How the Sun Compares to Massive Stars Like Stephenson 2-18

Although our closest star, the Sun, is the main reason we exist, most of us give it little thought as we go through our lives on our magical planet.

The Sun is nothing special when compared with many other stars. It is an average G-type main-sequence star, often called a yellow dwarf. While some stars are smaller than the Sun, many are so enormous that their sizes are almost impossible to imagine.

Stephenson 2-18 is considered to be one of the largest stars currently known. Because its exact size is still being studied, future observations may change its estimated dimensions, but it remains one of the true giants of our galaxy. It lies about 18,900 light-years from Earth.

That may not sound particularly impressive until you remember how fast light travels. Light moves at 1.08 billion kilometres (671 million miles) every hour, or 9.46 trillion kilometres (5.88 trillion miles) every year. By those standards, Stephenson 2-18 is unimaginably far away.

How can we visualise the size of Stephenson 2-18 when comparing it with the Sun?

Astronomers estimate that Stephenson 2-18 has a diameter about 2,150 times greater than the sun's.

The Sun has a diameter of 1,392,700 kilometres (865,400 miles). Using a simple calculation, that gives Stephenson 2-18 an estimated diameter of approximately 2,994,305,000 kilometres (1,860,660,000 miles).

Numbers this large are difficult for the human mind to grasp. Even seeing billions written on a page does little to help us understand just how enormous this star really is.

The next few paragraphs should make its size a little easier to imagine.

As you probably know, eight planets orbit the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and the farthest planet, Neptune. Now you will have to use your imagination.

Replace the Sun with Stephenson 2-18 so that it occupies exactly the same position at the centre of the solar system.

That would be disastrous news for the planets. Mercury, Venus, Earth and Mars would instantly disappear inside the star. Jupiter and Saturn would also lie within its enormous outer layers. The giant star would stretch almost as far as Saturn's orbit, swallowing the inner solar system completely. Only Uranus and Neptune would lie outside its surface.

Even that comparison is difficult to picture because our solar system feels unimaginably large. Yet this single star would fill almost everything from the centre of the solar system to Saturn.

Light travels at almost 300,000 kilometres (186,000 miles) per second. If it were possible for light to travel around the circumference of the Sun, the journey would take about 15 seconds. A trip around the circumference of Stephenson 2-18 would take an astonishing 8.7 hours.

Nothing with mass can travel at the speed of light. According to our current understanding of physics, this means humanity will probably never be able to journey very far beyond our own small area of the Milky Way.

We have never heard from intelligent extraterrestrials. If they exist, they too may be limited to exploring only their own local region of space, separated from us by distances so vast that communication could take thousands, or even millions, of years.

But could we be alone?

At this time, nobody knows. The Universe contains hundreds of billions of galaxies, most with billions of stars and countless planets. Somewhere among them, intelligent life may exist, or we may be the only civilisation to have looked up at the stars and wondered who else is out there.

Until we discover convincing evidence one way or the other, the answer to that question remains one of the greatest mysteries in science.

British spelling.

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