Astronomy/limiting magnitude


Dear Courtney

I have measured the limiting magnitude of a star in the NGC 869 cluster with a B and a V filter. I have noticed the difference in values between the two filter. At 60s, I have 12.37 for the B filter and 11.067 for the V filter. Does this mean that the star is blue hot? I'm not sure I fully grasp the concept of limiting magnitude.

Limiting magnitude represents the faintest thing you can see with the instrument you're using and current observing conditions. For instance, "normal" limiting magnitude for naked-eye viewing in a very dark sky is about 6 or 6.5. Binoculars used in a very dark sky typically have limiting magnitudes of 9 or 10. Small to moderate size telescopes used visually (that is, without a camera of any sort) generally have limiting magnitudes of 12 to 14. So I presume you're using a medium-size telescope, or a smaller telescope and a camera.

The numbers you list are called the apparent magnitude (as opposed to absolute magnitude, which is how bright the star would look if it was 10 parsecs away). The difference between the two values (B - V) is called the color index, and can be used as an estimate of temperature within a certain range of values. Positive values represent redder, cooler stars, while negative values represent hotter, bluer stars. Your values imply a B-V value of 1.3, which corresponds to a temperature of about 3800 Kelvins. That's on the cool side (the Sun is about 5800 Kelvins), so you were looking at a pretty cool, relatively red star.


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Courtney Seligman


I can answer almost any question about astronomy and related sciences, such as physics and geology. I will not answer questions about astrology and similar pseudo-scientific rubbish.


I have been a professor of astronomy for over 40 years, and am working on an online text/encyclopedia of astronomy, and an online catalog of NGC/IC objects.

Astronomical Journal, Publications of the Astronomical Society of the Pacific (too long ago to be really relevant, but you could search for Courtney Seligman on Google Scholar)

I received a BA in astronomy and physics and a MA in astronomy, both from UCLA. I was working on my doctoral dissertation when I started teaching, and discovered that I preferred teaching to research.

Awards and Honors
(too long ago to be relevant, but Phi Beta Kappa and Sigma Xi still keep trying to get me to become a paying member)

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