Ido not understand this ¯\_(ツ)_/¯

we know that the electromagnetic spectrum uses wavelengths and frequencies to determine a lot about outer space. how does it us find out the make-up of stars?

The answer is through light.

Light is unique it carries information it has no mass and it is insanely fast.

But what is light?

You can imagine it as a stream of photons (elementary particles) traveling from its source to surface of an object with a speed of light where they bounce of your eye. That very moment the information of a single photon in a form of energy is transmitted to your eyes and later on to your brain which processes the gathered information and constructs an image.

When you turn on the light you know that all the photons are coming from it the same thing is with nonanthropogenic light (natural light) we get about 99% from the sun and about 1% from the moon at night.

In the core of our sun elements are fused to make up heavier elements in this process energy is released (E=mc^2, where E is energy, m mass of a single atom fused inside star and c^2 the square of speed of light) in a form of photons and the type of photon (its frequency) determines if it will be visible to our eye and also determines the type of luminosity of stars.

Also if you were to track every single bit of information carried by any photon any time you would unravel the unfolding of the universe from the big bang to today.