University of Michigan - Department of Astronomy

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Version: Intro


Solar Observing

Wake! For the Sun, who scattered into flight
The stars before him from the field of night.

--Eward Fitzgerald

Overview

Introduction

The Sun is the closest star to us, which makes it the easiest star to observe and study.  In the spectroscopy lab, you learned how astronomers determine what the Sun is made of, by studying the light from the Sun's atmosphere. Each element has it's own unique "fingerprint" of specific colors of light that it can emit or absorb, called an emission or absorption spectrum.A hot, thin gas will produce an emission specrum. A thin gas with a light source behind it will absorb some of the light and produce an absorption spectrum. If you can identify the element that produces a particularspectrum, you can identify what the Sun is made of.  In fact, the absorption spectrum for helium were seen in the Sun’s spectrum long before the element was identified here on Earth.  In addition, a hot, opaque material will produce a black body spectrum.  The photosphere is the region where the sun becomes transparent, so the bottom of the photosphere is opaque and is where the Sun’s black body spectrum is produced. However, if we take a closer look, we can see the surface is not the uniform yellow-green of the black-body spectrum.  Chinese astronomers observing the Sun at sunrise noted there were dark spots, and when Galileo turned the telescope on the Sun, he too saw these spots.  Galileo also went blind from looking at the Sun through the telescope, so be sure to always use the proper filters when observing the Sun! 

The sunspots are dimmer than the surrounding area, and red in color, so we know that they are cooler than the rest of the surface.  Additionally, careful observation (and a safe filter) will show granulation: areas of slightly warmer and cooler plasma.  This tells us that right under the photosphere energy moves through the Sun by means of convection, so the region under the photosphere is called the convection zone.

We can learn even more by looking at only the light from specific wavelengths. Recall that the visible light part of the hydrogen spectrum has a red line at 656 nm.  This is known as the H-α line.  A filter that allows only this wavelegth to pass is an H-α filter. If we observe the Sun at this wavelength, we get a slice of the blackbody spectrum, but we also get emission from the transparent regions of the Sun’s atmosphere that would normally be overwhelmed by the black body spectrum.  This shows us what’s happening in the areas just above what we can see in white light, in the photosphere and the lower atmosphere of the Sun, called the chromosphere

Modern astronomers also use satellites to observe the Sun at wavelengths outside our visual range.  UV and X-ray observations let us observe even higher levels in the Sun’s atmosphere.

The Sun’s upper atmospher, the corona is very faint because it is so thin.  On the Earth, light scattered by the atmosphere makes it impossible to observe the corona at any time except during a total solar eclipse. Artificial satellites can create an artificial eclipse with a device called a coronagraph, so astronomers can observe the corona continuously.

The Sun’s magnetic field is generated in the convection zone. Magnetometers (“magnetic-field meters”) in orbit around the Sun measure changes in the Sun’s magnetic. This, together with the granulation from the visible light images, enables astronomers to map what’s going on in the convection zone. 

Putting all these observations together, we can come up with a nearly complete model of the Sun, inside and out.

Outside version

Ouside version with angular size

Computer based


updated: 2/24/09 by SAM

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