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.