3 March 2026 — Transcript

тур, две Альдебаран, очень известные звёзды.

В последовательности сверхгигантов особенно интересны Ригель и Бетельгейзе.

Ригель - это белый или бело-голубой сверхгигант, Бетельгейзе - это красный сверхгигант.

Почему они особенно интересны? Да хотя бы потому, что они находятся в одном созвездии.

Это созвездие, которое названо в честь персонажа греческой, древнегреческой мифологии охотника Ориона.

Это созвездие Ориона.

Вот так оно выглядит на небосводе.

Его хорошо видно в наших широтах, особенно начиная с поздней осени и до весны.

И вот здесь отлично, ну, правда, немножко подработано в фоторедакторе, но тем не менее, действительно, примерно так и выглядит.

Бетельгейзе - красный сверхгигант, Ригель - голубой или бело-голубой сверхгигант.

Две звезды последовательности сверхгигантов находятся в одном созвездии.

В созвездии Ориона.

Надо сказать, что диаграмма Герцшпрунга-Рассела может быть разделена не на три и не на четыре, а на большее количество ветвей.

То есть кривых, которые описывают разные последовательности.

В некоторых случаях это, наверное, действительно нужно.

И ещё, обратите внимание, что вот на данном изображении у нас есть так называемый спектральный класс.

Это система, которая просто использует буквенные обозначения, где буква О относится как раз к голубому спектральному классу, самому горячему.

А и, соответственно, буква М - к красному и самому холодному.

Что же означает вот эта классификация?

Что она значит применительно к тому, как живут, а в отношении звёзд в астрономии применяется глагол жить.

Или, например, словосочетание жизненный цикл.

Да-да, действительно так.

Так вот, как существуют во времени и чем заканчивают свой жизненный цикл представители разных последовательностей, разных ветвей.

Мы узнаем уже несколько позже.

На следующих лекциях.

Мы узнаем, чем они отличаются по каким-то существенным, важным признакам и характеристикам.

А сегодня, чтобы подытожить и закрепить весь рассмотренный материал, я вам предлагаю два небольших видеоролика.

Которые отлично иллюстрируют всё, что мы с вами разобрали.

Более того, там есть и дополнительный материал.

Нюанс или особенность.

Эти видеоролики на английском языке.

Но там есть титры, субтитры.

Там есть текст, который выводится на экран.

Особенно это касается второго ролика, который как раз и описывает систему классификации.

И поэтому я даю задание.

Самые главные понятия и выводы, конечно, из этих роликов взять.

Перевести это даже с использованием автоматического переводчика не составит никакого труда.

Но, вообще-то говоря, для того, чтобы это просто перевести даже без переводчика, с использованием какого-нибудь словаря.

Достаточно школьного уровня английского языка.

Поэтому, пожалуйста, проработайте этот материал.

Итак, смотрим видеофрагменты.

So we're going to be looking at the proton-proton cycle.

The proton-proton cycle is the predominant mechanism through which the sun generates its energy.

So it's a type of thermonuclear fusion reaction.

It's also known as nucleosynthesis.

Now, one of the really interesting things about how the sun and stars generate energy is that they also generate elements this way.

So your body is actually entirely composed of stardust.

So in the Big Bang, we had a lot of hydrogen created, a lot of helium created, and then just trace amounts of lithium and beryllium.

Any of the heavier elements that we find here on Earth, such as carbon, which is a big component of your body, was created in the center of stars.

Okay, so let's look at the proton-proton cycle.

As its name suggests, it starts with one proton and another proton.

The protons combine, most of the time when the protons meet each other, they'll just bounce off, but every once in a while, they'll combine with each other to form deuterium, which is a hydrogen nuclei still, but now it's got one proton and one neutron.

They also create a positron and an electron neutrino.

Now, because the sun is predominantly matter, that positron quickly meets another electron and they annihilate, creating two gamma rays.

So we've now got a deuterium nuclei.

That deuterium nuclei will meet another proton in the center of the sun, and it will form helium-3.

So this is a helium nuclei with two protons and one neutron, and in this process, it also emits gamma radiation.

So the protons are meeting each other very often, and even though these reactions are fairly unlikely to take place, because it's happening so frequently, we create around about 10 to the 12 kilograms of deuterium each second in the sun.

So once we have created our helium-3 nuclei, the next step in our proton-proton cycle takes quite a lot longer.

Because it relies on that helium-3 nuclei finding another helium-3 nuclei.

And these nuclei are relatively rare in the sun.

So when it does happen, we have the helium-3 plus the other helium-3, these combine to create helium-4, so an alpha particle with two protons and two neutrons, and then we also get two protons given off in this process.

So that's the entire cycle.

We can combine it all into one equation to describe the entire cycle.

So in the entire cycle, we require four protons, we also require two electrons, and these combine to give us helium-4, and then we also get two electron neutrinos and six gamma-rays given off.

There's also additional energy given off because the elements on the left-hand side have a higher mass than those on the right-hand side.

So this mass difference is converted into energy using the equation E is equal to mc squared.

So our sun has been undergoing this proton-proton cycle for around five billion years now.

The best estimates are that it will continue to undergo this proton-proton cycle for approximately another five billion years.

At that point, the concentration of protons in the sun's core will be slightly lower, and so we won't have this first step in the reaction taking place quite as frequently.

So at that point, the gravity will win out and will end up with a red dwarf, sorry, a red giant and then a white dwarf, as we've discussed previously.

It's Professor Dave, let's check out some stars.

He knows a lot about the science stuff, Professor Dave explains.

Now that we are about a billion years into the history of the universe, we can see a panorama of stars swirling around in galaxies, which have in turn collected into clusters and superclusters.

So what happened next?

The answer to this question will require that we learn more about stars and their characteristics, which determine the way we categorize them.

So let's learn the basics about this system now.

When we first started to observe stars in telescopes, we divided them into color classes: white, yellow, red, and deep red.

This was later refined and each color was broken up into letters: A to D for white, E to L for yellow, M and N for red.

Later, it was realized that things made more sense if stars were categorized by surface temperature, but this letter system was retained because all the work to classify stars had already been done.

So from hottest at around 25,000 Kelvin to coolest at around 3,500 Kelvin, we now have O, B, A, F, G, K, and M stars, a classification system called the Harvard System, which was developed by early astronomer Annie Jump Cannon.

This sequence of letters is rather unintuitive, but to remember the order, we can use the following mnemonic: Oh, be a fine girl, kiss me!

Feel free to replace girl with guy, depending on your persuasion.

Or if you find the whole thing terribly sexist, just make up your own, such as: Omniscient beings are firing gigantic knowledge missiles!

As we can't stick a thermometer into a star to see how hot it is, this classification based on temperature is actually derived from Wien's law regarding blackbody radiation, which we saw in the modern physics series.

As well as other types of data, like emission spectra, we analyze the light we receive from a star and correlate it with a particular temperature, as well as with specific elements, just like when we learned about the Bohr model in general chemistry.

The hotter the star, the more of the hydrogen and helium nuclei that have been stripped of their electrons, forming the phase of matter known as plasma.

The hottest stars, O stars, show very little hydrogen because most of the hydrogen is without an electron and thus can't absorb and emit light.

Helium is still able to retain one or both electrons and thus we do see emission correlating with helium.

Cooling down a little with A stars, suddenly hydrogen can hold on to an electron, so the spectrum changes.

Getting cooler still, some bands show up that correspond with metals like calcium.

So the convention is derived from temperature, but this happens to correlate with color and size as well.

Hotter objects, like O and B stars, are blue, and cooler objects, like K and M stars, are red.

Also, hotter stars tend to be larger and burn brighter, with the additional heat resulting from the fact that so much more fuel is being burned.

All of this data regarding temperature and luminosity, as well as indirect information on mass and radius, can be represented on something called a Hertzsprung-Russell Diagram, or an H-R Diagram for short.

In this diagram, the horizontal axis shows temperature decreasing to the right, and the vertical axis shows luminosity, or the amount of energy emitted by a particular star per unit time, increasing going up.

We can see that the majority of stars fall on a continuous curve, which we call main sequence stars.

90% of all stars follow this trend, including our own sun, which is part of this yellow region here.

Some stars, like red giants, are very cool yet luminous, while others, like white dwarfs, are very hot yet dim.

But the majority belong to this main sequence.

Even though this diagram lists only temperature and luminosity, we can infer many things about other variables.

Larger stars are always more luminous, as more surface area means more energy emitted.

We can also see color clearly correlating with temperature as we move from left to right.

Size is also represented, with main sequence stars decreasing in size from left to right, but with red giants and white dwarfs deviating from this trend.

This data, collected by looking at hundreds of thousands of stars in the early 20th century, reveals certain facts about stars, such as the mass-luminosity relationship that we just described.

It explains why the blue stars in this corner of the main sequence burn brightest, getting dimmer as we go towards the smaller red stars.

This has to do with the fact that the gravity crushing the star inwards increases exponentially with its radius, so larger stars have to generate much more outward pressure to prevent collapse.

So that's some basic information about all the stars in the universe.

Remember, for the main sequence, blue stars are big and hot and bright, up to about 100 to 200 solar masses, or 1 to 200 times the mass of our sun.

Red stars are small and cool and dim, down to around 1/10 the mass of our sun.

Yellow are in between, these are about the size of our sun.

Then beyond main sequence stars, there are red giants and there are white dwarfs.

Those are the three main classes of stars.

Most of the stars that have existed in the past, and most of the stars that exist today, fall into one of these categories.

But they are not static, they will move between these categories over time.

So, how does this happen? And how is it that stars eventually die?

There is a lot to discuss here, so let's move forward and learn about the lifetime of stars.

Thanks for watching, guys.

Subscribe to my channel for more tutorials.

Support me on Patreon so I can keep making content, and as always, feel free to email me, [email protected].

Professor Dave explains.