Chapter 14.2 The Evolution of Stars

Introduction Newton’s law states that gravity is an attractive force that exists between all masses, and this force gets stronger the closer the objects become. Gravity is the force that helps to create and build stars, but it also is the force that eventually causes them to die.

Nebulae These are vast clouds of gas and dust that scientists speculate are the birthplace of stars. Inside nebulae gravity acts to pull “inter-stellar” material together. As gas accumulates, the temperature inside center of the nebula rises. When the temperature reaches about 10 million degrees, nuclear fusion begins, and a star “turns on”.

This is a Nebula: This image composite compares visible-light and infrared views from NASA's Spitzer Space Telescope of the glowing Trifid Nebula, a giant star-forming cloud of gas and dust located 5,400 light-years away in the constellation Sagittarius.

Main Sequence Stars When fusion begins, stars consume hydrogen as fuel. Helium begins to accumulate in the center of the star. The star continues to heat up, increasing pressure and temperature. Gravity balances the forces, keeping the star stable. All main sequence stars are stable (like our sun). It eventually changes to another phase.

Low Mass Stars These “red dwarf” stars consume their hydrogen slowly, perhaps over as long as 100 billion years. They lose a lot of mass during this time. They essentially evaporate into a white dwarf.

Intermediate Mass Stars These stars (like our Sun) will burn a little slower, consuming their hydrogen in about 10 billion years. When it burns out, the core will collapse, and the exterior will expand greatly, sometimes up to 100 times as big! AT this point it is much cooler, and appears red. Our sun will do this in about 5 billion years, and expand past the orbit of Mars.

Intermediate Mass Stars 2 The outer layers of these stars will then peel away, leaving a fuzzy ball called a “planetary nebula”. Over time they just dissolve into space, with the cooling core becoming a white dwarf. Finally, completely burned out, they become a black dwarf.

Massive Stars These stars burn so fast, they expand to huge sizes. They burn very fast compared to other stars, some only 7 million years. When the core of these stars transform into iron, it takes only 1 day. This is called a “supernova” which is a huge, spectacular explosion.

Neutron Stars & Black Holes “Neutron stars” are the remaining cores of supernovae. They have massive gravity crushing them to only 10-20 km in diameter. “Black Holes” are even more dense. They have so much gravity that not even light escapes. They appear as a black dot of “nothing” in space.