Sun
Everyone knows the Sun, but how many stop to think about that big, bright circle in the sky? Most don’t, which is why you can learn all about the Sun here!
Formation
From your perspective the Sun has always existed, and seemingly will always exist. But how did the Sun get to be where it is today? The birth of the Sun is not unique, hundreds of billions of stars exist in the Milky Way, and there are an estimated ten trillion galaxies in the Universe. That leads us to know around 1 septillion (1 with 24 zeros) stars in the observable Universe! To put that into perspective, there is an estimated 10 sextillion (1 with 22 zeros) grains of sand on Earth, which gives us a ratio of 100 stars per grain of sand on Earth! So how does the Sun, and all those stars, come to exist?
At the beginning of the Universe, the hot energy from the Big Bang eventually cooled into the particles that we are familiar with today, protons and neutrons. Protons are particles that are positively charged, while neutrons have no charge. A single proton is a nucleus, which we have called hydrogen (H). Under right conditions, 2 protons can come together with 2 neutrons to form a nucleus, one which we have called helium (He). The early universe is dominated by these two nuclei, with 75% being H and 25% being He. Note that there were small traces of lithium (Li) produced with three protons and four neutrons, but the amount is negligible for our discussion (about 10^(-10)%). These particles will be the key players in the formation of the Sun.
Around 4.5 billion years ago, the hydrogen and helium, or as astronomers call it gas and dust, in the Solar vicinity experienced an instability that caused it to collapse. With that little bit of instability, the gas and dust began to spin and collapse, just like an ice skater who increases their spin as they tuck themselves into a smaller space. This spin is what we today experience on Earth as the passing of the year! Due to gravity, the gas and dust collapsed towards the center of the once massive cloud, increasing the temperature through gravitational energy being converted to thermal energy upon collisions between particles. As the amount of gas and dust increased in the center of the gas cloud, the temperature and pressure reached a point where the hydrogen ignited in spectacular fashion through nuclear fusion. The Sun is born!
Composition
The Sun is mostly made from hydrogen and helium, just like most ordinary stars. Around 75% of the Sun’s mass is composed of hydrogen, while 24% is composed of helium. The other 1% is formed from metals, or what astronomers call any element heavier than helium. Yes, even oxygen is a metal to astronomers! The hydrogen in the core is what powers the Sun, and gives us the energy on Earth to enjoy life.
Structure
Core
The core of the Sun is hot, at around 27 million degrees Fahrenheit (or 15 million degrees Celsius)! These temperatures, and pressures equivalent to 265 billion times the Earth’s atmospheric pressure, allow nuclear fusion to occur turning hydrogen nuclei into helium nuclei. Even though you would expect hydrogen nuclei to be rapidly producing helium nuclei at those extremes, it takes on average 9 billion years for any given hydrogen nuclei to undergo fusion into a helium nuclei! The Sun has existed for a long period of time as a result, allowing for life on Earth to start and evolve into the world we observe today.
Radiative Zone
The radiative zone allows energy transfer between the core and the outer layer of the Sun. Due to the size of the Sun, this zone acts much like a fireplace, moving energy through the release of photons from one atom to the next. Smaller stars do not have a radiative zone, but rather only have a convection zone.
Convection Zone
The Sun’s convection zone occurs as the density of the Sun decreases, allowing the matter to move more freely in chunks. Convection is essentially the hydrogen itself mixing as hot layers of gas rise to the surface as cooler layers sink to the center, much like the boiling pot of water attempting to find a stable temperature under the application of heat.
Photosphere
The visible surface of the Sun, this is the portion that we are familiar with on a daily basis. Due to the temperature of the photosphere being 9,900 degrees Fahrenheit (5,500 degrees Celsius), we view the Sun as being a yellow color, though it puts out all shades of light, as evidenced by the green leaves, blue skys, red flowers, and the orange juice you drink.
Atmosphere
The atmosphere of the Sun is something that most people do not get to see as the layer is only viewable during a Solar eclipse, where the moon passes in front of the Sun as viewed from a point on Earth. This atmosphere has tendrils that reach far out into space, driving home the scale on just how imperfect the Sun is compared to our everyday notion of the photosphere.
Magnetic Field
The Sun’s magnetic field is created by charged particles moving around in the core. This field is massive, covering all the planets and possibly even the entire solar system in what’s known as the heliosphere. The Sun’s magnetic field is dynamic, undergoing an 11-year cycle which are bookended by a Solar minimum and Solar maximum. When the Sun is experiencing the minimum, the magnetic field is smooth across the surface. Due to the Sun being a ball of gas, the poles rotate faster compared to the middle, causing the magnetic field to bunch in certain spots. Over time this bunching gets more pronounced, leading to sunspots which are dark, cool areas on the Sun where convection is suppressed thanks to the strong magnetic fields. The Sun’s magnetic field resets itself through switching polarities at the poles, essentially taking the North pole and making it the South pole, and vice versa.
The Sun’s magnetic field is both a friend and a foe. The heliosphere protects us from the Galaxy’s interstellar particles, and the predictable nature of the 11-year sunspot cycle makes the Sun a very friendly star when it cooperates. However, as with all things, sometimes the magnetic field of the Sun is less predictable, and even harmful. Strong magnetic fields can cause coronal mass ejections, spitting out large amounts of charged particles in one direction. If one of these hit the Earth, the current electric grid and communication satellites would be taken out with the influx of charged particles. Back whenever telegraphs were used in 1859, a coronal mass ejection caused them to short circuit and have to be repaired. The little ice age in the seventeenth century corresponds to a prolonged Solar minimum releasing less energy to heat the Earth. European and North American seasons became more harsh, with longer and colder winters, shorter and milder summers, and prolonged drought in the regions.
Life Cycle
The Sun, just like every other star, has a life cycle that will take it from birth to death. The formation of the Sun is spectacular, igniting a dense cloud of gas with hydrogen fusion putting the Sun into the main sequence phase. The Sun is now a planetary nursery, allowing the gas and dust in its disk to form into planets. As the Sun ages, the energy output increases eventually leading it to the red giant phase where the core of the Sun will be burning helium instead of hydrogen. In this stage, the Earth may be entirely engulfed by the expanded Solar atmosphere, but don’t worry, this won’t happen for another 5 billion years! After the red giant phase, the Sun will have an inert carbon core and become a white dwarf, shedding off it’s outer atmosphere into a spectacular planetary nebula (has nothing to do with planets!). This white dwarf will slowly cool for a really long time, and eventually turn into a black dwarf, not shining any more light into the vastness of space. At this point, the Sun is dead. These events happen on a period of billions of years, luckily we won’t have to deal with this. Our ancestors will have hopefully figured out interstellar travel at that point, allowing the survival of the human race to flourish away from our Sun.
Satellites
The first satellites launched to study the Sun were Pioneer 6, 7, 8, and 9. Sitting both inside and outside Earth’s orbit, these satellites helped to detect parts of the Sun before they revealed themselves to observers on Earth. Studying the Sun allowed these satellites to detect space weather such as ejections from the Sun. Even back in the sixties whenever the Pioneer missions were launched, there was a very real threat of the Sun messing with the electronics on Earth!
Helios 1 and 2 were launched as a collaboration between Germany and the US to study the Sun. Helios 2 set the record for the closest probe to the Sun for 30 years before the Parker Solar probe was able to navigate even closer to the Sun. These probes studied the Solar wind.
The Solar Maximum Mission set out to observe the Sun’s activity at a Solar maximum. Upon launch, it was discovered that there was a failure, so three years later the Challenger space shuttle serviced the satellite. Successfully repaired, the Solar Maximum Mission was able to measure Solar activity in the ultraviolet, x-ray and gamma ray spectrums.
The Yohkoh probe was launched by Japan as a collaboration with the US and UK. The mission tackled x-ray observations with a modern digital camera instead of using film as previous missions employed. This allowed for more novel observations than in the past, revealing the Solar wind in x-ray in spectacular detail.
The Solar and Heliospheric Observatory, referred to as SOHO, was a joint mission between the ESA and NASA that is still running today after about 25 years of operation. SOHO was almost lost in the early stages of the mission, with the on-board gyroscopes failing. The teams at ESA and NASA developed a plan to create a gyroscope-free operations mode which they successfully implemented to recover the mission. SOHO carries twelve instruments in order to investigate the outer layers of the Sun, the interior of the Sun, and investigate the Solar wind.
The Solar Dynamics Observatory was the spiritual successor to SOHO after its amazing success. While SOHO measured the Sun in a large-scale view, the Solar Dynamics Observatory measured the Sun on a more small-scale view, particularly focusing on its magnetic fields.
The Ulysses probe was the first satellite to orbit around the Sun perpendicular to the Earth’s orbit in order to study the Sun’s poles. This mission revealed that the Sun has weaker magnetic fields at the poles than previously thought, allowing more than 30 times more gas and dust into the Solar System from the Galaxy than previously expected.
Genesis was launched as a sample collector that collected particles from the Solar wind to return to Earth. Scientists have an easier time analyzing samples in laboratories with all of their instrumentation and techniques available to their disposal. Genesis almost failed as the parachute failed to deploy on its return, causing a crash that rendered most of the samples useless. Luckily, some of the samples survived, allowing the study of the Solar composition through the Solar wind.
The Solar Terrestrial Relations Observatory, known as STEREO, was a mission designed by NASA to have twin probes observe the entirety of the Sun at once. This mission had the twin STEREO satellites orbit around the Sun at different speeds, causing the Sun to have overlapping images from different angles that are not visible from Earth over the lifetime of the mission. Allowing a full scale view of the Sun can help to know what space weather the Earth may be subject to days before that part of the Sun rotates into view of the Earth. Sadly, STEREO B lost contact after nine years of operation while STEREO A is still going strong.
The Parker Solar Probe is the only satellite named after a living person, Eugene Parker, who studied the Sun and coined the term Solar wind. The Parker Solar Probe’s mission allows the study of the corona, or outer layer, of the Sun directly, making it the closest satellite to the Sun at only a distance of 5% Earth’s orbit. The extreme radiation calls for a massive Solar shield, without which the probe would last up to a minute before the electronics were damaged beyond repair. The satellite hopes to uncover the magnetic field structure close to the Sun and help to determine how the Solar wind is accelerated as it leaves the Sun’s surface.
The Solar Orbiter will provide a successor to the Ulysses probe, observing the Sun’s polar regions. The Solar Orbiter will be at an inclined orbit, using digital cameras to take pictures of the Sun at the closest distance yet. This orbiter will allow for the detection of changing surface activity of the Sun, as well as the change in the magnetic field in the polar regions.
Mysteries
One of the biggest mysteries of the Sun today is how the Corona is heated to 1.7 million degrees Fahrenheit, compared to the surface of the Sun being only about 9,900 degrees Fahrenheit. Many explanations have been put forward to explain this discrepancy, as clearly normal radiative transfer techniques such as convection or radiation cannot explain this jump in temperature. The most popular one is known as the Alfvén wave which is a magnetic wave that carries energy, thus increasing the temperature of the Corona as the waves are ejected from the photosphere.
Another unsolved problem is the faint young Sun issue. Models suggest that the Sun was about as 75% as bright as it is today compared to when it was young, yet geological records show that the Earth was just a little hotter then compared to now. Researchers believe that the surface of the Earth at the time was less reflective, causing more energy to be locked into the atmosphere raising the temperature beyond what is expected from the Solar output. Research into these areas are still ongoing to determine the solution.