The sun is a star. But is every star a sun?

One of humanity’s greatest coming-of-age moments was realizing that the sun is a star, somewhat inconspicuously within the range of the stars we see; made special only by our relationship with it. However, stars are not clones of each other. Do all other stars count as suns? Do something? If only some do, which ones? There are no universally accepted answers, but we can investigate.

Like many things, astronomical categories seemed so simple before we learned more about them. There was THE Sun, THE Moon, five planets and thousands of stars. We had to make up stories to explain what they were, but it wasn’t difficult to figure out which category something fell into.

What is a sun?

Then things got messy, as anyone bruised by Pluto’s deplanetation can tell you. New categories such as asteroid and dwarf planet were created and not everything fit exactly. Objects orbiting other planets came to be called moons, rather than creating a new term, although we also use “satellite.” Our own moon may still be there The Moon for us, but some fairly insignificant pieces of rock and ice orbiting the gas giants also get a rating of moons. It wasn’t until we heard about objects with a more informal association with their planets that we created a new category, quasimoon.

As we began to learn the scale of the universe and understand the nature of the stars, it became clear that the Sun clearly belongs to that category of stars. It’s not exactly typical: there are many cooler and less massive stars than stars of the Sun’s luminosity or greater, but if there’s anything truly exceptional about it, we have yet to discover it.

Most categories are created by humans to make it easier for us to understand an essentially vague universe, rather than being surrounded by immutable boundaries. So if we had wanted to, we could all have agreed that “The Sun” was a specific name for the star’s orbits around the Earth, and not to refer to any other body.

Some people do believe this. Website Little Astronomy states: “The use of the term sun to refer to any other star is incorrect. Sun is not a synonym for star.”

However, language changes with use. The use of ‘Sun’ to describe stars very similar to our own is now so widespread that those who cling to the belief that the word Sun should be reserved for just one star are in the minority. Stars that are quite similar to the Sun are often called other suns. As an example among many: when people refer to a famous scene in the original Star Warsthey talk about the demise of Tatooine’s “twin suns”.

If you prefer the word authority over democracy in a case like this, just look at NASA’s children’s website: “Many other solar systems have multiple suns…”

Which stars count?

Just because some stars are suns doesn’t mean they all are.

The Merriam-Webster dictionary provides “A celestial body such as the sun; star” as an acceptable definition. Notice the “like the sun” part. There is no official designation of how sun-like a star must be to be called a sun. Instead, it’s a matter of how speech evolved. Many science fiction books are set on planets orbiting Tau Ceti, the closest star to the Sun in our vicinity. These often call it a sun. Few scientists would dispute this.

On the other hand, it is unlikely that anyone would call a white dwarf, a dead star that is radiating its residual heat but no longer undergoing fusion, a sun. The same goes for a neutron star. But then again, if that’s what you want, no one has the right to stop you.

In general, however, stars are only called suns if they have at least two characteristics: they are still fusing elements, releasing heat, and they are surrounded by planets.

However, meeting these criteria does not mean that everyone will welcome a star to the club.

Brown dwarfs, for example, are not large enough to join the main sequence by fusing ordinary hydrogen. They still (barely) qualify as stars because they fuse deuterium, requiring less pressure. However, Deuterium is such a rare star that these types of stars do not perform very good solar imitations.

Stars that only have gas giants orbiting them, rather than planets that astronauts could one day land on, are also less likely to be seen as suns. Part of the use seems to be to imagine that aliens or future colonists can watch sunrises and sunsets. Starsets just sounds wrong.

Other people would be more restrictive. Red dwarfs (M-type stars) are definitely stars, and we know that many of them have orbiting planets with physical characteristics similar to those of Earth. Yet they are not that sunny. We still don’t know for sure if it’s possible for life to exist in their environment. Planets must be very close together to be warm enough for such faint stars for liquid water to be present, but at such close distances their atmospheres can be stripped by the frequent outbursts. Given all this, many would hesitate to call them suns.

It is up to you

Ultimately, this is a case of humans making the rules, and assuming all our readers are humans, we say it’s up to you. If you want to consider that there is only one sun, then you are right. The same goes if you even want to call white dwarfs suns, but in either case you would be a true outlier. Feel free to stand strong in your beliefs, but don’t expect many allies. Whether some modest sun-like stars deserve the name – use it at your discretion.

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