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The Question

(Submitted September 30, 1997)

I am a 6th grader and I know a lot of the constellations by name and enjoy looking at the stars and sky at night. This past weekend while camping our Girl Scout Troop went star gazing in an open field. It was really dark and really beautiful.

I really like to look at stuff on the Internet about the Universe. I need some help in understanding how I might find specific information on the Sun, planets, or stars.

For example, I was wondering: "How hot is the sun?"; "What star might be hotter than the sun?"; and "What star might be cooler than the sun?". Is there someplace that I can find just facts such as temperature, size, distance from earth of stars?

The Answer

You are asking how to find answers to questions on astronomy, using the Internet. I will tell you how I would find out these answers.

First of all, the Internet is not always the best place to learn things. If you can go to a library and look through their books on the Sun and stars, you will probably find a book which explains most of what you want to know. If I wanted to answer the questions you asked, the easiest thing for me to do would be to grab some books from my shelf and say: "The temperature of the surface of the Sun is 5770 Kelvin which is 5,500 Celsius (or 10,000 F). Stars which appear bluish are hotter than the Sun, stars which appear reddish are cooler than the Sun. For example: Rigel, Vega and Sirius are blue (hot), Arcturus, Aldebaran, and Betelgeuse are red (cool)." These numbers and examples come out of an astronomy textbook.

But sometimes you can't get to a good library, sometimes the Internet is more practical or convenient. If I were looking for information on starfish instead of stars, all the astronomy books in my office wouldn't be much help. There is an encyclopedia in an office two floors down, and that might be the fastest way to learn things. But if I wanted to know a specific obscure question, such as 'are there were any poisonous starfish?', the encyclopedia probably wouldn't help me. I'd either go to the library, ask a biologist, or go to the net where, with enough digging, I would find that the 'Crown of Thorns' is the only known venomous starfish.

It is easiest to find things with search engines if you know the exact words or phrases commonly used. Searching for "list of stars" will be less useful than searching for "star catalog", and searching for "star colors" is less useful than "spectral classification". And you will not try to find "Hertzsprung-Russell diagram" unless you already know what it is. If you do not know the exact words that people use, a web directory (such as Yahoo) will often make it easier to find

than a search engine such as Alta-Vista.

There are many places to learn things on the Internet, if you can find them. You found one: 'Imagine the Universe'. We also have a site called Starchild (

The Astrophysics Data System is a good place for astronomy: It has star catalogs and other information. It also has a copy of the 'Handbook of space Astronomy & Astrophysics' by Martin V. Zombeck This is one of the (paper) books on my shelf that I often refer to. Most of the pages in it are very hard for anybody but an astronomer to understand. The book includes a list (a 'star catalog') of the brightest stars in the sky at ( and the following pages. The catalog includes distances (labeled 'd' and given in units called 'parsecs', each of which is 3.26 light years) and a quantity called 'B-V', which tells how blue a star is: numbers below 0.65 are bluer and hotter than the Sun, numbers above 0.65 are redder and cooler.

If you look at a Hertzsprung-Russell diagram you will often find them marked with both the spectral classification and the temperature. If you look in the 'spec' column of the table, you will see entries like 'A0 p'--the first letter means that the spectral class is 'A', and so the temperature is about 10,000 Kelvin.

As you can see, it is easiest to use the web to find out about astronomy if you are already an astronomer. But I hope my reply has been helpful.

David Palmer
for Ask an Astrophysicist

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