There is a method called "Astronomical Distance Ladder". But it is too complicate.
And there are some method that scientists use!
Parallax - is the only direct method that we have to determine distances to objects outside the solar system.
Open Clusters
Open clusters form the second rung on the distance ladder and they are used to bridge the gap between parallax and Cepheid variables. The key concept behind using open clusters is that the only variation in brightness and color (temperature) among stars in an open cluster is due to the different masses of the stars. All other effects - metallicity, age, distance - are not present.
Cepheid Variables
In short, Cepheids are stars that vary in brightness very regularly, and the time each variation cycle takes is directly linked to their absolute magnitude. Thus, by measuring the period of the cycle, astronomers know how bright the star really is, and they can then calculate how far away it is based upon how bright it appears at Earth.
RR Lyrae Variables
Much like Cepheid variable stars, RR Lyrae variable stars pulsate with a period directly related to their absolute luminosity. These stars are red giants of similar or slightly less mass than the sun, and so they are generally much fainter than Cepheids. Because of this, they are useful only out to globular clusters in our own galaxy, and they cannot be observed in other galaxies. Consequently, they are useful in calibrating the globular cluster luminosity function (discussed below) and provide a link between the parallax method and the globular clusters.
Type-Ia Supernovae
