Our Cosmic Home Study Guide (page 3)
Look out into the sky at night. That's right–look out, not just up. Because when you look at the sparkling stars, you are gazing out across vast distances of space and time. You see light from stars that has taken tens, hundreds, even tens of thousands of years to reach your eyes. That's how vast space is, And one of the major, remarkable findings of astronomy is that the universe is expanding and has been expanding ever since it began with what is called the Big Bang.
Here is the outline of the events that occurred in the birth of our universe (see Figure 1.1).
- The Big Bang occurred about 13.7 billion years ago (with an uncertainty of a few hundred million years).
- Time: Between the Big Bang and the one second mark. Temperature: Many billions of degrees above absolute zero. Event: Matter and antimatter nearly annihilated each other, leaving a small amount of residual matter.
- Time: One second after the Big Bang. Temperature: About 1 billion degrees above absolute zero. Event: Protons, neutrons, and electrons exist as stable particles, or what physicists call subatomic particles, because they are basic constituents of atoms.
- Time: 300,000 years after the Big Bang. Temperature: 3,000° above absolute zero (close to the surface temperature of our sun). Event: Atoms are born. This happened because the universe cooled enough for electrons to remain bound to nuclei of protons and neutrons. At this point, matter consisted of 76% hydrogen and 24% helium (with a trace of lithium). No other elements existed.
- Time: Millions of years to a billion years after the Big Bang. Temperature: Much cooler. Event: First stars and galaxies are born. Other elements are created in the nuclear furnaces of stars.
- Time: Today. Temperature: About 3 degrees above absolute zero (2.7 degrees Kelvin, in which a unit of temperature on the Kelvin scale is the same as a °C, a degree Centigrade or Celsius, except rather than the zero point references to the freezing temperature of water, zero Kelvin is absolute zero, about –476 degrees Fahrenheit).
Note this is only the average temperature. Event: We are here to discover this story.
Visible light consists of a mix of wavelengths. Wavelength determines the color of light. Red light has longer wavelengths than green light has, for example. Green light has longer wavelengths than blue light. Study the diagram in Figure 1.2, which shows the spectrum of different wavelengths.
What we call visible light is only a small part of the electromagnetic spectrum, which consists of all waves that are both electric and magnetic. Note the relative sizes of the wavelengths and the fact that some wavelengths are too long for our eyes to see (infrared), whereas other wavelengths are too short (ultraviolet).
All elements glow at particular wavelengths. Light that we normally see is a mixture of thousands and thousands of specific wavelengths. Scientists can examine light in great detail and see specific patterns of wavelengths, revealing specific elements that emitted the wavelengths. Thus, elements in space can be measured by the wavelengths that they emit. Furthermore, as light passes through gases that contain particular elements, the elements in the gases also absorb wavelengths in specific patterns. Thus, patterns of both emissions and absorptions can provide astronomers with information about the elements in outer space. When light from galaxies is examined, an interesting pattern is discovered. Study the diagrams in Figure 1.3.
The top figure shows some of the locations of wavelengths of light given off by calcium on Earth. Note that they occur in a particular location and pattern on the spectrum. The bottom figure shows that in light from a distant galaxy, the characteristic pattern of calcium occurs at longer wavelengths. The calcium pattern has been shifted toward the red. This is the famous red shift.
The shift toward the red in the patterns of wavelengths that are characteristic of specific elements could occur only if the galaxies are moving away from the earth. As the galaxies move away, the light from them is stretched by the motion. (If the galaxies were moving toward us, the shift in the wavelengths of the patterns would have been toward the blue, which is not observed.)
The red shift is the primary evidence for the expanding universe. By extrapolating the expansion back in time, astronomers have concluded that the expansion started with all matter concentrated in a very small point and a single explosive event known as the Big Bang. Many other kinds of evidence have confirmed this theory.
Question: If all galaxies are moving away from us, does that imply that we are at the center?
Answer: No, because inhabitants of any galaxy would also observe that they appear to be at the center. It's like raisins in an expanding raisin cake. To each raisin, all the others are moving away.
Galaxies and Stars
It is important to know that as we look out in space, we look back in time. That's because the speed of light, though fast, is finite. Light travels at 186 thousand miles per second (300,000 kilometers per second). The light from stars in our own galaxy, generated hundred of thousands of years ago, or from stars in other galaxies, generated billions of years ago, is just now reaching us. Study the diagrams in Figure 1.4 and see how the term light-year is an astronomical unit of distance.
The figure shows that light from the sun needs to travel 93 million miles to reach Earth, and this takes the light 8.3 minutes. Thus, we see the sun as it was 8.3 minutes ago. We simply cannot see the sun as it is right now, because it takes time for the light to travel. This has important implications when we look at stars and galaxies. The other portion of the figure shows light reaching the eye of an observer from a star that is 252 trillion miles away. (Many stars that you see are this far, and many are much farther.) Light take 43 years to make the journey from star to eye. Thus, the star is 43 light-years away. If the star were to explode today, we would not know it for 43 years!
A light-year is nearly 6 trillion miles of distance (5.86 trillion miles, which is 9.4 trillion kilometers). When we start to look at galaxies, this "looking back in time" gets really serious. For example, the nearest large galaxy that is similar to our own Milky Way galaxy is Andromeda, and it is 2 million light-years away. Today, we see the light that it emitted 2 million years ago, during a time when the ancestors of humans were just making the first crude stone tools in Africa.
Many galaxies were formed in the first billion years or so after the Big Bang, as clumps of matter floating in space condensed under the attractive power of gravity. Galaxies are cities of stars. Just as gravity made the galaxies, gravity also made the stars within the galaxies. Stars are created when gas clouds in space condense, pulled together by gravity. During the condensation, the gas becomes hotter and hotter. If the density and temperature are high enough, the protostar ignites and is sustained as a glowing star by nuclear fusion. New stars are being formed all the time. Astronomers today have found regions of star births. Stars also die (see the next section). Our own galaxy is called the Milky Way (see Figure 1.5).
Our Milky Way galaxy contains about 400 billion stars, an incredible number. It is shaped somewhat like two dinner plates put together face to face, with a bulge in the middle, and within the bulge, a zone exists that is extremely rich with stars. From above, these appear as giant spiral arms. Astronomers have also found evidence for a massive black hole in the center. Black holes occur when matter has contracted to such a high density that even light cannot escape. We know black holes by certain effects they have on radiation in the space around them. Note that our sun is located about three fifths away from the center of the galaxy.
How Elements Are Born
You, me, the other animals, all the trees, even the atmosphere and rocky Earth itself are made of chemical elements that were born (manufactured) in the nuclear furnaces of stars. Elements can be characterized by their atomic numbers, which is the number of protons in their nuclei. As we have seen, the elements that existed at 300,000 years after the Big Bang were only the ones with the lowest atomic numbers, the light elements of hydrogen, helium, and a trace of lithium. The key concept to the formation of all other elements is nuclear fusion.
Figure 1.6 shows a general diagram for how nuclear fusion works. Atomic nuclei from two or more elements are squeezed by hot temperatures and pressures in the center of a star to create a new fused nucleus. For atoms from hydrogen up to the atomic number of iron, energy is released when atoms are fused to make larger atoms. This is because the protons and neutrons inside the nuclei of the larger atoms (again, up to iron) contain less mass per subatomic particle and therefore less energy according to Einstein's equation E=Mc2 (where E is energy, M is mass, and c is the speed of light). The excess energy of fusion is released as heat and radiation. To us, this released energy is the warm sunlight we feel and all the light we see from stars.
Stars are hot and are able to emit vast quantities of radiation into space because of fusion reactions deep within their cores. Inside stars, the first element to be fused is hydrogen, the most abundant primordial element. Under intense temperature and pressure, two hydrogen atoms are fused into one atom of helium, releasing energy and making stars hot, thus sustaining further fusion reactions. When the hydrogen is used up, helium is fused into carbon, and then the carbon and some helium are fused into oxygen. All the elements up to iron can be made in this way. Note the sequence of how elements are made: Hydrogen (H) → Helium (He) → Carbon (C) → Oxygen (O). All these fusion reactions release energy.
Stars can run out of matter to fuel fusion, they can "die." Some stars die by throwing off gases then withering into small, smoldering white dwarfs. Don't worry; we still have billions of years to go before that!
Very massive stars, on the order of ten times the mass of our sun, can create supernova explosions at their deaths. One supernova, for example, occurred in our galaxy in A.D. 1066, which is now the Crab Nebula. Ancient people then observed this bright new star in the sky before it faded.
Supernovas are important parts of how our universe works. They do two special things. First, all elements heavier than iron (such as gold and uranium) are made in the intense heat and pressure of the supernova. Second, the supernovas disperse all the elements inside the former star out into space. We can see these elements in the emission and absorption spectra in the regions surrounding former sites of supernovas. Note that in the dispersal of elements by supernovas, two categories of elements can be found: those that were made earlier in fusion reactions during the long, ordinary lifetime of the star, as well as those that are new (that is made only in the supernova itself).
The elements dispersed into space can eventually gather into gas clouds and possibly contract, after mixing with remnants of other supernovas, into totally new stars and their planets. We are, literally, stardust!
Practice problems of this concept can be found at: Our Cosmic Home Practice Questions
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