Astronomy DSST Free Clep Study Guide

Heliocentric Theory

Places the sun at the center of the solar system, stating that ll other bodies in the solar system orbit it. This was in direct contradiction to Ptolemy's geocentric theory, which stated Earth was the center of the Universe.

Geocentric Theory

Earth is the center of the Universe

Nicolaus Copernicus

Widely credited as the father of the heliocentric theory. Although he was not the first to propose the concept, his was the first mathematical model which correctly predicted the movement of the planets. He later published a book which was instrumental in making the idea popular.

Hans Lippershey

Usually credited as the inventor of the telescope (though it wasn't called that for many years later). He was a German-Dutch lens maker who first applied for the patent of the "Dutch Perspective Glass". It is important to note that, at the time, these were not used for astronomy. That came a little later with Galileo.

Galileo Galilei

Was the first to use a telescope for astronomical studies. He later used that telescope to discover the four largest moons of Jupiter. Galileo was a firm believer in the heliocentric theory. Since most scientists of the time and, more importantly, the Church embraced the geocentric theory, this caused Galileo much trouble and was the reason he was later placed under house arrest until his death.

Celestial Sphere Theory

In the early days of astronomy, there were plenty of theories like the geocentric theory which were later disproven. Another of these theories was something called the celestial sphere theory. This theory said that what we see in the sky was actually numerous spheres. You can think of the spheres as clear globes around the Earth with planets, comets and stars inside each of these globes. Some of these were thought to be in our own atmosphere and others were farther away. Even Copernicus, who got it so right with the Heliocentric theory, still believed in these celestial spheres. For example his model of the universe had the starts at equal distance from the Earth in one of the most distant spheres.

Tycho Brahe

Disproved the celestial sphere theory that all celestial objects were unmoving objects in Earth's sky. He demonstrated this was untrue through observation of a supernova, which he proved was beyond the solar system and thus not a phenomenon within Earth's atmosphere.

Johannes Kepler

He was a mathematician who proposed the three Laws of Planetary Motion. These stated that the planets move in an elliptical orbit around the sun and mathematically showed how it was possible. Kepler's laws provide the building blocks for many theories to follow.

First Law of Planetary Motion (The Law of Ellipses)

This law says that the planets orbits are elliptical in nature and that the center of the sun is a focus for those ellipses.

Second Law of Planetary Motion (The Law of Equal Areas)

This law describes how fast the planets are moving around the sun. The closer the planet gets to the sun, the faster it moves. As it moves farther away from the sun, it will slow down. However, the second law also says that if we drew a line from the center of the sun to the center of the planet, we would see that every 31 days the area covered by the line would be the same. Hence the name - "Law of Equal Areas".

Third Law of Planetary Motion (The Law of Harmonics)

This one requires some math. I would spend a few minutes wrapping your head around this law in case you're asked about it on the exam. Basically, the third law of planetary motion says that the orbital periods of different planets are related. Specifically, that the ratio of the squares of the orbital periods to the cubes of the average distance from the sun are the same. In other words - You take the length of time the planet takes to complete an orbit around the sun and square that number. You then take the planet's Average Distance from the sun and cube it. The ratio between those numbers will be the same no matter what planet you use the formula for. Pretty cool, eh?

Isaac Newton

He invented the first functional reflective telescope (We'll cover what a reflective telescope is a bit later). Newton also proposed the three laws of motion and the law of universal gravitation.

First Law of Motion

This law (paraphrased) states that an object at rest will stay at rest unless acted upon by an outside force. Likewise an object in motion will not change its velocity unless acted upon by an outside force. This is also called the Law of Inertia.

Second Law of Motion

This law (paraphrased) states that an object's mass is directly proportional to how much force is going to be needed to move that object. In other words, pushing a large box of tissue paper is going to be much easier than pushing an equally large box of rocks. The box of rocks has a much larger mass than the box of tissue paper, even though they are the same size. Newton's second law provided a formula to determine just how much force was needed in order to accelerate an object based on its mass.

Third Law of Motion

This law states that to every action there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions.

Law of Universal Gravitation

States that everything in the universe has a gravitational field that affects other objects, and how strong that field is depends on the object's mass and how close the two objects are to each other.

Max Planck

German scientist who worked with Albert Einstein and is generally credited with creating Quantum Mechanics. His Quantum Theory revolutionized natural science and was the reason he was awarded the Nobel Prize in 1918. Quantum Mechanics deals with matter on the atomic and sub-atomic level and is used in Astronomy to explain the behavior and the existence of many cosmic bodies which would otherwise be difficult to explain using normal physics.

Edwin Hubble

Confirmed the existence of other galaxies outside our own in 1919. Also discovered the degree of Doppler shift in those galaxies and how they increased the further the galaxy was from our own. This led to Hubble's Law which allows the observation of the universe expanding.

Big Bang Theory

Is a theory which explains the origin of all matter in the universe as the result of an original cosmic explosion and is constantly expanding.

Arno Penzias and Robert Wilson

Accidentally discovered cosmic microwave background radiation (CMB) in 1964 which was an important part of the necessary evidence for the Big Bang Theory of the Universe. They were both awarded the Nobel Prize for their discovery.

Universe Contract Theory

Suggest that the universe will eventually begin to contract and ultimately the Big Bang will be reversed when the universe implodes on itself.

Panspermia

This is the theory that life's building blocks are carried around the Universe by meteors, comets, asteroids, and planetoids. When one of these falls to a planet, life has the chance to begin evolving. Many scientists believe that this is how life began on earth and point out that the same process may have taken place on many other planets in our Universe. There is some scientific evidence pointing toward this conclusion, as there have been meteors originating from Mars that seem to show signs of microorganisms having once lived there. The debate about this evidence is still ongoing.

Solar mass

Astronomers use Solar Mass as a way of determining the size of celestial bodies. One solar mass is equal to the size of our sun. Knowing this, we can easily get a frame of reference for other stars. If we say another star is 15 solar masses in size, we can instantly draw a comparison - It's fifteen times the size of our sun.

Astronomical Unit (AU)

A way of measuring distances in Astronomy. It equals roughly 93 million miles which is (not so) coincidentally the same distance between the Sun and Earth. Using this we can determine distances when a light year is too large a unit of measurement. For example, Jupiter is roughly 5 AU from the Sun which means it's 5 times the distance from the Sun that Earth is.

Light Year

The distance that light travels in one year, roughly 6 trillion miles or 63,000 AU.

Parsec

A unit of measurement in Astronomy, Equal to about 206,000 AU or 3.2 Light years.

Absolute Magnitude

A measure of intrinsic brightness used for celestial objects

Hertzsprung Russel Diagram

A Hertzsprung Russell Diagram is used to determine the relationship between a star's absolute magnitude, type and temperature in scatter-graph form. It's kind of hard to describe what it looks like without pictures, so I suggest you run a quick google search on this to see what it looks like for the exam, but the two sides of the Hertzsprung Russell Diagram show the values of luminosity and color (which is linked to temperature) against each other. A simpler way of putting it is this - The Diagram allows a comparison of many types of stars based on color, temperature, and brightness. This can show you at a glance where most of the stars fall in brightness and temperature as well as color. Astronomers can use this to see trends and spot stars that fall outside the norm.

Spectroscope

A Spectroscope is used to measure light over the electromagnetic spectrum. Often this means that it breaks down the light into its individual wavelengths, much like a prism breaks down sunlight into individual colors. This is useful for a variety of purposes, including identifying what objects are composed of based on their visible spectrum. When light from a source is broken down using a spectroscope, it's often done so on a 2 dimensional format with all of the colors in the visible spectrum present. You've probably seen test before, long strips of paper with colors starting at blue on one side and moving to red on the other.

Absorption Lines

Absorption Lines are when a certain shade of color on that strip is black, indicating that a particular frequency of light has been blocked. It's pretty easy to notice as there will be different shades of red for example and then all of a sudden there's a black stripe there. Let's use UV light as an example. In Earth's atmosphere there are substances that block particular frequencies of light. One of these is ozone (which block UV light). If we were looking at the Sun through Earth's atmosphere, the UV light would show as an absorption line (black stripe) on a spectrum since it's been blocked. If we really wanted to see the UV light clearly, we'd need a space born telescope to get above the ozone. So how do we use these absorption lines? The same way we just did in our example. If we put something in between a light source and ourselves, we can tell a lot about its makeup by what wavelengths of light are blocked. These blocked wavelengths show up as absorption lines on our spectrum.

Emission Lines

The opposite of an absorption line is an Emission Line. This indicates the emission of a particular wavelength of light (rather than the absorption), it shows up as a bright line on a spectrum. You'll need to know both of these most likely for your exam. Remember - an Absorption line on a spectrum means a wavelength has been blocked and an Emission line means that particular wavelength is present.

Angstrom

Used to measure wavelengths of electromagnetic radiation and named after Anders Angstrom. Radio waves have the longest wavelength and Gamma rays have the shortest. That's probably something you'll want to remember for the exam.

Doppler Effect

The Doppler Effect describes how a wave's frequency changes based on the motion of the observer in relation to the source of the wave. A common example is the change in the frequency of sound waves from a passing car or an ambulance siren. You can usually tell it's getting closer by the rise in pitch as it approaches. That's actually the sound waves getting more and more compressed the closer the source come to you, the listener. Once the Source passes, the sound waves get longer and longer which changes how they sound again. Light works almost the same way. A source of light getting closer to Earth has a higher frequency (shorter wavelength) which exhibits what is called Blue Shift. As a source moves away from the observer, the waves lengthen toward the red end of the spectrum which is known as a Red shift. What you need to know for the exam is that an object exhibiting Redshift is moving away from us and an object exhibiting Blue Shift is moving toward us. Using the Doppler Effect, astronomers can detect the relative motion of celestial objects and even use determines their speeds.

Radial Velocity

Knowing that objects are moving towards or away from the viewer is only half of the equation. Next we need to know how fast that object is moving. Radial Velocity is the speed at which an object is moving towards or away from the viewer here on earth. This is one of the ways that astronomers determine whether stars have exoplanets. If a star does have an exoplanet, the radial velocity of the star will slightly change when the planet's gravity affects its orbit. This is also the way that astronomers know that the universe is expanding, by measuring the radial velocities of galaxies as they are moving away from us.

Parallax

Parallax is the apparent shift in the position of an object when the angle of viewing is changed. That sounds kind of complicated but it really isn't. Think about it this way: Let's say you're standing in a field and there are two fence posts directly in front of you but a few hundred yards away. One is at 100 yards and the other is at 500 yards. If you take a few steps to the left, both fence posts will appear to move as your perspective changes, but the one at 100 yards will appear to move much more than the one at 500 yards. That's all parallax really is. As you make small changes to you location, the objects closest to you will appear to move more than those farther away.

Triangulation

Knowing what Parallax is, you can now understand how scientists use it (also called triangulation) to measure distances to stars. Astronomers will not the angle of a star which they wish to find out the distance. They will then wait a period of time (say 6 months) and will take the angle of the star again. Since the Earth has moved around the Sun during this time, the angle is completely different. Using a healthy dose of Trigonometry, they can accurately determine the distance by computing the difference of those two angles. This method works on stars up to 400 light years away.

Proper Motion

Unlike Triangulation/Parallax, Proper Motion is the movement of the star itself rather than the observer. It is measured by taking the angle of the star's movement over time compared to the center of the solar system. In this way, stars will often seem to move to different constellations over time. The more the angle changes over a short period of time, the closer the star can be assumed to be.

Telescope

A device that is used to see distances far beyond normal eyesight. The first telescope, invented in the Netherlands in 1608, was refractive and was not used for astronomy.

Refracting Telescope

The use of the telescope for astronomy was pioneered by Galileo. It operates by having two lenses (objective and eyepiece) that focus the light from an image so that the view can see it more clearly. This is your typical "Tube" telescope. The downside of refractive telescopes is that they cannot see the entire spectrum of light due to the glass and the size of the lens is limited. Creating a lens free imperfections is more difficult the larger the size. These problems were mostly solved with the invention of the reflective telescope.

Reflective Telescope

Uses curved mirrors to reflect gathered light and form an image. Light enters the telescope and is then reflected towards an eyepiece through a series of mirrors. The first practical reflective telescope was built by Isaac Newton. This type of telescope allows much larger lenses to be used than refracting telescopes since the entire mirror can be supported instead of just the edges of the lenses. As a general rule of thumb, the maximum useful magnification is between 50 and 60 times the diameter of the lenses or mirror. Most telescopes used by professional astronomers today are reflective.

Radio Telescope

A Radio Telescope is used to see celestial bodies in the radio spectrum. Since there is so much radio interference around population centers, they're usually placed far away from cities and inside valleys to avoid that interference. Radio waves have extremely long wavelengths, so it's important to collect as many of them as you can. Due to this, radio telescopes tend to have very large dishes. They are able to see past physical obstructions and are therefore very useful for seeing into places like our galaxy's core where stellar dust and other light sources may block conventional telescopes.

X-Ray Telescope

Like the Radio Telescopes, an X Ray Telescope is able to see beyond the visible spectrum and identify objects that may otherwise be obscured. They are usually required to be either in the upper atmosphere or space-based as the Earth's atmosphere absorbs X-Rays. An easy example of an X-Ray telescopes' use is when looking at black holes. While we can't see black holes with any normal telescope, the X-Ray energy coming from them is easily seen by an X-Ray telescope.

Galaxy

A group of stars and stellar matter held together by gravitational forces. Can contain anywhere from 10 million to 100 trillion stars. They are usually classified by their shape. The primary types of Spiral, Elliptical, and Irregular

Spiral Galaxy

Shaped like... a spiral! (Imagine water going down a drain) They usually have two arms but may have more. Usually made up of younger stars and many globular clusters. The Milky Way is a spiral galaxy.

Elliptical Galaxy

Spherical shaped and may be larger than spiral galaxies. Usually filled with older stars and open clusters.

Irregular Galaxy

A galaxy that does not fall into one of the other two classifications. This is usually due to interactions with other stellar objects, such as a collision between two galaxies.

Quasar

The most distantly observed and luminous objects in the universe. According to their high redshift, we know they are the most distant objects observed. The fact that they are observable at all from this distance points to their extreme luminosity. One quasar has the luminosity equivalent to a trillion suns. Astronomers believe they are the result of super massive black holes in the center of some of the earliest formed galaxies in the universe.

Supermassive Black Hole

It is believed there is one of these in the middle of the Milky Way and most other galaxies, forming the gravitational center that the other stars in the galaxy revolve around.

Galaxy Groups

A large group of galaxies (usually at least 50) which are gravitationally connected.

Galaxy Clusters

Larger than a Galaxy Group. Usually has between 50 and 1,000 galaxies in each cluster. Think about that one for a minute -1,000 galaxies each containing billions and trillions of stars!

Local Cluster

The collection of galaxies in which our own Milky Way is a part of. It numbers roughly 53 galaxies with the Milky way and the Andromeda Galaxy being two of the largest.

Nebula

The birthplace of stars. These large collections of hot gasses and dust are interstellar nurseries where stars are born.

Protostar

A precursor to a star which is born in a nebula. The dust and gas condenses and begins to heat up. Once it condenses enough, the hydrogen fuses to form helium and it turns into a Main Sequence Star. If the mass of dust and gas isn't large enough to begin this fusing process, it forms a Brown Dwarf instead.

Main Sequence Star

A Star in which the hydrogen is fusing to form helium. An example is our Sun in its current form. The smaller the star, the longer it burns. A star with one solar mass takes almost 10 billion years to complete the transformation of all of its hydrogen into helium. Larger stars fuse their hydrogen much more quickly and therefore end the Main Sequence stage sooner.

Red Giants and Blue Giants

A Main Sequence Star which has fused all of its hydrogen into helium. As the hydrogen disappears, the star begins to swell and cool. As it swells it will begin to lose gases around the white dwarf core in the form of a planetary nebula. Contrary to what the green bean farmers will have you believe, there is no such thing as a "Green Giant" and that includes stars, too.

White Dwarf

Comparable in size to Earth but comparable in mass to the sun. (A spoonful of Earth may weigh a few grams. A spoonful of white dwarf matter would weigh several tons. ) Instead of nuclear fusion like a Main Sequence star, they are powered by the leftover energy remaining in the core. (I like to think of them as glowing embers) They are less luminous than the Sun and usually take several billion years to fade.

Supergiant

A byproduct of a star with larger mass. Instead of turning into a Red Giant after its Main Sequence Stage, Larger stars (above 10 solar masses) become Supergiants. With a rapidly expanding shell of gases around the core, they can be used up to 1000 times the diameter of our Sun. The difference between a Supergiant and Red Giant, apart from size, is in how they die. A supergiant tends to do so in a violent explosion as the core collapses upon itself in less than a second. This is known as a Supernova.

Supernova

An event that occurs when a massive star undergoes a gravitational collapse. The resulting supernova explosion produces extreme luminosity which can temporarily outshine an entire galaxy. If the core survives the explosion, it forms a Neutron Star or Black Hole.

Neutron Star

The remnants of a star that dies through particular types of supernova gravitational collapses. Very small and very dense. Generally have less than three solar masses. Anything larger than three solar masses and the core will collapse onto itself forming a Black Hole

Black Hole

A gravitational vacuum from which even light cannot escape. Formed after a supernova when the core collapses upon itself, black holes consume nearby matter which may include stars or even other black holes.

Pulsar

A type of neutron star which emits radiation that appears as a pulse when observed from Earth. This is due to the star rapidly spinning and emitting energy. When the rotation points it towards Earth, we see the energy released as a "Pulse".

Binary Star

A star system made up of two stars orbiting around a common center point. Many of the stars you see in the night sky may actually be a binary star system. The two stars are so close that the light appears to be coming from a single point.

Blue Star

Otherwise known as Class O stars, they are the hottest of the Main Sequence Stars. A Blue Super giant, which can have surface temperatures around 40,000 degrees Kelvin or higher, is one of the hottest types of stars known to exist. Remember that a blue star burns hotter than a red star.

Cepheids

Pulsating variable stars. They change luminosity at very regular intervals and by very regular amounts. This behavior makes them excellent 'standard candles' and they are thus useful for determining distances in space

Metallicity Classification

The Population I, II, and III classification system indicates the metallicity of stars. This is important because it is believed that by using this we can determine a star's age

Population III Stars

The lowest in metal content and thus the oldest. When they were formed there wasn't much metal in the universe. These are still unobserved since it is believed most of them went supernova soon after the universe's creation.

Population II Stars

The oldest of today's observed stars and were formed from the metallic remnants of the Population I stars.

Population I Stars

The youngest and highest in metal content, recycling metal that was formed by other stars and stellar events before them. Our Sun is a Population I star.

Star Cluster

A group of stars gravitationally bound. There are two primary forms of star clusters which are Globular Clusters and Open Clusters.

Globular Cluster

Usually contains very old (Population II) stars. Can contain several million stars in a very small region. Usually yellow stars and red stars are present with the very rare blue star.

Open Cluster

Made up of young stars which are usually brighter and burning hotter than their globular cluster counterparts. Blue stars are more common through here and nowhere near as many stars in an open cluster as there are in globular cluster. Open clusters usually last for only a few hundred million years before being dispersed by gravitational affects of other stellar bodies.

Fusion

Process by which the stars convert their hydrogen into helium. This provides the luminosity and heat by which the stars are classified. The two types of fusion that are primarily seen in stars are the Proton-Proton Chain Reaction for smaller stars such as our sun and the CNO cycle for lager stars.

Proxima Centauri

The closest star to our own. It is "only" 4.2 light years away from the sun. The "Proxima" in its name is Latin for "nearest to" which is an easy way to remember this one.

Age of Our Solar System

The currently accepted estimates place the age of the solar system at approximately 4.578 billion years old.

Orion Arm

Location of our solar system in the Milky Way galaxy.

Comet

A ball of ice, rock, and dust which usually originates in the Kipper Belt or Oort Cloud. It orbits the sun, burning off gasses the closer it gets to the sun during its orbit.

Kuiper Belt

An area past the orbit of Neptune that is filled with icy bodies mostly gaseous in nature (frozen methane, water). It is believed that many of our Solar System's comets come from this area.

Oort Cloud

A hypothetical cloud of many billions of icy objects on the outskirts of the solar system. While we don't have any firm proof of its existence, models of solar system formation agree that it must be there. It is believed that many of our solar systems long-period comets do originate from this cloud.

Planets

Eight Planets that are currently known in our solar system. These are (in order from closest to the sun) Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto was recently de-listed as a planet and the new version of the DSST should take that into account.

Ringed Planets

Jupiter, Saturn, Neptune, and Uranus all have rings. An easy way to remember this is to know that these are the last four in the Solar System. As long as you have the order memorized, this is easy. Of the four, Saturn's rings are most easily visible.

Gaseous Surface

Jupiter, Saturn, Neptune, and Uranus have gaseous surfaces. The are also the largest planets in the solar system. Remember. The last four planets have rings, are the largest, and are gaseous.

Rocky Surfaces

Mercury, Venus, Earth, and Mars all have rocky surfaces. Again, just remember that the first four planets closest to the Sun are made of rock and the four farthest away have gaseous surfaces and rings

Venus and Mercury

The only two planets in our solar system with no natural orbiting moon

Venus

Has an atmosphere of roughly 95% carbon dioxide which acts as a greenhouse. Due to a combination of proximity to the sun and the atmosphere, it is extremely hot, with surface temperatures between 800 and 900 degrees Fahrenheit.

Earth's Age

Radiometric age dating indicates that the Earth is approximately 4.54 billion years old.

Great Oxidation

Scientist use this term to explain the formation of oxygen in the Earth's atmosphere. They believe it occurred roughly 2.5 billion years ago and was due to algae blooms.

Earth's Rotation

It takes 23 hours and 56 minutes for the Earth to rotate completely on its axis.

Earth's Seasons

They are affected by the orbit of the Earth around the sun in combination with its tilt (23.5 degrees) on its axis. This results in different parts of the world being colder or warmer at different times of a year. For example, when the Northern Hemisphere is going through summer (tilted toward the sun) the Southern Hemisphere is experiencing winter. That's why July is one of the coldest months in Australia when it's one of the hottest months in the US. Likewise, January is the middle of summer in Argentina.

Precession

So we know that the Earth is tilted as it rotates around the sun, but did you know that the tilt changes as well? The tilt actually makes a small circle (think of a top that's spinning, but is also wobbling in a small circle as it spins). Of course, with something the size of the Earth, even a small circle takes a while. In this case, it takes around 26,000 years for the Earth to make one full "wobble" or precessional cycle. Precession is caused by the gravitational forces of the Sun and our Moon pulling on the Earth. So even though Polaris may be our North Star right now, in a few thousand years there will be another star that takes its place as the Earth's precession causes the night sky to move.

Celestial Sphere

This isn't the same thing as the old Celestial Sphere theory, but rather is a tool used by astronomers. The Celestial Sphere is an imaginary sphere around the Earth which allows observers to attribute celestial objects to particular places on this sphere. The sphere is divided by an Equator which matches Earth's Equator and has its own Celestial North and South Poles

Equinox

The twice a year occurrence where the Earth is tilted neither towards nor away from the Sun. At the moment of an equinox, the center of the sun appears to be directly overhead a person who is standing on the Earth's equator.

Vernal Equinox

The equinox that occurs in March (the beginning of spring in the Northern Hemisphere).

Autumnal Equinox

The equinox that occurs in September (the beginning of fall in the Northern Hemisphere).

Solstice

The twice a year occurrence when the Earth is tilted the most towards the sun. Think of this as the opposite of the equinox (sun is directly overhead). Generally, the beginning of summer in the Northern Hemisphere is accepted as being the Summer Solstice (when the sun is farthest north in the sky) and the first day of winter is the Winter Solstice (when the sun is farthest south). Just remember that the Southern Hemisphere is opposite the North. So their Winter Solstice is actually when the sun is the farthest North rather than south.

Meteroid

A piece of rocky or metallic debris smaller than an asteroid which travels through the Solar System.

Meteor

The name given to the visible path of a meteoroid as it enters the atmosphere of the Earth.

Meteorite

The surviving remnant of a meteor on the surface of the Earth.

Asteroids

Bodies of solid rock and metal which ar relatively small compared to the planets. Most of the rocky asteroids in the solar system are found in the asteroid belt between Mars and Jupiter.

Mars

Has an atmosphere of roughly 95% carbon dioxide, but also has very low density so the gas does not actually have a greenhouse effect to the extent it does on Venus.

Olympus Mons

Found on Mars, it is the highest mountain in the solar system.