Ask An Astronomer 

Answer Newsletter

16 November 2002


Here are the answers to selected questions submitted between . Questions may have been edited for clarity or brevity. Click on a link to move directly to the answer.

  1. Why are observatories always painted white? Is this simply tradition?

    Irregularities in the atmosphere have a lot to do with how good (or bad) the clarity of your observations will be. While there's not much that can be done about turbulence high up in the atmosphere (though that doesn't mean people won't try-- ttp://, intelligent telescope design can go a long way towards reducing turbulence near the ground. This is why a lot of telescopes are built near the ocean, where the winds tend to be regular and predictable, rather than inland, where they tend to be more turbulent and subject to unpredictable eddies and whatnot. The Andes mountains in Chile and the summit of Mauna Kea in Hawaii are particularly popular.

    The telescope dome itself can introduce turbulence into the atmosphere, both by being something for the wind to bounce off of, and also via thermal emission. White is a common choice for telescope domes because it is highly reflective, and will not absorb a lot of heat during the day-- heat that it would have to radiate away once the sun set, a process very likely to distort any images taken with the telescope that night.

    I hunted around on the web to try and find additional information to back up my own personal understanding of the "reasons why." Here's an answer to a question similar to yours:

    Both that web site and this next one point out that observations made in the infrared will be more sensitive to any dome temperature fluctuations. This next one also has some generic mugshots of the telescope family on Mauna Kea.

    Here's another web site that relates to choosing a site for a new telescope, and as such, makes mention of paint choices, among other things.

  2. The space shuttle expends a very large percentage of its fuel before it even leaves the tower. Why can't you get into orbit by starting with a horizontal takeoff?

    In order for the Space Shuttle to achieve its stable orbit about 100 miles above the Earth, its speed has to reach about 17,400 miles per hour. To reach high speeds, you want as little air friction as possible. There is much less air to experience friction with if you travel mostly straight up, rather in a shallow diagonal line or horizontally. Though not obvious at the beginning of launch, the Space Shuttle's engines do maneuver to give it a more horizontal trajectory as it gets higher in the atmosphere.

    Additionally, Jules Verne theorized about launching a spacecraft from a gun-barrel like device cut diagonally into the side of a mountain. Trying to set up the Space Shuttle and its rockets at an angle would cause diagonal stresses on the large spacecraft itself, as well as the launch pad structure. Engineers prefer to keep stresses as balanced as possible, so letting the Space Shuttle sit straight up and down is easier.

    A good summary of Space Shuttle launches and flights is available at

  3. I live in Zambia and we have very clear skies at night. I regularly see orbiting objects. Is there any way I can find out what these items are?

    The objects you are seeing are satellites in orbit around the Earth. Some are communications satellites, which relay television and phone signals around the world, while others take detailed pictures of the atmosphere which we use to study and forecast the weather. Unlike the thousands of stars visible on a dark night, satellites do not shine by themselves---what you are seeing is reflected sunlight off of the satellite's shiny surface. You're very lucky to have such dark skies, as most people who live near cities are unable to see such objects.

    There are many web sites that predict when the brighter satellites will be visible from a particular location on the Earth's surface. One of the best is Heavens Above. It has a large database of observing locations, including some in Zambia! One of the best orbiting objects to observe is the International Space Station (ISS), which is listed on this web site. The American space agency NASA has a page which shows the location of the ISS projected onto a map of the globe.

    Another method of identifying satellites is to download a computer program, which can plot the expected satellite transits (passages overhead) for a given time and location from a database stored within the program. Such programs exist for nearly every operating system; you can find some good choices at:

    If you want to find more pages that might help, take a look at the Yahoo! listing for the Satellite category: /Space/Satellites/

  4. Is it possible to use the moon to measure the gravitational bending of starlight?

    The main disadvantage in trying to use the moon to observe gravitational lensing is the moon's mass. The moon is far too small for any lensing effects to be noticeable and is therefore never used to observe gravitational lensing. The moon was, however, critical in the first experimental confirmation of gravitational lensing -- in the early part of the last century an experiment was performed observing starlight lensing around the sun. Because the sun is so bright, the experiment could only be performed during a solar eclipse! For more information on lensing, see this web page .

  5. The universe began as a huge explosion where all the parts were flung out from a central point. After this big bang, all the parts would be moving out with a fixed velocity as the force propelling them had stopped acting. So why don't the distant galaxies keep the same relative velocity with respect to us ? After all, there is no force causing them to accelerate away from us.

    Newton's first law states: "Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it." Thus at first sight your argument should hold. No forces are being applied to the galaxies, so they should not be accelerating. The problem is that Newton's theory of gravity assumes a static Euclidean space, whereas today we know that space-time is certainly not static and possibly not Euclidean either.

    Newton's laws deal with forces and accelerations in a fixed reference frame. In his world view it should be possible to attach real rulers to space in order to measure distances between objects and clocks to synchronize measurements. Using these physical rulers and clocks an experimenter could measure the change in distance of two objects over time and from these measurements deduce a velocity and an acceleration, and using Newton's laws make deductions about the forces applied to these objects. An essential assumption of this way of thinking is that space and time itself remain constant. In other words, Newton's laws are concerned with motions and forces with respect to a fixed underlying space.

    The "real" universe isn't quite that simple. In order to fully appreciate the expansion of the universe it is necessary to let go of Newton's world view and adopt a more general one, such as the one provided by Einstein's theory of general relativity. This theory allows for the concept of a universe in which space-time is expanding. It is a common misconception to think of the Big Bang as a great explosion in which all galaxies were blown out into space and have been receding from each other ever since. No, it is the fabric of space-time itself that is expanding, and with it the galaxies. If it were possible to drive two nails into this fabric, then over time the distance between them would grow, even though the nails remain motionless with respect to the underlying space. The classic example, of course, is the balloon with pennies glued to it. As one inflates the balloon the pennies move away from each other, with larger "speeds" the more distant they are, yet clearly the pennies aren't moving with respect to the balloon.

    In this expanding space-time it is thus possible to increase your speed relative to another object without feeling a force or an acceleration. The cause is the expansion of space-time itself. What causes this expansion of space-time? What is causing the recently discovered acceleration of this expansion? These are questions of deep significance and are areas of active research in cosmology.

  6. How do astronomers measure the distances to nebulae?

    The distance to a planetary nebula (such as the Ring Nebula) is difficult to determine. If the nebula is close enough, the trigonometric parallax of the central star can be determined. Another option is to compare the angular expansion rate of the nebula with its radial expansion velocity. Unfortunately, this depends on the assumed geometry of the nebula, which is not always well known. For more information on various distance estimates to the Ring Nebula, see

    Large samples of planetary nebulae can also be used as distance indicators to galaxies by taking advantage of the fact that they emit a significant fraction of their light (about 15%) in the Oxygen II (5007 Angstrom) emission line. For more information on this technique, check out

  7. There is often much speculation on the possibility of additional planets in our solar system. Is there a distance at which the sun cannot hold a planetary sized body in an orbit?

    A good place to start:

    I'll refer here to distances in these units: 1 AU (astronomical unit) is the distance from the Earth to the Sun --- about 93 million miles. 1 parsec is 3.26 lightyears, or 206,265 AU, or about 20 trillion miles.

    Astronomers understand gravity pretty well. As early as the 1840s, they noticed that the orbit of Uranus seemed to be influenced by the gravity of a big object further out. This led to the discovery of Neptune. With our current extremely-high-precision measurements of all orbits in the Solar System, we'd definitely notice if there were any more major planets beyond Neptune.

    That being said, there are many objects beyond Neptune (30 AU) that aren't that big. Perhaps the most famous is the object that might be a planet, Pluto. Pluto is likely just a relatively-massive representative of a whole group of objects called the Kuiper Belt. Another Kuiper Belt object that recently got a lot of press is Quaoar. The Kuiper Belt and Oort Cloud are thought to be "leftovers" --- debris that has drifted around since the formation of the Solar System. The Kuiper Belt may extend out to 100 AU from the Sun, give or take, and our understanding of the Oort cloud is even more uncertain. Perhaps it extends to about 50,000 AU as Jan Oort first hypothesized. Our present observations are only beginning to penetrate beyond 50 AU, so many more Quaoar-like rocks could exist out there --- but if there was anything really big like a "Planet X" we would notice its gravitational influence.

    The nearest stars to the Sun tend to be a parsec or so away, so in general things that orbit the Sun at a distance of a half a parsec, about 100,000 AU, would "feel" the gravity from another star about as strongly as the gravity from the Sun. So in that sense, the gravitational extent of the Sun essentially ends where the gravitational influence of other stars picks up, around a half a parsec out. So at its very largest, we would expect the Oort cloud to fizzle out around 100,000 AU, perhaps blending in with an Oort cloud that surrounds a neighboring star.

  8. Why does Saturn give off more heat than it receives from the sun?

    Objects in the solar system give off energy through a few basic means. The sun is powered by nuclear fusion at its core. Many of the other objects will then absorb the solar radiation given off by the sun and then re-radiate it into space.

    Saturn, however, gives off more energy than can be accounted for from solar radiation. This energy comes from the physical process of gravitational collapse. Saturn is still involved in this process, such that its physical size is undergoing contraction. The contraction heats up Saturn, therefore more energy is released than it receives from the sun alone.

    The process of gaining energy from gravitational collapse can be understood from a simple analogy. Imagine a tennis ball, originally at rest, that is dropped from some height above the ground. After some time, the tennis ball will have obtained some velocity because of the gravitational pull of the earth. That is, the gravitational field of the earth has converted its potential energy into the movement (kinetic) energy of the tennis ball. Now instead of a tennis ball, imagine a particle of gas in the same situation. In the atmosphere of Saturn there will be a huge number of gas particles undergoing this process. As these particles fall, they will have an increase in kinetic energy and will scatter off each other. The end result is that the gas becomes heated, and some of this energy is radiated into space.

  9. Is it possible for there to be moons around moons within our solar system? That is, can the Earth's moon have an object orbiting it, for example?

    Moons in the solar system seem to exist on a wide range of scales. Even some asteroids have small orbiting bodies (click here for a picture). However, while it is possible for an object to be a stable orbit around our moon, (Mike Collins orbited in a spacecraft during the Apollo 11 moon landing) there is no evidence for such a body. If such an object existed it would probably have to form while the moon was forming. In order for fully formed moon to capture another object there would have to be a mechanism for dissipating the orbital energy of the incoming body. Large, partially gaseous objects such as Sun or Jupiter can dissipate this energy via internal friction. Our moon is not large enough for this process to occur. In the absence of such a mechanism, the asteroid would just fly by. Orbiting space-crafts have to put on the brakes in order to slow down to the correct orbital speed. Another possible formation scenario for a moon involves an impact large enough to eject material. If the ejected material formed a ring around the planet this material could subsequently coalesce into a moon. This is one of the leading theories for the formation of our moon; if a large body collided with the earth while it was still young, the ejected material could have gathered together to form our present day moon.

  10. Does the moon rotate in exact synchronization with the earth? Or is there a sufficient difference so that over time we begin to see new areas of the moon?

    To put it into a nutshell, the moon doesn't rotate in exact sync. with the earth, so we see slightly more than one face of the moon, but we won't be able to see new areas of the moon.

    Let's call the difference between the moon's rotation and the orbit motion of the moon a libration. Several different forces come into play to determine libration: the eccentricity of the orbit (that is, the departure of the orbit from a perfect circle), the permanent quadrupole moment of the moon (that is, permanent bulges or departure from sphericity), and the tides that the earth raises on the moon.

    Thus the behavior of libration is sort of like a pendulum, with some dampening and some short-period terms. As a result, the libration period is 2.86 years for the moon, but the amplitude of libration is only 15 arcsec and is too small to detect.

  11. Why don't gravitational forces cause star clusters to collapse?

    Gravitational forces don't cause most star clusters to collapse for two reasons: Conservation of angular momentum and conservation of energy. Conservation of angular momentum means that unless a particular star is moving exactly toward the center of the cluster (which is unlikely), it will pass some minimum distance from the cluster center and then continue moving toward the opposite edge of the cluster. Conservation of energy means that as a star falls toward the center of the cluster, it speeds up. This means that when it gets close to the center, it has enough speed to continue to toward the other side and reach the same distance from the center that it started at. This is what most stars in a cluster are doing: either oscillating through the center of the cluster or moving on more or less circular orbits around the center. Stars are small compared to the volume of a star cluster, so they almost never hit each other.

    However, some star clusters are actually experiencing core collapse. Hubble Space Telescope recently discovered black holes in the center of two globular clusters: M15 and G1. One black hole is about 4,000 times more massive than the sun; the other is about 20,000 times more massive than the sun. These black holes are most likely due to the core collapse of the clusters.

    There are two types of star clusters: open clusters and globular clusters.

    An open cluster usually contains a few hundred loosely bounded stars. Each star in an open cluster moves around the center. The mutual gravity among stars in an open cluster is too weak to collapse the cluster. Instead, some stars gradually escape from open clusters. An open cluster will usually disappear in a few million years.

    A globular cluster is a tightly bounded cluster of about 100,000 to 1,000,000 stars. A globular cluster is very stable and has a long life of about 12 billion years, although it may undergo a process of core collapse. Stars in a globular cluster are attracted to each other by mutual gravity. Meanwhile, each star also moves around the center. Stellar collisions can happen in the core of globular clusters, but the rate is extremely low.


Thanks to Alex McDaniel, David Lai, Shawfeng Dong, Gabe Prochter, Ian Dobbs-Dixon, Jay Strader, Justin Harker, Karrie Gilbert, Kyle Lanclos, Laura Langland-Shula, Lynne Raschke, Marla Geha, Michael Kuhlen, Nick Konidaris and Scott Seagroves

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