UNIVERSITY OF COLORADO - COLORADO SPRINGS Star Temperatures Background Information ike most astronomical data, we get information about the temperature of the sun and other stars from analyzing the light that comes from them. In many cases, we can find the temperature at the surface of a star by its color. Red is cooler, yellow is warmer, and L blue is hotter. How do we know this? It comes from something called "blackbody radiation". Blackbody radiation connects the temperature of an object to the wavelengths of light emitted (different colors) from the object. Each temperature emits a different range of colors. Below is a picture of intensity (brightness) as a function of wavelength for different temperatures. In addition, the visible light spectrum shown at the appropriate wavelengths. Our Sun GENERAL ASTRONOMY LAB I I Before we start on color, let's figure out the temperature issue. The temperatures on this graph are given in degrees Kelvin (K) which is the most used scientific temperature scale, because 0 K is the lowest possible temperature. You can convert from Celsius to Kelvin by: Kelvin = Celsius + 273.15o Remember, Celsius is a scale where 0o corresponds to water freezing and 100o corresponds to water boiling (at sea level). Now we can look at the question of color. Look at the 5,000 K curve. It has a maximum at the yellow color, and clearly has some contribution from red and green. Yet, the overall color is yellow, which indicates our sun has a surface temperature near 5,000 K. So, we might imagine that at a higher temperature of 5,700 K, the peak would be in the green and we would see a green star (the Sun’s actually at 5,700 K). But it doesn't quite work that way, since there aren't any green stars. You will notice that all the colors are present under each of the black body curves, the height that the color reaches represent that colors intensity. Just because a curve peaks at a certain color does not mean that color is seen. You have to mix all the colors at their respective intensities to arrive at the final color. That mix gives us colors that range from dark red, through yellow, white, and blue. Below is a diagram that connects overall color to temperature. By Bhutajata - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=58848767 The scale at the bottom gives the temperature in degrees Kelvin Star Temperatures - 2 GENERAL ASTRONOMY LAB I I What about things that are much hotter or much colder? They send out electromagnetic waves which we can't see with our eyes. It turns out there is a simple formula to find the wavelength (in nanometers, nm) of the peak in brightness for the curves in the first figure. It is given by 2,900,000 푊푎푣푒푙푒푛푡ℎ (푛 푛푚) = 푇푒푚푝푒푟푎푡푢푟푒 (푛 퐾푒푙푣푛) Below is a table showing the wavelengths for different types of electromagnetic waves. Name Wavelength X rays less than 10 nm (a 10 nm length is about that of 5 atoms in a row) Ultra-violet 10 – 400 nm Blue 400 nm Green 500 nm visible range Red 600 nm Infra-red 700 -15,000 nm Microwaves wavelengths of about 1 mm to a km (these are really long!) By the way, it is easy to misunderstand what we are measuring here. We see the surface of a star, not its interior. Surface temperatures of stars are typically in the 2,000 K to 100,000 K range in temperature. But the interior of a star is much hotter. Our sun, for example, has an interior temperature of about 25 million K. Star Temperatures - 3 .