FreeSchool 101!
Of all the astronomical objects in space, none are more widely recognized than stars.
Stars are the building blocks of galaxies, responsible for the formation of heavy elements,
and tied to the fate of planetary systems that develop around them. You may be surprised
to learn that although they look similar to the naked eye, stars are as varied as anything
else in the universe. Studying the types, age, and composition of stars can help us
learn the history of the galaxies they are in, and to assist with this we can classify
stars in many different ways.
One of the first things that early astronomers realized when studying the stars was that
some of them are brighter than others. This may have seemed like a good method to use
to classify stars, but it has a major problem that might already be obvious to you.
Just like a flashlight that is farther away will seem dimmer than an identical one
that is closer, if two stars of equal brightness are at different distances from Earth, one
will appear brighter and one will appear dimmer than the other.
With the development of telescopes, it became easier to see that stars glowed in different
colors - blue, white, yellow, orange and red - but the boundaries between visible colors
can be subjective, so that wasn't quite enough to sort them into universally-agreed upon
categories.
It was the invention of spectroscopy that really revolutionized stellar classification.
Spectroscopy, or breaking up light into individual wavelengths, began with Isaac Newton's experiments
with prisms where he demonstrated that ordinary sunlight was made up of all the colors of
the rainbow. Later scientists realized that there were gaps in the sun's spectrum which
appear as dark lines. Further investigation revealed that other stars also had black lines
missing from their spectra - but in different places.
These black bands are called absorption lines. Absorption lines occur when gas in the outer
layer of stars absorbs some of the light emitted by them. By matching the pattern of absorption
lines with those from known elements, astronomers were able to learn more about the chemical
composition of stars. They realized that stellar spectra seemed to fall into several distinct
patterns, and began to group stars with similar patterns together.
Amateur astronomer Henry Draper began the work of photographing stellar spectra in 1872,
and following his death in 1882 Edward Pickering continued it at the Harvard College Observatory.
The Draper Catalogue of Stellar Spectra was published in 1890, classifying more than 10,000
stars by assigning them letters from A through O, based on the strength of their hydrogen lines.
Much of the work of classifying these thousands of stellar spectra was done by a group of
women called the Harvard Computers. Before computer machines, of course, a computer was
a person who performed computations, calculations, and data analysis. One of these women, named
Annie Jump Cannon, proposed a simpler method of classifying stars by their surface temperature,
combining categories until there were only seven left.
Since no one wanted to redo the classification that had already been done, the original letter
categories were kept, but rearranged into a seemingly random order: O B A F G K and
M. Fortunately Annie Jump Cannon also created a mnemonic to help remember them: "Oh be a
fine guy (or girl), kiss me." This new classification system was called the Harvard System. Under
the Harvard System, O type stars are the hottest, and M type stars are the coolest. Our sun
is a G type star, near the middle.
Of course, there is much more to classifying stars than temperature or color. Let's take
a look at something called the Hertzsprung-Russell Diagram (or the HR diagram, for short.) The
HR Diagram is a scatter plot of stars that allows us to sort them by brightness, or luminosity,
and temperature. As you can see, there is a correlation between temperature and color,
as well as size and luminosity.
The majority of stars belong to something called the "main sequence." Stars on the main
sequence are fusing hydrogen in their cores to form helium during the longest phase of
their life cycle, and account for about 90 percent of all known stars. If you observe
the main sequence on the HR Diagram, you can see that stars fall in a diagonal line from
the top left to the lower right. Stars at the top left are hotter and brighter, and
stars at the bottom right are dimmer and cooler.
Stars that have depleted their hydrogen cores develop into giants or supergiants, visible
in the HR diagram above the main sequence. Their cores contract, creating enough heat
and pressure to trigger fusion of heavier elements, and their outer layers expand to
many times the star's original size. Due to their massive size, giant stars are many times
more luminous than the sun, even if they are cooler on the surface.
Once a giant star has exhausted all possible fusion reactions in its core, it's off to
the next phase of its life. Smaller stars, like our sun, will expel their outer layers
and leave a dense, dim, slowly cooling core called a white dwarf, visible in the lower
left of the HR diagram. White dwarfs are extremely hot when originally formed, but as they have
no means of producing additional energy they will cool and darken over time, eventually
becoming a black dwarf, a dense lump made mostly of carbon that emits neither heat nor
light.
Now, let's discuss a few types of stars that do not appear on the HR diagram. A supergiant
at the end of its lifespan may explode in a supernova and produce either a black hole,
which is not a star at all, or a small, super-dense body called a neutron star. Neutron stars
are composed mostly of densely packed neutrons and have incredibly strong magnetic fields.
Though many neutron stars are undetectable due to the fact that they are not emitting
very much radiation, a special type of neutron stars called pulsars are the exception.
Pulsars are rapidly spinning neutron stars that emit beams of electromagnetic radiation
out of their poles. This radiation is only detectable at certain positions in the pulsar's
rotation, and as it spins the beam disappears and reappears at regular intervals. These
pulses of radiation gave pulsars their name.
Finally, let's talk about brown dwarfs. Technically not stars, brown dwarfs are classified as
substellar objects. They are sometimes referred to as failed stars because they did not accumulate
enough mass to jump start nuclear fusion of hydrogen into helium. Brown dwarfs are much
more massive than gas giants, but may be smaller than the planet Jupiter because they are more
dense. Although brown dwarfs have comparatively low surface temperatures and do not emit much
in the way of visible light, they do emit in the infrared wavelengths.
As you can see, the answer to the question 'how are stars classified' is complicated.
The sun is classified as a G-type main sequence star, but it is also referred to as a yellow
dwarf. Stars can be classified by their color, size, brightness, spectra, or phase in their
life cycle. All of this is an attempt to understand and quantify the things that make stars different
from each other and understand how they relate to the larger universe. As with so much out
there in space, there is more to a star than meets the eye.