All images were snagged, with some thought and care but without credit, from Google Images although you can imagine that most of the photographs came from Hubble and, therefore, from NASA.
A star is a luminous sphere of mainly hydrogen, the lightest and most plentiful element in the universe, and helium, the second lightest and most plentiful. A spherical shape is attained when a balance (hydrostatic equilibrium) of two forces, gravity at its surface pulling inward toward the core and nuclear reaction at its core pushing outward toward the surface, is achieved.
The nuclear reaction fuses together the nuclei of the lightest atoms (hydrogen) to form heavier atoms (helium) that in turn if the sun is large enough, fuse with other like atoms once the lightest atoms are mostly used up. For instance, within the Sun’s core, two hydrogen atoms fuse to form a helium atom that, because of the process of fusion, releases a photon (light) of radiation (electromagnetism) that will push outward toward the Sun’s surface, a journey that could take thousands of years, before finally radiating into space at a speed of 186,000 miles per second that, if heading towards the Earth, will shine down on us 8 minutes later as a single particle of light.
With suns the size of ours, this fusion process doesn’t continue much beyond helium, but with massive stars, due to gravity’s inward pressure, this shelling of heavier to ever heavier elements towards their core continues well beyond helium. Consequently, massive stars burn much hotter, they use up their fuel much quicker and they die much younger than non-massive stars. Once a massive sun has fused all its silicon to iron, it will die within hours as iron cannot be fused.
First: a graphic showing a massive sun’s core with the shelling of its elements from hydrogen (H) to helium (He) to carbon (C) to neon (Ne) to oxygen (O) to silicon (Si) to iron (Fe)
Second: from http://science.gsfc.nasa.gov/, a photographic of the Sun
Protostar: Lasting around 100 thousand years, this is when gases formed in nebulae (a stellar nurseries) have collapsed to form a giant molecular cloud which, due to gravity, will continue to collapse inward into a T Tauri star.
T Tauri Star: Lasting around 100 million years, this is when the star looks like a star but hasn’t yet begun the nuclear fusion at its core. It hasn’t yet begun to shine.
Red Dwarf Star: the most common of all star types, they are much smaller than the Sun, which is believed to have a 10 billion year lifespan, and so, these stars could have up to a 10 trillion year lifespan.
Main Sequence Star: The majority of stars in the universe are main sequence stars because they are believed to be in the middle phase of their lifespan where they are the most stable. The Sun and its closest neighbors, Sirius and Alpha Centauri A. Main, are all main sequence stars. While they do vary in size, mass (defined by the amount of gravitational pressure pulling inward toward the core) and luminosity, they all still convert hydrogen into helium in their cores, thereby releasing a tremendous amount of energy in the form of photon (light) radiation. Once they have burned up all their hydrogen, they will move out of their main sequence phase into their next phase.
Graphic: Shows a sun-sized star from its beginnings through its planetary nebula phase.
Red Giant Star: When a star between 0.5 to about 2.5 times the mass of the Sun has consumed the hydrogen in its core, fusion stops and the star no longer generates an outward nuclear pressure to counteract the inward gravitational pressure that had been keeping it in balance. Consequently, with the star’s temporarily collapse, the shell of hydrogen around its core ignites. This continues the lifespan of the star by causing it to swell dramatically, 100 times larger than it was during its main sequence phase as, for, a time, gravity gives way to the nuclear pressures from within and the star become a red giant.
When the hydrogen fuel in this shell is used up, further shells of helium and even heavier elements (not with the Sun which is too small to continue the nuclear process) can be consumed in fusion reactions.
The red giant phase of a star’s lifespan will only last a few hundred million years before it runs out of fuel completely and becomes a white dwarf which is its original core only now exposed.
Artists’ rendering: Red Giant with Earth for scale
Graphic: Shows comparisons of size between small to supergiant stars.
Supergiants: These stars come in colors: Blue, which burn cooler and are smaller at around 8 solar masses and Red, which bun hotter and are much larger, anywhere from 10 to 70 solar masses. Because they last only 10 to 50 million years, supergiants tend to be found in relatively young cosmic structure. These will all die as supernovae exploding out and then either as a neutron star or a black hole imploding in depending on their size.
Hypergiants: These are larger even still, at 70 to 120 solar masses and all end, after a couple of million years, as supernovae and as black holes. One, VY Canis Majoris, a red hypergiant star, is the largest star known, between 1800 and 2100 solar diameters in width.
White Dwarf Star_the remnant of a low-mass star_When a star has completely run out of hydrogen fuel in its core and lacks the mass to force higher elements into fusion reactions, it becomes a white dwarf. Because the outward photon radiation pressure from the fusion reaction at its core has stopped, the star collapses from the inward pull of its own gravity. At the same time it slowly expels most of its outer material forming a planetary nebula around itself.
A white dwarf shines because it was a hot star once, but there is no longer any fusion reactions taking place. A white dwarf will continue to cool until its temperature matches the background temperature of the universe. This process will take many more billions of years than the universe has existed so no white dwarfs have actually reached this end process yet. A typical white dwarf is half as massive as the Sun, yet only slightly bigger than Earth.
Graphic: White Dwarf with Earth for scale
Second: a binary pulsar as seen from the bridge of the Starship Enterprise by Captain Janeway and the doctor on Voyager.
Third: cool artist's rendition of some distant planet's nightsky pulsar
Neutron Star_the remnant of a high-mass star_When stars are between 1.35 and 2.1 times the mass of the Sun, they do not form into white dwarfs when they die, but instead simultaneously explode their outer material into a supernova and implode their inner core to a point where all the electrons and protons crush together to form neutrons. Neutron stars are so dense that even though they are about 1.4 times the mass of the Sun, they are only 20 km (12.5 miles) in diameter.
Pulsar Star: (from PULSAting stARs)_the remnant of a high-mass star, are among the most exotic objects found in the galaxy that, even after 40 years of study, are still not well-understood. In fact, 1967, they were dubbed little green men (LGM) because it was thought that one possibility for their very regular sounds came from some extraterrestrial life form. First observed as radio sources they are now also observed using optical, X-ray and gamma-ray telescopes.
Currently, there are about 1500 known to exist. They are a type of neutron star only with a strongly anisotropic (directionally dependent) emission of electromagnetic radiation, mostly thought to emanate from the object's magnetic poles. The strength of the radiation pulses changes according to a regular period of time, which is thought to match to the period of time in which the star rotates. The time interval between consecutive pulses is called the pulsar's period. Periods of one second are typical, meaning the pulsar rotates once every second, although pulsars have been discovered with periods from a few milliseconds up to eight seconds. The time between pulses is incredibly regular, some are as consistent as an atomic clock, and can be measured very precisely.
All stars are born out of stellar nebulae, or nurseries, that are made of clouds of dust and gas that are formed in one of two ways.
Way One: Soon after the universe’s birth, atoms formed which would eventually coalesce into the first clouds of dust and gas.
Pictured: The Omega & The Carina Nebulae (ooooh, pretty!!)
Way Two: Later, when suns the size of the Sun and larger die, they form two types of nebulae, either Planetary Nebulae or Supernova Nebulae depending on how they ejected outer matter.
Pictured: A bunch of very cool planetary nebulae.
Also, the origins of nebulae are not always so clear cut as they can be a mixture of both kinds of gas and clouds -the ‘beginning of the universe’ and the ‘death of a star’.
Nebulae_5 Different Types_Three ‘Beginning of the Universe’ & Two ‘Death of a Star’.
Emission_ Beginning of the Universe
Emission nebulae, the most colorful of the five main types, are lit from within by young stars. The varying colors are caused by the different gases and the composition of the dust in the nebulae. They emit radiation in the form of ionized gas, mostly hydrogen, and are diffuse nebulae meaning they are extended and have within them no well-defined boundaries.
Photograph: of theThe Eagle Nebula (first time I saw this photograph, I was blown away)
Reflection_ Beginning of the Universe
Also diffuse, reflection nebulae do not emit significant amounts of visible light themselves, but are visible because they reflect the light of other stars that are either inside it or nearby to it. The stars from the Pleiades, pictured below, are thought to have all formed at roughly the same time, about 100 million years ago, which would make them about 1/ 50 of the age of the Sun. They are believed to currently be plowing through the thin, wispy nebula that is seen as blue wisps and glows around the stars.
Photograph: of the The Pleiades Nebula
Dark_ Beginning of the Universe
Since no nebulae produce visual light of their own, when astronomers refer to a ‘dark nebula’, they mean one that from our line of sight is blocking the light of something else behind it, as if it were a wall. It is difficult to see visible light very far into the Milky Way Galaxy for this very reason as there are too many lanes of dust and gas (dark nebulae) in the way. To combat this, astronomers use other forms of light, such as infrared to see into the Milky Way.
Photograph: of the The Horsehead Nebula_Taken by the Hubble Space Telescope, this image shows the horsehead portion that is the dark nebula known as Barnard 33 that lies in front of the emission nebula known as IC 434.
Planetary_The death of a Star
In the 19th Century, when the Milky Way Galaxy was thought to be the only galaxy in the universe (the educated guesstimate now is 125 billion), astronomers, using primitive telescopes, saw that these nebulae looked somewhat like the newly-discovered Uranus and Neptune planets so they called them the misnomer of planetary, when they are actually the second kind of nebulae, those formed by the death of a star.
There are at least 10,000 planetary nebulae thought to exist in our Milky Way Galaxy alone, with 1,500 of these already observed and catalogued. It is estimated that eventually 95% of all the stars seen in the Milky Way will become planetary nebulae including the Sun.
Planetary nebulae are created when a main sequence star evolves into a red giant which slowly ejects its outer layers of material into space. Since the material is thrown off the star in a roughly symmetrical manner, usually a somewhat spherical shape is achieved.
First Photograph: of the Cat’s Eye Nebula: At the center is its at-one-time Sun-sized dying star which will one day become a white dwarf.
Second Photograph: of the The Retina Nebula (IC 4406) The circular disk is viewed on its side, like looking at a doughnut's edge. The star's spin and magnetic fields caused the material to expand in more of a circular disk, than as a spherical shape.
Supernova Remnant Nebulae_The death of a Star
The other 5% of the stars in the Milky Way, those with masses more than eight times larger than the Sun, will end their lives as supernovae remnant nebulae.
These massive stars explode their outer material away from their remnant, whether neutron star, pulsar star, or black hole, as opposed to the other 95% of Milky Way stars whose outer material slowly drifts away from their white dwarf remnant to form planetary nebulae. As with planetary nebulae though, these supernovae nebulae also release the heavy elements that were created during the nuclear fusion reaction at their cores such as carbon and oxygen, but it is also believed they create heavy elements during the process of the explosion itself such as gold and silver, spreading them throughout the universe.
Through this process of heavy element seeding, everything we find on Earth was created, its atmosphere, its soil and water, even the very carbon contained within the cells of every living organism. And so, as Carl Sagan once famously said, ‘we are made of star stuff’ literally.
The first astronomical event to be discovered, by Chinese and Arab astronomers in 1054, was the supernova event which resulted in the Crab Nebula in the constellation of Taurus.
First Photograph: of the The Crab Nebula
Second Photograph: of the The Cassiopeia Nebula
Digression Two: It was all of these beautiful, beautiful Hubble photographs that initially got me interested in finding out what these amazing objects were. And year after year, the photographs and the amzing objects they depict just keep on getting more and more beautiful and so, naturally, my interest grew. I now know only some small bit and I am stunned to my very core.
This is what I believe in.
#60_The Standard Model_The Twelve Fundamental Matter Particles and Three of the Four Fundamental Force Carrying Particles
#59_Quasars, Pulsars & Black Holes