What is the Universe?
The universe is all matter and energy, including the Earth, the galaxies, and the contents of intergalactic space regarded as a whole.
How large is the Universe?
The universe is expansive. To give you sense of the size of the universe, let's compare it to us: human beings.
Here we have everyday people like you and me on the surface of the Earth.
Let's expand a bit and go up to an altitude of 100km. We have now reached space, according to The Federation Aeronautique Internationale, and can see the Earth as a whole.
Now let's compare the Earth to our whole solar system: containing our many planets and the sun.
Let's get a bigger picture of where our solar system is located. Our solar system, which is centered around our sun, is but a spec of light in our galaxy: The Milky Way Galaxy.
Our galaxy is a spiral galaxy, we'll get into that more later on. It consists of hundreds of billions of stars, which are similar to our sun. And each star most likely consists of its own version of a solar system.
You can see now that we're pretty small in our galaxy, but let's expand one last time so you can really see how small we are.
If you venture out an unknown number (but most likely, very large) amount of light-years, we can see that there isn't just one galaxy, but also hundreds of billions of them. Each containing hundreds of billions of stars, that contain their own versions of a solar system, which are bound to consist of masses or planets of many sizes.
The Life of a Star
Now that you have a sense of the scale of the universe, let's get into the main thing that fills up almost every corner of the universe. Let's talk about stars. What is a star? A star is a self-luminous celestial body consisting of a mass of gas held together by its own gravity in which the energy generated by nuclear reactions in the interior is balance by the outflow of energy to the surface, and the inward-directed gravitational forces are balanced by the outward-directed gas and radiation pressures.
The life of a star begins in a nebula. Nebulae are the birthplaces of stars. A nebulas is a diffuse of interstellar of dust or gas or both, visible as luminous patches or areas of darkness depending on the way the mass absorbs or reflects incident radiation. Nebulas often consist of Hydrogen an Helium. There are five types of nebulae:
Emission Nebula: clouds of high temperature gas.
Reflection Nebula: clouds of dust which are simply reflecting the light of a nearby star or stars.
Dark Nebula: clouds of dust which are simply blocking the light from whatever is behind.
Planetary Nebula: shells of gas thrown out by some stars near the end of their lives.
Supernova Remnant: appears when a massive star ends its life.
Inside a nebula, certain varying regions cause this dust and gas to cluster. As these clusters collect more atoms, which results in an increase of mass, it collects even more atoms in the process. As the materials pull in tighter and contract, it achieves and maintains equilibrium. In this case, equilibrium is the balance between gravity pulling atoms towards the center and gas pressure pushing heat and light away from the center. This cluster is now a Protostar. A circumstellar disk of additional matter surrounds the Protostar. Parts of this disk still spiral inwards towards the center of the Protostar to layer on more mass, while other parts remain in order to form a planetary system. This phase lasts for about 100,000 years.
T Tauri Star
The T Tauri phase begins when matter cease to spiral into the Protostar and the star releases an enormous amount of energy. The T Tauri phase lasts for about 100 million years.
This is the phase that a star spends most of its life in. Once a star has achieved nuclear fusion, converting protons of hydrogens into atoms of helium, it exudes a tremendous amount of energy into space. Over the span of billions of years, the star slowly contracts in order to compensate for the amount of energy it is releasing. As it slowly contracts, the temperature, density and apressure at its core continue to increase. This contraction due to gravity pulling in and gas pressure pushing out will last throughout the entire life span of the star to maintain equilibrium.
Since throughout its life a star is constantly converting hydrogen into helium, the hydrogen fuel runs out while the helium builds up. When a star depletes its fuel of hydrogen, its internal reactions cease. Without this gas pressure, the star begins to contract inward due to gravitational forces. In order to still maintain equilibrium between gravity and gas pressure, the star must re-ignite fusion by increasing temperatures in its core. To maintain stability, the star is forced to burn up its supply of helium. Helium burns inside the core, but a hydrogen reaction occurs faster in the shell of around the core. As the temperature of the shell increases, the outer layer of the star expands. At this stage, the star is larger, but less stable due to fusion releasing more energy during helium burning than the main sequence phase.
The Red Giant will eventually burn up its helium fuel supply. In order to maintain equilibrium, the star will contract again to commence the last type of fusion: carbon burning. To contract, the star must expel its outer layers into space. The Red Giant has now become a White Dwarf. A White Dwarf will start out hot, but over time (hundreds of billions of years), it will gradually cool down.
A Supernova can be divided into two basic physical types:
Type Ia.: These result from some binary star systems in which a carbon-oxygen white dwarf is accreting matter from a companion.
Type II.: These occur at the end of a star's lifetime, when its nuclear fuel and is depleted and it is no longer supported by the release of nuclear energy. If the star's iron core is massive enough, it will collapse and become a supernova.
Neutron stars are created in the cores of massive stars during a supernova explosion. When the core of the sar collapses, portons are crushed together with a corresponding electron, which transforms every electron-proton pair into a neutron. In this case however, the neutrons remain to form a neutron star.
Black holes are believed to be formed when a massive star collapses in on itself. When a supernova occurs, a neutron star is able to form due to neutron degeneracy. But if the degenerating neutrons are not able to prevent the collapse of the star because of gravitational forces, it contracts and compresses into an infinite void of blackness- also known ans a stellar mass black hole. The gravitational pull from a black hole is so strong that nothing can escape them, including light. Their density is immeasurable. Black holes are able to distort space around them and even absorb neighboring matter, including stars.
How do we know so much about stars you ask? Well, you can thank Hans Lippershey for that. Thanks to him, we have now been able to modify and improve his integral invention: the telescope, in order to conduct our study of astronomy and the universe. Lippershey's patent for the invention of the telescope is credited as the earliest, on September 25, 1608. Most people credit Galileo Galilei for its invention, though this is inaccurate. Galileo was the first to use a telescope for the purposes of astronomy in 1609, not invent it.
The Hertzsprung-Russel Diagram
This diagram was created by Danish astronomer, Ejnar Hertzsprung and American astronomer, Henry Norris Russell in 1919. The Hertzsprung-Russell Diagram helped create a classification system for discovered stars. This diagrams has become an important tool in stellar astronomy.
Each dot represents a star. The diagram is a plot of stars' luminosity (absolute magnitude) and its temperature. The color of the stars range from the high-temperature, blue-white stars on the left side of the diagram, to the low-temperature, red stars on the right side. The Y-axis represents the star's luminosity or absolute magnitude. Luminosity is basically the amount of energy a star radiates in one second or how bright it is. In either case, the scale is a ratio scale in which stars are compared to each other based on the reference of our star, the sun. On the other hand, the X-axis represents the surface temperature of a star. Going to the left side of the x-axis means that the stars are hotter, while going to right of the axis means that the stars are cooler. The X-axis is usually represented in either Kelvins or Colour (B-V).
1. What process has to occur in order for a star to achieve/maintain equilibrium?
b. nuclear fusion
Firstly, eliminate answers that are wrong. For example, a. is contraction. The meaning of equilibrium is balance. Contraction is only one side of it. And if you did your reading, there has to be another process that takes place to counter contraction and achieve equilibrium. So a. can't be the answer. Let's eliminate another answer. It's a safe bet that you can take out c. supernova. This due to the fact that supernova is the phase in which a star dies. How can a star achieve/maintain anything if it dies? So let's cross that off. Now you have two answers, leaving you with a 50/50 chance. If you knew the definition of accretion, you can take it right off and be left with the right answer. But if you didn't, then look at the other answer b. nuclear fusion. Like I mentioned before: if equilibrium means balanced and the word fusion means combine, then you can add one and one together and get the answer. For equilibrium to happen, there needs to be two things happening. The word fusion means combine, which implies there are two things occurring. By using the elimination method, you were able to figure out the correct answer.