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Q. Suppose Earth orbited the star Proxima Centauri instead of the Sun. Where would we be in the Milky Way? — Sarah W.
A. Earth is one of nine planets orbiting a star we call the Sun. In turn, our Sun is one star among 100 billion stars in a galaxy we refer to as the Milky Way. Proxima Centauri is a "sun" just like our star.

Not far apart. If you were to look at the entire Milky Way galaxy at one time, you would see that our Sun and Proxima Centauri are very close together among the 100 billion stars in the galaxy.

In fact, if you could look at the whole galaxy, the Sun and Proxima Centauri might even appear to be lost in the vast cloud of stars that make up the galaxy.

However, in terms we can understand better here on Earth, Proxima Centauri is a long way off — about 25 trillion miles away from the Sun.

Yet, it's very close. Among the 100 billion stars in the galaxy, the three closest neighbors to our Sun are the stars Alpha Centauri A, Alpha Centauri B and Proxima Centauri (also called Alpha Centauri C).

Proxima Centauri is 4.2 lightyears away from us. [One lightyear is the distance light travels through space in one year, about 5.9 trillion miles.]

Viewed from afar, our Milky Way galaxy would look like a flaming pinwheel, a spiral of stars, a flat disk of stars 100,000 lightyears in diameter with a bulge of stars at the center.

What would be different about our location if the Earth were moved to an orbit around Proxima Centauri? Not much on the grand scale our galaxy, yet we would be in a different position on the same outer spiral arm of the galaxy.

Can we see Proxima? When we look up from Earth, stars are a long way off, yet all the stars visible from Earth, even with telescopes, are within our own Milky Way galaxy.

Proxima Centauri is too dim to be seen from Earth with the naked eye. However, Alpha Centauri A and Alpha Centauri B can be seen as a bright star. Together, A and B form a "binary" or double star as they orbit each other. Alpha Centauri A and B are about 4.4 lightyears from the Sun.

Proxima Centauri, the star nearest to Earth, is so far away it's only a pinpoint of light in the largest telescopes on Earth. However, Proxima Centauri is very close to us in the giant scale of our galaxy.

From our perspective on Earth, stars seem to hold the same positions in the sky year after year. Over many centuries they do appear to move, however, as our Solar System slowly circles the core of our Milky Way galaxy.

Does Proxima have planets? Many other stars may be ringed by planets and other small bodies, just as our Sun has its Solar System of planets, moons, comets and asteroids. For all we know, Proxima Centauri may have its own solar system of planets.

By the way, Earth is only about 93 million miles away from the Sun and it is locked in orbit around the Sun.

The average distance between Earth and Sun is 92.96 million miles. Astronomers refer to that distance as one astronomical unit (AU). An AU is a unit of measurement.

The outer edge of the Solar System is estimated to be 75 to 100 times the distance from the Earth to the Sun. That is 75 to 100 AU.

Think of a human spacecraft flying away from Earth and out of the Solar System — for example, the interplanetary probe Voyager-2. When it passed by the planet Neptune back in 1989, on its way out of the Solar System, Voyager-2 was 47 astronomical units from the Sun.



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