What can change the expansion rate of the Universe over time? O Gravity: Attractive forces increase the expansion rate by pulling mass together making gravity stronger. O Gravity decreases the expansion rate, while electromagnetic forces are always repulsive and speed-up expansion. O Attractive forces such as gravity decrease the expansion rate. Repulsive forces would speed-up expansion by pushing spacetime apart. O Nothing, since the expansion rate remains always constant, and it is measured from Hubble's law.
Stellar evolution
We may see thousands of stars in the dark sky. Our universe consists of billions of stars. Stars may appear tiny to us but they are huge balls of gasses. Sun is a star of average size. Some stars are even a thousand times larger than the sun. The stars do not exist forever they have a certain lifetime. The life span of the sun is about 10 billion years. The star undergoes various changes during its lifetime, this process is called stellar evolution. The structure of the sun-like star is shown below.
Red Shift
It is an astronomical phenomenon. In this phenomenon, increase in wavelength with corresponding decrease in photon energy and frequency of radiation of light. It is the displacement of spectrum of any kind of astronomical object to the longer wavelengths (red) side.
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