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Structure of Black Holes

Although black holes come in a variety of masses and sizes, their structures are all alike. A black hole's entire mass is concentrated in an almost infinitely small and dense point called a singularity. This point is surrounded by the event horizon - the distance from the singularity at which its escape velocity exceeds the speed of light. And a rotating black hole is surrounded by the ergosphere, a region in which the black hole drags space itself.

The singularity forms when matter is compressed so tightly that no other force of nature can balance it. In a "normal" star, like the Sun, the inward pull of gravity is balanced by the outward pressure of the nuclear reactions in its core. In the collapsed stars known as white dwarfs or neutron stars, other forces prevent the ultimate collapse. If there is too much mass in a given volume, though, the object reaches a critical density where nothing can prevent its ultimate collapse to form a black hole. Because gravity overcomes the other forces of nature, a singularity follows its own bizarre rules of physics. Time and space as we know them are crushed out of existence, and gravity becomes infinitely strong. As the distance from the singularity increases, the escape velocity decreases. Escape velocity is the speed at which an object must move to get away. For Earth, the escape velocity is around seven miles (11 km) per second. In other words, a spacecraft must go at least that fast to escape Earth's gravitational pull and travel to another planet. At a certain distance from the singularity, the escape velocity drops to the speed of light (about 186,000 miles/300,000 km per second). This distance is known as the Schwarzschild radius, in honor of Karl Schwarzschild, who first defined it. This radius depends on the mass of the black hole. For a black hole as massive as the Sun, the radius is about two miles (3 km). For every extra solar mass, the radius increases by two miles.

This radius enfolds the singularity in a zone of blackness - in other words, it makes a black hole black. It gives the black hole a visible surface, which is known as the event horizon. This is not a solid surface, though. It is simply the "point of no return" for anything that approaches the black hole. Once any object - from a starship to a particle of light - crosses inside this horizon, it cannot get back out. It is trapped inside the black hole. Anything that enters the black hole increases its mass. And as the mass goes up, the size of the event horizon gets bigger, too. So if you feed a black hole, it gets fatter!

If the black hole doesn't rotate, then its gravitational influence on its environment is straightforward. If the black hole is spinning, though, then its gravitational effects are more complicated. It actually pulls the fabric of spacetime along with it - an effect called frame dragging. This area is known as the ergosphere. Seen in cross-section, it is oval-shaped, with the region of influence extending farther into space at the black hole's equator than at its poles.

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Structure of the Sun

Let’s begin by asking what the solar atmosphere is made of. As explained in Radiation and Spectra, we can use a star’s absorption line spectrum to determine what elements are present. It turns out that the Sun contains the same elements as Earth but not in the same proportions. About 73% of the Sun’s mass is hydrogen, and another 25% is helium. All the other chemical elements make up only 2% of our star. The 10 most abundant gases in the Sun’s visible surface layer are listed in Table 1. Examine that table and notice that the composition of the Sun’s outer layer is very different from Earth’s crust, where we live. Although not like our planet’s, the makeup of the Sun is quite typical of stars in general. The Sun’s layers are different from each other, and each plays a part in producing the energy that the Sun ultimately emits. We will begin with the core and work our way out through the layers. The Sun’s core is extremely dense and is the source of all of its energy. Inside the core, nuclear energy is being released. The core is approximately 20% of the size of the solar interior and is thought to have a temperature of approximately 15 million K, making it the hottest part of the Sun.Above the core is a region known as the radiative zone named for the primary mode of transporting energy across it. This region starts at about 25% of the distance to the solar surface and extends up to about 70% of the way to the surface. The light generated in the core is transported through the radiative zone very slowly, since the high density of matter in this region means a photon cannot travel too far without encountering a particle, causing it to change direction and lose some energy.The convective zone is the outermost layer of the solar interior. It is a thick layer approximately 200,000 kilometers deep that transports energy from the edge of the radiative zone to the surface through giant convection cells, similar to a pot of boiling oatmeal. The plasma at the bottom of the convective zone is extremely hot, and it bubbles to the surface where it loses its heat to space. Once the plasma cools, it sinks back to the bottom of the convective zone.Now that we have given a quick overview of the structure of the whole Sun, in this section, we will embark on a journey through the visible layers of the Sun, beginning with the photosphere the visible surface. Earth’s air is generally transparent. But on a smoggy day in many cities, it can become opaque, which prevents us from seeing through it past a certain point. Something similar happens in the Sun. Its outer atmosphere is transparent, allowing us to look a short distance through it. But when we try to look through the atmosphere deeper into the Sun, our view is blocked. The photosphere is the layer where the Sun becomes opaque and marks the boundary past which we cannot see. As we saw, the energy that emerges from the photosphere was originally generated deep inside the Sun. This energy is in the form of photons, which make their way slowly toward the solar surface. Outside the Sun, we can observe only those photons that are emitted into the solar photosphere, where the density of atoms is sufficiently low and the photons can finally escape from the Sun without colliding with another atom or ion.

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Structure of the Eyes

The structures and functions of the eyes are complex. Each eye constantly adjusts the amount of light it lets in, focuses on objects near and far, and produces continuous images that are instantly transmitted to the brain.

The orbit is the bony cavity that contains the eyeball, muscles, nerves, and blood vessels, as well as the structures that produce and drain tears. Each orbit is a pear-shaped structure that is formed by several bones. The outer covering of the eyeball consists of a relatively tough, white layer called the sclera (or white of the eye). Near the front of the eye, in the area protected by the eyelids, the sclera is covered by a thin, transparent membrane (conjunctiva), which runs to the edge of the cornea. The conjunctiva also covers the moist back surface of the eyelids and eyeballs. Light enters the eye through the cornea, the clear, curved layer in front of the iris and pupil. The cornea serves as a protective covering for the front of the eye and also helps focus light on the retina at the back of the eye. After passing through the cornea, light travels through the pupil (the black dot in the middle of the eye). The iris—the circular, colored area of the eye that surrounds the pupil—controls the amount of light that enters the eye. The iris allows more light into the eye (enlarging or dilating the pupil) when the environment is dark and allows less light into the eye (shrinking or constricting the pupil) when the environment is bright. Thus, the pupil dilates and constricts like the aperture of a camera lens as the amount of light in the immediate surroundings changes. The size of the pupil is controlled by the action of the pupillary sphincter muscle and dilator muscle. Behind the iris sits the lens. By changing its shape, the lens focuses light onto the retina. Through the action of small muscles (called the ciliary muscles), the lens becomes thicker to focus on nearby objects and thinner to focus on distant objects. The retina contains the cells that sense light (photoreceptors) and the blood vessels that nourish them. The most sensitive part of the retina is a small area called the macula, which has millions of tightly packed photoreceptors (the type called cones). The high density of cones in the macula makes the visual image detailed, just as a high-resolution digital camera has more megapixels. Each photoreceptor is linked to a nerve fiber. The nerve fibers from the photoreceptors are bundled together to form the optic nerve. The optic disk, the first part of the optic nerve, is at the back of the eye. The photoreceptors in the retina convert the image into electrical signals, which are carried to the brain by the optic nerve. There are two main types of photoreceptors: cones and rods. Cones are responsible for sharp, detailed central vision and color vision and are clustered mainly in the macula. Rods are responsible for night and peripheral (side) vision. Rods are more numerous than cones and much more sensitive to light, but they do not register color or contribute to detailed central vision as the cones do. Rods are grouped mainly in the peripheral areas of the retina. The eyeball is divided into two sections, each of which is filled with fluid. The pressure generated by these fluids fills out the eyeball and helps maintain its shape. The front section (anterior segment) extends from the inside of the cornea to the front surface of the lens. It is filled with a fluid called the aqueous humor, which nourishes the internal structures. The anterior segment is divided into two chambers. The front (anterior) chamber extends from the cornea to the iris. The back (posterior) chamber extends from the iris to the lens. Normally, the aqueous humor is produced in the posterior chamber, flows slowly through the pupil into the anterior chamber, and then drains out of the eyeball through outflow channels located where the iris meets the cornea. The back section (posterior segment) extends from the back surface of the lens to the retina. It contains a jellylike fluid called the vitreous humor.

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Structure of the Brain

The brain is composed of the cerebrum, cerebellum, and brainstem.

Cerebrum: is the largest part of the brain and is composed of right and left hemispheres. It performs higher functions like interpreting touch, vision and hearing, as well as speech, reasoning, emotions, learning, and fine control of movement. Cerebellum: is located under the cerebrum. Its function is to coordinate muscle movements, maintain posture, and balance. Brainstem: acts as a relay center connecting the cerebrum and cerebellum to the spinal cord. It performs many automatic functions such as breathing, heart rate, body temperature, wake and sleep cycles, digestion, sneezing, coughing, vomiting, and swallowing.

Right brain & left brain

The cerebrum is divided into two halves: the right and left hemispheres. They are joined by a bundle of fibers called the corpus callosum that transmits messages from one side to the other. Each hemisphere controls the opposite side of the body. If a stroke occurs on the right side of the brain, your left arm or leg may be weak or paralyzed. Not all functions of the hemispheres are shared. In general, the left hemisphere controls speech, comprehension, arithmetic, and writing. The right hemisphere controls creativity, spatial ability, artistic, and musical skills. The left hemisphere is dominant in hand use and language in about 92% of people.

Lobes of the brain

The cerebral hemispheres have distinct fissures, which divide the brain into lobes. Each hemisphere has 4 lobes: frontal, temporal, parietal, and occipital. Each lobe may be divided, once again, into areas that serve very specific functions. It’s important to understand that each lobe of the brain does not function alone. There are very complex relationships between the lobes of the brain and between the right and left hemispheres.  Frontal lobePersonality, behavior, emotionsJudgment, planning, problem solvingSpeech: speaking and writing Body movement Intelligence, concentration, self awarenessParietal lobeInterprets language, wordsSense of touch, pain, temperature Interprets signals from vision, hearing, motor, sensory and memorySpatial and visual perceptionOccipital lobeInterprets vision Temporal lobeUnderstanding language MemoryHearingSequencing and organizationLanguageIn general, the left hemisphere of the brain is responsible for language and speech and is called the "dominant" hemisphere. The right hemisphere plays a large part in interpreting visual information and spatial processing. In about one third of people who are left-handed, speech function may be located on the right side of the brain. Left-handed people may need special testing to determine if their speech center is on the left or right side prior to any surgery in that area.Aphasia is a disturbance of language affecting speech production, comprehension, reading or writing, due to brain injury – most commonly from stroke or trauma. The type of aphasia depends on the brain area damaged.Broca’s area: lies in the left frontal lobe. If this area is damaged, one may have difficulty moving the tongue or facial muscles to produce the sounds of speech. The person can still read and understand spoken language but has difficulty in speaking and writing  – called Broca's aphasia. Wernicke's area: lies in the left temporal lobe. Damage to this area causes Wernicke's aphasia. The individual may speak in long sentences that have no meaning, add unnecessary words, and even create new words. They can make speech sounds, however they have difficulty understanding speech and are therefore unaware of their mistakes.CortexThe surface of the cerebrum is called the cortex. It has a folded appearance with hills and valleys. The cortex contains 16 billion neurons  that are arranged in specific layers. The nerve cell bodies color the cortex grey-brown giving it its name – gray matter . Beneath the cortex are long nerve fibers that connect brain areas to each other — called white matter. The cortex contains neurons, which are interconnected to other brain areas by axons. The cortex has a folded appearance. A fold is called a gyrus and the valley between is a sulcus. The folding of the cortex increases the brain’s surface area allowing more neurons to fit inside the skull and enabling higher functions. Each fold is called a gyrus, and each groove between folds is called a sulcus. There are names for the folds and grooves that help define specific brain regions.Deep structuresPathways called white matter tracts connect areas of the cortex to each other. Messages can travel from one gyrus to another, from one lobe to another, from one side of the brain to the other, and to structures deep in the brain. Coronal cross-section showing the basal ganglia.Hypothalamus: is located in the floor of the third ventricle and is the master control of the autonomic system. It plays a role in controlling behaviors such as hunger, thirst, sleep, and sexual response. It also regulates body temperature, blood pressure, emotions, and secretion of hormones.Pituitary gland: lies in a small pocket of bone at the skull base called the sella turcica. The pituitary gland is connected to the hypothalamus of the brain by the pituitary stalk. Known as the “master gland,” it controls other endocrine glands in the body. It secretes hormones that control sexual development, promote bone and muscle growth, and respond to stress.Pineal gland: is located behind the third ventricle. It helps regulate the body’s internal clock and circadian rhythms by secreting melatonin. It has some role in sexual development.Thalamus: serves as a relay station for almost all information that comes and goes to the cortex. It plays a role in pain sensation, attention, alertness and memory.Basal ganglia: includes the caudate, putamen and globus pallidus. These nuclei work with the cerebellum to coordinate fine motions, such as fingertip movements.Limbic system: is the center of our emotions, learning, and memory. Included in this system are the cingulate gyri, hypothalamus, amygdala  and hippocampus .MemoryMemory is a complex process that includes three phases: encoding , storing, and recalling. Different areas of the brain are involved in different types of memory. Your brain has to pay attention and rehearse in order for an event to move from short-term to long-term memory – called encoding.

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Structure of the Heart

The heart is a complex muscle that pumps blood through the three divisions of the circulatory system: the coronary, pulmonary, and systemic. Coronary circulation intrinsic to the heart takes blood directly from the main artery coming from the heart. For pulmonary and systemic circulation, the heart has to pump blood to the lungs or the rest of the body, respectively.

The heart muscle is asymmetrical as a result of the distance blood must travel in the pulmonary and systemic circuits. Since the right side of the heart sends blood to the pulmonary circuit, it is smaller than the left side, which must send blood out to the whole body in the systemic circuit.

In humans, the heart is about the size of a clenched fist. It is divided into four chambers: two atria and two ventricles. There are one atrium and one ventricle on the right side and one atrium and one ventricle on the left side. The atria are the chambers that receive blood while the ventricles are the chambers that pump blood. The right atrium receives deoxygenated blood from the superior vena cava, which drains blood from the veins of the upper organs and arms. The right atrium also receives blood from the inferior vena cava, which drains blood from the veins of the lower organs and legs. In addition, the right atrium receives blood from the coronary sinus, which drains deoxygenated blood from the heart itself. This deoxygenated blood then passes to the right ventricle through the right atrioventricular valve, a flap of connective tissue that opens in only one direction to prevent the backflow of blood. After it is filled, the right ventricle pumps the blood through the pulmonary arteries to the lungs for re-oxygenation. After blood passes through the pulmonary arteries, the right semilunar valves close, preventing the blood from flowing backwards into the right ventricle. The left atrium then receives the oxygen-rich blood from the lungs via the pulmonary veins. The valve separating the chambers on the left side of the heart is called the biscuspid or mitral valve.The blood passes through the bicuspid valve to the left ventricle where it is pumped out through the aorta, the major artery of the body, taking oxygenated blood to the organs and muscles of the body. Once blood is pumped out of the left ventricle and into the aorta, the aortic semilunar valve closes, preventing blood from flowing backward into the left ventricle. This pattern of pumping is referred to as double circulation and is found in all mammals.