Podcaster: Richard Drumm
Title: UNAWE Space Scoop – Dizzy, crazy-spinning brown dwarfs
Organization: 365 Days Of Astronomy
Description: Space scoop, news for children.
Often called ‘failed stars’, brown dwarfs are halfway between a star and a planet. They form like stars but are more like giant planets. In fact, they are bigger than planets, but smaller than stars: they can have a pretty similar diameter to Jupiter, so they’re not much bigger in physical size, but they have much more mass than Jupiter.
Bio: Richard Drumm is President of the Charlottesville Astronomical Society and President of 3D – Drumm Digital Design, a video production company with clients such as Kodak, Xerox and GlaxoSmithKline Pharmaceuticals. He was an observer with the UVa Parallax Program at McCormick Observatory in 1981 & 1982. He has found that his greatest passion in life is public outreach astronomy and he pursues it at every opportunity.
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This is the 365 Days of Astronomy Podcast. Today we bring you a new episode in our Space Scoop series. This show is produced in collaboration with Universe Awareness, a program that strives to inspire every child with our wonderful cosmos.
Today’s story is…
Dizzy, crazy-spinning brown dwarfs
Often called ‘failed stars’, brown dwarfs are halfway between a star and a planet. They form like stars but are more like giant planets.
In fact, they are bigger than planets, but smaller than stars: they can have a pretty similar diameter to Jupiter, so they’re not much bigger in physical size, but they have much more mass than Jupiter.
The extra mass has caused the brown dwarf to have a higher density, it’s, well, compressed by its own gravity.
It may be called a dwarf, but we’re still talking about enormous dimensions here.
If you were to take Earth and squish it into a cube and start stacking the cubes into a sphere, you’d need more than 1,300 Earths to equal Jupiter’s size.
Jupiter’s a big, honking planet!
So brown dwarfs are big, honking stars, too! In a way…
If the thing has less than 13 times Jupiter’s mass, it’s a planet. Over that mass it’s a brown dwarf because it’s massive enough to fuse deuterium, the heavy isotope of hydrogen, and that generates heat and light.
When the brown dwarf is 80 times Jupiter’s mass it can fuse hydrogen in its core and it’s a spectral class K star, not a brown dwarf any more.
Like planets, brown dwarfs rotate around their own axis, quite like a spinning top or an ice skater.
The catch is that these objects rotate fast, very fast. And a team of astronomers has found the fastest-spinning one they’ve ever seen. It rotates ten times faster than normal brown dwarfs!
In fact, they found 3 of them!
By using NASA’s Spitzer Space Telescope and ground-based telescopes like the Gemini North in Hawaii and the Magellan Baade Telescope in Chile, astronomers found brown dwarfs that complete a full rotation every hour.
Think about it: our Earth makes a full rotation every 24 hours, once each day. If we were standing on one of these fast-spinning brown dwarfs, a whole day would last only one hour!
And our shoes would be on fire!
But that’s a story for another time…
Brown dwarfs are much more massive than Earth. They can have from 4,000 to 25,000 times the mass of our planet.
Spinning this fast, they’re at dizzying speeds: about 350,000 kilometers per hour, or around 220,000 miles per hour at their equator.
Our Earth spins fast too, at about 1,600 kilometers per hour or 1,000 miles per hour at the equator, but these brown dwarfs are over 200 times faster!
The team that found them concluded that if these objects rotated much faster, they’d tear themselves apart.
In other words, these brown dwarfs have reached their speed limit and cannot spin any faster!
Hey, Here’s a cool fact!
Theorized about in the 1960s, and finally discovered in 1995, brown dwarfs could be the hottest planets or the coldest stars in the Universe.
According to a 2017 study, our Milky Way is home to possibly 100 billion brown dwarfs, roughly half of the stars in our galaxy!
As each brown dwarf rotates, light from the hemisphere turning toward us appears blueshifted, while light from the hemisphere turning away from us appears redshifted, because of the Doppler effect.
This causes absorption lines in the brown dwarf’s spectrum to appear broadened or stretched both toward the red end of the spectrum and the blue end of the spectrum.
By matching this broadening of the line widths to a computer model, the astronomers determined how fast each brown dwarf is spinning.
Thank you for listening to the 365 Days of Astronomy Podcast!
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365 Days of Astronomy
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