Universe and Solar System
2.4 The Sun
Consider Earth, the Moon, and all the other planets and satellites in the solar system. The mass of all of those objects together accounts for only 0.2 percent of the total mass of the solar system. The remaining 99.8 percent of the solar system’s mass is within the Sun. The Sun is the center of the solar system and the most massive object in the solar system. This nearby star provides light and heat and supports almost all life on Earth.
The Sun is a sphere, composed almost entirely of the elements hydrogen and helium. The Sun is not solid or typical gas. Most atoms in the Sun exist as plasma, the fourth state of matter made up of superheated gas with a positive electrical charge. (The Sun | Earth Science, n.d.)
Because the Sun is not solid, it does not have a defined outer boundary. It does, however, have a definite internal structure with identifiable layers.
The Sun’s central core is plasma with a temperature of around 27 million degrees Celsius. At such high temperatures, hydrogen combines to form helium by nuclear fusion, a process that releases vast amounts of energy. This energy moves outward towards the outer layers of the Sun.
The radiative zone, just outside the core, has a temperature of about 7 million degrees Celsius. The energy released in the core travels exceptionally slow through the radiative zone. A particle of light, called a photon, travels only a few millimeters before it hits another particle. The photon is absorbed and then released again. A photon may take as long as 50 million years to travel through the radiative zone.
In the convection zone, hot material from near the radiative zone rises, cools at the Sun’s surface, and then plunges back downward to the radiative zone. Convective movement helps to create solar flares and sunspots.
The Outer Layers
The next three layers make up the Sun’s atmosphere. Since there are no solid layers to any part of the Sun, these boundaries are fuzzy and indistinct.
The photosphere is the visible surface of the Sun, the region that emits sunlight. The photosphere is relatively cool, only about 6,700 degrees Celsius. The photosphere has several different colors; oranges, yellows, and reds, giving it a grainy appearance.
The chromosphere is a thin zone, about 2,000 km thick, that glows red as energy heats it from the photosphere. Temperatures in the chromosphere range from about 4,000-10,000 degrees Celsius. Jets of gas fire up through the chromosphere at speeds up to 72,000 km per hour, reaching heights as high as 10,000 km.
The corona is the outermost plasma layer and is called the Sun’s halo or crown. The corona’s temperature of 2 to 5 million degrees Celsius is much hotter than the photosphere.
The Sun’s surface features are quite visible for humans to observe, but only with specialized equipment. The most noticeable surface feature of the Sun are cooler, darker areas known as sunspots. Sunspots are located where loops of the Sun’s magnetic field break through the surface and disrupt the smooth transfer of heat from lower layers of the Sun, making them cooler and darker and marked by intense magnetic activity. Sunspots usually occur in pairs. When a loop of the Sun’s magnetic field breaks through the surface, a sunspot is created where the loop comes out and where it goes back in again.
There are other types of interruptions of the Sun’s magnetic energy. If a loop of the sun’s magnetic field snaps and breaks, it creates solar flares, which are explosions that release vast amounts of energy. A strong solar flare can turn into a coronal mass ejection.
A solar flare or coronal mass ejection release streams of highly energetic particles that make up the solar wind. The solar wind can be dangerous to spacecraft and astronauts because it sends massive amounts of radiation that can harm the human body. Solar flares have knocked out entire power grids and disturbing radio, satellite, and cell phone communications.
Another highly visible feature on the Sun is solar prominences. If plasma flows along a loop of the Sun’s magnetic field from sunspot to sunspot, it forms a glowing arch that reaches thousands of kilometers into the Sun’s atmosphere. Prominences can last for a day to several months. Prominences are also visible during a total solar eclipse.