The Star Killer: Facts About Black Holes

 In Space

What is a Black Hole?

Some initial Facts About Black Holes

Black Holes are defined as a region of space-time that is so gravitationally dense that it folds in upon itself.  It has an escape velocity greater than the speed of light, thus supposedly preventing mass or electromagnetic radiation from escaping.  But, is that actually true? What are the real facts about black holes?

Once mass accumulates sufficiently, any object begins to compress under its own mass.  If the mass is large enough, it forms a ball (which is why planets and stars are round).  If it is too small a mass, they end up looking like asteroids. Essentially, this looks like a loose collections of ice and rock with no particular shape other than “lumpy”.

Some immense things keep compressing until fusion starts to take place and that forms a star.  The fusion reaction pushes back against gravity and a balance is reached.  However, once all its nuclear fuel is exhausted, a star will collapse.

Eventually, if a star is not massive enough, it becomes a white dwarf star, until it stops emitting energy all together and becomes a black dwarf.  That is the future of our own Sun in about five billion years. The is a key fact about black holes, and it means that only massive stars have the required gravity to collapse and form a black hole.

Bigger is Better?

The concept and general facts about black holes are perhaps a little difficult to wrap your mind around. Further, people have some strange notions about what they are, what they look like, where they exist, and the danger they represent.  Typically, for people who have read a little about it, they visualize it like this image, a black body with a glowing halo.

Facts About Black Holes

As scientists, we have to take responsibility for not explaining clearly, or for using bad examples.  A black hole isn’t a black body surrounded by a hyper-accelerated, superheated, glowing ring of matter being drawn inside forever.

Yes. It can have an accretion disk, if it is stealing mass from a companion star. However, it glows because the material is “star stuff” that is ridiculously hot, and glows from friction as it rubs together, not because of the “event horizon”.


Accretion Disks

Since one part of a companion star is a bit closer than the rest of that star, and the direction of the flow of incoming matter isn’t perpendicular (falling straight in), the black hole will grow a disk (reminiscent of Saturn’s rings), as it steals from that closest part.

The ring will be perpendicular to its spin direction and will eventually fall into the black hole.  Just to be clear, the event horizon is invisible.  If there is no matter falling into a black hole, it cannot be seen.

We do not know what is inside a black hole, since by its very nature it is unobservable.  Anyone who lays claim to knowing is mistaken, overconfident, or lying.  By mathematical extrapolation we suspect that whatever is inside collapses to a single point or at least much smaller than the event horizon (aka Schwarzschild Radius).

So even if we could delineate the event horizon itself it is simply a limit to what we can see.  It is not the black hole.

So, How Do We Find Black Holes?

Einstein Cross, Facts About Black Holes


There are, however, still ways to know that it is there.  This image is something that perplexed astronomers when it was first discovered.

How could these four identical stars be in such perfect formation around another?  Was it evidence of a super civilization that had mastered engineering on an unimaginable scale to set up a cosmic “Look at me” sign for other intelligences?

Unfortunately, that was not the case.  What we’re actually seeing here is called Einstein’s Cross.  That “star” in the center is actually a galaxy. Located directly behind it is what would ordinarily be an invisible quasar, obscured by the galaxy in the foreground.

Though not visible to the naked eye, there is actually a fifth image of the quasar dead center over top of the intervening galaxy.

Quasar Confusion

But how does one simple quasar become five quasars?  When Albert Einstein defined space-time as a continuum we learned that gravity could “bend” space.  If you have a sufficiently large mass, light doesn’t travel in straight lines in the vicinity of it.

This galaxy and quasar combination provided prima facie evidence of how the light of that quasar was being bent around the mass of the galaxy. This shows up as four (five) identical stars with identical metallic chemistries.

Einstein was absolutely correct and this little asterism proved it.  Of course you may be asking yourself why it doesn’t appear as a distorted ring of light instead of four discrete images.

This Einstein Ring happens in most cases of gravitation lensing because galaxies are generally elongated.  How it renders is entirely dependent on the shape of the intervening galaxy.

Black holes do exactly the same thing.  They bend the light of distant objects. So, when we see a star moving against the background in an uncharacteristic fashion, it is almost always because a large mass is bending space, giving the appearance of motion.

If you click here, you can see a model of a black hole passing in front of a distant galaxy (simulation obviously because of the time scales involved).


Where are they?

More Numbers and Facts About Black Holes

Over time we’ve come to the conclusion that every galaxy has a supermassive black hole at its center.

We used to think Black holes might be 3-10 solar masses (the mass of our own Sun being equal to “1”).  We thought a practical upper limit might be in the thousands.

Now we know that black holes, particularly in the centers of galaxies where stars are nearby each other, coalesce and have unimaginable masses.

Our own (rather large) Milky Way Galaxy has a black hole at its center named Sagittarius A* (pronounced “a-star”). Sagittarius A* has a mass of 4.3 million stars the size of our Sun.  It may sound like a lot, but it is actually only 0.00086 percent of the stars in our galaxy. For this reason, it is not really all that significant. Makes you think about how big space is, doesn’t it?

Black holes can be found anywhere, usually individually, resulting from a supernova.  They are not dangerous to us, since there are none in the immediate vicinity.

If our Sun was replaced with a black hole of equal mass, nothing would happen (other than it getting cold and dark).  It would have the same gravity as the Sun so the planets would continue to orbit undisturbed.

History of Black Holes

Black Holes were first hypothesized in the 1700s by Pierre-Simon Laplace who created the notion of gravitation collapse, and John Michell, the first published advocate of black holes, who ordinarily specialized in magnetometry and seismology.

It was Karl Schwarzschild who first found a solution that could characterize black holes within general relativity, in 1916, only a year after Einstein had published his work on General Relativity for the first time.

The idea languished as a curiosity until David Finkelstein republished the work in 1958 while describing a gravitation phenomenon from which nothing could escape.

No one seriously believed in them until neutron stars were discovered in 1967 by an Irish research student named Jocelyn Bell (now knighted Dame Susan Jocelyn Bell Burnell, DBE, FRS, FRSE, and FRAS).  Oddly she was excluded from the Nobel Prize and her teacher got the credit… <sarcasm>Gee, how did that happened? </sarcasm>

Of course, to be entirely fair, the teacher was recognized for work on pulsars. It was some years later when the importance of the spinning neutron stars was made apparent.  Nevertheless Bell Burnell did well for herself, being President of the Royal Astronomical Society, President of the Institute of Physics (twice), President of the Royal Society of Edinburgh, and now in her mid-70s, Pro-Chancellor of the University of Dublin.

Neutron stars did lead researchers to seriously consider high gravity objects as possible candidates for supernova remnants (star corpses). At this point we finally discovered black holes!

Do they exist “forever”?

We’ve used terms like Event Horizon or Schwarzschild’s Radius throughout our facts about black holes to give us an idea off a cutoff point where matter is lost forever.  But is it?

Steven Hawking, probably the most brilliant mind of the 20th Century, postulated that even black holes aren’t permanent, and that they “evaporate” as they leak radiation.  This was eventually shown to be correct and the phenomenon was consequently (and eponymously) named Hawking Radiation.

Among all the facts about black holes, this is perhaps the most interesting, because it leads to many questions about how black holes store the “data” of our universe. Take a look at this video, I really enjoyed watching it:

The Takeaway

So, let’s take a second to summarize the concepts and facts about black holes that we’ve discussed today.

Black holes can be any size from three solar masses and upwards to the number of available stars or other mass.  The leak at the event horizon is complex.

The easiest way to think about it is that the energy leaked is so cold (on the scale on nano-Kelvins, barely above absolute zero) it is undetectable, but it is mathematically provable.

We know that the radiation rate is inversely proportional to its size as well. Thus, small black holes evaporate quickly and large black holes evaporate slowly (think “Ice cube” vs. Glacier).  Black holes…a fascinating subject!

Give our other space articles a read too, you would be surprised what you could learn!

Matt Cameron

About Matt Cameron

Hard-working, dedicated, and passionate are three traits that describe me. I've spent my entire life learning the skills that I need to be able to be a successful entrepreneur. Whether I'm doing work for my companies, or writing content for my blogs, I'm always giving it my best effort.

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