Brown Dwarfs are Probably Much More Common in the Milky Way Than Previously Believed

Brown dwarfs are strange things. They are in the middle ground between planets and stars. A star is defined as an object massive enough for helium to fuse into hydrogen into its core, while a planet is too small for core fusion to occur. It seems a simple distinction until you learn about fusion. Anything with a mass below about 13 Jupiters is too small for fusion to occur, and is thus a planet. If your mass is about about 80 Jupiters, then you can fuse helium and are therefore a star. But if your mass is between 13 and 80 Jupiters, things get interesting. You can’t fuse hydrogen to shine brightly, but you can fuse lithium into other elements. This is known as lithium burning. It doesn’t provide lots of energy, but it is technically nuclear fusion.

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Brown Dwarfs can Spin so Fast They Almost Tear Themselves Apart

We tend to image planets as spheres. Held together by gravity, the material of a planet compresses and shifts until gravity and pressure reach a balance point known as hydrostatic equilibrium. Hydrostatic equilibrium is one of the defining characteristics of a planet. If a planet were stationary and of uniform density, then at equilibrium, it would be a perfect sphere. But planets rotate, and so even the largest planets aren’t a perfect sphere.

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We Now Have a 3D Map of The 525 Closest Brown Dwarfs

Zooniverse brings out the best of the internet – it leverages the skills of average people to perform scientific feats that would be impossible otherwise.  One of the tasks that a Zooniverse project called Backyard Worlds: Planet 9 has been working on has now resulted in a paper cataloguing 525 brown dwarfs, including 38 never before documented ones.

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Astronomers see Swirling Weather on the Closest Brown Dwarf

An artist's conception of a brown dwarf. A new study identifies CK Vulpeculae as the remnant of a collison between a brown dwarf and a white dwarf. Image: By NASA/JPL-Caltech (http://planetquest.jpl.nasa.gov/image/114) [Public domain], via Wikimedia Commons

Brown dwarfs are the weird not-planets but not-stars in the universe, and astronomers have wondered for decades if their atmospheres are striped like Jupiter’s, or splotchy like the sun’s. A team of astronomers based at the University of Arizona used NASA’s TESS Observatory to find the answer: if you saw a brown dwarf for yourself, it would look more like a giant planet than a star.

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Astronomers Can Actually See the Clouds and Weather on Brown Dwarf 6.5 Light-Years Away

This artist's conception illustrates the brown dwarf named 2MASSJ22282889-431026, observed by NASA's Hubble and Spitzer space telescopes. Brown dwarfs are more massive and hotter than planets but lack the mass required to become stars. Image credit: NASA

Brown dwarfs are in a tough spot. Not quite a star, not quite a planet, they occupy a place between gas giants and stars. They have more mass than gas giants like Jupiter, but not enough to ignite fusion and become a star.

But astronomers still study them. How could they resist?

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Astronomers Measure the Wind Speed on a Brown Dwarf for the First Time. Spoiler: Insanely Fast

This artist's conception illustrates the brown dwarf named 2MASSJ22282889-431026, observed by NASA's Hubble and Spitzer space telescopes. Brown dwarfs are more massive and hotter than planets but lack the mass required to become stars. Image credit: NASA

In some ways, brown dwarfs are nature’s stellar oddballs. A lot of stars exhibit strange behaviour at different times in their evolution. But brown dwarfs aren’t even certain that they’re stars at all.

But that doesn’t mean astronomers don’t want to study and understand them.

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348 Years Ago, a French Astronomer Monk Might have Witnessed the Collision Between a White and Brown Dwarf Star

This hourglass-shaped figure is named CK Vulpeculae. It was discovered by French Monk-Astronomer Per Dom Anthelme in 1670. A new study identifies it as the remnant of a collision between a white dwarf and a brown dwarf. Image Credit: ALMA (ESO/NAOJ/NRAO)/S. P. S. Eyres

There’s something poignant and haunting about ancient astronomers documenting things in the sky whose nature they could only guess at. It’s true in the case of Père Dom Anthelme, who in 1670 saw a star suddenly burst into view near the head of the constellation Cygnus, the Swan. The object was visible with the naked eye for two years, as it flared in the sky repeatedly. Then it went dark. We call that object CK Vulpeculae.

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Astronomers Observe a Pulsar 6500 Light-Years From Earth and See Two Separate Flares Coming off its Surface

Astronomy can be a tricky business, owing to the sheer distances involved. Luckily, astronomers have developed a number of tools and strategies over the years that help them to study distant objects in greater detail. In addition to ground-based and space-based telescopes, there’s also the technique known as gravitational lensing, where the gravity of an intervening object is used to magnify light coming from a more distant object.

Recently, a team of Canadian astronomers used this technique to observe an eclipsing binary millisecond pulsar located about 6500 light years away. According to a study produced by the team, they observed two intense regions of radiation around one star (a brown dwarf) to conduct observations of the other star (a pulsar) – which happened to be the highest resolution observations in astronomical history.

The study, titled “Pulsar emission amplified and resolved by plasma lensing in an eclipsing binary“, recently appeared in the journal Nature. The study was led by Robert Main, a PhD astronomy student at the University of Toronto’s Dunlap Institute for Astronomy & Astrophysics, and included members from the Canadian Institute for Theoretical Astrophysics, the Perimeter Institute for Theoretical Physics, and the Canadian Institute for Advanced Research.

The system they observed is known as the “Black Widow Pulsar”, a binary system that consists of a brown dwarf and a millisecond pulsar orbiting closely to each other. Because of their close proximity to one another, scientists have determined that the pulsar is actively siphoning material from its brown dwarf companion and will eventually consume it. Discovered in 1988, the name “Black Widow” has since come to be applied to other similar binaries.

The observations made by the Canadian team were made possible thanks to the rare geometry and characteristics of the binary – specifically, the “wake” or comet-like tail of gas that extends from the brown dwarf to the pulsar. As Robert Main, the lead author of the paper, explained in a Dunlap Institute press release:

“The gas is acting like a magnifying glass right in front of the pulsar. We are essentially looking at the pulsar through a naturally occurring magnifier which periodically allows us to see the two regions separately.”

Like all pulsars, the “Black Widow” is a rapidly rotating neutron star that spins at a rate of over 600 times a second. As it spins, it emits beams of radiation from its two polar hotspots, which have a strobing effect when observed from a distance. The brown dwarf, meanwhile, is about one third the diameter of the Sun, is located roughly two million km from the pulsar and orbits it once every 9 hours.

Image of the pulsar surrounded by its bow shock. White rays indicate particles of matter and antimatter being spewed from the star. Its companion star is too close to the pulsar to be visible at this scale. Credit: NASA/CXC/M.Weiss

Because they are so close together, the brown dwarf is tidally-locked to the pulsar and is blasted by strong radiation. This intense radiation heats one side of the relatively cool brown dwarf to temperatures of about 6000 °C (10,832 °F), the same temperature as our Sun. Because of the radiation and gases passing between them, the emissions coming from the pulsar interfere with each other, which makes them difficult to study.

However, astronomers have long understood that these same regions could be used as “interstellar lenses” that could localize pulsar emission regions, thus allowing for their study. In the past, astronomers have only been able to resolve emission components marginally. But thanks to the efforts of Main and his colleagues, they were able observing two intense radiation flares located 20 kilometers apart.

In addition to being an unprecedentedly high-resolution observation, the results of this study could provide insight into the nature of the mysterious phenomena known as Fast Radio Bursts (FRBs). As Main explained:

“Many observed properties of FRBs could be explained if they are being amplified by plasma lenses. The properties of the amplified pulses we detected in our study show a remarkable similarity to the bursts from the repeating FRB, suggesting that the repeating FRB may be lensed by plasma in its host galaxy.”

It is an exciting time for astronomers, where improved instruments and methods are not only allowing for more accurate observations, but also providing data that could resolve long-standing mysteries. It seems that every few days, fascinating new discoveries are being made!

Further Reading: University of Toronto, Nature