Star Light, Star Bright

“The main thing we can detect from stars is the EM radiation they produce.” All matter absorbs EM radiation from its surroundings. Some of the radiation is reflected but at every wavelength, some of it is always absorbed. So, everything is sort of a “black body”… it absorbs every wavelength. Conversely, all matter also emits electromagnetic radiation when it has a temperature above absolute zero. The radiation represents a conversion of a body's thermal energy into electromagnetic energy. Every object emits radiation of some kind, called Black Body Radiation. The types (wavelengths) of radiation are dependent upon the temperature. Therefore, we can look at the color and other types of radiation coming from any matter and make a pretty accurate estimation of the temperature!

Electromagnetic Spectrum

The temperature of a Pāhoehoe lava flow can be estimated by observing its color. Estimating temperature from color agrees well with measured temperatures of lava flows. This lava is about 1000° to 1200 °C. It works pretty well with stars, too!

SATELLITES & TELESCOPES We have satellites and land-based telescopes which can measure the radiation from celestial bodies. Satellites are great because we can just simply measure how much radiation is reaching them from a particular star. The land-based telescopes pose a challenge, though. Some incoming radiation is absorbed by our atmosphere. So, astronomers measure the radiation reaching the telescope/array and have to “add back” the amount that would be lost to our atmosphere. The Karl G. Jansky Very Large Array (VLA) is a radio astronomy observatory located at about 7000 feet above sea level, in New Mexico, USA. There are 27 independent antennas, each of which has a dish diameter of 25 meters (82 feet) and weighs 230 US tons.

Black Body Radiation – good to know All objects emit radiation of some kind, called BBR The hotter the object, the higher the energy, the shorter the dominant wavelength of radiation. So, hotter objects are blue/violet. Blue stars are very hot. Yellow-white (like our sun) are cooler. Our eyes can fool us, so we use instruments to measure the spectrum and verify the radiation. The temperature is related to the peak of the intensity spectrum. 400 nm = 7000 Kelvin Wien’s Law:

What Does EM radiation tell us? From EM radiation, we can determine: The color of the star The surface temperature of a star What elements are present The percentages of each element present Astronomers use this information to classify stars. More on this Tuesday once you’re well rested from the 3 day weekend!

Emission & Absorption Spectra Remember those Absorption and Emission lines we learned about during Ch. 3? The emission lines are made when electrons jump from one level to another, releasing energy in the form of light. The energy spacing between ‘orbitals’ is different for each element. So, when an electron jumps from one level to another, the color / frequency is related to the change in energy levels, and is specific to each element present. When astronomers look at the emission spectra for stars, they can tell what elements are present.

Reading: “Watts More” Pages 490 – 491

Watt’s More – My Notes A Watt is a unit of power. Power = Energy / Time Apparent Brightness – what we see Luminosity = power output, in Watts. Comparing brightness with luminosity gives information about how far away a star is located from Earth. Stellar parallax does not work when stars are really far away… they don’t “move” enough! Instead, astronomers use a light-meter to measure apparent brightness and compare (using math!) the relative brightness of closer stars to the “distant” star.

Watt’s More – Extra Info You should see that your energy consumption is measured in “kilo-watt hours”. One kWh means you used 1000 watts of energy in an hour. Your bill represents how much energy your household is consuming. The bill below shows 600 kWh for a month. Find your household’s energy consumption for a recent month. Also, find your electricity rate. Electricity rates (cost per kWh) vary widely. Rates can range from about 12¢ to 50¢ per kWh from the same provider.

Star Light, Star Bright (Day 2)

Exploration: Temp vs. Distance (Results) Temperature vs. Distance from Light Bulb

Flash Light Decrease over Distance Exploration: Flash Light Decrease over Distance

Another Application of the Inverse Square Law