Stars

Stars are huge spheres of mainly hydrogen gas. Starlight comes from nuclear fusion produced in their cores. This energy is measured in visible light and in the high (UV and X-ray) and low (Radio waves) ends of the light spectrum.

Stars range in size from 1/10th to 100 times the sun's mass. The sun is about average physically, larger stars live short lives, smaller stars live longer.

Stars are born out of huge but tenuous clouds of hydrogen gas and dust. As areas of these clouds shrink into local densities the gas becomes dense enough for hydrogen to fuse into helium. The average star becomes stable and continues to "burn" for billions of years.

Stars usually form alongside other forming stars (one or more) and remain gravitationally bound. Multiple star systems are more common than single-star systems. About a 7/10 ratio.

Stars vary in mass, temperature (internal and external), and color. These physical properties change with time. Small stars may exist relatively unchanged for 12-15 billion years eventually fading into a cinder. "Average" mass stars live 8-12 billion years and eventually 'puff-away' their atmosphere to become "planetary nebula" (An old fashioned name), and later burn down into a cinder.

Some stars may "nova" which is a fairly violent event. Mass from one star is gravitationally transferred to a companion star and this star will blow this material into space one or many times.

Large mass stars end their short lives (10,000 years or more) by exploding nearly their entire mass. The remaining core will become a fast spinning neutron star.

Stars vary in brightness throughout their lives based primarily on their mass. Many vary in brightness by high amounts later in life. Some appear to vary from Earth's line of sight because a gravitationally-bound companion passes in front of the "primary star".

Star Clusters

Stars tend to form in loose clusters in the arms of galaxies. Many examples in our Milky-Way galaxy can be seen with the naked-eye or telescope. Clusters vary greatly in density, number and types of stars as well as age. The youngest exhibit hot new stars with some original gas gust surrounding them.

Globular Clusters

Some stars form in giant spherical masses of stars. These clusters are very old (as old as their galaxies) formations, probably 10-14 billion years. Often visible to the unaided eye from a very dark sight and many more are visible through a telescope.