Places that seem dark to our eyes, or to regular telescopes, burn bright in radio waves. Places where stars form, for example, are full of dust. That dust blocks the light from getting to us, so the whole area looks like a black blob. But when astronomer turns radio telescopes to that spot, they can see straight through the dust: they can see a star being born.
Stars are born in giant clouds of gas in space. First, that gas clumps together. Then, because of gravity, more and more gas is attracted to the clump. The clump grows bigger and bigger and hotter and hotter.
When it is huge and hot enough, it starts smashing hydrogen atoms, the smallest atoms that exist, together. When hydrogen atoms crash into each other, they make helium, a slightly bigger atom.
Then, this clump of gas becomes an official star. Radio telescopes take pictures of these baby stars [ 3 ]. Radio telescopes show the secrets of the nearest star, too. The light we see from the Sun comes from near the surface, which is about 9,oF. But above the surface, the temperature reaches ,oF. Radio telescopes help us learn more about these hot parts, which send out radio waves.
The planets in our solar system also have radio personalities. Radio telescopes show us the gases that swirl around Uranus and Neptune and how they move around. If we send radio waves toward Mercury, and then catch the radio waves that bounce back using a radio telescope, we can make a map almost as good as Google Earth [ 4 ].
When they look much farther away, radio telescopes show us some of the weirdest objects in the universe. Most galaxies have supermassive black holes in their centers. Black holes are objects that have a lot of mass squished into a tiny space. This mass gives them so much gravity that nothing, not even light, can escape their pull.
These black holes swallow stars, gas, and anything else that comes too close. As it gets closer, it goes faster and faster. Huge jets, or columns, of electromagnetic radiation and matter that does not make it in to the black hole sometimes taller than a whole galaxy is wide form above and below the black hole. More energy is emitted at the average vibration or motion rate the highest part of each curve , but if we have a large number of atoms or molecules, some energy will be detected at each wavelength.
Second, note that an object at a higher temperature emits more power at all wavelengths than does a cooler one. In a hot gas the taller curves in Figure 3 , for example, the atoms have more collisions and give off more energy. In the real world of stars, this means that hotter stars give off more energy at every wavelength than do cooler stars. Third, the graph shows us that the higher the temperature, the shorter the wavelength at which the maximum power is emitted.
Remember that a shorter wavelength means a higher frequency and energy. It makes sense, then, that hot objects give off a larger fraction of their energy at shorter wavelengths higher energies than do cool objects.
You may have observed examples of this rule in everyday life. When a burner on an electric stove is turned on low, it emits only heat, which is infrared radiation, but does not glow with visible light. If the burner is set to a higher temperature, it starts to glow a dull red. At a still-higher setting, it glows a brighter orange-red shorter wavelength. At even higher temperatures, which cannot be reached with ordinary stoves, metal can appear brilliant yellow or even blue-white.
If one star looks red and another looks blue, which one has the higher temperature? Because blue is the shorter-wavelength color, it is the sign of a hotter star.
Note that the temperatures we associate with different colors in science are not the same as the ones artists use. Likewise, we commonly see red on faucet or air conditioning controls to indicate hot temperatures and blue to indicate cold temperatures. We can develop a more precise star thermometer by measuring how much energy a star gives off at each wavelength and by constructing diagrams like Figure 3.
The location of the peak or maximum in the power curve of each star can tell us its temperature. The average temperature at the surface of the Sun, which is where the radiation that we see is emitted, turns out to be K. Throughout this text, we use the kelvin or absolute temperature scale. On this scale, water freezes at K and boils at K.
All molecular motion ceases at 0 K. The various temperature scales are described in Units Used in Science. There are stars cooler than the Sun and stars hotter than the Sun. For the Sun, the wavelength at which the maximum energy is emitted is nanometers, which is near the middle of that portion of the electromagnetic spectrum called visible light.
Characteristic temperatures of other astronomical objects, and the wavelengths at which they emit most of their power, are listed in Table 1. If the emitted radiation from a red dwarf star has a wavelength of maximum power at nm, what is the temperature of this star, assuming it is a blackbody?
We can also describe our observation that hotter objects radiate more power at all wavelengths in a mathematical form. If we sum up the contributions from all parts of the electromagnetic spectrum, we obtain the total energy emitted by a blackbody. What we usually measure from a large object like a star is the energy flux , the power emitted per square meter.
It turns out that the energy flux from a blackbody at temperature T is proportional to the fourth power of its absolute temperature. The substance that a wave moves through is called the medium. That medium moves back and forth repeatedly, returning to its original position. But the wave travels along the medium. It does not stay in one place.
Imagine holding one end of a piece of rope. If you shake it up and down, you create a wave, with the rope as your medium. When your hand moves up, you create a high point, or crest. But the crests and troughs do move away from your hand as the wave travels along the rope. The same thing happens in other waves. If you jump in a puddle, your foot pushes on the water in one spot. This starts a small wave. The water that your foot hits moves outward, pushing on the water nearby.
This movement creates empty space near your foot, pulling water back inwards. The water oscillates, moving back and forth, creating crests and troughs.
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