Ask An Astronomer 
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.
The most important things acting on the comet and its tail are the gravitational field of the sun, the solar wind (which is a flow of fast-moving charged particles such as protons out of the sun), solar radiation (photons from the sun hitting the comet and particles in the comet's tail) and the gravitational field of the comet itself.
The gravitational field of the sun just causes the comet to orbit the sun. The solar wind and solar radiation "blow" the comet's tail so that it always points away from the star. Comets are commonly referred to as "dirty snowballs," meaning that they are mostly ice with some dust and other "dirt" mixed in. The part of the comet's tail that we can see is mostly due to the sun's light reflecting off of dust in the comet's tail.
In principle there could be a stable "atmosphere" around a comet if the comet's gravitational field were strong enough to hold on to water and dust. However, comets are very small (typically a few kilometers in diameter), and it turns out that their gravitational field is just too small to hold on to any material that's not physically stuck to the comet.
To be a bit more quantitative, the escape velocity from the surface of a comet (how fast something needs to be moving to leave the comet and never return) is about 1 meter per second, around 2 miles per hour. So, if you were standing on the surface of a comet, you wouldn't find it too difficult to just pick up a snowball and throw it straight up so that it never came back. In fact, you could probably bend down and jump, and you'd exceed the escape velocity and therefore float off into space and never return to the surface of the comet. (Such a thing happened to an astronaut in the movie "Deep Impact.")
Now, an object's temperature is one way of measuring the average speed of the particles that make up that object. Hot objects are made up of very energetic, fast moving particles, while cool objects are made up of sluggish, slowly moving particles. In order to make the average speed of water molecules faster than 1 meter per second, you have to have the gas warmer than about 0.001 degrees Kelvin, very close to absolute zero (this is about -459 degrees Fahrenheit). This is extremely cold, and a comet in the solar system will never get this cold, even when it's far from the sun.
The upshot of all this is that as soon as the sun heats up the comet enough to cause some of the ice to turn to water vapor, these water molecules are moving fast enough to completely overwhelm the gravitational field of the comet and escape. Then the dust and water vapor are blown back by the solar wind and solar radiation so that the comet's tail points away from the sun.
So no, there is no point where the comet's gravitational field balances the sun's radiation pressure. But this is only because comets are typically so small and therefore have weak gravitational fields. If comets were much more massive, there would be such a point. A good example of this is the Earth, with gravity strong enough to hold on to a fairly thick, warm atmosphere.
SOHO, the SOlar and Heliospheric Observatory, is an international collaboration between ESA (European Space Agency) and NASA, with many scientists at various institutions throughout the world working together. This is not to be confused with Soho, a famous part of London.
SOHO orbits L1, a point in space about 1.5 million kilometers towards the Sun, at which the combined gravitational force of the Sun and Earth equals the force necessary to orbit the sun at the same rate as the Earth. Orbiting this Lagrange point, SOHO remains more or less fixed with respect to the Earth and Sun and has an uninterrupted view of the Sun.
SOHO carries with it a number of instruments, twelve in all, used to study the interior and the atmosphere of the sun, as well as the solar wind and associated phenomena like Coronal Mass Ejections.
Researchers can learn about the solar interior by studying surface oscillations of the solar disk, just like we can learn about the interior of the Earth by studying earthquakes. The interior structure of the Sun determines what types of oscillations make it to the surface and their relative strengths. By matching oscillation observations with results from theoretical models we can gain insights about parts of the Sun that remain hidden to direct observation. GOLF, VIRGO, and SDI/MDI are instruments on SOHO concerned with these types of measurements.
Although the solar atmosphere, or corona, is much hotter than the visible surface of the Sun, it is also much less dense. As such it is impossible for us to see the corona, except during spectacular but rare full solar eclipses. SOHO doesn't wait for natural eclipses, which occur only every 18 months, and beyond that, SOHO is beyond the moon's orbit. Instead SOHO provides its own eclipses by employing a so-called occulter disk, which blocks the light from the solar disk. Alternatively it is possible to observe only in a narrow wavelength band at which the solar disk isn't too bright. SOHO also carries spectrometers which provide spectral information necessary to study the composition of the sun. SUMER, CDS, EIT are instruments designed to study the inner corona, and UVCS and LASCO can observe both the inner and outer corona. Using these instruments SOHO scientists can measure the temperature, density, composition, and velocity of the corona.
Several of the instruments on board SOHO perform local measurements of the solar wind. CELIAS, COSTEP, and ERNE capture solar wind ions and other energetic particles released by the sun, and measure their electric charge and isotopic abundance ratios.
Coronal Mass Ejections are powerful ejections of charged matter from the atmosphere of the sun. These days, during the maximum of the solar activity cycle, the sun produces about 3 to 4 CME's per day. Most of them don't hit the Earth, but those that do can produce marvelous Northern Lights in addition to occasionally knocking out urban power grids and endangering satellites and other spacecraft. SOHO has proved to be an excellent tool for early detection, study of, and to some degree prediction of CME's.
EIT, the Extreme ultraviolet Imaging Telescope, provides images of the full solar disk or sub-fields at four bandpasses centered around common ultraviolet transitions (Fe IX/X @ 171 A, Fe XII @ 195 A, Fe XV @ 284 A, and He II @ 304 A). EIT images active regions, filaments and prominences, coronal holes, coronal "bright points," and polar plumes. LASCO, the Large Angle Spectrometric COronagraph, is the most useful instrument for detecting CME's. Its wide field of view and occulter disk allows it to take full images of the corona from 1.1 to 32 solar radii.
A CME that is seen in profile, as in the many beautiful pictures shown at SOHO's website (http://sohowww.nascom.nasa.gov/) will not hit the Earth, as the material is ejected perpendicular to the line of sight. CME's ejected along this line of sight, either towards or away from us, appear as so-called halo events. In such an event the gas will spread out wider and wider on the field of view, eventually engulfing the solar disk and appearing as a halo. To break the degeneracy between approaching and receding line of sight CME's EIT comes in handy. A CME that was ejected from the near side towards us passes through the solar atmosphere and causes disturbances that can be picked up with EIT. Using both LASCO and EIT it is possible to identify earthward heading CME's and to subsequently ring the alarm bells.
In addition to these early detection efforts, the Coronal Diagnostics Spectrometer (CDS) has been used to try and predict CME's. The following is from a IAU Press Release (3 August 2000): "[...] on 25 July 1999, CDS saw a magnetic loop rising through the atmosphere at 10 km/s for two hours before a CME, apparently carrying with it the gas that provided the mass of the mass ejection. Then a magnetic explosion occurred, releasing the CME and also causing a flare."
For much more information on SOHO and excellent pictures and movies visit the SOHO homepage. As an added incentive to visit, the planet Venus is visible in images taken with LASCO until February 14. Feel free to make your own comment about the goddess of love leaving on Valentine's Day. See Venus here.
Many of the "Own-your-own or Name-your-own" star companies seem to report slightly inaccurate coordinates for their stars, though it shouldn't be a problem to obtain accurate coordinates. The stars "sold" are more-or-less randomly chosen from a list of catalogued stars, and the names/ownerships are not recognized officially or legally by any professional astronomical organization or government agency. (So don't try and collect royalties if the Hubble Telescope images "your" star!) Still, a lot of people enjoy the gift, and there is absolutely no harm in it.
You can find pictures of these stars at the online Digitized Sky Survey -- pictures of the entire sky that have been scanned in to digital format. Go to the Digitized Sky Survey website and enter the coordinates. You may have to switch the "J2000" button to "B1950," depending on which coordinates your star registry gave you. Be sure to change "File Format" from "FITS" to "GIF." Chances are, your star is the brightest one in the field.
Lick Observatory does not take pictures of "your" star for people (unless you have several million dollars you'd like to donate), and it's appearance certainly has not changed since the sky survey images were taken. Re-taking the image would be a waste of the observatory's very limited resources.
Before "buying" a star, please read the International Astronomical Union's position on the naming of stars. Remember, you are buying a novelty gift. What you get (often a star chart with "your" star circled, a certificate with the star's name and coordinates, a spot in the company's records as owning a particular star, and perhaps a booklet with quasi-factual information about stars in general) is all that you are getting for your money.
In the modern Universe, gravity is the main (often only!) force shaping the large-scale structure of the universe (galaxy-sized objects, groups of galaxies, and clusters of galaxies). The expansion of the universe wants to push galaxies further apart, but if enough mass is in a small enough area, gravity can stop the expansion of the universe in that region. This is why the planets are not moving away from the sun, why the galaxy doesn't fly apart, and why the Local Group of galaxies (the Milky Way, the Andromeda Galaxy, and several smaller galaxies) stays together.
Another question some people have is whether the "dark energy" of the universe, the "cosmological constant" that may be causing the expansion in the universe to accelerate, will result in our galaxy getting ripped apart in the future. Again, gravity wins, and everything up to our local group of galaxies will remain together in the future. There are larger structures in the Universe bound together by gravity, such as clusters of galaxies, where the local expansion of space has ceased. Superclusters of galaxies are known to exist, and these have enough gravitational pull to eventually overcome the expansion of the universe, but they haven't overcome the expansion yet.
The nearest cluster of galaxies is the Virgo cluster of galaxies. Currently, the Virgo cluster is receding from us at a rate of about 1200 kilometers per second. If there is no cosmological constant, the gravity of the Virgo cluster would stop this expansion, and the Local Group of galaxies would fall into the Virgo Cluster in the very distant future. But with the possible strength of the cosmological constant, it is no longer certain if the gravitational pull of the Virgo Cluster is strong enough to stop the Local Group of galaxies. If this is the case, then in the very distant future, the acceleration of the universe will push all the galaxies except for those in the Local Group beyond our sight, and we'll be left alone in a much duller Universe.
Groundhog Day is indeed an astronomical holiday. First, let's remind ourselves of the legend. If a groundhog sees its shadow on February 2, there will be six more weeks of winter. Now, let's look at the calendar. There are 13 weeks in winter, and winter started 6 1/2 weeks ago. So there are 6 1/2 weeks of winter left... suspiciously close to the 6 weeks of winter the groundhog claims are left.
In reality, February 2 is a "cross-quarter" day, a day halfway between a solstice and an equinox. There are three other cross-quarter days in a year; all of these have (or had) celebrations associated with them. May Day and Halloween are the remaining well-known cross-quarter days; the final one is "Lammas" at the start of August.
So, for February 2, groundhogs become honorary astronomers. Now, if we could only get the day off from work...
Normal groundhog on the
astronomical holiday of February 2.
Punxsutawney Phil
, the "Carl
Sagan" of groundhog-astronomers.
Thanks to Marla Geha, Mike Kuhlen, and Greg Novak for helping to answer these questions!