The Boomerang Nebula, in our galaxy, is even colder than completely empty space. Here’s how that’s possible.
Imagine the coldest place you possibly can. Inside it, the particles that make up matter move as slowly as you can possibly imagine, approaching the quantum limit of what it means to be truly at rest. There will be no major interior heat sources nearby for those particles inside to absorb; there will be no significant exterior sources of energy heating them up from the outside.
Physically, that means you need to be as far away as possible as you can from all sources of moving particles and radiation. You’d want to be at as great a distance as you could from stars, galaxies, and contracting gas clouds. You’d want to screen out any external sources of photons. If you headed to the deepest recesses of intergalactic space, shielded from starlight, the only thing that would heat you up would be the Big Bang’s leftover glow: the cosmic microwave background at 2.725 K. And yet, our own galaxy has a place — the Boomerang Nebula — that’s even colder than that.
Anywhere you go in the Universe, there are heat sources to contend with. The farther away you are from all of them, the colder it gets. At a distance of 93 million miles from the Sun, Earth is kept at a modest ~300 K, a temperature that would be nearly 50º cooler if it weren’t for our atmosphere. Move farther out, and the Sun becomes progressively less and less able to heat things up. Pluto, for instance, is just 44 K: cold enough that liquid nitrogen freezes. And we can go to an even more isolated place, like interstellar space, where the nearest stars are light years away.
The cold molecular clouds that roam, isolated, throughout the galaxy are even colder, just 10 K to 20 K above absolute zero. As stars, supernovae, cosmic rays, stellar winds and more all provide energy to the galaxy as a whole, it’s hard to get much cooler than that within the Milky Way. Only in intergalactic space, millions of light years from the nearest stars, will the cosmic microwave background be the only heat source that matters.
At less than 3º C above absolute zero, these barely-detectable photons are the only heat source around. Since every location in the Universe is constantly bombarded by these infrared, microwave and radio photons, you might think that 2.725 K is the coldest you can ever get in nature. To experience something colder, you’d have to wait for the Universe to expand more, stretch the wavelengths of these photons, and cool down to an even lower temperature.
This will happen, of course, in time. By time the Universe is twice as old as it is today — in another 13.8 billion years — the temperature will be just barely a single degree above absolute zero. But there’s a place you can look, right now, that’s colder than even the deepest depths of intergalactic space.
You don’t even need to go anywhere special! This is the Boomerang Nebula, located just 5,000 light years away in our own galaxy. In 1980, when it was first observed from Australia, it looked like a two-lobed, asymmetrical nebula, and hence it was given the name “Boomerang” as a result. Better observations have shown us this nebula for what it really is: a preplanetary nebula, which is an intermediate stage in a dying, Sun-like star’s life.
All Sun-like stars will evolve into red giants and end their lives in a planetary nebula/white dwarf combination, where the outer layers are blown off and the central core contracts down to a hot, degenerate state. But in between the red giant and the planetary nebula phases, there’s the preplanetary nebula phase.
Before the internal temperature of the star heats up, but after the expulsion of the outer layers begins, we get a preplanetary nebula. Sometimes in a sphere, but more often in two, bipolar jets, the ejecta make their way out of the star’s solar system and into the interstellar medium. This phase is short-lived: only a few thousand years. There are only a dozen or so stars that are found to be in this phase. But the Boomerang Nebula is special among them. Its gas gets expelled about ten times faster than normal: moving at about 164 km/s. It loses its mass at a higher rate than normal: about two Neptune’s worth of material every year. And as a result of all of this, it’s the coldest natural place in the known Universe, with some portions of the nebula coming in at just 0.5 K: half a degree above absolute zero.
Every other planetary and preplanetary nebula is much, much hotter than this, but the physics underlying why is some of the simplest of all to understand. Inhale a deep breath, hold it for three seconds, and then let it out. You can do this two different ways, holding your hand about 6″ (15 cm) away from your mouth both times.
- Exhale with your mouth wide open, and you’ll feel the warm air gently blow onto your hand.
- Exhale with your lips puckered, making a tiny opening, and that same air feels cold.
In both cases, the air in your body has been warmed, and remains at that high temperature until just before it passes your lips. With your mouth wide open, it simply exits slowly, warming your hand slightly. But with only a tiny opening, the air expands rapidly — what we call adiabatically in physics — and cools as it does so.
The outer layers of the star that’s birthing the Boomerang Nebula has all of these same conditions:
- a large amount of hot matter,
- being ejected incredibly rapidly,
- from a tiny point (well, two points),
- that has all the room it could ask for to expand and cool.
The amazing thing about the Boomerang Nebula is that it was predicted before it was found! Astronomer Raghvendra Sahai calculated that preplanetary nebulae with just the right conditions — the ones outlined above — could actually achieve a cooler temperature than anything else that naturally occurred in the Universe. Sahai was then part of the team in 1995 that made the critical long-wavelength observations that determined the temperature of the Boomerang Nebula, now known as the coldest natural place in the Universe.
As far as why the Boomerang Nebula is ejecting all this matter so quickly and in such a collimated fashion, that’s a controversial and highly active area of research. So far, the Boomerang Nebula is the only preplanetary nebula whose temperature has dropped below that of the Big Bang’s afterglow, but there’s no way it’s the only one that ever has. There’s likely an even colder place out there. We just have to keep looking. And who knows? Perhaps, someday, the star at the center of our Solar System — the Sun — will take the record for its own.
Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.