This telescope is 2 and a half feet long.
How do I know? I used a tape measure.
I was just looking at the Andromeda Galaxy.
It’s 2.5 million light
years from Earth.
How do I know? Certainly not a tape measure!
Here, let’s talk about finding interstellar
distances.
To determine the distances to nearby stars,
astronomers use parallax, the effect where
an object seems to change position depending
on where it is viewed from. We humans use
it every day using our two eyes.
Notice how from my perspective, the tractor
appears to change position while the background
remains still. The same concept can be used
with stars. Using telescopes, we take a picture
of the star, and then we take another picture
of the star 6 months later when the Earth
is at the other end of its orbit. By measuring
this change in the star’s apparent position
while keeping the background stars stationary,
we can triangulate the distance to the object.
While this method works for stars that are
relatively close, most stars are simply too far away for the
apparent change in position to be measurable.
Farther out, the methods of finding distance
all revolve around the concept of the standard
candle.
Standard candle doesn’t refer to an actual
candle, it refers to an astronomical object
that has a known luminosity.
When it's close to me, this candle it’s actually
pretty bright. However,
*struggling to blow out candle*
When it's moved away from me, it gets significantly dimmer.
In fact the brightness of an object
decreases with the square of the distance.
And while it looks dim to me, I know that
the candle itself is still producing the same
amount of light that it did before. It has the same luminosity.
So, by knowing the luminosity of an object,
and it’s apparent brightness,
we can determine our distance to it.
The catch here is that we need to know the
luminosity of the object, and the method of
determining luminosity depends on the object
in question. For example, cepheid variable
stars periodically change in luminosity. Their
luminosity at any given time can be determined
by the period of the oscillation. For other
stars, we can determine their luminosity simply
by observing the spectrum of light they produce,
and comparing this to the spectrum of stars
of a known luminosity.
Using the appropriate method, we can
find the approximate distance to virtually
any object in the universe.
Measuring interstellar distances may not be
as glamorous as some other topics in astronomy,
but it is certainly one of the most important.
Without it, we would never have discovered
dark energy and the accelerating expansion
of expansion of the cosmos. Evidence for theories
regarding everything from the big bang to quasars would be far less concrete. Being able
to accurately determine astronomical distances
has played a fundamental role in helping us
understand the size and structure of the cosmos.
