Gauge the temperature of a star by its color

Saturday, May 11, 2019 4:03 AM

Greetings, Stargazers.

Last month, I began a discussion of how stars are classified by their brightness and color, or more technically, by their luminosity and spectroscopic classification. I spent most of that column on the first of those features, and how that relates to how bright a star looks to us. The second major characteristic of a star is its color. Although stars come in many spectral types, most are so dim that our eyes just perceive them as a pale white.

Many of you know Roy G. Biv, but if you haven’t heard of him, this is just a simple way to remember the colors in a rainbow – red, orange, yellow, green, blue, indigo, violet. I must admit that I have often wondered how indigo squeezed into the list, but I have learned that suggesting the removal of indigo can be as emotionally charged as removing Pluto from the list of planets.

The color of a star (or any radiating object) is directly related to its temperature. They radiate at all wavelengths, but some are more intense than others depending on the temperature. To try to understand this, think of the heating element in an electric oven. When you first turn on the oven, the element is radiating heat, but you can’t see any light coming from it. That is because the radiation is in the infrared, at wavelengths too long for your eyes to see. As the element gets hotter, we can begin to see it glow red as some of the radiation becomes energetic enough to be in the red end (the lower energy end) of the visible spectrum. As the element gets even hotter and hotter, the color becomes more orange, and then maybe even yellow. (You might need to think of the filament in an incandescent light bulb, as your oven won’t get this hot.) This same effect is what we see in stars of different temperatures.

A question I often get is what happens after yellow – why don’t we see green stars? The answer is that all colors are being radiated all the time, just in different relative amounts. The red stars emit a fair amount of yellow, but hardly any blue. The yellow stars just happen to have a bit more red than blue. When it gets a little hotter than yellow is when all wavelengths are being radiated in approximately equal amounts, so what we see is white. When the star gets a little hotter than that, there is then more blue than red and we see the overall color as blueish white.

The label that a star gets that describes its color is called its spectroscopic class, and it is the primary indication of the star’s temperature. For historic reasons, the categories are usually listed from hot to cold, which would be from blue to red, and the categories have the peculiar letter designations of OBAFGKM for the different classes. The letters are from a somewhat archaic system that indicated the intensity of hydrogen as seen in the spectrum of the star. “A” stars had the most intense hydrogen lines, “B” stars the second most and so on. Most of those letter designations have been eliminated, and the order has been updated to that shown above. “O” stars are the hottest and “M” stars the coolest. The sun is a G-type star. If you want to remember the order of these spectroscopic classes, you can use the mnemonic “Oh, Be A Fine Girl/Guy Kiss Me.”

This month

The full moon this month is a blue moon (the real kind). It is the third full moon in a season that has four full moons. Most seasons have only three full moons, and this definition is much more restrictive than the popular one that says the second full moon in a month is a blue one.

Jupiter and Saturn are both near the Milky Way. Jupiter now rises before 11 p.m. and (other than the moon) will be the brightest thing in the sky until Venus comes up. Jupiter will reach opposition early next month, which should be the best time to view it through a telescope. Saturn follows Jupiter by a couple of hours, and Venus is now rising a little after 5 a.m. as the morning star.

This is the time of year to plan for some summer stargazing activities. Look for events at Chimney Rock National Monument and at the San Juan College planetarium. The Powerhouse Science Center is planning a “Chasing the Moon” activity on the afternoon of June 9 with at least one solar telescope set up.

Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Email him at hakes_c@fortlewis.edu.