Space is a cold place, but the coldest place in the universe cranks things down even more than normal, thanks to the effects of its rapidly dying star.
The Boomerang Nebula, located about 5,000 light-years away, is the coldest natural environment we've ever identified, and it is a blistering-cold stretch of the cosmos, despite having a star within it that should have been keeping things warm. Instead, it appears to be making things worse.
How can something so active, even in its death throes, produce temperatures that are barely a degree Celsius above the point where all molecular motion stops? Let's find out.
What is the coldest point in space?
The Boomerang Nebula is a reflecting planetary nebula full of dust and ionized gases about 5,000 light-years from Earth in the Centaurus constellation. Unlike other planetary nebulae with a white dwarf in the center, the Boomerang Nebula is a young planetary nebula, so the red giant inside it is still shedding its outer layers of material.
Though it was once very similar to our sun, the red giant at the heart of the Boomerang Nebula has exhausted its fuel, and so nuclear fusion cannot continue. The mass of the original star was low enough that this exhaustion does not produce a supernova with a resulting neutron star or black hole, but instead goes through something of a "deathbed" phase.
In this final state, a red giant's end is both near and certain, and it is shedding the heavy elements in its outer layers so that it no longer has the energy to fuse into even heavier elements. This process produces a typically vibrant nebula around the resulting white dwarf — the super-condensed core of the star that radiates residual heat and energy, but does not produce any new energy of its own.
This process normally takes some time to complete (cosmically speaking) but the red giant in the Boomerang Nebula just plays the game differently, shedding its mass at a rate that is around 100 times faster than similar stars in their end-stage of life according to Space.com.
And while this is fast, this is nothing compared to the speed it is shedding mass when compared to our Sun — about 100 billion times faster — an absolutely gobsmacking pace. It is thought that the central star in the heart of the nebula has shed about 1.5 times the mass of the Sun in just about 1,500 years.
This is likely the main cause of the extremely cold conditions there, since all of this rapid venting of material into space is carrying away the star's heat energy at a velocity of just over 100 miles per second, or about 164 kilometers a second.
Why is the Boomerang Nebula so cold?
In order to understand how the Boomerang Nebula could be the coldest place in the universe with a natural cause (i.e., it isn't the product of transient artificial conditions in a lab), it's necessary to explain what temperature actually is.
While we describe temperature in terms of hot and cold, what we are actually feeling during the sweltering days of summer is the energy released by molecular collisions on an atomic scale. The more collisions are happening at any given time, the hotter the resulting temperature. This is why applying energy to an object (via electricity, for instance) heats the object up.
What's actually happening is that this energy is exciting the molecules in the object and causing them to collide with each other faster and more frequently. The flipside of this is that the colder an object is, the slower and less frequently these collisions occur.
This is the best theory for why the Boomerang Nebula is so frigid: the incredible speed of its expansion into space is reducing the number of molecular collisions in the expanding gas.
This produces a subsequent "drop" in temperature, and as the expansion of gas passes through the relative vacuum of space, it sweeps everything before it like a plow, incorporating that material as a part of the rapidly expanding cloud, spreading the material out and reducing the number of collisions even further, dragging the temperature of the surrounding expanse of space down as well.
That's the theory anyway. The question is what caused this particular star to so violently shed its material, especially considering that similar stars developing into a planetary nebula do so at a tiny fraction of the rate of the Boomerang Nebula.
Recently, researchers came up with an interesting hypothesis that would explain the incredibly rapid shedding of mass from the Boomerang Nebula's central star: the consumption of a companion star.
According to this research—published in The Astrophysical Journal in 2017—, as the red giant swelled, a smaller companion star was encompassed and its orbital motion tore away the red giant's outer layers, resulting in the violent and rapid ejection of so much material from the red giant.
"These new data show us that most of the stellar envelope from the massive red giant star has been blasted out into space at speeds far beyond the capabilities of a single, red giant star, " said Raghvendra Sahai, the lead author of the study and an astronomer at NASA's Jet Propulsion Laboratory in Pasadena, California. "The only way to eject so much mass and at such extreme speeds is from the gravitational energy of two interacting stars, which would explain the puzzling properties of the ultra-cold outflow [of the Boomerang Nebula]."
What's more, the central star of the nebula seems to eject its mass from just two relatively small points. Since gas cools as it expands far more rapidly when released through a smaller opening than a larger one, you really have a situation where you are stacking heat-sapping physical circumstances one on top of the other.
How cold is the Boomerang Nebula, exactly?
To put the Boomerang Nebula in perspective, absolute zero is the temperature at which all molecular motion stops. Molecules cannot slow any further than total motionlessness, and now with our proper understanding of what temperature actually is, it's obvious that this would be the absolute coldest any physical thing can get. There is simply nothing colder. This temperature is, appropriately enough, called absolute zero (or 0° Kelvin, –459.67° Fahrenheit, or –273.15° Celsius).
The Boomerang Nebula is about –458° Fahrenheit (-272° Celsius, about 1.15° Kelvin), or a little more than one degree Celsius above absolute zero.
This puts the temperature of the Boomerang Nebula below that of the cosmic background radiation (-454.7° Fahrenheit, -270.4° Celsius), which is the residual heat left over from the Big Bang. That means the Boomerang Nebula actually absorbs the leftover heat produced by the sudden emergence of the universe itself.
If that isn't enough for you, the Boomerang Nebula is the only object in space known to be colder than the cosmic microwave background. When it comes to frigid temperatures, the Boomerang Nebula stands alone.
How was it discovered and how did we measure its temperature?
The Boomerang Nebula was first identified in 1980 by astronomers Keith Taylor and Mike Scarrott using the Siding Spring Observatory in Australia, though at the time the most exciting thing about it was that—from the perspective of the ground-based observation anyway—the nebula looked like a boomerang.
Subsequent observations from Hubble have revealed its "shape" to be more like an hourglass.
More than two decades later, Sahai hypothesized that the rapid expansion of gases from the central star of the nebula would naturally reduce the nebula's temperature, a hypothesis confirmed by observations in 2003.
As for measuring the temperature, this was done by comparing carbon monoxide readings from the nebula and comparing it to similar measurements from the cosmic background radiation, the temperate of which has long been established.
"One can say the Boomerang acts as a refrigerator," astronomer Lars-Ake Nyman, who carried out the temperature measurements in 2003, told the Sydney Morning Herald at the time. "Normally the cosmic microwave background photons would excite the gas to at least its own temperature. [But] the gas self-shields itself, and the photons from the microwave background do not penetrate deep into the outflow. The low temperature of the gas in the outflow, therefore, stays low."
However, this won't last forever since there is a limited amount of mass remaining to eject from the nebula's central star. Eventually, the force driving the frigid gas cloud into the cosmos will dissipate, and the laws of thermodynamics will establish an equilibrium, ultimately raising the temperature of gases from the Boomerang Nebula up to the level of the cosmic microwave background.
Until then, bundle up, as it will remain the coldest place in the known universe and isn't likely to surrender that crown any time soon.