About five thousand light years from Earth, in the direction of the southern-sky constellation Centaurus, lies the coldest place in the universe — or at least the coldest natural object astronomers have yet detected.
It’s called the Boomerang Nebula on account of its appearance in telescope images (and because the astronomers who found it were Australian). Space.com reports that the object is believed to be the shroud of a dying red giant star that will in due course be “reborn” as a tiny but very hot white dwarf star. But for a brief cosmic moment, we’re seeing it in a transitional phase when the shroud, formed from the dying star’s outer layers, has become extremely cold.
The Low End of the Thermometer
How cold is the Boomerang Nebula? We can’t (yet!) send a thermometer to take its temperature, but we can measure its radiation spectrum, a reliable guide to temperature. And according to Science Alert, it is cold: a frosty -458.5 degrees F, or -272.5 degrees C, less than half a degree (Centigrade or Kelvin) above absolute zero, the theoretical temperature of matter with no heat content whatsoever.
In fairness, asking what is the coldest place in the universe is a bit of a trick question because the coldest places we know of are in laboratories right here on Earth. As Astronomy magazine wryly noted in 2015, one could even give the address (a lab at Harvard), but some other lab may might be the current record holder.
Earthly labs use deliberate refrigeration technology to achieve such low temperatures, but how did the Boomerang Nebula get so cold? The minimum temperature of the universe as a whole is set by the so-called cosmic microwave background (CMB), left over from the Big Bang more than 13 billion years ago.
The CMB temperature comes out to -270.4 C, more than two degrees Centigrade warmer than the Boomerang Nebula. In fact, per Science Alert, astronomers only realized how cold the Boomerang was when they discovered that it was absorbing CMB radiation that passed through it. That could only happen if the nebula was colder than the CMB temperature.
In effect, the Boomerang Nebula is a naturally occurring laboratory-grade refrigerator.
Shroud of a Dying Star
Apart from the puzzle of it being the coldest place in the universe, the Boomerang Nebula is a familiar type of object called a protoplanetary nebula. In spite of the name, it has nothing to do with protoplanets, or planets of any sort. Rather, it is an initial stage in the formation of a planetary nebula, so named because it looks like a cluster of distant planets when viewed through a small telescope.
These objects are formed when a sunlike star reaches the final stages of its life cycle. As its nuclear fuel runs low, internal processes cause the star to expand into a red giant. Then, as the internal nuclear furnace runs out, the star’s core collapses while its already-distended outer layers are puffed away into space.
The core finally becomes a white dwarf, a “dead” (but still very hot) stellar cinder. Ultraviolet light from the white dwarf causes the surrounding shroud to fluoresce and become visible as a planetary nebula. The nebula expands and ultimately fades away, leaving just the slowly cooling white dwarf.
The Boomerang is still early in the process of forming a planetary nebula, hence the term “protoplanetary,” but how did it become the coldest place in the universe?
A Hidden Binary
The likely answer, says Space.com, was first proposed by astronomer Raghvendra Sahai, who wrote in a 1990 scientific paper that the expansion of a red giant’s outer layers into a protoplanetary nebula could act as a natural refrigerator.
Just as compressing a gas heats it up, reducing its pressure by allowing it to expand cools it down. Astronomy magazine notes you can see this process at home: Spraying from a can of whipped cream or tire-inflating gas causes the container to feel cooler.
Ordinarily the gentle expansion of a protoplanetary nebula would not produce the very low temperature of the Boomerang, but measurements show that the Boomerang is expanding much faster than usual.
The likely culprit is now believed to be a companion star to the dying red giant that formed the Boomerang. As it swelled into a red giant, says Science Alert, it actually absorbed the companion stars, but the companion’s orbital motion tore the red giant’s outer layers away, hurling them off into space at nearly 100 miles per second, ten times the usual expansion rate.
According to Universe Today, the result is that we are watching the Boomerang in the brief cosmic moment of maximum cooling. According to astronomer Lars-Åke Nyman, co-author of the paper describing the process, “We see this remarkable object at a very special, very short-lived period of its life. It’s possible these super cosmic freezers are quite common in the universe, but they can only maintain such extreme temperatures for a relatively short time.”
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