THE SUN
HOW DO SCIENTISTS KNOW HOW BIG THE SUN IS?
In order to determine the size of the Sun, we have to figure out its distance. Then we can use geometry to calculate the Sun’s size, which is directly proportional to its distance. Information on determining the distance to the Sun should be in any good introductory astronomy text book written for people who know geometry.
In the past, methods involving geometry of the Earth, Moon and Sun were used. The ancient Greek astronomer Aristarchus of Samos used a method involving phases of the Moon. He underestimated the distance and the size of the Sun, but only because the measurements he used were not accurate. You can read a description of the calculation here:
http://astrosun.tn.cornell.edu/courses/astro201/aristarchus.htm
(Aristarchus)
Currently we can measure the distance to the Sun by bouncing radio waves off of it (radar). We know the speed of light and so we can tell how far the signal has gone by measuring how long it takes to go there and back: distance = time * speed of light/2
WHAT KIND OF STAR IS THE SUN?
The Sun is basically a very ordinary star, about mid-way through its “life cycle.” It is a dwarf star (astronomers call stars either “giants” or “dwarfs” – the normal stars are “dwarfs”) with a surface temperature of about 6,000 degrees Kelvin. This makes it appear yellow in color. Hotter stars are more blue in color, and cooler stars are more red.
Technically, the Sun is classified as a G2V star. Stars are classified according to their surface temperature and luminosity (brightness). The Sun appears very bright because it is much closer to us than other stars, but in fact it is not unusually luminous. If we put the Sun at the same distance as other “nearby” stars, it would look about the same as many other stars. The main temperature classes are denoted (in decreasing temperature) by the letters O, B, A, F, G, K and M.
Each class is then divided into ten subclasses using numbers from 0 to 9, so a G2 star is roughly 20% of the way between a G0 and a K0 star. There are also five luminosity classes, denoted by the Roman numerals I, II, III, IV, V and VI. The Sun belongs in luminosity class V, and temperature (or spectral) class G2, hence the designation G2V.
The reasons for this messy system of classification are historical, and no doubt we could come up with a better system. However, astronomers have gotten used to this system, and with some experience you can use these classifications to tell an awful lot about a star. For example, knowing the surface temperature and luminosity class allows you to compute a star’s size (a big star at a certain temperature is brighter than a small one at the same temperature).
HOW FAST DOES THE SUN SPIN? WHICH WAY IS N, S, E, AND W?
The rotation period of the Sun varies from about 25 days at the equator to over 30 days at the poles, so it takes roughly two weeks for a feature to cross the solar disk.
Sunspot groups can last more than one rotation. However, each time a region comes around, it gets a new number – that keeps it simpler because it is hard to be absolutely positive it is the same region without being able to watch its evolution on the far side of the Sun.
When you look at something in the sky east and west get reversed (think about lying on the ground with your head point north – it is the opposite of the normal map convention in which you are floating above the ground looking down), so we say the sunspots go from east (which is the left hand side of our maps) to west ( on the right hand side).
IF IT IS TRUE THAT ALL HEAVIER SUBSTANCES MOVE TOWARDS THE CENTER, THEN WHY IS IT THAT THE SUN IS THE CENTER OF OUR SOLAR SYSTEM WHEN IT IS MADE OF HYDROGEN?
If you had ten cotton balls they would not weigh very much, but if you had 10,000 they would. Each cotton ball is very light, but if you have enough of them together they are very heavy.
Hydrogen atoms are like the cotton balls. Each individual hydrogen atom is very light, but if you have enough of them together yo can still get something, like the Sun, which has a lot of mass and a lot of gravitational pull.
Another thing to mention is that it is the density of an object that makes it sink rather than their total weight. Density is how much stuff is put in a particular volume. That is why a giant iceberg the size of a mountain will float, but a rock the size of your hand will sink. The rock weighs less, but is denser than the iceberg.
WHY DOES THE SUN’S CENTER HAVE A GREATER DENSITY THAN ITS OUTSIDE?
Imagine yourself under water in the ocean. If you are near the surface, there isn’t much pressure on you and you can swim around comfortably. But you might know that as you go deeper and deeper, the pressure would increase. Even submarines can’t go too deeply, or they would be crushed. Well, the same sort of pressure increase occurs as you go deeper and deeper into the Sun.
The difference between water and gas is that we can’t compress water. If we push down on it, it doesn’t get any smaller. But if we increase the pressure on gas, it will get smaller. Which means the individual atoms in the gas all get closer together, increasing the density.
DOES THE SUN HAVE AN ORBIT? WHERE IS IT HEADED?
The Sun orbits the center of the Milky Way galaxy, in the same way as the Earth orbits the Sun. It takes about 250 million years to complete one orbit traveling at about 200-300 kilometers/second. In an orbit, you follow an elliptical path, so you are not really going towards or away from something in particular, but at the moment the motion of the Sun is towards the constellation of Hercules.
WHAT CAUSES THE SO CALLED “WAVES” ON THE SUN?
The so called waves that you see on the Sun in some of the beautiful SOHO movies are just a surface response to an explosion that ejects large quantities of gas and magnetic field upwards and out from the Sun. Think of an underwater explosion: you will get water forced upwards as a great spray, but you will also get a wave on the surface of the water that travels outwards from the explosion site. The waves on the Sun are similar, although in this case the explosion occurs just above the surface rather than below it.
If the explosion (or coronal mass ejection, to use the solar physicists’ term) happens to be directed towards Earth, then we can get geomagnetic storms and spectacular auroral effects when it arrives here, usually a few days later.
HOW IS THE AMOUNT OF HYDROGEN IN THE SUN MEASURED?
There are several ways we can measure the relative amount of hydrogen in the Sun.
One way is to measure the particles directly in the solar wind. The solar wind particles are ionized. That means that they have been heated so much that one or more of the electrons orbiting the atomic nucleus have been kicked off. The absence of the electrons gives the atom a positive charge, and makes it sensitive to the electric and magnetic fields in the solar atmosphere. These ions (and electrons) then get accelerated into the solar wind. It is possible to measure the mass, charge, and energy of these particles by observing their reaction to electric and magnetic fields, and thus deduce their composition. SOHO is one of a number of satellites which measure the properties of particles in the solar wind.
Another, and much older, way to determine the composition of the Sun is through spectral analysis. Every element will have characteristic wavelengths (colors) associated with it. This is a result of quantum mechanics. The electrons surrounding the atomic nuclei can only be found in specific orbits. For example, an electron can’t be halfway between one allowed orbit and another. If a photon of light has exactly the right amount of energy, which is directly given by its wavelength, then it will be able to kick an electron from one orbit into a higher one. The photon will be absorbed by the atom, and we’ll see a reduction of light at that wavelength. We call these diminished areas in the spectrum absorption lines, because they appear as dark lines in a slit spectrum. They’re also known as Fraunhofer lines, after the person who discovered them. It’s possible, by measuring the depth and width of these lines, to determine the total number of atoms of the element which produced the absorption line.
Of course, these measurements only tell us about the outer layers of the Sun. We can’t measure the inner parts of the Sun directly, but we can infer things about it through theoretical models, and by observing stars like the Sun at various ages in their development, some of which have shed their outer atmospheres. Also, the relatively new science of helioseismology allows us to infer some of the properties of the solar interior.
DOES A DOPPLER SHIFT OCCUR BETWEEN THE EARTH AND THE SUN? WHAT EFFECT DOES IT HAVE ON THE COLOR SPECTRUM?
There are actually a number of doppler shift effects one needs to worry about when measuring velocities on the Sun. One of these doppler shifts, is caused by the motion of the Earth about the Sun. If the Earth’s orbit was exactly circular, then there would be no doppler shift. However, the orbit is slightly elliptical, and Earth is closest to the Sun in January, and farthest away in July. The maximum doppler shift, though, occurs halfway between these dates, in April and October. The size of this doppler shift is about 0.5 kilometers per second.
For a telescope on the Earth, another doppler effect which needs to be accounted for is the Earth’s daily rotation. This is also on the order of 0.5 kilometers per second. On the other hand, a satellite in orbit about the Earth would have a much larger effect. Since a typical orbital period in low Earth orbit is once every 90 minutes, the doppler shift would then be about 7.5 kilometers per second.
(Because of SOHO’s special orbit around the L1 Lagrange point between the Earth and the Sun, the doppler shifts are much more complicated than what is described above. The NASA Flight Dynamics Facility keeps track of SOHO’s position for us, and one of the products that they provide us is the relative velocity between the spacecraft and the Sun, on 10 minute intervals. This is then used to calibrate SOHO data.)
One also has to take into account the fact that the Sun itself is rotating, about once every 25 days at the equator. Since the Earth is also revolving in the same direction, it makes it look like the Sun takes a little bit longer to rotate, about 27 days. This produces a doppler effect of up to 2 kilometers per second. This doppler shift can be clearly seen in MDI data from SOHO.
HOW BIG WAS THE LARGEST SUNSPOT GROUP?
The largest sunspot group since 1900 was one in 1947 which measured 6132 millionths of the solar disk, or 18 times the surface area of the Earth.
WHY IS IT THAT THE AREAS THAT APPEAR BRIGHTEST IN THE X-RAY PICTURES CORRESPOND TO SUNSPOT LOCATIONS?
Sunspots are areas where the Sun’s magnetic field is very high. We believe they are dark at the photosphere level (what we see in visible light, generally considered to be the Sun’s surface) because they inhibit the convection that transports energy up to the photosphere. X-ray pictures show the Sun’s corona-its hot outer atmosphere. We are still trying to work out the exact details of how this happens.
WHAT IS THE ORIGIN AND STRUCTURE OF THE DESIGNATION PROCESS FOR SUNSPOTS?
Sunspot groups are given numbers by the Space Environment Center, a part of the National Oceanic and Atmospheric Administration (NOAA). Sunspot groups are numbered sequentially as they are identified. If a group rotates around the far side of the Sun and then back into view it is given a new number when it is spotted again at the east limb of the Sun.
The numbering system got started in the mid 1960s by solar physicist Pat McIntosh who soon after joined the then forming NOAA. Pat says that the number system was set back to zero at the start of 1970, but that the numbers have been continually increasing since around 1972 or 1973.
DO SUNSPOTS ONLY APPEAR ON CERTAIN LATITUDES?
Sunspots have a preferred latitude, depending on where you are in the solar cycle. At the beginning of each 11-year solar cycle, sunspots form at fairly high latitudes. Then, as the solar cycle progresses, the preferred latitude for sunspots shifts down towards the equator.
The magnetic field at the poles is substantially different from that at the Sun’s equator. At the equator, the magnetic field lines generally do not stray far from the surface. They come out from one point in the surface and go back in again not far away, forming loops. Sunspots are found at the bases of really strong loops. At the poles, however, the magnetic field lines go out to far distances away from the Sun. If they go back again at all, it’s far away from where they came out. Sunspots can’t form under these conditions.
WHY DO SUNSPOTS SEEM TO ENHANCE RADIO WAVE PROPAGATION ON EARTH?
Sunspots themselves don’t enhance radio communications, but rather magnetic storms on the Sun which cause changes in the Earth’s ionosphere that enhance radio communication.
DOES THE NUMBER OF SUNSPOTS HAVE ANY EFFECT ON THE CLIMATE HERE ON EARTH?
Sunspots are slightly cooler areas on the surface of the Sun, due to the intense magnetic fields, so they radiate a little less energy than the surroundings. However, there are usually nearby areas associated with the sunspots that are a little hotter (called falculae), and they more than compensate. The result is that there is a little bit more radiation coming from the Sun when it has more sunspots, but the effect is so small that it has very little impact on the weather and climate on Earth.
However, there are more important indirect effects: sunspots are associated with what we call “active regions”, with large magnetic structured containing very hot material (being held in place by the magnetism). This caused more ultraviolet (or UV) radiation (the rays that give you a suntan or sunburn), and extreme ultraviolet radiation (EUV). These types of radiation have an impact on the chemistry of the upper atmosphere (e.g. producing ozone). Since some of these products act as greenhouse gases, the number of sunpots (through association with active regions) may influence the climate in this way.
Many active regions produce giant outflows of material that are called Coronal Mass Ejections. These ejections drag with them some of the more intense magnetic fields that are found in the active regions. The magnetic fields act as a shield for high-energy particles coming from various sources in our galaxy (outside the solar system). These “cosmic rays” (CRs) cause ionization of molecules in the atmosphere, and thereby can cause clouds to form (because the ionized molecules or dust particle can act as “seeds” for drop formation).
If clouds are formed very high in the atmosphere, the net result is a heating of the Earth – it acts as a “blanket” that keeps warmth in. If clouds are formed lower down in the atmosphere, they reflect sunlight better than they keep heat inside, so the net result is cooling. Which processes are dominant is still a matter of research.
WHY DO SOLAR FLARES HAPPEN?
(The Solar Flare Theory Page) This has a number of descriptions, questions and answers, and links to other interesting pages including information on how Solar flares may affect YOU! Below are a couple of sections taken from this page.
A flare is defined as a sudden, rapid, and intense variation in brightness. A solar flare occurs when magnetic energy that has built up in the solar atmosphere is suddenly released. Radiation is emitted across virtually the entire electromagnetic spectrum, from radio waves at the long wavelength end, through optical emission to x-rays and gamma rays at the short wavelength end. The amount of energy released is the equivalent of millions of 100-megaton hydrogen bombs exploding at the same time!
The first solar flare recorded in astronomical literature was on September 1, 1859. Two scientists, Richard C. Carrington and Richard Hodgson, were independently observing sunspots at the time, when they viewed a large flare in white (visible) light.
Solar flares are thought to result from the build up and explosive release of magnetic energy in the solar atmosphere. The outer layer of the Sun is convective, meaning that the gas rolls up and down like in a pot of boiling water. This ionized gas (plasma) drags the Sun’s magnetic field with it, twisting it and strengthening it. In some regions the magnetic field becomes particularly strong and breaks out into the solar atmosphere as discrete, loop-like structures. In active regions where flares occur, these structures either interact or become internally unstable, giving a flare. The signs of a flare are gas rapidly heated to high temperatures, electrons and ions accelerated to high energies, and bulk mass motions. The energy in the magnetic field is thought to be converted into these things through a process called magnetic reconnection, in which oppositely directed magnetic field lines “break” and connect to each other and part of their energy is transferred to the gas in the solar atmosphere. This is the basic picture. Some aspects of it may not be entirely correct and many of the details are not yet understood.
HOW LONG DOES IT TAKE A CME TO REACH THE EARTH?
The average distance from the Sun to the Earth is 150 million kilometers which can be written as 150 x 10^6 Km.
CME’s can vary in speed. They can range from 200 km/s to 1000 km/s, although typically, most of them travel at about 424 km/s (Avg 1996-1998 speed St. Cyr et al., 2000).
Assuming that CME’s don’t accelerate or decelerate on their way from the Sun to the Earth, we can easily compute from the numbers given, the minimum travel time, the maximum travel time, as well as the average travel time.
time=distance/speed
So, the average time it would take a CME to get to the Earth is 98 hrs. At 1000 km/s, a CME would take 42 hrs, and at 200 km/s, a CME would take 208 hrs.
DO YOU WEAR SAFETY GOGGLES TO OBSERVE THE SUN?
I mostly observe the Sun with telescopes on the spacecraft, so I don’t need to look directly at it. Astronomers using telescopes on the ground observe with special filters that block out much of the Sun’s light.
There are kinds of “safety glasses” you can use to look at the Sun briefly, but unless you are very sure you have the right kind, it is better to project a light from the Sun on a wall or piece of paper and look at that. One way to do that is with a “pin hole camera.”
Above all, NEVER look at the Sun with your naked eyes — not even for a quick glance. Obey this rule and your retinas will thank you.
WHICH REACTION RELEASES MORE ENERGY: FUSION OR FISSION?
For a particular reaction formula (e.g. 1 hydrogen + 1 deuterium -> 1 helium + 1 neutron + 16.6 MeV of energy) fission reaction are about 10 times more energetic than fusion reactions. However, the particles that participate in fusion are much less massive than those of fission reactions. Thus, if the reactions are described in terms of energy per unit mass, fusion reactions produce a few times more energy than fission reactions. (Reference: Chp. 11 of Modern Physics by Tippler, 1st ed.)
ARE THERE ANY SPACE PICTURES OF THE SUN FROM DISTANCES MUCH GREATER THAN EARTH?
One candidate is my very favorite Mars Pathfinder image, because there is something special about this sunset:
http://antwrp.gsfc.nasa.gov/apod/ap970804.html
Mars is only about 50% further out from the Sun than the Earth, so the Sun still largely “looks like” the Sun.
In the early 1990s, one of the two Voyager spacecraft took several images of the inner solar system from a distance of many billions of kilometers. The late Carl Sagan published some of them in the book Pale Blue Dot. The title comes from the appearance of the planet Earth at such a distance. Here’s what we look like from 6.4 billion km away:
As you can see, the Sun is 40 times smaller in diameter when viewed from such a distance.
Remember that most spacecraft — the Hubble Space Telescope, for example — try very hard to keep their instruments pointed away from the Sun. Staring at the Sun can be as damaging to a spacecraft sensor as it can be to your own eyes. Mars Pathfinder was one exception, as is SOHO.
WHY DO THE EARTH AND OTHER PLANETS ORBIT AROUND THE SUN?
The motion of something in orbit (like a planet around a star) is a combination of the effect of inertia and of gravity. “Inertia” means that something moving will continue going in a straight line at a constant velocity unless it is stopped (this is Newton’s first law). If there were no gravity, for instance, something moving past the Sun would just keep going in the same direction.
The force of gravity from the Sun pulls the planet towards the Sun, but the material which formed the planets was moving originally and the inertia from the original motion does not go away; instead you get a sum of the two. That makes for a diagonal motion at any one instant. However, the direction the force of gravity and thus the planet’s direction of motion changes as the planets changes its position relative to the Sun. The result is an orbit around the Sun.
WHAT COLOR IS THE SUN?
The Sun is white. To demonstrate this, take a sheet of white paper, go into a dark room, and shine a red flashlight on it. What color do you see the paper? The white sheet of paper appears red. This works for any color of illumination. A green laser pointer shined onto the paper will make the white paper appear green, and so on. The white paper will appear the same color as its source of illumination. Now take the same sheet of white paper out into the full noonday sunlight. What color does the white sheet look like? White!
Further discussion can be found at the Stanford SOLAR Center:
http://solar-center.stanford.edu/FAQ/Qsuncolor.html
(Ask a Solar Physicist: Why does the Sun appear orange?)
HOW DO SCIENTISTS KNOW THAT THE SUN HAS A CORE?
Obviously, nobody can see the center of the Sun and we’ve never sent a space probe into the Sun, either. However, we think we know what it is like in there.
The only thing that provides enough energy to heat a star for billions of years is nuclear fusion. We know the pressures and temperatures needed for this to take place, so we can figure out from that what the center of the Sun (the “core”) must be like.
HOW DOES SPACE WEATHER AFFECT US ON EARTH?
We can observe these phenomena better than before, thanks to new space observatories and spacecraft such as SOHO, ACE, and TRACE.
Magnetic storms, such as the coronal mass ejection interact with the Earth’s magnetic field which in turn can interfere with radio, television, and telephone signals, upset the navigation systems of ships and airplanes, and cause blackouts. Sun-induced storms can damage satellites and spacecraft or force them to re-enter the atmosphere. In some instances, it can be dangerous to astronauts out in space and especially on space walks. However, on a more positive note, solar wind also causes the Aurora Borealis also known as the northern lights.
Because we have more satellites, larger power grids, smaller cell phones, greater reliance upon GPS and such, we are much more susceptible to the effect of Space Weather.
WHAT CAN WE EXPECT FROM THE SUN IN THE FUTURE? IS IT GETTING BRIGHTER OR DIMMER? IS IT EXPANDING OR CONTRACTING?
(For an overview of the controversy over solar variability, I recommend John Gribbin’s pre-SOHO book, Blinded by the Light: The Secret Life of the Sun, published in 1991) The Sun’s brightness changes by only 0.1% between the minimum and maximum of a cycle. We do not have enough data to see any longer-term changes from cycle to cycle. However, the Sun has not changed drastically in recent decades. Models of solar evolution indicate that the Sun is gradually increasing in brightness at a rate of about 6% per billion years.
The Sun is not “currently” expanding or contracting to any measurable extent. I know of some observations made by an astronomer indicating that the Sun changes size over the 11 year solar cycle – decreasing in size from the maximum activity to the minimum and then increasing in size again as solar activity picks up again. Others have tried similar measurements and found no change in size. However, it is believed that in 4 to 5 billion years, the sun will expand as it fuses the last of its core hydrogen. The outer layers of gas will swallow some inner planets (possibly even the Earth). This is commonly referred to as the red giant phase. Then the inner parts of the Sun will stop fusing, contract, and become a white dwarf.
HOW LONG DOES IT TAKE FOR THE SUN’S LIGHT TO REACH THE EARTH?
The Sun is about 93 million miles, or 150 million kilometers, away from the Earth. The speed of light is 186,000 miles per second or 300,000 kilometers per second. Therefore it takes about 500 seconds or a little over 8 minutes for the Sun’s light to reach the Earth.
WHY ARE THE EIT IMAGES DIFFERENT COLORS AND WHAT DO THEY MEAN?
All of the EIT images are actually produced by extreme ultraviolet (EUV) light from the Sun. This is light that is between ultraviolet light and x-ray light in the electromagnetic spectrum and is not visible to our eyes directly.
EIT images are taken at four different wavelengths and four colors in order of wavelength (bluer=shorter wavelength, redder=longer) were assigned to represent each of them. Just as the human eye is capable of discriminating different colors in the visible, so EIT’s four bandpasses discriminate among four “colors” in the extreme ultraviolet. In addition, each color table was carefully constructed to bring out typical features of its particular wavelength.
The red images have a wavelength of 304 Angstroms, the yellow are 284 Angstroms, the green are 195 Angstroms, and the blue are 171 Angstroms. So, just like in the visible spectrum red is the longest wavelength and blue is the shortest with yellow and green in between, the EIT image colors were chosen so that the longest wavelength is reddish and the shortest is bluish.
Source: http://www.thesuntoday.org/faq/
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