There are some strange animals in the cosmic zoo, many of which are stellar remnants.
..1 RED GIANT - A red giant is a star at the end of its life. A star becomes a red giant when the temperature rises at its core, causing it to expand. Betelgeuse and Antares are red giants. Our sun will become one in 5 billion years, engulfing the inner planets.
..2 WHITE DWARF - A white dwarf is a stellar remnant, small and dense! The sun will become a white dwarf after its red giant phase. Its core will shrink. A shell of expanding gas will surround it, making it a "planetary nebula." Sirius' companion was the first known white dwarf.
..3 NEUTRON STAR - Neutron stars are remnants of supernova explosions. When the cores of these stars collapse, protons and electrons merge to form neutrons. Neutron stars are dense. A spoonful weighs a billion tons!
..4 PULSAR - Pulsars are neutron stars that spin. They pulse like lighthouses. There is a pulsar inside the Crab Nebula, a supernova remnant.
..5 NOVA - There is a difference between a nova and a supernova. A nova is an explosion of a white dwarf caused by the accretion of hydrogen from its companion.
..6 SUPERNOVA - A supernova is a massive star blowing up. Tremendous energy is released! Heavy elements are created and distributed through space. The elements in our bodies come from supernovas. A supernova occurs in the Milky Way once every century. Tycho's star was a supernova.
..7 CEPHEID VARIABLES - First discovered in Cepheus, these variable stars are used to determine distances to galaxies.
..8 DOPPLER EFFECT - The Doppler effect applies to both sound waves and light waves. Both can be compressed and stretched. An approaching ambulance compresses sound waves from its siren so that the pitch is high and annoying. After the ambulance passes, the waves are stretched, and the pitch is lower. With light waves, a galaxy moving toward us will have its spectral lines shifted toward the blue. A galaxy moving away from us will have its spectral lines shifted toward the red. The farther away galaxies get, the more stretched out their light waves become.
..9 EXOPLANETS - 4000 exoplanets were confirmed as of 2018. These are planets outside our solar system, orbiting stars other than our sun. Most exoplanets found thus far are like Jupiter, although Earth-like planets are being discovered as techniques improve. Indirect methods are used. The transit method records a star's dimming as a planet passes in front of it. The radial velocity method measures a planet's gravitational influence on its parent star. Astronomers search for planets in habitable zones, ones that might have water and support simple life. The Kepler Space Telescope searched for exoplanets. There are billions in the Milky Way. There may or may not be exoplanets with intelligent life.
10 DARK MATTER - Dark matter is believed to make up 25% of the universe. Normal matter accounts for 5%. Astronomers think dark matter is real because of its effect on normal matter. There are many theories as to what it is. Dark matter tries to hold the universe together while dark energy tries to push it apart.
11 DARK ENERGY - When the expansion of the universe was found to be accelerating, the force responsible was given the name "dark energy." It is driving galaxies apart at a greater and greater speed. It overwhelmed gravity to become the dominant force. Dark energy is believed to make up 70% of the universe.
12 BLACK HOLE - A black hole is a dead star whose gravity is so strong that even light cannot escape. It is invisible. The gravity is strong because its mass has collapsed into an infinitely small space. Black holes come in all sizes, the largest being supermassive. Galaxies have supermassive black holes at their centers, and there is one at the center of the Milky Way.
Black holes form when the biggest stars collapse. Astronomers detect them by their gravitational effect on nearby stars. The event horizon is the boundary beyond which nothing escapes. The Chandra X-ray telescope showed black holes to be common.
13 WHITE HOLE - White holes exist hypothetically in General Relativity as the opposite of black holes. Light can enter black holes but cannot escape them. Light can escape white holes but cannot enter them. Einstein's influence is profound.
14 QAUNTUM MECHANICS - A branch of physics dealing with the very small, the behavior of atoms and molecules.
15 QUASARS - Quasars are the cores of young, energetic galaxies. Tremendous radiation is emitted! They are up to 12 billion light-years away and probably no longer exist.
16 COSMIC RAYS - Cosmic rays are particles coming from the Galaxy. They are pieces of atoms, remnants of supernova explosions traveling close to the speed of light. They are not part of the electromagnetic spectrum.
17 GAMMA RAY BURST - A GRB is a flash of energy! Gamma rays are the most brilliant form of radiation. They come from galaxies colliding in the early universe. We cannot see them, nor can they penetrate our atmosphere. We know of them from satellites.
18 NEUTRINOS - It used to be protons, neutrons & electrons. Sub-atomic particles now proliferate: quarks, bosons, muons, gluons & neutrinos. Neutrinos are like electrons but have no electrical charge. Billions of neutrinos from the sun pass through our bodies.
19 INFLATION - Inflation theory has to do with the rapid expansion of the universe after the Big Bang. The universe expanded faster than the speed of light.
20 ANTIMATTER - Small amounts of antimatter exist. Because it annihilates matter, it poses a threat to space travel.
21 STRING THEORY - String theory maintains that strings of energy and matter triggered the formation of galaxies.
22 M-THEORY - M-theory combines versions of string theory to explain everything from the microverse to the macroverse. It assumes parallel universes. The universe is a membrane. Thus, M-theory!
23 GRAVITATIONAL WAVES - These are ripples in the fabric of space/time caused by the merging of black holes.
24 WORMHOLES - Wormholes are theoretical. They are tunnels in space/time. Traveling through a wormhole would get you from point A to point B in less time. A shortcut! Wormholes broach the topic of time travel.
25 MULTIVERSE - The multiverse theory captured the imaginations of cosmologists. The idea is that our universe is one of an infinite number, each with its own Big Bang. Some cosmologists think the laws of physics vary from universe to universe and that some universes teem with life while others are sterile. If there are other universes, they are beyond our horizon.