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(originally published to Helium writing site, now gone)

Astronomy is the study of stars, planets, moons and other celestial bodies, and of entire groups of heavenly objects such as solar systems and galaxies. So solar astronomy is simply the study of the sun, the word solar meaning “of or relating to the sun”.

The sun is about 93 million miles from earth. How do we know this? Italian astronomer Gian Cassini in the 17th century used a method called parallax. To understand this technique, put your thumb straight out in front of you and observe it with one eye and then the other. The thumb looks to have moved to a slightly different spot. The difference is the thumb’s parallax. Cassini was able to observe the sun and the planets from different places and determine the distance to these bodies using the parallax technique. His calculations turned out to be quite accurate.

Solar astronomers were then able to calculate the sun’s size. As a yellow dwarf, the sun is not a large star. Yet its diameter of nearly 900,000 miles is 109 times that of earth. That’s like putting a pea next to a medicine ball. The sun’s surface area is 12,000 times greater than earth’s and its volume is 1.3 million times that of earth.

Astronomers have found that the sun’s rotation time is about 25 days at its equator, although from earth it appears to take about 27 days due to the earth’s orbital motion around the sun. Rotation time can be measured by observing the movement of sun spots. Nearer the poles, the sun spots move slower, and rotation time at the poles is about 34 days. The difference in rotation times is possible because the sun isn’t a solid mass but is made of gases, mainly hydrogen (75 per cent) and helium (24 per cent).

The interior of the sun can’t be seen but scientists have been able to determine its inner structure through helioseismology, which involves detecting the movement of the sun’s pressure waves beneath its surface. This is rather akin to seismology on earth which studies the waves produced by earthquakes. Some of the sun’s waves are amplified and this is transmitted to the surface. Changes in the waves allow astrophysicists to find out numerous things about the sun’s interior.

At its middle or core, the sun is 150 times heavier than water. The core extends from the very center a fifth of the way to the surface or 0.2 solar radii. The temperature at the core is about 25 million degrees Fahrenheit, compared with a surface temperature of 10,000 degrees. Data from the Solar and Heliospheric Observatory spacecraft suggests the sun’s core rotates faster than the radiative zone which is the next layer out from the core. Most of the sun’s heat is produced in the core, by nuclear fusion, converting hydrogen into helium.

The resultant photons or gamma rays travel at 186,000 miles per second, or the speed of light, but they collide with matter billions of times before reaching the surface and escaping as sunlight. At the sun’s core, photons travel an average distance of a 250th of an inch between each charged particle. Near the surface, this increases to about a tenth of an inch. Solar astronomers estimate the time taken for photons to complete their journey through the layers of the sun to be around 10,000 to 170,000 years. Yet the time taken for light to travel from the sun to earth is a little over eight minutes.

Moving out from the core is the radiative zone which covers the area between 0.2 and 0.7 solar radii. This layer is hot and dense, allowing heat to transfer upwards by thermal radiation. The final layer is the convection zone where heat is carried to the surface via thermal columns. The matter cools at the surface and falls back down to the bottom of this zone, before heating up and rising to the surface again. The sun’s photosphere or surface is opaque and 250-300 miles thick.

Above the photosphere is the solar atmosphere. This too is divided into several layers. The temperature minimum layer includes the area of lowest temperature, about 7,000 degrees Fahrenheit, some 300 miles above the surface. This is cool enough to support water and carbon monoxide, detectable by their absorption spectrum. The chromosphere is about 1,500 miles thick and becomes visible as the colored flash of light at the start and finish of total eclipses. Temperatures here can be up to 180,000 degrees.

The transition layer can be up to 1.8 million degrees. Solar astronomers observe this region more clearly via spacecraft rather than directly from earth. The corona is hotter still, at several million degrees, but astronomers are not totally sure why it is so hot. Lastly, the heliosphere is made up of hydrogen and helium blown by solar wind to the far reaches of the solar system at over 600,000 miles an hour for much of the journey.

Astronomers also study sunspots and the solar cycle. The latitudinal differences in the sun’s rotation time result in its magnetic field lines becoming twisted, leading to magnetic field loops erupting from the surface which results in sunspots. The twisting action produces a solar cycle which lasts on average 11 years. The number of sunspots varies over this cycle. The cycle has a significant impact on our weather and climate. During longer solar cycles, we experience hotter temperatures. The fewer the number of sunspots, the colder it seems to be. Europe experienced a mini ice age in the 17th century when the cycle seemed to stop and there were very few sunspots.

The sun is located on the Orion Arm of the Milky Way, which is towards the outside of the galaxy and about 24,800 light years from its center. The sun, and the solar system, takes 225-250 million years to revolve around the galaxy at a speed of about 150 miles a second relative to the center of the galaxy. Using stellar evolution models and cosmochronology, astronomers have determined the age of the sun as 4.57 billion years. This means the sun has completed barely 20 orbits of the Milky Way.

The sun is about halfway through the main part of its life. It isn’t large enough to become a supernova but will become a red giant in about five billion years. The sun will be bigger than earth’s current orbit. Astronomers initially thought a much larger sun would push the earth’s orbit outwards and it would survive, but the latest research shows that earth might be swallowed up by the expanding sun due to tidal interactions. Terrestrial life will probably end in a billion years anyway because by then the sun will be 10 per cent brighter and the additional heat will make it too hot for water to exist. This will mean the end of all life on earth.

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