Surely you’d need one of those measurements to calculate the others?
How can they tell the difference between a ‘small’ object closer to us to a ‘large’ object further away from our limited vantage point?
In: Physics
Speed can be done for free by using the doppler effect. We know the exact spectral ‘fingerprint’ of certain elements like Hydrogen. For example Hydrogen always has a a strong emission line at 656nm. If an object is receding from us then its light gets redshifted proportionally to its velocity, so if this Hydrogen emission spectrum is shifted then we can measure the shift and therefore determine the velocity.
Size we can measure as an angular size fairly easily, just the size it is in the sky. But to turn this in to an actual measurement we do need the distance.
Finding distance is a lot more tricky and we’ve developed a kind of ladder of methods which work for different objects. For close stars we can use parallax, basically if you view a nearby object from two different vantage points it will move relative to faraway background objects (you can test this by holding up a finger in front of your face and then closing one eye, swap to the other eye and your finger has shifted relative to the room).
We can take advantage of the Earth’s orbit around the Sun to get two different perspectives, one from each side. Then we just do some trigonometry to find the distance to the star.
This works up to a few thousand light years.
For objects further than this we can use standard candles which are certain objects or events we know the intrinsic brightness of, things get dimmer as they get further away so by comparing this intrinsic brightness with the measured brightness we can find the distance. One example of a standard candle is a Cepheid variable star. This is a type of star which has a periodically fluctuating brightness, the period of this fluctuation is dependent on the intrinsic brightness of the star so by measuring the period we can find the brightness and thus distance. This is how it was discovered that the Andromeda Galaxy was a different galaxy and not just a nebula in the Milky Way.
Standard candles work for a lot of the objects in the Universe but outside our galactic cluster we need to use more mathematical methods, for example we can compare the rotation of galaxies to find their mass which gives us an estimate of their intrinsic brightness.
The furthest objects we can measure the distance to using redshift again, it turns out that the universe is expanding and the further an object is the faster it is receding from us. So for very very distant objects the speed of expansion is much greater than any relative velocity they have with us, that means we can measure their redshift and use it to estimate their distance.
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