Thanks to Squarespace for sponsoring today’s video.
You may have wondered, with the vast distances between objects in space, and with the lack
of a tape measure that big, how can scientists be sure they know the distances between us
and other celestial bodies?
Well, there are a few methods available depending on how far away the object is, each with varying
degrees of accuracy.
The first method is the most accurate, and in fact gives us very precise measurements.
This is the parallax method.
For those of you that don’t know what the parallax effect is, it is where nearby objects
appear to move more compared to objects far away, as you travel parallel to them.
For instance, when you look out of a side window in a car.
Everything close whizzes by, but objects in the background stay reasonably still.
How does this equate to measuring the distance between stars?
Well, Earth orbits the Sun, taking 6 months to reach from one side to the other.
Scientists can look at a star, and record its position compared to distant stars beyond
it.
In six months’ time, scientists can again record where the star is.
Because we know the diameter of Earth’s orbit is roughly 300,000,000km, using simple
trigonometry we can work out the distance to the star.
Is the star close to us?
Then the differences in its apparent location will be much bigger.
Is it further away?
The star’s position will only change very slightly because the angle is much smaller.
This method works up until a distance of about 400ly, as beyond that the change in its apparent
position can’t be measured anymore.
Earth’s orbit would have to be a lot bigger before you could use this method for farther
distances.
Which is unfortunate because most things in space are further than 400ly away from us.
But thankfully there are very clever scientists out there that have come up with another method
to judge distances, without having to use trigonometry, although it should be mentioned
that this method is slightly less accurate, it’s just simply the best we’ve got.
It seems that stars tend to follow a pattern, which can be seen on this chart.
Main sequence stars, which make up the majority of stars in the universe, can all be found
somewhere along this band.
Their temperature corresponds to their colour, and most importantly, their brightness.
Using stars that have a confirmed distance thanks to the parallax method, we can see
how much a star dims due to the distance between it and us, and extrapolate that far beyond
400ly.
So, say we see a very blue star that we want to know the distance to.
Once we know the stars precise colour, we can tell how bright it would be if it were
right next to us.
We can then measure the star to see how bright it is from our perspective.
Combining this with our extrapolated data, we can predict how far away the star really
is.
Obviously though, there is some margin of error in our predictions, this band is quite
thick after all, not a precise thin line.
So, these two methods would work for stars in our own galaxy, where they are still close
enough to be resolved individually.
But what about other galaxies?
How do we measure the distances to them?
Well, we would have struggled, were is not for the universe being kind to us by gifting
us Cepheid Variables.
A Cepheid Variable is a very special type of star that changes in brightness periodically
depending on how bright it is.
And some of these stars are very bright indeed, so much so that the changes in brightness
can be detected by us all the way in a different galaxy.
Timing the pulses in a Cepheid Variable, we can know exactly how bright the Cepheid Variable
should be, and how dim it is to us, allowing us to use the extrapolated data again to work
out the distance to it, and to the galaxy it resides in.
Beyond that, it gets much more complicated and less accurate.
With galaxies billions of light years away, you have to start taking into account the
expansion of the universe and redshift, and the distances makes seeing the galaxies at
all very difficult, as they require long exposure photos, which will blur the variations in
light produced from Cepheid Variables, assuming they are bright enough to be seen at all.
However, the universe has given us one more measuring stick to work with, Type Ia supernovae.
These are a very specific type of supernova, where in a binary star system a dense white
dwarf starts to cannibalise a larger red dwarf.
Once the white dwarf hits a critical mass, the star becomes unstable and undergoes a
runaway nuclear fusion reaction, producing an extremely bright event that can rival an
entire galaxy in brightness.
Because these supernovae always happen to a white dwarf that hits a very narrow range
of mass, the rise and fall of their brightness is very predictable, and given they are very
bright events, they have been used to measure distances of up to 13.2 billion light years.
However, there aren’t always these types of supernova going off, which means if scientists
want to use this method, then they need to be patient.
There are a few other methods out there too, but this covers the main ones.
To me, these are incredibly clever methods to measure distances, and credit to the scientists
who came up with them in the first place.
The only sad thing about realising how far away everything is, is knowing how hard it
will be to cover those distances should we ever want to explore beyond our solar system.
Here’s hoping for more bright minds to come to the rescue in a few generations’ time!
Thanks again to Squarespace for sponsoring this video.
Squarespace gives people a powerful and beautiful online platform from which to create your
own website.
I’ve been having a go with their platform, and I like the fact that you can integrate
videos and audio seamlessly.
They also have a mailing list and email campaign feature to get your message across to your
subscribers that matches the voice and style of your website.
So, if you’re looking to build a website, give it a go!
If you use the link squarespace.com/astrum you can try it out for free and get 10% off
your first purchase.
Thanks for watching!
If you liked this video, subscribe, and check out the other Astrum Answers videos in this
playlist.
Want to help pick the next question?
Become a Patron or Member and join in the polls.
Links are all in the description!
All the best and see you next time.