Quick! What’s the nearest star to Earth? Not counting the Sun, because this is not a trick question. If you answered Proxima Centauri, you can count yourself among the astronomically knowledgeable. Its distance of 4.25 light years means it is practically next door in interstellar terms.
This star is one of a trio making up our nearest stellar neighbors. The Alpha Centauri system comprises two Sun-like stars that appear as one to the naked eye, and Proxima Centauri, a red dwarf star too dim to be seen without a telescope. The two larger stars orbit a common center, and Proxima Centauri orbits them at a much greater distance.
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Proxima takes more than half a million years to complete one orbit around its two other stars and is currently moving closer to Earth. In another few thousand years it will have receded, and the two larger stars of the system will swap nearest neighbor status every 80 years.
How do we know this?
Stars that are relatively nearby can be measured with the stellar parallax method. To understand how this works, hold a finger in front of your face and view it with first one eye shut, and then the other. Your finger will appear to shift against a more distant background due to the different positions of your two eyes.
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For something much more distant than an arm’s length, we can make use of the Earth’s travel around the Sun. If we observe a nearby star against a background of far more distant ones in January, in July we will have traveled to the other side of the Sun, a distance of 2 astronomical units from our original spot.
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A little basic trigonometry gives us the distance to the star. (Yes, people do use math after high school.) And if you’ve ever wondered what a parsec is…
The distances to even the nearest stars are so great that these apparent position shifts are very small and difficult to measure. For Proxima Centauri—the nearest star with the largest parallax angle—the angle amounts to the diameter of a nickel about three and a half miles away. This limits the distances discernible by this method to a little over 300 light years for Earth based observations. Space based observations that escape the distortions of our atmosphere have pushed these distances to tens of thousands of light years, albeit with decreased precision at the greatest distances.
The New Horizons spacecraft that flew by Pluto in 2015 turned its camera toward two nearby stars in April 2020. At a distance 43 times that of the Earth from the Sun, the distance of the spacecraft from an Earth based telescope is much greater than the two positions of that telescope six months apart. The parallax angle of Proxima Centauri was large enough to see easily!
https://www.nasa.gov/sites/default/files/styles/side_image/public/thumbnails/image/nh-proxima_cap.gif?itok=n51k_3Dz
Stellar parallax is a standard used to calibrate other means of distance measurements that are part of the cosmic distance ladder. The Hipparcos and Gaia spacecraft together measured the parallax of almost two billion stars and other astronomical objects, giving us a precise three dimensional map of our section of the Milky Way Galaxy. Enjoy this tour!
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