Space is cold. Very cold. In fact, empty space, far from any star or other hot object, is about -270 degrees C.
While downright frigid—a temperature low enough to freeze hydrogen on Earth—that's still about 2.7 degrees above absolute zero, the lowest possible temperature. The source of those couple of degrees is primordial: the leftover glow of the big bang that gave birth to our universe.
The entire cosmos is bathed in this radiation, called the cosmic microwave background. As a result, it's hard to avoid this bit of heat, meaning that in most of the cosmos, -270 degrees is as cold as it gets.
But not everywhere.
5,000 light years away in Centaurus, a large constellation in the southern sky, is the Boomerang Nebula, a cloud of gas being expelled from a dying star.
This is how stars die
This cloud is one of the most bizarre and mysterious objects in the universe. Here, within the gas streaming outwards, astronomers have found that the temperature drops as low as half a degree above absolute zero.
It is, as far as anyone knows, the coldest place in the universe.
It may also prove to be quite important. Because this most frigid place, and objects like it, albeit a tad warmer—may help astronomers unravel a host of cosmic conundrums, from the violent yet spectacular deaths of stars and the formation of galaxies to cosmic explosions and the origin of life itself.
Death of stars, birth of life
In many respects the Boomerang Nebula is unremarkable. All stars have to die some day. When smaller stars end, those up to about eight times as massive as our own sun, they produce a similar display of gas and dust.
During this transformation, each dying low-mass star will cool and swell, becoming what's called a red giant. In a few billion years, when our own sun exhausts its nuclear fuel, it will similarly cool and grow, until it engulfs Mercury, Venus, and possibly even Earth.
The temperature of the star's outer layers drops low enough such that molecules start clumping together, condensing into dust particles. Starlight radiating from below smacks into these particles and ejects them outward. The particles drag the star's outer gas layers along, creating vast clouds like the ones seen in the Boomerang.
In every which way we look at this object, it's extreme
Ultraviolet radiation from the dying star heats the gas, making it glow. Eventually, the radiation strips away the electrons from the atoms that make up the clouds. Once this ionisation process is complete, what’s left is called a planetary nebula, which is a misnomer of a name, originating when astronomers a century ago mistook these bright objects for planets. Meanwhile, the dying star collapses into its final stage: a hot and dense object called a white dwarf. Our sun will collapse into a white dwarf the size of Earth.
"This is how stars die," says Sun Kwok, an astronomer at the University of Hong Kong. "They are born; they have a long life—billions of years of life. And they die very suddenly over a very short period of time.”
But that also means that objects such as the Boomerang Nebula are incredibly useful; as by studying them astronomers can solve the mysteries of stellar death. "We're interested in how they die, why they die," Kwok says. "The good thing is that before they die, they put up a huge spectacular show—like fireworks."
And the deaths of stars play a crucial role in the birth of life. Astronomers have long known that many elements such as carbon, oxygen, and even iron are fused inside the cores of stars. When the stars die, those elements are distributed across the galaxy. And when very massive stars die—those more than about eight times the mass of the sun—they explode instead of creating planetary nebulae, creating even heavier elements that become the building blocks for rocks, planets, and even life.
In the last decade, Kwok says, he and his colleagues are learning that even planetary nebulae may be contributing to life by producing complex organic compounds. Some of these compounds may have made their way to our solar system as the planets were forming. And, they may have been key ingredients for the origin of life on Earth.
A special nebula
The Boomerang Nebula, however, is special.
For a start, the planetary nebula phase of a star's lifecycle lasts only a few tens of thousands of years. The Boomerang is not yet a full-fledged planetary nebula, since its central star hasn't ionised its surroundings. So it's a pre-planetary nebula, a transition stage that lasts only about a thousand years—a mere blink in cosmic time, and one that we are lucky to witness.
Pre-planetary nebulae are important to astronomers such as Kwok because they provide a glimpse for how stars transform from a swollen red giant to a complex and dazzling planetary nebula. Although a dying star is round, a planetary nebula is not. It often has a bipolar shape, with lobes expanding out from two ends. As the Hubble Space Telescope has revealed in dramatic fashion, from our point of view on Earth, these nebulae sometimes appear to have intricate structures of interlocking rings and arcs.
I don't think there's any theoretical explanation yet as to how this object is what it is right now
The metamorphosis of a round star into a planetary nebula is akin to a caterpillar turning into a butterfly, says Kwok, who did pioneering work on pre-planetary nebulae in the 1990s. Looking at the Boomerang, he says, is like peering into a cocoon just before a butterfly emerges.
But none of that explains why the Boomerang is so cold?
The Boomerang Nebula got its name because it appeared to have a curved shape like a boomerang. In 1995, Raghvendra Sahai, an astronomer at NASA's Jet Propulsion Laboratory in Pasadena, California, and Lars-Åke Nyman, now at the Atacama Large Millimeter/submillimeter Array (ALMA) telescope in Chile, took a closer look with a telescope in Chile, observing in millimetre wavelengths that revealed clouds of gas molecules. They found that the Boomerang wasn't a boomerang, but a round cloud expanding at a prodigious rate.
"In every which way we look at this object, it's extreme," Sahai says.
He and Nyman discovered that the gas was gushing out at 164 km/s, almost 4,000 times faster than the average high-speed train and ten times faster than the typical speeds seen in similar objects. Such high speeds meant that for the last 1,500 years, the central star was losing mass at a rate of one-thousandths of a sun every year, ten times faster than what's been measured in similar stars that are ejecting gas.
This speed is why the Boomerang is so cold, Sahai explains.
Gas gets cold as it expands, which you can feel if you place your hand over a tyre nozzle as air is being let out. And if the gas expands as fast as it does in the Boomerang, it can get really cold. The nebula also contains a lot of gas, which makes it difficult for the ambient heat from the cosmic microwave background to seep in, helping the gas remain at a low temperature. With the exception of the artificial conditions created in certain laboratories on Earth, there's no known colder place in the universe.
The cold wasn't a complete surprise, however.
Sahai previously hypothesised that if certain conditions were just right, and if the central star were ejecting gas fast enough, the temperature could drop below the cosmic microwave background. Still, it was just a theoretical possibility. When he started analysing that Boomerang data nearly 20 years ago, however, he realised that his prediction was coming true. "My hair stood on end," he recalls. "That was one of the most exciting parts of my career."
Still, exactly how the central star ejects gas so fast remains a mystery, Sahai says.
According to conventional theory, it's the radiation from the star that's pushing out all that material. But, the star inside the Boomerang is nowhere bright enough to produce the radiation needed to cause gas to be ejected at 164 km/s. "I don't think there's any theoretical explanation yet as to how this object is what it is right now," Sahai says.
The Boomerang has also perplexed scientists in other ways. In the nearly two decades since the Boomerang was discovered to be the coldest region in the universe, Sahai and his colleagues have continued to explore the extreme object, slowly peeling back layers of complexity and mystery.
And one of the first puzzles to solve was its shape.
Sahai and Nyman's observations in submillimetre wavelengths revealed that the Boomerang consisted of a round, expanding molecular cloud. But what did it look like in visible light? In 1998, astronomers pointed the Hubble Space Telescope at the Boomerang to find out. The nebula didn't look round nor did it look like a boomerang. Instead, it boasted an hourglass figure.
Astronomers didn't know why the Boomerang looked so different in visible light compared to submillimetre, and the problem wasn't solved until last year, when Sahai and his colleagues described their latest observations using the new ALMA telescope in Chile, which allowed them make the most detailed observations of the nebula yet.
The researchers discovered that the Boomerang has a complex structure consisting of three parts. First, there is the large, round, expanding molecular gas cloud—the same cloud that was observed earlier. But zooming in, the astronomers found a denser, doughnut-shaped cloud of dust surrounding the central star.
This dusty doughnut, the astronomers realised, acts like a mask, blocking the starlight emanating from the star's equator. Because light can only escape from the two poles, it illuminates the surrounding gas like two flashlights pointing in opposite directions. So the two lobes seen in the Hubble images are the beams of those flashlights shining through the gas—just as how you can see the beams of a car's headlights on a foggy night.
These are not just beautiful objects. They hold many secrets
The new ALMA observations showed why the Boomerang could appear both round and hourglass-shaped. But zooming in further, the astronomers found yet another structure: a hollow cylindrical nebula surrounding the central star. Sahai suspects that the cylindrical walls were formed by powerful jets of hydrogen or helium gas blasting from the star's poles, carving out a tunnel in the ambient gas.
Where do these jets come from?
It turns out that jets are a common phenomenon in the universe, shooting out from many kinds of stars and even enormous black holes billions of times more massive than the sun. Although the details are unknown, they happen when a disc of gas and dust spirals into the star or black hole. The falling matter carries energy, which is released via narrow jets shooting out in opposite directions.
In planetary nebulae, these jets are made of gas. But in the supermassive black holes that reside at the centre of galaxies, they're likely charged particles blasting out at extreme speeds. These black-hole powered jets are so powerful that they can blow bubbles in the hot gas that permeates the space between the galaxies in a galaxy cluster. The way these jets inject heat and gas into their environments influences how galaxies form and evolve.
Even though the bubbles blown by these jets are up to a million times bigger and even though the gas, at tens of million of degrees, is far from cold, the general process is the same as what happens in systems such as the Boomerang, says astronomer Noam Soker of Technion University in Israel. So by studying the jets in the Boomerang and other planetary nebulae, astronomers can learn about galaxies and the supermassive black hole at their centres.
Jets are also thought to be involved in the strange explosions known as gamma-ray bursts, which are some of the most powerful cosmic phenomena observed, Soker says. He also thinks they may help drive supernovae—the explosive deaths of very massive stars.
"This is a highly controversial subject," he notes, as most astronomers think supernovae are propelled by an eruption of energetic particles called neutrinos. Still, current theories aren't satisfactory and the ubiquity of jets makes them a plausible mechanism, he says.
As for the Boomerang, there's still much to learn about what Sahai calls one of his favourite objects in the universe.
He and his colleagues plan to study it further with ALMA later this year. Their earlier observations showed that in the inner regions, the gas moves at a mere 35 km/s. With more detailed data, they hope to map exactly how fast different regions of the expanding gas cloud are moving. They also want to better understand the dusty doughnut at the centre.
The Boomerang is bizarre because it's a frigid place. But for astronomers, the nebula and its brethren are more than that. "These are not just beautiful objects," Soker says. "They hold many secrets."