Eli5: how do we know the properties of things in space like stars and planets if we’ve never been on them?

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Eli5: how do we know the properties of things in space like stars and planets if we’ve never been on them?

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We use the scientific model. Also depends on what properties? Luminosity, mass, velocity, density, composition, distance, magnetic field, spin, etc. Everything uses a different form of the scientific model. We use technology and theories to determine properties of such objects.

We measure them from a safe distance. We’ve only been on the Moon. Everyplace else in the Universe is only known through distant observation or near observation by robotic space probes.

That said, you don’t measure the temperature of lava in a volcano by going to the lava either. From a safe distance you make your measurements because lava is not human-friendly.

We measure mostly the light we receive. From position of the peak intensity in the spectrum we can derive the temperature, combined with the color can give us the current state the star is in.
From specific lines in the spectrum we can derive what element is present and have further hint ln the stars current state. For observing periodically changing brightness we can see if it is a special type of star or if it has a planet in orbit.

From this period we can estimate the orbital speed of the planet. If we can see the star move, we can derive the mass of the planet.

All of this is based on physics that habe been around for a long time. Gravity, Thermodynamics, Electrodynamics, Plasmatheory. Even Quantummechanics is needed for the explanation of why a star shines.
Most of this is theoretically know and verified by experiments in labs. And because physics is universal, laws that work here in labs also work anywhere in the universe and can tell us all the wonderful things.

Most things we know about distant objects are found out by studying light.

There are two main pieces of information we can receive from light: brightness (intensity) and color (wavelength).

Brightness tells us the amount of light that reaches us from distant objects such as stars and galaxies. If every star gave off the same amount of light, we would know that brighter stars are closer to us, and dimmer stars are farther from us. However, it turns out that there are many different types of stars, some giving off much more light than others, so if we want to know the distance to a star, we need to measure that separately. For relatively close stars, we use a method called “parallax”, in which we measure how much the star moves in the sky as Earth orbits the sun. Just like when you look out the window while driving, closer objects appear to move by faster than more distant ones.*

Some stars change in brightness over time (called “variable stars”). This can be due to activity within the star itself, which we can use to study how stars change over time. Changes in brightness can also be due to a planet (or another star) passing in front of the star. This is one way we can tell whether or not a star has any planets. By how often these changes in brightness occur, we can tell how long it takes a planet (or star) to orbit (another) star. The properties of these orbits can give us information about the masses of the objects involved. Occasionally, we can observe cataclysmic events such as supernovas, which involve an extreme change in brightness in a short time.

Color tells us the specific wavelengths of light that are emitted from distant objects. Not just the colors of the rainbow; stars and galaxies emit colors that the human eye can’t perceive, such as the more familiar infrared and ultraviolet, as well as microwaves, radio waves, x-rays, and gamma rays. From physics experiments, we know that there is a direct relationship between surface temperature and the distribution of colors emitted by an object. From this, we can determine the surface temperature of distant objects. We also know from physics experiments that different types of atoms absorb (or emit) specific colors at certain temperatures. This can tell us what types of atoms are found within the object. A list (often visual) of the colors being emitted by an object is called its “spectrum”.

Although light still remains the primary way to observe the cosmos, we can also study distant objects through cosmic rays and neutrinos (subatomic particles originating from our sun or other star systems), and, as of the 2015 detection from LIGO, through gravitational waves (ripples through spacetime itself).

*Extra: Parallax only works well in our own stellar neighborhood. To measure the distance to farther places, we use objects or events that are known to always give off the same amount of light, known as “standard candles”. One example of a standard candle is a certain type of supernova. If we observe this type of supernova in a distant galaxy, we can compare our measurements of its brightness to how much energy we know it gave off in order to determine the distance.

Because those same properties exist here.

There are lots of way to test a matter. One of which is light. If hydrogen is blue, then the blue light shining off a distance planet or sun implies that it has hydrogen.