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The Boomerang Nebula is colder than space! And it's not really shaped like a boomerang!

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Reid: Here on SciShow Space, we’re fans of all things cool and extreme. We’ve talked about some of the coldest places in the universe that we know of: man-made laboratories here on Earth and on the International Space Station. But what about the coldest natural place that we know of?

That’s the Boomerang Nebula -- a dying star that’s spewing out a cloud of dust and gas. And by studying it over the past few decades, astronomers have learned a lot about some of the weird things that can happen when a star dies, and what might happen to our very own sun in billions of years.

The Boomerang Nebula was first observed in 1980 by an Australian team of astronomers using a big ground-based telescope. But it’s pretty far away from us -- head about 5000 light years in the direction of the constellation Centaurus and you’re there. All the astronomers could really see was a lopsided shape, and they decided it kind of looked like a boomerang. Which is how the nebula got its name. The nebula is actually something called a pre-planetary nebula, or a low-mass star that’s near the end of its life.

First, a dying low-mass star will expand outward and become a red giant. Then, the outermost molecules will start growing colder and clumping into dust particles, and energy from the center of the star pushes all this gas and dust outward. Eventually the star will collapse into a hot white dwarf core that glows because it’s emitting lots of UV radiation. The Boomerang Nebula is still in this dusty cloud of gas phase, but that phase only lasts for a couple thousand years -- a tiny span of time, on a universal timescale. That made it a really interesting target for astronomers to study.

In the 1990s, a different group of scientists was observing this strange, asymmetrical nebula with a radio telescope in Chile. That’s when they realized just how cold the Boomerang Nebula was. They compared radiation signals from vast empty stretches of space with signals from carbon monoxide gas particles in the Boomerang Nebula, and realized that even though the background temperature of empty space is cold -- like around 2.7 or 2.8 Kelvin -- this was even colder -- at around 1 Kelvin. Which is just barely above the coldest temperature possible -- 0 Kelvin, or absolute zero.

But it does kind of make sense that the Boomerang Nebula would be so cold. See, temperature is a measure of how much molecules in a certain area are moving around -- their kinetic energy. So when a gas expands and fewer molecules cover more area, it gets colder. The Boomerang Nebula is expanding incredibly quickly and expelling a lot of gas outward from the star -- at a rate of about 164 kilometers per second for the past 1,500 years. That’s about 10 times faster than other similar gas-spewing space objects.

As the gas expands, the nebula gets colder -- and it’s hard for the ambient heat of the universe to seep back in. Which might explain how the Boomerang Nebula ended up with the lowest natural temperature we’ve ever seen. But its temperature isn’t the only strange thing about the Boomerang Nebula -- because it turns out that it isn’t shaped like a boomerang at all. In the late 90s, a research group observed the nebula with the Hubble Space Telescope, and found that it looked more like a hazy bow-tie.

Astronomers call this a double-lobe structure, and many other planetary nebulae end up with a similar shape because of the gases they’re expelling. But then, in 2013, astronomers took a closer look at the nebula’s shape, using the ALMA radio telescopes in Chile. And it turns out that the Boomerang Nebula is actually a much rounder cloud of expanding dust and gas. The bow-tie structure was just a distortion of the visible light that we could see from Earth -- there are a lot of small chunks of dust around the star, which form a kind of ring that partially obscures the light coming from the nebula. That ring was affecting the images from telescopes like Hubble, making the nebula look more like a bow tie.

No matter what it’s shaped like, scientists studying the Boomerang Nebula are hoping to learn more about how stars die. Especially because in a few billion years, our Sun might become a ridiculously cold, gaseous nebula, too.

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