The nearest star to Earth (other than the Sun, of course) is not one of the bright, famous stars we can spot in the night sky. That star is Proxima Centauri, in the constellation of Centaurus; the “Proxima” name derives from the same root as “proximity”. It’s a part of the star system that includes Alpha Centauri. Despite the fact that Alpha Centauri is one of the brightest stars in the southern sky (well, it’s a binary star, though we can’t tell that without binoculars), Proxima Centauri is too faint to see without a telescope.[1] That’s why it escaped discovery until 1915.

Proxima Centauri is actually a pretty common variety of star known known as an M dwarf, a subgroup of the red dwarfs. As their name suggests, these stars are much smaller in mass and size than the Sun, so even though they are common, they don’t stand out: no M dwarf is visible from Earth without a telescope. Their commonality is important, though: surveys have found many exoplanets orbiting M dwarfs, including several in the habitable zone, where liquid surface water theoretically could exist.

To date, nobody has spotted a planet orbiting Proxima Centauri. (An earlier possible detection was ruled out by follow-up observations.) However, its relative proximity to the Solar System gives us a good chance to study M dwarfs up close: at only 4.24 light-years distance, we can observe its fluctuations in more detail than farther stars. That’s important because, like many M dwarfs, Proxima Centauri is a flare star: one that rapidly increases in brightness, probably due to magnetic activity in the star’s atmosphere. This is also the cause of solar flares on the Sun, but the greater size of our host star makes its brightness less variable; on a dwarf star, a single flare can double the brightness for a time.

With many exoplanets orbiting M dwarfs, astronomers would like to know if flares could disrupt their atmospheres. The habitable zone for many dwarf stars is really close in: so close as to tidally lock the exoplanet, so that it presents one face to the star the same way the Moon always presents the same side to Earth. Strong stellar flares could conceivably erode the atmosphere away, not a good situation for a water- or life-bearing world.

I don’t know the specific reason why researchers pointed the Hubble Space Telescope at Proxima Centauri recently to obtain the image above. Even at the Hubble’s high-resolution, the star appears as a single bright point of light, with the common “diffraction spikes” (the four lines you often see in star images) produced by support rods inside the telescope.[2]
However, Hubble isn’t just about making pictures: it has the ability to measure both the brightness and the spectrum of what it’s looking at, in visible light, ultraviolet light, or infrared. So, there’s literally more to this picture than meets the eye.

A Proxima shun

(OK, maybe that joke doesn’t work unless you say it out loud quickly.)

As I mentioned, Proxima Centauri is an M dwarf. The “M” refers to the color classification of the star, which I described in some detail in an earlier post. M type stars, a category that also includes some really huge stars like Betelgeuse nearing the end of their life, have relatively low surface temperatures, giving them a reddish color. (A lot of their light is actually infrared.) Proxima Centauri has a surface temperature of about 3,000 Kelvins, compared with the Sun’s temperature of 5,800 Kelvins.

Unlike Betelgeuse, M dwarfs are main sequence stars, meaning they are in the stable middle part of their evolution. The Sun is a main sequence star, about halfway through its 10 billion year life cycle; Proxima Centauri is much less massive, and is expected to live about 4 trillion years because of its much slower rate of converting hydrogen to helium.

The brightness of a star — its luminosity — depends both on its temperature and its size. Proxima Centauri has about 14% of the radius of the Sun, so its surface area is only about 2% of the Sun’s. That’s a lot less surface to produce light, so combined with the lower temperature, Proxima Centauri emits only about 0.17% of the light our Sun emits. No wonder we can’t see it from Earth!

Betelgeuse, for comparison, is about 640 light-years away, and has a slightly higher surface temperature: fluctuating between about 3,100 and 3,600 Kelvins. (Partly that’s because the star is at the end of its life, pulsating and shedding huge amounts of material.) However, the real reason it’s so bright is its size: Betelgeuse has a radius as much as 1,200 times the Sun’s, meaning it emits around 120,000 times as much light. That’s roughly 71 million times as much light as poor little Proxima Centauri.
Close as Proxima Centauri is to the Solar System, it’s still out of reach with current technology. Voyager 1, which is the most distant spacecraft from Earth, is a bit more than 17.5 light-hours away as of this writing; Proxima Centauri is more than 4 light-years distant. The Hubble and its telescopic kin are the best we can do for studying the stars for the foreseeable future.

Notes

1. I’ve always wanted to see Alpha Centauri, ’cause I’m that kind of person. However, I’ve never been far enough south, or at least not during the right season of the year. (Evidently you can see it in the southernmost points of the United States during summer months.)
2. Most large telescopes, including the Hubble, consist of two mirrors: the large primary and the smaller secondary. The support structure for the secondary mirror blocks part of the light entering the telescope, and the resulting image contains the light passing around – diffracting from – that support structure.