Back ground information on Astronomy

Light travels extremely quickly at 300,000 kilometers per second (over 670 million miles per hour); this is known as the speed of light. It only takes a ray of light from the Sun 8.3 minutes to reach Earth. Since light can travel so quickly, astronomers use a unit of distance known as the light year to measure distances between stars and other celestial bodies. A light year is the distance light can travel in one year on Earth, or 365 days. One light year equals 9,460,000,000,000 kilometers, or roughly 6 trillion miles. The light year makes it much easier for astronomers to measure distances between celestial bodies as well as the sizes of those bodies.
It is important to remember that when a scientist snaps a picture of a star, by the time the light enters the lens of the telescope it has been travelling for thousands, maybe millions, of years depending on how far away the star is. It is possible that at the moment the picture of the star is taken, that star no longer exists and the scientist just does not know it yet.
There are many different forms light energy can take, and we describe these different forms according to their positions on the electromagnetic spectrum. The electromagnetic spectrum is the range of all types of electromagnetic radiation, or energy, that travels through space. The spectrum is described in terms of wavelengths. As energy travels, it moves like a wave with crests and troughs (similar to how a slinky moves on the floor if one shakes it from left to right with one end fixed). The distance between two crests is a wavelength. Short wavelengths have high amounts of energy (as one would have to shake the slinky frequently to decrease the distance between crests). Long wavelengths have relatively lower amounts of energy. Knowing the type of energy a celestial body emits helps scientists to determine that body's movement and location relative to Earth.
The longest wavelengths in the electromagnetic spectrum are radio waves, which have low amounts of energy. Radio waves are used in radios, televisions, cellular phones, and other wireless networks to transmit data over long distances. Radio telescopes were the first telescopes to measure a portion of the electromagnetic spectrum outside the visible spectrum and are used to explore space.
Moving to a smaller wavelength (more energy) from radio waves on the electromagnetic spectrum, there are microwaves. Scientists have observed a constant amount of microwave radiation all over the universe and have used it to estimate the age of the universe and distances between galaxies. Microwave radiation is also what is used in microwave ovens to heat food quickly. Infrared radiation is next in the electromagnetic spectrum and has a shorter wavelength than microwaves. This type of radiation comes primarily in the form of heat. Earth radiates energy from its surface in the form of infrared radiation.
Visible light has more energy and smaller wavelengths than infrared radiation and is divided into the different colors humans see. Moving from longer wavelengths (less energy) to shorter wavelengths (more energy) one finds red, orange, yellow, green, blue, and violet. The color of the light emitted from an object can indicate its energy; for example, a yellow flame is hotter (more energy) than a red flame. Most stars emit energy within the visible spectrum. Since we know the speed of light, we can measure how long it takes for light to reach an object and then calculate the distance to it. Ultraviolet radiation is next on the spectrum and has more energy than visible light. Often called UV radiation, this type of energy emitted from the Sun is what produces sunburns.
X-rays and then gamma rays are the last two regions on the electromagnetic spectrum. X-ray radiation is so powerful that it can be used to see the bones inside the body, and too much exposure to it can be bad for organisms' health. Gamma rays are a dangerous form of radiation with high amounts of energy that can be lethal to living things. Gamma rays are released from solar flares and when stars explode. X-rays and gamma rays from other celestial bodies cannot penetrate Earth’s atmosphere, and because of this, gamma-ray telescopes have to be placed on satellites in space. Gamma-ray telescopes are used to study black holes and subatomic particles in space.
All energy and matter are found within the universe. Larger celestial components of the universe include galaxies, nebulae, black holes, stars, and planets. Scientists believe that the universe began ~14 billion years ago with the Big Bang. The Big Bang Theory says that matter did not exist before the Big Bang occurred. The universe consisted of only energy, and all this energy was contained in an infinitely small area (smaller than an atom). Before the Big Bang, space and time were set to zero. After the Big Bang occurred, the four basic forces (gravity, electromagnetic force, strong nuclear force, and weak nuclear force) began to function, and the universe started expanding rapidly.
There are two strong pieces of evidence that support the Big Bang Theory. The first piece of evidence is that the universe is still expanding. When scientists use telescopes to look at the night sky, they can see galaxies moving away from our galaxy and each other all over the universe at very high speeds. Using telescopes, they can take pictures of these galaxies moving away from us. Specifically, they want to measure the wavelengths of the light they see from these galaxies. Bodies that are moving away from us emit light that appears to have a lower energy wavelength than what it actually is. This is called redshift. Knowing how much a celestial body’s energy is “redshifted” allows scientists to estimate the distance between us and the body. Knowing the distance and the speed of light allows scientists to estimate the age of that body, and eventually the universe. By measuring the distance between celestial bodies all over the galaxy, scientists can use this information to calculate how long ago all the galaxies were together at one point (in other words, the age of the universe). Pictures of celestial bodies, particularly galaxies, can give information not only about their age and distance from the Milky Way galaxy but also about the galaxies’ sizes, shapes, number of stars, and even number of planets.
The second piece of evidence which supports the Big Bang Theory is that scientists can measure the background radiation of the universe by mapping microwave radiation. There is a background radiation of 2.7 degrees above absolute zero (-273 °C), which is believed to be the leftover energy from the Big Bang.
Scientists also look for the oldest stars and other celestial bodies in the universe. By studying these bodies, understanding how long it took them to form, and knowing how old they are, scientists can create a bound for the age of the universe. That is, if the oldest star is so many billion years old, then the universe is at least that old.
Major components of the universe include galaxies, black holes, nebulae, and stars. A galaxy is a large grouping of stars and interstellar gas and dust. These components are held together in the galaxy by their gravity. There is a wide range of galaxy shapes and sizes. There are four classifications of galaxies, based on their shapes: elliptical, spiral, lenticular (lens-shaped) or irregular. Our galaxy, the Milky Way, is a spiral galaxy.
A black hole is a large body thought to have zero volume but infinite density, which forms a dense gravity well in space. Its gravity is so strong that not even light is fast enough to escape it, hence the name black hole. One way a black hole can be observed is by watching the light and energy of the bodies it is pulling into its gravity well. Scientists hypothesize that black holes are remnants of supernova explosions. (When a star uses up all its fuel and explodes, the gases and dust that remain collapse to form the black hole.)
A nebula is a hot cloud of gas and dust. Stars are formed in the nebula as it spins and its materials slowly condense. As these materials get packed more and more tightly together, their temperature rises, and eventually a star is formed. Therefore, a star is a large ball of gas that is held together by its own gravity. Stars generate their own energy through the process of fusion in which two hydrogen atoms are combined to form a helium atom, and large amounts of energy are released during the process. Some of the energy generated by the star is in the form of light.
The size and composition of a star determine the amount of gravity (more mass generates more gravity) and energy it emits. Scientists use the Herztsprung -Russell diagram to understand the relationship between a star’s brightness and its surface temperature. On the Herztsprung -Russell diagram there is a band known as the main sequence which contains about 90% of all stars. This shows that there is a strong relationship between a star’s brightness and its surface temperature. The other 10% of stars are special types (supergiants, giants, and white dwarfs), and each group falls in a distance area on the diagram.
Our Sun is a medium-sized star relative to other stars and falls within the main sequence of the Herztsprung-Russell diagram. It is the center of our solar system, but within the Milky Way galaxy it is only one star out of millions and resides on the edge of one of the spirals, far away from the center of the galaxy. Out of the 50 stars closest to Earth, the Sun is the 4th largest in mass and the closest, ~149.6 million kilometers away. By percentages of the total number of atoms, the Sun is about 91.2% hydrogen, 8.7% helium, and less than 1% other elements; it is 333,000 times heavier than Earth. Without the Sun, life on Earth would not be possible.