“As a boy I believed I could make myself invisible. I’m not sure that I ever could, but I certainly had the ability to pass unnoticed.” -Terence Stamp
When we look up at the night sky from a dark location here on Earth, somewhere around 6,000 stars greet you on a clear night.
This is just a tiny fraction of the hundreds of billions of stars that actually make up our galaxy, which makes sense, considering how large our galaxy is and how vast the distances between the stars is. You’d probably think that the stars we can see are pretty representative of the closest stars to us, but the story is actually much richer than that.
If you took a star like the Sun and moved it ten times as far away, it would appear just one-hundredth as bright. But if you took a star that was the mass of the Sun and compared it with a star that was ten times as massive as the Sun, the more massive one would be about five thousand times brighter! The most massive stars — more than 100 times as massive as our own — can outshine the Sun by literally millions of times.
In other words, the stars you see are a combination of stars that are relatively close to us, but moreso stars that are very bright intrinsically. In fact, of the ten nearest star systems to us, only two of them are visible to the naked eye!
Take the closest star to us, for example: Proxima Centauri. You’ve probably heard of the Alpha Centauri system, a binary pair of stars just 4.3 light years away. But even closer lies Proxima Centauri, a red dwarf star that’s just 12% the mass of the Sun, and only 0.0056% as luminous in visible light. The photo below shows Alpha and Beta Centauri, the 3rd and 9th brightest star systems in the sky, along with Proxima Centauri, circled and pointed out.
That’s the closest star to us, and it wasn’t even discovered until 1915, less than 100 years ago. And, as a hydrogen-fusing, main sequence star, it’s not even close to the dimmest object out there.
This is the “standard” Hertzsprung-Russell diagram, showing a huge variety of stars, ranging from the low-mass, cool, M-class red dwarfs (you can find Proxima Centauri among them) all the way up to the ultra-massive, bright blue O-class stars.
But this diagram cuts off stars that are even lower mass: too low in mass to fuse Hydrogen into Helium. Instead, they generate their light by fusing the trace amounts of deuterium they were born with into somewhat heavier elements, with some of them literally trillions of times less luminous than the Sun.
Known as brown dwarfs (even though they’d be faintly magenta to the naked eye), these things can be so cool that they give off practically no visible light, and need to be hunted in the infrared. Even today, only a few thousand (confirmed) are known, with the coolest one, WISE 1828+2650, having a temperature so low that — at atmospheric pressure — it couldn’t even boil water!
WISE — the Wide-field Infrared Survey Explorer — is presently the best tool we have for finding these objects. Shown below is Ned Wright’s graph of a standard Brown Dwarf spectrum, as well as the sensitivities of various space-based missions. As you can clearly see, until the James Webb Space Telescope comes along, WISE is the best instrument we have for finding these elusive objects.
And WISE has just really outdone itself, finding a pair of brown dwarfs just 6.5 light years away, making this the third-closest star system (if you count brown dwarfs as stars) to our Sun!
That’s right — once more, for emphasis — we’ve only just now, in 2013, found the third-closest star system to us.
The pair — known as WISE 1049-5319 — was first observed by WISE back in 2010, but was very close to the plane of our galaxy. Because of how dense stars are in the galactic plane (where we are, too, incidentally), it’s very difficult to detect faint sources. So this might lead you to ask:
If we can have a pair of brown dwarfs just 6.5 light years away, how many of them could possibly be out there in our galaxy?
In other words, we’re sure that there are unseen lights right here in our own backyard. But how many of them could there be?
The best constraint we have comes from gravitational microlensing. That is, we don’t directly observe-and-count the number of brown dwarfs to measure their density. I mean, we do, but we’re sure that there are many that we’re missing. The observe-and-count method gives us a lower limit on how many there could possibly be, but not an upper limit.
To get an upper limit, we observe a distant patch of sky, and each time a brown dwarf (or dimmer object) passes between us and the light source, it causes a characteristic brightening-and-unbrightening of the background light source we’re observing.
These objects are generically known as MACHOs, or MAssive Compact Halos Objects. And they exist! But they exist in very small numbers, at least as a percentage-of-total-mass of our galaxy.
This was once a legitimate candidate for dark matter, but thanks to groups of MACHO-hunters, we know for certain that there aren’t enough of them to account for the missing mass of the Universe.
What this work has done is effective rule out that dark matter could be explained by MACHOs with a mass between 0.00000001 Solar Masses (about the mass of the Moon) to 100 Solar Masses (which rules out black holes of these masses, too).
Now this doesn’t mean that brown dwarfs couldn’t be a substantial fraction of the baryons in our galaxy; there could be — in theory — as much mass locked up in these brown dwarfs as there are in all other known stars, combined, or there could be just a few per thousand cubic light-years.
The future James Webb Space Telescope should allow us to measure just how many brown dwarfs are here in our local neighborhood, and then we’ll better know exactly what our local Universe looks like!
But isn’t that amazing? More than 400 years after the invention of the telescope, we still don’t know how many (and what types) of stars there are just in our own backyard in space. There are unseen lights in our own backyard, and cooler, lower mass brown dwarfs than these could, in principle, be even closer than our nearest star! (Although not likely to be as close as the once-hypothesized Nemesis; WISE has taken care of that!)
We’re still coming to terms with the Universe around us, on both the largest scales and also the smallest. Keep your eyes on the skies, and remember, there’s a whole Universe out there, and even with perfect skies, most of it is invisible to us!