All stars eventually run out of hydrogen in their cores. Once hydrogen fusion stops the star collapses under its own gravity. This collapse causes the star to heat up, and other elements to fuse. The increased heat, like a rapidly boiling pot, swells to become a red giant, and sends much of its mass back into space as a stellar wind. Depending on the size of the star (at the moment of its demise) it may stop at fusing helium or continue on to fuse heavier elements up to iron.
Small stars, up to about 1.4 solar masses, collapse to become white dwarfs while quietly sending out beautiful planetary nebulae.
Medium stars, 1.4 to 8 solar masses, explode in a violent supernovae while what remains collapses to become neutron stars. Some of these are spinning pulsars, that send pulses of x-rays out into space.
Large stars, born with many solar masses, explode even more violently into supernovae. Sometimes they blow themselves to bits and no central star remains. Sometimes only enough of the star remains to form a neutron star ... but if more than about 2 times the mass of the Sun remains after the explosion, it collapses to become a dense BLACK HOLE, with so much mass and so much gravity, not even light can escape!
When small stars, less than the Chandrasekhar Limit of 1.4 solar masses have fused all the hydrogen in their core, the core starts to collapse, heats, fuses helium, creates a red giant, and finally becomes a white dwarf while sending out a beautiful planetary nebula.
A Type G2 star, like the Sun, will live 10 billion years. Since the Sun has been around for 5 billion years, in another 5 billion years it will become a red giant and swell as far as the Earth's orbit or further.
The huge mass of the Sun will then collapse to become a white dwarf the diameter of Earth, while sending off a planetary nebula. The electrons in the white dwarf, stripped from the atoms, will make an electron gas with a pressure sufficient to support this extremely compressed star. A teaspoon of this strange white dwarf material is so dense it would weigh several tons.
Planetary nebulae can be very beautiful and peaceful looking, but they are excited to emit by ultraviolet light from the star, and the gases are flowing outward at many kilometers each second.
They come in all varieties, and are typically two-lobed because of rotation of the parent star. They appear round only when seen from one end.
Although planetary nebulae are 5 or more light years across, they seem small compared to the star-birth nebulae from which they came. Giant star forming nebulae produced the star clusters known as M46 and M47, shown below. Although they appear in the sky together, they are not associated with one another because M46, the bigger one, is farther away from us. A planetary nebula, apparently belonging to M46, is almost lost in the images. Can you find it in all three?
The Helix Nebula is, also, about 6 light years across, about three times wider than our cold cometary Oort Cloud. Stars are too hot to form compounds, but as the planetary nebula spreads into interstellar space, it cools. As a rule of thumb, if density and temperature are right for chemical reactions to happen ... they will. When spectra were taken of the edges of the Helix nebula, primitive compounds show up, CO (carbon monoxide), CN, CH, HCN, HNC, CCH, H2, and ions such as HCC+ CO+ N2H+ CH+. These are found within the knots being thrown out. Without these fragmentary compounds, the more complex compounds would not have had time to form in starbirth nebulae. The raw materials to make the next generation of stars and planets come in part from the gases ejected by old stars.
All small isolated single stars similar to the Sun end as planetary nebulae. But, occasionally, a white dwarf may start to accrete matter from a companion star, typically an aging red giant. When the white dwarf reaches the Chandrasekhar Limit, about 1.4 solar masses, a Type Ia Supernova occurs! If enough matter remains after the explosion, a neutron star may be left in the center. Type Ia supernovae all happen at the Chandrasekhar Limit, and all have the same mass and same brightness. If the explosion appears very dim, we know it must be very far away, and astronomers can calculate the distance to the galaxy in which the supernova appears. Type Ia supernovae may be briefly brighter than the entire galaxy they belong to, which means we can see them even when they are very far away. Their properites allow us to measure distances in the Universe.
These Type Ia supernovae become visible suddenly and then fade gradually over a few months. Afterwards, a supernova remnant is left to spread out through its galaxy for millions of years. A Type Ia supernova and its remnant are recognizable because their spectra show hydrogen from the gases of the white dwarf's companion, and there are few heavy elements in its spectrum.
Nearby supernovae are rare, and those recorded in history were so bright they have been visible both day and night. A few historic Type Ia supernovae are Tycho's Star (seen by Tycho in 1572), Kepler's Star (seen by Kepler in 1609), and SN 1006 (seen worldwide in 1006).
Middle sized stars, born with over the Chandrasekhar Limit 1.4 times the mass of the Sun to 10 solar masses, explode in a supernova which sends out a huge supernova remnant. Most of their mass is blown off in the supernova, but If a star of 1.4 to about 2.1 solar masses survives the explosion, it will collapse to become a neutron star, whose gravity is so great that its electrons are pressed into its protons creating only neutrons. Instead of being the size of Earth, because the space between the electrons and atomic nuclei disappears, the star collapses to the size of a small city about 24 km in diameter. One teaspoon would weigh a billion tons.
As these neutron stars collapse, they spin faster and faster, like a spinning ice skater pulling in his or her arms. Pulsars are spinning neutron stars, with misaligned magnetic poles. Jets of material shoot out along the axis. The pulsars also beam x-rays, visible light, and radio waves into space along the axis of their magnetic field, and since it flips around as the pulsar rotates, we see a bright flash, or pulse, when the beam of light sweeps past us. The pulses were first discovered with a radio telescope, but they can be seen in visible light too. There is a pulsar in the center of the Crab Nebula, the remnant neutron star of a 1054 CE supernova explosion. This pulsar spins 30 times a second, but slowed down it looks like this.
Stars fuse hydrogen into helium, and helium into carbon as the source of the energy they radiate during their lifetime. The more massive stars make heavier elements too, up to iron, towards the ends of their lives. Only the powerful shockwaves of a supernova create even heavier elements such as gold and silver and copper.
The largest stars, born with over 10 solar masses, also explode in supernovae, leaving behind a pulse of light lasting a few months, a short pulse of x-rays and gamma-rays lasting a few seconds, gaseous remnants we can still see today, and possibly a black hole.
The cores of these stars have fuse all their hydrogen into helium, and most of that into carbon, oxygen, and silicon. In the next step they turn silicon into iron, squeezing out the last bit of energy available by fusion. That goes quickly, and the stars own gravity compresses the iron which also will fuse into heavier elements. Unlike the lighter elements, however, when iron fuses it consumes energy. There is no stopping the consequential catastrophy for the star. The outer parts of the star fall onto the collapsing core, bounce outward as a shockwave, and create a supernova. After this is over, there are remants of the outer part of the star scattered into the space between stars, to be used again in the next generation. What happens to the core of the star?
If there is less than about 2.1 times the Sun's mass left in the core, it can turn into a neutron star. The electrons, protons, and neutrons are so compressed that they make a closely packed ball of neutrons held together by gravity. The pressure of the neutrons on one another can resist the force of gravity and make a stable remant that we see as a pulsar. If there is too much mass left, however, then gravity is too strong for the neutron pressure to resist.
If what remains after the explosion is over 2.1 solar masses, the star will collapse down to an infinitely small BLACK HOLE with gravity so great not even light can escape. There is no way to know what sort of matter lies within a black hole, because gravity changes space and time making everything that happens inside the black hole inaccessible to the outside universe, except the influence of its gravity (and its charge and spin).
Neutron stars are all neutrons, which themselves are made of particles known as quarks. How much further down may matter be split? When all are crushed together in one, the constituent fundamental particles appear, under conditions close to what existed at the very beginning of the universe. From the outside of the black hole, however, all we can tell is that there is gravity. The star disappears. Its gravity remains.
A supernova was seen in the Large Magellanic Cloud in 1987, and is known now as SN1987A.
No neutron star has been found in the debris. Three possibilities for the 'missing' neutron star are being considered. The first is that the neutron star is enshrouded in dense dust clouds so that it cannot be seen. The second is that large amounts of material fell back on the neutron star, so that it further collapsed into a black hole. The third possibility is that the collapsed core became a hypothetic quark star.
Similar looking stars to the one that became this supernova are large blue stars (Wolf-Rayet stars and Type O stars), which do not live more than a few million years, and are already dying while still in their star-birth nebula. Thus in some nebulae such as the one below we can see all stages of stellar evolution at once.
Eta Carinae (η Carinae or η Car) is a hypergiant luminous blue variable star in the constellation Carina. Its luminosity is about four million times that of the Sun and, with an estimated mass of between 100 and 150 solar masses, it is one of the most massive stars yet discovered. Because of its mass and the stage of life, it is expected to explode in a supernova in the "astronomically" near future (tomorrow, or 1000 years from now).
Eta Carina is a relatively close 7,500 to 8,000 ly away. While it will only be visible from the Southern hemisphere, telescopes close to the equator or in the southern hemisphere, such as ours at Mt Kent in Australia, or research telescopes in telescopes in Hawaii and Chili, will allow us to see the event. It will be so bright, however, that large telescopes will be overwhelmed by the light.
Eta Carinae last flared up around 1843, and afterward it faded away to an 8th magnitude star. It doubled its brightness in 1998-9, and can now be seen with the naked eye because it is brighter than magnitude 5. Some of its changes in brightness may be due to the periodic influence of an unseen companion. When it explodes it is expected to be as bright as a crescent Moon, and visible both day and night.
Only second, third or later generation stars, made with a sprinkling of supernova dust, could have such things as solid ground, plants, trees, and living creatures on terrestrial planets. Are all made of stardust ... including intelligent life as we know it. The Veil Nebula is a supernova remant faintly glowing in the constellation Cygnus.
At the centers of galaxies, even bigger black holes exist than those that come from single stars. Black holes may grow, capturing matter from many stars. Messier object M87 is a giant elliptical galaxy 55 million ly beyond our Milky Way galaxy with a 3 billion solar mass black hole at its center! Although most galaxies with central black holes are spiral, like our own Milky Way Galaxy with a smaller 6 million solar mass black hole, in the center of M87 there is an accretion disk, where matter is streaming into its enormous black hole. It is a strong source of multi-wavelength radiation, particularly radio waves, X-rays and gamma rays, and of charged particles shooting out from the axis of rotation for a distance of 5,000 ly and more. The energy which creates these jets derives from the infalling matter itself.