When we imagine the coldest corners of space, our minds often drift to the dark, empty voids between distant galaxies. We think of places untouched by starlight, freezing in the vast nothingness. Yet, the coldest object ever discovered in the universe isn’t hiding in some remote, lightless abyss. It is actually a star located right inside our own galaxy, relatively close to home.
Known as the Boomerang Nebula, this cosmic freezer sits about 5,000 light-years from our Solar System in the constellation Centaurus. While outer space generally hovers around a chilly -454 degrees Fahrenheit (-270°C), this particular nebula plunges even lower.
It defies the natural background temperature of the cosmos, making it a unique laboratory for understanding the death of stars and the behavior of matter at extreme extremes.
So, how did a glowing star transform into an object colder than the empty vacuum of space itself? Let’s explore the science behind this frozen phenomenon.
Meet the Boomerang Nebula
The Boomerang Nebula is a planetary nebula—a cloud of ionized gas expelled by a dying star. Despite the name “planetary,” it has nothing to do with planets. Instead, this is the final, spectacular act of a star that was once very similar to our own Sun.
As the central star reached the end of its life, it began to shed its outer layers. What makes the Boomerang Nebula unique is the speed and violence of this process.
The star is ejecting gas at a velocity of roughly 310,000 miles per hour (500 km/h). This rapid expansion is the key to its record-breaking temperatures.
A Temperature Below Absolute Zero? Almost.
The temperature within the Boomerang Nebula drops to a staggering -272 degrees Celsius. To put that in perspective, absolute zero—the theoretical point where all molecular motion stops—is -273.15 degrees Celsius.
This means the Boomerang Nebula is only about one degree warmer than the lowest temperature theoretically possible.
It is significantly colder than the cosmic microwave background radiation (the afterglow of the Big Bang), which permeates the rest of the universe at roughly -270 degrees Celsius. In essence, this nebula is a natural refrigerator that is colder than the empty space surrounding it.
How Can a Star Be So Cold?
It seems counterintuitive that a star, a massive ball of burning plasma, could generate such freezing conditions. The answer lies in the physics of expansion.
Think about how an aerosol can feels cold when you spray it continuously. As the gas inside expands rapidly upon leaving the nozzle, it cools down. A similar thermodynamic effect is happening on a cosmic scale within the Boomerang Nebula.
The star is losing mass about 100 times faster than other similar dying stars—shedding about one-thousandth of a solar mass every year for the last 1,500 years.
This ultra-fast ejection of mass causes the gas to expand at breakneck speeds. This rapid expansion absorbs heat energy so efficiently that the gas cools down well below the temperature of the surrounding environment.
The Illusion of the “Boomerang”
The nebula was first studied in detail in 1995 by astronomers Lars-Åke Nyman and Raghvendra Sahai using the 15-meter Swedish-ESO Submillimetre Telescope in Chile.
From Earth-based optical telescopes, the gas cloud appeared slightly curved, resembling the shape of a boomerang, which earned it its nickname.
However, more recent observations using the powerful ALMA (Atacama Large Millimeter/submillimeter Array) radio telescope have revealed that this shape is somewhat of an illusion.
The nebula is actually roughly spherical. The “boomerang” shape seen in visible light is caused by a band of dust blocking parts of the star’s light, creating an hourglass-like shadow that obscures the true spherical structure of the gas.
What This Means for Our Sun
Studying the Boomerang Nebula offers astronomers a glimpse into a potential future for our own solar system. The central star of this nebula was once a yellow dwarf, just like the Sun.
While our Sun is not expected to lose mass at the violent rate seen in the Boomerang Nebula, it will eventually exhaust its hydrogen fuel. It will swell into a Red Giant, engulfing the inner planets, before shedding its outer layers to form a planetary nebula and leaving behind a white dwarf.
The Boomerang Nebula serves as a dramatic, extreme example of this stellar life cycle—a ghost of a star freezing itself in its final moments.
Looking Forward: The Future of Cold Space!
The Boomerang Nebula won’t stay the coldest place forever. This phase of rapid expansion and extreme cooling is a fleeting moment in cosmic time.
Eventually, the central star will heat up enough to ionize the surrounding gas, warming the nebula and likely causing it to glow with vibrant colors, much like other famous planetary nebulae such as the Ring Nebula.
For now, however, it remains the reigning champion of cosmic cold—a fascinating reminder that the universe is full of extremes that challenge our understanding of physics and nature!