Astronomy: The Study of the Universe
An astronomer answers students' questions about this field of science.
- Grades: 3–5, 6–8, 9–12
The following questions were answered by astronomer Dr. Cathy Imhoff of the Space Telescope Science Institute.
What is astronomy?
Astronomy is the scientific study of the universe — stars, planets, galaxies, and everything in between. It's a pretty big topic!
Are there any different fields of astronomy?
Yes! First, many astronomers consider themselves to be either theoreticians, instrumentalists, or observationalists. The theoreticians specialize in creating models using computer programs to simulate a star, or a supernova, or whatever it is that they are studying. The instrumentalists specialize in designing and building new instruments to make measurements or designing new telescopes. The observationalists specialize in obtaining, analyzing, and interpreting the data. Of course there are also some astronomers who do all of these things.
We also tend to classify ourselves by what type of astronomical object we study. There are those who specialize in studying the solar system, and they usually concentrate on just the gaseous planets, the rocky planets, comets, asteroids, etc. There are astronomers who specialize in studying stars. Usually they concentrate on just hot stars, cool stars, or certain types of stars like binaries, variable stars, etc. The same is true for nebulae, galaxies, and so on.
How long is a light-year?
A light-year is the distance that a beam of light can travel in one year. Since light moves very fast, that is a long distance. It is over 5,000,000,000,000 miles! The term "light-year" is very confusing to many people. It sounds like a measure of time, but it is actually a measure of distance. The nearest star to our sun is a little over one light-year away.
How many light-years are in one parsec?
There are 3.26 light-years in 1 parsec. So the nearest star, Alpha Centauri, is about 4.3 light- years away, but 1.3 parsecs away.
Astronomers normally use parsecs in our research, as you may know. But putting distances in terms of light-years is useful too because it tells you how long it took for the light to get to you.
What are the characteristics of light? For example, why do all of the colors in a rainbow always appear in the same distinct pattern?
As you may know, light acts like a wave, and that means it has a wavelength. Each photon, or bit of light, has its own wavelength. The wavelength tells us how much energy the photon carries and also what color it is.
The light from the sun is made of light of many wavelengths. When the light passes through a raindrop or a prism, the light is refracted (bent). The amount that the light is bent depends on its wavelength. The violet light is bent the most, the blue next, then green, yellow, orange, and red. That is because violet has the smallest wavelength, then blue, then green... So the raindrop or prism has spread the mixed white light out by wavelengths, which correspond to colors as perceived by our eyes. It was Sir Isaac Newton who proved this about light. You might want to try his experiment that proved that white light is made up of many colors and that those colors are distinct and unchanging. First he passed light through a prism making the familiar rainbow also known as a spectrum — that's the scientific term. If you pass some of that colored light, say the blue, through another prism, only blue light will come out. That is, you can break white light into its different colors (wavelengths) but you can't break blue apart (because they are all about the same wavelength).
P.S. Isaac Newton was an interesting guy! You might want to read about him and his experiments with light.
Do you know what's in the ozone? Do you know where the ozone hole is and what it is over right now?
You asked about ozone. Ozone is actually just a special form of oxygen. The oxygen that we breathe is a molecule formed of two oxygen atoms. Ozone is a molecule formed of THREE oxygen atoms. We don't like to have ozone near the ground — it isn't good for us to breathe. But it is great to have up high in the atmosphere, because it absorbs the sun's ultraviolet light. Normally there is a layer of ozone high in the atmosphere all around the earth.
Our concern though is that the ozone layer is very thin — a "hole" — in an area above the South Pole. We are trying to understand how the hole forms so that hopefully we can keep from making the hole bigger or affecting the rest of the earth.
Have astronomers found life on other planets?
Astronomers have been looking for planets outside our solar system for some time. It's a really tough thing to do. Planets are small and very dim compared to stars. It's kind of like trying to see a little moth flying around a huge bonfire. This is one of the tasks that the Hubble Space Telescope is working on. It is above Earth's atmosphere and has very excellent mirrors (despite the bad press). So it can see faint things and things that are very close together in the sky very well. It seems to most scientists that there must be life elsewhere. The elements and chemicals that make up life on Earth are very common throughout the universe. We've even found amino acids in meteors! It's hard to believe that among all those billions of galaxies, each with billions of stars, that we are the only living creatures. But finding life "out there" is very hard. We looked very hard on Mars and so far, no life (but we're going to look some more — maybe we picked a bad spot for the Viking Lander back in 1976). We've listened for the extremely faint radio signals that might come from another civilization on a planet around some nearby star. So far, nothing — but we'll keep looking!
How does the Hubble Telescope take pictures of things and then send them down to Earth?
The Hubble Space Telescope has several instruments onboard. The ones that take pictures are called the Wide Field/Planetary Camera and another is the Faint Object Camera. These are electronic cameras that record the images with numbers, not with film. Then those numbers are sent by radio to antennas on the ground, relaying to computers, which can then put the numbers back together again into a picture.
Has the Hubble Telescope allowed you to find out about any new galaxies?
Yes, astronomers have been especially excited to look at the most distant, youngest galaxies. They turn out to be much more irregular, less organized than the more familiar, nearer galaxies. We think the young galaxies must interact with each other a lot, sometimes even colliding. Later they move further apart and settle down into the more regular shapes (spirals, ellipticals) that we are familiar with.
What does an astrolabe look like and how do you use it?
The astrolabe is an early instrument used to help measure time and the position of the sun and stars in the sky. Typically it is made of brass and is about 6 inches across. It consists of several flat, circular plates all rotating on a pin. The plates are inscribed with circles of altitude and azimuth for a given latitude on the earth. I've never used an astrolabe, but I understand that by rotating the disks to the proper places, you could use the position of the sun during the day to tell time, or the position of the stars at night to tell time. Astrolabes were used mostly between A.D. 800 and 1650, after which more sophisticated devices like the sextant became available.
How do computers help you in studying the universe?
You may be surprised to learn that astronomers use computers a whole lot for almost everything we do. Here are some of the ways: (1) We use computers to help run big telescopes, the instruments that collect data, and the satellites that study stars and planets. (2) We use computers to analyze the data and try to understand what the data mean. (3) We use computers to make mathematical models of how stars and galaxies behave. (4) We use computers to help access storehouses of data, known as archives. For instance, the satellite that I work on has taken over 100,000 images, which are stored on a computer. (5) We use computers to communicate with other astronomers, by e-mail, the World Wide Web, and so forth. (6) We use computers when writing papers describing our results and graphing the data. I never realized how much astronomers use computers until I had some teachers and students working with me on a research project. We discovered that they had to learn about the computers before they could help work on the research analysis! I use about six computers of various kinds every day in my work!
How was space made?
Boy, you ask hard questions! I'll tell you how astronomer think the universe was formed. We think that it was created in a great big explosion that occurred about 15 billion years ago. People have called it the "Big Bang." That probably sounds a little crazy. But when we look far out into the universe, we can see that everything is moving away, just as if it is all being blown apart by a big explosion!
How is gravity measured?
We measure it by dropping something!
Of course to measure gravity correctly, we have to be careful. For instance, the air helps to slow something that is falling. So to do the measurement properly, we would have to have a long tube with no air in it, then very carefully measure how long the tube is and how long it takes for something to fall.
If gravity is what holds things together, is gravity everywhere? And what is gravity made of?
Gravity is one of the fundamental forces in the universe. Anything which has mass (weight) also has gravity. So yes, gravity is everywhere. Also the more massive something is, the more gravity it has. But how much gravitational pull we feel from something depends also on how far away we are from it. So even though Earth is much smaller than the sun, we are much much closer to Earth, so its gravitational pull on us is larger. Everyday objects, like a chair or bus, actually do have gravity too, but they are so much smaller that their gravitational pull is extremely small.
Which direction would a compass point in outer space?
It depends on where you are in space. If you were in orbit around Earth, like in the space shuttle, it would follow Earth's magnetic field there, which is pretty much like on the surface of the earth. If you were near the sun, though, your compass would respond to the magnetic field around the sun. Even way out in space, there is generally a weak magnetic field that your compass would respond to.
Interesting question! But I don't think the astronauts are going to use compasses in space to find their way around!
Could you explain the oscillating theory of the beginning of the universe?
I believe that you are referring to the "Big Crunch" — the idea that the universe is expanding now but later it will stop, turn around, and collapse again to a point (the "crunch"). Then presumably this will bring about another "Big Bang" as it all explodes outward again. So the universe would go "bang," expand, collapse, "crunch," "bang," expand, collapse, and so on.
This idea came from the fact that although we know that the universe is expanding, we also know that the gravity of all the matter in the universe is slowing that expansion. If there is enough matter in the universe, it would then have strong enough gravity to stop the expansion, and cause a collapse.
The other idea is, of course, that there is NOT enough matter and gravity, so the universe will keep expanding forever. So far, the observations that we have taken have not answered this question.
Do you know of any other theories other than the big bang and the oscillating?
There is an old theory called the "steady state" theory. It says that matter is continually being formed throughout the universe and that there was no "Big Bang." In fact British astronomer Fred Hoyle, who was one of the main supporters of this theory, gave the "Big Bang" theory its name (he intended it as sarcasm, but the name stuck). There are only a few astronomers who still subscribe to this theory.
A new idea is of the "inflationary universe." This theory says that our expanding part of the universe was caused by a "big bang" but that this is only one part of the universe. There are bubble universes all around, each one caused by a "big bang." The characteristics of each universe differ according to the details of what happened during its particular "big bang." So what we call the laws of physics (how matter and energy behave in our universe) would not be the same in some other bubble universe.
One of the ideas that Albert Einstein put forth is that what we think of as "space" is determined by the presence of matter and energy. Matter has gravity, mass, energy of motion, and so forth. These are the things that we can measure. So these things are what make up the universe. Suppose we think of a "place" where there is no matter and energy — nothing. It is "not-space." How big is it? We can't measure it in any way. We can't go there or there would be something in it. We can only imagine it. So it is "undefined." You can't use science to describe it.
Now we know that our universe is expanding. That's because there are things inside it that we can use for measurement. For instance, we know the speed of light. We know how far it is from Earth to the sun. So we — creatures inside this universe — can make measurements and can show that the galaxies in the universe are pretty much moving away from each other. As they move outward, they extend what we can "space."
Does the universe have an end?
We think it has a beginning — the Big Bang. As the end, there seem to be two possibilities.
One is that the universe will keep expanding forever. If that happens, though, all the stars will eventually burn out and the universe will become a cold, dark place.
The other possibility is that at some point the universe will quit expanding and then collapse in on itself. If it collapses in on itself, there will be a "Big Crunch," which would pretty much be the end as far as you and I are concerned!
When two galaxies collide, what happens?
You may have seen in the news recently some pictures of two galaxies colliding. When that happens, they sometimes merge together. Probably nothing much happens to the stars, because there is actually a lot of space in between stars. But the clouds of gas and dust collide. Big streams of gas, dust, and stars get thrown out, making a pretty wild-looking couple of galaxies! Cool!
Does space have an end to it?
This is a tough concept! One of the ideas that Albert Einstein put forth is that what we think of as "space" is determined by the presence of matter and energy. Matter has gravity, mass, energy of motion, and so forth. These are the things that we can measure. So these things are what make up the universe.
Suppose we think of a "place" where there is no matter and energy — nothing. It is "not-space." How big is it? We can't measure it in any way. We can't go there or there would be something in it. We can only imagine it. So it is "undefined." You can't use science to describe it.
Now we know that our universe is expanding. That's because there are things inside it that we can use for measurement. For instance, we know the speed of light. We know how far it is from Earth to the sun. So we — creatures inside this universe — can make measurements and can show that the galaxies in the universe is pretty much moving away from each other. As they move outward, they extend what we can "space."
Is it true that you can tell people's future by the stars and sun? Do they tell people what they will do next?
Astrology is based on an ancient religion. There is no scientific basis for believing that the stars control our lives. For instance, I once computed that the tiny amount of gravity from the doctor who delivers a baby is greater than the gravity from a nearby star.
How long ago did the universe form?
We think that it formed about 12 to 20 billion years ago. The number is still pretty uncertain, but we know that there are stars in our galaxy about 12 billion years old, so it has to be at least that.
Is there really extraterrestrial life?
It is very hard to answer your questions because the only life we know of for sure is on Earth! Almost 20 years ago, we landed the Viking spacecraft on Mars. One of its tasks was to search for life. It tested for bacteria or microbes, but it didn't find any. There is a big debate about what life on another planet would be like. Life on Earth is very complicated, so some people argue that it would be very unlikely for life to arise somewhere else that would be like us. But others point out that the chemicals and processes involved in life on Earth are very common in the universe and would be expected to occur anywhere under the right conditions, so life elsewhere might be similar to that on Earth.
I have heard of being a tiny bit younger after traveling through space than you were when you first started traveling through space. How is this possible?
Our astronauts don't grow younger while in space, but they do age just a little more slowly than the rest of us on the surface of the earth for the time they are in space. This is one of the effects of relativity, as described by Albert Einstein. When something is moving very fast, time seems to slow down. This effect is very small unless you are moving close to the speed of light (186,000 miles per second!). The astronauts aren't moving that fast — only about 17,000 miles per hour (or five miles per second)!
All of the maps that I look at are in the same direction. How do I know that they are in the correct direction?
You can draw a map in any direction that you want. But to avoid confusion most maps are drawn so that north is up and east is to the right. Often there is a little "compass" mark that shows the directions of north, south, east, and west. I have seen a few maps with the directions turned, but there is always a compass mark somewhere on the map to tell you which way is which.
It does make some sense to put either the North Pole or South Pole at the top, because of the earth's rotation. That defines north and south. I understand that the reason the North Pole is at the top is that many of the early mapmakers were from Europe and thus live in the Northern Hemisphere. I have seen some maps drawn the other way round — with the South Pole at the top — usually done by people who live in the Southern Hemisphere trying to make this point!
How was celestial navigation discovered? Do people still use it today? What are the most important stars to navigate by?
We are still using celestial navigation but in a new way. Many of our satellites steer by the stars. The Hubble Space Telescope and also the satellite that I work on, the IUE, use computer and motion sensors to move around the sky. But to point exactly in the right place, we must locate one or two known stars for which we know the positions. From those stars we can then point precisely at whatever point in the sky we want. I believe that celestial navigation began with sailors. Out on the ocean there is only the water, the sun, and the stars. So early sailors thousands of years ago probably figured out some basic navigation.
Probably the most important star for navigation, both then and now is Polaris, the Pole Star. You may have learned to find the Big Dipper (Ursa Major) constellation. The two stars at the end of the dipper point at the Pole Star (which is part of a fainter constellation, the Little Dipper, or Ursa Minor). As long as you are in the Northern Hemisphere of the earth, you can use Polaris to find north at night (if it's not cloudy, raining, or snowing).
How do people use celestial navigation? Are there any other things in the sky that they use except stars?
I think that celestial navigation is not used so much any more by ships. Ships and planes use radio beacons to determine where they are. If you can pick up two or more radio beacons, you can figure out where you are pretty precisely. Recently we have been using radio beacons from space! There are several satellites in orbit that are used just to figure out where you are. This is called the Global Positioning System, or GPS. If I recall correctly, it was developed by the U.S. military but is now available to everyone to use. People can now buy a GPS device and put it into their own boat, even if it's just a yacht or a rowboat. It is very accurate and is now available commercially. It has built into it all the radio sensors and a computer to do the calculations for you.
Who gets the credit for saying the sun is the center of the solar system, and that the planets rotate around it?
The idea that the sun is the center of our solar system goes back to a Polish astronomer named Nicolaus Copernicus. He first published this idea in 1514. But this idea was not immediately accepted.
A Danish astronomer named Tycho Brahe performed very careful observations of the motion of the planets, the best that had ever been done. These observations were the test of any theory about the orbits of the planets. They were done during the late 1500s (he discovered a supernova in 1572).
It was the German astronomer Johannes Kepler who came up with the mathematical theory that really worked to explain the motion of the planets (using the careful observations by Tycho). He showed that the planets actually move in ovals, not circles, around the sun. His work on planetary orbits was published in 1609–1627.
Finally, Galileo was the first person to look at the night sky with a telescope. He found moons in orbit around Jupiter, that Venus has phases, and that the planets appeared larger and smaller as they moved through the sky. He found that these observations could only make sense if the sun is the center of the solar system. His ideas were published in 1632. He ran into trouble though, because the Catholic Church at the time insisted that the earth was the center of the universe.
So the idea comes from Copernicus, but it took awhile before it could be proved and before it was generally accepted as correct.
How does a magnet work in space?
A magnet would work fine in space. It doesn't need air or gravity or anything else to work. In fact the earth is a big magnet. Its magnetic fields help to produce the aurora, as particles given off by the sun interact with the magnetic field. Those fields are called the Van Allen belts.
Is there rain or lightning in space?
If by space, we are talking about out in space away from planets and stars, then no, there is no rain and lightning, because there are no water clouds.
But there can be rain on another planet if there are water clouds. Mars comes pretty close. It has a little water, but it is cold, so it shows up as frost and icy fogs. We have also seen lightning on Jupiter. It has different clouds — methane, ammonia, stuff like that. But lightning is basically an electrical discharge, and that can happen. I would guess that lightning occurs in the clouds of some of the other planets as well.
Can you hear yourself talk in space?
Talking is sound. Sound is vibrations traveling through something — air if you are talking, but sound can travel through liquids (the ocean) and through solids (the earth) as well. Space is very empty, nearly a vacuum. So there would be no sound. All those great whooshes and pows in science fiction movies are great special effects, but not real.
What is it like in space?
Empty, dark, hot on one side (where the sun shines), and cold on the other (in shadow)!
Could you explain what is meant by curved space, as I believe Einstein described?
We usually talk about curved space with respect to gravity. A large mass like the sun distorts space by its gravity, causing both matter and energy to "fall" in toward it. The usual analogy is to picture a two-dimensional universe. If nothing were in it, it would be flat, but put a "star" in the middle and it "sags" in toward the star.
How do the northern lights (also called the aurora borealis) get their color?
The northern (and southern) lights occur when charged particles emitted by the sun encounter the earth's magnetic field. These particles slide along the lines of magnetic force toward the North and South poles. When the particles hit the earth's atmosphere, they can excite (add energy to) the molecules in the air. If I remember correctly, the green color in the northern lights is from the nitrogen (or is it oxygen?) in the air.
There is an absolute zero where there is no kinetic energy in the motion of an atom/molecule. Is there a temperature where there can be no more kinetic energy, the opposite of absolute zero?
That is a very interesting thought. Let's see — the most kinetic energy an atom or molecule could have would be if it could move at the speed of light. It must have been almost that hot at the formation of the universe during the Big Bang. Also it may be possible to speed up a few atoms to near the speed of light in a particle accelerator. Otherwise it would be difficult to reach this "maximum temperature." Of course it is difficult to reach absolute zero too. So I think that in practice, although one can't actually reach these values, we can get pretty close so the concepts are valid.