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The Question

(Submitted December 15, 1996)

I need some information on stars. What I need is the age and if it is binary or trinary or alone. The stars that I am interested in are 1. Deneb, 2. Rigel, 3. Betelgeuse, and 4. Antares. I have looked in our local library and could not find the information. I am in the 6th grade. Thank you very much.

The Answer

The stars you ask about are some of the brightest stars in the sky, so there is lots of information on them. The place I look for information like this is:

The Peterson Field Guide Series #15
Stars and Planets
by D.H. Menzel and J.M. Pasachoff
Copyright 1983, by the authors $12.95

You should be able to find it in your local library.

Most of the information you need is in table 1 on page 8 of the book mentioned above. For other information you might need the tables in back of the book or the sky charts.

However, ages of stars are harder to measure, because it is not something we can directly observe. We can measure directly a star's brightness. From its color we can measure its temperature (blue is hot and red is cold). Close stars appear to move very slightly in the sky throughout the year, because the Earth is going around the Sun. From this we can measure the star's distance. By observing binary stars' orbits, we can also measure their mass.

So, the book I mentioned will tell you much of this information, but not the age.

A few things to be aware of to understand the tables in the book:

  1. Brightness is measured in Magnitudes (the smaller the magnitude the brighter the star). This is how bright it looks to your eye. On a dark night in a remote area we can see with the naked eye stars of 6th magnitude or brighter. In a city, we can see stars of 1st, 2nd or 3rd magnitude or brighter. A star can be bright either because it is near by, or because it is really bright. All of the stars you mention are much brighter than the Sun (Sirius is 24 times brighter -- table A4). If someone lived on a planet near those stars, they would not be able to see the Sun with their naked eye.
  2. Temperature is measured in degrees Kelvin (= degrees Celsius + 273), or by color or by "spectral type". The spectra types are, from hottest to coolest, O B A F G K M. These have historically been remembered by, "Oh Be A Fine Girl/Guy Kiss Me." Later three types of strange cool stars were added as R, N and S for Right Now Smack. See table A-3 for a conversion between each of these.
  3. Mass is measured in "solar masses", so the Sun has a mass of 1 solar mass.
  4. Distance is often measured in light years (how long it takes light to travel in one year -- or about a Million years by jet airliner).
Now, back to your question of age. The more massive a star is the brighter it is, thus the faster it runs out of fuel. So, in general, the more massive a star is the shorter its life-span. So a star like Rigel must be quite young, as compared to our Sun (which is 4 billion years old).

In general, the exact age of one star is quite hard to measure. If there is a cluster of stars, you can assume they are all the same age, so we can find the age of all the stars. Even this is sometime hard to do too, because the exact age depends on many things, including the exact chemicals that make up the stars.

I hope this helps.

Jonathan Keohane
for Imagine the Universe!

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