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Birth of a Star
Stars are formed when swirling gases and dust in space reach what is called critical density and then are collapsed together by gravity. Gravity causes friction, and hydrogen molecules begin to fuse, forming helium and powering the star. A star will spend about 90 percent of its life cycle fusing hydrogen into helium, and during this phase it is called a main sequence star.
Red Giants and Supergiants
Eventually a star will burn up all the hydrogen at its core, and the core will begin to collapse as it burns through whatever other elements are remaining. At the same time the outer shell of the star will expand as it fuses its last stores of hydrogen. Because the hot, outer shell is expanding, the star is called a red giant, or supergiant for massive stars. Our sun, already 4.5 billion years old, will burn through its hydrogen and become a red giant in around 5 billion years. After gravity loses its hold, the outer shell of a red giant will move away from the core, leaving what is known as a white dwarf, a hot, white star.
Death of a Supergiant
Some supergiants become unstable as the fusion in their cores continues to burn elements other than hydrogen and they explode, causing a supernova. The explosion flings much the material making up the star into space, and what is left is a very small and very dense star called a neutron star. Neutron stars spin very quickly, and some emit light and radio waves as they spin, creating what looks like a pulse to astronomers. These stars are called pulsars. Some supernovas explode and then collapse in on themselves, creating black holes.
Twinkling Stars
Stars don't have an atmosphere, but when we view them either just with our eyes or through telescopes, we view them through Earth's atmosphere. The Earth's atmosphere is always moving, and this disturbance refracts the light from the star, making it look as if it twinkles or pulses.
A Star's Points
When you look at a photograph of a star or look at it through a telescope, it might appear to have four points aligned on a perpendicular axis, like a cross. But the star isn't shaped that way; the cross of light through a star is caused by light refracting off the supports in the secondary lens in the telescope or camera. The four points are called diffraction spikes.
The Shape of a Star
On May 31, 2007, the first image from the CHARA array telescope at the Mount Wilson Observatory in California showed the Altair star. It was oblong in shape due to its high rate of spin. Although stars don't come in predetermined shapes, gravity, rotation, proximity to other celestial bodies and their stage along the star life cycle can determine what they look like. On Oct. 2, 2008, scientists used NASA's RHESSI spacecraft to see how round the sun is. They found that although the sun is generally round, it does ripple and bulge as its core spins and reactions take place at its surface. The dynamic aspects of stars, and all celestial bodies, mean that they will never be constant. Stars can go from clouds of dust and gas to a black hole in the course of their evolution.