25 May 2024 - Mission Day: 10403 - DOY: 146

Take a look at our Sun 101 page for some basic introductory information on our star the Sun!


See also our list of not-so-frequently asked questions and the questions and answers at Cornell University's "Curious about Astronomy?" website.


What is SOHO and how can society benefit from its science?

SOHO stands for Solar and Heliospheric Observatory and is a satellite that studies the Sun 24 hours a day, 365 days a year without interruptions. The spacecraft has 12 scientific instruments collecting information about the Sun ranging from activity in the Sun's corona to vibrations deep in the Sun's interior.

Because the Sun is the only star close enough to have real and dramatic effects on our life here on Earth, we certainly expect and hope that improving our observations and our understanding of this beautiful, awesome object will in the course of time bring about beneficial applications. While it's never possible to tell where the quest for knowledge will lead, at this time the area where we have the greatest expectation of useful fallout is in the "space weather" arena.

"Space Weather" may sound abstruse, but it's a concept that is growing in importance as mankind pushes further and further against the limits within which we live. When a farmer had only an acre or two to worry about, a look out the window was a good enough weather forecast for the day's plowing. When he has thousands of acres to plow, seed, and fertilize, he may find it necessary to plan on a much broader scale in order to avoid disaster; thus, we need weather satellites and global forecasting systems for tropospheric weather. Similarly, when communication and electrical grids connected only local communities, the worst threat might have been a lighting strike on a local utility pole. But today, our electrical power grids span entire continents, and our communication lines reach across hemispheres, linked by synchronous-altitude satellites. It's not too early to be thinking about the effects on these extended systems, of vast clouds of atomic particles and magnetic fields thrown out by the Sun on an almost daily basis.

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What is SOHO's orbit?

SOHO is in orbit between the Earth and the Sun. It is about 150,703,456 kilometers (92 million miles) from the Sun and only about 1,528,483 Kilometers (1 million miles) from the Earth (three times farther than the moon). This orbit is around a mathematical point between the Earth and the Sun known as the Lagrange point or the L1 point. The L1 point is a point of equilibrium between the Earth's and Sun's gravitational field, that is to say that the pull is equal from both the Sun and the Earth. The L1 point is a point of unstable equilibrium (like a bowl round side up with a marble balanced on it). As a result, we have to compensate for perturbations due to the pull of the planets and the Earth's moon. Every few months we use a little fuel to fine tune our orbit and keep it from getting too far off track. This is known as "station keeping manoeuvres"

No spacecraft is actually orbiting at the L1 point. For SOHO there are two main reasons: the unstable orbit at the L1 point and facility of communication in a halo orbit. If SOHO was sitting directly at the L1 point, it would always be right in front of the Sun. The trouble is that the Sun is very noisy at radio wavelengths, which would make it very difficult to tune into the radio telemetry from the spacecraft. By putting it into a halo orbit, we can place it so that it's always a few degrees away from the Sun, making radio reception much easier.

Check out this website for more on SOHO's orbit.

Bowl and marble demo - stable and unstable equilibria

(Lagrange Points)

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What are those flying saucer-shaped objects in the LASCO images?

The "funny-looking spheroid" is a typical response of the SOHO LASCO coronagraph CCD detector to an object (planet or bright star) of small angular extent but so bright that it saturates the CCD camera so that "bleeding" occurs along pixel rows. There is a bright horizontal streak on either side of the image, because the charge leaks easier along the direction in which the CCD image is read out by the associated electronics.

CCD stands for charge-coupled detector, and refers to a silicon chip, usually a centimeter or two across, divided into a grid of cells, each of which acts like a small photomultiplier in that an incoming photon knocks loose one or more electrons. The electrons are "read out" by row (fast direction) and column (slow direction), the current converted to a digital signal, and each cell or picture element ("pixel") thus assigned a digital value proportional to the the number of incoming photons in that pixel (the brightness of the part of the image falling on that pixel). This is the same kind of detector as is used in a hand-held video camera, though until recently, the analog-to-digital conversion was left out in consumer devices.

If you point a video camera at a very bright source (say, the Sun), the image "blooms" or brightens all over --- there are so many electrons produced in the pixels corresponding to the bright source that they spill over into adjacent rows and column, perhaps over the entire detector. Better CCD's will "bleed" only along the fast readout direction (a single row), and perhaps a few adjacent rows.

The LASCO and EIT CCD cameras include "anti-bleed" electronics which limit the pixel bleeding around bright sources to less than the full row (and usually no adjacent rows). In the case of a marginally too-bright object, the pixel bleeding will be only a few pixels in either direction along the fast readout direction. Thus, the "flying saucer" images.

A few of the LASCO images that have appeared on the "extraterrestrial" Web sites show much larger and brighter, but still saucer-like features. These images are in fact obtained with the instrument door closed, but with an incorrectly long exposure. The big "saucers" result from massive pixel bleeding along every row of the detector containing part of the image of the "opal," or small diffusing lens, in the instrument door, that is used for obtaining calibration data.

If your correspondents still prefer to believe that the pixel-bled images of planets or bright stars are something else, ask them why the extended part of the "saucers" (i.e., the pixel bleeding) always occurs in the same direction relative to the image --- even when the spacecraft is rolled relative to its normal orientation relative to the Sun.

More discussion with images on our Hot Shots page:

Check out these web sites for more on LASCO:
(LASCO web site)

Flying Saucer image

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What color is the Sun?

The Sun is white. To demonstrate this, take a sheet of white paper, go into a dark room, and shine a red flashlight on it. What color do you see the paper? The white sheet of paper appears red. This works for any color of illumination. A green laser pointer shined onto the paper will make the white paper appear green, and so on. The white paper will appear the same color as its source of illumination. Now take the same sheet of white paper out into the full noonday sunlight. What color does the white sheet look like? White!

Further discussion can be found at the Stanford SOLAR Center:
(Ask a Solar Physicist: Why does the Sun appear orange?)

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How do scientists know that the Sun has a core?

Obviously, nobody can see the center of the Sun and we've never sent a space probe into the Sun, either. However, we think we know what it is like in there.

The only thing that provides enough energy to heat a star for billions of years is nuclear fusion. We know the pressures and temperatures needed for this to take place, so we can figure out from that what the center of the Sun (the "core") must be like.

Check out this web site for more on the Sun's core:
(The Virtual Sun: Its Core)

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How does Space Weather affect us on Earth?

We can observe these phenomena better than before, thanks to new space observatories and spacecraft such as SOHO, ACE, and TRACE.

Magnetic storms, such as the coronal mass ejection interact with the Earth's magnetic field which in turn can interfere with radio, television, and telephone signals, upset the navigation systems of ships and airplanes, and cause blackouts. Sun-induced storms can damage satellites and spacecraft or force them to re-enter the atmosphere. In some instances, it can be dangerous to astronauts out in space and especially on space walks. However, on a more positive note, solar wind also causes the Aurora Borealis also known as the northern lights.

Because we have more satellites, larger power grids, smaller cell phones, greater reliance upon GPS and such, we are much more susceptible to the effect of Space Weather.

Check out these web sites for more on Space Weather:
(NOAA Space Weather Forecast)

(Windows to the Universe Space Weather pages)

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What can we expect from the Sun in the future? Is it getting brighter or dimmer? Is it expanding or contracting?

(For an overview of the controversy over solar variability, I recommend John Gribbin's pre-SOHO book, Blinded by the Light: The Secret Life of the Sun, published in 1991) The Sun's brightness changes by only 0.1% between the minimum and maximum of a cycle. We do not have enough data to see any longer-term changes from cycle to cycle. However, the Sun has not changed drastically in recent decades. Models of solar evolution indicate that the Sun is gradually increasing in brightness at a rate of about 6% per billion years.

The Sun is not "currently" expanding or contracting to any measurable extent. I know of some observations made by an astronomer indicating that the Sun changes size over the 11 year solar cycle - decreasing in size from the maximum activity to the minimum and then increasing in size again as solar activity picks up again. Others have tried similar measurements and found no change in size. However, it is believed that in 4 to 5 billion years, the sun will expand as it fuses the last of its core hydrogen. The outer layers of gas will swallow some inner planets (possibly even the Earth). This is commonly referred to as the red giant phase. Then the inner parts of the Sun will stop fusing, contract, and become a white dwarf.

Check out these web sites for more on the lives of stars:
(Strobel's Astronomy Notes)
(Total Solar Irradiance, 1978-1999)

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How long does it take for the Sun's light to reach the Earth?

The Sun is about 93 million miles, or 150 million kilometers, away from the Earth. The speed of light is 186,000 miles per second or 300,000 kilometers per second. Therefore it takes about 500 seconds or a little over 8 minutes for the Sun's light to reach the Earth.

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Why are the EIT images different colors and what do they mean?

All of the EIT images are actually produced by extreme ultraviolet (EUV) light from the Sun. This is light that is between ultraviolet light and x-ray light in the electromagnetic spectrum and is not visible to our eyes directly.

EIT images are taken at four different wavelengths and four colors in order of wavelength (bluer=shorter wavelength, redder=longer) were assigned to represent each of them. Just as the human eye is capable of discriminating different colors in the visible, so EIT's four bandpasses discriminate among four "colors" in the extreme ultraviolet. In addition, each color table was carefully constructed to bring out typical features of its particular wavelength.

The red images have a wavelength of 304 Angstroms, the yellow are 284 Angstroms, the green are 195 Angstroms, and the blue are 171 Angstroms. So, just like in the visible spectrum red is the longest wavelength and blue is the shortest with yellow and green in between, the EIT image colors were chosen so that the longest wavelength is reddish and the shortest is bluish.

Check out these web sites for more on EIT and its rainbow of colors:
(EIT web site)

EIT images

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What's a CCD bakeout?

See the SOHO/EIT page,
(What's a CCD bakeout, anyway?)
For the answer to this and other related questions.

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Didn't find the information you were looking for? Check out our Not So Frequently Asked Questions List!


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Last modification: July 27, 2020

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