The Coldest Place in the Universe Is a Nebula That Cools Itself

The coldest place in the universe is not deep space. Deep space, on average, sits at about 2.7 Kelvin, which is the residual temperature left by the cooling of the cosmic microwave background after the Big Bang.

Almost everything in the universe is at least that warm. One thing is colder. The Boomerang Nebula, a young pre-planetary nebula about 5,000 light-years from Earth in the constellation Centaurus, has a temperature of about 1 Kelvin.

That is one degree above absolute zero, the theoretical point at which all molecular motion stops. It is the coldest naturally occurring object ever found in the universe, and the only known place colder than the background radiation of space itself.

Where the Coldest Place in the Universe Is

The Boomerang Nebula was first observed in 1980 by astronomers Keith Taylor and Mike Scarrott using the Anglo-Australian Telescope at the Siding Spring Observatory in New South Wales. They named it Boomerang because in their early observations, taken with relatively low-resolution equipment, the nebula appeared to have a curved, lopsided shape.

Later observations with the Hubble Space Telescope revealed a bow-tie shape. Even later observations with the Atacama Large Millimeter/submillimeter Array (ALMA) telescope in Chile, published by the National Radio Astronomy Observatory in 2013, showed that the actual structure is more like an hourglass with extended cold material the earlier observations couldn't see.

The temperature was measured in 1995 by astronomers Raghvendra Sahai and Lars-Åke Nyman using the 15-meter Swedish ESO Submillimetre Telescope. They found that the nebula was absorbing the cosmic microwave background radiation rather than emitting it, which can only happen if the absorbing material is colder than the background.

That single measurement made the Boomerang Nebula famous. It is still the only known natural object colder than the universe's average temperature.

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How Cold Is the Coldest Place in the Universe

The exact temperature is approximately 1 Kelvin, or about minus 272 degrees Celsius. In Fahrenheit, that is about minus 458 degrees. For comparison:

• The coldest temperature ever recorded on Earth is around minus 89 Celsius (minus 128 Fahrenheit), at the Soviet Vostok Station in Antarctica in 1983.
• The surface of Pluto is around minus 230 Celsius.
• The cosmic microwave background, the average temperature of deep space, is about minus 270 Celsius.
• Absolute zero is minus 273.15 Celsius. The Boomerang Nebula is about 1 degree above it.

In other words, the Boomerang Nebula is so cold that the rest of the universe, on average, is comparatively warm. That's the part that breaks intuition. Space is not cold enough. The nebula is.

The cold is not uniform across the entire nebula. Some regions are warmer, particularly closer to the central star. The 1 Kelvin figure refers to the coldest outer regions of the outflowing gas, which expand and cool rapidly enough to drop below the cosmic background.

Why the Boomerang Nebula Is So Cold

The cold is the byproduct of an extreme version of the same principle that makes refrigerators work. The Boomerang Nebula contains a dying central star roughly similar in original mass to the Sun. As the star approaches the end of its life, it is shedding its outer layers in a high-speed outflow of gas.

The outflow is moving outward at about 500,000 kilometers per hour, which according to research published in the Astrophysical Journal makes it one of the fastest-known stellar outflows of its kind.

When gas expands rapidly, it cools. This is called adiabatic cooling, and it is the principle behind why an aerosol can feels cold when you spray it, or why your breath frosts in cold weather, or why expanding refrigerant cools the inside of a refrigerator.

Same physics, different scale. The Boomerang Nebula's gas is expanding from a very small volume to a vast one in cosmic terms, and it is expanding fast. The combination produces an extreme cooling effect, dropping the gas temperature below the surrounding background radiation. Nothing else in the universe has been found doing this on the same scale.

This is also why the cold is temporary. As the nebula continues to age and expand, the gas density drops, the rate of expansion slows, and the cooling effect weakens. The NRAO has reported that some regions of the Boomerang Nebula appear to be warming back toward the cosmic background temperature. It may not be the coldest place in the universe forever. For now, though, it is the only known place that is.

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How Astronomers Measure the Temperature of a Distant Cloud

Measuring the temperature of an object 5,000 light-years away requires indirect methods. Astronomers can't put a thermometer in the Boomerang Nebula. They use the way the nebula's gas interacts with the cosmic microwave background radiation instead.

Cold gas absorbs photons from the CMB at specific wavelengths. By measuring how much of the background light is being absorbed and at which wavelengths, astronomers can calculate the temperature of the gas doing the absorbing.

This technique works because the cosmic microwave background is incredibly uniform across the entire sky, at about 2.7 Kelvin. Any region absorbing rather than emitting CMB photons must be colder than the background. The amount of absorption gives the precise temperature.

The work has been a long-running project involving multiple telescopes, including ALMA, Hubble, and the James Webb Space Telescope, which has been turning up new structural detail on distant galaxies and nebulae since its launch.

How the Boomerang Nebula Fits Into the Larger Story of the Cosmos

The Boomerang Nebula is a snapshot of a single phase in stellar evolution that most sun-like stars will eventually go through. When the Sun reaches the end of its life in roughly five billion years, it will go through a similar process. It will swell into a red giant, then shed its outer layers in slower stages, eventually leaving behind a hot white dwarf at the center of a planetary nebula.

Whether our future planetary nebula will produce a record-breaking cold zone like the Boomerang's is unclear, but the basic mechanism, an expanding outflow cooling by adiabatic expansion, is universal.

The discovery of one place colder than the cosmic background has also affected how astronomers model the lifecycles of stars and the distribution of matter across galaxies. It suggests that mass-loss rates from dying stars can be much higher than older models assumed. The Boomerang Nebula is losing material at roughly 1,000 times the rate of typical stars at the same stage.

This is one of the reasons astronomers keep going back to the Boomerang. As cosmic objects go, it is fairly close to us, fairly easy to observe with the right equipment, and fundamentally weird in ways that intersect with the biggest open questions in stellar astrophysics. New observations of much older galaxies in the early universe continue to push back the timeline of cosmic history, but the Boomerang remains the local outlier.

It is the only place in the known universe where a dying star has produced a region colder than the universe's own ambient temperature. Until something colder turns up, it holds the record.