Class notes
Now that’s a stellar sequence
- Course
- Astro 101 (ASTRO1O1)
- Institution
- University Of Alberta (UofA)
Now that’s a stellar sequence
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A Hertzsprung Russell diagram or HR diagram is a tool common in astrophysics, used for thepurpose of analyzing properties of populations of stars. It's a simple two-axis plot with luminosity
increasing as you go up on the vertical axis and temperature increasing as you move leftward on
the horizontal axis. The temperature axis is ipped. By observing a large number of stars and
plotting each one as a point on one of these diagrams, we can begin to notice several patterns.
Perhaps the most striking of which is what we call the main sequence. The main sequence of stars
represented on the HR diagram is a roughly diagonal swath of points, stretching from the low
luminosity, low-temperature region in the diagram to the high luminosity, high-temperature region in
the diagram. These are the stars which originated from the formation scenario we described in the
previous section. Fusion rates have stabilized in their cores and they're living out their adulthood in
a state of hydrostatic equilibrium. The main sequence phase of a star's life, when considered
relative to formation and to retirement, meaning prior to the death of a star, is the longest. As a note,
we consider the death of a star to be any end state such as the formation of a white dwarf, neutron
star or a black hole.
Temperature axis
is flipped
T
We'll see more about these in the coming sections. During the main sequence phase, a stable
source of fuel is present in the form of hydrogen, which the star consumes converting it through
fusion processes into helium. Elderly stars which have left the main sequence source that energy
not only from hydrogen but from other elements as well.
TMass TLuminosity
For main sequence stars, there's a strong relationship between mass and luminosity. The more
mass of the star, the brighter it is. The intense gravity of a massive star means its core will be
denser and as a result, hotter. This is important because fusion rates, meaning the rate at which
energy is produced, is highly dependent on core temperature. So, the more mass of a star, the more
energy per unit time it produces and as this energy leaves the core and eventually reaches the
surface, we observe a greater luminosity, meaning a brighter star. You'll notice in taking this course
that we often refer to stars by their color. So when we say color, what do we mean? Well, as we
learned in module one, electromagnetic radiation or light is a spectrum, visible light being just one
portion of it. What we call blue is just an even smaller portion of the spectrum. Instead of simply
calling it blue, we could de ne it in numbers because as we know, light is characterized by its
wavelength. Blue light has a wavelength of about 450 nanometers. So when we refer to a blue star,
what we are saying is that much of its radiation is coming from this portion of the spectrum. When
we say a star is bluer than another star, what we mean is that the bulk of the bluer star's radiation is
coming from even shorter wavelength light. The same can be said of red and redder stars. As a
redder star will have the bulk of its radiation in longer wavelength light. Just like the lament of a
light bulb, a star's light is produced by incandescence or formally, blackbody radiation. Blackbody
radiation is temperature dependent. The hotter a blackbody radiator is, the brighter it is. As the
temperature of a blackbody emitter increases or decreases, it also changes color. This is why colder
, stars appear dim and red, and hotter stars are brighter and bluer. The hottest stars in the sky, blue
hyper-giants are upwards of 40,000 degrees centigrade and can shine about ve million times
brighter than our own sun. We say that hotter stars are blue and the colder stars are red but the
reality with blackbody radiation is that every star produces at least a little bit of every color of the
spectrum. When we say a star is blue, we're saying that the majority of its radiation is being
produced in this portion of the spectrum. This majority exists because each blackbody emitter has
a spectrum that is peaked. Meaning there's a wavelength at which the star produces more light
than at any other wavelength.
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These peak wavelength are directly related to the surface temperature of a star. The relationship is
described by Wien's law which takes the form of this equation.
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1 peak T
The peak wavelength Lambda peak is equal to 0.0029 meters Kelvin divided by the temperature T
of the blackbody emitter. As we mentioned earlier, more massive stars are brighter because they're
more luminous as a result of having higher fusion rates. Despite having more material to burn, more
massive stars live shorter lives. The more massive a main sequence star is, the quicker it exhaust
the fuel in its core. Hot blue star might live on the main sequence for 10 million years whereas the
dim red star could live as long or longer than a trillion years, that's 100,000 times longer. This is why
we like to personify stars. We'd like to think of blue stars as rock stars that live fast and die young.
Red stars live long and much more uneventful lives.
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