The sun is just one of billions and billions of stars in the part of the universe we can see, but it's the star that gives life to Earth, so it's the one in which humans are rightfully most interested. If beings from civilizations in other parts of the galaxy ever do communicate with us publicly, though, they'll probably shatter any illusions of grandeur we might have about our home star.
Sure, it looks big and hot from here, but compared to other stars, it's small and relatively cool. It may be home to a system of worlds, but that's par for the course, as far as stars go. "Nothing to see here, folks," the aliens might quip as they aim their interdimensional space pods toward more dramatic star systems.
There would be no need to be discouraged by such a jaded encounter, were it ever to occur. The physical properties of the sun may not be special when compared to other stars, but those properties have engendered human life, and that's not just special; it's miraculous.
There are innumerable features of the sun to appreciate, but here are five of the most notable, plus a bonus look into the sun's future.
1 – The Sun Is Just Your Normal, Average Star
Astrophysicists classify the sun as a yellow dwarf, which immediately gives you an idea where it stands in terms of the other stars that populate the universe, some of which are giants. In scientific terms, the sun is classed as a population I, G2V star (V is the Roman numeral 5).
Most stars in our part of the galaxy are population I stars. They are metal-rich, which means that are relatively young. Metals are produced during the dying stages of large stars, and population I stars are born out of the debris from those stars. Population I stars are typically no more than a few billion years old. The sun's age is estimated to be 5 billion years.
The letter G refers to the sun's spectral classification, which is a measure of how hot and bright it is in comparison to other stars. There are seven star classifications, denoted by the letters O, B, A, F, G, K and M. O designates gigantic stars that are so hot they emit blue light, and M designates cool dwarf stars that emit light in the infrared range. As a yellow dwarf, the sun is below average in size and temperature.
The Roman numeral V signifies that the sun is a main-sequence star, which means it is in the middle part of its life, during which the fusion of hydrogen into helium occurring at its core generates enough pressure to prevent gravitational collapse. The number 2 refers more specifically to spectral characteristics.
The length of time a star remains in the main sequence is mostly dependent on its mass. The sun has been in the main sequence for 5 billion years and will remain there for another 5 billion years.
2 – The Structure of the Sun Is Layered
Far from being just a big ball of burning gas, the sun has a complex internal structure that forms four distinct layers. Scientists further divide the outer layer, the atmosphere, into three sublayers. The six layers of the sun include the core, the radiative zone, the convection zone, the photosphere, the chromosphere and the corona.
The core: The hottest part of the sun, the core, is where hydrogen fusion takes place. The gravitational forces are so strong at the core that they squeeze hydrogen into a liquid with about 150 times the density of water. The temperature at the core is 15 million degrees Celsius, or 28 million degrees Fahrenheit.
The radiative zone: The zone directly surrounding the core decreases in density with increasing radius, but it's still dense enough to prevent light from escaping. Radiation produced by the fusion reaction continuously occurring at the core takes 100,000 years to bounce around in the radiative zone before it escapes into space.
The convection zone: The convection zone is an area of high turbulence that extends from a depth of 200,000 km to the visible surface. In this zone, density falls to a level that allows light from the core to be converted into heat. Superheated gases and plasmas rise, cool and fall again, forming a complex cauldron of large bubbles, called convection cells.
The photosphere: The layer of the sun's atmosphere that is visible from Earth is the photosphere. The temperature has cooled to 5,800 C (10,000 F). The photosphere is pockmarked by solar flares and sunspots, which are dark, cool areas formed when the sun's magnetic field breaks through to the surface.
The chromosphere: In the chromosphere, which extends about 2,000 km above the photosphere, the temperature rises to 20,000 C (36,032 F). This layer has the name it does because the color of the emitted light turns reddish.
The corona: The outermost layer of the sun, the corona, is usually invisible, but it becomes visible from Earth during a total solar eclipse. The density of the gases is about a billion times less than water, but the temperature can be as high as 2 million C (3.6 million F). The reason for this rise isn't completely understood, but scientists suspect it has to do with magnetic storms that are constantly occurring there.
3 – From a Human Perspective, the Sun Is Really, Really Big
To other stars in the universe, the sun may be a dwarf, but to people on Earth, it's incomprehensibly huge. One of the most-often cited features of the sun is that you could stuff 1.3 million Earth-sized planets inside it. If you arranged those planets side-by-side, you would need 109 of them to span the sun's diameter.
In terms of statistics, the sun's diameter is about 1.4 million km (864,000 miles), and its circumference is about 4.4 million km (2.7 million miles). It has a volume of 1.4 × 1027 cubic meters and a mass of 2 × 1030 kilograms, which is about 330,000 times the mass of the earth.
Even though the sun is so large compared to Earth, it's important to remember that scientists have observed stars that are many times larger. One of the largest stars so far observed is the red giant Betelgeuse. It's about 700 times larger than the sun and about 14,000 times brighter. If it took the sun's place, it would extend as far as the orbit of Saturn.
4 – The Sun's Surface Activity Is Cyclical
The sun's magnetic field switches polarity every 11 years, and this creates a corresponding cycle of sunspot and solar flare activity. At the beginning and end of each cycle, sunspot activity is weak to nonexistent, and activity is at a maximum at the midpoint of each cycle.
The sun's surface activity affects everyone on Earth. During periods of high surface activity, when solar flares are common, the aurora becomes more pronounced, and increased radiation affects communications and can even constitute a health hazard.
The best known solar flare disturbance occurred in 1859. Known as the Carrington super flare, it disrupted global telegraphic systems. If such an event occurred today, some scientists believe it would cause a global catastrophe.
Because solar activity can have such an impact on Earth, scientists have been monitoring it since 1755, when the start of the first cycle was observed. Since then, the sun has experienced 24 complete cycles. The 25th cycle began in 2019, and the transition from cycle 24 was unusually quiet, a fact that puzzled scientists who track the sun's activity.
5 – The Whirling Sun's Magnetic Field
Astronomers believe that the sun and all the planets were formed from a cloud of space gas. As the gas contracted under the force of gravitation, it began to spin, and as you might expect, the sun still spins. Being a big ball of gas, it doesn't give away this fact readily. Scientists know because they are able to watch the motion of sunspots on the surface.
Because the sun is mostly gas, different parts of it rotate at different rates. The equatorial region has a rotational period of 25 days, but the rotation at the polar regions takes 36 days. Moreover, the core and radiative zone behave like a solid body and rotate as a unit whereas the rotation in the convection zone and photosphere is more chaotic. The transition between these two rotational zones is known as the tachocline.
Remember that the sun is a population I star, which means it contains metals. One of these is iron, and the presence of iron in a spinning body is the recipe for a magnetic field. The sun's magnetic field is about twice as strong as Earth's, but because the sun is so much bigger, its field extends much farther. Carried by the stream of charged particles known as the solar wind, the farthest reaches of this magnetic field extend even beyond the edge of the solar system.
The Sun Is Going to Swallow the Earth
No one will likely be around so see it, but the sun will eventually turn into one of the most picturesque objects in space – a planetary nebula. Before that happens, though, the yellow dwarf we have come to know and depend upon will grow and expand until its outer radius reaches beyond Earth's orbit. The sun will engulf the earth, which will cease to exist, but there's no tragedy involved. It's just what happens to stars the size of the sun.
Unlike very large, hot stars, which collapse under their own weight to go supernova and contract into neutron stars or even gravitational singularities known as black holes, stars the size of the sun age much more sedately.
When the sun runs out of hydrogen to burn in its core, it will begin to collapse, but the intensified gravitational forces will begin the process of helium fusion, and the collapse will turn into a new period of expansion. The outer shell will balloon out to almost the orbit of Mars and cool down, and the sun will become a red giant.
When the core runs out of fusible material, it will collapse again, but the outer shell will be too far away to be attracted and will simply drift away. Meanwhile, the super-hot core will send out ionizing beams of radiation, which will turn the diffusing cloud, which is now a planetary nebula, into a riotous color show.
Well-known images of the Helix Nebula, Ring Nebula and other interstellar marvels give a taste of what is in store for the sun in about 5 billion years, give or take an eon.
- University of Maryland: Introductory Astronomy: Stellar Populations
- Enchanted Learning: Star Types
- The Sun Today: Solar Structure
- ARS Technica: Why Is the Sun’s Corona So Hot? Two Words: Solar Tornadoes
- Ultimate Science: How Big Is the Sun?
- Universe Today: Are We Witnessing the Start of Solar Cycle 25?
- Windows to the Universe: The Sun's Magnetic Field