Stellar Evolution

cannot be determined exactly, but it was roughly 6 billion years ago.

Main Sequence

No they can't sleep because thet are not like us

Yes, a brown dwarf is a star that failed to ignite hydrogen fusion because it did not have enough mass for a strong enough gravitational collapse.

Brown dwarf stars glow dimly with residual heat for a very short time.

More than 74% of the matter in the universe is hydrogen, so when a star forms and collects surrounding matter most of that matter will be hydrogen.

It is a simple matter of the availability of different elements.

It is iron.

The endpoints of stellar evolution are:

White Dwarf

Neutron Star

Black Hole

The endpoint is dependent upon birth mass of the star.

Elements are the same size regardless of how they are synthesized. It is true, however, that small stars create fewer elements, and that the elements they create are lighter. A normal G-type star can, during the course of its evolution along the Main Sequence, produce elements up to iron. For elements heavier than that, larger stars are required; when they go nova, they can produce elements as heavy as uranium and beyond.

Type II supernova. Formation of a neutron star or black hole.

The theory of evolution states that most living organisms share common ancestry. Charles Darwin proposed this theory 150 years ago to account for the remarkable diversity of life around the globe, and to explain apparent successions of fossils.

Since Darwin's time the theory of evolution has been modified to incorporate discoveries in genetics. We now know the source of the mutations that create change, for example. We have also learned a great deal about earlier stages of life. Multicellular organisms clearly originated in a marine environment. We have fossils of fish that predate any fossils of land plants or animals. Given that fact, evolution would predict the earliest terrestrial vertebrates should be amphibians.

It turns out this is what we find. Fossils of amphibious creatures like ichthyostega, eusthenopteron, acanthostega, and eogyrinus all bear remarkable resemblance to sarcopterygian lungfish of that era. One telling find was discovered by paleonotologists who predicted (on the basis of evolutionary theory) that there should be a creature with particular features in specific strata. When they searched that strata they found Tiktaalik roseae.

There are hundreds of scholarly papers published in peer reviewed journals on the subject of biological evolution each and every year, and this has been the case for decades. Research into this subject is ongoing and pervasive--it is of keen interest in numerous prestigious universities around the world. Essentially no research has been conducted that casts any serious or substantial doubt on the accuracy of Darwin's theory. On that basis I am led to the conclusion the theory is essentially correct.

Stellar Evolution [See Link] - or Life of a Star.

Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years (for the most massive) to trillions of years (for the less massive), considerably more than the age of the universe.

Birth:

Star formation begins with the gravitational collapse of a giant molecular cloud. As the gases coalesce, heat and pressure increase until they condense into a rotating sphere of superhot gases. This is known as a protostar.

Protostars with masses less than roughly 1.6×1029 kg never reach temperatures high enough for nuclear fusion and become brown dwarfs.

For larger protostars, the core temperature will eventually reach 10 megakelvins, and nuclear fusion will begin.. The onset of nuclear fusion leads relatively quickly to a hydrostatic equilibrium in which energy released by the core exerts a "radiation pressure" balancing the weight of the star's matter, preventing further gravitational collapse.

The star thus evolves rapidly to a stable state.

Main Sequence.

Small, relatively cold, low mass red dwarfs burn hydrogen slowly and will burn for hundreds of billions of years

Massive hot supergiants will live for just a few million years.

A mid-sized star like the Sun will remain on the main sequence for about 10 billion years.

Maturity:

After millions to billions of years, depending on the initial mass of the star, the continuous fusion of hydrogen into helium causes a build-up of helium in the core. Eventually, the core exhausts its supply of hydrogen. Depending on the mass of the star, the outcome can vary.

Low Mass will become red dwarfs, such as Proxima Centauri, some of which will live thousands of times longer than the Sun.

Mid Sized will become red giants, such as Aldebaran and Arcturus .

Massive Stars will become red supergiants, such as VY Canis Majoris, Betelguise and Antares.

Stellar Remnants (The End)

After a star has burned out its fuel supply, depending on its mass, one of three things can happen.

For a star with a mass similar to the Sun, it will turn into a white dwarf and radiate the remaining heat into space for billions of years. Finally ending it's life as a black dwarf. (Though none exist at the moment, as the universe is not old enough).

For larger stars, depending on the chemical composition and temperature, the star explodes as a supernova. and either collapses into a neutron star or, if the remaining mass is large enough, the pressure will be insufficient to stop the total collapse and the star will become a black hole.

The longest stage of stellar evolution is the main sequence phase.

Stars cannot fuse any other elements heavier than iron simply for the fact that it does not produce energy. However, what comes next mainly depends on how much mass is contained within the star itself. If the mass of the star is 1.4 times the size of our sun, the electron degeneracy pressure (what holds up the dying star. the lower limit to size--electrons in star are squeezed together so tightly, further contraction is impossible) cannot hold the star, so the electrons are "squeezed" together, creating neutrons. The star will shrink until neutrons are packed as close together as possible and a neutron is the result. Neutron stars do not glow like white dwarfs but can be detected.

Pressure and gravity

Black dwarf stars, they have cooled off so much they emit no detectable light (but some emit small amounts of microwaves that are barely detectable).

not enough info

A nebula.

The heaviest elements occurring in nature are formed inside supernovae, through nucleosynthesis.

Good question. Three part answer.

First, all stars convert gravitational potential energy into radiative energy. White dwarfs have a mass that's in the same order of magnitude as the stars that became them, but with a much higher density and a much smaller surface area. So the amount of energy radiated per inch would be higher, resulting in higher surface temperatures.

Second, the degenerate matter that makes up the bulk of a white dwarf has a very low opacity, because any absorption of a photon requires an electron transition to a higher empty state, which may not be available given the energy of the photon.

Third, since the heat-generating capacity of the white dwarf is not replenished by nuclear fusion - and IF there is no companion star present from which the dwarf gains new mass - the star will slowly cool; the high surface temperatures do not last.

Only those which aren't absorbed too much by the atmosphere. Those are visible light, and radio waves.

It is not old enough. It is estimated that it would take trillions of years for a white dwarf to a black dwarf. The universe is only about 13.8 billion years old.

A red giant can have a radius of 50 million to 500 million kilometres.

Yes, a G-type star, like our Sun, can eventually collapse into a white dwarf. After exhausting its nuclear fuel, the star goes through a red giant phase, shedding its outer layers and leaving behind a hot core. This core, composed mostly of carbon and oxygen, becomes the white dwarf. Over time, the white dwarf will cool and fade, but it will no longer undergo fusion reactions.