Is the Earth bigger than the Sun?

All week, in honor of the summer solstice, I’ve been writing about the sun and about astronomy. Why? Because my son unleashed a torrent of questions, once we started talking about Monday having been the longest day of the year. So far I’ve answered questions about why the sun doesn’t melt, what the hottest star is, and what would happen if the sun turned into a black hole. So now, we’re on to the final question:

“Is the Earth bigger than the Sun?”

This one made me chuckle, just a little. “No,” I told him, “the sun is lots bigger.”

“Well,” he said thoughtfully, “my friend says the Earth is bigger.”

“It isn’t,” I assured him. “It just looks bigger, because we stand on the earth and the sun is really far away.”

He thought about that for a minute. “But it could be bigger!”

No. It really couldn’t.

This is a clear cut answer. The sun has an equatorial diameter of 1,391,400 kilometers and masses 1.988 x 10^30 kg, while the Earth has an equatorial diameter of 12,756.2 kilometers and masses 5.972 x 10^24 kg. Put another way, the Sun’s diameter is about 109 times that of earth, and it is 333,000 times as massive. There is no way the sun could be smaller than the earth.

So why does it look so small in the sky?

I’ll be honest here:  this is not something I ever recall stopping to ask.  Not until I started writing this article.  Like so many things, I just took it for granted that things that are close look bigger than things that are far away.  It never occurred to me to ask “why”, before.  (Which is one of the cool things about writing these articles – I end up answering questions I never thought to ask.)

Researching this, I ended up on a couple of different physics forums, and both of them agreed with Cognition and the Visual Arts:

The size of the retinal image varies in inverse proportion to the distance of an object.  Near objects appear larger than far objects because they occupy more space on the retina.  In the perception of real world stimuli, an object 5 feet away casts an image on the retina twice the size as the same object viewed from 10 feet away.

ZWiXS

The object on the left is an eye, and the two stick figures are identical.  However, the stick figure closer to the eye occupies a greater percentage of that eye’s field of vision, and so will appear larger.  Likewise, a small object held close can appear to be the same size (or even larger) than a large object that is far away.

The sun, on average, is 93 million miles away.  As a result, even though it’s more than a hundred times as wide as the Earth, it appears small enough that you could make your own solar eclipse by holding a quarter (or similar coin) at arm’s length in front of it.

Don’t try that, though.  You could really damage your eyes.

 

What if the Sun turned into a Black Hole?

This week, I’ve been writing about the sun. I blame the summer solstice for this, because the news that Monday was the longest day of the year fired my son’s imagination and got him asking question after question about the sun, and about the stars, and about related astronomical phenomena. So far, I’ve answered his questions about whether or not the sun can melt (it can’t) and what the hottest star is (H1504+65). Now it’s time to move on to his next question, one which demonstrates that he’s learned some interesting things.

“What if the sun turned into a black hole?” he asked, as we walked up the stairs to the front door of our condominium building. “Would it swallow the earth and all the planets?”

That one took me off guard, because I’m pretty sure that when I was five I didn’t even know what a black hole was. But then, I also realized that the first black hole was discovered the year I was born, so it’s not surprising the term wasn’t in common usage when I was five.

It’s a chilling thought, isn’t it? “Nothing escapes a black hole,” science fiction tells us. “Not even light.” Black holes are the great white sharks of space – remorseless predators consuming everything in their path. And we’d never see them coming. But they have one other thing in common with sharks.

black-holes-opener-615

They have an exaggerated reputation.

Newton’s Laws of Motion and Universal Gravitation

Although aspects of his laws have been superceeded by Einstein and his General and Special Theories, Newton’s laws remain an excellent (if ever so slightly inaccurate) model of motion. In brief, his three laws of motion state:

  1. If no forces act upon it, a body in motion will remain in motion and a body at rest will remain at rest, and velocity will remain constant in either case.
  2. If force is applied to an object, there will be a change in velocity proportional to the magnitude to which the force is applied.
  3. If body A exerts force on body B, then body B will exert a force of equal strength but in opposite direction on body A. This is also stated as “for every action there is an equal and opposite reaction”.

In addition, Newton put forth a law of universal gravitation. This law states that “two particles having masses m1 and m2 and separated by a distance r are attracted to each other with equal and opposite forces directed along the line joining the particles. The common magnitude F of the two forces is

eq4-02

where G is an universal constant, called the constant of gravitation, and has the value 6.67259×10^-11 N-m^2/kg^2.”

Yeah? What does this have to do with black holes?

I’ll get to that. But first, let’s cover what a black hole actually is.

Fine. What’s a black hole?

Does it surprise you to know that NASA has some good resources about black holes? It really shouldn’t.

A black hole is a region in space where the pulling force of gravity is so strong that light is not able to escape. The strong gravity occurs because matter has been pressed into a tiny space. This compression can take place at the end of a star’s life. Some black holes are a result of dying stars.

Because no light can escape, black holes are invisible. However, space telescopes with special instruments can help find black holes. They can observe the behavior of material and stars that are very close to black holes.

Black holes come in four size categories, representing both their mass and their physixal size. There are:

  1. Micro black holes. These can run all the way up to about 7.342 x 10-8 M (the mass of our Moon), and can get as big as 0.1 millimeters. Yes, it would suck if one hit you.
  2. Stellar black holes. These range up to 10 M in mass, and can be up to about 30 kilometers in diameter (0.5 x 10-4 R).
  3. Intermediate-mass black holes, which can get up to 1,000 M and up to about the mass of the Earth itself.
  4. Supermassive black holes. These are the monsters that lurk at the center of most galaxies, massing up to 1010 M and up to 400 astronomical units in size.

Wow. So, why do you say they have an exaggerated rep?

It’s true that the escape velocity of a black hole exceeds the speed of light, which is what it means to say that “no light can escape”. However, no black hole will be larger or more massive than the sum of all of the mass that went into making the black hole in the first place. So, outside the event horizon (the point at which gravity becomes too powerful to escape), the black hole has the same effect as any other object of the same mass. With that in mind, Newton’s law of universal gravitation tells us that – if the sun were to be instantly replaced with a 1 M black hole – there would no impact on our solar system. the r2 figure in the equation is measured from center of m1 to center of m2, so nothing changes.

Nothing?

Well, all right. That’s not true. Black holes have no luminosity – no energy would be generated and nothing would reach the Earth. So, to quote Randall Munroe’s Sunless Earth article, “We would all freeze and die.”

What is the Hottest Star?

This week, I’m writing about the sun. And about stars in general. Why? Because two days ago, while walking home from preschool, we started talking about the summer solstice and things escalated from there. Five year olds are fully capable of unleashing an avalanche of questions.

Yesterday, if you recall, I answered his question about why the sun didn’t melt. He accepted the explanation I gave him, but it led him to two more: “What is the hottest star? Is it the biggest?”

I was honest with im. I had no idea what the hottest star is, or if that star is the biggest star. Let’s find out the answer together, shall we?

Measuring Stars

Most commonly, stars are measured in “solar” or “stellar” units, based on the measure of our own star (aka “the sun”).

  • Solar Mass (M): the mass of the sun, which is 1.98855 x 1030 kilograms.
  • Solar Luminosity (L): the energy output of the sun, which is 3.828 x 1026 watts.
  • Solar Radius (R): the radius of the sun, which is 6.960 x 105 kilometers.

To put that in perspective, the Earth has a mass of 5.927 x 1024 kg and an equatorial radius of 6.378 x 103 kilometers. So that means that the sun has the mass of roughly a million earths, and is approximately 100 Earths wide.

None of these address temperature, though. The sun’s core is modeled to be 1.57 x 107 Kelvin (K), the photosphere is 5,772 K, and the corona is 5 x 106 K. (If you aren’t familiar, the kelvin is a measure of temperature; it’s identical to Celsius, except that 0 degrees kelvin is absolute zero, and water begins to melt at 273.15 kelvin.)

Harder. Better. Faster. Stronger.

So, what’s the brightest star? Based on luminosity it’s R136a1, located about 163,000 light years from Earth. It’s luminosity is 8,710,000 L, and it’s not a slouch in other matters as well. It’s the most massive star we know of as well, with an estimated 315 M. The radius, however, is “only” estimated at 28.8 to 35.4 R, so it’s a long way from being the largest star.

The honor of being the largest star belongs (right now) to Westerlund 1-26, which has a radius around 1,530 R. Although not the brightest or hottest, it does its best. Its luminosity is 380,000 L, but it’s photosphere temperature is a paltry 3,600 K – only 62% of the Sun’s and far cooler than R136a1’s 53,000 K.

R136a1 isn’t the hottest star going, though. That title belongs to H1504+65, which has an estimated photosphere temperature of 200,000 K – 34 times hotter than the sun.

Is there anything bigger?

Of course there is. There’s plenty of room for things to go big in space, after all. For example, there is a thing called a pair-instability supernova – technically a hypernova – that happens when a star with 130 to 250 M explodes. It generates 1011 L at peak output.

The single most massive, brightest thing we know of is S5 0014-81, “a distant, compact, hyperluminous, broad-absorption line quasar or blazar located near the high declination region of the constellation Cepheus”. It comes in with 40,000,000,000 M and about 300,000,000,000,000 L. To put that in perspective, if it was 280 light years away from us – about the distance to Theta Scorpii – it would give us as much energy as the Sun. Fortunatly, it’s more like 12,000,000,000 light years away.

Next to S5 0014-81, or Sagittarius A* at the heart of our own galaxy, or even our fairly ordinary home star, our Earth is a tiny speck in the universe.  But it’s home.

Why doesn’t the sun melt?

This week, it seems, I’ll be writing about the sun. Why? Well, when we were walking home from preschool yesterday, I asked my son if he knew what day it was. “It’s the first day of summer!” he declared. And then he asked me a whole bunch of questions, one right after the other:

To be honest, I felt like I’d been hit by an avalanche of curiosity. But they’re all great questions.

What is melting?

Let’s start with the Dictionary.com, and a definition of melt that states:

  1. to become liquefied by warmth or heat, as ice, snow, butter, or metal.
  2. to become liquid; dissolve
  3. to pass, dwindle, or fade gradually (often followed by away)
  4. to pass, change, or blend gradually (often followed by into)
  5. to become softened in feeling by pity, sympathy, love, or the like
  6. Obsolete. to be subdued or overwhelmed by sorrow, dismay, etc.

From a more technical perspective, ‘melting’ is a first-order phase transition in which a material’s latent heat increases and it’s density or volume decreases sufficiently that it moves from the solid phase to the liquid phase.

Wait. Phases? Phase transitions? What now?

You’ve probably seen a diagram like this before:

Phase_diagram_of_water.svg

That is a phase diagram for water, showing the different states (solid, liquid, or vapor) that water can be in based on temperature and pressure. Under one atmosphere of pressure (1 bar or 100 kPa), it is a solid at or below 0 degrees Celsius, a liquid between 0 and 100 degrees Celsius, and a vapor above 100 degrees Celsius. Phase is just the technical term for these states, and a phase transition is simply where the material changes (or makes a transition) from one phase to another.

While researching these phase changes, I ran into the terms “first-order phase transitions” and “second-order phase transitions” a lot. I’ll be honest and say I don’t fully understand them, because the best definitions all seem to involve a whole lot more physics than I understand. But here’s my best attempt at an explanation, after reading several articles and staring hard at Wikipedia:

  • A first-order phase transition is driven by heat, and the material transitions from one phase to another at a set rate based on the energy added to the system. Think of melting ice, or boiling water.
  • A second-order transition is also called a continuous phase transition, and appears to a uniform change across the material – imagine a block of ice instantly becoming water, for example.

340px-Phase_change_-_en.svg
The first-order phase changes

The first-order phase changes are the ones we’re all familiar with. There are a whole lot of other types of phase change, so have fun reading up on them some time.

Right. So what does all of this have to do with a star?

All of this is a long walk to the answer I originally gave my son, when he asked me why the sun didn’t melt. “The sun can’t melt,” I told him. “Melting is when a solid turns into a liquid, and the sun is made of gas and plasma. It’s way hotter than melting.” Which is more or less true, and also led to the next question he asked me. But we’ll handle that tomorrow.

What is a star?

I’ll be honest here. I thought the International Astronomical Union would have a formal definition of a star, much like they do for planet and dwarf planet and the like. But, if they do, I couldn’t find it. So, in brief, a star is “a luminous sphere of plasma held together by its own gravity”. It can’t melt, because “melt” is not a phase transition available to plasma. Anything in it that could have melted has already melted, then vaporized, then ionized around 4.6 billion years ago.

Can you come with me, daddy?

It’s Father’s Day tiday, and we’re at church. The service is over, and we’re in the Great Hall taking part in the monthly potluck lunch when my so asks me if he can go to the bathroom. Of course, I say yes. Then he looks at me and asks his question:

“Can you come with me, daddy?”

For a moment, I consider saying no. I’m in a conversation, after ai, and the church isn’t very big. He knows how to find the bathroom by himself. But he’s staring up at me with his big blue eyes, and I sigh and say yes.

As soon as I say yes, he smiles big and wide and grabs my hand and we’re off. The “whole way” (maybe a hundred feet) he’s chattering to me about five year old things, right until I ask him why he wanted me to go with him.

“In case I get scared,” he tells me.

Wait. Where’s all the research?

This is a different entry, folks. There’s no crazy facts, no research, nothing like that. Just a Father’s Day musing on being a father.

My own father died over 20 years ago and, although the pain of that loss has faded over the years, there hasn’t been a day that I haven’t missed him. I’ve got a lot of memories of him, naturally, and there’s one in particular I’m thinking of right now.

I was about my son’s age -five, maybe sux – and I’d had a particularly vivid nightmare. A monster if some sort, with purple skin the texture of a football, had wanted to cut off my skin with safety scissors and eat it. I woke up screaming and crying, and this time it was my dad who came to see what was wrong. I told him I’d had a bad dream, and asked if I could sleep in his bed.

He said no. But then he said he’d stay with me so the bad dream wouldn’t come back.  Then he tucked me in, and sang to me until I was asleep.  He kept his promise, too – the bad dream didn’t come back.

As a grown-up, I know that he probably went back to bed once I fell asleep. It was late, after all, and he had to get up early to go to work. But, despite needing sleep himself, he was there when I needed him.

He always was. Right up to the day he was too sick to do it any more. And then it was my turn to be there for him, until he wasn’t there any more.

So, can I go with my son?  In case he gets scared?  Yes. Yes, of course I can. As long as he needs me. Because I want to be as good a father for him as my dad was do me.  As long as I can.

Happy Father’s Day, everyone.

Where Does The Potty Go?

We’re out shopping, and my son announces – as small children are wont to do – that he needs “to go to the potty”. So I’m standing in the men’s room with him as he goes about his business, back to him as he finishes and flushes. And then he asks the title question: “where does the potty go?”

It hits me that I have, at best, an extremely vague idea. But I do my best. “Well, it goes through pipes into a sewer.”

“And then into the river?” he asks, lighting up. We’ve looked at storm water drains before, walking home form daycare.

“No,” I tell him. “It goes to a treatment plant. Like in Curious George.”

“Oh,” he responds. Then another thought strikes him. “How do they clean the pee out?”

Now I’m stumped. Stumped, and slightly grossed out. But curious.

How Does A Toilet Work?

toilet

I’ll be honest, here. When I searched for “how does a toilet work”, I never anticipated that I’d find myself on a web page hosted by the Massachusetts Institute of Technology. But they have a straight-forward article from a student titled How A Toilet Works, and it turns out to be all about siphons.  And physics.

Everyone understands that, when you pull the flush handle, water empties from the tank into the bowl. What happens next is based on the “trapway” in that diagram up there – the famous “u-bend”. The water level rises in the bowl, and because of the way water works that also causes the water level to rise in the trapway. This puts you in a position of having greater pressure in the bowl than in the air filling the trapway, and the water moves from the area of greater pressure to the area of lower pressure until the pressure equalizes.

What makes flushing especially interesting has to do with water velocity. See, as described in the Bernoulli Equation, fluids under constant pressure will move faster through narrow channels. That’s why you can spray water faster from a hose when you put your thumb over the opening – the velocity increases as you constrict the channel. Well, the trapway channel is significantly narrower than the mouth of the bowl. So, as water is pumped in, the bowl drains faster than it fills until the bowl is completely drained. Then the siphon effect breaks down and the bowl refills with clean (for “you still don’t want to drink that” values of “clean”) water.

John Harrington and his successors were pretty ingenious, weren’t they?

Once You’ve Flushed, Where Does It Go?

Once you’ve flushed, there are three primary places (in the United States, at least) that the waste water can go: into a septic drain field, into a septic tank, or into a sewer. Which one you use really depends on where you are, what sort of access you have to municipal water and sewer services, and local ordinances.

Septic drain fields are also known as leach fields or leach beds. They consist of “a network of perforated pipes that are laid in underground gravel-filled trenches to dissipate the effluent from a water-based collection and storage/treatment or (semi-)centralized treatment technology.” Basically, your waste water flows out the into the pipes, which should be buried deeply (the article mentions ten feet of soil). Wastewater leaks out of the pipes, with the water being absorbed and percolating downwards and the (I love this phrase) “organic materials” being consumed by bacteria and other microfauna.

The leach bed is a somewhat more sophisticated version of a soak pit, which is pretty much what it sounds like: a pit that receives wastes or wastewater and holds it as it soaks into the surrounding soil. If you see an old-fashioned outhouse, you’ve seen the surface component of a soak pit.

A septic tank is, quite literally, a big tank. YOur waste water empties into that tank, and then is emptied out and disposed of at a later date.

By no stretch of the imagination are these the only means of disposing of waste water. The SSWM (Sustainable Sanitation and Water Management) web site discusses several other techniques and technologies.

How Do You Clean It?

If you’ve got a leach bed or a soak pit or something similar, cleaning the waste water is relatively straight-forward. You let nature handle it, and you don’t dump toxic chemicals in your toilet while you do. However, if you’ve got something like a septic tank or a sewer hookup, then you turn to wastewater and sewage treatment plants. And according to the Wastewater treatment page on the Food and Agriculture Organization of the United Nations website, you’ve pretty much got two options for treatment: conventional treatment and natural biological treatment.

Conventional wastewater treatment is what you probably first think of when you hear about a “sewage plant”. It “consists of a combination of physical, chemical, and biological processes and operations to remove solids, organic matter and, sometimes, nutrients from wastewater,” and it generally goes through four stages with the imaginative names of preliminary, primary, secondary, and tertiary (or advanced).

  • Preliminary treatment removes “coarse solids and other large materials often found in raw wastewater”, generally through screens and filters designed to trap large objects (which, in this context, includes “grit”). water velocity is generally high in preliminary treatment to keep particulate solids from settling.
  • Primary treatment is “the removal of settleable organic and inorganic solids by sedimentation, and the removal of materials that will float (scum) by skimming. The goal is to remove 25% – 50% of the biochemical oxygen demand (the dissolved oxygen in water used by microorganisms to degrade and consume organic compounds), 50% – 70% of the suspended solids, and 65% of the oil and grease in the water. The resulting settled sludge is pumped out intermittently for processing in – wait for it – sludge processing units (generally by allowing it to be eaten by bacteria in an anaerobic process, allowing it to dry before processing further).
    • Once water has gone through primary treatment it is generally considered acceptable for wastewater irrigation, which can be used for all crops not intended for human consumption and some crops that are directly consumed by humans (orchards and vineyards, for instance).
  • Secondary treatment “follows primary treatment and involves the removal of biodegradable dissolved and colloidal organic matter using aerobic biological treatment processes”.  The process seems counter-intuitive after primary treatment, because microorganisms are added back to the water, along with more air, to consume more of the organic matter remaining in the water.  Further settling tanks (or sometimes rotating tanks) are then used to separate out the microorganisms in a process that removes 85% f the biochemical oxygen demand and suspended solids.
  • Tertiary treatment kicks in when there are specific contaminates that are not removed by the primary and secondary treatments.  They are many and varied, and depend on what contaminate or contaminates are to be removed.

Natural biological treatment often follows the same process as conventional treatment, but makes use of natural processes (such as evaporation) or plant life (such as algae or trees) to filter organic contaminates from the water.  The processes are generally slower than conventional treatment, but also tend to cost less to maintain and operate.

So, where does the potty go?  Out into our infrastructure, and eventually into the ecosystem.

Why Didn’t Buffaloes Go To Heaven?

My son asked me that question right out of the blue one day. I think I might have been cooking dinner, and he just walks over and asks “why didn’t buffaloes go to heaven?” I’m pretty sure my answer was a largely unintelligible “unh?” Followed by some quick thinking – not to mention wondering if this was going to be a “do animals go to heaven” kind of question – and then a counter-question of my own. “What do you mean?”

Hey, they can’t all be winners.

He thinks about that for a moment. “Why didn’t they die when the others did?”

Others? Now I’m even more lost. What on Earth is he talking about?

“The saber-tooths!” he informs me. “Did they run away from the poisonous gas?”

And suddenly, it all clicks. He knows the dinosaurs went extinct, but he’s settled on the idea that it was poisonous gas that did them in. I think that comes from a dinosaur program we watched (I don’t recall which one) that showed a bunch of dinosaurs dying in a limnic eruption. So, he’s asking why dinosaurs didn’t go extinct when the saber-tooths did. Which is not a bad question at all.

Why did saber-toothed tigers go extinct?

At one point, it was a popular theory that saber-tooth cats went extinct because they were ‘superpredators’ that were too successful and wiped out their prey. After this, they starved. Recent studies, however, seem to contradict this. In Implications of Diet for the Extinction of Saber-Toothed Cats and American Lions, researchers examined wear patterns of teeth and jaws for animals recovered from the famous La Brea tar pits with those of living large carnivores. In tough times, our living large carnivores will put more effort into “bone consumption” – cracking and chewing bone for whatever nutrition can be obtained, and the research team believed it was reasonable to assume that extinct large carnivores would exhibit the same behavior. The results of the study?

DMTA here suggests that extinct carnivorans at La Brea may have utilized carcasses less than do some carnivorans today. This idea is inconsistent with interpretations of high incidences of tooth breakage in extinct Pleistocene carnivorans from La Brea compared with extant taxa. We suggest that tooth breakage data may be recording damage from both carcass utilization and prey-capture, with greater tooth breakage occurring due to increased prey size. Lower mean values for DMTA attributes consistent with greater durophagy (i.e., Asfc and Tfv) in both S. fatalisand P. atrox compared with both P. leo and C. crucuta, suggest that the late Pleistocene at La Brea was not any “tougher” (or perhaps “harder”) than the African savanna is today. Further, dental microwear texture comparisons through time offer no evidence that carcasses were utilized consistently more over time, especially for P. atrox. Thus, DMTA provides no support for the idea that prey-resources became scarcer over time. While competition with humans for prey is unlikely to explain the extinction of P. atrox and S. fatalis via competition for prey resources at La Brea, further work is necessary to assess the situation at other sites. Collectively, there is no evidence for greater carcass utilization during the Pleistocene; however, high levels of anterior tooth breakage could instead result from hunting megafauna and/or conspecific competition at La Brea. Thus, times may have been “tough,” but not as originally proposed.

So, what did kill them?

The Quaternary extinction

The Quaternary extinction is the name given to the mass extinction between 40,000 and 10,000 years ago. It impacted every continent to a greater or lesser degree, and in North America it rendered the majority of all animals over 44 kg (100 lbs) extinct. Nobody’s quite sure why it happened, but there are several hypotheses:

  • Hunting: human predation wiped the megaherbivores out, and then the megapredators starved because they couldn’t eat enough prey to make up for the calorie count of hunting. Like all hypotheses it has supporters and detractors. The supporters point out a strong correlation between human arrival and extinction (particularly in Australia and New Zealand), while detractors point out that (for example) most of the North American megafauna were already extinct before humans are known to have arrived. Also (in North America) one megafaunal species that was a human prey species (bison) did not go extinct.
  • Climate change: either an ice age or the end of an ice age drive the extinctions.
  • Hyperdisease: humans and/or the animals travelling with them brought diseases that the local animals were not able to adapt to.

All of these arguments have multiple and ongoing arguments for and against them. The only clear scientific consensus is that the Quaternary extinction did happen. Barring some sort of smoking gun (aliens calling us and telling us they wiped out all the giant sloths, for instance), there will probably never be a clear consensus. My gut feeling is that it’s some mixture of predation and climate change and disease all hitting in some sort of extinction-level perfect storm, but that’s just me.

Bison

So, with apologies to my son, the better question might be “why didn’t the buffaloes go to heaven?” After all, North America lost 33 of 45 genera of large mammals – including wild horses, tapirs, camels, giant beavers, giant tortises, some really huge birds, and ground sloths. Why did the bison survive?

The answer is that they almost didn’t. Before the Quaternary extinction, there were five distinct species of bison in North America: the ancient bison (Bison antiquus), the long horned (or giant) bison (Bison latifrons), the steppe bison (Bison priscus), the American bison (Bison bison), and Bison occidentalis. The giant bison went extinct between 30 and 20 thousand years ago, during the Quaternary extinction. The ancient bison lived until about 10,000 years ago, and was the ancestor of the American bison, and the steppe bison went extinct around the same time. B. occidentalis went extinct around 5,000 years ago, and was most likely bred out of existence (DNA studies have demonstrated that they interbred with what became the American bison).

Of course, even after surviving the Quaternary extinction, they very nearly still went extinct. By 1900, they had been so extensively hunted that there were only 39 bison left alive in the United States, all residing in Yellowstone National Park. They’ve recovered with extremely careful management, but a population of 75,000 is a minuscule fraction of the pre-European bison population. So maybe the “hunting’ hypothesis for the Quaternary extinction isn’t so outlandish.