Video Transcript
In this video, our topic is
classifying stars by brightness and temperature. When we think about the billions
and billions of stars in our galaxy alone, it makes sense we would want to come up
with a way of organising or classifying them. Two ways of doing this are
considering the brightness of a star as well as its temperature. And as we’ll see later on, the
temperature of a star is related to the color of light that it gives off.
As we look into how to classify
stars using these standards, let’s start by considering the brightness of a
star. On one hand, the brightness of a
star can seem like a pretty straightforward thing. As we look up into the sky at
night, some stars just look brighter. That is, more light seems to be
coming from them than from other stars. It’s important to realise though
that this assessment, that one star is brighter than another, is based on observing
these stars from a particular perspective. That is, it’s based on the
viewpoint that our eye has while we’re on the surface of the Earth looking up at the
sky.
When we think about star brightness
this way, then sometimes appearances can be deceiving. For example, consider this
scenario. Let’s say that we’re on the surface
of the Earth looking up at the sky. And we see a couple of stars. As we look at these two stars, this
one here and this one much farther away, based on our perspective and our vantage
point, we make an assessment of which star looks to be brighter. From where we’re standing and
looking, it seems as though this closer star is giving off more light. And so we would naturally say that
this star is brighter than the one farther away.
We can see, though, that this
assessment of relative star brightness would change if our eye was positioned
somewhere else. For example, let’s say we were
somehow able to observe these two stars from over here. Well, in that case, this closer
star would now look brighter to our eye than the one farther away. And so we might conclude that this
star is the brighter of the two. So we can see that when our
assessment of star brightness depends on our perspective, the particular place from
which we’re looking at the stars, those assessments could disagree. From one perspective, this star
looks brighter. And from another, another one looks
brighter.
What we’d really like is an
assessment of star brightness that doesn’t depend on our particular viewpoint. The name for this is the absolute
magnitude of a star’s brightness. This measure of star brightness is
called absolute because it only depends on the stars themselves and how much light
they give off. It’s got nothing to do with where
or how we look at a star. Measured on the scale of absolute
magnitude then, we can get an objective reading of a star’s brightness. For example, the absolute magnitude
of the brightness of our Sun is a little bit less than five, 4.83. And note that this number has
nothing to do with how we look at the Sun, whether we’re near to it or far away,
whether we view it from an angle or straight on. This value indicates the intrinsic
brightness of the Sun in and of itself.
Now, an interesting thing about the
absolute magnitude scale is that it’s an inverse scale. This means that the lower our
absolute magnitude gets, the brighter a star it indicates. Consider, for example, two other
stars beside our Sun, Sirius and Betelgeuse. The absolute magnitude of the
brightness of Sirius is 1.4. And that of Betelgeuse is negative
7.2. And because of the way that this
measurement scale works, these stars are actually brighter than the Sun, even though
these numbers are lower. And of all three stars, Betelgeuse
is the brightest because it has the lowest absolute magnitude.
By the way, here we can see an
example where the absolute magnitude of a star’s brightness is a negative number,
which may seem confusing since we’re calling it an absolute magnitude. Keep in mind, though, that this
word absolute refers to the fact that this is a measure of the star’s intrinsic
brightness, rather than its brightness with respect to any particular vantage
point. So a lower absolute magnitude
number indicates a brighter star. This is one way of classifying
stars.
Another approach is to organise
stars by their temperature. Now, when we talk about the
temperature of a star, we need to be careful to indicate which temperature we’re
talking about. Recall that, for a large portion of
their lifetime, stars, in their core, have a process called nuclear fusion going
on. The conditions required for this
process tell us that the core of a star is often very hot. For example, the core of our sun is
at about 15 million degrees Celsius. A star’s core temperature can be
very different from the temperature of its surface. The surface temperature of our Sun,
for example, is thousands of times cooler than the temperature of its core. And when we see light coming from a
star, that light is indicative of the temperature of the surface, rather than an
interior layer of the star. All this to say, when we look at
the light that comes from a star and work back from that to solve for the
temperature of the star, we’re talking about its surface temperature.
Now, earlier on, we said that we
can tell something about the temperature of a star by looking at the color of the
light that comes from it. At first though, this idea may seem
a bit strange. After all, when we look up at the
night sky, don’t all the stars we see appear basically white? Does this mean they all have the
same temperature? That they’re all basically like our
Sun? Well, it turns out that this
perception, that all stars are white in color, has more to do with our eyes than the
actual light coming from the star. The way our eyes work, they’re much
more sensitive to brightness than they are to color. So if we see a very dim source of
light, like the light coming from a distant star, our eyes are able to tell if
there’s some light there. But we’re not able to see any color
in it. As a result, it looks like that
light is white.
The way to get around this
limitation of our eyes is to increase the relative brightness of these light
sources. One way to do that is to look
through an optical device, like a pair of binoculars or a telescope. When we do this and look at a
particular star, it’s not uncommon to see the light from that star, take on a color
other than white. So just like in the case of star
brightness, when we look at the color of light coming from a star, we need to be
careful to keep our perspective in mind. From now on, when we talk about the
color of light a star gives off, we’ll be speaking of the true color, the one that
doesn’t depend on our perspective. And when we know that true color,
we can tell something about the temperature of the star’s surface.
We can better understand this
connection between star temperature and star color by looking at the colors of light
our eyes can see. Recall that the name for this is
the visible spectrum. And we can remember further that
the visible spectrum is just one small chunk of what’s called the electromagnetic
spectrum, the collection of all possible electromagnetic waves. This spectrum is organised by
wavelength, where, towards one end, the wavelength is very small. And then, as we move towards the
other end, the wavelength gets progressively larger. Now, interestingly, if wavelength
gets larger as we move from right to left, then the energy of a wave gets smaller as
we move that direction. Or another way to say it,
wavelength and energy work in opposite directions. As one increases, the other
decreases and vice versa.
Now, this fact, that energy
increases as we go from left to right across the visible spectrum, is useful to
us. It means that if we pick two
different colors in the visible spectrum, say we pick yellow and green, then we can
tell which of these two colors indicates a more energetic wave of light. It’s green light because that color
appears farther to the right on the visible spectrum than yellow light. And if we picked another color, say
blue light, we could see that light of that color is more energetic than green or
yellow. And again, that’s because it’s
farther to the right in the visible spectrum. So we’re now seeing a connection
between the color that light has and the energy that it has. But then, where does the energy
that we’re talking about come from? Well, it comes from the star, which
is giving off light of some particular color. And the more energy that star has,
the higher its temperature will be. After all, temperature is an
expression of heat energy.
So the color of light coming from a
star tells us how energetic that star is, which tells us about its surface
temperature. This means that, along with
wavelength and energy as they vary in the visible spectrum, we can also write down
how temperature varies with color. So here’s what we’re seeing. Hotter stars, those with a higher
surface temperature, will have light that’s in the blue-to-violet-to-purple
range. And relatively cooler stars will
have light that looks red or orange.
At this point, we may wonder about
stars which look neither blue nor red in color. For example, what about our Sun,
which gives off white light? Well, we know that white light is a
combination of all the visible colors. So a star that truly gave off light
that was white would be one that has a temperature in the middle range between these
hotter, bluer stars and the cooler, redder ones. And so we see how we can connect
the wavelength of light coming from a star, that is, the color of light it gives
off, with the temperature of that star’s surface.
Knowing all this about classifying
stars by brightness and temperature, let’s get a bit of practice with these ideas
through an example.
Two stars, A and B, are known to
have the same brightness. When observed by an astronomer on
Earth, however, star B appears brighter than star A. Which star is farther away from
Earth?
Okay, so in this exercise, we have
these two stars, star A over here and star B we’ll say is over here. And an important fact about these
two stars that we’re told is that they have the same brightness. In other words, they give off the
same amount of light. But then, we’re told that when
these two stars are observed by an astronomer on Earth, they don’t appear to have
the same brightness. But rather, star B appears brighter
than star A. Based on this information, we want
to figure out which of these two stars is farther away from Earth.
On one level, the two bits of
information we have about these star brightnesses seem to contradict one
another. At first, we’re told that they’re
the same. But then, we’re told that star B
appears brighter than star A. What we need to realise though is
that these are two different assessments of the star brightness. The first one that tells us they
have the same brightness is a measure of what’s called the absolute magnitude of the
brightnesses of these stars.
When we talk about absolute
magnitude, we’re describing the brightness of a star in terms of the intrinsic
properties of the star all by itself. It’s got nothing to do with how the
star is looked at or perceived. So the first sentence of our
statement is telling us that, in an absolute sense, regardless of how these stars
are looked at, they have the same brightness. They give off the same amount of
light. But if we introduce an observer,
the astronomer on Earth, then it may not appear that these stars have the same
brightness.
Our statement tells us that the
astronomer, based on Earth, perceives star B to be brighter than star A. And this makes perfect sense if
star B is closer to the astronomer than star A is. In that case, more of the light
given off by star B, compared to the light given off by star A, would reach the
astronomer’s eye. So then, rather than these two
pieces of information about star brightness contradicting one another, they actually
give us more information about the space relationships of these stars to the
astronomer. Since star B appears brighter than
star A, that must mean that star A is farther away from the astronomer. And since the astronomer is on
Earth, that must mean that star A is also farther away from Earth than star B.
Let’s look now at a second example
exercise.
An astronomer looks at the light
coming from two stars: star A and star B. Star B is emitting bluer light than
star A. Which star is hotter?
Okay, so we have these two
stars. Star A we’ll say is over here and
star B we’ll say is over here. And we’re told that when an
astronomer looks at the light coming from both stars, the light from star B appears
bluer than the light coming from star A. The question then is which of these
two stars is hotter, that is, higher in temperature. Now, what we’ve been given in this
exercise is the information about the color of light that comes from these
stars. We want to take that information
and make an inference about the relative temperature of these stars. So what does the color of light
that comes from a star tell us about its temperature?
To figure this out, it will be
helpful to look at all the light our eyes are capable of seeing, called the visible
spectrum. For all the light in this spectrum,
we know that not only does that light have a color associated with it, but it also
has a wavelength. And that as we go from the right
side of the spectrum to the left side, that wavelength increases. But we know that, in general, for
waves of light as their wavelength grows gets bigger, the amount of energy carried
by that wave decreases. This means that while wavelength
gets bigger right to left on the spectrum, energy gets bigger left to right. In other words, the most energetic
visible waves of light are blue and purple, while the least energetic are red and
orange. So we see a connection between the
color of light and the energy that it has. But we want to find a connection
between color and temperature.
Thankfully, though, we’re not far
away because we can recall that the energy in these light waves is supplied by the
stars that emit the waves. And the more energy a star has, the
hotter that star will be, the higher its temperature will be. So just as energy increases left to
right across the visible spectrum, so does star temperature. This is to say that the hottest
stars give off light that looks blue or purple. And the coolest ones give off light
that looks red or orange.
And knowing that, we can now answer
our question. We know that star B is emitting
light that is bluer in color than that of star A. This means that the light coming
from star B is farther to the right on the visible spectrum than the light coming
from star A. And therefore, the light from star
B has more energy, which indicates a higher temperature of the star emitting that
light. Because the light coming from star
B is bluer than that from star A, that means that star B is hotter than star A.
Let’s summarise now what we’ve
learned in this lesson about classifying stars by brightness and temperature. Starting off, we saw that stars
vary in their brightness, how much light they emit, as well as their
temperature. And that these are two ways of
classifying or organising stars. We then saw that star brightness is
measured using absolute magnitude, an objective brightness scale where lower values
indicate greater brightness. And finally, we learned that star
color corresponds to star surface temperature, where bluer light indicates hotter
stars and red light corresponds to cooler stars.
