Galaxies and Cosmology
The Milky Way Galaxy
A galaxy is a collection of stellar and interstellar matter (stars, gas, dust, neutron stars, black holes) isolated in space and held together by its own gravity. Our galaxy is the Milky Way Galaxy (also called simply Galaxy).
The Galactic Disk is an immense, circular, flattened region containing most of the Galaxy’s luminous stars and interstellar matter.
The Andromeda Galaxy is the nearest major galaxy to our own. It lies roughly 800 kpc (about 2.5 million light years) away. It consists of a galactic disk that fattens into a Galactic bulge (thick distribution of warm gas and stars around the galactic center) at the center. The disk and bulge are embedded in a roughly spherical ball of faint old stars known as the Galactic halo (region of a galaxy extending far above and below the galactic disk, where globular clusters and other old stars reside).
The twin ideas that (1) the Sun is not at the center of the Galaxy, and (2) the Galaxy is not at the center of the universe, both required time and hard observational evidence before they gained widespread acceptance.
The 18th century astronomer William Herschel constructed a map of the Galaxy by counting the number of stars he saw in different directions of the sky. He assumed all stars were of about equal brightness. The Sun appears to lie near the center of the distribution. The long axis of the diagram lies in the plane of the Galactic disk. This was called Herschel’s Galaxy Model.
(above image from Wikimedia Foundation)
The Galaxy is known today to be several tens of kpc across. The Sun is known to lie far from the center. Older observations of the Galaxy (such as Herschel’s) were flawed because they were made in the visible part of the spectrum, and the absorption of light by interstellar gas and dust was then unknown. It wasn’t until the 1930s that astronomers began to realize the true extent and importance of interstellar medium. Interstellar dust hides objects more than a few kpc away.
During the 1st quarter of the 20th century, studies focused on two items: globular clusters and spiral nebulae. There are about 150 globular clusters known in our Galaxy. The spiral nebulae are now known to be spiral galaxies (a galaxy composed of a flattened, star forming disk component which may have spiral arms and a large central galactic bulge).
Variable stars are stars whose luminosity changes with time, some erratically, others more regularly. Cataclysmic variables involve novae and supernovae in binaries, because of their sudden, large changes in brightness. An intrinsic variable is a star where the variability is a trait of the star, not because it is part of a binary. Pulsating variable stars vary cyclically in luminosity in very characteristic ways. Two types of pulsating variable stars are the RR Lyrae and Cepheid variables. RR Lyrae stars have more or less the same average luminosity. Cepheid variables stars have luminosity that varies in a characteristic way, with a rapid rise in brightness followed by a slower decline. The period of a Cepheid variable is related to its luminosity, so a determination fo this period can be used to obtain an estimate of the star’s distance. Pulsating variable stars can be recognized and identified just by observing the variations in the light that they emit.
Main sequence stars do not go through the instability that causes them to puff up and shrink down, and be variable stars. They occur in post-main sequence stars as they pass through the instability strip of the H-R Diagram (See page 622). High mass stars entering the instability strip become Cepheid variables. Low mass stars become RR Lyrae variables.
For RR Lyrae stars, the luminosity is known to be about 100 times that of the Sun. For cepheids, the period-luminosity relationship is used. Those with long periods (vary slowly) have large luminosities; those with short periods have low luminosities.
The Galactic Center is the center of any galaxy – the point about which the disk of a spiral galaxy rotates. The center of our Galaxy is about 8 kpc from the Sun.
Globular Cluster Distribution outlines the true distribution of stars in the Galactic Halo.
The Shapely-Curtis Debate concerned whether or not the spiral nebulae were part of the Galaxy, and if our Galaxy was the entire universe. Here are some of the key elements of the debate:
1. Size of the Milky Way.
2. Distribution of the nebulae on the sky.
3. Observations of novae.
4. Brightness and spectra of the nebulae.
5. Rotation of the nebulae.
Due to the observations of the day, the disagreements could not be resolved. A few years later in 1925, Edwin Hubble observed Cepheids in the Andromeda Galaxy, finally succeeding in measuring its distance, and firmly establishing it as a separate galaxy.
Much of our knowledge of the galactic structure comes from using 21 cm radio emission lines produced by atomic hydrogen.
(above image from Penn State University - Class not taken there - only image)
The galactic disk is about 30 kpc in diameter, and about 300 pc thick. At the bulge, it is about 4 kpc thick, and about 6 kpc across (a football shape). As stars age, their abundance above and below the disk slowly increases.
The halo contains almost no gas or dust – the opposite of the disk and bulge where interstellar medium is common. Stars in the bulge and halo are found to be redder than those in the disk. The gas-rich galactic disk has ongoing star formation and contains stars of all ages. The halo has only old stars, due to the absence of dust and gas in the halo. Astronomers refer to young disk stars as Population I stars, and old halo stars as Population II stars.
Careful study of the positions and velocities of stars and gas clouds near the sun lead to two important conclusions about the motion of the Galactic disk. First, the entire disk is rotating. The orbital speed in the vicinity of the Sun is about 220 km/s. At a distance of 8 kpc from the Galactic center, it takes about 225 million years for the Sun to make one orbit around the Galaxy (this is called a Galactic year). Second, the rotation period depends on the distance from the Galactic center. Rotation is shorter closer to the center, and longer farther away from the center. Therefore the disk rotates differentially rather than as a solid object.
Stars in the halo and Galactic center do not have an orderly circular motion as just described. The orbital rotation for them is largely random. They orbit the Galactic center, but move in all directions.
The table below compares some key properties of the three basic components of the Galaxy.
The Galaxy possibly formed through the merger of several smaller systems. Astronomers reason that early on, our Galaxy was irregularly shaped, with gas distributed throughout its volume. When stars formed during this stage, there was no preferred direction in which they moved and no preferred location in which they were found. Their orbits carried them throughout an extended three dimensional volume surrounding the newborn Galaxy. In time, the gas and dust fell to the Galactic plane and formed a spinning disk. The stars that had already formed were left behind in the halo. New stars forming in the disk inherit its overall rotation and so orbit the Galactic center on ordered, circular paths.
Knowing direction, distance, and density, astronomers can use observations along different lines of sight to map out the radio-emitting gas in our Galaxy.
Due to the differential rotation of the Galaxy, it is impossible for any large-scale structure (such as the spiral arms) to be ”tied” to the disk material to survive. The leading explanation then for the spiral arms is that they are spiral density waves – coiled waves of gas compression that move through the Galactic disk, squeezing clouds of interstellar gas and triggering star formation as they go.
There are two theories concerning the waves and star formation. One is that as matter catches up to the waves, the waves crush them and form stars. The second is that the formation of stars drives the waves. The death of a star creates a wave that causes the birth of another star. It is possible for this wave of star formation to take on the form of a partial spiral and for the pattern to persist for some time. This is called self-propagating star formation.
The Galaxy’s mass can be measured using a modified version of Keppler’s third law
Total mass (in solar masses) = [orbital size (AU)]3/[orbital period (years)]2
The sun’s distance to the center is about 8 kpc and orbital period is about 225 million years around the galaxy. This gives us a mass of about 9 x 1010 solar masses – 90 billion times the mass of our sun. This is the mass within the orbit of the Sun (not the total mass of the Galaxy). To determine the mass of the entire Galaxy, we have to use distances and rotations further out than our Sun. Through radio observations of gas in the Galactic disk, astronomers have measured the rotation speed at various distances away from the Galactic center. The resultant plot of rotation speed versus distance from the center is called the Galactic rotation curve. Within the luminous part of the Galaxy, out to about 15 kpc, the mass is about 2 x 1011 solar masses. However, the Galaxy actually stretches out further, about 40 to 50 kpc. Mass at 40 kpc is about 6 x 1011 solar masses. Therefore, we have to conclude that at least twice as much mass lies outside the luminous part of the Galaxy as lies within.
Astronomers therefore consider the luminous part of the Galaxy to be the tip of the iceberg. The luminous region is surrounded by an invisible dark halo, the region of the galaxy beyond the visible halo where dark matter is believed to reside. Dark matter is the term used to describe the mass in galaxies and clusters whose existence we infer from rotation curves and other techniques, but that has not been confirmed by observations at any electromagnetic wavelength. Even the visible portion of our Galaxy contains dark matter. Dark matter is undetectable at any wavelength, but we know of its existence due to its gravitational pull. It is not hydrogen gas, nor made up of ordinary stars.
One idea for dark matter is that they are MACHOs (MAssive Compact Halo Objects). This is the collective name for stellar candidates for dark matter, including brown dwarfs, white dwarfs, and low mass red dwarfs. Another is that they are WIMPs (Weakly Interacting Massive Particles). This is a class of subatomic particles that might have been produced early in the history of the universe.
Gravitational lensing is the effect induced on the image of a distant object by a massive foreground object. Light from the distant object is bent into two ore more separate images, thus making it brighter. The foreground object is referred to as the gravitational lens.
The galactic nucleus is the small, central, high-density region of a galaxy. Almost all the radiation from active galaxies is generated within the nucleus. Sagittarius A (or SGR A*) is a strong radio source corresponding to the supermassive black hole at the center of the Milky Way. Observations show that SGR A* cannot be more than 10 AU, and is most likely much smaller than that.
Images of galaxies look distinctly nonstellar. They have fuzzy edges and are elongated. The Hubble classification scheme classifies galaxies into four groups: spiral, barred spiral, elliptical, and irregular.
A spiral galaxy is a galaxy composed of a flattened, star-forming disk component which may have spiral arms and a large central galactic bulge. They contain a flattened galactic disk, central galactic bulge with a dense nucleus, and an extended halo of faint old stars. These are denoted by the letter S and given a classification of a, b, or c based on size of their galactic bulge. Sa have the largest bulges, and Sc the smallest. Sc contain the most interstellar matter while Sa the least.
A barred spiral galaxy is a spiraled galaxy in which a bar of material passes through the center of the galaxy, with the spiral arms beginning near the ends of the bar. Barred spirals are designated by the letters SB, and subdivided into categories SBa, SBb, and SBc just as a spiral galaxy, depending on the size of the bulge. As with the spiral, the tightness of the spiral pattern is correlated with the size of the bulge.
Elliptical galaxies are galaxies in which the stars are distributed in an elliptical shape on the sky, ranging from highly elongated to nearly circular in appearance. They have no spiral arms. They are denoted by the letter E, and subdivided by how elliptical they appear in the sky. The most circular are E0, slightly flattened E1, and so on up to the most elongated E7.
Giant elliptical can range up to hundreds of kpc and contain trillions of stars.
Dwarf elliptical may be as small as 1 kpc in diameter and contain fewer than a million stars.
Ellipticals contain little to no cool gas and dust. 21 cm radio emission is with few exceptions completely absent, and no obscuring dust lanes are seen. They are made of mostly old reddish low mass stars.
Between E7 ellipticals and Sa spirals is a class with a thin disk and a flattened bulge, but contain no gas and no spiral arms. If no bar is evident, they are called S0 galaxies, and if a bar is present SB0 galaxies. These are also called lenticular galaxies.
An irregular galaxy is a galaxy that does not fit into any of the other major categories of the Hubble classification scheme. They are divided into two subclasses: Irr I (which often look like misshapen spirals) and Irr II. The smallest are called dwarf irregulars.
Magellanic Clouds are two small irregular galaxies that are gravitationally bound to the Milky Way Galaxy. They are about 50 kpc from the center of our galaxy.
The Hubble Sequence is the variation in types across the tuning fork diagram, from elipticals to spirals to irregulars.
(above image from Penn State University - Class not taken there, only image)
Galaxies up to 25 Mpc can be detected and distances measured by the Cepheid stars in them. To measure the distances of galaxies further away, or that don’t have Cepheids, astronomers use standard candles. A standard candle is any object with an easily recognizable appearance and known luminosity, which can be used in estimating distances. Supernovae, which all have the same peak luminosity (depending on type) are good examples of standard candles and are used to determine distances to other galaxies. To be useful, a standard candle must be bright enough to be seen at large distances, and have a well defined luminosity. Planetary nebulae and Type I supernovae have been particularly reliable as standard candles.
The Tully-Fisher relation is used to determine the absolute luminosity of a spiral galaxy. The rotational velocity, measured from the broadening of spectral lines, is related to the total mass, and hence the total luminosity. This can be used to measure galaxies up to 200 Mpc.
The Local Group is the small galaxy cluster that includes the Milky Way Galaxy. It has about 50 galaxies in it. The diameter is a little over 1 Mpc A collection of galaxies held together by their mutual gravitational attraction is called a galaxy cluster.
All individual galaxies and galaxy clusters are moving away from us. Hubble’s Law states that the rate at which a galaxy recedes is directly proportional to its distance from us. Charts that plot the distance (x axis) and recession velocity (y axis) generally are showing that the points fall more or less in a straight line. These plots are called Hubble diagrams. The universal recession described by the Hubble diagram is sometimes called the Hubble flow.
As galaxies recede, their spectra are redshifted. Cosmological redshift is the component of the redshift of an object that is due only to the Hubble flow of the universe. Objects that lie so far away that they exhibit a large cosmological redshift are said to be at a cosmological distance – distances comparable to the universe itself.
Hubble’s constant (H0) is the constant of proportionality between recessional velocity and distance in Hubble’s Law.
Recessional velocity = H0 X distance
The constant is therefore the slope of the line: the recessional velocity divided by the distance. Generally, this is accepted as 70 km/s/Mpc (70 kilometers per second per megaparsec).
Normal galaxies are those whose overall energy emission is consistent with the summed light of many stars. These are the galaxies that fall into the various Hubble classes. Active galaxies are the most energetic galaxies, which can emit hundreds or thousands of times more energy per second than the Milky Way, mostly in the form of long-wavelength non-thermal radiation. Active galactic nuclei are regions of intense emission at the center of active galaxies, responsible for virtually all of the galaxy’s nonstellar luminosity.
Sefert galaxies are a class of astronomical objects whose properties lie between those of normal galaxies and those of the most energetic active galaxies known.
Radio galaxies are active galaxies that emit large amounts of energy in the radio portion of the electromagnetic spectrum. Radio lobes are roundish clouds of gas spanning about half a megaparsec and lying well beyond the visible galaxy.
A blazar is a particularly intense active galactic nucleus in which the observer’s line of sight happens to lie directly along the axis of a high speed jet of particles emitted from the active region.
A quasar or quasi-stellar object (QSO) is a starlike radio source with an observed redshift that indicates an extremely large distance from Earth. It is the brightest nucleus of a distant active galaxy.
As a class, active galactic nuclei have some or all of the following properties:
1. High luminosities, generally greater than the 1037 W of a bright normal galaxy.
2. Energy emission is mostly nonstellar – it cannot be explained as the combined radiation of even trillions of stars.
3. Energy output can be highly variable, implying it is emitted from a small central nucleus less than a pc across.
4. May exhibit jets and other signs of explosive activity.
5. Optical spectra may show broad emission lines, indicating rapid internal motion within the energy producing region.
6. Often the activity appears to be associated with interactions between galaxies.
The generally accepted explanation for the observed properties of all active galaxies is that their energy is generated by the accretion of galactic gas onto a supermassive black hole lying in the galactic center. The small size of the accretion disk explains the compact extent of the emitting region, and the high speed orbit of gas in the black hole’s intense gravity accounts for the rapid motion that is observed. Typical luminosities of active galaxies require the consumption of about 1 solar mass of material eery few years. Some of the infalling material is blasted out into space, producing magnetized jets that create and feed the galaxy’s radio lobes.
Synchrotron radiation is the type of nonthermal radiation produced by high-speed charged particles, such as electrons, as they are accelerated in a strong magnetic field.
Galaxies and Dark Matter
A galaxy rotation curve is the plot of rotation speed versus distance from the center of a galaxy.
Galaxies contain from 3 to 10 times more mass than can be accounted for in the form of luminous matter. Studies of elliptical galaxies suggest similarly large dark halos surrounding these galaxies as surround the Milky Way.
Some of the least luminous galaxies have the largest fraction of dark matter.
Upwards of 90% of the matter in the universe is dark, undetectable not just in the visual spectrum but in all electromagnetic wavelengths.
Intracluster gas is superhot (more than 10 million K), diffuse intergalactic matter filling the space among the galaxies.
Hierarchical merging is where there is repeated merging of smaller objects to form galaxies.
A starburst galaxy is a galaxy in which a violent event, such as a near-collision, has caused an intense episode of star formation in the recent past.
Computer simulations have shown that the dark matter halos surrounding galaxies are crucial to galaxy interactions. They make the galaxies much larger than their optical appearance would suggest.
Every bright galaxy – active or not- contains a central supermassive black hole. The largest black holes tend to be found in the more massive galaxies.
Quasar feedback is a process in which some fraction of the quasar’s enormous energy output is absorbed by the surrounding galactic gas. Some astronomers speculate this might explain the correlation of black hole and bulge masses in galaxies.
The difference between an active galaxy and a normal one is fuel supply. When the fuel runs out and a quasar shuts down, its central black hole remains behind, its energy output reduced to a relative trickle.
Superclusters are groupings of several clusters of galaxies into a larger, but not necessarily gravitationally bound unit. The local supercluster is about 40-50 Mpc across, contains some 1015 solar masses of material, and is very irregular in shape.
The distribution of galaxies on very large scales is nonrandom. Gal0061ies appear to be arranged in a network of strings, or filaments, surrounding large, relatively unpopulated regions of space known as voids. A void is a large, relatively empty region of the universe around which superclusters and “walls” of galaxies are organized.
Microlensing is the gravitational lensing by individual stars in a galaxy.
Cosmology is the study of the structure and evolution of the entire universe.
The Sloan Digital Sky Survey is the most extensive redshift survey to date. The largest known structure in the universe is the Sloan Great Wall. It is 250 Mpc long and 50 Mpc thick. It is an extended filament of galaxies near the center wedge about 300 Mpc from Earth, as seeon on the SDSS.
The largest known structures in the local universe are only 200-300 Mpc across. No larger voids, superclusters, or walls of galaxies are seen.
The universe is homogeneous (the same everywhere) on scales greater than a few hundred megaparsecs. The universe also appears to be isotropic (the same in all directions) on these large scales. These twin principles, that the universe is homogeneous and isotropic, n sufficiently large scales are known as the cosmological principle. The cosmological principle also includes the assumption that the laws of physics are the same everywhere.
The cosmological principle implies that there can be no edge to the universe, because that would violate the assumption of homogeneity. It also implies that there is no center, because that would mean that the universe would not be the same in all directions from any noncentral point, a violation of the assumption of isotropy. This is in connection with the Copernican principle that we are not central to the universe, and that no one can be central, because the universe has no center.
Olbers’s Paradox is a thought experiment suggesting that if the universe were homogeneous, infinite, and unchanging, the entire night sky would be as bright as the surface of the Sun. The fact that the night sky is not as bright as the Sun is why this is considered a paradox.
Using the Hubble constant of 70 km/s/Mpc, at a point 14 billion years ago, all the galaxies lay on top of one another. Astronomers think that everything in the universe at that instant were confined to a single point of enormously high temperature and density called the primeval fireball. Then came the Big Bang – the event that cosmologists consider the beginning of the universe in which all matter and radiation in the entire universe came into being. The primeval fireball began to expand, its density and temperature falling rapidly. Therefore, the age of the universe is about 14 billion years old.
The galaxies are not flying apart into the rest of the universe. The universe itself is expanding. To determine if it will continue to expand forever, or eventually stop and contract, is determined by the density of the universe. The dividing line between these two possibilities, the density corresponding to a universe in which gravity acting alone would be just sufficient to halt the present expansion, is called the universe’s critical density. For 70 km/s/Mpc, the critical density is about 9 X 10-27 kg/m3. If the universe contains sufficiently high density, then it contains enough matter to halt expansion and galaxy recession will eventually stop. Some astronomers refer to the final collapse of a high density universe as the “Big Crunch”. If there is not enough density, the universe will continue to expand, until an observer on Earth will not be able to see anything outside the Local Group. Radiation will have weakened to the point it does not reach us. All radiation, matter and life will eventually freeze as galaxies and stars burn out their fuel. This is referred to as a “cold death” for the universe.
When dealing with density, energy must also be calculated in (E=mc2). 1 J is counted as 1.1 X 10-17 kg. Therefore, the density of the universe includes atoms, molecules, and invisible dark matter that dominates the masses of galaxies and galaxy clusters, and everything that carries energy (photons, neutrinos, gravity waves, etc).
The ration of the universe’s actual density to the critical value is called the cosmic density parameter and is denoted by the symbol omega naught (?0). A universe with density equal to the critical value has ?0 = 1. A low density cosmos is less than 1, and a high density universe is greater than 1.
A closed universe is the geometry that the universe as a whole would have if the density of matter is above the critical value. A closed universe is finite in extent and has no edge, like the surface of a sphere. It has enough mass to stop the present expansion and will eventually collapse.
An open universe is the geometry that the universe would have if the density were less than the critical value. In an open universe there is not enough matter to halt the expansion of the universe. It is infinite in extent.
A critical universe is a universe in which the density of matter is exactly equal to the critical density. The universe is infinite in extent and has zero curvature. The expansion will continue forever, but will approach an expansion speed of zero.
The Great Attractor is the name given to a nearby huge accumulation of mass, with a total mass of about 1017 solar masses and a size of 100-150 Mpc, that has been suggested by the measured velocities of the overall motion of galaxies (including the Local Group) within the Local Supercluster.
Using supernova data, astronomers have shown that the universe is actually accelerating.
Dark energy is the generic name given to the unknown cosmic force field thought to be responsible for the observed acceleration of the Hubble expansion.
The cosmological constant is the quantity originally introduced by Einstein into generally relativity to make his equations describe a static universe. Now one of several candidates for the repulsive “dark energy” force responsible for the observed cosmic acceleration.
Quintessence (in ancient alchemy, this was the fifth element) is a possible form of dark-energy as a possible explanation for the expanding universe.
All approaches to calculate density yield consistent results: ?0 = 1 – the universe is of precisely critical density. This density includes dark matter and dark energy (converted into mass units). Current best estimates are that normal “luminous” matter is about 4% of the total, dark matter is 23%, and dark energy 73%. This assumption for the universe means that the universe is flat and will expand forever.
For the Hubble constant of 70 km/s/Mpc, current estimates put the Big Bang at 14 billion years ago. The first quasars appeared about 13 billion years ago (at a redshift of 6), the peak quasar epoch (redshifts 2-3) occurred during the next 1 billion years, and the oldest known stars in our Galaxy formed during the 2 billion years after that.
Cosmic microwave background is the almost perfectly isotropic radio signal that is the electormagentic remnant of the Big Bang.
The Early Universe
In the beginning, the universe consisted of pure energy at unimaginably high temperatures.
Dark energy dominates the density of the universe.
Even though dark energy dominates the density of the universe today, it was unimportant at early times. Astronomers estimate that the densities of matter and dark energy were equal about 4 billion years ago. Before then, in cosmological parlance, the universe was matter dominated.
Although radiation density is currently much less than that of matter, there must have been a time even farther in the past when they, too, were equal. Before that time, radiation was the main constituent of the cosmos, which is said to have been radiation dominated. The crossover point, where the densities of matter and radiation were equal, occurred about 50,000 years after the Big Bang.
Pair production is a process in which two photons give rise to a particle-antiparticle pair. Through pair production, matter is created directly from energy in the form of electromagnetic radiation. A particle and antiparticle can also annihilate each other to produce radiation.
For any given particle, the temperature above which pair production is possible and below which it is not is called the particle’s threshold temperature.
Everything we see around us was created out of radiation as the cosmos expanded and cooled.
At the time of the Big Bang, gravity and the other fundamental forces were one force, indistinguishable from one another.
Grand Unified Theories is the class of theories describing the behavior of the single force that results from unification of the strong, weak, and electromagnetic forces in the early universe.
A boson is a particle that exerts or mediates forces between elementary particles in quantum physics.
Electrons, muons and neutrinos are collectively known as leptons, after the Greek word meaning “light” (as in not heavy).
The production of elements heavier than hydrogen by nuclear fusion shortly after the Big Bang is called primordial nucleosynthesis.
Not only is most of the matter in the universe dark, but most of the dark matter is not composed of protons and neutrons.
Decoupling is the event in the early universe when atoms first formed, after which photons could propagate freely through space. This period is also referred to as recombination. This occurred at a redshift of 1100.
The horizontal problem is one of two conceptual problems with the standard Big Bang model, which is that some regions of the universe that have very similar properties are too far apart to have exchanged information within the age of the universe.
The flatness problem is one of two conceptual problems with the standard Big Bang model, which is that there is no natural way to explain why the density of the universe is so close to the critical density.
Epoch of inflation is the short period of unchecked cosmic expansion early in the history of the universe. During inflation, the universe swelled in size by a factor of about 1050.
Hot dark matter consists of lightweight particles – much less massive than the electron. Cold dark matter consists of very massive particles, possibly formed during the GUT epoch or even before.
Protons and neutrons are made up of subparticles called quarks. There are six distinct types of quark in the universe (up, down, charm, strange, top, and bottom).
The strong nuclear force is actually a manifestation of the interactions that bind quarks to one another.
Supersymmetry extends the idea of symmetry between fundamental forces to place all particles (those that are acted on by forces and those that tramsit those forces) on an equal footing.