What is a light year? (How Light Works)

A light year is a way of measuring distance. That doesn't make much sense because "light year" contains the word "year," which is normally a unit oftime. Even so, light years measure distance.

You are used to measuring distances in either inches/feet/miles or centimeters/meters/kilometers, depending on where you live. You know how long a foot or a meter is -- you are comfortable with these units because you use them every day. Same thing with miles and kilometers -- these are nice, human increments of distance.

When astronomers use their telescopes to look atstars, things are different. The distances are gigantic. For example, the closest star to Earth (besides our sun) is something like 24,000,000,000,000 miles (38,000,000,000,000 kilometers) away. That's the closeststar. There are stars that are billions of times farther away than that. When you start talking about those kinds of distances, a mile or kilometer just isn't a practical unit to use because the numbers get too big. No one wants to write or talk about numbers that have 20 digits in them!

So to measure really long distances, people use a unit called a light year. Light travels at 186,000 miles per second (300,000 kilometers per second). Therefore, a light second is 186,000 miles (300,000 kilometers). A light year is the distance that light can travel in a year, or:

186,000 miles/second * 60 seconds/minute * 60 minutes/hour * 24 hours/day * 365 days/year = 5,865,696,000,000 miles/year

A light year is 5,865,696,000,000 miles (9,460,800,000,000 kilometers). That's a long way!

Using a light year as a distance measurement has another advantage -- it helps you determine age. Let's say that a star is 1 million light years away. The light from that star has traveled at the speed of light to reach us. Therefore, it has taken the star's light 1 million years to get here, and the light we are seeing was created 1 million years ago. So the star we are seeing is really how the star looked a million years ago, not how it looks today. In the same way, our sun is 8 or so light minutes away. If the sun were to suddenly explode right now, we wouldn't know about it for eight minutes because that is how long it would take for the light of the explosion to get here.

COOL FACT

A light nanosecond -- the distance light can travel in a billionth of a second -- is about 1 foot (about 30 cm). Radar uses this fact to measure how far away something like an airplane is. A radar antenna sends out a short radio pulse and then waits for it to echo off an airplane or other target. While it's waiting, it counts the number of nanoseconds that pass. Radio waves travel at the speed of light, so the number of nanoseconds divided by 2 tells the radar unit how far away the object is!­

Origin of Light (How Light Works)

Scientists today accept the existence of photons and their weird wave-particle behavior. What they still debate is the more existential side of things, such as where light came from in the first place. To answer this question, physicists turn their attention to the big bang and the few moments that followed.

You might recall that the big bang is the birthing event that gave rise to the universe. You can read more in How the Big Bang Theory Works, but it will be useful to remind you of the basics here. About 15 billion years ago, all matter and energy were bottled up in a small region known as a singularity. In an instant, this single point of super-dense material began to expand at an incredibly rapid rate. As the newborn universe expanded, it began to cool down and become less dense. This allowed more stable particles and photons to form.

Here's what may have happened:

1. Immediately after the big bang, electromagnetism didn't exist as an independent force. Instead, it was joined to the weak nuclear force.
2. Particles known as B and W bosons also existed at this time.
3. When the universe was just 0.00000000001 seconds old, it had cooled enough for electromagnetism to break from the weak nuclear force and for the B and W bosons to combine into photons. The photons mingled freely with quarks, the smallest building blocks of matter.
4. When the universe was 0.00001 seconds old, quarks combined to form protons and neutrons.
5. When the universe was 0.01 seconds old, protons and neutrons began to organize into atoms.
6. Finally, when the universe was the tender age of 380,000 years old, photons broke free, and light streamed across the dark chasms of space.

This light eventually dimmed and reddened until, finally, the nuclear furnaces in stars kicked on and began generating new light. Our sun turned on about 4.6 billion years ago, showering the solar system with photons. Those photons have been streaming to our humble blue planet ever since. A few fell on the eyes of great thinkers -- Newton, Huygens, Einstein -- and caused them to stop, to think and to imagine.

Keep reading for more links to satisfy your curious mind.

Pigments and Absorption of Light (How Light Works)

Another way to make colors is to absorb some of the frequencies of light, and thus remove them from the white light combination. The absorbed colors are the ones you don't see -- you see only the colors that come bouncing back to your eye. This is known as subtractive color, and it's what happens with paints and dyes. The paint or dye molecules absorb specific frequencies and bounce back, or reflect, other frequencies to your eye. The reflected frequency (or frequencies) are what you see as the color of the object. For example, the leaves of green plants contain a pigment called chlorophyll, which absorbs the blue and red colors of the spectrum and reflects the green.

You can explain absorption in terms of atomic structure. The frequency of the incoming light wave is at or near the vibration frequency of the electrons in the material. The electrons take in the energy of the light wave and start to vibrate. What happens next depends upon how tightly the atoms hold on to their electrons. Absorption occurs when the electrons are held tightly, and they pass the vibrations along to the nuclei of the atoms. This makes the atoms speed up, collide with other atoms in the material, and then give up as heat the energy they acquired from the vibrations.

The absorption of light makes an object dark or opaque to the frequency of the incoming wave. Wood is opaque to visible light. Some materials are opaque to some frequencies of light, but transparent to others. Glass is opaque to ultraviolet light, but transparent to visible light.

Making Colors of light (How Light Works)

Visible light is light that the human eye can perceive. When you look at the sun's visible light, it appears to be colorless, which we call white. Although we can see this light, white isn't considered part of the visible spectrum. That's because white light isn't the light of a single color but instead many colors.

When sunlight passes through a glass of water to land on a wall, we see a rainbow on the wall. This wouldn't happen unless white light were a mixture of all of the colors of the visible spectrum. Isaac Newton was the first person to demonstrate this. Newton passed sunlight through a glass prism to separate the colors into a rainbow spectrum. He then passed sunlight through a second glass prism and combined the two rainbows. The combination produced white light. His simple experiment proved conclusively that white light is a mixture of colors.

You can do a similar experiment with three flashlights and three different colors of cellophane -- red, green and blue (commonly referred to as RGB). Cover one flashlight with one to two layers of red cellophane and fasten the cellophane with a rubber band (don't use too many layers or you'll block the light from the flashlight). Cover another flashlight with blue cellophane and a third flashlight with green cellophane. Go into a darkened room, turn the flashlights on and shine them against a wall so that the beams overlap, as shown in the figure. Where red and blue light overlap, you will see magenta. Where red and green light overlap, you will see yellow. Where green and blue light overlap, you will see cyan. You will notice that white light can be made by various combinations, such as yellow with blue, magenta with green, cyan with red, and by mixing all of the colors together.

By adding various combinations of these so-called additive colors -- red, green and blue light -- you can make all the colors of the visible spectrum. This is how computer monitors (RGB monitors) generate colors.

Lasers (How Light Works)

An interesting application of the quantum nature of light is the laser. You can get the whole story on lasers in How Lasers Work, but we're going to cover some of the key concepts here. Laser is an acronym for "light amplification by stimulated emission of radiation," which is a tongue-tying way to describe light in which the photons are all at the same wavelength and have their crests and troughs in phase. Research physicist Theodore H. Maiman developed the world's first working laser, the ruby laser, in 1960. The ruby laser contained a ruby crystal, a quartz flash tube, reflecting mirrors and a power supply.

Let's review how Maiman used these components to create laser light, starting with the characteristics of ruby. Ruby is an aluminum oxide crystal in which some of the aluminum atoms have been replaced with chromium atoms. Chromium gives ruby its characteristic red color by absorbing green and blue light and emitting or reflecting only red light. Of course, Maiman couldn't use a ruby in its naturally occurring crystalline state. First, he had to form the ruby crystal into a cylinder. Next, he wrapped a high-intensity quartz lamp around the ruby cylinder to provide a flash of white light. The green and blue wavelengths in the flash excited electrons in the chromium atoms to a higher energy level. As these electrons returned to their normal state, they emitted their characteristic ruby-red light.

Here's where it got interesting. Maiman placed a fully reflecting mirror on one end of the crystal and a partially reflecting mirror on the other. The mirrors reflected some of the red-wavelength photons back and forth inside the ruby crystal. This, in turn, stimulated other excited chromium atoms to produce more photons, until a flood of precisely aligned photons bounced back and forth within the laser. At each bounce, some of the photons escaped, which allowed observers to perceive the beam itself.

Today, scientists make lasers out of many different materials. Some, like the ruby laser, emit short pulses of light. Others, like helium-neon gas lasers or liquid dye lasers, emit a continuous beam of light.

Producing a Photon (How Light Works)

There are many different ways to produce photons, but all of them use the same mechanism inside an atom to do it. This mechanism involves the energizing of electrons orbiting each atom's nucleus. How Nuclear Radiation Worksdescribes protons, neutrons and electrons in some detail. For example, hydrogen atoms have one electron orbiting the nucleus. Helium atoms have two electrons orbiting the nucleus. Aluminum atoms have 13 electrons circling the nucleus. Each atom has a preferred number of electrons zipping around its nucleus.

Electrons circle the nucleus in fixed orbits -- a simplified way to think about it is to imagine how satellitesorbit the Earth. There's a huge amount of theory around electron orbitals, but to understand light there is just one key fact to understand: An electron has a natural orbit that it occupies, but if you energize an atom, you can move its electrons to higher orbitals. A photon is produced whenever an electron in a higher-than-normal orbit falls back to its normal orbit. During the fall from high energy to normal energy, the electron emits a photon -- a packet of energy -- with very specific characteristics. The photon has a frequency, or color, that exactly matches the distance the electron falls.

You can see this phenomenon quite clearly in gas-discharge lamps. Fluorescent lamps, neon signs and sodium-vapor lamps are common examples of this kind of electric lighting, which passes an electric current through a gas to make the gas emit light. The colors of gas-discharge lamps vary widely depending on the identity of the gas and the construction of the lamp.

For example, along highways and in parking lots, you often see sodium vapor lights. You can tell a sodium vapor light because it's really yellow when you look at it. A sodium vapor light energizes sodium atoms to generate photons. A sodium atom has 11 electrons, and because of the way they're stacked in orbitals one of those electrons is most likely to accept and emit energy. The energy packets that this electron is most likely to emit fall right around a wavelength of 590 nanometers. This wavelength corresponds to yellow light. If you run sodium light through a prism, you don't see a rainbow -- you see a pair of yellow lines.

Wave-Particle Duality of Light (How Light Works)

At first, physicists were reluctant to accept the dual nature of light. After all, many of us humans like to have one right answer. But Einstein paved the way in 1905 by embracing wave-particle duality. We've already discussed the photoelectric effect, which led Einstein to describe light as a photon. Later that year, however, he added a twist to the story in a paper introducing special relativity. In this paper, Einstein treated light as a continuous field of waves -- an apparent contradiction to his description of light as a stream of particles. Yet that was part of his genius. He willingly accepted the strange nature of light and chose whichever attribute best addressed the problem he was trying to solve.

Today, physicists accept the dual nature of light. In this modern view, they define light as a collection of one or more photons propagating through space as electromagnetic waves. This definition, which combines light's wave and particle nature, makes it possible to rethink Thomas Young's double-slit experiment in this way: Light travels away from a source as an electromagnetic wave. When it encounters the slits, it passes through and divides into two wave fronts. These wave fronts overlap and approach the screen. At the moment of impact, however, the entire wave field disappears and a photon appears. Quantum physicists often describe this by saying the spread-out wave "collapses" into a small point.

Similarly, photons make it possible for us to see the world around us. In total darkness, our eyes are actually able to sense single photons, but generally what we see in our daily lives comes to us in the form of zillions of photons produced by light sources and reflected off objects. If you look around you right now, there is probably a light source in the room producing photons, and objects in the room that reflect those photons. Your eyes absorb some of the photons flowing through the room, and that's how you see.

But wait. What makes a light source produce photons? We'll get to that. Next Post.

Light as Particles (How Light Works)

Maxwell's theoretical treatment of electromagnetic radiation, including its description of light waves, was so elegant and predictive that many physicists in the 1890s thought that there was nothing more to say about light and how it worked. Then, on Dec. 14, 1900, Max Planck came along and introduced a stunningly simple, yet strangely unsettling, concept: that light must carry energy in discrete quantities. Those quantities, he proposed, must be units of the basic energy increment, hf, where his a universal constant now known as Planck's constant and f is the frequency of the radiation.

Albert Einstein advanced Planck's theory in 1905 when he studied the photoelectric effect. First, he began by shining ultraviolet light on the surface of a metal. When he did this, he was able to detect electrons being emitted from the surface. This was Einstein's explanation: If the energy in light comes in bundles, then one can think of light as containing tiny lumps, or photons. When these photons strike a metal surface, they act like billiard balls, transferring their energy to electrons, which become dislodged from their "parent" atoms. Once freed, the electrons move along the metal or get ejected from the surface.

The particle theory of light had returned -- with a vengeance. Next, Niels Bohr applied Planck's ideas to refine the model of an atom. Earlier scientists had demonstrated that atoms consist of positively charged nuclei surrounded by electrons orbiting like planets, but they couldn't explain why electrons didn't simply spiral into the nucleus. In 1913, Bohr proposed that electrons exist in discrete orbits based on their energy. When an electron jumps from one orbit to a lower orbit, it gives off energy in the form of a photon.

The quantum theory of light -- the idea that light exists as tiny packets, or particles, called photons -- slowly began to emerge. Our understanding of the physical world would no longer be the same.

Light Frequencies (How Light Works)

Once Maxwell introduced the concept of electromagnetic waves, everything clicked into place. Scientists now could develop a complete working model of light using terms and concepts, such as wavelength and frequency, based on the structure and function of waves. According to that model, light waves come in many sizes. The size of a wave is measured as its wavelength, which is the distance between any two corresponding points on successive waves, usually peak to peak or trough to trough. The wavelengths of the light we can see range from 400 to 700 nanometers (or billionths of a meter). But the full range of wavelengths included in the definition of electromagnetic radiation extends from 0.1 nanometers, as in gamma rays, to centimeters and meters, as in radio waves.

Light waves also come in many frequencies. The frequency is the number of waves that pass a point in space during any time interval, usually one second. We measure it in units of cycles (waves) per second, orhertz. The frequency of visible light is referred to as color, and ranges from 430 trillion hertz, seen as red, to 750 trillion hertz, seen as violet. Again, the full range of frequencies extends beyond the visible portion, from less than 3 billion hertz, as in radio waves, to greater than 3 billion billion hertz (3 x 10¹⁹), as in gamma rays.

The amount of energy in a light wave is proportionally related to its frequency: High frequency light has high energy; low frequency light has low energy. So, gamma rays have the most energy (part of what makes them so dangerous to humans), and radio waves have the least. Of visible light, violet has the most energy and red the least. The whole range of frequencies and energies, shown in the accompanying figure, is known as the electromagnetic spectrum. Note that the figure isn't drawn to scale and that visible light occupies only one-thousandth of a percent of the spectrum.

This might be the end of the discussion, except that Albert Einstein couldn't let speeding light waves lie. His work in the early 20th century resurrected the old idea that light, just maybe, was a particle after all.

Light as Waves (How Light Works)

Unlike water waves, light waves follow more complicated paths, and they don't need a medium to travel through.

When the 19th century dawned, no real evidence had accumulated to prove the wave theory of light. That changed in 1801 when Thomas Young, an English physician and physicist, designed and ran one of the most famous experiments in the history of science. It's known today as the double-slit experiment and requires simple equipment -- a light source, a thin card with two holes cut side by side and a screen.

To run the experiment, Young allowed a beam of light to pass through a pinhole and strike the card. If light contained particles or simple straight-line rays, he reasoned, light not blocked by the opaque card would pass through the slits and travel in a straight line to the screen, where it would form two bright spots. This isn't what Young observed. Instead, he saw a bar code pattern of alternating light and dark bands on the screen. To explain this unexpected pattern, he imagined light traveling through space like a water wave, with crests and troughs. Thinking this way, he concluded that light waves traveled through each of the slits, creating two separate wave fronts. As these wave fronts arrived at the screen, they interfered with each other. Bright bands formed where two wave crests overlapped and added together. Dark bands formed where crests and troughs lined up and canceled each other out completely.

Young's work sparked a new way of thinking about light. Scientists began referring to light waves and reshaped their descriptions of reflection and refraction accordingly, noting that light waves still obey the laws of reflection and refraction. Incidentally, the bending of a light wave accounts for some of the visual phenomena we often encounter, such as mirages. A mirage is an optical illusion caused when light waves moving from the sky toward the ground are bent by the heated air.

In the 1860s, Scottish physicist James Clerk Maxwell put the cherry on top of the light-wave model when he formulated the theory of electromagnetism. Maxwell described light as a very special kind of wave -- one composed of electric and magnetic fields. The fields vibrate at right angles to the direction of movement of the wave, and at right angles to each other. Because light has both electric and magnetic fields, it's also referred to as electromagnetic radiation. Electromagnetic radiation doesn't need a medium to travel through, and, when it's traveling in a vacuum, moves at 186,000 miles per second (300,000 kilometers per second). Scientists refer to this as the speed of light, one of the most important numbers in physics.

Light as Rays (How Light Works)

Imagining light as a ray makes it easy to describe, with great accuracy, three well-known phenomena: reflection, refraction and scattering. Let's take a second to discuss each one.

In reflection, a light ray strikes a smooth surface, such as a mirror, and bounces off.  A reflected ray always comes off the surface of a material at an angle equal to the angle at which the incoming ray hit the surface. In physics, you'll hear this called the law of reflection. You've probably heard this law stated as "the angle of incidence equals the angle of reflection."

Of course, we live in an imperfect world and not all surfaces are smooth. When light strikes a rough surface, incoming light rays reflect at all sorts of angles because the surface is uneven. This scattering occurs in many of the objects we encounter every day. The surface of paper is a good example. You can see just how rough it is if you peer at it under a microscope. When light hits paper, the waves are reflected in all directions. This is what makes paper so incredibly useful -- you can read the words on a printed page regardless of the angle at which your eyes view the surface.

                                                 Refraction occurs when a ray of light passes from one transparent medium (air, let's say) to a second transparent medium (water). When this happens, light changes speed and the light ray bends, either toward or away from what we call the normal line, an imaginary straight line that runs perpendicular to the surface of the object. The amount of bending, or angle of refraction, of the light wave depends on how much the material slows down the light.Diamonds wouldn't be so glittery if they didn't slow down incoming light much more than, say, water does. Diamonds have a higher index of refraction than water, which is to say that those sparkly, costly light traps slow down light to a greater degree.

Lenses, like those in a telescope or in a pair of glasses, take advantage of refraction. A lens is a piece of glass or other transparent substance with curved sides for concentrating or dispersing light rays. Lenses serve to refract light at each boundary. As a ray of light enters the transparent material, it is refracted. As the same ray exits, it's refracted again. The net effect of the refraction at these two boundaries is that the light ray has changed directions. We take advantage of this effect to correct a person's vision or enhance it by making distant objects appear closer or small objects appear bigger.

Unfortunately, a ray theory can't explain all of the behaviors exhibited by light. 

What is Ionizing Radiation?

Introduction - Waves and Particles

Energy emitted from a source is generally referred to as radiation. Examples include heat or light from the sun, microwaves from an oven, X rays from an X-ray tube, and gamma rays from radioactive elements

Ionizing Radiation

Ionizing radiation is radiation with enough energy so that during an interaction with an atom, it can remove tightly bound electrons from the orbit of an atom, causing the atom to become charged or ionized.

Here we are concerned with only one type of radiation, ionizing radiation, which occurs in two forms - waves or particles.

Forms of electromagnetic radiation. These differ only in frequency and wave length.

• Heat waves
• Radiowaves
• Infrared light
• Visible light
• Ultraviolet light
• X rays
• Gamma rays

Longer wave length, lower frequency waves (heat and radio) have less energy than shorter wave length, higher frequency waves (X and gamma rays). Not all electromagnetic (EM) radiation is ionizing. Only the high frequency portion of the electromagnetic spectrum which includes X rays and gamma rays is ionizing.

Waves

Most of the more familiar types of electromagnetic radiation (e.g. visible light, radio waves) exhibit “wave-like” behavior in their interaction with matter (e.g. diffraction patterns, transmission and detection of radio signals). The best way to think of electromagnetic radiation is a wave packet called a photon. Photons are chargeless bundles of energy that travel in a vacuum at the velocity of light, which is 300 000 km/sec.

Particulate

Specific forms of ionizing radiation:
Particulate radiation, consisting of atomic or subatomic particles (electrons, protons, etc.) which carry energy in the form of kinetic energy or mass in motion.

Electromagnetic radiation, in which energy is carried by oscillating electrical and magnetic fields traveling through space at the speed of light.

Alpha particles and beta particles are considered directly ionizing because they carry a charge and can, therefore, interact directly with atomic electrons through coulombic forces (i.e. like charges repel each other; opposite charges attract each other).

The neutron is an indirectly ionizing particle. It is indirectly ionizing because it does not carry an electrical charge. Ionization is caused by charged particles, which are produced during collisions with atomic nuclei.

The third type of ionizing radiation includes gamma and X rays, which are electromagnetic, indirectly ionizing radiation. These are indirectly ionizing because they are electrically neutral (as are all electromagnetic radiations) and do not interact with atomic electrons through coulombic forces.

Isotopes and Activity

Isotopes

Atoms in their normal state are electrically neutral because the total negative charge of electrons outside the nucleus equals the total positive charge of the nucleus .

Atoms with the same number of protons and different number of neutrons are called ISOTOPES. An isotope may be defined as one or two or more forms of the same element having the same atomic number (Z), differing mass numbers (A), and the same chemical properties.

These different forms of an element may be stable or unstable (radioactive). However, since they are forms of the same element, they possess identical chemical and biological properties.

The simplest atom is the hydrogen atom. It has one electron orbiting a nucleus on one proton. Any atom which has one proton in the nucleus is a hydrogen atom, like both of the ones shown here. Hydrogen-2 is called deuterium, hydrogen-3 is called tritium. However, while their chemical properties are identical their nuclear properties are quite different as only tritium is radioactive.

Activity

• The activity of a radioisotope is simply a measure of how many atoms undergo radioactive decay per a unit of time.
• The SI unit for measuring the rate of nuclear transformations is the becquerel (Bq). The becquerel is defined as 1 radioactive disintegration per second.
• The old unit for this is the curie (Ci), in honour of Pierre and Marie Curie who discovered radium and polonium. The curie is based on the activity of 1 gram of radium-226, i.e. 3.7 x 1010 radioactive disintegrations per second.

Dose and Source

Dose

• Only the amount of energy of any type of ionizing radiation that imparted to (or absorbed by) the human body can cause harm to health.
• To look at biological effects, we must know (estimate) how much energy is deposited per unit mass of the part (or whole) of our body with which the radiation is interacting.
• The international (SI) unit of measure for absorbed dose is the gray (Gy), which is defined as 1 joule of energy deposited in 1 kilogram of mass. The old unit of measure for this is the rad, which stands for "radiation absorbed dose." - 1 Gy = 100 rad.
• Equivalent dose – the biological effect depends not only on the amount of the absorbed dose but also on the intensity of ionisation in living cells caused by different type of radiations.
• Neutron, proton and alpha radiation can cause 5-20 times more harm then the same amount of the absorbed dose of beta or gamma radiation.
• The unit of equivalent dose is the sievert (Sv). The old unit of measure is the rem. - 1 Sv = 100 rem.

Sources of Radiation Exposure

• Radiation is permanently present throughout the environment, in the air, water, food, soil and in all living organisms.
• Large proportion of the average annual radiation dose received by people results from natural environmental sources.
• Each member of the world population is exposed, on average, to 2.4 mSv/yr of ionizing radiation from natural sources.
• In some areas (in different countries of the world) the natural radiation dose may be 5 to 10-times higher to large number of people.

Comet Hyakutake :The Great Comet of 1996

On January 30, 1996, Comet Hyakutake was discovered using 25×150 binoculars. The comet was designated Comet C/1996 B2 (Hyakutake). As subsequent observations of the new comet were obtained, the IAU Central Bureau was able to compute the comet’s orbital elements, and these computations indicated that the comet passed as close as 0.10 AU from the Earth on March 25, 1996! The comet became a bright unaided-eye object and remained so in March, April and May in 1996. The comet had exceeded expectations, becoming the brightest comet since Comet West in 1976. A long tail of up to 100 degrees was reported, and small fragments have been observed to break off the main nucleus. Comet Hyakutake was indeed the Great Comet of 1996.

Comet Hyakutake led to several scientific discoveries. Most surprising to cometary scientists was the first discovery of X-rays being emitted from a comet. They are believed to have been caused by ionized solar wind particles interacting with neutral atoms in the coma of the comet. The Ulysses spacecraft unexpectedly crossed the comet’s tail at a distance of more than 500 million km from the nucleus, showing that Comet Hyakutake had the longest tail known for a comet. Hyakutake is a long period comet. Its orbital period was 17,000 years, but the gravitational influences of the large planets this trip through the inner solar system have extended that to every 100,000 years.

Comet Hyakutake became visible to the naked eye in early March 1996. By mid-March, the comet was still fairly unremarkable, shining at 4th magnitude with a tail about 5 degrees long. As it neared its closest approach to Earth, it rapidly became brighter, and its tail grew in length. After its close approach to the Earth, the comet faded to about 2nd magnitude. It reached perihelion on May 1, 1996, brightening again and exhibiting a dust tail in addition to the gas tail seen as it passed the Earth. By this time, however, it was close to the Sun and was not seen as easily. It was observed passing perihelion by the SOHO Sun-observing satellite, which also recorded a large coronal mass ejection being formed at the same time. Its distance from the Sun at perihelion was 0.23 AU, well inside the orbit of Mercury.

Galaxy Clusters and Large-Scale Structure

Groups and clusters of galaxies

Galaxies are preferentially found in groups or larger agglomerations called clusters. The Local Group consists of our own galaxy, the larger spiral galaxy Andromeda (M31) and several smaller satellites, including the Large and Small Magellenic Clouds.

Fornax is a small cluster of spiral and elliptical galaxies near our Local group.

Regular clusters have a concentrated central core and a well-defined spherical structure. These are subdivided according to their richness, that is, the number of galaxies within 1.5 Mpc of the centre (known as the Abell radius). Typically, they have a size in the range 1-10Mpc and a mass M ~ 10^15 solar masses (one followed by 15 zeros, that is, a million billion suns). The Coma cluster shown here is a very rich cluster with thousands of ellipticals inside the Abell radius.

The central region of the Coma cluster populated with large elliptical galaxies. This is one of the densest known regions on this scale in the universe.

Irregular clusters have no well-defined centre, a similar range of sizes, but they are generally poorer with a mass 10^12 - 10^14 solar masses (i.e. a thousand to a hundred thousand million suns). An example is the nearby Virgo cluster.

Virgo, an irregular cluster, is the nearest large cluster of galaxies.

Large-scale structures

Superclusters: These usually consist of chains of about a dozen clusters which have a mass of about 10^16 solar masses (ten million billion suns). Our own Local Supercluster is centred on Virgo and is relatively poor having a size of 15Mpc. The largest superclusters, like that associated with Coma, are up to 100Mpc in extent. The system of superclusters forms a network permeating throughout space, on which about 90% of galaxies are located.

The Great Attractor: Measurements of peculiar velocities---deviations away from the Hubble flow - are achieved by comparing redshifts and galactic distance indicators. These have revealed enormous coherent motions on scales in excess of 60Mpc. Consistent with these flows, our own galaxy is moving at about 600km/s towards a distant object dubbed the `Great Attractor'. This lies at a distance of 45Mpc and has a mass approaching 5x10^16 solar masses.

Voids, sheets & filaments: Deep redshift surveys reveal a very bubbly structure to the universe with galaxies primarily confined to sheets and filaments. Voids are the dominant feature and have a typical diameter of about 25Mpc. They fill about 90\% of space and the largest observed, Bootes void, has a diameter of about 124Mpc. Other features that have been observed are the `Great Wall', an apparent sheet of galaxies 100Mpc long at a distance of about 100Mpc.

The CfA survey showing large scale structures out to a distance of 150 Mpc, that is, about 2% of the distance to the edge of the observed universe. Galaxy positions are plotted as white points and large filamentary and sheet-like structures are evident, as well as bubble-like voids (CfA).

Deep field surveys

A particularly exciting recent development in the study of large-scale structure has been the advent of very deep galaxy surveys, notably those currently being made by the Hubble Space Telescope. These images (below) show galaxies just a couple of billions years after the Big Bang. One of the remarkable puzzles presented by this work is that galaxies appear to form earlier than predicted in most theoretical models.

Star clusters

Star cluster -Star cluster: A bunch of stars (ranging in number from a few to hundreds of thousands) which are bound to each other by their mutual gravitational attraction.

When stars are born they develop from large clouds of molecular gas. This means that they form in groups or clusters, since molecular clouds are composed of hundreds of solar masses of material. After the remnant gas is heated and blow away, the stars collect together by gravity. During the exchange of energy between the stars, some stars reach escape velocity from the protocluster and become runaway stars. The rest become gravitationally bound, meaning they will exist as collection orbiting each other forever.

When a cluster is young, the brightest members are O, B and A stars. Young clusters in our Galaxy are called open clusters due to their loose appearance. They usually contain between 100 and 1,000 members. One example is the binary cluster below:

And the Jewel Box cluster:

Early in the formation of our Galaxy, very large, globular clusters formed from giant molecular clouds. Each contain over 10,000 members, appear very compact and have the oldest stars in the Universe. One example is M13 (the 13th object in the Messier catalog)shown below:

The Age of the Universe

 Universe - The Universe is commonly defined as the totality of existence, including planets, stars, galaxies, the contents of intergalactic space, and all matter and energy. Definitions and usage vary and similar terms include the cosmos, the world and nature.
The age of universe is 14-18 billion years.

Hubble Time -

The inverse of the Hubble constant H has the units of time because the Hubble law is

v = H d where v is the velocity of recession, H is the Hubble constant, and d is the distance. Thus, from this equation, we have that 1/H = d/v. but d/v is distance divided by velocity, which is time (e.g., if I travel 180 miles at 60 miles/hour, the time required is t = d/v = 180/60 = 3 hours).
Thus, the Hubble time T is just the inverse of the Hubble Constant:

T = 1 / H Taking a value of H = 20 km/s/Mly (where Mly means mega-light years),

where all the factors are necessary to convert the time units to years and I've rounded some numbers to simplify the display.
The physical interpretation of the Hubble time is that it gives the time for the Universe to run backwards to the Big Bang if the expansion rate (the Hubble "constant") were constant. Thus, it is a measure of the age of the Universe. The Hubble "constant" actually isn't constant, so the Hubble time is really only a rough estimate of the age of the Universe.


Reasonable assumptions for the value of the Hubble constant and the geometry of the Universe typically yield ages of 10-20 billion years for the age of the Universe. For example, H near 50 km/s/Mpc gives a larger value for the age of the Universe (around 16 thousand million years), while a larger value of 80 km/s/Mpc gives a lower value for the age (around 10 thousand million years). Therefore, we shall take this information, and additional information from other methods to estimate the age of the Universe that we have not discussed, to indicate an age of approximately 15 billion years for the Universe.

The Fate of the Universe

The Universe is currently expanding. One extremely important cosmological question is whether this expansion will continue forever. As we shall see later, this is a question that does not yet have a definitive answer. Ultimately, this will turn out to be a question of how much mass is contained in the Universe. If it is below a critical amount, the Universe will expand forever. If it is above the critical amount, the expansion will eventually reverse and the Universe will collapse on itself, leading to what has been termed the big crunch. If it is exactly equal to the critical amount, the expansion will slow, but will only stop after an infinite amount of time. Thus, in this case the Universe will expand forever too.

Does vacuum have friction?

A BALL spinning in a vacuum should never slow down, since no outside forces are acting on it. At least that's what Newton would have said. But what if the vacuum itself creates a type of friction that puts the brakes on spinning objects? The effect, which might soon be detectable, could act on interstellar dust grains.

In quantum mechanics, the uncertainty principle says we can never be sure that an apparent vacuum is truly empty. Instead, space is fizzing with photons that are constantly popping into and out of existence before they can be measured directly. Even though they appear only fleetingly, these "virtual" photons exert the same electromagnetic forces on the objects they encounter as normal photons do.

Now, Alejandro Manjavacas and F. Javier García de Abajo of the Institute of Optics at the Spanish National Research Council in Madrid say these forces should slow down spinning objects. Just as a head-on collision packs a bigger punch than a tap between two cars one behind the other, a virtual photon hitting an object in the direction opposite to its spin collides with greater force than if it hits in the same direction.

So over time, a spinning object will gradually slow down, even if equal numbers of virtual photons bombard it from all sides. The rotational energy it loses is then emitted as real, detectable photons.The strength of the effect depends on the object's make-up and size. Objects whose electronic properties prevent them from easily absorbing electromagnetic waves, such as gold, may decelerate little or not at all. But small, low-density particles, which have less rotational momentum, slow down dramatically.The rate of deceleration also depends on temperature, since the hotter it is the more virtual photons pop in and out of existence, producing the friction. At room temperature, a 100-nanometre-wide grain of graphite, the kind that is abundant in interstellar dust, would take about 10 years to slow to about one-third of its initial speed. At 700 °C, an average temperature for hot areas of the universe, that same speed decrease would take only 90 days. In the cold of interstellar space, it would take 2.7 million years.

Could this effect be tested in the lab? Manjavacas says the experiment would require an ultra-high vacuum and high-precision lasers to trap the nanoparticles, conditions that are "demanding but reachable in the foreseeable future".

John Pendry of Imperial College in London calls the analysis a "fine piece of work" and says it could provide insights into whether quantum information is ever destroyed, for example, when it falls into a black hole. He says the real photons emitted during the deceleration process should contain information about the quantum state of the spinning particle, much as the photons thought to escape from black holes as Hawking radiation are thought to encode information about the holes.

"This is one of the few elementary processes that converts what appears to be purely classical mechanical energy into a highly correlated quantum state," Pendry says.

What is a vacuum? Is it matter?

A vacuum, to us, is a space with no matter in it. As a practical matter though, it's really a space with very little matter in it. You might already know that it's REALLY hard to get all the matter out of any space. Believe it or not, vacuums are very important and are becoming more useful every day. There is actually a whole branch of science dedicated to creating and studying vacuums.

Many modern devices (like the integrated circuit chips that make everything from cars to computers work), have to be fabricated in a vacuum. Jefferson Lab uses vacuums for thermal insulation. A lot of our equipment will only work at extremely cold temperatures. We operate at 2 degrees above the lowest possible temperature in the universe - you bet we're paying attention to insulation! If you could insulate your home with the same insulating vacuum that we use for our accelerator then you wouldn't need a furnace at all!

Even outer space, which is considered a vacuum and has less matter in it than anything mankind can reproduce, still has some atoms bouncing around.

Is vacuum matter? What are ten things that are not matter?
Vacuum, by definition, is the absence of matter. Matter, of course, is something that has mass and occupies space.

What is a Star?

A star is a huge ball of burning plasma that is held together by gravity.
The key to a star's existence is a phenomenon known as hydrostatic equilibrium. The inward gravitational pressure created by the star's mass is balanced by the outward radiation pressure created by the nuclear fusion taking place in the core.

The mechanism driving the outward radiation pressure in a star's core is the nuclear fusion process where hydrogen is fused into helium via the proton-proton chain. This reaction is exothermic, that is it produces more energy than it takes to initiate the reaction.
fusion is a natural process, but it is a difficult one to achieve. It takes a tremendous amount of energy in order to initiate enough fusion reactions to actually balance the force of gravity in a star.
Specifically, a star's core needs to reach temperatures in excess of about 10 million kelvin to energize the hydrogen enough to fuse. Our Sun, for instance has a core temperature around 15 million kelvin.
Therefore a star isn't said to have actually formed until the core temperature reaches this level and fusion begins. Prior to this the object is said to be a protostar.

Stellar Death

A star will continue existing on this primary part of its life, known as the main sequence, until it has used virtually all of the hydrogen fuel in its core. At this point the core will contract because the outward radiation pressure is no longer sufficient to balance the gravitational force.

This process, though, causes the core temperature to rise allowing helium to fuse into carbon. At this point the star has expanded and become a red giant.

The next phase in the star's evolution is completely dependent on the mass of the star. If it is a low mass star, like our Sun, it will eventually blow off its outer layers, creating a planetary nebula with a white dwarf in the middle.

High mass stars, however, will explode in a supernova. The core of the original star is left behind as either a neutron star or a black hole.

Did Galileo Invent the Telescope?

No. This is one of those things that "everyone knows," yet is absolutely incorrect. I'll talk about the reason for this mistake in a moment. First, let's give credit to the person who really invented the telescope. Who invented the telescope?

I don't know. No one does, really. However, there is some good evidence and many people believe that Leonard Digges invented both the reflecting and refracting telescopes. He was a well known mathematician and surveyor as well as a great populariser of science. His son, the famous English astronomer, Thomas Digges, posthumously published one of his father's manuscripts, "Pantometria," and wrote of the telescopes used by his father. Political problems may have prevented Leonard from capitalizing on his invention.

In 1608, Dutch eyeglass maker, Hans Lippershey offered a new device to the government for military use. This new device made use of two glass lenses in a tube to magnify distant objects. He may not have invented the telescope (in fact, at least two other Dutch opticians were also working on the idea at the time), but Hans Lippershey has been credited with its invention. He, at least, applied for the patent for it first.

Now, why do people think of Galileo Galilei as the inventor of the telescope?
 
As soon as he heard about the wonderous device coming out of the Netherlands, Galileo Galilei was fascinated. He began constructing telescopes, himself, before ever seeing one in person. By 1609, he was ready for the next inevitable step. He began using telescopes to observe the heavens, becoming the first astronomer to do so.

While Galileo Galilei did not invent the telescope, he made great improvements in the technology. His first construction was a three power instrument, which he quickly improved to eight, twenty and then thirty power. With this new tool, he found mountains and craters on the moon, discovered that the Milky Way was composed of stars, and discovered the four largest moons of Jupiter.

What Happens To The Human Body In A Vacuum?

In the 1981 movie "Outland", starring Sean Connery, there is a scene where a construction worker in space gets a hole in his suit. As the air leaks out, the internal pressure drops and his body is exposed to a vacuum, we watch in horror through his faceplate as he swells, and explodes.

A somewhat similar scene is in the 1990 Arnold Schwarzenegger movie, "Total Recall." In that movie, Schwarzenegger leaves the pressure of the habitat of a Mars colony and begins to blow up like a balloon in the much lower pressure of the Mars atmosphere, not quite a vacuum. He is saved by the creation of an entirely new atmosphere by an ancient alien machine.

what happens to the human body in a vacuum?

No, the body won't blow up. Your blood won't boil, either. 

There are a number of things about being in space, in a vacuum, which could cause harm to the human body. You wouldn't want to hold your breath. This would cause lung damage. You would probably remain conscious for several seconds, until the blood without oxygen reaches your brain.

It would be pretty darn cold, but the human body doesn't lose heat that fast, so you'd have a little time before you froze to death. It's possible you could have some problems with your eardrums, including a rupture, but maybe not. It would be worse if you had a cold, and were stuffy headed, with no way for the pressure to equalize.

You could get a bad sunburn, and you might actually swell some, but not to Arnold Schwarzenegger, "Total Recall" proportions. The "bends" are also possible, just like a diver who surfaces too quickly.

While your own normal blood pressure will keep your blood from boiling, the saliva in your mouth could very well begin to do so. In 1965, while performing tests at the NASA facility now known as Johnson Space Center a subject was accidentally exposed to a near vacuum (less than 1 psi) when his space suit leaked while in a vacuum chamber. He did not pass out for about 14 seconds, by which time unoxygenated blood had reached his brain. Technicians began to repressurize the chamber within 15 seconds and he regained consciousness at around the equivalent of 15,000 feet of altitude. He later said that his last conscious memory was of the water on his tongue beginning to boil.

The human body is amazingly resilient. The worst problem would be lack of oxygen, not lack of pressure in the vacuum. If returned to a normal atmosphere fairly quickly, you would survive with few if any irreversible injuries.

There have actually been cases of parts of astronauts bodies being exposed to vacuum, when suits were damaged. The results were negligible.

How Long Does a Star Live?

The length of a star's life depends on how fast it uses up its nuclear fuel. Our sun, in many ways an average sort of star, has been around for nearly five billion years and has enough fuel to keep going for another five billion years. Almost all stars shine as a result of the nuclear fusion of hydrogen into helium. This takes place within their hot, dense cores where temperatures are as high as 20 million degrees. The rate of energy generation for a star is very sensitive to both temperature and the gravitational compression from its outer layers. These parameters are higher for heavier stars, and the rate of energy generation--and in turn the observed luminosity--goes roughly as the cube of the stellar mass. Heavier stars thus burn their fuel much faster than less massive ones do and are disproportionately brighter. Some will exhaust their available hydrogen within a few million years. On the other hand, the least massive stars that we know are so parsimonious in their fuel consumption that they can live to ages older than that of the universe itself--about 15 billion years. But because they have such low energy output, they are very faint.

When we look up at the stars at night, almost all of the ones we can see are intrinsically more massive and brighter than our sun. Most longer-lasting stars that are fainter than the sun are just too dim to view without telescopic aid. At the end of a star¿s life, when the supply of available hydrogen is nearly exhausted, it swells up and brightens. Many stars that are visible to the naked eye are in this stage of their life cycles because this bias brings them preferentially to our attention. They are, on average, a few hundred million years old and slowly coming to the end of their lives. A massive star such as the red Betelgeuse in Orion, in contrast, approaches its demise much more quickly. It has been spending its fuel so extravagantly that it cannot be older than about 10 million years. Within a million years, it is expected to go into complete collapse before probably exploding as a supernova.

Stars are still being born at the present time from dense clouds of dust and gas, but they remain deeply embedded in their placental material and cannot be seen in visible light. The enveloping dust is transparent to infrared radiation, however, so scientists using modern detecting devices can easily locate and study them. In so doing, we hope to learn how planetary systems like our own come together.