Where is the coldest place in the Universe?

The Boomerang Nebula is the only known place colder than the glow left over from the Big Bang.

The Boomerang Nebula: a planetary nebula considered to be the coldest known place in the entire Universe. Credit: European Space Agency, NASA
Published: April 11, 2022 at 10:58 am
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The coldest place in the Universe is the Boomerang Nebula, a glowing cosmic cloud located 5,000 lightyears away in the constellation Centaurus.

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The nebula’s title as the coldest place in the Universe is a result of a 1995 study by astronomers Raghvendra Sahai and Lars-Åke Nyman.

Using the 15-metre Swedish ESO Submillimetre Telescope in Chile, Sahai and Nyman observed the Boomerang Nebula and discovered its temperature to be -272°C, making it just 1°C warmer than absolute zero.

Considering that the Cosmic Microwave Background - the background glow left over from the Big Bang - is about -270°C, this gives you some idea of just how cold the Boomerang Nebula is, relative to the rest of the cosmos.

It seems it has rightly earned its title as the coldest place in the Universe.

A snapshot of the Cosmic Microwave Background - heat left over from the Big Bang - when the Universe was just 380,000 years old, as seen by the Planck Telescope. It shows tiny temperature fluctuations that correspond to regions of different densities: the seeds that would grow into the stars and galaxies of today. Credit: ESA and the Planck Collaboration
A snapshot of the Cosmic Microwave Background - heat left over from the Big Bang - when the Universe was just 380,000 years old, as seen by the Planck Telescope. Credit: ESA and the Planck Collaboration

The Boomerang Nebula was first observed in 1980 by astronomers Keith Taylor and Mike Scarrott, who used the Anglo Australian Telescope at Siding Spring Observatory to discover its double-lobed shape.

This symmetrical formation is a result of the star shedding its material out into space in two opposite directions, which is a occurring because the Boomerang Nebula is actually a planetary nebula (more on this later).

As it appeared to Taylor and Scarrott when viewed through the telescope - and perhaps because they were in Australia - the nebula's shape reminded the two astronomers of the famous Australian throwing tool, and the Boomerang Nebula got its enduring nickname.

More recent observations by the Hubble Space Telescope, however, reveal its double-lobed shape, and that it in fact doesn't look much like a boomerang at all.

More recent observations of the Boomerang Nebula, like the image on the left captured by the Hubble Space Telescope in 2005, have revealed it doesn't bear much resemblance to a boomerang after all. Credit: NASA, ESA and The Hubble Heritage Team STScI/AURA
More recent observations of the Boomerang Nebula, like the image on the left captured by the Hubble Space Telescope in 2005, have revealed it doesn't bear much resemblance to a boomerang after all. Credit: NASA, ESA and The Hubble Heritage Team STScI/AURA; Michael Reusse / Getty Images

Fast-forward to the mid 1990s, and astronomer Raghvender Sahai was considering the notion that somewhere in the Universe there must be a place that's colder than the aforementioned background glow left over from the Big Bang.

Dr Sahai and his colleague Dr Nyman studied the Boomerang Nebula and concluded that it was absorbing background radiation.

This led them to infer that the Boomerang Nebula is the coldest place we know of in the Universe: it's the only region known to be colder than the background glow.

Why is the Boomerang Nebula so cold?

Planetary nebula NGC 2022. Credit: ESA/Hubble & NASA, R. Wade
Many planetary nebulae are round, puffed-out objects like NGC 2022. Credit: ESA/Hubble & NASA, R. Wade

The Boomerang Nebula is an object known as a planetary nebula, which is one of the final stages of a Sun-like star’s life.

As the star runs out of fuel, instability at its core causes the star to shed its outer layers.

A stream of charged particles known as a stellar wind pushes this material outwards, often creating a round, puffed-out object.

This spherical appearance appearance is what gives planetary nebulae their name: they don’t actually have anything to do with planets.

The Boomerang Nebula may not have a rounded appearance typical of a planetary nebula, but it is expelling vast amounts of cosmic material at breakneck speeds, and has been doing so for about 1,500 years.

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And this is why the Boomerang Nebula is the coldest place in the Universe: it is expelling mass, which is projected outwards and rapidly expanding, causing it to cool down to the extend that it is actually colder than the radiation left over from the Big Bang.

Authors

Iain Todd, BBC Sky at Night Magazine
Iain ToddScience journalist

Iain Todd is BBC Sky at Night Magazine's Staff Writer. He fell in love with the night sky when he caught his first glimpse of Orion, aged 10.

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