Physics Photo of the Week

Physics Photo of the Week

March 8, 2024

The Sun

I photographed this image of the Sun recently (Feb 26, 2024) using a 4-inch (104 mm aperture) telescope fitted with a special solar filter and a digital SLR camera.  The solar filter has a transmission of 1/100,000 amount of the incident light.  The solar filter also reddens the image.  Since the Sun's surface, or photosphere, has a temperature of about 5700 deg K, its true color is blue white - about the color of old-fashioned fluorescent light tubes.

Notice the large sunspot dominating a group of sunspots on the center right of the Sun's image.  The Earth is only about 1/100 the diameter of the Sun, smaller than the size of the large sunspot.  Sunspots are cooler regions of the Sun's surface (about 2000 deg K cooler than the surrounding photosphere).  Sunspots are also known for strong magnetic fields directed out of the sunspots.  Thus a sunspot  resembles the north pole or south pole of a strong magnet.  This strong magnetic field is about 2500 times the magnitude of the Earth's weak magnetic field.  These strong magnetic fields of a sunspot are responsible for the cooler temperature of the sunspot, hence the relative darker appearance of the sunspot.  The magnetic field directs ions to travel along the field lines leaving the solar surface and cooling that part of the solar surface (an over-simplification).

Notice in the Solar image above that there is a subtle darkening of the image toward the limbs of the Sun.  The limb of an astronomical body is the term given to the apparent edge of the Sun, Moon, or planetary body.  Since the Sun, Moon, and Planets are basically spheres - not disks - they don't have any true "edges", so a different term is used to describe the apparent edge in an image.  There is no resemblance of an anatomical appendage of the same name.

This subtle darkening of the Sun's image toward the limbs demonstrates the spherical nature of the Sun, and that the Sun has an atmosphere.  The limb darkening is caused by partial absorption of the light from the photosphere.  For light from the solar limb to reach the Earth the path of the light has to travel further through the Sun's atmosphere than light coming from the center of the apparent solar disk.  The light from the center regions only has to pass through the Sun's atmosphere perpendicular to the layer of the atmosphere.  Sunlight from the solar limbs leaves almost tangentially from the photosphere's surface, thus it's path length in the solar atmosphere is longer than the path perpendicular to the surface, thus more of the light is absorbed by the Sun's atmosphere.  On Earth, the Earth's atmosphere also affects the perceived color of the Sun or the Moon when the objects are close to the horizon, especially when more water vapor aerosols or pollutants are present.  The light path from objects low to the horizon travel further through the Earth's atmosphere than light coming from the zenith, thus the objects low to the horizon are dimmer and reddened, because the blue is absorbed more than the red.

I measured the relative brightness of the light from the surface of the Sun by reading the digital pixels produced by the digital camera.  The color camera also lets us separate the blue pixels from the red pixels - all this being done with the aid of digital image processing software.  To the right of the image of the Sun with the scan-line shown as the dotted line right through the large sunspot is a graph of the brightness of each pixel for each of the three primary colors: red, green, and blue.  The horizontal axis represents the pixel number starting from the left of the image; the vertical axis represents the the relative brightness in analog digital units (ADU).  The ADU's of a digital camera are proportional to the intensity of the light received by each pixel sensor.  The prominent dip in the brightness curves represents the low brightness of the center of the sunspot.  Also by examining the relative brightness's of the three curves, we can get an idea of any changing color of the Sun's surface.

The Sun also rotates with a period of about 28 Earth days.  The animated photo at right was made with the same telescope and filter during about a week of almost daily photos in the spring of 2016.  We can see from the single sunspot of those images that the Sun rotates toward the right (East in this video clip). 

 

 

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Physics Photo of the Week is published periodically during the academic year on Fridays by Donald F. Collins, professor emeritus of Warren Wilson College. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.

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