Which definition describes a half-life?

the time required for half of a radioactive sample to decay.

the time required for half of a radioactive sample to become unstable.

the time required for all of a radioactive sample to change because of heat.

the time required for all of a sample to break down due to pressure change.

Look at the spectrums of a star moving towards earth and a motionless star. which of these is a correct inference that can be draw from the observation of the two spectrums? (2 points) the spectrum of a motionless star is difficult to be viewed separately using oridinary telescopes. the spectrum of a motionless star is identical to the spectrum of a star which moves towards earth. the spectrum of a star shifts towards the red region when the star moves towards earth. the spectrum of a star shifts towards the blue region when the star moves towards earth.