Ask An Astronomer 

Answer Newsletter

29 October 2001


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. Can people who are color blind be astronomers?

    Yes, color blind people can be astronomers. We very rarely do any work involving color photographs. The pretty pictures that you see from the Hubble Space Telescope are actually a compilation of black and white photographs taken through different colored filters. An artist then combines the pictures to create an aesthetically pleasing picture that, more-or-less, reproduces what colors the eye would see.

    When we investigate an image scientifically, though, we use the black and white images taken through a single colored filter. We can combine information from images taken through different filters to create a number that we call a "color," but this research work does not involve looking at actual color images.

    One example of this comes from work I am doing. I am looking for the remnants of dead stars called "white dwarfs." White dwarfs start off as very hot, very blue objects. To find them, we take a picture of a cluster of stars through a green filter and a second picture through a blue filter. We then calculate how bright each star is in each image. Hot white dwarfs are very blue, so they appear brighter in the blue-filter picture than in the green-filter picture. If you were to place the two black-and-white images side by side, you can see that the white dwarfs are brighter in the blue image with your own eye. But I never create a color picture to look for blue stars; I just use the numbers to tell me if a star is blue or not. It works very well!

  2. The Drake equation predicts the number of extraterrestrial civilizations in the galaxy. The equation requires knowing the odds of many different events: the frequency of solar systems, the number of solar-systems with Earth-like planets, and so on. Now that we are learning about planets outside our solar system, how has our understanding of the equation changed?

    The Drake equation you refer to is named after Frank Drake, who happens to be an emeritus professor here at Santa Cruz. There is a nice explanation of the Drake equation and the meaning of it available here . Essentially, the equation allows you to estimate the number of technologically advanced civilizations in our galaxy by estimating a series of individually (somewhat) more tractable quantities. In particular, the Drake equation requires you to estimate the number of stars that have planets around them and the number of earthlike planets in each system that does have planets.

    It is true that in the last five years, the study of extrasolar planets (ESPs) has gone from total speculation to an active field of research. Observers are rapidly building statistics about how many stars may harbor planetary systems. Further, the currently known systems allow theorists to test their ideas about how the solar system may have formed, or glean detailed and surprising insights about the past history of known systems.

    Current research allows us to get a much better handle on the number of stars that have planetary systems. Current searches suggest that 6-8% of stars similar to the sun harbor giant (Jupiter mass) planets closer than about 4 AU from the star (An Astronomical Unit (AU) is the distance from the earth to the sun, about 93 million miles). Current detection methods are not precise enough to find planets with masses significantly smaller than Jupiter, and planets far from the star cannot be seen since scientists haven't been taking measurements for long enough for the planet to complete an entire orbit.

    However, the answer is not so simple. Researchers must select candidate stars to be observed for the presence of planets, and they must take many things into account. The star in question must not have much activity in its atmosphere so that the precision measurements required to reveal the presence of planets can be performed. This limits observations to fairly old, low mass stars. Scientists also often select stars with high iron abundances because planets seem to occur more frequently around such stars. What people know now, then, is that giant planets occur about 7% of the time around nearby stars of a very specific type.

    The next factor in the Drake equation is the number of earth like planets (with a mean temperature close to Earth's, thought to be necessary to sustain life) occur in systems that have planets. Current observations offer very little insight into the answer to this question since current, methods cannot detect earth mass planets (Giant planets are thought to be incapable of supporting life because they have no surface, and conditions get quite inhospitable very quickly (high pressure and temperature) as you descend into their atmospheres).

    What we can say is that the current crop of extrasolar planets are unlikely to harbor life. This is because they typically have very massive (several Jupiter masses) planets on very eccentric orbits. Researchers have numerically solved the equations of motion for earth mass planets in such systems and find that a large fraction (in some cases all) of the possible orbits are unstable. This means that the small planet would collide with the large planet or the star, or else be ejected from the system, before life had a chance to form. However, the future looks promising. There are many long term projects underway that will hopefully be able to detect earth mass planets, and even obtain spectra of these planets. The spectrum of a planet can tell you all kinds of interesting information about the chemical composition of the planet's atmosphere and even whether or not there are active biological processes going on on the planet.

    There are many interesting web sites devoted to extrasolar planets:

  3. Are there stars that are not in galaxies? If so how do you classify them?

    We believe that all stars were formed inside of galaxies. However, over time, it is possible for a star to be ejected out from it parent galaxy. This is a very rare occurrence for our own Milky Way. In regions with a very high density of galaxies- a galaxy cluster- ejection may happen more frequently. I do not know of any isolated stars in the regions near the Milky Way, but there are discoveries of free floating stars in galaxy clusters. These objects are identified as stars from their spectrum.

  4. Why are the moon's rotation and revolution periods the same? It is unusual that they are the same is there any explanation to it or is it just by luck?

    The time it takes the moon to rotation on its axis and to revolve around the earth are the same. That is, the moon's 'day' is equal to it 'year'. It is not a coincidence that these motions are the same, the phenomenon is called "tidal locking". The moon's motion did not start out this way, but has been slowly forced into this situation by the gravitational force of the earth. This is also the reason why we only see one side of the moon.

    Many moons of Jupiter and Saturn also exhibit tidal locking. In fact, the planet Mercury is also tidally locked to the sun, although its case is slightly different --- Mercury's rotational period is exactly two-thirds the length of its revolution around the sun ("year").

    Tidal locking occurs because one body (say, the Earth) exerts a tidal force on another body (say, the moon). We see the effects of tidal forces in our own oceans, as the moon's tidal force on the Earth causes our daily tides. The tidal force tries to speed up or slow down the rotation of the other body until the period of rotation and the period of revolution match. The closer a body is to a much larger body, the greater the tidal forces. This is why Mercury is tidally locked to the sun, but the Earth isn't.

    The moon, in fact, is slowing the Earth's speed of rotation, as its tidal forces try to do the same thing to the Earth. The length of a day in the time of the dinosaurs was only 23 hours! So if you, like me, wish you could have an extra hour in the day, just wait another 100 million years or so, and the length of the day will be 25 hours.

    Another good source of information on tidal forces with deeper explanations of the cause of the tidal force is Phil Plait's Bad Astronomy website.

  5. If sound does not travel in space , how do we expect to recieve raido waves from other planets?

    Sound waves are pressure waves travelling through a medium. We are most familiar with sound waves travelling through air, but sound waves also propagate through water, even through solids. A sound waves is made up of a sequence of alternating compressed and rarefied regions of the medium it is travelling through. A "middle A" has a frequency of 440 Hertz, which means that every second 440 of these waves hit your eardrum.

    Sound waves, however, are not the only types of waves that we know of. Some examples of waves that are not sound waves are: ocean waves, gravitational waves, and electromagnetic waves. Of these the first requires a medium, just like sound waves. Ocean waves travel on water. (It wouldn't make sense to talk about an ocean wave in space!) But not all waves require a medium to travel through. Electromagnetic waves are one example of these types.

    Electromagnetic waves are oscillations in electric and magnetic fields. Electric (E) and magnetic (B) fields can permeate space without a medium to support them. This is strange but true -- electromagnetic fields can exist in a vacuum. It's true that E- and B-field can penetrate things such as air, water, or brick walls, but these materials are not necessary for electromagnetic waves.

    What are some example of electromagnetic waves? Well, light is one example. Blue light has a frequency of roughly 700 TeraHertz. What this means is that every second the E- and B-field oscillates about 700 trillion times. We know that electromagnetic waves can propagate through a vacuum because we can see planets and stars at night time. The light originating from far away stars had to travel through interstellar space, which is a vacuum, for many years before it reached our eyes or telescopes here on Earth.

    Now radio waves are just another form of electromagnetic radiation. To be more precise, radio waves are just "light" with a much smaller frequency. When you listen to FM radio, you're tuning in stations that are being broadcast at anywhere from 88 to 108 MegaHertz. So this means that the E- and B-fields are oscillating 88 to 108 million times a second. That's still awefully fast, but much slower than optical light.

    So, here's the summary. Sound waves can't travel through space, because they require a medium to carry along the compressions and rarefactions. Electromagnetic waves, however, of which radio waves are one example, can travel through space because they are made up of oscillations of electric and magnetic fields which have no problem permeating a vacuum. So, if E.T. tried to send a sound wave to us using a giant speaker, nothing would happen. There is no material in space to carry the sound wave! But if E.T. sent a radio wave, it could travel across all of space toward us. This is why searches for extraterrestrial life use radio waves.

  6. I read an article in our local newspaper about an asteroid, 1997 FX11. The prediction is that it will pass close to Earth in late 2028. Do you know anything about it?

    There was a lot of news many years ago (and a few times since) about an asteroid that might hit the Earth in the near future. Our page on asteroid impacts talks about the chances of an asteroid impact and what it would do to the Earth. In short, though, there is no known asteroid that is going to impact the Earth in the foreseeable future.

    A big discussion among astronomers is how to deal with announcing potentially dangerous asteroids. In the past, when an asteroid was discovered, it would be announced in a telegram that went to other observatories. The purpose of the telegram was to alert astronomers to the discovery and call for follow-up observations. When an asteroid is first discovered, its orbit is poorly known. Follow-up observations permit its orbit to be calculated much more accurately.

    When an asteroid is discovered that orbits near the Earth, the "error ellipse," or uncertainty in the orbit, often includes the possibility that the asteroid will hit the Earth within a few hundred years. Including this tidbit of information in the discovery telegram has encouraged enough follow-up observations to permit the orbit to be tightly constrained, an in each case the asteroids that seemingly had the potential to hit the Earth will pass safely by.

    Unfortunately, these telegrams had often been read by the media and misinterpreted to say that the Earth was going to be hit by the asteroid. Certainly none of the astronomers intended to scare people, they only wanted the asteroid to be checked out to insure that the Earth was safe.

    More recently, astronomers have started keeping their information more private until the orbit of the asteroid is confirmed. This way, the public will not tire of constant false alarms (now numbering several a year!). Hopefully this method of confirming asteroid orbits will prevent us from losing public confidence while still following up potentially dangerous asteroids.


Thanks to Marla Geha, Mike Kuhlen, and Greg Novak for helping to answer these questions!

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