Pepsin

Pepsin is a digestive enzyme that is secreted by the chief cells in the stomach. It breaks down food proteins into peptides. Pepsin is one of three protein-degrading, or proteolytic enzymes in the digestive system. The other two are chymotrypsin and trypsin. These enzymes work together to break proteins down to peptides and amino acids, so they can be readily absorbed by the intestinal lining.

Pepsin is first produced as a pepsinogen, which is a zymogen whose primary structure has an additional 44 amino acids. The hormone gastrin and the vagus nerve trigger the release of both pepsinogen and Hydrochloric acid from the stomach lining when food is ingested. Then this pepsinogen is activated by the hydrochloric acid, which is released from parietal cells in the stomach lining. Hydrochloric acid creates an acidic environment which allows pepsinogen to unfold and cleave itself in an autocatalytic fashion, thereby generating pepsin.

Pepsin was discovered in 1836 by Theodor Schwann who also coined this enzyme's name from the Greek word pepsis, meaning digestion (peptein: to digest). It was the first animal enzyme to be discovered, and in 1929 it became one of the first enzymes to be crystallized, by John H. Northrop. Pepsin is a digestive protease.

Trypsin

Trypsin is a digestive, proteolytic enzyme which breaks down proteins. It cleaves peptide chains at the carboxyl side of the amino acids lysine and arginine, except when either is followed by proline. Secreted by the pancreas, trypsin continues the process of digestion in the duodenum (small intestine) where a slightly alkaline environment with pH 8 promotes its maximal enzymatic activity.

Trypsin acts to hydrolyze peptides into their smaller building blocks, namely amino acids. These peptides are the result of the enzyme pepsin breaking down the proteins in the stomach. This is necessary for the uptake of protein in the food. Although peptides are smaller than proteins, they are still too big to be absorbed through the lining of the ileum. Trypsin catalyzes the hydrolysis of peptide bonds. Trypsin is remarkably similar in chemical composition and in structure to the other chief pancreatic proteinase, chymotrypsin.

Trypsin is secreted by the pancreas in the form of inactive zymogen and trypsinogen. It is then released into the small intestine where the enzyme enteropeptidase activates it into trypsin by proteolytic cleavage. The resulting trypsins themselves activate more trypsinogens, so only a small amount of enteropeptidase is necessary to start the reaction. This activation mechanism is common for most serine proteases, and serves to prevent autodigestion of the pancreas.

Pancreatic Lipase

Pancreatic lipase is a digestive enzyme secreted by the pancreas that breaks apart fat molecules through hydrolysis. Bile salts secreted from the liver and stored in the gallbladder are released into the duodenum where they coat and emulsify large fat droplets into smaller droplets, thus increasing the overall surface area of the fat, which allows the lipase to break apart the fat more effectively. The resulting monomers are then moved by way of peristalsis along the small intestine to be absorbed into the lymphatic system by a specialized vessel called a lacteal. This protein belongs to pancreatic lipase family. Unlike some pancreatic enzymes like trypsin, which are first secreted in the inactive form of trypsinogen, pancreatic lipase is secreted as an active enzyme.

Pancreatic lipase is released into the duodenum through the duct system of the pancreas. Usually, its concentration of serum is very low. Found in the intestinal lumen, the basic pancreatic lipase used to digest the fat droplets is Streapsin. Under extreme disruption of pancreatic function, such as pancreatitis, the pancreas may begin to autolyse and release pancreatic enzymes into serum. Thus, through measurement of serum concentration of pancreatic lipase, pancreatitis can be diagnosed.

Amylase

Amylase is an enzyme which breaks starch down into sugar. Human saliva contains Amylase. Foods that contain much starch but little sugar, such as potato and rice, taste slightly sweet, because when they are chewed, amylase turns some of their starch into sugar in the mouth. The pancreas also secretes amylase (alpha amylase) to break down dietary starch into di- and trisaccharides which are converted by other enzymes to glucose to supply the body with energy. Plants and some bacteria also produce amylase. As amylase was the first enzyme to be discovered and isolated by Anselme Payen in 1833. Specific amylase proteins are designated by different Greek letters. All amylases are glycoside hydrolases and act on α-1,4-glycosidic bonds.


The α-amylases are calcium metalloenzymes, which are completely unable to function in the absence of calcium. By acting at random locations along the starch chain, α-amylase breaks down long-chain carbohydrates, ultimately yielding maltotriose and maltose from amylose, or maltose, glucose and "limit dextrin" from amylopectin. Because it can act anywhere on the substrate, α-amylase tends to be faster-acting than β-amylase. In animals, it is a major digestive enzyme and its optimum pH is 6.7-7.0. In human physiology, both the salivary and pancreatic amylases are α-Amylases, which are also found in plants (barley) , fungi and bacteria (Bacillus).

Cirrhosis

Cirrhosis is a chronic liver disease characterized by replacement of normal, healthy liver tissue by fibrous scar tissue and generative nodules, which lead to progressive loss of liver function. Because the liver plays a vital role in keeping the body functioning properly, removing poisons from the blood, producing bile to help absorb fats and fat-soluble vitamins, and synthesizing proteins that regulate blood clotting, one can not live without one. So, the straight consequence of Cirrhosis is death.

In cirrhosis of the liver, scar tissue replaces normal, healthy tissue, blocking the flow of blood through the organ and preventing it from working as it should. Cirrhosis is the twelfth leading cause of death by disease, killing about 26,000 people each year. Also, the cost of cirrhosis in terms of human suffering, hospital costs, and lost productivity is high.

Cirrhosis is most commonly caused by chronic alcoholism, hepatitis B and C and fatty liver disease but has many other possible causes, including long-standing inflammation, poisons, infections, and heart disease. Alcohol can poison all living cells, causing liver cells to become inflamed and die. The death of liver cells leads your body to form scar tissue around veins of your liver. Healing liver cells form nodules, which also press on the liver veins. This scarring process occurs in 10-20 percent of alcoholics and is the most common form of cirrhosis in the United States.

People with cirrhosis have no symptoms in the early stages of the disease. However, as scar tissue replaces healthy cells, liver function starts to fail and a person may experience exhaustion, fatigue, loss of appetite, nausea, weakness, abdominal pain, weight loss, and spider-like blood vessels that develop on the skin.

In cirrhosis, a damaged liver cannot remove toxins from the blood, causing them to accumulate in the blood and eventually the brain. There, toxins can dull mental functioning and cause personality changes, coma, and even death. Signs of the buildup of toxins in the brain include neglect of personal appearance, unresponsiveness, forgetfulness, trouble concentrating, or changes in sleep habits.

Usually, liver damage from cirrhosis cannot be reversed, but treatment could stop or delay further progression and reduce complications. A healthy diet is encouraged, as cirrhosis may be an energy-consuming process. Close follow-up is often necessary. Alcoholic cirrhosis caused by alcohol abuse is treated by abstaining from alcohol. Treatment for hepatitis-related cirrhosis involves medications used to treat the different types of hepatitis, such as interferon for viral hepatitis and corticosteroids for autoimmune hepatitis.

Gallstones

Gallstones are small, stone-like substances that develop in the gallbladder or anywhere within the biliary tree, including the bile duct. This happens when the bile, which is produced by the liver and stored in the gallbladder, hardens into pieces of pebble-like material. Gallstones (choleliths) are crystalline bodies formed by accretion or concretion of normal or abnormal bile component. If the bile contains too much cholesterol or bilirubin, it can harden into gallstones.

There are two types of gallstones; cholesterol stones and pigment stones. Cholesterol stones are usually yellow-green and are made primarily of hardened cholesterol. They account for about 80 percent of gallstones. Pigment stones are small, dark stones made of bilirubin. Gallstones can be as small as a grain of sand or as large as a golf ball. The gallbladder can develop just one large stone, hundreds of tiny stones, or a combination of the two.

Obstruction of the common bile duct is choledocholithiasis; obstruction of the biliary tree can cause jaundice; obstruction of the outlet of the pancreatic exocrine system can cause pancreatitis. Cholelithiasis is the presence of stones in the gallbladder—chole- means "bile", lithia means "stone", and -sis means "process".

At the beginning gallstones remains asymptomatic. But once the stones reach a certain size, they start causing symptoms. A main symptom of gallstones is commonly referred to as a gallstone "attack", also known as biliary colic, in which a person will experience intense pain in the upper abdominal region that steadily increases for approximately thirty minutes to several hours. These attacks are sharp and intensely painful, similar to that of a kidney stone attack.

Often, these attacks occur after a particularly fatty meal and almost always happen at night. Other symptoms include abdominal bloating, intolerance of fatty foods, belching, gas, and indigestion. One way to alleviate the abdominal pain is to drink a full glass of water at the start of an attack to regulate the bile in the gallbladder. Another way is to take magnesium followed by a bitter liquid such as coffee or swedish bitters an hour later. Bitter flavors stimulate bile flow. A study has found lower rates of gallstones in coffee drinkers. But if the above symptoms coincide with chills, yellowing of the skin or eyes, and clay-colored stool, a doctor should be consulted immediately.

As treatment, gallstones can be broken up using a procedure called lithotripsy, which is a method of concentrating ultrasonic shock waves onto the stones to break them into tiny pieces. They are then passed safely in the feces. However, this form of treatment is only suitable when there are a small number of gallstones. When this procedure is not possible, then the gallbladder is surgically removed. The gallbladder removal is called cholecystectomy, which has a 99% chance of eliminating the recurrence of cholelithiasis. Only symptomatic patients must be indicated to surgery. The lack of a gall bladder does not seem to have any negative consequences in many people.

Bile

Bile is a bitter yellowish green alkaline fluid secreted by liver cells called hepatocytes. Bile is stored in the gallbladder between meals and upon eating is discharged into the duodenum where it helps break down fats into fatty acids, which can be taken into the body by the digestive tract. Bile has various components. Its constituents include: water, cholesterol, bile pigments, and bile acids.

The bile acids are typically conjugated with table salt or glycine and are produced by the liver from cholesterol. They are secreted by hepatocytes along the bile canaliculi, which then join the bile duct, and thence into the gall bladder. Usually the concentration of bile salts in bile is 0.8%, however the gall bladder removes water from the bile, concentrating it between meals.

Gallbladder

The gall bladder is a small, sac-like organ, which is located under the liver. It stores the bile secreted by the liver and aids in the digestive process. Bile is released from the gallbladder into the upper small intestine, duodenum, in response to food, especially fats. The cystic duct connects the gall bladder to the common hepatic duct to form the common bile duct. This common bile duct then joins the pancreatic duct, and enters through the hepatopancreatic ampulla at the major duodenal papilla.

Liver

Located in the upper abdomen, the liver is the largest organ and one of the most vital one as it does important jobs such as protein synthesis, glycogen storage, and removing waste products and worn out cells from the blood. The liver is necessary for survival, and a human can only last up to 24 hours without liver function. It also produces bile, an alkaline compound which aids in digestion by breaking down fat. It weighs about 3.5 pounds, measures about 8 inches horizontally and 6.5 inches vertically, and is 4.5 inches thick. It is a soft, pinkish-brown, triangular organ. The liver is the only internal human organ capable of natural regeneration of lost tissue. So, as little as 25% of a liver can regenerate into a whole liver. Medical terms related to the liver often start in hepato- or hepatic from the Greek word for liver.

Gross anatomy divides the liver into four lobes based on surface features. The falciform ligament is visible on the front of the liver. This divides the liver into a left anatomical lobe, and a right anatomical lobe. If the liver flipped over, to look at it from behind (the visceral surface), there are two additional lobes between the right and left. These are the caudate lobe (the more superior), and below this the quadrate lobe.

The bile secreted by the liver is collected in the bile canaliculi (tiny canals), which merge to form bile ducts. These eventually drain into the right and left hepatic ducts, which in turn merge to form the common hepatic duct. Bile can either drain directly into the duodenum via the common bile duct or be temporarily stored in the gallbladder via the cystic duct. The common bile duct and the pancreatic duct enter the duodenum together at the ampulla of Vater. The branchings of the bile ducts resemble those of a tree, and indeed the term "biliary tree" is commonly used in this setting.

The splenic vein joins the inferior mesenteric vein, which then together join the superior mesenteric vein to form the hepatic portal vein, bringing venous blood from the spleen, pancreas, stomach, small intestine, and large intestine, so that the liver can process the nutrients and by-products of food digestion. The hepatic veins of the blood can be from other branches such as the superior mesenteric artery. Approximately 60% to 80% of the blood flow to the liver is from the portal venous system, and one fifth of the blood flow is from the hepatic artery.

Pancreatitis

Pancreatitis is inflammation of the pancreas. The pancreas produces and releases digestive enzymes into the duodenum through a tube called the pancreatic duct. Pancreatic enzymes join with bile, which is produced in the liver and stored in the gallbladder, to digest food. Normally, digestive enzymes secreted by the pancreas do not become active until they reach the small intestine. But when the pancreas is inflamed, the enzymes inside it attack and damage the tissues that produce them, causing then pancreatitis. Pancreatitis can be acute or chronic. Either form is serious and can lead to complications. In severe cases, bleeding, infection, and permanent tissue damage may occur.

Acute pancreatitis occurs suddenly and usually resolves in a few days with treatment. Without appropriate treatement, acute pancreatitis can be a life-threatening illness with severe complications. Each year, more than 200,000 people in the United States are admitted into hospitals with acute pancreatitis. The most common cause of acute pancreatitis is the presence of gallstones, which cause inflammation in the pancreas as they pass through the common bile duct. Chronic, heavy alcohol use is also a common cause. Acute pancreatitis can occur within hours or as long as 2 days after consuming alcohol. Abdominal trauma, medications, infections, tumors, and genetic abnormalities of the pancreas can also cause acute pancreatitis.

Acute pancreatitis usually begins with gradual or sudden pain in the upper abdomen that sometimes extends through the back. The pain may be mild at first and feel worse after eating. But the pain is often severe and may become constant and last for several days. A person with acute pancreatitis usually looks and feels very ill and needs immediate medical attention. Other symptoms may include a swollen and tender abdomen, nausea and vomiting, fever, and rapid pulse. Severe acute pancreatitis may cause dehydration and low blood pressure. The heart, lungs, or kidneys can fail. If bleeding occurs in the pancreas, shock and even death may follow.

Chronic pancreatitis is inflammation of the pancreas that does not heal or improve—it gets worse over time and leads to permanent damage. Chronic pancreatitis, like acute pancreatitis, occurs when digestive enzymes attack the pancreas and nearby tissues, causing episodes of pain. Chronic pancreatitis often develops in people who are between the ages of 30 and 40.

The most common cause of chronic pancreatitis is many years of heavy alcohol use. The chronic form of pancreatitis can be triggered by one acute attack that damages the pancreatic duct. The damaged duct causes the pancreas to become inflamed. Scar tissue develops and the pancreas is slowly destroyed. Other causes of chronic pancreatitis are hereditary disorders of the pancreas, cystic fibrosis, and high levels of calcium in the blood.

Type 2 Diabetes

Diabetes type 2 is a chronic disease characterized by high levels of glucose (sugar) in the blood. It begins when the body does not respond correctly to insulin, which is a hormone produced by the pancreas. Type 2 diabetes is the most common form of diabetes and it is caused by a problem in the way your body uses insulin. Insulin is needed to move glucose into cells, where it is used for energy. If glucose does not get into the cells, the body cannot use it for energy. As a result, too much glucose will stay in the blood, causing the symptoms of diabetes. There are several types of diabetes. Diabetes type 2 usually occurs with obesity and insulin resistance.

Because sugar is not getting into the tissues, abnormally high levels of sugar build up in the blood. This is called hyperglycemia. Many people with insulin resistance have hyperglycemia and high blood insulin levels at the same time. People who are overweight have a higher risk of insulin resistance, because fat interferes with the body's ability to use insulin. Type 2 diabetes usually occurs gradually. Most people with the disease are overweight at the time of diagnosis. However, type 2 diabetes can also develop in those who are thin, especially the elderly. Low activity level, poor diet, and excess body weight (especially around the waist) significantly increase your risk for type 2 diabetes.

Type 1 Diabetes

Diabetes type 1 is a form of diabetes mellitus. Type 1 diabetes is an autoimmune disease that results in destruction of insulin-producing beta cells of the pancreas. Thus far, such destruction has been permanent, but there is informed speculation that reversing the immune system malfunction may allow recovery of beta cell function. Healthy people has between 70 and 120 mg/dL glucose level in the blood, but when insulin is lacking, there is an increase of fasting blood glucose, that begins to appear in the urine above the renal threshold, about 190-200mg/dl, thus connecting to the symptom by which the disease was identified in antiquity, sweet urine. Glycosuria or glucose in the urine causes the patients to urinate more frequently, and drink more than normal (polydipsia).

Type 1 diabetes is lethal unless treatment with exogenous insulin, usually via injections which replaces the missing hormone formerly produced by the now non-functional beta cells in the pancreas. In recent years, pancreas transplants have also been used to treat Type 1 diabetes. Islet cell transplant is also being investigated and has been achieved in mice and rats, and in experimental trials in humans as well. Use of stem cells to produce a new population of functioning beta cells seems to be a future possibility, but has yet to be demonstrated even in laboratories as of 2008.

Although type 1 diabetes was formerly known as "childhood", "juvenile" or "insulin-dependent" diabetes, it is not exclusively a childhood problem as the adult incidence of Type 1 is noteworthy. Many adults who contract Type 1 diabetes are sometimes misdiagnosed with Type 2 due to confusion on this point.

Diabetes Mellitus

Diabetes mellitus is a disease characterized by high levels of glucose (sugar) in the blood, caused by inadequate production of insulin, a hormone produced by the beta cells in the islets of Langerhans in the pancreas and which allows the body to use and store glucose. It is a leading cause of death in the United States and is especially prevalent among African Americans. The treatment of diabetes was revolutionized when F. G. Banting isolated insulin in 1921.

Under normal conditions, blood glucose levels are tightly controlled by insulin, which lowers the amount of glucose in the blood. When the blood glucose elevates, for example, after eating food, insulin is released from the pancreas to normalize the glucose level. In patients with diabetes, the absence or insufficient production of insulin causes hyperglycemia, which is high sugar levels. Hyperglycemia symptons are: listlessness, dry mouth, impotence in male, frequently urinating (polyuria), frequently thirsty (polydipsia), frequently hungry (polyphagia), and poor wounds healing.

Digestive Enzyme

Digestive enzymes are enzymes that break down the complex macromolecules that make up food into smaller and simpler molecules which can be absorved and/or stored into the blood and organs. They are found in the digestive tract of animals or humans where they aid in the digestion of food as well as inside cells, especially in their lysosomes. Digestion enzymes are also found in your saliva, which are produced by your salivary glands.

Digestive enzymes are secreted by different glands which are located in the human digestive system: the salivary glands, the glands in the stomach, the pancreas, and the glands in the small intestines. The actions of digestive enzymes are as follows:

• Salivary amylase produced by salivary glands in the mouth breaks down starch into sugar.
• Pepsin and rennin produced by stomach gastric pit breaks down protein into peptides.
• Amylase produced by pancreas breaks down starch into glucose.
• Lipase produced by pancreas breaks down lipids into fatty acids and glycogen.
• Trypsin produced by pancreas breaks down peptides into amino acids.
• Sucrase produced by ileum breaks down sucrose into glucose.
• Lactase produced by small intestine breaks down lactose into glucose and galactose.

Insulin

Insulin is a hormone produced by the endocrine pancreas. It lowers the sugar level in the blood, causing most of the body's cells to take up glucose from the blood, including liver, muscle, and fat tissue cells, storing it as glycogen in the liver and muscle. When insulin is absent, or low, glucose level in the blood goes up. As its level is a central metabolic control mechanism, its status is also used as a control signal to other body systems such as amino acid uptake by body cells.Composed of 51 amino acid, insulin is a peptide hormone which is produced in the islets of Langerhans in the pancreas. Insulin's structure varies slightly between species of animal. Pig insulin is especially close to the human version.

Glucagon

Glucagon is an hormone secreted by the endocrine pancreas. Along with insulin it is involved in carbohydrate metabolism. Glucagon is released when the glucose level in the blood is low, which is called hypoglycemia, causing the liver to convert stored glycogen into glucose and release it into the bloodstream. The action of glucagon is thus opposite to that of insulin, which instructs the body's cells to take in glucose from the blood in times of satiation.

Glucagon is a 29-amino acid polypeptide, which is synthesized and secreted from α (alpha) cells of the islets of Langerhans in the endocrine pancreas. The amino acid sequence of glucagon was described in the late-1950s. A more complete understanding of its role in physiology and disease was not established until the 1970s, when a specific radioimmunoassay was developed.

Islets of Langerhans

Islets of Langerhans is the area in which the endocrine cells of the pancreas are grouped. They constitute approximately 1 to 2% of the mass of the pancreas. There are about one million islets in a healthy adult human pancreas, which are distributed evenly throughout the organ. The islets of Langerhans are clusters of specialized cells that produce insulin and glucagon. Named after the German pathologist Paul Langerhans (1847-1888), who discovered them in 1869, these cells sit in groups that Langerhans likened to little islands in the pancreas.

The islets of Langerhans make up the endocrine pancreas and consist of five different types of cells which secret hormones directly into the bloodstream; α (alpha), β (beta), δ (delta), PP, and ε (epsilon) cells. α cells produce glucagon; β insulin; δ somatostatin; PP polypeptide; and ε ghrelin. Glucagon raises the level of sugar (glucose) in the blood, whereas insulin lowers the level of glucose.

Pancreas

The pancreas is a fish-shaped spongy organ about 6 inches long. Stretching across the back of the abdomen, behind the stomach, the head of the pancreas is on the right side of the abdomen and is connected to the first section of the small intestine. The tail extends to the left side of the body.

The pancreas is a gland organ in both the digestive and endocrine systems of vertebrates. It is both an endocrine gland that produces several important hormones which play an important role in the breaking down of carbohydrates, as well as an exocrine gland, secreting digestive enzymes that pass into the small intestine. These enzymes help in the further breakdown of food for absortion of nutrients in the small intestine. According to its functions, the pancreas can be divided into two parts; the endocrine and the exocrine.

The part of the pancreas with endocrine function is made up of cell clusters called islets of Langerhans. There are four main cell types in the islets. Although they are difficult to distinguish using standard staining techniques, they can be classified by their secretion: α cells secrete glucagon, β cells secrete insulin, δ cells secrete somatostatin, and PP cells secrete pancreatic polypeptide. The islets are a compact collection of hundreds of thousands of endocrine cells arranged in clusters and cords which are crisscrossed by a dense network of capillaries. The capillaries of the islets are lined by layers of endocrine cells in direct contact with vessels, and most endocrine cells are in direct contact with blood vessels, by either cytoplasmic processes or by direct apposition.

The exocrine pancreas produces digestive enzymes and an alkaline fluid, and secretes them into the small intestine through a system of exocrine ducts in response to the small intestine hormones secretin and cholecystokinin. Digestive enzymes include trypsin, chymotrypsin, pancreatic lipase, and pancreatic amylase, and are produced and secreted by acinar cells of the exocrine pancreas. Specific cells that line the pancreatic ducts, called centroacinar cells, secrete a bicarbonate and salt-rich solution into the small intestine.

The pancreas receives regulatory innervation via hormones in the blood and through the autonomic nervous system. These two inputs regulate the secretory activity of the pancreas.

Digestive System

The human digestive system is a series of organs and glands that processes food. It is made up of the digestive tract, which is a series of hollow organs joined in a long, twisting tube from the mouth to the anus, plus other organs that help the body break down and absorb food. Organs that make up the digestive tract are the mouth, esophagus, stomach, small intestine, large intestine—also called the colon—rectum, and anus.

Inside these hollow organs is a lining called the mucosa. In the mouth, stomach, and small intestine, the mucosa contains tiny glands that produce juices to help digest food. The digestive tract also contains a layer of smooth muscle that helps break down food and move it along the tract. Two solid digestive organs, the liver and the pancreas, produce digestive juices that reach the intestine through small tubes called ducts. The gallbladder stores the liver’s digestive juices until they are needed in the intestine. Parts of the nervous and circulatory systems also play major roles in the digestive system.

When you eat foods—such as bread, meat, and vegetables—they are not in a form that the body can use as nourishment. Food and drink must be changed into smaller molecules of nutrients before they can be absorbed into the blood and carried to cells throughout the body. Digestion is the process by which food and drink are broken down into their smallest parts so the body can use them to build and nourish cells and to provide energy. Digestion involves mixing food with digestive juices, moving it through the digestive tract, and breaking down large molecules of food into smaller molecules. Digestion begins in the mouth, when you chew and swallow, and is completed in the small intestine.

The digestive glands that act first are in the mouth—the salivary glands. Saliva produced by these glands contains an enzyme that begins to digest the starch from food into smaller molecules. An enzyme is a substance that speeds up chemical reactions in the body.

The next set of digestive glands is in the stomach lining. The stomach produce gastric acid and an enzyme that digests protein. A thick mucus layer coats the mucosa and helps keep the acidic digestive juice from dissolving the tissue of the stomach itself. In most people, the stomach mucosa is able to resist the juice, although food and other tissues of the body cannot.

After the stomach empties the food and juice mixture into the small intestine, the juices of two other digestive organs mix with the food. One of these organs, the pancreas, produces a juice that contains a wide array of enzymes to break down the carbohydrate, fat, and protein in food. Other enzymes that are active in the process come from glands in the wall of the intestine.

The second organ, the liver, produces yet another digestive juice—bile. Bile is stored between meals in the gallbladder. At mealtime, it is squeezed out of the gallbladder, through the bile ducts, and into the intestine to mix with the fat in food. The bile acids dissolve fat into the watery contents of the intestine, much like detergents that dissolve grease from a frying pan. After fat is dissolved, it is digested by enzymes from the pancreas and the lining of the intestine.

Turbine Airflow

Air enters the LM6000 turbine at the inlet of the variable inlet guide vanes (VIGVs) and passes into the low pressure compressor (LPC). The low pressure compressor compresses the air by a ratio of approximately 2.4:1. Air leaving the low pressure compressor is directed into the high pressure compressor (HPC) and is regulated at idle and at low pressure by variable bypass valves (VBVs) arranged in the flow passage between the two compressors.

The airflow in the 14-stage high pressure compressor is regulated by variable inlet guide vanes and five stages of variable stator vanes (VSVs). The high pressure compressor compression ratio is approximately 12:1. High pressure compressor discharge and stage 8 bleed air are extracted for emissions control. Compressor discharge air is then directed to the combustor section.

Air entering the combustor is mixed with fuel and ignited. Once combustion becomes self-sustaining, the igniter is de-energized. The combustion gases then exit to the high pressure turbine (HPT). The hot gases from combustion that enters into the high pressure turbine flow out of this to enter the low pressure turbine (LPT), which drives both the low pressure compressor and the output load. The exhaust gases that pass through the low pressure turbine exit through the exhaust duct.

The discharge of the low pressure compressor (LPC) enters into the compressor front frame (CFF). Based on generator loading, the required mass air flow for the high pressure compressor (HPC) varies. At low loads , the low pressure compressor provides excessive airflow which must be vented before entering the high pressure compressor. This is accomplished by the actuation of 12 variable bleed valves (doors) mounted on the external casing of the compressor front frame. The excess air is then vented to the atmosphere through ducting. Primary air from the compressor front frame is then drawn into the high pressure compressor compressed again and distributed to the compressor rear frame.

Front Frame Assembly

The front frame of the LM6000 gas turbine is a major structure that provides support for the low pressure compressor rotor and the forward end of the high pressure compressor rotor through No 1B, No 2R, and No 3R bearings. The frame also form an airflow path between the low pressure compressor and the high pressure compressor inlet. Front engine mount provisions are located on the front frame 3 o’clock and 9 o’clock positions. One pad is included on the frame outer case for mounting high pressure compressor inlet temperature sensors T2.5 and high pressure compressor pressure sensor P2.5. Sensors provide control information to the fuel management system.

Front frame is made from a high-strength stainless steel casting. Twelve equally spaced radial struts are used between the hub and outer case to provide support for the inner hub. Twelve variable-position bypass valve doors are located on the outer wall for the low pressure compressor discharge bleed.

The front frame contains the engine A-sump, which includes a thrust bearing (1B) and roller bearing (2R) that support the low pressure compressor rotor, and a roller bearing (3R) that support the forward end of the high pressure compressor rotor. Lubrication oil supply and scavenge lines for the A-sump are routed inside the frame struts. The inlet gear box is located in the A-sump with the radial drive shaft extending outward through the strut located at the 6 o’clock position.

Ignition System

The LM6000 ignition system produces the high energy sparks that ignite the fuel-air mixture in the combustor during starting. The system consists of high energy spark igniters, a high-energy capacitor-discharge ignition exciter, and an interconnecting cable. The ignition cables interconnect directly between the package-mounted exciters and the igniters, which are mounted on the engine compressor rear frame.

During the start sequence, fuel is ignited by the igniter, which is energized by the ignition exciter. Once combustion becomes self-sustaining, the igniter is de-energized at _> 400º F.

Proper installation of the igniter plug on the combustor chamber is essential for long operating life. The igniter plug has a special distance ring that must be installed between the plug and the compressor rear frame. The correct distance of the plug in the rear frame is important, both for operation and cooling, and it can be adjusted with the distance ring. Cooling is achieved with compressor air flowing alongside the igniter plug tip. Also 12 holes in the plug tip are present for cooling purposes, and, finally 6 holes provide cooling air for the igniter plug shank.

To ensure a successful light off, the ignition system is comprised of two independent ignition systems. Due to already increased air temperature from compression through the compressor, and fuel atomization from the fuel nozzle, it is possible to achieve ignition with only one igniter. Running two independent systems ensures the ability to maintain normal operations even with the complete loss of one system.

Hydraulic Start System

The starter, or hydraulic starting motor, drives the LM6000 gas turbine high pressure rotor through the accessory gear box (AGB) starter drive pad which has a 0.9562 gear ratio to the high pressure rotor. The starter is required to crank the gas turbine for starting, water wash, and inlet/exhaust duct purge.

The hydraulic start equipment is located in the auxiliary module and consists of the reservoir, filters, air-oil heat exchanger, charge pump and motor, a SOV-actuated valve providing pressurized hydraulic fluid to the variable displacement main pump, and a hydraulic starter motor mounted on the turbine auxiliary gear box (AGB).

The hydraulic starter consists of a variable displacement type hydraulic motor. Piston stroke controlled by a wobble (swash) plate. Displacement is controlled by varying the angle of the wobble (swash) plate by means of a pressure compensator. The starter is equipped with an over-running clutch to prevent the motor from being driven by the high pressure rotor when the hydraulic supply pressure and flow are reduced to zero.

Hydraulic Starter Operation
For starting a minimum pressure drop of 4,200 psi is applied. As starter speed increases, the flow will increase from 0 to 55 gpm. At 55 gpm flow, an internal pressure compensator in the starter maintains starter inlet pressure and accelerates the starter while the hydraulic supply system maintains the 55 gpm flow. When the gas turbine is fired, it will eventually attempt to drive the starter above above its maximum speed. This is referred to as self-sustaining speed. At this point an internal over-running clutch allows the gas turbine to continue to accelerate while the starter continues to run at its maximum speed. The hydraulic supply system for the hydraulic starter will then be shut down.

If the gas turbine is unfired, such as for purge or compressor cleaning, the starter speed will approach a steady 2200-2400 rpm, depending upon ambient conditions.

Compressor Stall

A stall can happen within the compressor if the air shifts from its general direction of motion (also known as the angle of attack). At this point the low pressure on the upper surface disappears on the stator blade. This phenomenon is known as a stall. As pressure is lost on the upper surface, turbulence created on the backside of the stator blade forms a wall that will lead into the stall. Stall can be provoked if the surface of the compressor blade is not completely even or smooth. A dent in the blade, or a small piece of material on it, can be enough to start a turbulence on the backside of the blade, even if the angle of attack is fairly small.

Each stage of compression should develop the same pressure ratio as all other stages. When a stall occurs the front stages supply too much air for the rear stages to handle, and the rear stage will choke. If the angle of attack is too high, the compressor will stall. The airflow over the upper airfoil surface will become turbulent and destroy the pressure zone. This will decrease the compression airflow. Any action that decreases airflow relative to engine speed will increase the angle of attack, increasing the tendency to stall.

If there is a decrease in the engine speed, the compression ratio will decrease with the lower rotor velocities. With a decrease in compression, the volume of air in the rear of the compressor will be greater. This excess volume of air causes a choking action in the rear of the compressor with a decrease in airflow. This in turn decreases the air velocity in the front of the compressor and increases the tendency to stall.

Normal Start Sequence

LM6000 Normal Start Sequence

Refer to the following sequence when performing a normal start:

1-Select Normal Mode

2-Permissive

3-Select “Start” from HMI menu to initiate start sequence

4-Verify N25 reference is set at 6025 rpm and N2 reference is set at 3600 rpm

5-AC lube oil pump motor MOT-0033 energizes

6-Generator and turbine compartement fans energize

7-Observe dP for generator and turbine compartements

8-Observe lube oil pressures and rundown tank level

9-Before initiating crank, generator stator, generator bearing, and generator lube oil supply temperatures must be met

10-Hydraulic pump motor MOT-6015 energizes and 10-second delay timer starts

11-After 10-second timer has expired, hydraulic pump selenoid valve SOV-6019 angles starter swash plate to 100% output and jacking lube oil pump motor MOT-0085 energizes

12When N25>1700 rpm, 2-minute duct purge timer starts

13-Liquid fuel pump motor energizes (if liquid fuel is selected)

14-After 2-minute timer has expired the SOV-6019 destroke the starter swash plate to 0% and holds until N25<1700>

15-When N25 goes 1700 rpm (gas fuel) or N25<1200>

16-When N25 reaches 1700 rpm (gas fuel) or 1200 rpm (liquid fuel), igniter energizes and engines controller commands FUEL ON

17-Gas block valves and gas metering valve open

18-When N25 reaches 4600 rpm, SOV-6019 destrokes the starter swash plate to 0% and hydraulic pump motor MOT-6015 de-energizes after 10-second delay

19-Jacking lube oil pump de-energizes when N2>1000 rpm

20-AC lube oil pump MOT-0033 de-energizes when N2>3000 rpm

21-When N25>6050 rpm and N2>1250 rpm, N25 ramps to sync idle and the warm-up timer starts

22-Unit is ready to load after warm-up timer has expired

Radial Drive Shaft

The radial drive shaft assembly is located in the 6 o’clock compressor front frame (CFF) strut of the LM6000 turbine. The shafts serve to transmit torque from the inlet gear box to the transfer gear box. The drive shaft assembly consists of three machined tubular steel shafts, housing and bearing.

The upper radial shaft is splined at the upper end to the inlet gear box and the lower end to the radial mid shaft. The shaft is enclosed by the front frame and supported by a ball bearing at its lower end. The radial mid-shaft is splined at the upper end to the upper shaft and at the lower end to the lower shaft. The mid-shaft is enclosed in a housing and supported by a ball bearing at its lower end. The lower radial shaft is splined at the upper end to the mid-shaft and at the lower end to the transfer gear box.

Accessory Gear Box

LM6000 Accessory Gear Box
Engine starting, lubrication, and speed monitoring of the high pressure rotor shaft is accomplished by accessories mounted on the accessory gear box (AGB). The accessory gear box is mounted beneath the gas engine at the compressor front’s frame. Fitted to the aft side of the gear box is the hydraulic starting motor clutch, which drives the tranfer gear box, radial drive shaft, and inlet gear box in A-sump to rotate the high pressure compressor rotor.

The following accessories can be mounted on the Accessory Gear Box:
1- Hydraulic starting motor
2- Clutch assembly
3- Variable-geometry control unit
4- Engine lube oil pump
5- Fuel-metering valve hydraulic oil pump
6- Two magnetic speed pickups (XN25-A and XN25-B)
7- Transfer Gear Box
8- Radial drive shaft

Thrust Balance Piston

A thrust balance piston system has been included in the aft-end of the LM6000 engine to control thrust loading on the No 1B bearing. These loads are imposed by the low pressure compressor and the low pressure turbine and vary with output power. Forward axial loads are applied by varying air pressure in the balance piston air cavity to maintain thrust loads within the capability of the bearing.

The balance piston system consists of the balance piston disk, the balance piston casing, their associated seals, and the domed shaped cavity, formed by these parts. This cavity is pressurized by the stage 11 high pressure compressor bleed air, controlled by a modulating valve mounted externally on the turbine rear frame. The balance piston casing is attached to the aft inner half of the turbine rear frame. The balance piston disk is attached to the low pressure turbine shaft. Thrust is monitored by a total-pressure probe (P48) and static-pressure probe (PS55).

Turbine Rear Frame

The LM6000 turbine rear frame (TRF) is one piece-casting that provides the gas turbine exhaust flow path and the supporting extructure for the D and E sump, the low pressure turbine rotor thrust balance assembly, the low pressure turbine rotor shaft, and the aft drive adapter. Fourteen radial struts function as outlet guide vanes to straighten the exhaust airflow into the exhaust diffuser for enhanced performance. Lubrication oil supply and scavange lines for the D and E sump and the low pressure turbine rotor speed sensors(XNSD-A and XNSD-B) are routed through the struts.

The low pressure turbine rotor thrust balance system is designed to maintain the axial thrust loading on the No 1B thrust bearing within designed limits. The balance piston static seal is mounted to the turbine rear frame hub. Stage 11 high pressure compressor bleed air is routed through the three turbine rear frame struts to generate the required axial loading through the rotor thrust balance system.

NGC 604

The NGC 604 is a colorful, cavernous nebula situated in the Triangulum Galaxy M33, three million light years away. The NGC 604 is a site in which new stars are being born in a spiral arm of the galaxy. The nebula NGC 604 is so vast it is easily seen in ground-based telescopic images. The bright stars in NGC 604 are quite young by astronomical standards, having formed only 3 million years ago.

Discovered by William Herschel on September 11, 1784, the NGC 604 is a gigantic star-forming region which contains more than 200 brilliant blue stars within a cloud of glowing gases some 1,500 light-years across, and is nearly 100 times the size of the Orion Nebula. By contrast, the Orion Nebula contains just four bright central stars.

At the heart of NGC 604 there are over 200 hot stars, much more massive than our Sun (15 to 60 solar masses). They heat the gaseous walls of the nebula making the gas flouresce. Their light also highlights the nebula's three-dimensional shape, like a lantern in a cavern. By studying the physical structure of a giant nebula, astronomers may determine how clusters of massive stars affect the evolution of the interstellar medium of the galaxy.

Triangulum Galaxy

Also known as Messier 33, the Triangulum Galaxy is a spiral galaxy approximately 3 million light-years away in the constellation Triangulum. It is the third largest galaxy in the Local Group, a group of galaxies that also contains the Milky Way Galaxy and the Andromeda Galaxy, and it may be a gravitationally bound companion of the Andromeda Galaxy. The Pisces Dwarf, one of the small Local Group member galaxies, is possibly a satellite of Triangulum. The Triangulum Galaxy can only be seen with the naked eye under exceptionally good conditions.

The Triangulum Galaxy was discovered by Giovanni Batista Hodierna before 1654, grouping it together with open cluster NGC 752. It was independently discovered by Charles Messier in 1764, who catalogued it as M33 on August 25. Triangulum Galaxy is over 50,000 light-years in diameter, and 3 million light-years away from the Milky Way.

The Milky Way

The Milky Way (Latin: Via Lactea) is a barred spiral galaxy; a gravitationally bound collection of roughly two hundred billion stars. As part of the Local Group of galaxies, the Milky Way is one of billions of galaxies in the observable universe and is the home galaxy of our Solar System. The plane of the Milky Way galaxy is visible from Earth as a band of light in the night sky, and it is the appearance of this band of light which has inspired the name for our galaxy. Our Sun is one of these stars and is located roughly 24,000 light years (or 8000 parsecs) from the center of our the Milky Way.

The Milky Way, along with Andromeda Galaxy, is the most massive member of the the Local Group and has a system of satellite galaxies. Viewed from the Earth, the Milky Way galaxy appears in the night sky as a hazy band of white light which originates from stars and other material that lie within the galactic plane. The center of the galaxy is in the direction of Sagittarius. The stellar disk of the Milky Way galaxy is approximately 100,000 light-years in diameter, and is believed to be, on average, about 1,000 ly (9.5×1015 km) thick and is estimated to contain at least 200 billion stars.

The age of the Milky Way is estimated to be about 13.2 billion years, nearly as old as the universe itself. This estimate is based on research by a team of astronomers in 2004 using the UV-Visual Echelle Spectrograph of the Very Large Telescope to measure, for the first time, the beryllium content of two stars in globular cluster NGC 6397.

Constellation

A constellation is a group of celestial bodies that are imaginary connected together to form a visible figure in a sector in the sky. The term is used to mean any group of stars visibly related to each other, if they are considered as a fixed configuration or pattern in a particular culture. Some well-known constellations contain striking and familiar patterns of bright stars. Examples are Orion (forming a figure of a hunter), Leo (containing bright stars outlining the form of a lion), Scorpius, and Crux.

The constellations are totally imaginary things that poets, farmers and astronomers have made up over the past 6,000 years. The real purpose for the constellations is to help us tell which stars are which, and as a guide for sailors to navigate at night. On a really dark night, you can see about 1000 to 1500 stars. Trying to tell which is which is hard. The constellations help by breaking up the sky into more managable bits. The International Astronomical Union divides the sky into 88 official constellations with exact boundaries, so that every direction or place in the sky belongs within one constellation. These are mostly based upon the constellations of the ancient Greek tradition, passed down through the Middle Ages, and contains the signs of the zodiac.

Black Hole

A black hole is a region of space in which the gravitational field is so powerful that nothing can escape its pull after having fallen past its event horizon. A black hole has so much mass concentrated in it that there is no way for a nearby object to escape its gravitational pull.The term derives from the fact that the absorption of visible light renders the hole's interior invisible, and indistinguishable from the black space around it. Imagine an object with such an enormous concentration of mass in such a small radius, that even a beam of light would be pulled back by gravity.

Although its interior is invisible, a black hole may reveal its presence through an interaction with matter that lies in orbit around it. A black hole can be perceived by tracking the movement of a group of stars that orbit its center. One may observe gas from a nearby star that has been drawn into the black hole. The gas spirals inward, heating up to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and earth-orbiting telescopes.

A black hole is the evolutionary endpoint of star at least 10 to 15 times as massive as the Sun. If a star that massive undergoes a supernova explosion, it will leave behind a fairly massive burned out stellar remnant. With no outward forces to oppose gravitational forces, the remnant will collapse in on itself, that is to say it will implode, collapsing to the point of zero volume and infinite density, thus creating what is known as a singularity. As the density increases, the path of light rays emitted from the star are bent and eventually wrapped irrevocably around the star. Any emitted photons are trapped, too, by the intense gravitational field. Because no light escapes after the star reaches this infinite density, it is called a black hole.

Supermassive black holes that contain hundreds of thousands to billions of solar masses are believed to exist in the center of most galaxies, including our own Milky Way. They are thought to be responsible for active galactic nuclei, and form either from the coalescence of smaller black holes, or by the accretion of stars and gas onto them. The largest known supermassive black hole is located in OJ 287 weighing in at 18 billion solar masses.

The Local Group

The Local Group is the group of galaxies that includes the Milky Way, the galaxy in which our solar system is found. The group comprises over 35 galaxies, with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy. The Local Group has a 10 million light-year diameter and have a binary (dumbbell) shape. The group is estimated to have a total mass of (1.29 ± 0.14)×1012M. The group itself is one of many within the Virgo Supercluster. The two most massive members of the group are the Milky Way and the Andromeda Galaxy. These two barred spirals each have a system of nearby satellite galaxies.

The Milky Way's satellite system consists of Sagittarius Dwarf Galaxy, Large Magellanic Cloud, Small Magellanic Cloud, Canis Major Dwarf, Ursa Minor Dwarf, Draco Dwarf, Carina Dwarf, Sextans Dwarf, Sculptor Dwarf, Fornax Dwarf, Leo I, Leo II, and Ursa Major Dwarf. Andromeda's satellite system comprises M32, M110, NGC 147, NGC 185, And I, And II, And III, And IV, And V, Pegasus dSph, Cassiopeia Dwarf, And VIII, And IX, and And X. The third-largest galaxy of the Local Group is the Triangulum Spiral M33.

The Local Group was first identified by Edwin Hubble in 1935, and later mentioned in the chapter VI of his book The Realm of Nebulae published in 1936.

Andromeda Galaxy

Andromeda is the nearest major galaxy to the Milky Way. These two galaxies dominate the Local Group of galaxies. Also known as M31, Andromeda is a spiral galaxy which lies about 2.5 million light years away in the constellation Andromeda. It is visible with the naked eye as a faint smudge on a moonless night. Although Andromeda is the largest galaxy, it may not be the most massive, as recent astronomical observations suggest that the Milky Way contains more dark matter and, in consequence, may be the most massive in the group.

The first description of the Andromeda Galaxy based on telescopic observation was given by Simon Marius in 1612, and Charles Messier catalogued it as object M31 in 1764. In 1785, the astronomer William Herschel noted a faint reddish hue in the core region of the M31. He believed it to be the nearest of all the great nebulae and, based on the color and magnitude of the nebula, he wrongly estimated that it was no more than 2,000 times the distance of Sirius. In 1864, William Huggins observed that the spectrum of M31 differed from a gaseous nebula. The spectra of M31 displayed a continuum of frequencies, superimposed with dark lines. This was very similar to the spectra of individual stars. From this it was deduced that M31 had a stellar nature and, therefore, that it was a galaxy.

Galaxy

A galaxy is one of billions of systems in the universe, including stars, nebulae, star clusters, globular clusters, and interstellar matter, held together by gravitational bound. The solar system is in a galaxy called the Milky Way. Astronomers estimate that there are more than 100 billion galaxies scattered throughout the visible universe. Millions of them have been photographed through telescopes from astrophysical observatories. The most distant galaxies ever photographed are as far as 10 billion to 13 billion light-years away.

Galaxies have been categorized according to their visual morphology. A common shape is the elliptical galaxy, that has an ellipse-shaped profile. Spiral galaxies are disk-shaped assemblages with curving, dusty arms. Galaxies with irregular forms are known as peculiar galaxies, and typically result from disruption by the gravitational pull of neighboring galaxies. Galaxies have diameters of 3,260 to 326,000 light years. The Milky Way has a diameter of about 100,000 light years with the solar system lying about 25,000 ligh years from the center of the galaxy.

The word galaxy derives from the Greek and means “milky.” Aside from the the Milky Way, there are only three other galaxies which are visible to the naked eye. In the northern hemisphere, people can see Andromeda Galaxy, that is about 2 million light years away. In the southern hemisphere, the Large Magellanic Cloud and the Small Magellanic Cloud can be seen.

Galaxies are unevenly scattered throughout the universe. Some stand alone with no close neighbor. Others occur in pairs, with each orbiting the other. But most galaxies are found in groups called clusters. A cluster may contain from a few dozen to several thousand galaxies. It may have a diameter as large as 10 million light-years. Clusters of galaxies, in turn, are grouped in larger structures called superclusters.

Pulsar

Pulsar from PULSating stAR

Pulsars are the relics of massive stars that have ended their lives in a tremendously powerful explosion called a supernova and that have turned into highly magnetized rotating neutron stars. Emitting a beam of electromagnetic radiation in the form of radio waves, a pulsar appears to flash on and off many times a second.The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name.

Neutron stars also have very large magnetic fields. The magnetic field on Earth, which makes compasses point north, is a trillion (1,000,000,000,000) times weaker than the typical neutron star magnetic field. The magnetic field is so strong that it causes most of the light and radiation that the neutron star emits to be concentrated into cones of emission, like beams from a lighthouse. In fact, the key to a pulsar is the combination of the extraordinary magnetic field and the rotation of a neutron star. If the neutron star is spinning, like the Earth rotates on its axis, and if the Earth happens to lie in the path of the beams, we see a pulse of light each time a beam sweeps across the earth.

Population I Stars

Population I stars are those young stars whose metallicity is highest. The Earth's Sun is an example of a metal-rich star. These are common in the spiral arms of the Milky Way galaxy. Usually, the youngest stars, the extreme Population I, are found farther in and intermediate Population I stars are farther out, etc. The Sun is considered an intermediate Population I star. Population I stars have regular elliptical orbits of the galactic centre, with a low relative velocity. The high metallicity of Population I stars makes them more likely to possess planetary systems than the other two populations, since planets, particularly terrestrial planets, are thought to be formed by the accretion of metals.

Sun

The Sun is the star at the center of the Solar System. The Earth and other planets along with their natural satellite orbit the Sun. With a mass of 99.8% of the Solar System, the Sun is the gravity center of the system, exerting a gravitational pull over all the planets and matter that orbit about it. Energy from the Sun, in the form of sunlight, supports almost all life on Earth via photosynthesis, and drives the Earth's climate and weather.

The Sun consists of hydrogen (about 74% of its mass), helium (about 24% of mass), and trace quantities of other elements, including iron, nickel, oxygen, silicon, sulfur, magnesium, carbon, neon, calcium, and chromium. The Sun has a spectral class G2V and, like most stars, is a main sequence star. This means that it generates its energy by nuclear fusion of hydrogen nuclei into helium. There are more than 100 million G2 class stars in our galaxy.

With an orbital speed of 251 km/s, the Sun orbits the center of the Milky Way galaxy at a distance of approximately 26,000 to 27,000 light-years from the galactic center, moving generally in the direction of Cygnus and completing one revolution in about 225–250 million years. Because of a high abundance of heavy elements such as gold and uranium in the Solar system, it has been suggested that the formation of the Sun may have been triggered by shockwaves from one or more nearby supernovae. This makes the Sun a Population I, or heavy element-rich, star.

The Sunlight is the Earth's primary source of energy. Sunlight on the surface of Earth is attenuated by the Earth's atmosphere so that less power arrives at the surface, closer to 1,000 watts per directly exposed square meter in clear conditions when the Sun is near the zenith. Ultraviolet light from the Sun has antiseptic properties and can be used to sanitize tools and water. It also causes sunburn, and has other medical effects such as the production of Vitamin D. Ultraviolet light is strongly deadened by Earth's ozone layer, so that the amount of UV varies greatly with latitude and has been partially responsible for many biological adaptations.

Nebula

(Nebula from Latin: Cloud)

A nebula is an interstellar cloud of dust, hydrogen gas and plasma. It is the first stage of a star’s cycle. Nebulae often form star-forming regions, such as in the Eagle Nebula. This nebula is depicted in one of NASA's most famous images, the "Pillars of Creation". In these regions the formations of gas, dust and other materials 'clump' together to form larger masses, which attract further matter, and eventually will become big enough to form stars. The remaining materials are then believed to form planets, and other planetary system objects.

Some nebulae are formed as the result of supernova explosions. The material thrown off from the supernova explosion is ionized by the supernova remnant. But many nebulae originate from the gravitational collapse of gas in the interstellar medium. As the material collapses under its own weight, massive stars may form in the center, and their ultraviolet radiation ionizes the surrounding gas, making it visible at optical wavelengths. An example of this type of nebula is the Rosette Nebula or the Pelican Nebula. The size of these nebulae varies depending on the size of the original cloud of gas, and the number of stars formed can vary too. As the sites of star formation, the formed stars are sometimes known as a young, loose cluster.

Supernova

A supernova is a stellar explosion; one of the most energetic explosive events in the universe. This supernova explosion occur at the end of a star lifetime when its nuclear fuel is exhausted and it is no longer supported by the release of nuclear energy. They are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy before fading from view over several weeks or months. During this short interval, a supernova can radiate as much energy as the Sun could emit over its life span. The explosion expels much of a star's material at a velocity of up to a tenth the speed of light, driving a shock wave into the surrounding interstellar medium. This shock wave sweeps up an expanding shell of gas and dust called a supernova remnant.

With the development of the telescope, the field of supernova discovery has enlarged to other galaxies, starting with the 1885 observation of supernova S Andromedae in the Andromeda galaxy. The last supernova event to be seen in our galaxy was Kepler's star in 1604. This remnant has been studied by many X-ray astronomy satellites, including ROSAT. There are, however, many remnants of Supernovae explosions in our galaxy, that are seen as X-ray shell-like structures caused by the shock wave propagating out into the interstellar medium. Supernovae provide important information on cosmological distances. During the twentieth century, successful models for each type of supernova were developed, and scientists' comprehension of the role of supernovae in the star formation process is growing. Some of the most distant supernovae recently observed appeared dimmer than expected. This has provided evidence that the expansion of the universe may be accelerating.