Frequently Asked Questions - And Some Answers

Ask your question to the address given below.

QUESTION: What are gamma ray bursts?

ANSWER: Gee, I'm glad you asked; our guys in x-ray astronomy just put up a page on these powerful waves of radiation that sweep through our galaxy, if not the entire universe.

QUESTION: What are the chances a close supernova could threaten life on Earth?

ANSWER: It seems likely that this may already have happened several times in the billion or so years of life on earth. There are several mass extinctions in the geologic record that do not bear the Iridium and soot signature of the asteroid impact in the Yucatan some 65 million years ago which seems to have caused the Cretaceous-Tertiary geological boundary associated with the demise of the dinosaurs. “Nearby” supernovae are a leading suspect for some of the other mass extinctions.

The Cygnus Loop is the remnant of a supernova only some 25 pc (80 Ly) away, which would have been seen on Earth 20,000 years ago, in the time of Cro-Magnon man. Everyone would have seen for many days a star of magnitude -17; a thousand times brighter than the full moon, but still a thousand times dimmer than the sun. The spectrum of a Type II supernova peaks in the UV and the earth would have received 1000 times the normal UV flux at the top of the atmosphere, leading to greatly increased ionization of the upper layers of the atmosphere. Yet, it is expected that little of this UV would have reached the ground and there is no evidence of massive die-offs. As the expanding remnant bubble reaches the solar system in a few tens of thousands of years, we may see a several-fold increase in primary cosmic rays.

The likelihood of a supernova going off within 25 pc is only once every few million years, and once every 100 million years within 10 pc. Cosmic radiation from such an event would increase the genetic mutation rate a bit for species with a short reproduction cycle and a lot for those with longer cycles, such as ourselves. Think of the monolith in the movie “2001” as an analogy for these events.

The Crab Nebula, first seen on July 4, 1054, is located some 2000 pc or 7000 light-years away. Eta Carina, a giant and unstable star, is some 3700 ly away. Fifty ly has been suggested as a survivable distance, although living in tunnels (or caves) might be a good idea for a while. There are no known red giants within 50 ly, so we may be safe from Type II supernovae for a while. Alas, Type I supernovae, some 10 times more powerful, are the endpoint in the lives of some accreting white dwarf stars, which could lurk nearby without our seeing them!

Further reading: Shklovskii, "Stars, Their Birth, Life and Death", 1978, Freeman; also, any modern textbook, eg. Seeds, "Horizons - Exploring the Universe" 1993, Wadsworth and “Foundations of Astronomy” 1992, Wadsworth.

QUESTION: What are the nearest and brightest stars?

ANSWER: Please see our lists of the nearest and brightest stars to, and as seen from, Earth. Perhaps surprisingly, these lists are not the same. This is because stars come in a wide range of brightnesses, there are many more dim ones than bright ones, and thus there are more dim ones closest to us.

QUESTION: When is the solstice, exactly?

ANSWER: The solstice is when the Earth reaches the point in its orbit when the Earth's spin axis is pointed closest to or fartherest away from the Sun. The north pole is closest at the summer solstice. Geocentrically, in the northern hemisphere, this is the time that the days stop getting shorter and begin to get longer, even though the latest sunrise and earliest sunset may occur a day or so before and after. In prehistoric times, this date was determined by seeing the furtherest south (for the northern hemisphere) sunset or sunrise, and this position was recorded at many archaeo-astronomical sites. You can still do this yourself! Incidently, I've had people ask exactly when the solstice occurs, and always am a bit concerned; sacrifices of any sort were never necessary to start the Sun edging back the other way along the sunrise or sunset horizon.

QUESTION: What was that bright light I saw in the West last night (or East this morning)?

ANSWER: Jupiter and Venus can be amazingly bright, depending on their orbital positions relative to the Earth. They stay so for weeks at a time, so look again. If it was moving slowly, it was probably an aircraft. If it moved quickly, you probably saw a meteoric bolide or fireball. You did make a wish, didn't you? A meteor coming right at you might not have much apparant motion, but would grow steadily brighter. Serious wishing time!

QUESTION: Are we going to be sucked into the black hole in relatively-nearby Cygnus X-1?

ANSWER: No, we are not being sucked into this black hole: it is no more a danger to us than any other star. While it is the closest known black hole, it is still some 1000 light years distant and no more dangerous than any other star at that distance. All matter in the universe exerts gravitational attraction on all other matter, no matter what form its in. In the words of Richard Harms: "Black holes may eat, but they can't hunt!"

QUESTION: How can I learn more about astronomy?

ANSWER: Check out your library for books, your newsstand or bookstore for magazines with current information and pretty pictures, your local astronomy club, the other WWW sites listed and the fine collection of astronomy education sites listed on accompanying pages.

QUESTION: What sort of telescope should I buy?

ANSWER: Have you considered the far more versatile binocular? 7x35 minimum, 10x80 maximum for hand holding. If you really want a scope, see the magazines and the clubs for data and prices.

This page, http://cass.ucsd.edu/public/faq.html, last updated May 8, 1996
Comments and especially questions via "mailto:" Bill Baity or contact Bill Baity, CASS, UCSD 0424, 9500 Gilman Drive, La Jolla CA 92093-0424, tel(619) 534-3209, wbaity@ucsd.edu