Stellar Lifecycle: Death

   One day, stars will stop shining. The Sun will rise over the Earth, whatever state it may be in in five billion years, and it will have a final day as the Sun we know it. Low-mass stars like the Sun undergo fusion for a long time. In the later stages of their life when fusion has resulted in a core composed mainly of helium, the star will start to fuse hydrogen in the outer envelopes of the star. During this time, the star grows, it expands its diameter outward. When this happens in our own solar system, the Sun will swallow Mercury, Venus, and most likely the Earth. We will be on the inside of our Sun, our planetary surface either a burnt husk of what it was or the heat could melt the Earth and the planet simply dissipates into the newly formed red giant. Either way, no more Earth. But we will either be long extinct or long gone in the stars before that happens, so no worry. After this red giant phase, the star throws off its outer layers. The gas and plasma (super-heated, ionized gas) will be thrown outward to form a planetary nebula rich with newly formed elements. The core remains, and current stellar models suggest the remnant will be a dense white dwarf star. Over possibly trillions of years, this white dwarf will cool into a black dwarf, a truly dead star. It is a slow, rather boring process or these low-mass stars to die.

Artist's rendition of planets orbiting their white dwarf, the product of a once magnificent star

   High-mass stars have a far more exciting death. As mentioned in the previous post, these stars are massive enough to keep fusing up to iron. When iron is reached, heat is absorbed and the star collapses from its own gravity. The star implodes, fusing elements heavier than iron like uranium very quickly. Then, by a process still not completely understood, the star rebounds and creates a massive explosion in space. This explosion, called a supernova, is bright. Its luminosity is greater than the combined luminosity of all the stars in its own galaxy. In 1054 AD, a star's light that had gone supernova reached Earth and illuminated the night sky. It was bright enough to be seen in the daytime as well. These extraordinary events release gamma ray bursters, an event that would have catastrophic consequences for a planetary system near enough. Scientists believe Earth was hit by a gamma ray burster that rendered 99% of Earth's inhabitants extinct some time in prehistory, long before the dinosaurs. 

The image shown above is the Crab Nebula, it is the aftermath of the supernova witnessed in 1054 AD

   After the supernova, stellar cores of less than three solar masses become neutron stars. Basically a star composed of neutrons, they are incredibly dense. Their density is equivalent to sticking the mass of the Sun into the area of Manhattan. A teaspoon of neutron star material would weigh more than a battleship. Due to the law of conservation of angular momentum, the core rotates phenomenally fast, often time spewing material out of its magnetic poles. This is known as a pulsar and is akin to a cosmic lighthouse.

Pulsars are highly magnetized, so they have strong magnetic fields. The image shown is not what a real pulsar would look like

   The final fate for a stellar core of more than three solar masses is a black hole. Incredibly perplexing objects, it was only until the 70's before they were discovered in space. Black holes are a region of space-time that is so dense that light itself cannot escape the gravity. Everything has what is called an escape velocity. An escape velocity is the speed required to escape gravitational attraction. The Earth's is seven miles per second, meaning rockets that leave orbit must at least travel that speed. But a black hole's escape velocity is greater than the speed of light. If you will remember the posts about Einstein, it is impossible to travel faster than light, thus nothing escapes a black hole once it has passed through the event horizon. The event horizon is the point where the object chiefly becomes black. It is at this point that gravity is too powerful for light and photons are stuck in orbit around the black hole. At the center of a black hole is a singularity. A point where the laws of physics deconstruct and Einstein's theory of relativity fails. We do not yet know what occurs at the singularity, but it is one of the most sought out answers in physics. Black holes also grow as they "feed" on other objects. Their mass adds to the black hole's mass. The gravitational distortion of a black hole is so tremendous, that there is a point that to someone observing an object falling into the black hole, the object seems to stop. In fact, time itself seems to halt. But to the thing falling, time passes normally and it starts to stretch. Known as spaghettification, the objects is pulled apart, molecule by molecule, atom by atom, particle by particle and then is taken into the black hole's mass.

Artist's rendition of what a black hole looks like. Notice the distortion of the light around the object itself. This is called gravitational lensing and occurs when objects of high gravity distort light passing near them

   Stars form, stars live, stars die. One day, the last star will blink out of existence, but tonight when you look up and see just one, enjoy it. We are living during a period of the universe where stars are able to form and it will not last forever. Cherish the stars, for they died so their atoms created in their core could coalesce into you and I. We take stars for granted, but they fade into darkness one day. But enjoy them until then. 

   Hi all, I am going to be starting on quantum mechanics next post so we can build into subjects too complex to be explored with only relativity and classical mechanics. Ask questions about the post, criticize, praise, be indifferent to stars, whatever. Hopefully you come back and read. Subscribe to the posts and keep checking back for some quantum physics! Until then, salutations!