Stellar Temperature Measurement

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Equilibrium Temperature

Objects in outer space that do not have their own heat source will heat up or cool off until their temperature is such that they emit exactly as much thermal energy as they receive from the Sun. This temperature is called the equilibrium temperature. A planet that is d  times further away from the Sun than the Earth and that is pitch-black so that it absorbs all the sunlight that falls on it will have an equilibrium temperature that is equal to

Teq = 279/SQRT(d) kelvin or
Teq = 279/SQRT(d) - 273 degrees C, or
Teq= 502/SQRT(d) - 460 degrees F

where SQRT is the square-root function. If the planet is not pitch-black but actually reflects part of the sunlight, then its equilibrium temperature will be lower because it does not get heated up quite so much. If the planet has an atmosphere, then its surface temperature may be much higher than the equilibrium temperature because an atmosphere works like a blanket.

If a pitch-black planet without an atmosphere always shows the same side to the Sun and if the heat does not travel, then the point at which the Sun is straight overhead gets a temperature of

Thot = 394/SQRT(d) kelvin, or
Thot = 394/SQRT(d) - 273 degrees C, or
Thot = 710/SQRT(d) - 460 degrees F

Teq is an upper estimate for the average temperature on the outside of the atmosphere (if any) of planets that rotate fast or that have a thick atmosphere that rotates fast, which means all planets except Mercury. Thot is an upper estimate for the hottest temperatures on planets without atmospheres that rotate slowly, such as Mercury, and also for the Moon. There is no lower limit on how cold the dark side of a planet or Moon can get; this mostly depends on how fast the planet rotates. Slow rotators cool off more than fast rotators.

First...... follow this link Sun

Back?  Ok...

solar surfer iconIt's time for some "Solar Surfing"solar surfer icon

As you surf each web site listed below, look for as much as you can about equilibrium temperature. At the next section, you will be given an opportunity to check your understanding.

How do scientists find the temperatures of stars ?
How does the sun get heated ?


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Welcome to the National Solar Observatory / Sacramento Peak


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Yohkoh Public Outreach Project Classroom  at Montana State University 


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How is the Solar Corona Heated? Welcome to the home page for the High Energy Solar Spectroscopic Imager (HESSI) at NASA Goddard Space Flight Center (GSFC) in Greenbelt, Maryland, USA. 

Now.......Let's look at some HOT images of the Sun

The Extreme-Ultraviolet Imaging Telescope EIT.

Take The On-LINE SUN Quiz Here Sun quiz icon link

Want to know more about Astronomy ?

general facts about the Sun exploring temperature development thermometers thermal radiation & equilibrium temperature scales Hertzsprung-Russell Diagrams
Ask An Astronomer @ Lick Observatory

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This page last updated  06/25/02