this the limit? The answer is no.

When high-energy events such as supernova explosions and neutron star collisions occur in the universe, they usually produce unusually powerful gamma-ray bursts and high-speed particle streams with velocities close to the speed of light, and although gamma rays are invisible to the naked eye, the high-speed particle streams will violently impact the interstellar matter in their vicinity, thus producing very strong visible light, so much so that hundreds of millions or even billions of light years away, we can see directly through the naked eye. directly.

In the past, astronomers have observed such a phenomenon many times, including the farthest away from Earth, which occurred on March 19, 2008, in this day, the "Rain Bird" satellite orbiting over the Earth suddenly observed the direction of gamma-ray bursts from the Makemake, and then the relevant ground-based observation equipment was quickly The observation equipment on the ground was then quickly aimed at this direction.

Soon after, the visible light from this high-energy event was observed, and the observed data showed that its "apparent magnitude" peaked at about 5.7 and lasted for about 30 seconds.

In other words, under good observing conditions, if we look up at the sky at this point, we can see the sudden appearance of a dull "star" in the direction of the constellation Makemake, only to disappear after about 30 seconds.

The gamma-ray burst was numbered "GRB 080319B", and subsequent studies have shown that the distance between the source and the Earth was as high as 7.5 billion light-years, which became a new record for the "farthest object visible to the naked eye", and has remained so far.

What is the concept of the brightness of the light emitted from a distance of 7.5 billion light-years that we can see directly through the naked eye? Let's put it this way, if "GRB 080319B" is placed in the position of the Sun, then its "apparent magnitude" will reach -67.57, which is equivalent to about 223 million times the brightness of the Sun seen on Earth, I must say, this is amazing. It's amazing, don't you think?