A pulsar is a spinning neutron star that appears to give off radio waves
The Doppler Effect has helped astronomers determine the distances of stars from Earth.
a protostar reaches a temperature high enough for nuclear fusion to begin
The size. The gaseous Jovian planets are far larger than the inner terrestrial planets.
Red stars have the coolest surface temperature. Blue color stars have the highest surface temperature. The Sun belongs to the main sequence stars.
It's Alto which usually means high but when describing clouds it means the middle.
On the main sequence, red dwarfs are the least massive.
Astronomers conclude that the universe is expanding, because wavelengths are increasing.
These are called refractory telescopes.
Reflectory telescopes use mirrors to focus the light.
No. They do not have enough mass to become black holes. Depending on the mass they will either become white dwarfs or neutron stars.
Helium is the product of nuclear fusion in stars. The stars generate its energy by nuclear fusion of hydrogen nuclei into helium.
I think you are referring to a phenomenon called Nutation.