What are phages and how do they kill bacteria?

Bacteriophages (phages) are viruses that infect bacteria. Typical phages have hollow heads (where the phage DNA or RNA is stored) and tunnel tails, the tips of which have the ability to bind to specific molecules on the surface of their target bacteria. The phage DNA is then injected through the phage tail into the host cell, where it directs the production of progeny phages [See Graphic], often >100 in 30 minutes. These young phages burt from the host cell (killing it) and infect more bacteria. Click here to see a simulation of the above-described process (Requires RealPlayer).
Phages are very specific, They can only infect their targeted bacteria, and they have no effect on any human, other animal, plant, insect, etc. cells.

How common are bacteriophages in nature?

Bacteriophages are the most common and ubiquitous organisms on Earth. Their total number is estimated to be approximately 10^32.
Some additional facts or examples are

• More than 100 million phage species exist
• 1 milliliter of non-polluted water may contain 100,000,000 phages (one milliliter equals one cubic centimeter, or about two drops from an eyedropper.)
• Poultry products, fruits and vegetables, and cheese sold at retail often contain more than 100, 000, 000 phages per gram of food.
• Phages have been found in commercial area.
• Phages have been found in human vaccines.
• Phages are common in the human mouth, where they are harbored in dental plaque and saliva.
• Phages are found, in prodigious numbers, in the gastrointestinal tracts of humans and other animals.

Have bacteriophages been found or on foods?

Yes bacteriophages have been isolated from drinking water and from a wide range of food products including ground beef, pork sausage, chicken, farmed freshwater fish, common carp and marine fish, oil sardines, raw skim milk, and cheese. In one published study, bacteriophages were recovered from 100% of examined fresh chicken and pork sausage samples and from 33% of delicatessen meat samples. In another study, phages were found in 48 to 100% of the samples of fresh chicken breasts, fresh ground beef, fresh pork sausage, canned corned beef, and frozen mixed vegetables. Several other studies have suggested that 100% of the ground beef and chicken meat sold at retail contain various levels of various bacteriophages. Phages also have been found in animal feed. Humans consume phages daily by drinking water and by eating unprocessed foods.

Do organically-grown foods contain phages?

Yes. Organically-grown foods are likely to contain more phages than do non-organic foods treated with various chemicals, because many chemicals used to improve the safety of foods, or to preserve and increase the shelf-life of foods, kill phages as well as potentially pathogenic bacteria and potentially beneficial probiotic bacteria.

Summer Hair Care Tips

Summer is here and with it come all sorts of fun outdoor things to do. It is also that time of year when we encounter the highest percentage of damaged hair, some of it beyond repair.

Here are a few tips that may save you a lot of heartache and money.

1.  Stay away from hair lightening products

Most contain some form of peroxide or metallic crystals and can cause severe damage to your hair, not to mention such undesirable effects such as orange, green, or pink hair.  Worse, if the product does contain metallic crystals and you then perm your hair, it may literally turn your hair into mush and wash down the drain.

Although a chelator, (an ingredient in some shampoos), can help remove some of the metals residing on the hair shaft, they are not strong enough to remove metal deposits that have penetrated the shaft.  The only chemicals that we know of that will do this are chemicals used in the development of film.  These chemicals are highly toxic and we do not recommend that you mess with them.

Bottom line, just let nature do its own thing, or invest in a highlight.  In the long run the highlight will be far less expensive than a color correction.

2.  Use a hair conditioner that contains a sunscreen
 

If you can't find one and you are heading out to swim or tan, simply massage some of your sunscreen into your hair, it should wash out.

If you are not going to use a sunscreen because you want a deep tan quickly, try using safflower oil.  It provides your hair with much needed fatty acids, and makes a great tanning oil to boot.

3.  After a day at the pool, clarify your hair

A clarifier is a stronger shampoo designed to remove toxins from you hair such as auto emissions and other environmental pollutants, as well as build up from some ingredients in over the counter shampoos such as silicon.  A clarifier is too strong to use daily, but once or even twice a week is okay.  It should also help remove chlorine from your hair, but if you are at the pool daily you will need to invest in a special shampoo to control chlorine.

Avoid 'green hair' before it begins.  Thoroughly wet your hair before you dip in the pool and add a bit of conditioner too.  Wash your hair immediately upon exiting the pool, BEFORE YOUR HAIR HAS A CHANCE TO DRY.  These precautions alone will go a long way toward dodging the 'swamp look.' 

Although not exactly a fashion statement, you can always wear a bathing cap too.

If you already have the 'Greenies' you can look around for a shampoo called 'Malibu 2000.' It may or may not work. You can also try soaking your hair in tomato juice, cap it and let it sit on the hair for about twenty minutes, then rinse, shampoo, rinse, and condition the hair.

These few tips should see you well through the summer and leave you with fond memories instead of bitter tales.

Eye Makeup Tips

If you don't like the sharpness of your eyeliner, you can soften it using your eye shadow brush by adding a little powder shadow of a similar shade.

Remedy For Puffy Lidded Eyes or Eyes With No Defined Crease

1. Apply a touch of black eyeshadow in the center of the upper lashline.

2. Sweep a taupe or natural color eyeshadow over the crease and slightly above it. .....(Remember, we're working with minimal space here...not Cher's lids! ).....

3. Use an eyeshadow shade that's lighter on the browbones and lids.

That way, when the light hits your peepers, the highlighted area of the eyes will stand out as the crease appears, well, creased! The upper lids lined with black shadow in the center provides a contrast with the highlighted lid to create more visible lids.

Eyeliner Too Harsh? - An easy remedy...
If you don't like the sharpness of your eyeliner, you can soften it using your eye shadow brush by adding a little powder shadow of a similar shade.

Using An Eyeshadow Pencil - Did you know the secret?
You should not draw the pencil across your eyelid as this could cause stretching of the skin.

Instead, apply the color to the palm of your hand, then pick up some of the color with your pinky and apply gently to your eyelid.

Younger Looking Eyes - Oh, the tricks...
The first step is to reduce fine lines. So apply a little eye cream on the brow bone and under the eye.

Next dab on a concealer that matches your skin tones.

The last step is to line the eye with a white pencil just beneath the lash line. This will give the impression of larger and brighter eyes. 

The Perfect Eyeshadow For Your Eye Color - Secrets revealed

FOR BLUE EYES
1. Tried and True: taupe, gray, violet, purple, deep blue (a darker shade than your eye color makes your eyes really blue), black (mix it with bright blue for a smoky effect)

2. Funky Favorites: silver, turquoise, fuschia (brightens any shade of blue)

GREEN or HAZEL EYES
1. Tried and True: brown, apricot, purple, plum, deep khaki or forest green (because they are in the same greenish family, they brighten green eyes)

2. Funky Favorites: gold, lime-green, really light green, bright purple (super modern)

BROWN EYES
1. Tried and True: copper, bronze, champagne (soft pink with a touch of apricot), brown (for a doe-eyed look), beige, and khaki-green (lighter shades add highlight)

2. Funky Favorites: tangerine, royal blue, hot pink, lime-green (the contrast adds punch to brown)

ALL EYES
1. Tried and True (Classic): navy or charcoal base to define and a powder-blue shadow for highlighting (it brightens your brow bone so any eye color pops)

2. Funky Favorites: silver-sparkle shadow makes all eyes look edgy

How to Pluck Your Eyebrows

Tweezing your eyebrows is the most dramatic way to change your face without makeup or surgery. It can make your eyes look larger and give your face a clean, polished look.

Steps: 

1.   Sit near a window to get the best light. 
 
2.   Wash the area thoroughly so it's not oily. 
 
3.   Decide what shape you want for your eyebrows.
Styles change: It may help to flip through fashion magazines for ideas. 
 
4.   Draw in a brow line on your eyebrow with a brow pencil to serve as
a guide. Follow the brow's natural line by conforming to the curve of your upper eyelid. 
 
5.   Pull the skin at the outer end of the eyebrow taut against the brow bone, and use the brow bone as an additional guide. 
 
6.   Use a pair of angled eyebrow tweezers to pluck the hairs below the brow; never shape your brow by plucking above it. Pluck only one hair at a time. 
 
7.   Start plucking in the middle of the eyebrow and pluck toward the outer end; then go back to the middle and pluck toward the nose.

Your brows should extend a little beyond each corner of your eye. 
 
8.   Use a cotton ball or pad soaked in pure tea tree oil or witch hazel to soothe your plucked brows.  
  
Tips

Consider having your brows waxed once professionally to get exactly the shape you want. You can then pluck the strays as they grow in.  
 
Habitual plucking may make some hairs stop growing permanently, so pluck with caution.  
 
There are tweezers on the market that are specially designed for plucking eyebrows. They cost a bit more but make the experience less painful.  
  
Eyebrow Tweezing

Always grasp the hair as close to the root as possible. The further away from the root you grasp, the more it hurts. Hold the skin down and pull slowly in the opposite direction to the tweezers. This also reduces pain, as well as the risk of ingrown hairs. If you yank the tip of the hair with the tweezers without supporting the skin, you risk snapping the hair just below the skins surface. This is one of the causes of ingrown hair. Exfoliate the skin regularly with a gentle exfoliant, it helps prevent ingrown hairs also. Be sure both skin and hair are dried before tweezing (skin is more sensitive when it is wet). A heat pad is ideal to prepare
the skin. Failing that, use a hot flannel and dry off before tweezing.  
 
The beauty of Threading

Threading is much easier than waxing. When you wax, you run the risk of pulling out wanted hairs. Tweezing is painful, as you're pulling out each hair one by one. The best thing to do is thread. Go to a salon - - threading is not for amateurs. No product is being used on your skin (such as wax). You can pluck the hairs as they grow in. I advise you to put an ointment on after threading, as some people experience redness. Be aware of the 'green shadow' that may come from waxing/threading/tweezing. When the hair grows back in its place, it leaves a green shadow. I find threading to be the best way to remove it!

So it's OK if your hair grows in for a week or two - - you can remove the shadow all at once!

Acne Do's and Don'ts

Whether you have severe acne or just a few pimples, this section helps you prevent blemishes and tells you how to keep your skin clear and beautiful.

It lists some great home remedies and also products that work wonders to clear up your skin.

Acne Don'ts

A very important no-no is going to bed without washing your face. Your skin heals most while you are sleeping and if you have got leftover makeup and dirt from that day sitting on your face, you will wake up with some nasty zits on that face. Do not use soap on your face for this can irritate your skin. Use a facial cleanser such as Clean and Clear's Cream Cleanser or Biore's Facial Cleanser. I will list more of my favorite products later. Also, never sleep with your face directly on the pillow. The oils and dirt from your hair get on the pillow and then resting your face on the pillow may make you breakout the next morning. Do not pick the zits on your face or try to squeeze them. This usually causes you to push the zit into your skin, making healing time even slower.

Acne Do's

Definitely use a benzoyl peroxide or salicyic acid cleanser to fight pimples. A very good product for washing your face is Proactive Solution. It gets rid of that acne within a week, in most cases!

Getting Rid Of Existing Acne

Here are some great tips and home remedies on getting rid of acne you already have.

• Cleanse the skin with a benzoyl peroxide or salicylic acid cleanser, apply an alphy hydroxy, and then use a toner. Then use an acne fighting medication to the skin along with a moisturizer.

• To dry out a pimple overnight, try using toothpaste (not gel) directly on the zit. This should help dry it out overnight.

• A great face mask is to use the white part of an egg and apply it to your face for about 15 minutes. The vitamin A in the whites is also good for your skin.

• The egg white is also a great overnight remedy, but just use a Q-tip to dab on the egg-white directly to the pimple.

• To rid redness from an inflamed zit, use visine eyedrops and apply to the red spots.

• Another way to get rid of redness: try hydrocortisone cream or pop two Advil.

Since it`s not possible to think about everything all at once, most experienced writers handle a piece of writing in stages. Roughly speaking, those stages are planning, drafting, and revising. You should generally move from planning to drafting to revising, but be prepared to circle back to earlier stages whenever  the need arises.

C1
--------------------------------------------------------------------------------------------------------
Planning
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c1-a Assess the writing situation.

Begin by taking a look at the writing situation in which you find ourself. The key elements of the writing situation include your subject, the sources of information available to you, your purpose, your audience, and constraints such as length, document design, review sessions, and deadlines.
    It is unlikely that you will make final decisions about all of these matters until later in the writing process - after a first draft, for example. Nevertheless, you can save yourself time by thinking about as many of them as possible in advance. For quick check list, see page 4.

What counts as good writing varies from culture to culture and even among groups within cultures. In some situations, you will need to become familiar with the writing styles - such as direct or indirect, personal or impersonal, plain or embellished - that are valued by the culture or discourse for which you are writing.

C1-b Experiment with techniques for exploring ideas.

Instead of just plunging into a first draft, experiment with one or more techniques for exploring your subject - perhaps listing, clustering, asking questions, freewriting, annotating texts, or simply talking and listening. Whatever technique you turn to, the goal is the same which is to generate a wealth of ideas. At this early stage of the writing process, you should aim for quantity, not necessarily quality, of ideas. If an idea proves to be off the point, trivial, or too far-fetched, you can always throw it out later.

Solar Energy
For most people, what matters most about the Sun is the energy that it radiates into space. Without the Sun`s warming rays, our atmosphere and oceans would freeze into an icy layer coating a desperately cold planet, and life on Earth would be impossible. To understand why we are here, we must understand the nature of the Sun.

The Sun is the only object in the solar system that emits substantial amounts of visible light. The light that we see from the Moon and planets is actually sunlight that struck those worlds and was reflected toward Earth.

 Luminosity - is the amount of energy emitted

Astronomers have collected a wealth of data about the stars, but merely having tables of numerical data is not enough. Like all scientists, astronomers want to analyze their data to look for trends and underlying principles. One of the best ways to look for trends in any set of data, whether it comes from astronomy, finance, medicine, or meteorology, is to create a graph showing how one quantity depends on another. For example, investors consult graphs of stock market values versus dates, and weather forecasters make graphs of temperature versus altitude to determine whether thunderstorms will form. Astronomers have found that a particular graph of stellar properties shows that stars fall naturally into just a few categories. This graph, one of the most important in all astronomy, will in later chapters help us understand how stars form, evolve, and eventually die.

H-R Diagrams
which properties of stars should we include in a graph? Most stars have about the same chemical composition, but two properties of stars -their luminosities and surface temperatures - differ substantially from one star to another. Stars also come in a wide range of radii, but a star`s radius is a secondary property that can be found from the luminosity and surface temperature. We also relegate the positions and space velocities of stars to secondary importance. (In a similar way, a physician is more interested in our weight and blood pressure than in where you live or how fast you drive.) We can then ask the following question - What do we lean when we graph the luminosities of stars versus their surface temperatures?
    The first answer to this question was given in 1911 by the Danish astronomer Ejnar Hertzsprung. He pointed out that a regular pattern appears when the absolute magnitudes of stars (which measure their luminosities) are plotted against their colors (which measure their surface temperatures). Two years later, the American astronomer Henry Norris Russell independently discovered a similar regularity in a graph using spectral types (another measure of surface temperature) instead of colors. In recognition of their originators, graphs of this kind are today know as Hertzsprung-Russel diagrams, or H-R diagrams (Figure 17-15).

    Figure 17-15a is a typical Hertzsprung-Russel diagram. Each dot represents a star whose spectral type and luminosity have been determined. The most luminous stars are near the top of the diagram, the least luminous stars near the bottom. Hot stars of spectral classes O and B are toward the left side of the graph and cool stars of spectral class M are toward the right.

    CAUTION! You are probably accustomed to graphs in which the numbers on the horizontal axis increase as you move to the right. (For example, the business section of a newspaper includes a graph of stock market values versus dates, with later dates to the right of earlier ones.) But on an H-R diagram the temperature scale on the horizontal axis increases toward the left. This practice stems from the original diagrams of Hertzsprung and Russel, who placed hot O stars on the left and cool M stars on the right. This arrangement is a tradition that no one has seriously tried to change.


The Birth of Stars

The stars that illuminate our nights seem eternal and unchanging. But this permanence is an illusion. Each of the stars visible to the naked eye shines due to thermonuclear reactions and has only a finite amount of fuel available for these reactions. Hence, stars cannot last forever. They form from material in interstellar space, evolve over millions or billions of years, and eventually die. In this chapter our concern is with how stars are born and become part of the main sequence.
Stars form within cold, dark clouds of gas and dust that are scattered abundantly throughout our Galaxy. One such cloud appears as a dark area on the far right-hand side of the photograph at the top of this page. Perhaps a dark cloud like this encounters nearby. From the shock of events like these, the cloud begins to contract under the pull of gravity, forming protostars - the fragments that will one day become stars. As a protostar develops, its internal pressure builds and its temperature rises. In time, hydrogen fusion begins, and a star is born. The hottest, bluest, and brightest young stars, like those in the surrounding interstellar gas. The result is a beautiful glowing nebula, which typically has the red color characteristic of excited hydrogen (as shown in the photograph).
In Chapters 19 and 20 we will see how stars mature and grow old. Some even blow themselves apart in death throes that enrich interstellar space with the material for future generations of stars. Thus, like the mythical phoenix, new stars arise from the ashes of the old.

To understand where we see objects on the sky you will need to understand what is meant by the following terms when used(this is especially true for those of you taking the lab - Ast10 lab)

Celestial sphere - the dome of the sky
Celestial Equator and Celestial Poles - projection of the Earth`s equator and poles on the sky
Constellations - the 88 regions into which the sky is segmented
Ecliptic - Path of the sun on the sky
Signs of the Zodiac - Constellations that Lie along the Ecliptic
Local Meridian - line on the celestial sphere drawn from the north point on the horizon through the zenith to the south point on the horizon.
Zenith - point directly overhead on the sky
Vernal Equinox & Autumnal Equinox - points on the sky where the sun on the ecliptic crosses the Celestial Equator going from south to north (V.E.) and north to south (A.E.)

2. Angular Measure - degrees, minutes of arc, seconds of arc

There are 360 degrees in a circle
There are 60 arcminutes (60`) in a degree and 60 arcseconds (60``) in an arcminute
There are 360 degrees all the way around the horizon.

Q - Why do we use angular measure rather than miles or yards to indicate the separation of objects on sky? HINT - Do you need to know the distance to the objects?
Q - How big an angle does the full Moon subtend on the sky? Go ahead and use the width of your finger to measure it! Your finger at arm`s length subtends about 1 degree on the sky.
Q - How many degrees from the south point on your horizon to your zenith? From the South point on your horizon through your zenith to the North point on your horizon?

The Sun's magnetic field changes all the time. In fact, the Sun has a cycle that repeats itself every 11 years. During that time the structure of the magnetic field changes dramatically. At the beginning of the cycle, the lines of magnetic force run north and south between the Sun's magnetic poles. This is the period of minimum magnetic activity called the "Solar Minimum." However, this condition does not last.

As the Sun rotates, the Convection Zone spins faster at the equator than it does at the poles. Beneath the Convection Zone, the Radiation Zone spins as a sold mass. The different ways that these two zones move causes the Sun's magnetic field to stretch at the equator.     As the solar cycle continues, these lines of magnetic force continue to stretch. Like a rubberband that's twisted too much, the magnetic field begin to buckle. Eventually, the magnetic force, which is generated beneath the Convection Zone, breaks the surface of the Sun.

When this happens, all sorts of strange activity occurs: Sun spots form the Corona heats up solar flares and loops erupt from the Sun's surface These phenomena are like giant magnetic storms that not only alter the Sun's surface, but also eject powerful bursts of energy out into the Solar System. The peak of all this activity is called the "Solar Maximum." At these times, we on Earth can experience magnetic disturbances like disruptions in satellite communications and atmospheric events like the Aurora Borealis. Following the Solar Maximum, the magnetic field begins to unwind and activity on the Sun subsides. Gradually, the Sun returns to the Solar Minimum and the cycle begins again. The Sun reached a Solar Minimum around 2006. The next Solar Maximum should occur around 2012 to 2015.

WHAT IS TRACE UP TO

TRACE works together with the Solar and Heliospheric Observatory (SoHO), another satellite that is studying the Sun. TRACE and SoHO will coordinate their observations and provide scientists with detailed information about current conditions on the Sun. The TRACE mission is investigating several important solar phenomena including: Plasma Confinement Plasma Heating Solar Flares.
	Plasma Confinement Maybe you've heard the Sun referred to a burning ball of gas. To be more precise, the material that makes up the Sun is actually called plasma. Plasma is a state of matter that forms when tremendous amounts of energy cause the bonds that hold atoms together to break apart. Plasma acts in some ways just like a gas. But on the Sun, plasma also does some pretty strange things. This photograph shows a solar phenomenon called loops. Loops are gigantic arches of plasma that extent thousands of miles into space. They can last for weeks, or even months. Scientists believe that powerful disturbances in the Sun's magnetic field hold, or confine the plasma in these great looping formations. One of the objectives of the TRACE mission is to better understand the forces that might be responsible for these solar plasma formations.
	Plasma Heating Loops are not the only weird thing that happen on the Sun. One of the weirdest, and most confusing of all solar phenomena is the plasma heating that takes place in the Transition Region, just above the Sun's surface. Think of a candle, or a space heater. Our experience is that the farther you get away from a source of heat, the cooler it will get. Makes sense, right? Not on the Sun. This graph shows what happens to the temperature as we move from the solar surface out into the solar atmosphere (Corona). For some reason, the temperature increases dramatically throughout the Transition Region. The Sun's surface is a mere 5,000 degrees Kelvin. But 100,000 kilometers out into the Corona, the temperature jumps to nearly 2 million degrees Kelvin. Why? Again, scientists believe that the action of the Sun's magnetic field has something to do with it. TRACE will collect data to help explain this strange increase in temperature.
	Solar Flares A solar flare is a sudden eruption of energy on the Sun's surface. Flares are important. Even though they do not make any noticeable change in the brightness of the Sun, they can have an effect on our lives here on Earth. While they only last a couple of minutes, large flares on the Sun throw out sudden bursts of high energy radiation which can disrupt and even damage communications system on Earth. Once again, solar physicists believe that disruptions in the Sun's powerful magnetic field plays an important role in the creation of solar flares. Another objective of the TRACE mission is to search for clues to help explain the relationship between flares and the Sun's magnetic field.

What color is Sunlight?

Sunlight is composed of every color. When Sunlight passes through a prism, the light is separated into a rainbow of different colors.

Separation of Sunlight by a Glass Prism

Red Orange Yellow Green Blue Indigo Violet

HINT: Many people remember the colors of the rainbow by remembering the name ROY G. BIV ( RO Y G. BI V)

Sunlight your eyes CAN see (visible) red orange yellow green blue indigo and violet

Light can be regarded as a wave with the different colors representing different wavelengths. (This is why we sometimes talk about "light waves.") Our Sun emits almost every wavelength of light, even light our eyes can't see. Each different part of Sunlight tells scientists different information about our Sun.

Sunlight your eyes CANNOT see (invisible) radio microwaves infrared ultraviolet X-Rays Gamma Rays

http://solar.physics.montana.edu/YPOP/Spotlight/Tour/tour03.html

Kirchhboff`s Laws

1. A hot opaque body, such as a perfect blackbody, or a hot, dense gas produces a continuous spectrum - a complete rainbow of colors without any spectral lines.
2. A hot, transparent gas produces an emission line spectrum - a series of bright spectral lines against a dark background.
3. A cool, transparent gas in front of a source of a continuous spectrum produces an absorption line spectrum - a series of dark spectral lines among the colors of the continuous spectrum. Furthermore, the dark lines in the absorption spectrum of a particular gas occur at exactly the same wavelengths as the bright lines in the emission spectrum of that same gas.

NOTICE - Absorption lines are seen if the background is hotter than the gas, and emission lines are seen if the background is cooler.

Hot blackbody ------> cloud of cooler gas ------> prism ------> Absorption line spectrum
(atoms in gas cloud absorb light of certain specific wavelengths, producing dark lines in spectrum).

Cloud of cooler gas ------> prism ------> Emission line spectrum
(atoms in gas cloud reemit absorbed light energy at the same wavelengths at which they absorbed it).

Hot blackbody ------> prism ------> Continuous spectrum
(blackbody emits light at all wavelengths).

By identifying the spectral lines present in the solar spectrum, we can determine the chemical composition of the Sun`s atmosphere.

Red - hydrogen gas

Spectral lines are produced when an electron jumps from on energy level to another within an atom.

The Doppler Shift - The Doppler shift enables us to determine the radial velocity of a light source from the displacement of its spectral lines.

The spectral lines of an approaching light source are shifted toward short wavelengths (a blueshift), the spectral lines of a receding light source are shifted toward long wavelengths (a redshift).

The size of a wavelength shift is proportional to the radial velocity of the light source relative to the observer.

To learn about objects in the heavens, astronomers study the character of the electromagnetic radiation coming from those objects. Such studies can be very revealing because different kinds of electromagnetic radiation are typically produced in different ways. As an example, on Earth the most common way to generate radio waves is to make an electric current oscillate back and forth ( as is done in the broadcast antenna of a radio station). By contrast, X rays for medical and dental purposes are usually produced by bombarding atoms in a piece of metal with fast-moving particles extracted from within other atoms. Our own Sun emits radio waves from near its glowing surface and X rays from its corona (see the photo that opens Chapter 3). Hence, these observations indicate the presence of electric currents near the Sun`s surface and of fast-moving particles in the Sun`s outer-most regions. ( We will discuss the Sun at length in Chapter 18.)

Radiation from Heated Objects

The simplest and most common way to produce electromagnetic radiation, either on or off the Earth, is to heat an object. The host filament of wire inside an ordinary light bulb emits white lights, and a neon sigh has a characteristic red glow because neon gas within the tube is heated by an electric current. In like fashion, almost all the visible light that we receive from space comes from hot objects like the Sun and the stars. The kind and amount of light emitted by a hot object tell us not only how hot it is but also about other properties of the object.
    We can tell whether the hot object is made of relatively dense or relatively thin material. Consider the difference between a light bulb and a neon sign. The dense, solid filament of a light bulb makes white light, which is a mixture of all different visible wavelengths, while the thin, transparent neon gas produces light of a rather definite red color and, hence, a rather definite wavelength. For now we will concentrate our attention on the light produced by dense, opaque objects. (We will return to the light produced by gases in Section 5-6.) Even though the Sun and stars are gaseous, not solid, it turns out that they emit light with many of the same properties as light emitted by a hot, glowing, solid object.
    Imagine a welder or blacksmith heating a bar of iron. As the bar becomes hot, it begins to glow deep orange light  red, (you can see this same glow from the coils of a toaster or from an electric range turned on ~high.~) As the temperature rises further, the bar begins to give off a brighter orange light. At still higher temperatures, it shines with a brilliant yellow light. If the bar could be prevented from melting and vaporizing, at extremely high temperatures it would emit a dazzling blue-white light.
    As this example shows, the amount of energy emitted by the hot, dense object and the dominant wavelength of the emitted radiation both depend on the temperature of the object. The hotter the object, the more energy it emits and the shorter the wavelength at which most of the energy is emitted. Colder objects emit relatively energy, and this emission is primarily at long wavelengths.
    These observations explain why you can`t see in the dark. The temperatures of people, animals, and furniture are rather less than even that of the iron bar in Figure 5-9a. So, while these objects emit radiation even in a darkened room, most of this emission is at wavelengths greater than those of red light, in the infrared part of the spectrum (see Figure 5-7). Your eye is not sensitive to infrared, and you thus cannot see ordinary objects in a darkened room. But you can detect this radiation by using a camera that is sensitive to infrared light.
   To better understand the relationship between the temperature of a dense object and the radiation it emits, it is helpful to know just what ~temperature~ means. The temperature of a substance is directly related to the average speed of the tiny atoms - the building blocks that come in distinct forms for each distinct chemical element - that make up the substance. (Typical atoms are about 10^-10 m = 0.1 nm in diameter, or about 1 over 5000 as large as a typical wavelength of visible light.)
    If something is hot, its atoms are moving at high speeds, if it is cold, its atoms are moving slowly. Scientists usually prefer to use the Kelvin temperature scale, on which temperature is measured in kelvins (K) upward from absolute zero. This is the coldest possible (they can never quite stop completely). On the more familiar Celsius and Fahrenheit temperature scales, absolute zero (0 K) is -273 degree C and -460 degree F. Ordinary room temperature is 293 K, 20 degree C, or 68 degree F. Box 5-1 discusses the relationships among the Kelvin, Celsius, and Fahrenheit temperature scales.
    Figure 5-11 depicts quantitatively how the radiation from a dense object depends on its Kelvin temperature. Each curve in this figure shows the intensity of light emitted at each wavelength by a dense object at a give temperature - 300 K (the temperature at which molten gold boils), 6000K (the temperature of an iron-welding arc), and 12,000 K (a temperature found in special industrial furnaces). In other words, the curves show the spectrum of light emitted by such an object. At any temperature, a hot, dense object emits at all wavelengths, so its spectrum is a smooth continuous curve with no gaps in it.
    The shape of the spectrum depends on temperature, however. An object at relatively low temperature (say, 3000 K) has a low curve, indicating a low intensity of radiation. The wavelength of maximum emission, at which the curve has its peak and the emission of energy is strongest, is at a long wavelength. The higher the temperature, the higher the curve (indicating greater intensity) and the shorter the wavelength of maximum emission.
    Figure 5-11 shows that for a dense object at a temperature of 3000 K, the wavelength of maximum emission is around 1000 nm. Because this is an infrared wavelength well outside the visible range, you might think that you cannot see the radiation from an object at this temperature. In fact, the glow from such an object is visible, the curve shows that this object emits plenty of light within the visible range, as well as at even shorter wavelengths.
    The 3000-K curve is quite a bit higher at the red end of the visible spectrum than at the violet end, so a dense object at this temperature will appear red in color. Similarly, the 12,000-K part of the spectrum, at a wavelength shorter than visible light. But such a hot, dense object also emits copious amounts of visible light ( much more than at 6000 K or 3000 K, for which the curves are lower) and thus will have a very visible glow. The curve for this temperature is higher for blue light than for red light, and so the color of a dense object at 12,000 K is a brilliant blue or blue-white. These conclusions agree with the color changes of a heated rod shown in Figure 5-9. The same principles apply to stars - A star that looks blue, such a Bellatrix in the constellation Orion, has a high surface temperature, while a red star such as Betelgeuse has a relatively cool surface.
    These observations lead to a general rule

The higher an object`s temperature, the more intensely the object emits electromagnetic radiation and the shorter the wavelength at which it emits most strongly.

    We will make frequent use of this general rule to analyze the temperatures of celestial objects such as planets and stars.
    The curves in Figure 5-11 are drawn for an idealized type of dense object called a blackbody. A perfect blackbody does not reflect any light at all, instead, it absorbs all radiation falling on it. Because it reflects no electromagnetic radiation, the radiation that it does emit is entirely the result of its temperature. Ordinary objects, like tables, textbooks, and people, are not perfect blackbodies, they reflect light, which is why they are visible. A star such as the Sun, however, behaves very much like a perfect blackbody, because it absorbs almost completely any radiation falling on it from outside. The light emitted by a blackbody is called blackbody radiation, and the curves in Figure 5-11 are often called blackbody curves.
    Figure 5-12 shows the blackbody curve of a temperature of 5800 K. It also shows the intensity curve for light from the Sun, as measured from above the Earth`s atmosphere. (This is necessary because the Earth`s atmosphere absorbs certain wavelengths.) The peak of both curves is at a wavelength of about 500 nm, near the middle of the visible spectrum.Note how closely the observed intensity curve for the Sun matches the blackbody curve. This is a strong indication that the temperature of the Sun`s glowing surface is about 5800 K - a temperature that we can measure across a distance of 150 million kilometers! The close correlation between blackbody curves and the observed intensity curves for most stars is a key reason why astronomers are interested in the physics of blackbody radiation.
    Blackbody radiation depends only on the temperature of the object emitting the radiation, not on the chemical composition of the object. The light emitted by molten gold at 2000 K is very nearly the same as that emitted by molten lead at 2000 K. Therefore, it might seem that analyzing the light from the Sun or from a star can tell astronomers the object`s temperature but not what the star is made of. As Figure 5-12 shows, however, the intensity curve for the Sun (a typical star) is not precisely that of a blackbody. We will see later in this chapter that the differences between a star`s spectrum and that of a blackbody allow us to determine the chemical composition of the star.

Visible and Nonvisible Light

Maxwell`s equations place no restrictions on the wavelength of electromagnetic radiation. Hence, electromagnetic waves could and should exist with wavelengths both longer and shorter than the 400-700 nm range of visible light. Consequently, researchers began to look for invisible forms of light. These are forms of electromagnetic radiation to which the cells of the human retina do not respond.
    The first kind of invisible radiation to be discovered actually preceded Maxwell`s work by more than a half century. Around 1800 the British astronomer William Herschel passed sunlight through a prism and held a thermometer just beyond the red end of the visible spectrum. The thermometer registered a temperature increase, indicating that it was being exposed to an invisible form of energy. This invisible energy, now called infrared radiation, was later realized to be electromagnetic radiation with wavelengths somewhat longer than those of visible light.

Frequency and Wavelength

Astronomers who work with radio telescopes often prefer to speak of frequency rather than wavelength. The frequency of a wave is the number of wave crests that pass a given point in one second. Equivalently, it is the number of complete cycles of the wave that pass per second (a complete cycle is from one crest to the next). Frequency is usually denoted by the Greek letter v (nu). The unit of frequency is the cycle per second, also called the hertz (abbreviated Hz) in honor of Heinrich Hertz, the physicist who first produced radio waves. For example, if 500 crests of a wave pass you in one second, the frequency of the wave is 500 cycles per second or 500 Hz.
    In working with frequencies, it is often convenient to use the prefix mega- (meaning ~million,~ or 10^6, and abbreviated M) or kilo- (meaning ~thousand,~ or 10^3, and abbreviated k). For example, AM radio stations broadcast at frequencies between 535 and 1605 kHz (kilohertz), while FM radio stations broadcast at frequencies in the range from 88 to 108 MHz (megahertz).
    The relationship between the frequency and wavelength of an electromagnetic wave is a simple one. Because light moves at a constant speed c = 3 x 10^8 m per s, if the wavelength (distance from one crest to the next) is made shorter, the frequency must increase (more of those closely spaced crests pass you each second). Mathematically, the frequency v of light is related to its wavelength lamba by
 
Frequency and wavelength of an electromagnetic wave

v = c divided by lamba

where v = frequency of an electromagnetic wave (in Hz)
            c = speed of light = 3 x 10^8 m per s
  lamba = wavelength of the wave (in meters)

That is, the frequency of a wave equals the wave speed divided by the wavelength.
    For example, hydrogen atoms in space emit radio waves with a wavelength of 21.12 cm. To calculate the frequency of this radiation, we must first express the wavelength in meters rather than centimeters - lamba = 0.2112 m. Then we can use the above formula to find the frequency v -

v = c divided by lamba = (3 x 10^8 m per s) divided by 0.2112 m = 1.42 x 10^9 Hz = 1420 MHz

    Visible light has a much shorter wavelength and higher frequency than radio waves. You can use the above formula to show that for yellow-orange light of wavelength 600 nm, the frequency is 5 x 10^14 Hz or 500 million megahertz!
    While Young`s experiment (Figure 5-5) showed convincingly that light has wavelike aspects, it was discovered in the early 1900s that light also has some of the characteristics of a stream of particles and waves. We will explore light`s dual nature in Section 5-5.

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