Answers to some Astronomical Questions

A collection of questions and answers which I've collected over the years, in no particular order.

Will a nearby supernova kill all life on Earth?
I worked up this response a few years ago; thanks to David Palmer and Robert Spiker for their help in putting it together. I've also tried to calculate how frequently a star will explode close enough to the Earth to be dangerous. That document is in Postscript, I'm afraid; there are too many equations for HTML or plain ASCII text.

What is the evidence for ice on the moon?
There's information from the Lunar Prospector mission, and from the Clementine spacecraft.

What are some limitations on distances derived from heliocentric parallax?

How are supernovae classified?
available in HTML table format (thanks to Jim Benet!), or ASCII text format.

Is there evidence for periodicity of impact events on the Earth?

What do we know about Martian meteorites,
and how do we know it? A very nice summary by James N. Head of the Lunar and Planetary Lab.

Is it good for astronomers not to fall from observing ladders?
A short rant in which I warn against the increasing distance of professional astronomers from the acquisition of their data.

Why aren't CCDs the answer to every astronomical imaging application?
A comparison of some astronomical detectors by Brian Skiff. Originally posted to sci.astro.amateur.

Are quasars really at distances indicated by their redshifts?
or The Redshift Controversy; a discourse by Bill Keel of the claim that (some) quasars aren't at distances indicated by their redshifts. Originally posted to sci.astro.

What is the format of a simple 16-bit integer FITS file?

What is the "dust" in space?
An essay on the evolution of dust in the ISM, by Amara Graps. Originally posted to sci.astro. Latest update: July 29, 2000.

What sort of telescope should I buy as a Christmas present?

What does a radio galaxy look like?
A brief summary by Martin Hardcastle, originally posted to sci.astro.research.

Is it possible to detect supernovae with neutrino experiments?
Alex Habig explains the process by which this might be possible, in a note originally posted to VSNET.

Does the Cassini spacecraft present a significant health risk?
No, but please do read Jeff Cuzzi's analysis, originally posted to sci.astro on September 16, 1997.

What type of film is best for astrophotography?
Thanks to Michael Covington (I enjoy my copy of his book).

Why are there rings around the remnant of Supernova 1987A?
Read Phil Plait's fine description and find out.

Why do astronomers measure distances in parsecs?
My answer was originally posted to sci.astro on Jan 29, 1998.

Why don't Moths and astronomy mix?
To see why, read a description of moth-related problems at the Apache Point Observatory. Or take a look at this picture, taken in June 2003.

What is the "L" class of stars?
From a sci.astro posting by Neill Reid.

What is Dark Matter?
A brief answer from a posting to

What's the relationship between visual and photoelectric/CCD magnitudes?
Arne Henden supplies a brief answer in this posting to VSNET.

Why do astronomers think there are supermassive black holes at the centers of some galaxies?
Martin Hardcastle explains, in a message originally posted to sci.astro.

What's the difference between GMT and UT and all those other timescales?
Paul Schlyter explains, in a message originally posted to sci.astro.fits.

Are there any upcoming high-energy space satellite missions? What about the "Swift" mission?
David Palmer gives a nice summary of the "Swift" mission, scheduled for launch in 2003, which will detect GRBs and locate them precisely.

Where can I find information about a particular star?
Brian Skiff provides a guide to some excellent on-line resources (from a post to VSNET-chat, Sep 10, 2001). But do read his warning in the next item ...

Can you always trust those giant astronomical databases?
Brian Skiff shows that sometimes, just occasionally, one needs to take the results of a query with a grain of salt ....

Some comments on astrometry with recent all-sky surveys
Brian Skiff rates several of the all-sky surveys.

How have astronomers mapped the sky at different wavelengths?
Ned Wright's NGSS proposal has a very nice figure showing a number of recent surveys, with their sensitivity and wavelength.

Why do some stars pulsate?
Toshihito Ishida outlines the kappa mechanism briefly , from a message originally sent to vsnet-chat, May 19, 2000.

Why are there tides?
My short answer , with pointers to some more detailed explanations. Originally mailed to a curious student, Oct 6, 2000.

How do globular clusters stay spherical?
My short answer , which points out the key is lack of dissipation in stellar systems. Originally posted to sci.astro on Dec 10, 2000.

Is light from the distant universe scattered, or not?
Radiation from the CMB is, but light from distant stars is not. Originally posted to sci.astro on Dec 10, 2000.

Can we see Apollo hardware on the Moon?
The short answer is "no", but there are some pictures taken from spacecraft in orbit around the moon which show evidence for the Apollo hardware. Based in part on a posting to sci.astro by Alex Blackwell of the University of Hawaii, Jan 3, 2001.

How can I figure out the appropriate exposure time for my CCD astrophotos?
Try using Michael's signal-to-noise calculator

Which colleges and universities offer undergraduate programs in astronomy?
Check out Tom Balonek's compilation of undergrad programs in astronomy

Why do the gas giant planets (Jupiter, Saturn, Uranus, Neptune) emit more energy than they receive from the Sun?
Marshall Perrin of UC Berkeley posted a short answer to sci.astro which includes some good references: read Marshall's explanation

Where can I find information on a particular star?

Who invented the term "parsec"?
A discussion appeared in sci.astro in April 2002. The short answer is probably Herbert Hall Turner, in 1913.

How many measurements does it take to determine an asteroid's period?
Kim Lang provides some good advice. Originally posted to the Minor Planet Mailing List in Aug 2002.

Why do AM radio stations fade out (or in) at night?
David Knisely explains it all. Originally posted to sci.astro in January, 2003.

What are the first results of the Microwave Anisotropy Probe (MAP) mission?
Ted Bunn gives a summary. Originally posted to sci.astro.research in February, 2003.

How faint a star can a person see?
Brian Skiff provides some information Originally posted to sci.astro.amateur some years ago, and then re-posted in vsnet-chat in February, 2003.

What is "differential" photometry?
Arne Henden describes the basic photometric techniques. Originally posted to CCD-astrometry-photometry on Nov 6, 2003.

What would happen if 10^(75) ergs were released in intergalactic space?
This was a particularly interesting question which came my way as part of the MadScientist Network . You can read the full question and my take on the answer, though of course there are many, many other ways to analyze the situation.

How can I avoid common mistakes while doing astrometry?
This question came up on the Minor Planet Mailing List and it seemed appropriate to save it here for future reference...

How dangerous is a chunk of weapons-grade uranium?
Originally posted to the Bad Astronomy forums on Jan 25, 2007.

Why doesn't dark matter accumulate at the center of the Sun?
Originally posted to the Bad Astronomy forums on Mar 3, 2007.
Another good source of information is Phil Plait's Bad Astronomy page.

Last modified Mar 3, 2007
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