Science Snippet:

Explore The Closest Stars

On a clear night with extremely dark conditions, there can be as many as 6,000 stars visible to the naked eye. Some of these stars are bright and noticeable whereas others are barely visible pinpricks of light. You might expect that the bright prominent stars are the closest ones. If you are stargazing in winter, the brightest star in your sky is one of the closest stars. But there are other close stars that you can’t see at all. Of course, the closest star of all is visible in the daytime rather than at night. In fact, this star is responsible for our definitions of day and night. The sun, the closest star to earth, is only 93 million miles away. 93 million miles doesn’t sound very close. But to see how comparatively close that is, we will explore the seven closest stars to Earth besides the sun.

When discussing the distance between objects in space, miles aren’t the best option. Another way to measure these distances is by comparing them to the distance between the sun and the Earth. Astronomers call the average distance between the sun and the Earth one astronomical unit (abbreviated AU). If an object is twice as far away as the sun, we would say that it is two AU away (equivalent to 186,000,000 miles).

The closest star to Earth besides the sun is the dim star Proxima Centauri. Proxima Centauri is almost 40 times dimmer than Neptune. Not surprisingly, it is not a star we can find. Proxima Centauri is part of a group of three stars known as the Alpha Centauri star system. Although Proxima Centauri is too dim to find, the other two stars are easy to find. They are in fact some of the brightest stars in the sky but they are only visible in the southern hemisphere. The Alpha Centauri star system is over 275,000 AU away from Earth. And they are the closest stars!

The closest stars are not the brightest ones. Another of our near neighbors is Barnard’s star. Barnard’s star is a red dwarf only about a tenth the size of our sun. It’s too dim to see without a telescope. But astronomers spend a lot of time studying this dim star. It is one of the oldest stars in our galaxy. Furthermore, it moves across the sky in relation to the stars around it. All stars move in our sky over the course of thousands of years. But Barnard’s star moves across the sky relatively quickly. Over the course of a human lifetime, Barnard’s star moves about half a full moon width across the sky. (Even a relatively fast-moving star doesn’t move that fast in our sky.) Barnard’s star is currently 377,000 AU away from the Earth but it is currently moving closer to Earth. In 11,000 years it will be only 237,000 AU away. But even when it is this close, you would still need a telescope to see Barnard’s star.

Stars must be a certain size to convert hydrogen into helium. Some stars are too small to do this. But they can fuse deuterium, a special form of hydrogen, into helium. This process produces a lot less heat so these stars are relatively cool. Such stars are called brown dwarfs. For many years, astronomers theorized that brown dwarfs existed although they had never found any. But as it turns out, we have brown dwarfs as close stellar neighbors. In 2011, two brown dwarfs were discovered only 412,000 AU away from Earth making them the third closest star or star system to Earth besides the Sun. These two stars are called the star system Luhman 16. Brown dwarfs are very small. The stars in Luhman 16 are smaller than Jupiter although they are much more massive. Being so small, they are not visible without a telescope. But they aren’t as dim as you might expect. Luhman 16 is only about 2.5 times fainter than Barnard’s star.

Some objects are too small even for deuterium fusion. Such objects are called sub-brown dwarfs. These objects have similar masses to large planets. They aren’t planets, however, because they formed in the same way as stars. But they aren’t stars either because they don’t function like stars. In 2014, one of these objects was discovered 470,000 AU away making it the fourth closest star (if it is a star). It was given the name WISE 0855-0714. Because sub-brown dwarfs don’t undergo fusion they are quite cold. WISE 0855-0714 has a temperature range between -55 and 8 °F. We don’t know much about sub-brown dwarfs because studying such small dim objects is difficult. WISE 0855-0714 is about 400,000 times dimmer than Luhman 16. That’s too dim to find even with a fairly large telescope. Scientists can only see it from the largest observatories.

Fortunately, the stars don’t keep getting dimmer the further one looks away from Earth. The next closest star is much brighter although you still can’t see it without a fairly large telescope. Its name is Wolf 359 which is kind of a fun name. Wolf 359 is about 500,000 AU away. It is a red dwarf like Proxima Centauri. It is much smaller than Proxima Centauri, however. In fact, Wolf 359 is only just large enough to be a star instead of a brown dwarf. It produces only one-thousandth the light that the sun produces. Of course, since Wolf 359 is much further away than the Sun it is not just 1,000 times dimmer. If Wolf 359 was as close as the sun, it would be ten times brighter than the full moon. That might sound very bright but the sun is actually 40,000 times brighter than that.

Red dwarfs are dim. The next closest star, Lalande 21185, is the brightest red dwarf visible from the northern hemisphere. It still is only about as bright as the planet Neptune which means you would need at least a pair of binoculars to see it. Lalande 21185 is about 525,000 AU away. It’s fairly large for a red dwarf at about a third the size of the sun. It has two confirmed planets orbiting it and possibly another one.

It might seem that all the closest stars are dim. But the next closest star is the brightest star in our sky. This is the star Sirius. Sirius is about twice the size of the sun. It is about 556,000 AU away. Stars are incredibly bright. It is amazing to think that something as far away as Sirius could be so easily visible. How far away is 556,000 AU? That’s over 50 trillion miles away. Imaging that miles are inches won’t really help visualize this distance. But we could imagine that AU were inches. So if we decided to make a scale model of our stellar neighbors we could put the sun an inch away from the Earth. Something as close as the moon would be only half the thickness of a piece of paper away. I don’t know how you’d correctly measure that distance. But the marker for Sirius would be about nine miles away. So wherever we are building this model, it certainly won’t be crowded!