It’s Professor Dave, let’s check out some
stars. 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 onto 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. Ninety percent 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 twentieth 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. We can also categorize stars by their luminosity
rather than color, using Roman numerals one through five, one being the brightest. 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 around a hundred to two hundred solar masses, or one
to two hundred times the mass of our sun. Red stars are small and cool and dim, down
to around one tenth 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.
