This right here is a picture
of Henrietta Swan Leavitt. And she made, a little
over 100 years ago-- this is in the early 1900s, while
working for Edward Charles Pickering, who was a Harvard
astronomer, while working for his observatory, she
made what is arguably-- well, definitely, one of the
most important discoveries in all of astronomy. And I would say it ranks their
top three, because it really enabled people like
Hubble to start realizing that the
universe is expanding or even being able to think
about how to measure distances to objects in space well
beyond the reach of our tools with parallax. We saw with parallax, you have
to have extremely sensitive instruments just to
even measure distances to stars relatively close to us. Very sensitive
instruments to get to stars maybe further
out into our galaxy. And we don't have the
instruments, even today, to measure things
beyond our galaxy. But because of
Henrietta Swan Leavitt, we were able to
approximate or get good senses of the distance
to objects beyond our galaxy. So let's just think
about what she did. So her job was literally
to classify stars in the Large Magellanic, I have
trouble saying that, Magellanic Cloud and the Small
Magellanic Clouds. And this is what they look like
from the Southern Hemisphere. This is the large,
right over here. And that this is the
small, right over here. And remember, this is before
Hubble realized that there-- or showed the world-- that there
are stars beyond our galaxy, or that there are galaxies
beyond our galaxy. So at this point in time,
people didn't even fully appreciate that these
were separate galaxies. We just said, hey, these
are kind of these blobs or these clusters
of stars that we see in the Southern Hemisphere. And just to get a
sense of where they are relative to our galaxy,
the Milky Way Galaxy-- this is obviously not
an actual picture. We can't take a picture
from this vantage point. This would have to be
very, very far away. But this is the
Milky Way right here. And this is the Small
Magellanic Cloud. And this is the Large
Magellanic Cloud. I'm getting better at saying it. So her job was literally just
to classify the different stars that she saw. But while she was
classifying, she looked at these things
called variables. And it turns out
what she was looking at were a class of stars
called Cepheid or Cepheid, Cepheid variable stars. And what's interesting
about them is two things. They're super-duper bright. They're up to 30,000 times
as luminous as the sun. And they're 5 to 20
times more massive than the sun, 5 to 20
times the sun's mass. But what makes them interesting
is one, they're really bright. So you can see them
from really far away. You can see these
Cepheid variable stars in other galaxies. In fact, we can see it
well beyond even the Small Magellanic Cloud or the
Large Magellanic Cloud. But you could see these
stars in other galaxies. And what's even more
interesting about them is that their
intensity is variable. That they become
brighter and dimmer with a well-defined period. So if you're looking at
a Cepheid variable star-- and this is just kind of
a simulation, a very cheap simulation-- it
might look like this. And then over the course of
the next three, four days, it might reduce in intensity
to something like this. And then after three, four days
again, it will look like this. And then it'll look
like this again. So it's actual intensity
is going up and down with a well-defined period. So if this takes three
days and then this is another three
days, then the period, one entire cycle of its
going from low intensity back to high intensity,
is going to be six days. So this is a six-day period. And what Henrietta
Leavitt saw, and this wasn't an obvious
thing to do, she assumed that everything
in each of these clouds are roughly the
same distance away. Everything in the
Large Magellanic Cloud is roughly the
same distance away. And it's obviously not exact. This is an entire galaxy. So you have obviously things
further away in that galaxy and things closer up. You have stars here and here. And their distance isn't going
to be exactly the same to us, even though we're sitting
maybe over here someplace. But it's going to be close. It wasn't a bad approximation. And by making that assumption,
she saw something pretty neat. If she plotted-- so let me
plot this right over here. So she plotted on
the horizontal axis, if she plotted the
relative luminosity. So really, the only way
that she can measure this is just how bright
did they look to her? And she's assuming that
they're same distance. So obviously, if you have a
brighter star, but it's much, much further away, it's
going to look dimmer. So if you assume that they're
all roughly the same distance, then how bright it
is will tell you how bright it is
at the actual star. So she plotted relative
luminosity of the star on one axis. And on the other axis,
she plotted the period of these variable stars. She plotted the period. And what I'm going
to do is I'm going to do this on a
logarithmic scale. So let's say this is in days. So this is one day. This is 10 days. This is 100 days,
right over here. It's a logarithmic scale because
I'm going up in powers of 10. If we take the log of
these, this would be 0, this would be 1,
this would be 2. And so that's what
I'm using as a scale. So I'm using the
log of the period or I'm just marking
them as 1, 10, 100. But I'm giving each of these
factors of 10 an equal spacing. But when you plot it on this
scale, the relative luminosity versus the period,
she got a plot that looked something like this. And this is obviously not exact. She got a plot that looks
something like this. It was a fairly
linear relationship when you plot the
relative luminosity against the log of the period. So this is obviously a
logarithmic scale over here. And so you could fit a line. And why, I'd argue and I
think most people would argue, this is one of the most
important discoveries in astronomy is if you
know-- because think about what the problem here is. We can look at all of
these stars in space. Let's say you look at
a fraction of the sky and you look at something
that looks like that. So it's really bright. And then you see something
dim that looks like that. So if you have a very
superficial understanding, you say, oh, this
star is brighter. You would say that this is a
fundamentally brighter star. But how do you know that? Maybe, instead of
being brighter, maybe it's just a
dimmer, closer star. Maybe this is a closer star. Maybe this is an
entire galaxy, but it's so far away that
you can't even tell. But all of a sudden, by the
work that Henrietta Leavitt did, if you see one of
these Cepheid variable stars in another galaxy, you
know its relative brightness compared to other
Cepheid variable stars. And so if you can place just
one of these Cepheid variable stars, if you know exactly
the distance to one of them, and then you know its
absolute luminosity, you then know the
absolute luminosity of any other Cepheid
variable stars. So let's say using parallax,
which is our other tool, we find-- let's say there
are some star in our galaxy. And let's say using
parallax we're able to come up with
a pretty good measure that it is, I don't know, let's
say it's 100 light years away. And this star is a Cepheid,
this is a Cepheid variable star. And let's say its
period is one day. It's one day. So we now know
something interesting. We know variable stars
with a period of one day, at 100 light years away,
will look like this, will look like this
drawing right over here. So if we later on, if we later
on see a Cepheid variable star with a period of one day,
so it gets brighter and dim over the course of one day and
maybe it's red shifted as well, but maybe it looks
a little bit dimmer. It looks like this. We now know that if it
was 100 light years away, it would have this luminosity. So based on how
much dimmer it is, we can then figure out
how much further away this Cepheid variable star is. If that confuses
you a little bit, I'll do a little bit more
details in the next few videos so we can get a closer sense
of how the math would work. But this was a big discovery. Just discovering this class of
stars, this Cepheid variable class-- she wasn't on the
one who discovered them. People knew before
her that there were these stars that
got brighter and dimmer. But what her big
discovery was is seeing this linear relationship
between the relative luminosity of these stars and their period. Because then, if we see
Cepheid variable stars in completely different
galaxies or galactic clusters, by looking at their
period we know what their real
relative luminosity is. And then we can guess how
far those things really are. We could estimate how far
those things really are.