STELLAR OBJECTS

"Black Holes are among the most
mysterious of all stellar remnants."

Animation © SkyMarvels.com
derived from NASA/GSFC SVS animation.

PROTOSTARS, STARS & STELLAR REMNANTS

Stars are the mighty lampposts that light the Universe.  In addition, they are the miraculous nuclear foundries that have fabricated the heavier elements that make all life as we know it possible.

Our own Sun is a star—in fact, a rather ordinary one.  Like all stars, it produces its prodigious energy through self-sustaining nuclear reactions.  These proceed stably, depending on a star's type, from millions to billions of years.

From NASA's CHANDRA X-Ray Observatory site, here are two posters that begin to acquaint you with Stellar Objects and their evolution.  Stellar Evolution poster 1   Stellar Evolution poster 2.

NASA's  "Life and Death of a Planetary System"  likewise has a good introduction to how stars may form.

And from NASA's Imagine the Univers, here's a good page on  Stars and Stellar Evolution.

PROTOSTARS

© SkyMarvels, enhancement of NASA/JPL-Caltech image

Protostars, the precursors of stars, are massive objects born from the gravitational collapse of immense clouds of interstellar gas and dust.  Protostars represent the "toddler" phase of star development, which lasts only a tiny fraction of a star's overall age.  For example, for a star with a mass similar to our Sun's, its phase as a protostar may last only 100,000 years.

Generally, protostars form in four major stages.  "Early accretion" has been reached when a collapsing cloud of interstellar dust and gas has grown in density to become substantially less transparent to internal radiation.  This causes its temperature to rise greatly.  "Main accretion" is achieved when the protostar begins to outshine its enveloping donut of dust and gas.  What has been called the "T-Tauri" stage proceeds when the protostar, finally enveloped by a flattening "protoplanetary disc"      or "accretion disc", is radiating prodigious quantities of energy.  Nonetheless, this is only the energy released by the cloud's gravitational collapse, for temperatures in the core of the protostar have not yet risen adequately to support the fusion of hydrogen nuclei.  Eventually the "weak-line T-Tauri" stage is reached as the protostar continues to collapse substantially, and this cosmic lamppost is now destined to develop into a star of the main-sequence—in some tens of millions of years!

Wikipedia's Protostar page

STARS

STAR CLASSIFICATION

Stars are incredibly massive objects that, for at least a portion of their lives, radiate prodigious quantities of energy produced through nuclear fusion.  This colossal outflow is balanced by the relentless pull of each star's own gravity, which holds it in equilibrium for millions to billions of years.  This imposes a predictable order on stars, which are most commonly classified according to their spectra and brightness, as shown in the venerable H-R diagram below.

Wikipedia's Stellar Classification page

From UNL, here is an excellent  HTML5 interactive H-R Diagram.  Click anywhere to create a star. Then drag your star or the triangles on the graph axes.  Awesome!

From the Las Cumbres Observatory, here's a fantastic interactive showing how a star's properties (mass, age, brightness) determine its location on the HR-diagram.  Star in a Box: Interactive H-R Diagram.

Here's another excellent  Interactive H-R Diagram.

And though they are a little slow to load, from ESA Science here are the intriguing and informative  Gaia's Stellar Family Portrait  and   ESA Star Mapper.

Though the  100,000 Stars  Google Chrome Experiment shows all our Solar System's planetary orbits in the same plane, it does show our nearest stars faithfully, and you can zoom out to the edge of the galaxy!  Click on labels for more info on particular stars.

NASA/JPL's  The Lives of Stars  poster

List of Brightest Stars  (apparent magnitude)

List of Most Luminous Stars  (absolute magnitude)

List of Named Stars    List of Nearest Stars

Closest Stellar Neighbors

Star Name Pronunciation

image credit: NASA

STAR SIZES

After seeing star-types like "supergiants", "giants" and "dwarfs" in the H-R diagram above, it should come as little surprise that stars come in a wide variety of sizes.

You can get a good idea of the relative size of the stellar realm with Nikon's extraordinary demo:  UniverseScale.

Another similarly awesome online tool that reveals the sizes of many types of stars is  Scale of the Universe 2.

NUCLEAR FUSION IN STARS

For stars of roughly 1¼ solar masses and lower, the  Proton-Proton (P-P) cycle    is the primary way in

which Hydrogen-1 ions (protons) are fused to yield helium and energy.  The process involves several interactions of 6 overall protons, which ultimately yield one Helium-4 nucleus, two positrons, two gamma rays (energy), two neutrinos and two protons.

Here's an excellent  Proton-Proton Cycle Interactive.

For stars of greater than roughly 1¼ solar masses, the  Carbon-Nitrogen-Oxygen (C-N-O) cycle  dominates in producing helium and energy from hydrogen ions (protons).  This process involves several interactions of 4 overall protons along with a carbon-12 nucleus, the latter acting as a catalyst as it is converted to isotopes of nitrogen, oxygen and finally carbon-12 again.

In massive stars, heavier elements are progressively built-up in great shells around the core, resulting in a structure likened to the layers of an onion!.

From HubbleSite  Massive Stars: Engines of Creation  is an excellent poster that shows build-up of layers in more massive stars and their eventual supernova stage.

MORE INTERESTING STAR LINKS

Wikipedia pages:  Star,  Stellar Classification,  Stellar Nucleosynthesis,  Binary Star,  Star System.

NASA Science: Astrophysics: Stars.

NASA: Herschel Space Observatory: Stars.

STELLAR REMNANTS

WHITE & BLACK DWARFS, NOVAE & SUPERNOVAE, NEUTRON STARS,
QUARK STARS AND BLACK HOLES

All stars eventually succumb to natural processes and "die".  They go through a process of expansions and contractions, shed a percentage of their outer layers, and undergo a final collapse.  But this does not occur in the same way for all stars.

By far, most stars (perhaps 95% or more) approach their final stages as white dwarfs!  These are fantastically dense objects.  Roughly Earth-sized, a typical white dwarf may still be half as massive as the Sun!  This means that one is so dense that a  white dwarf's gravity can noticeably bend the light of stars passing behind it!     Nonetheless, at this stage it can no longer radiate any energy through fusion, but only by virtue of its final contraction.  Eventially it will cool to become a black dwarf, though not for an incredibly long time.  In fact, astronomers now estimate that the Universe is not yet old enough to have produced any black dwarfs!  Here are Wikipedia's  White Dwarf  and  Black Dwarf  pages.

Some stars approach their final stages with a series of explosions (novae) or a single devastating explosion (supernovae).  These return great amounts of matter back to the interstellar medium, matter that eventually contributes to the immense clouds that may one day spawn other stars.  From the University of Arizona's astronomy site, here's a nice page highlighting  Novae and Supernovae.  From NASA/Conceptual Image Lab at the Goddard Space Flight Center, here are nice ani- mations depicting  V407 Cygni "Going Nova"  and a  Type Ia Supernova".  Also from NASA is the high-reso- lution  A Guide to Supernovae  .  And here are Wikipedia's  Nova  and  Supernova  pages.

When a star's final collapse yields an object of between 1.4 and 3.2 solar masses, its gravity becomes sufficient to literally squeeze its electrons into its atomic nuclei!  This yields a  neutron star, an object with a diameter of roughly 12 miles (20 km) but a mass that's greater than that of our Solar System!  Ranking among the most exotic objects in the universe, neutron stars can take several forms, as these next images and animations from NASA show.  One may simply be a  single pulsar, or may be a  pulsar drawing matter from a much larger companion, which can generate strong  gravitational waves!  Or it may also be a  magnetar, a neutron star with an incredibly strong magnetic field.  It has even been theorized that in some cases there are  Binary Neutron Stars That Merge!  Here are links to NASA's Imagine the Universe:  Neutron Stars and Pulsars  and   Neutron Stars, Pulsars and Magnetars  pages.  And here are Wikipedia's  Neutron Star,  Pulsar,  and  Magnetar  pages.

It has been theorized that there may be objects that are too massive to remain neutron stars, yet not massive enough to become black holes.  These objects are  quark stars, whose gravity is sufficient to disassociate its neutrons' "bound quarks" into "free quarks!"  Quark stars would not be much smaller than neutron stars, as this intriguing comparison from Chandra's site shows:  A Neutron Star and Quark Star in the Grand Canyon!  But it must be stressed that this comparison is only illustrative of relative sizes.  If a neutron star and quark star were ever so near to us, Earth would have already been destroyed by their gravity!  Here is Wikipedia's  Quark Star  page.

With black holes we reach the "end of the line" of our stellar remnants!  When stars of a sufficient mass eventually collapse, their gravity is so powerful that nothing, not even light, can escape from them!  They may even become  Star Eaters!  
    Wikipedia's  Black Holes  page.
    UCLA Galactic Center Group's
        Stars Orbiting MWs Supermassive Black Hole 
    ESA's  Black Hole Visualization Tool 
    cK-12's  Does A Black Hole Bend Time?