Measuring Cosmic Distances
The Unimaginable Scale of Our Universe
Most of us can Google how far our favorite pizza place is and how long it would take to deliver to our home. We can also find out how long it will take to get to our favorite beach. But if we zoom out of our daily lives, scales start to grow quickly. It took the Apollo 11 crew 3 days to travel the 240,000 mile journey to the Moon. It took the New Horizons spacecraft 9 years, traveling at 36,000 miles per hour, to cover the 3.7 billion mile journey to Pluto.
That far out, it’s easier to measure distances in the Astronomical Unit(AU) — which historically was the average distance between Earth and the Sun. On the AU scale, Pluto is 39 AU away from the Sun. Voyager 1, our furthest messenger to the cosmos, is at the distance of 163 AU. It has effectively left our Sun’s field of influence.
No human has gone that far out. Only our robotic ambassadors and our imagination has traversed interstellar space.
As far out as that sounds, these distances are minuscule compared to the distances between stars and galaxies. The nearest star outside of our solar system is Proxima Centauri. It’s at a distance of about 268,770 AU. Which makes measuring in AU not very useful as cosmic distances grow. We have to switch our scale to the one that measures distances in light-years. A light-year is the distance traveled by light in one year. Light travels at an incredible, but constant, speed of 186,000 miles per second. So with that, the distance to Proxima Centauri becomes a more manageable 4.3 light-years.
This also means that the further out you look, the further back in time we’re looking. This is because light travels at a constant speed. So when we look at the Sun, we are looking at it as it was 8 minutes ago because it takes 8 minutes for the Sunlight to reach us.
How do we measure these vast distances?
Instead of a ruler, astronomers use a “cosmic distance ladder” to determine the best tool for the job. Which simply means that there’s no single way to measure these unimaginable cosmic distances. The specific method depends on how far the object is from us. Here are the various steps of the ladder —
Radar for object in the solar system
We can bounce light waves off objects in our solar system to get their distances. Since light waves (radio, infrared, visible) travel at constant speed, we can measure the time it takes for the response to come back and estimate distances. We can precisely measure the distance to our Moon by bouncing a beam of light from mirrors left on the Moon. That’s how we know at what rate the Moon is drifting away from us. This is also how we can now plan to do a more precise landing on the Moon.
Parallax for nearby stars
For nearby stars, at about 100 light-years away, this is a great way to measure their distances. This almost becomes obvious when you try this method on your own. Extend your hand out and hold your thumb pointing up. Now look for some object further away from you; clock, chair, window. Look through one eye and then the other. You’ll notice that the object behind your thumb appears to move as you switch looking through each eye.
Similarly, Earth is orbiting around the Sun and its relative motion to the far away star allows us to see that star from different positions in space. This method works because the stars are relatively far out compared to Earth’s orbit around the Sun. It’s similar to your eyes seeing the change in relative position of object in your house.
Cepheids for nearby galaxies
Cepheids are special stars that brighten and dim periodically. Henrietta Swan Leavitt discovered that the star’s brightness and the period between dimming & brightening again were related. Specifically, the brighter the star, the longer its period of dimness. So if you can measure the period, you can get the star’s brightness. Which indirectly gives us its distance as brightness reduces when distances increase. Cepheids became cosmic lighthouses for astronomers to measure distances at a few million light-years.
This discovery laid the foundation for modern astronomy and later helped Edwin Hubble establish that our Universe was made up of other galaxies when he studied a Cepheid star in the nearby Andromeda Galaxy.
Supernovae for distant galaxies
At even larger distances, Cepheids appear too dim and the parallax effect becomes too small to be useful for measuring distances. Astronomers then rely on another dependable and bright source of light — the Supernova. A massive star can go supernova at the end of its life. This explosion isn’t great for nearby life, but is a great source for dependable measurements as their brightness can be used to calculate distances to its host galaxy.
Specifically, a supernova of type Ia has proved to be a dependable cosmic candle for astronomers. Supernova explosions are also the source of heavier elements like zinc, silver, tin, gold, mercury and lead that are found on Earth.
Redshift for even distant objects and the early Universe
For distances of more than a billion miles, astronomers have to rely on redshift in light. Edwin Hubble, after destroying the single galaxy theory, then got rid of the static Universe theory. Not only are humans not at the center of the Universe (there is no such place), but we are also not living in a static Universe.
The underlying principle for redshift can be easily understood by the doppler effect of sound waves. A car horn coming towards you will sound at a higher pitch than when it’s speeding away from you. A similar effect is happening with light waves — they get red-shifted when the object emitting them is moving away from us and blue-shifted when it’s moving towards us.
Hubble found that other galaxies were not only moving away from us, but with greater speeds the further away they were from us. The exact reasons for this accelerated expansion are still unclear, but it is confirmed by recent observations from our space telescopes. This is one of the mysteries keeping Astrophysicists awake.
So greater the redshift, further away is that object. Greater blueshift means the object is getting closer to us.
Astronomers are always looking for ways to improve these measurements by building new telescopes, creating more precise star maps and constructing more innovative techniques. From studying nearby stars to taking pictures of the early Universe, this is an active field of development.
It’s a wonderful example of human ingenuity using physics and mathematics as tools for understanding our Universe!
