A Side of Science: Measuring the speed of things

In my latest video, I talked about how the Gaia mission discovered some “Milky Way interlopers” in our galaxy that were moving differently than expected to if they had been born in the the galaxy. How are these velocities measured, though?

Credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO

There are two aspects to measuring velocities of objects in space – there’s how fast they’re moving toward or away from us, called “radial velocity,” and how fast they’re moving in the plane of the sky, called the “proper motion.”

It’s actually pretty easy to measure the radial velocity of stars – that is how fast they’re moving toward or away from is. This is generally done by measuring what’s called the redshift of the stars. What does that mean?


When you heat up an element – think hydrogen, helium, or carbon – it emits light at very specific wavelengths. These “emission lines” form a unique pattern for each different element. It’s kind of like a fingerprint for that element. If you see a specific pattern, you know what element you’re looking at.

If you have a star that has hydrogen in its outer layers, we see the pattern of light we’d expect from hydrogen. However, if that star is moving with respect to us, then that pattern of light will be shifted in wavelength from what we’d expect to see. That shift is directly related to the speed of the star! That’s how we can get the radial speed – by measuring that shift.

Credit: NASA, ESA, K. Sahu and J. Anderson (STScI), H. Bond (STScI and Pennsylvania State University), M. Dominik (University of St. Andrews), and Digitized Sky Survey (STScI/AURA/UKSTU/AAO)

What about the proper motions?

Watching the stars move takes a lot of time, or a very high level of precision in position measurements. The image above shows how much our nearest neighbor star, Proxima Centauri, appeared to move across the sky in about 40 years. Stars that are even more distant will appear to move even less.

The first step is to measure the distance and position of the star in the sky. This is done using a method called parallax. Whether you know it or not, you’ve experienced parallax.

Try this: extend your arm with your pointer finger up. Close your right eye and line your finger up with something – the corner of the room, the edge of the TV, or something else in the distance. Then, without moving your arm, open your right eye and close your left eye. Your finger will appear to move relative to that object you lined it up with. If you know the distance between your two eyes and the angle of the apparent change in position, you can find out the distance to your finger.

Image Credit: NASA/ESA, A.Feild/STScI

Applying that to stars, astronomers measure the apparent position of a star against more-distant background objects (stars or galaxies), and then measure again to see how much they have appeared to move. To maximize the apparent change, they use the Earth’s orbit to their advantage – measuring the star at one point in the Earth’s orbit and then again six months later. The physical distance between the measurements is the diameter of Earth’s orbit.

That’s just measuring distance. Then, to determine the motion, the measurement needs to be repeated several times to find the change in position.

Image Credit: ESA/ATG medialab; background: ESO/S. Brunier

The Gaia mission’s primary purpose is to make these distance and velocity measurements. The last mission to do this was called Hipparcos, and Gaia represents an improvement in accuracy of about 200 times. According to the ESA website, “if Hipparcos could measure the angle that corresponds to the height of an astronaut standing on the Moon, Gaia will be able to measure his thumbnail!”

And, in case you missed my Milky Way Interlopers video, be sure to check it out.

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