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Black holes are some of the most extreme astronomical objects out there, but there are some that really standout. Let's look at black holes that grow larger, consume more, and spin faster than the rest.

Host: Reid Reimers

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Sources:
https://www.universetoday.com/143135/a-monster-black-hole-has-been-found-with-40-billion-times-the-mass-of-the-sun/
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https://academic.oup.com/mnras/article/471/2/2321/3906599
https://www.gizmodo.com.au/2018/05/the-hungriest-black-hole-ever-detected-gobbles-a-sun-sized-star-every-two-days/
https://astrobites.org/2018/06/11/the-biggest-baddest-quasar-of-them-all/
https://www.cambridge.org/core/services/aop-cambridge-core/content/view/6FB5687AE7326F09557AD625C2889A0F/S132335801800022Xa.pdf/div-class-title-discovery-of-the-most-ultra-luminous-qso-using-span-class-italic-gaia-span-skymapper-and-wise-div.pdf
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Images:
https://svs.gsfc.nasa.gov/12854
https://svs.gsfc.nasa.gov/11086
https://svs.gsfc.nasa.gov/11206
https://svs.gsfc.nasa.gov/12005
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(Intro)

Black holes are some of the most extreme astronomical objects out there.  They're dense enough to pack the mass of the Earth into the size of a marble, and at the heart of each one is a point of infinite density where space and time cease to exist, but even among these extreme objects, there are extraordinary ones.  We're talking black holes that grow larger, consume more, and spin faster than all the rest.  By finding and studying these outliers, astronomers get a glimpse of physics at its limits.  Here are three of the most intense black holes helping them do just that.

The most massive black hole astronomers have directly measured is located 825 million light years away in the galaxy Holm 15A.  It weighs in at around 40 billion solar masses, but it didn't start out that big.  To reach its mass, it had to consume almost all the matter around it, leaving basically a ghost town around its galaxy's center.

According to a study submitted to The Astrophysical Journal in July 2019, this likely happened when two black holes merged.  As they danced around each other, whatever matter they didn't devour got ejected out of the region.  Now, while this whopper holds the current record, pinning the blue ribbon on the most massive black hole isn't an easy job.  

Since black holes swallow up light, we can't directly see them, which means astronomers have to get creative about their measurements.  They got this one's mass by looking at how its gravity pulls on nearby stars, but they can also use less direct methods to gauge the mass of a black hole, like looking at the spectral lines or chemical fingerprints in the light surrounding it.  The shape of those lines can tell them how fast the source of that light is moving.

This method is less precise, but scientists used it to measure one black hole that might be around 66 billion solar masses, and another could be as much as 100 billion.  It's hard to say for sure how accurate these measurements are, but regardless of whoever ends up with the crown, all of these objects will help astronomers figure out if there's an ultimate mass black holes can achieve, because we're not actually sure about that.

The next type of extreme black hole consumes an enormous amount of matter.  Most supermassive black holes, including the one at the center of our own galaxy, are kind of lazy.  The occasional morsel might fall to its doom, but for the most part, stuff orbits far enough away that they barely eat anything.  There's a small group of them, though, literally the 1%, that astronomers call 'active', meaning they're constantly gobbling up matter around them.  We find this type of black hole in a region called an active galactic nucleus or AGN.

The AGN contains the black hole, its accretion disc, and the surrounding donut of dust and gas.  As these black holes devour matter, the AGN emits light, in some cases outshining all the stars in the galaxy.  The hungriest black hole ever detected fuels an AGN around 12 billion light years away in one of the oldest galaxies we can detect.  It devours a mass equivalent to our Sun every two Earth days and it shines as bright as 700 trillion suns.  For comparison, if this thing were at the center of the Milky Way, we could see it as a pinprick of light shining 10 times brighter than a full moon, and that's about twice as luminous as typical AGNs.

This discovery was so surprising that the team of astronomers that found it had to check that the brightness they detected wasn't exaggerated by any other phenomena.  For example, many AGNs have bright jets of particles streaming out of either side, but follow-up research suggested no jets and nothing else that could be boosting the signal.  So it looks like this AGN is just naturally brilliant.

The third extreme black hole is the fastest spinner.  That prize goes to the black hole known as, well, this, and it's located in our very own Milky Way.  Black holes spin because of a principle called Conservation of Angular Momentum.  You've seen that principle in action if you've ever watched an ice skater spin faster as they pull their arms closer to their bodies.  A similar thing happens with black holes.

Most black holes start out as the collapsed cores of massive stars.  Those stars have some small amount of spin and as the center collapses into a black hole, that spin gets faster, except with black holes, there's a little catch.  For a given mass, there's a maximum speed.  If a black hole has too much spin, it will actually release some of the energy in its system until it slows down to below that limit.  So rather than saying that a black hole rotates however many times a second, astronomers assign it a so-called spin parameter.  That gives its speed as a percent of how fast it physically could spin.  The current prize holder as of January 2019 is rotating at between 92 and 95% of its speed limit, so it might not be the fastest spinning black hole ever, but so far, it's the one spinning closest to its physical limit.

Astronomers can't measure a spinning black hole itself, of course, so they rely on clues in the light around it, but actually measuring a black hole's spin is pretty complicated, since there's a lot of light interfering with the signal.  As a result, astronomers only have accurate measurements for half a dozen black holes with really high spin parameters, meaning again, this title could be up for grabs.

But as our technology improves and we find new record-setters, these extreme black holes will keep giving us ways to test our predictions about the limits of physics and reality as we know it. 

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