This post was edited by Hinge at 2017-5-7 16:54|
If this article is too long for you, enjoy the pictures.
(by Ethan Siegel from Forbes)--
This is the Milky Way from Concordia Camp, in Pakistan's Karakoram Range. While many of the stars seen here may have already died, their stellar remnants continue to shine on.
(Anne Dirkse / http://www.annedirkse.com)
Ever since the first star in the Universe ignited some 13.7 billion years ago, the Universe has been flooded with light. When enough matter — mostly hydrogen and helium gas — gravitates together into a single, compact object, nuclear fusion will take place inside the core, giving rise to a true star. But as time goes on and fusion continues, eventually that star will run out of fuel. Sometimes, the star is massive enough that additional fusion reactions will take place, but at some point, it all must stop. When those stars finally die, however, their remnants shine on. In fact, the Universe hasn't been around long enough for even a single remnant to stop shining. Here's the story of how long we'll need to wait for the first star to go dark.
It all begins from a cloud of gas. When a cloud of molecular gas collapses under its own gravity, there are always a few regions that start off just a little bit denser than others. Every location with matter in it does its best to attract more and more matter towards it, but these overdense regions attract matter more efficiently than all the others. Because gravitational collapse is a runaway process, the more matter you attract to your vicinity, the faster additional matter accelerates to join you.
Dark, dusty molecular clouds, like this one within our Milky Way, will collapse over time and give rise to new stars, with the densest regions within forming the most massive stars.
While it can take millions to tens of millions of years for a molecular cloud to go from a large, diffuse state to a relatively collapsed one, the process of going from a collapsed state of dense gas to a new cluster of stars — where the densest regions ignite fusion in their cores — takes only a few hundred thousand years.
Stars come in a huge variety of colors, brightnesses and masses, all of which are predestined from the moment of the star's birth. When you create a new cluster of stars, the easiest ones to notice are the brightest ones, which also happen to be the most massive. These are the brightest, bluest, hottest stars in existence, with up to hundreds of times the mass of our Sun and with millions of times the luminosity. But despite the fact that these are the stars that appear the most spectacular, these are also the rarest stars, making up far less than 1% of all the known, total stars, and also the shortest-lived stars, as they burn through all the nuclear fuel (in all the various stages) in their cores in as little as 1–2 million years.
Hubble space telescope of the merging star clusters at the heart of the Tarantula Nebula, the largest star-forming region known in the local group. The hottest, bluest stars are over 200 times the mass of our Sun.
[NASA, ESA, and E. Sabbi (ESA/STScI); Acknowledgment: R. O'Connell (University of Virginia) and the Wide Field Camera 3 Science Oversight Committee]
When these brightest stars run out of fuel, they die in a spectacular type II supernova explosion. As this occurs, the inner core implodes, collapsing all the way down to a neutron star (for the low-mass cores) or even to a black hole (for the high-mass cores), while expelling the outer layers back into the interstellar medium. There, these enriched gases will contribute to future generations of stars, providing them with the heavy elements necessary to create rocky planets, organic molecules, and in rare, wonderful cases, life.
When the most massive stars die, their outer layers, enriched with heavy elements from the result of nuclear fusion and neutron capture, are blown off into the interstellar medium, where they can help future generations of starsby providing them with the raw ingredients for rocky planets and, potentially, life.
[NASA, ESA, J. Hester, A. Loll (ASU)]
You don't have to wait long for a black hole to go dark. In fact, by definition, black holes go "black" immediately. Once the core collapses sufficiently to form an event horizon, everything inside collapses down to a singularity in a fraction of a second. Any remnant heat, light, temperature, or energy in any form in the core simply gets converted into the mass of the singularity. No light will ever emanate from it again, except in the form of Hawking radiation, when the black hole decays, and in the accretion disk surrounding the black hole, which is constantly fed and refueled from the surrounding matter.
But neutron stars are a different story.
Forming from the remnant of a massive star that's gone supernova, a neutron star is the collapsed core that remains behind.
You see, a neutron star takes all the energy in a star’s core and collapses incredibly rapidly. When you take anything and compress it quickly, you cause the temperature within it to rise: this is how a piston works in a diesel engine. Well, collapsing from a stellar core all the way down to a neutron star is maybe the ultimate example of rapid compression. In the span of seconds-to-minutes, a core of iron, nickel, cobalt, silicon and sulfur many hundreds-of-thousands of miles (kilometers) in diameter has collapsed down to a ball just around 10 miles (16 km) in size or smaller. Its density has increased by around a factor of a quadrillion (10^15), and its temperature has grown tremendously: to some 10^12 K in the core and all the way up to around 10^6 K at the surface. And herein lies the problem.