Stars?

Yeah.

You always talk about stuff.

Yeah.

So this was just a question that came out.

Were just chatting about this, and we're talking about the kinds of stars there are in particular, you know, big stars, little stars.

All right, What's the biggest star?

That's all I care about.

Is that what you want?

Okay, we could do the biggest star.

There it is.

Doesn't look terribly inspiring.

That's because it's quite a long way away Has the inspiring name of all 136 a one.

It's actually in the large Magellanic Cloud, which is one of our nearby neighbors.

So the biggest star that we know is not even in the Milky Way.

Absolutely not.

There's two reasons why it's hard to find a really big star in the Milky Way.

One easy.

When you're looking in the Milky Way, you always peering through all the Marchal, the dust that's there between us and the center of the galaxy.

Where is the large Magellanic Cloud is conveniently well out of the plane in the Milky Way so we can actually see it.

The other thing is that there's a lot of very strong star formation going on in the large Magellanic Cloud.

Very massive stars.

A very rare.

And so actually you need to have an awful lot of star formation happening for there to be just one of these popping up.

And secondly, they don't live very long.

They live a few 1,000,000 years, which means that actually, you really gotta, at least in astronomical terms, catch them just at the moment they're forming because actually, if you blink and you miss it in astronomical terms, so actually you've really got see that star active star formation that's happening right now.

So are 136 is the general region.

It's in a is the star cluster within that region on one because it's the brightest star in that star cluster.

It's about 300 times the mass of the sun, which is big, and it's about I think it's about 10 million times as bright as the sun.

When I were a lad, it was thought that the world any stars more massive than about 100 150 solar masses, and there is actually there's a physical reason why you might expect there to be a limit for how big a star can be, Which is that So what makes the star's gravity pulls it together.

But there is, ah, kind of, ah countervailing force, an outward force which is just due to the Starlight itself.

The radiation pressure from the star itself tries to push the star apart, so the light itself is trying to push the star apart.

And the interesting thing is, as you go up to more or more massive stars, the mass goes up.

They get more massive, but they get a lot more luminous.

So if you had a bit more master star, it gets a bit more massive, but quite a lot more luminous on.

That means that even though in a star like the Sun, this radiation pressure really doesn't matter very much.

Gravity always wins over the radiation pressure.

As you go up in Mass, the mass goes up so the gravity goes up.

But the brightness, the luminosity, goes up a lot more.

And so actually, the radiation pressure becomes more and more important on a fairly naive calculation, says that the crossover point where radiation pressure winds over gravity so the star would literally blow itself apart by its own radiation by its own Starlight is about 150 solar masses.

A sent Paul Crowther, the guy who did this, an email saying, Why is it that there are now stars more massive than we thought this limit ought to be?

And he says that the explanation is actually quite interesting.

But although for low mass stars as you go up a bit in mass, the luminosity goes up a lot When you get to these very, very high mass stars.

It turns out the two go up together, that the mass goes up a bit and the luminosity goes up a bit.

So there's no longer this kind of crossover point where the radiation pressure wins in the end.

So actually, these very massive stars, you haven't quite reached that crossover point, and then you get never get any closer to it because gravity and the luminosity go up together in the same way as it evolves.

It tends to lose mass stuff gets thrown off the outside so it becomes less massive and it gets brighter.

So actually, these very massive stars, even though they conform eventually they will cross over that line because Suddenly they get brighter and less massive.

So suddenly the radiation pressure wins out over the gravity, and they will blow themselves apart quite quickly.

Let me ask you the question.

How do you know something's a star, right?

What's the star?

Well, a star is like a bowl of mainly hard region.

Yes, that has, like, spontaneously started nuclear fusion.

So you required.

That's the interesting definition, part of the definition as to whether you have nuclear fusion in there.

Of course, by that definition, a white dwarf isn't a star because it doesn't have nuclear fusion anymore.

It did at some point in its life, but not any longer.

There's no longer a source of energy in it is just glowing because it's hot.

But for example, Jupiter gives out more light than it absorbs the actually glows in the dark on, not because of nuclear fusion, but just because actually, it's very slowly contracting with time.

Tend to me, tour's over a year on that gravitational contraction is actually liberating energy, liberating the potential energy.

So, actually, if Jupiter um, it's more light than it absorbs it.

Turns out both these ends of the spectrum are hard to actually detect the high mass.

Stars are hard to detect because they're very rare and they don't live very long.

The low mass stars that plentiful there's loads and loads of them around, but they're so faint that they're actually hold a spot on, Actually is only in our neighborhood that we can find them because they're so pathetically fine.

So here it is, the lowest mass stars known.

There it is.

It's called SC or 18 45 minus 6357 for about 15 light years away.

Actually, it has a companion that's a brown dwarf, and we see this one because we're seeing the reflected light from the star.

But it's very, very faint.

We're seeing a brown dwarf that we normally wouldn't see because it's being shown apart by its neighbor, the Sun.

The most important step, where is that in the distribution of star size is boring.

Lee in the middle really is it really is not quite in the middle.

So if you think about so the lowest mass, so usually you think about these things on kind of a log arrhythmic scale.

The lowest mass stars are about attend the mass of the sun on the highest mass stars for a few 100 times the mass of the sun.

So it's not quite in the middle.

It's sort of somewhat towards the lower mass end, which I guess you sort of expect.

It's just there's more stars towards that end.

Pistol pistol stars about 150 times the mass of the sun.

It's one of those that's the kind of the brightest ones in the Milky Way.

It might well be the biggest star in the Milky Way, the most massive star in the Milky Way.

And it's quite spectacular to look at to.

The interesting thing about these very massive stars is that they tend to be in regions of intense star formation, so there's lots of gas around them, so they light it up and look very nice.

And actually, the other thing you sometimes find with them is because they tend to blow off their outer layers because they're not very well bound together.

You sometimes find features around them that had to do with that as well.

One of the things that people interested in into the first stars that formed on what properties they had although we've never observed them yet, the prejudice is the expectation is that the first stars to form in the universe were very massive stars.

They have lifetimes of a few 1,000,000 years, and they wouldn't have hung around very long, but they would have seeded the universe with heavy elements.

So you start off with a big bang, which makes your hydrogen and helium lithium, but anything heavier than that you need stars for the oldest stars that we can see around us today actually have heavy elements in them.

So we know they weren't the first generation of stars.

The the earliest kind of second generation of stars.

There's two at least two reasons.

To think that those first generation of stars might have been very massive.

One is to make a star, you need gas clouds, the kind of fragment and collapse on.

To get something to fragment and collapse, you need to be able to cool down, and one of the things that the heavy elements do is they sort of accelerate the cooling process.

They allow gas to cool down, and so, with heavy elements, you can kind of create quite low temperature regions and actually have little things collapsing to make little stars.

If you don't have the heavy elements, it's very hard for gas to cool down, which means you tend to make big things because it just doesn't fragment into those little bits.

So we think probably that just that process of collapse would probably tend to make biggest stars in the early universe, because it's hard for little things to call down and collapse because you don't have the heavy elements.

The other is that process we were talking about about, like competition between gravity and radiation, that what's happening is the radiation is kind of driving its way out on smacking into things.

And the interactions are much stronger if you have the heavy elements there.

The opacity that how opaque the story's depends enormously on how much of those heavy elements there are.

So stars are much more transparent if you don't have heavy elements.

Which means that rather than pushing the star apart, that radiation tends to be able to escape more easily.

So that process competition between the gravity pulling things in and the radiation pushing it apart.

If you don't have the heavy elements, it's harder for the radiation to push things apart.

So therefore you can actually make bigger and bigger stars.

So that side of the process tends to make more massive star stable.

If you haven't got the heavy elements, as we did in the early parts of the universe, we won't be able to see them were looking back in time.

That's the plan s next generation of telescopes.

James Webb, Space Telescope.

Next generation of ground based telescopes Next generation of Space Telescope We really want to be looking for those first stars to form.

We will actually have the sensitivity to find particularly, very massive, very massive things.

A very bright.

So hopefully we will in on the time scale of maybe a decade or so, be able to see those first stars as they formed.

Wow, Cool.