Solar System
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"Sun." Space, Solar System, Q-files Encyclopedia, 12 May. 2023.
https://www.q-files.com/space/solar-system/sun.
Accessed 19 Mar. 2024.
Sun 2023. Space, Solar System. Retrieved 19 March 2024, from
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Space, Solar System, s.v. "Sun," accessed March 19, 2024.
https://www.q-files.com/space/solar-system/sun
Sun
The Sun is a typical star, a spinning ball of hot gas made up almost entirely of hydrogen (three-quarters of its mass) and helium. To us it is of crucial importance since no life could exist without it, but it is simply one of hundreds of billions of stars in the Milky Way Galaxy—itself one of billions of galaxies in the Universe. The Sun's diameter is more than 100 times that of Earth. Although larger than most stars, it is tiny when compared to supergiants; it is sometimes classified as a “yellow dwarf”. The Sun contains more than 99% of all the matter in the Solar System—planets included.
Internal layers
At the centre of the Sun is the core, a region of incredible pressure (200 billion times that on the Earth’s surface) and intense heat—about 15 million°C (25 million°F). This is the Sun’s nuclear furnace, where the energy that keeps it shining, is created. It produces massive amounts of energy by “burning” about four million tonnes of hydrogen every second. Hydrogen atoms fuse together to form helium. Energy from this reaction flows out from the core through the radiative zone to the convective zone. Here, in a continuous cycle, hot gas bubbles up to the surface before sinking down to be reheated again.
Photosphere and chromosphere
The Sun’s visible surface, the photosphere, is only about 500 kilometres (300 miles) thick and, at 5500°C (9900°F), much cooler than the core. It is in a state of constant motion, like soup in a saucepan. Surrounding the photosphere is the chromosphere, a hotter, transparent layer. Hundreds of thousands of flaming gas jets, called spicules, leap up to 10,000 kilometres (6000 miles) out from the chromosphere into the outermost layer of the Sun's atmosphere, the corona.
This picture of the Sun (right) was taken just 77 million kilometres (48 million miles) from its surface. One of a set from the European Space Agency's Solar Orbiter probe in July 2020, it is the closest photograph ever captured by cameras. The image reveals mini-flares, which astronomers have called "camp fires" (the white arrow points to one), on the solar surface. They are minute in comparison to the Sun's giant flares observed by Earth-based telescopes.
Arches, flares and sunspots
Invisible lines of magnetic force that twist throughout the Sun’s globe and beyond are the cause of many extraordinary features. Huge arches, called prominences, can be held up above the Sun by magnetism. Flares, sudden, massive explosions of energy, burst forth when the magnetic field shifts. Where magnetic field lines erupt through the photosphere, there are dark, cooler areas (about 4300°C / 7800°F) known as sunspots.
Beyond the chromosphere lies the corona, the Sun’s hot outer atmosphere. This, along with the reddish chromosphere, is visible from Earth only during a total solar eclipse.
Solar eclipse
By coincidence, the Moon and Sun appear to be the same size in the sky. So when the Moon passes between the Earth and the Sun, its New Moon phase, it may block out the view of the Sun from some parts of the Earth. This is called a solar eclipse. In a total eclipse, the Moon covers the Sun’s surface entirely and the corona, the Sun's broad outer atmosphere shines out from behind a black disc. For a short while, dusk falls in the zone of totality, which is the long, narrow area on the Earth’s surface where the Moon’s shadow falls.
This sequence (pictured right) shows the Moon passing in front of the Sun to bring about a total eclipse. If the Moon were on exactly the same orbital plane as Earth, there would be a total eclipse every month. But because the Moon's orbit is slightly tilted relative to Earth's, its shadow at New Moon usually misses our planet.
On average, two total eclipses are visible from somewhere on Earth every three years.
The umbra is the name given to the Moon's full shadow. People in places where it falls see the Sun’s photosphere completely covered and experience a total eclipse. The penumbra is the partial shadow which causes a partial eclipse, and people in places where this falls—a somewhat wider area—see the Moon‘s black disc covering only a part of the Sun. The light dims only slightly for the duration of a partical eclipse.
Because the Moon's umbra traces only a narrow path on Earth's surface, total eclipses are rare at any particular location: it may be 500–600 years before another one is visible from the same point. Partial eclipses are more common. Total eclipses last a maximum of 7 minutes 32 seconds, but are usually shorter than that.
A star in the Galaxy
The Sun is a star, just like many of the others we see in the night sky. It only seems much larger and brighter—bathing the Earth in light during the day—because it lies so much nearer: "only" 150 million kilometres (93 million miles) away. The Sun is positioned on one of the spiral arms, the Orion Arm, about 25,000 light years—halfway out—from the central point of the Milky Way Galaxy. The Sun's region of the spiral revolves at about 250 kilometres (150 miles) per second.
Other stars are much more distant. The next nearest, Proxima Centauri, is so far away it would take a supersonic jet aircraft about 5 million years to reach it (were there air in space for it to fly through). Other stars in our Galaxy are tens of thousands of times farther away than that. And the Galaxy is one of billions of galaxies in an unimaginably vast Universe. The nearest spiral galaxy like our own, the Andromeda Galaxy, lies 2.25 million light years distant.
Future Sun
To astronomers, the Sun is a main sequence star, classified as a yellow dwarf (although its light is more white than yellow). It formed approximately 4.6 billion years ago, and is now middle aged—roughly halfway through its life. The Sun has not changed dramatically so far, and will remain fairly stable for more than another five billion years. From its birth, however, it has been gradually getting warmer and brighter. This is because it is burning through its hydrogen fuel at an increasing rate. It means that, in about 800 million years, the Sun's fierce heat will have killed off all life on Earth except the simplest micro-organisms.
By about 1.1 billion years into the future, the Sun's increased radiation will cause the Solar System's "habitable zone" to shift outwards: the Earth's surface will have become too hot for water to exist in liquid form—and liquid water is essential for life. At the same time, Mars's surface temperature will gradually rise, releasing carbon dioxide and water, currently frozen, into its atmosphere. This may produce conditions similar to those on Earth today—perhaps even providing a haven for life. By contrast, in about 3.5 billion years from now, Earth could resemble Venus, the hottest of all the planets, where all life on the surface is impossible.
Death of the Sun
About 5 billion years from now, the hydrogen in the core that the Sun uses as fuel to create energy will start to run out. Eventually, the Sun will balloon into a red giant, about one hundred times larger than it is now, engulfing Mercury and Venus—and possibly Earth. The Sun will lose about half of its mass as its outer layers are blown off. Radiation produced by the the core’s heat will turn these layers into a brightly glowing, colourful ring of plasma (electrically-charged gas): a planetary nebula. It will shine for about 10,000 years.
The Sun will eventually shed all its layers into space. All that remains will be, at first, a small, extremely dense star—a white dwarf—before it, too, eventually cools and becomes a black dwarf.
Death of a planet
In 2023, astronomers witnessed for the first time the intense burst of light from a planet being swallowed by its host star—the same fate that awaits Earth when the Sun expands rapidly near the end of its life. The ageing star swells so much that a nearby planet starts to skim the surface, sending streams of gas and dust into space, before finally plunging into the depths of the star.
The flash detected by astronomers came from a star 12,000 light years away, near the constellation of Aquila. Over just 10 days, the star became several hundred times brighter than normal. A planet the size of Jupiter circled the star incredibly fast, completing an orbit in less than a day. It continued to skim the star’s surface for a few hundred orbits before eventually plunging into the star, causing it to brighten rapidly and then fade.
Factfile
Diameter: 1.4 million km (900,000 miles)
Density (water = 1): 1.4
Mass (Earth = 1): 330,000
Rotation period at equator: 26 days
Rotation period at poles: 34 days
Average distance from Earth: 149.6 million km (93 million miles)
Surface (photosphere) temperature: 5500°C (9900°F)
Composition: hydrogen (74.4%); helium (24.9%) traces of oxygen, carbon and other elements
Consultant: Mike Goldsmith