Last week the big astro news was the discovery of two really, really big Black Holes.  These cosmic mystery monsters were so huge that astrophysicists threw up their hands, not knowing how such beasts could form.  Since everybody loves Black Holes (after aliens, it's what people ask me about most), I thought I’d give some background for context. 

Why was this news important and what’s the problem with such massive black holes?

To get started, these kinds of black holes are the tombstones of massive stars.  We astronomers put stars into 3 categories based on how much mass they contain when they’re born (how stars are born is a story for another time).  How much mass a star has at birth is what determines how it will die millions or billions of years later.

Stars hold themselves up against their own gravity by “burning” matter ivia nuclear fusion reactions.  These reactions happen in the stellar core where temperature and pressures are insanely high. When the fusion fuel at the stellar center runs out, gravity wins. The dying star contracts on itself and the kind of corpse left over depends on the birth mass. 

Stars less than 8 times the mass of the Sun (8 solar masses) end their lives as White Dwarfs (yes, astronomy’s nomenclature is weird). White Dwarfs are basically Earth-sized balls of diamond (i.e carbon).  Stars that are not too much bigger than 8 solar masses get squeezed so tightly they basically become a giant atomic nuclei. These are called neutron stars.  But for really massive stars - the ones much bigger than 8 solar masses - gravity is so strong that once fusion stops the star gets squeezed down to… well, the squeezing never stops.  The star, with all its zillions of tons of matter, gets slammed down into a single geometric point. Since a point as zero dimensions all the mass of the star is still there but it has space for to occupy. That’s what we call a black hole and that’s one reason why they are so weird (there is more to it but that will also be a story for another time).

Ok, on to last week’s news.  About half of all stars with masses like our Sun’s are actually part of a binary system. They do not exist alone but orbit another star.  Massive stars, however, almost always form with a companion. As the two orbiting massive stars evolve, each one eventually dies and becomes a black hole.  At some point the two orbiting stars become two orbiting black holes.  

Over time the orbital separation between the two black holes shrinks. They draw ever closer to each other in a slow spiral. Why does this happen?  Black Holes, with all that mass but no where to put it, are so weirdly intense they make the fabric of Space and Time vibrate like the head of a drum when they move.  These vibrations are called gravitational waves. If they’re strong enough, we can detect them using recently developed gravitational wave telescopes.  As the distance between the black holes shrinks, the orbits gets faster and the space-time waves get more intense.  Eventually the black holes merge violently. Binary black hole mergers release many apocalypses worth of energy which gets spewed into the Universe as gravitational waves. What was announced last week were new observations that caught exactly this kind of merger. 

But there was a problem.

The two black holes doing the hokey pokey weighed in at 137 solar masses and 103 solar masses respectively.  That’s too big.  None of our theories about stellar evolution predict black holes this large.  Why? What ends up as a black hole is really just a fraction of what the original star weighed when it was born.  Stars constantly lose matter into space in “stellar winds”.  They are always losing weight as they age. That fact implies that if one of these black holes is 137 solar masses now, the star that led to it must have been much, much bigger.  But no star anywhere in the Universe is that big.  The largest stars we’ve ever seen weigh in at about 120 solar masses.

So, what gives? 

Researchers do have some ideas. Maybe there were actually four or more stars to begin with.  Then, perhaps, there were earlier mergers that led to the monsters we just discovered.  Ok, fine. That’s a nice possibility but for now it’s just a conjecture.  The truth is we don’t know. 

Isn’t that awesome?  We don’t know.  The Universe just showed us something new that we don’t understand and now we gotta figure it out.  That’s why science kicks ass.  Always something new, always something unexpected, always something to learn.