1.2 - Space |
Day 2 - Due 9/24(A) & 9/25(B) |
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Electromagnetic Radiation
Earth is separated from the rest of the universe by very large expanses of space. Very rarely matter from outside Earth’s environment reaches us, such as when a meteorite makes it through the atmosphere from elsewhere in the solar system. But for the most part, astronomers have one main source for their data — light. Light can travel across empty space, and as it does, so it carries both energy and information. Light is one type of electromagnetic (EM) radiation, energy that is transmitted through space as a wave.
The Speed of Light
Light travels faster than anything else in the universe. In the almost completely empty vacuum of space, light travels at a speed of approximately 300,000,000 meters per second (670,000,000 miles per hour). To give you an idea of how fast that is, a beam of light could travel from New York to Los Angeles and back again nearly 40 times in just one second. Even though light travels extremely fast, objects in space are so far away that it takes a significant amount of time for light from those objects to reach us. For example, light from the Sun takes about 8 minutes to reach Earth.
Since astronomical distances are so large, it helps to have a unit of measurement that is good for expressing those large distances. A light-year is a unit of distance that is defined as the distance that light travels in one year. One light-year is approximately equal to 9,500,000,000,000 (9.5 trillion) kilometers, or 5,900,000,000,000 (5.9 trillion) miles (Figure below ). That’s a long way! But by astronomical standards, it’s actually a pretty short distance.
Proxima Centauri, the closest star to us after the Sun, is 4.22 light-years away. That means the light from Proxima Centauri takes 4.22 years to reach us. The galaxy we live in, the Milky Way Galaxy, is about 100,000 light-years across. How long does it take light to travel from one side of the galaxy to the other? 100,000 years! If an astronomer looks through a telescope at a star that is 1,000 light years away, is she seeing the star as it is now?
Earth is separated from the rest of the universe by very large expanses of space. Very rarely matter from outside Earth’s environment reaches us, such as when a meteorite makes it through the atmosphere from elsewhere in the solar system. But for the most part, astronomers have one main source for their data — light. Light can travel across empty space, and as it does, so it carries both energy and information. Light is one type of electromagnetic (EM) radiation, energy that is transmitted through space as a wave.
The Speed of Light
Light travels faster than anything else in the universe. In the almost completely empty vacuum of space, light travels at a speed of approximately 300,000,000 meters per second (670,000,000 miles per hour). To give you an idea of how fast that is, a beam of light could travel from New York to Los Angeles and back again nearly 40 times in just one second. Even though light travels extremely fast, objects in space are so far away that it takes a significant amount of time for light from those objects to reach us. For example, light from the Sun takes about 8 minutes to reach Earth.
Since astronomical distances are so large, it helps to have a unit of measurement that is good for expressing those large distances. A light-year is a unit of distance that is defined as the distance that light travels in one year. One light-year is approximately equal to 9,500,000,000,000 (9.5 trillion) kilometers, or 5,900,000,000,000 (5.9 trillion) miles (Figure below ). That’s a long way! But by astronomical standards, it’s actually a pretty short distance.
Proxima Centauri, the closest star to us after the Sun, is 4.22 light-years away. That means the light from Proxima Centauri takes 4.22 years to reach us. The galaxy we live in, the Milky Way Galaxy, is about 100,000 light-years across. How long does it take light to travel from one side of the galaxy to the other? 100,000 years! If an astronomer looks through a telescope at a star that is 1,000 light years away, is she seeing the star as it is now?
Looking Back in Time
When we look at astronomical objects such as stars and galaxies, we are not just seeing over great distances—we are also seeing back in time. Because light takes time to travel, the image we see of a distant galaxy is an image of how the galaxy used to look. For example, the Andromeda Galaxy, shown in (Figure below), is about 2.5 million light years from Earth. If you look at the Andromeda Galaxy through a telescope, what are you seeing? You are seeing the galaxy as it was 2.5 million years ago. If the galaxy ceased to exist 1 million years ago, when would you know that? If you want to see the galaxy as it is now, you will have to wait and look again 2.5 million years into the future.
When we look at astronomical objects such as stars and galaxies, we are not just seeing over great distances—we are also seeing back in time. Because light takes time to travel, the image we see of a distant galaxy is an image of how the galaxy used to look. For example, the Andromeda Galaxy, shown in (Figure below), is about 2.5 million light years from Earth. If you look at the Andromeda Galaxy through a telescope, what are you seeing? You are seeing the galaxy as it was 2.5 million years ago. If the galaxy ceased to exist 1 million years ago, when would you know that? If you want to see the galaxy as it is now, you will have to wait and look again 2.5 million years into the future.
Electromagnetic Waves
Light is one type of EM radiation; light is energy that travels in the form of an electromagnetic wave.
The distance between two adjacent oscillations is called wavelength. A related value is frequency, which measures the number of wavelengths that pass a given point every second. Wavelength and frequency are reciprocal, which means that as one increases, the other decreases.
Light is one type of EM radiation; light is energy that travels in the form of an electromagnetic wave.
The distance between two adjacent oscillations is called wavelength. A related value is frequency, which measures the number of wavelengths that pass a given point every second. Wavelength and frequency are reciprocal, which means that as one increases, the other decreases.
The Electromagnetic Spectrum
Visible light — the light that human eyes can see — comes in a variety of colors. The color of visible light is determined by its wavelength. Visible light ranges from wavelengths of 400 nm to 700 nm, corresponding to the colors violet through red. EM radiation with wavelengths shorter than 400 nm or longer than 700 nm exists all around you — you just can’t see it. The full range of electromagnetic radiation, or the electromagnetic spectrum, is shown in Figure below.
Visible light — the light that human eyes can see — comes in a variety of colors. The color of visible light is determined by its wavelength. Visible light ranges from wavelengths of 400 nm to 700 nm, corresponding to the colors violet through red. EM radiation with wavelengths shorter than 400 nm or longer than 700 nm exists all around you — you just can’t see it. The full range of electromagnetic radiation, or the electromagnetic spectrum, is shown in Figure below.
Like our Sun, every star emits light at a wide range of wavelengths, all across the visible spectrum and even outside the visible spectrum. Astronomers can learn a lot from studying the details of the spectrum of light from a star.
Some very hot stars emit light primarily at ultraviolet (UV) wavelengths, while some very cool stars emit mostly in the infrared. There are extremely hot objects that emit X-rays and even gamma rays. Light from some of the faintest, most distant objects is in the form of radio waves. In fact, a lot of the objects most interesting to astronomers today can’t even be seen with the naked eye. Astronomers use telescopes to detect the faint light from distant objects and to see objects at wavelengths all across the electromagnetic spectrum.
Redshift
If you look at a star through a prism, you will see a spectrum, or a range of colors through the rainbow. The spectrum will have specific dark bands where elements in the star absorb light of certain energies. By examining the arrangement of these dark absorption lines, astronomers can determine the composition of elements that make up a distant star. In fact, the element helium was first discovered in our Sun — not on Earth — by analyzing the absorption lines in the spectrum of the Sun.
While studying the spectrum of light from distant galaxies, astronomers noticed something strange. The dark lines in the spectrum were in the patterns they expected, but they were shifted toward the red end of the spectrum, as shown in Figure below. This shift of absorption bands toward the red end of the spectrum is known as redshift.
Some very hot stars emit light primarily at ultraviolet (UV) wavelengths, while some very cool stars emit mostly in the infrared. There are extremely hot objects that emit X-rays and even gamma rays. Light from some of the faintest, most distant objects is in the form of radio waves. In fact, a lot of the objects most interesting to astronomers today can’t even be seen with the naked eye. Astronomers use telescopes to detect the faint light from distant objects and to see objects at wavelengths all across the electromagnetic spectrum.
Redshift
If you look at a star through a prism, you will see a spectrum, or a range of colors through the rainbow. The spectrum will have specific dark bands where elements in the star absorb light of certain energies. By examining the arrangement of these dark absorption lines, astronomers can determine the composition of elements that make up a distant star. In fact, the element helium was first discovered in our Sun — not on Earth — by analyzing the absorption lines in the spectrum of the Sun.
While studying the spectrum of light from distant galaxies, astronomers noticed something strange. The dark lines in the spectrum were in the patterns they expected, but they were shifted toward the red end of the spectrum, as shown in Figure below. This shift of absorption bands toward the red end of the spectrum is known as redshift.
Redshift occurs when the light source is moving away from the observer or when the space between the observer and the source is stretched. What does it mean that stars and galaxies are redshifted? When astronomers see redshift in the light from a galaxy, they know that the galaxy is moving away from Earth.
If galaxies were moving randomly, would some be redshifted but others be blueshifted? Of course. Since almost every galaxy in the universe has a redshift, almost every galaxy is moving away from Earth.
Redshift can occur with other types of waves too. This phenomenon is called the Doppler Effect. An analogy to redshift is the noise a siren makes as it passes you. You may have noticed that an ambulance seems to lower the pitch of its siren after it passes you. The sound waves shift towards a lower pitch when the ambulance speeds away from you. Though redshift involves light instead of sound, a similar principle operates in both situations.
Questions
1. Betelgeuse is around 640 light-years from Earth. Light travels 9.5 trillion kilometers in one year. How far away is Betelgeuse in kilometers?
2. Identify four regions of the electromagnetic spectrum that astronomers use when observing objects in space.
3. What is redshift, and what causes it to occur? What does redshift indicate?
4. What is the doppler effect?
If galaxies were moving randomly, would some be redshifted but others be blueshifted? Of course. Since almost every galaxy in the universe has a redshift, almost every galaxy is moving away from Earth.
Redshift can occur with other types of waves too. This phenomenon is called the Doppler Effect. An analogy to redshift is the noise a siren makes as it passes you. You may have noticed that an ambulance seems to lower the pitch of its siren after it passes you. The sound waves shift towards a lower pitch when the ambulance speeds away from you. Though redshift involves light instead of sound, a similar principle operates in both situations.
Questions
1. Betelgeuse is around 640 light-years from Earth. Light travels 9.5 trillion kilometers in one year. How far away is Betelgeuse in kilometers?
2. Identify four regions of the electromagnetic spectrum that astronomers use when observing objects in space.
3. What is redshift, and what causes it to occur? What does redshift indicate?
4. What is the doppler effect?