The Sun

 

The Sun’s diameter is 864,000 miles, or 109 times that of Earth.  Over a million Earth’s can fit inside it.  Its mass is 330,000 times that of Earth.  The sun accounts for 99.8% of the total mass of the Solar System.  The Sun is made up of 73% hydrogen and 25% helium.  The rest of the mass is made up of oxygen, carbon, neon, and iron. 

The sun formed about 4.6 billion years ago from the gravitational collapse within a region of a large molecular cloud.  It is considered a yellow dwarf (G-type main-sequence star – G2V) among stars.  However, its light is closer to white than yellow.  It appears yellow due to Earth’s atmosphere.

The Sun’s core fuses about 600 million tons of hydrogen into helium every second.  It converts 4 million tons of matter into energy every second.  It takes between 10,000 and 170,000 years for the energy to escape from the core. 

In 5 billion years, the Sun will transform into a red giant and will become large enough to engulf the orbit of Venus and making Earth uninhabitable.  

Light travels from the Sun’s horizon to Earth’s horizon in 8 minutes and 19 seconds.  Light from the closest points of the Sun and Earth takes 8 minutes and 17 seconds.  

The rotational period of the Sun is 25.6 days at the equator and 33.5 days at the poles.  Viewed from Earth as it orbits the Sun, the apparent rotational period of the Sun at its equator is 28 days.  Viewed from above its north pole, the Sun rotates counterclockwise. 

Sunlight at the top of Earth’s atmosphere is 50% infrared light, 40% visible light, and 10% ultraviolet (UV) light.  The atmosphere filters out over 70% of the UV light. 

The surface temperature of the sun is 9,941 degrees Fahrenheit (5,505 degrees Celsius or 5,778 degrees Kelvin). 

The core of the Sun extends from the center to about 25& of the solar radius.  It has a density of 150 grams per cubic centimeter, or about 150 times the density of water.  The core’s temperature is  about 15.7 million Kelvin.  The Sun’s surface temperature is about 5,800 Kelvin.  

The corona, the sun’s outer atmosphere, is hotter than the sun’s surface.  It can be heated up to 10.8 million degrees Fahrenheit, whereas the sun’s surface is only 10,000 degrees Fahrenheit.  For over a century, since 1869, solar physicists have been mystified by the sun’s ability to reheat the corona.  The mechanics of coronal heating are currently unknown.  It may be related to processes in the sun’s magnetic field.  The lowest layer of the sun’s atmosphere is called the photosphere, which is about 300 miles thick.   [source: Sharp, “Atmosphere of the Sun: Photosphere, Chromosphere & Corona,” space.com, Nov 2, 2017]

It takes 250 million years for the sun to complete one orbit through the Milky Way.  It has complete 25 orbits during the lifetime of the Sun.  The sun’s orbital speed in 156 miles per second.  The last telemetry data was received on Feb 10, 1986.

On December 10, 1974, Helios-A (Helios-1) was launched into heliocentric orbit to study solar processes. Built by West Germany’s space agency, it was the first space probe built outside the USA and the USSR to leave Earth orbit.  In February 1975, Helios-1 came closer to the Sun than any previous spacecraft.  The solar panels reached a temperature of 270 degrees Fahrenheit. 

On January 15, 1976, Helios-B (Helios-2) was launched into heliocentric orbit to study solar processes.  The probe set a maximum speed record of 157,076 miles per hour.  Its closest approach to the Sun was 26,987,000 miles.  Maximum temperature on the spacecraft was 306 degrees Fahrenheit.

On October 6, 1990, the Ulysses spacecraft was launched from the Space Shuttle Discovery (STS-41).  It made three complete orbits of the Sun during more than 17 years of service, ending on June 30, 2009.  The mission showed that there was a weakening of solar wind over time.  In 2008, the solar wind was at a 50-year low.  The mission also helped determine that the Sun’s magnetic field “reverses” in direction every 11 years.  This was the first mission to survey the space environment above and below the poles of the Sun.  On May 12, 1995, the spacecraft made its closes approach to the Sun of about 124 million miles.  The spacecraft used a gravity assist from Jupiter to get above the Sun’s poles. 

In August 2001, NASA launched the Genesis spacecraft to collect solar wind samples and return them to Earth to study.

In September 2004, the spacecraft Genesis returned to Earth, but had an unplanned hard landing when its parachute failed to deploy, and the capsule hit the Utah desert at 193 mph.  Still, it marked NASA’s first sample retune since the final Apollo lunar mission in 1972, and the first material collected beyond the moon.  Using the solar wind samples, researches found evidence that the Earth possibly formed from different solar nebula materials than those that created the sun. 

In 2013, physical construction began of the Advanced Technology Solar Telescope (ATST), now known as the Daniel K. Inouye Solar Telescope (DKIST), at Haleakala Observatory in Maui, Hawaii.  It is the world’s largest solar telescope, with a 4-meter aperture.  It obtained its first solar images in December 2019. 

On August 12, 2018, the Parker Solar Probe was launched aboard a Delta IV Heavy rocket.  In 2025, it is expected to fly low into the solar corona, about 4.3 million miles from the surface of the sun.  At its closest approach, the spacecraft will be travelling at 430,000 miles per hour.  This spacecraft is the first to be named after a living person, honoring Dr. Eugene Newman Parker (1927- ), professor emeritus at the University of Chicago.  It was Parker who developed the theory of the supersonic (500 miles per second) solar wind.  The Parker Solar Probe has 4 instruments to study magnetic fields, plasma, energetic particles, and solar wind.  It does not have a camera, which could not survive the intense heat.

In February 2020, the ESA Solar Orbiter was launched.  It completed its first images of the Sun in July 2020.  It will begin routine science operations in November 2021.  Solar Orbiter will take images of the Sun from closer than any spacecraft before and for the first time it will look at the Sun’s polar regions.  Solar Orbiter will be closest to the Sun in October 2022.  On board the spacecraft is Metis, the coronagraph of the scientific payload.  Its main objective is the investigation of the global corona in polarized visible light and in ultraviolet light.  It simulates a solar eclipse.