The Cosmos in a Nutshell

The Constellations

There are officially 88 official constellations – at least according to the International Astronomical Union. These constellations, and their boarders, divide the entire sky into eighty-eight segments. Each star falls into a specific segment and is associated with that constellation. Although the boundaries are exact, the artwork and stick figures for the constellations can be drawn in different and arbitrary ways. Forty eight of these constellations were identified in 150 CE by Claudius Ptolemy in the Almagest. The rest have a more modern origin. For more on this, see A History of the Constellations.

Asterisms are unofficial star patterns that we recognize. They typically have names, but are not official constellations. The Big Dipper is probably the best-known asterism. However, this is a name given to it by those of us in the states. In England it is called The Plow.

Constellations toward the southern horizon follow a seasonal pattern with a different ones appearing after sunset over each of the four seasons. As you look to the south, the following constellations and asterisms will be visible.

• Spring (Leo, Virgo, and Boötes)
• Summer (Scorpius, Sagittarius, and the Summer Triangle)
• Autumn (Pegasus and Andromeda)
• Winter (Orion, Taurus, Canis Major, Canis Minor, Gemini, and Auriga)

Look to the north and you will see that the constellations follow a circumpolar path as they appear to circle the North Star. Ursa Major (Big Dipper), Ursa Minor (Little Dipper), and Cassiopeia are just a few easily visible in this direction.

There are 12 (or maybe 13) “special” constellations. Some call these the constellations of the zodiac, others the constellations of the ecliptic. For more on this, see Astrology to Astronomy.

When we look up at the night sky, we can see many “deep-sky objects.” These are the favorite targets of amateur astronomers. Groups of stars, called open star clusters and globular clusters, are found within the Milky Way galaxy. Others, called nebulae, are also found within the Milky Way galaxy. Outside of the Milky Way there are other galaxies, many of which are visible through even small telescopes.

The distance to other stars and these deep-sky objects is vast. It is impractical to measure it in miles, so astronomers use something called a “light year.” This is the distance light travels in a year and is around 5.8 trillion miles. Distances are also measured in terms of a parsec (or a million parsecs, which is a Megaparsec). A parsec is 3.262 light years. For more on this, see Cosmic Distances.

The night sky is assigned a coordinate system. All objects are given a position defined by their Right Ascension (around the sky) and Declination (north or south). This is very similar to the way places on the surface of the Earth have coordinates (Latitude and Longitude). For more on this, see Measuring the Night Sky (Coordinates and Field of View).

To learn more about the constellations and other objects in the night sky, see The Stars & Constellations.

The Solar System

Our solar system contains a star known as the Sun. It is somewhat of an average star, officially designated as a yellow dwarf star. It is still large enough so that over one million Earth’s could fit inside with room left over. The temperature at the core of the Sun is around 27 million degrees (F), while at the surface, it drops to a mere 10,000 degrees (F). Although, the Sun accounts for most of the mass of the Solar System, there are still many interesting objects gravitationally bound to the Sun.

The Solar System has eight planets. For a long time there were nine planets. Many of us learned their names at a very young age. Do you remember the mnemonic, “My Very Eager Mother Just Served Us Nine Pizzas?” Of course, today with the reclassification of Pluto, the Nine Pizzas have become Nachos.

• Mercury, Venus, Earth, and Mars are small terrestrial planets in the inner Solar System.
• Jupiter and Saturn are large gas giants in the outer Solar System.
• Uranus and Neptune are large ice giants farther out in the Solar System.

• Mercury is the smallest of the planets and resembles the Earth’s moon. It speeds around the Sun with an orbit lasting just 88 days.
• Venus is around the size of the Earth. However, it is very un-Earth like with an atmosphere of carbon dioxide and clouds of sulfuric acid. It is the hottest planet with a surface temperature over 850 degrees (F).
• Earth, our home, is only planet that supports life – at least that we know of. The Earth is tilted as it orbits the Sun. This is why we have seasons.
• Mars is the red planet due to iron oxide (rust) on its surface. Olympus Mons, the largest volcano in the Solar System, and Valles Marineris, a huge cannon, are found on Mars.
• Jupiter is the largest planet (in our solar system) at around 11 times the width of the Earth. It has alternating bands of clouds (darker belts and lighter zones) on its “surface.” The great red spot is a storm that’s been raging for, maybe 300 years, and one that could swallow the Earth.
• Saturn is know for its rings, which are composed of chunks of water ice. Saturn is a little smaller than Jupiter at 9 times the width of the Earth.
• Uranus is the planet that spins on it side. The big question is how to pronounce its name? Most of us learned you-RAIN-us (U-RAIN-us), today most astronomers call it YUR-ah-nus (Ur-a-nus). 
• Neptune is the farthest of the planets from the Sun. It once had a great dark spot that’s disappeared.

NASA keeps a count of the moons of the Solar System and last time we checked there were over 200. Because of the availability of ice, the outer solar system is home to many icy moons. The moons are possibly more interesting and more Earth-like than the planets. Nineteen are large enough to be spherical. Seven are larger than the dwarf planets, including Pluto. Two, Ganymede and Titan, are larger than the planet Mercury. Titan is the most interesting moon with a thick hazy orange atmosphere, icy dunes, and methane filled lakes and seas.

Pluto used to be planet. Today, it is one of five official dwarf planets, although not everyone is happy with this classification. Both a planet and a dwarf planet must be spherical and must orbit the Sun (no moons allowed). However, planets are large enough to clear their neighborhood of other objects, dwarf planets do not – they are too small. Therefore, we find the dwarf planets in or near the Asteroid belt or the Kuiper belt. These are the five dwarf planets.

• Ceres is the only dwarf planet in the asteroid belt.
• Haumea (how-MAY-ah) is shaped like an egg or maybe a football.
• Makemake (MAH-kay MAH-kay) has a reddish surface.
• Eris (Air-is) is almost the same size as Pluto and was considered to be the 10^th planet for a short time.
• Pluto, of course, is everyone’s favorite ex-planet.

In addition to the planets and dwarf planets, there are two belts of objects – both containing material left over from the formation of the Solar System.

• The Asteroid belt has rocky and metallic objects that failed to condense into a planet. Most asteroids are found in the main belt between Mars and Jupiter. Some, called “Near Earth Objects” cross the Earth’s orbit, and a few pass close to the Earth. Contrary to the way the asteroid belt is depicted in the movies, it is mostly empty space. There is on the average one million miles between any two asteroids, and, if we bundled them all up into a ball, they would weigh less than the Earth’s moon.
• The Kuiper belt has icy objects that failed to condense into larger objects. These objects are found outside of the orbit of Neptune and the Kuiper belt is the home of most dwarf planets.

Finally, way out beyond everything else is the Oort Cloud. This is the home to the comets, which are chunks of dirty ice. Every so often one of them gets nudged and falls toward the Sun. We see it as an object with a tail that passes close the Sun before heading back out into the depths of space.

The Universe

For a long time we only knew of one solar system – ours. However, In 1995, Michel Mayor and Didier Queloz discovered 51 Pegasi b, a planet orbiting another star and the flood gates opened. Alien solar systems were found around many other stars in the Milky Way, and we think there might be billions of them. At last count, according to the folks at NASA, there are over 5,000 known “exoplanets” – planets orbiting other stars.

All stars, including the Sun, are composed mostly of hydrogen and are large enough so gravitational pressure causes the star to “ignite”. Nuclear fusion in a star’s core fusses hydrogen to helium. This releases a huge amount of energy, which we eventually see as light and heat.

The characteristics (size, color, and temperature) of stars are specified on the Hertzsprung-Russell diagram. For most of their life, stars appear somewhere along the main sequence, with large blue stars to the upper left, mid-sized white and yellow stars in the middle, and small red dwarf stars toward the lower right. As they age, stars become blue or red supergiants, red giants, or white dwarfs. As an aside, the largest known star is UY Scuti, in the constellation Scutum (The Shield), with a diameter over 1,800 times that of the Sun.

A solar system begins with a large cloud of gas, dust and ice. Over time, gravity pulls most of the material to the center and a star is formed. The left over material swirls around the star and gravity again takes over, forming planets and moons. The terrestrial planets formed inside the “frost line”. Here there was little gas or ice available, so these planets formed from the available rock and metals. The outer planets formed outside the “frost line”. Here there was plenty of gas and ice, so these planets grew much larger and are composed mostly of gas. Uranus and Neptune are far enough out that their interior contains icy material (water, methane, and ammonia).

Stars eventually use up their supply of hydrogen. Large stars do this every quickly within a few million years. Stars like the Sun might take a few billion years. Small red dwarfs can last for trillions of years. Low mass stars, like the Sun, eventually expand into a red giant and leave their core behind as a white dwarf. Larger stars explode as a supernova and leave behind either a neutron star or, if they are massive enough, a black hole.

Our Solar System resides in a galaxy called the Milky Way. The Milky Way is a barred spiral galaxy some 100,000 light years across and around 12 or so billion years old. The Solar System is around 1/2 way out from the galactic center in the Orion spur off the minor Sagittarius arm. The Milky Way has two satellite galaxies, the Large and Small Magellanic Clouds, which are visible in the night sky from the Southern Hemisphere.

The Milky Way is found in the Local Group of galaxies, which includes around 80 other smaller galaxies and one other large spiral galaxy, Andromeda. The Local Group, in turn, is found within the Virgo supercluster of galaxies. By the way, the Milky Way and Andromeda are slowly moving toward each other. They will collide in another 4-5 billion years and form a combined galaxy known as Milkomeda.

Galaxies come in different shapes. There are spiral galaxies like the Milky Way, elliptical galaxies, and irregular galaxies. Supermassive black holes are found at the center of most of these galaxies. Sagittarius A* is the black hole found at the center of the Milky Way.

The Sun is one of around 200 billion stars within the Milky Way. There may be around 200 billion to a trillion galaxies in the visible universe, which extends over some 92 billion light years in diameter. We don’t know what is outside of this horizon. Light from there hasn’t had enough time to reach us.

As best as we can determine, the Universe began with something called the Big Bang around 13.8 billion years ago. Shortly after the Big Bang, we think that the universe underwent a period of cosmic inflation when it grew exponentially in a fraction of a second. Then it began a more steady expansion, which we still see today. What this means is that most galaxies are moving away from us (and from each other). We can’t see all the way back to the moment of the Big Bang. We can, however, detect the Cosmic Microwave Background, which is radiation from the time (380,000 years after the Big Bang) when the universe cooled enough so that light first became visible.

The large scale structure and motion of the universe and objects within it are governed by Einstein’s Special and General Theories of Relativity.

• Einstein’s Special Theory tells us that the universe is not as it seems. Our measurements of time and distance are not absolute, but depend on the motion of the observer. Energy and matter are interchangeable (i.e., e = mc²). The only constant is the speed of light in a vacuum (around 186,000 miles per second).
• The General Theory of Relativity is the modern theory of gravity. It too provides some surprises. Einstein’s equations aren’t the easiest to understand. They look something like this, R_mn – ½ g_mn R = -kT_mn, and say that space (and time) is curved and the amount of curvature depends on the mass of objects. Physicist John Wheeler put it this way – “matter (with mass) tells space-time how to curve” and “space-time tells matter (with mass) how to move.” Once again, time is not absolute, but varies with gravity. As gravity becomes more intense, time slows down.

Einstein’s general theory leads to probably the strangest objects in the universe – black holes. A black hole is something that is so dense (i.e., it has so much gravity) that even light cannot escape. Anything that falls into a black hole disappears from the universe forever. The point of no return is the “event horizon”, which is at the Schwarzschild radius from the center of the black hole. In theory, if you squeeze any object down to within its Schwarzschild radius, it will become a black hole. There are two types of black holes: 1. Stellar mass black holes that form after a large star explodes as a supernova, and 2. Supermassive black holes found at the center of most galaxies.

At the lowest level, the universe is made up of a few fundamental particles. This is captured in something scientists call The Standard Model of Particle Physics. Everything we see and touch is composed of molecules. Molecules, in turn, are built from atoms. Atoms are composed of a nucleus with protons and neutrons, and electrons, which exist as waves surrounding the nucleus. Electrons are thought to be fundamental particles. Protons and neutrons, however, are not. They are composed of quarks. A proton is two up quarks and a down quark, while a neutron is two down quarks and an up quark. Quarks within protons and neutrons are held together by gluons.

There are four fundamental forces in nature that make everything work.

• Gravity is by far the weakest, but is everywhere, so we notice it more than the other three. In theory, this force might be carried by gravitons – although we don’t know for sure.
• Electromagnetism powers our modern world. We see this as light and other electromagnetic radiation such as infrared rays, ultraviolet rays, and x-rays. This force is carried by massless photons.
• The weak nuclear force is responsible for radioactivity where particles spontaneously decay into other particles. This weak force is short ranged and is carried by “intermediate vector bosons.” The intermediate vector bosons (W^+/- and Z⁰ particles) get their mass by interacting with the Higgs field/bosons.
• The strong nuclear force binds the nucleus of atoms together. This strong force is also short ranged and is carried by those gluons. It is the binding energy of this force that gives protons and neutrons their mass.

The universe contains around 5% “baryonic matter”, which is the ordinary matter that what we can see. The rest is somewhat of a mystery. There appears to be too much gravity that can be accounted for by ordinary matter. Because we don’t know where this extra gravity comes from, it is called dark matter. We think dark matter is the “scaffolding” upon which galaxies are built. In the 1990s, astronomers tried to measure the rate with which the expansion of the Universe was slowing down due to gravity. However, they discovered that the expansion is accelerating – it is speeding up. We don’t know what is causing this, so it is called dark energy. Dark matter (27%) and dark energy (68%) account for around 95% of the universe.

Is our Universe alone? Or is there a multiverse? In 2003, Max Tegmark introduced four possible types (or levels) of multiverses. Is one of these a description of reality? We really don’t know. So, choose one.

1. The infinite universe
2. The bubble universe
3. The quantum (many worlds) universe
4. The mathematical universe