# How do we know how far away a star is?

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How do we know how far away a star is?

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Thinking back 15 years ago to Astonomy 101, but I think they measure the parallax, and red/blue shift in its spectrum.

Parallax is the apparent shift in position of an object due to a shift in the position of the observer. If you open and close your eyes one at a time, and see how things appear to move, that is parallax.
measuring the apparent MOA (minutes of angle) shift of a celestial body when observed from two different points (as long as we know the distance beyween them) enables us to calculate the distance to the celestial body.

There’s a bunch more math to account for Earth’s rotation and orbit, and I think spectrometry plays a role as well, but IIRC the parallax is the fundamental mechanic that enables us to judge distance from earth.

There are several ways:

When measuring relatively close stars, we use parallax measurement. Take an accurate measurement of a stars position, and wait 6 months. Take that measurement again. Just like when you close one eye and then the other, the apparent location shifts because of the changing viewing angle. We can use the known diameter of the orbit of the earth to figure out the stars distance.

There are certain types of objects which are referred to as “standard candles”. These are objects or events which, thanks to their physical properties (which have been mathematically figured out), we know their intrinsic brightness, or absolute magnitude.

Type 1a supernova are caused when a binary system has one large star and one small. The large pulls matter off the small, and at a very predictable amount it all ignites in nuclear fusion. This always happens with the same amount of mass, so the brightness is always the same. By comparing that with the apparent brightness, we can find the distance.

Cepheid variable stars are stars which fade in and out, the time it takes is directly related to the maximum brightness. Time them and you can see how bright the absolute magnitude is, and again compare to the apparent brightness.

There are many other methods, using gravitational effects, spectroscopy, Doppler effects etc etc but these are the main ones.

For objects that are relatively close astronomers use a method based on parallax. That is done by measuring the viewing angle to the star from a point on Earth, or an orbiting telescope, then waiting a period of time, often 6 months, and measuring it again. The difference in that angle lets them calculate the distance. The bigger the distance of the telescope between the first and second measurements allows us to measure objects that are farther away. 6 months is the time for the Earth to be on the opposite side of the orbit of the sun, and us the farthest away we can get the two points. This method only works for objects about a maximum of 400 light-years away.

For objects even farther away, where parallax angles aren’t measurable, they commonly look at the spectrum of light emitted by the star. From that they can determine about how bright the star really is. By comparing that to how dim it looks from Earth they can get an approximation of the distance.

For very distant objects like far away galaxies they can also use red-shift to get an approximate distance. The farther away something is, the more the light from it is shifted to the red end of the spectrum.

We can now use direct measurements for things very close. Radar, and lasers can be used to measure their distance.

A variety of methods. It all depends on how far away the star is.