Author: Fraser Cain

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The Sun is hot. Really really hot. But all of the heat and light coming from the Sun comes from the fusion process happening deep inside the core of the Sun. The core of the Sun extends from the very center of the out to about 0.2 solar radii. Inside this zone, pressures are million of times more than the surface of the Earth, and the temperature reaches more than 15 million Kelvin. This is where fusion in the Sun happens.

Every second, 600 million tons of hydrogen are being converted into helium. This reaction releases a tremendous amount of heat and energy.

The process of fusion in the Sun is known as the proton-proton chain. The Sun starts with protons, and though a series of steps, turns them into helium. Since the total energy of helium is less than the energy of the protons that went into it, this fusion releases energy.

Here are the steps.

1. Two pairs of protons fuse, forming two deuterons
2. Each deuteron fuses with an additional proton to form helium-3
3. Two helium-3 nuclei fuse to create beryllium-6, but this is unstable and disintegrates into two protons and a helium-4
4. The reaction also releases two neutrinos, two positrons and gamma rays.

As we said, a helium-4 atom has less energy than the 4 protons came together. All of the heat and light streaming from the Sun came from this fusion reaction.

Here’s an article about how the conditions inside supernovae have been recreated in the lab, and another about a white dwarf star that just shut down its fusion reactions.

Here’s an article from NASA that helps explain how the fusion process works. And here’s a project that lets your students understand the process by making their own fusion reactions.

We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.

Many of the brightest, most familiar stars in the sky have names. For example, have you ever heard of Sirius – the brightest star in the sky? Or Polaris, also known as the North Star. If all these stars have names, does the Sun have a name?

Actually, the Sun doesn’t have its own name, apart from “the Sun”. But “sun” is also a generic name that you can use for any star. Sometimes people say that a star has the mass of 20 suns, or planets orbit other suns. You might have heard the term “sol”, but that’s just another name for Sun, based on the Roman God of the Sun.

We now know that the Sun is just a star. And so, it can be classified into categories like the other stars in the Universe. Just in case you were wondering, the Sun is a G2V star. The G2 part refers to the spectral class, and the V part is the luminosity. Stars with the “V” designation are in the main-sequence, or hydrogen burning, phase of their lives.

So it’s kind of strange to say, but Sun has no scientific name or designation, apart from, “the Sun”. Every other star in the sky does have a scientific designation.

We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.

Astronomers and astrologists have used various symbols to depict all of the planets, and many of the minor objects in the Solar System. Perhaps two of the most commonly used are the Sun and Moon symbols.

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The symbol for the Sun looks like circle with a dot in the middle of it. Historians aren’t sure what it represents any more, but it’s the same symbol as the one used by the ancient Egyptians to represent Ra… the Sun god. It’s also possible that it looks like a shield.

The symbol for the Moon is… a picture of the Moon. Specifically, the symbol for the Moon looks like a crescent Moon in the last quarter. This symbol is very obvious, as it’s what ancient peoples saw in the sky for thousands of years, and it’s the same thing we see today.

Astronomers use both Sun and Moon symbols when they’re writing research journals. It’s much faster to just put in the symbol for the object.

Want more astronomical symbols? Here’s the symbol for the Earth, and here’s the symbol for Mars.

And Wikipedia has a great list of all the astronomical symbols.

We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.

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Have you ever looked up and noticed that there’s a big ring around the Sun? These rings are caused by ice crystals within thin cirrus clouds, and there several different kinds of sun rings you can see depending on the weather conditions.

One of the most common ones is called a 22° halo. They get this name because the ring is located 22 degrees away from the Sun itself. Both the Sun and the Moon block a 1/2 degree region of the sky at a time, so the ring around the Sun is about 44 times larger than the Sun itself.

Why do you get a ring at exactly 22°? The ring is formed because of the ice crystals suspended in the cirrus clouds. If you could look at the crystals under the microscope, you would see that they’re hexagonal in shape, and act as prisms for the Sun’s light. As light passes through the two sides of the prism, it’s deviated by exactly 22°. Since the ice crystals are jumbled up randomly in the sky, most of the light is deflected away. But from every position you’re always able to see the deflected light from some of the crystals in the sky. And this is why you see the bright ring around the Sun.

When you’re looking for halos, or rings around the Sun, make sure you always shield both eyes from the Sun. Even looking at the Sun for an instant can cause permanent eye damage.

Here’s an article from Universe Today that includes instructions for looking for Sun halos.

Here’s a great article from Atmosphere Optics that helps to explain the process.

We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.

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The amount of the Sun’s energy falling at any point on the Earth depends on the angle of the Sun. This is reason why the seasons have different temperatures, and the polar regions are colder, on average than regions around the equator. Let’s take a look at why the angle of the Sun is so important, and how its change effects the Earth’s climate.

To understand how various parts of the Earth receive less energy, imagine holding a flashlight, and pointing straight at a piece of paper. Light comes out of the flashlight and forms a perfect circle on the paper. At this point, the energy from the flashlight is most concentrated in each square centimeter on the paper. Now imagine angling the paper so that the flashlight’s beam creates a big ellipse on the paper. The same amount of energy is coming out of the flashlight, but it’s being spread out across a much larger area of paper. Each square centimeter of paper is receiving less light than it was before.

Take this analogy to the Earth. When the Sun is directly overhead, like for people in the tropics, the maximum amount of energy is being soaked up by each square meter of Earth. This causes temperatures to rise. For the polar latitudes, the Sun is at a steep angle, so the same amount of energy from the Sun is falling over a much larger area.

During summer in the northern horizon, the Sun is at its maximum angle in the sky, and we get the most energy. But in the winter, the Sun is at a much steeper angle, and so we get less energy from the Sun. And this is why we experience different seasons – it’s all in the angle of the Sun.

Here’s more information from Universe Today about how the Earth has seasons. And Mars has seasons too.

Windows on the Universe has a great description of this. Here’s a handy tool you can use to calculate sunrise and sunset times, as well as the angle of the Sun.

We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.

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Modern science tells us that the Sun is a big hot ball of hydrogen at the center of the Solar System, and all the planets orbit around it. But ancient people didn’t have access to the same scientific tools we have today. Their understanding about the Sun was much more primitive, and often… wrong. Let’s investigate the history of the Sun.

Most life on Earth evolved with the Sun in mind; the rising and setting Sun defined the cycle of daily life for almost all life. Ancient peoples were entirely dependent on the Sun for light; only the light from a full Moon gave any way to see in the night. It wasn’t until the invention of fire that humans had any way to get any work done after the Sun went down.

Since the Sun was such an important object, many ancient people treated it with reverence and considered the Sun a god. Many worshipped the Sun, and built monuments to celebrate it. Monuments like Stonehenge in England, and the Pyramids of Egypt were used to mark the position of the Sun over the course of the year.

The first accurate measurement of the distance to the Sun was made by Greek philosopher Anaxagoras. Of course, he was threatened with death for his ideas that the Sun was a burning ball of fire, and not a god.

It was long thought that the Sun orbited around the Earth, but it was Nicolaus Copernicus who first proposed a Sun-centered Solar System. This theory gained evidence from Galileo and other early astronomers. By the 1800s, solar astronomy was very advanced, with astronomers carefully tracking sunspots, measuring absorption lines in the spectrum of light from the Sun, and discovering infrared.

For the longest time, astronomers were puzzled by how the Sun generated so much energy. It wasn’t until the 1930s when astrophysicists Subrahmanyan Chandrasekhar and Hans Bethe finally developed the theoretical concept of nuclear fusion, which explained the Sun (and all stars) perfectly.

NASA has a great website with photographs of ancient building used to mark the position of the Sun, and more about solar eclipses of historical interest.

Want more history? Here’s an article about the history of Venus, and another about the history of Saturn.

We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.

Reference:
NASA Sun-Earth Day: 2009, Issue # 64
NASA Ancient Observatories

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I’m sure you know that we live in the Milky Way galaxy, but where is the Sun located? And how did astronomers figure out where the Sun is located, since we’re living inside the galaxy?

The Milky Way is a grand spiral galaxy, which astronomers think has four major spiral arms: Perseus, Cygnus, Scutum-Crux, Sagittarius. Some astronomers think we might actually just have two arms, Perseus and Sagittarius. The Sun is located in the inner rim of the Orion Arm, which is thought to be an offshoot of the Sagittarius Arm. The Sun is located about 26,000 light-years away from the center of the galaxy.

Before telescopes, the Milky Way just looked like a bright area in the sky, but when Galileo first turned his telescope on the region in 1610, he realized that it was actually made up of faint stars. The astronomer Immanuel Kant correctly guessed that this might be a cloud of stars held together by gravity, like the Solar System.

The famous astronomer William Herschel attempted to map out the stars in the Milky Way to get a sense of the galaxy’s size and shape, and determine the Sun’s position in it. From Herschel’s first map, it appeared the Sun was at the center of the Milky Way. It was only later on that astronomers realized that gas and dust was obscuring our view to distant parts of the galaxy, and that we were actually in the outer region of the Milky Way.

The astronomer Harlow Shapley accurately determined where the Sun is in the MIlky Way in the early 20th century by noticing that globular clusters were uniformly located above and below the Milky Way, but they were concentrated in the sky towards the constellation Sagittarius. Shapely realized that many globular clusters must be blocked by the galactic core. He created one of the most accurate maps of the Milky Way.

It wasn’t until the 20th century, with the development of larger and more powerful telescopes that astronomers could see the shape of other spiral galaxies, located millions of light-years away. In 1936, Edwin Hubble used cepheid variables as yardsticks to measure the distances to many galaxies, and prove conclusively that the Universe was filled with galaxies, each with as many stars as our own Milky Way.

Here’s an article from Universe Today about how the Milky Way might actually just have two spiral arms, and the largest picture ever taken of the Milky Way.

Here’s an article about the Great Debate that Harlow Shapley had about the nature of the Milky Way. And here’s Shapley’s obituary, published in Nature in 1972.

We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.

Reference:
NASA’s Imagine the Universe!

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In ancient times, everyone thought the Earth was the center of the Universe – it was obvious to anyone who just looked up. The Sun, Moon, stars and planets were thought to be attached to crystal spheres that turned around us. We now know that the Earth goes around the Sun, but how do we know this?

In astronomy, putting the Sun at the center of the Solar System is known as heliocentrism, while putting the Earth at the center is called geocentrism. As astronomers put in more and more time studying the heavens, they realized that this model didn’t match reality. The Sun didn’t follow an exact path every day, and the planets didn’t move how they were supposed to.

It wasn’t until the 16th century that the Polish astronomer Copernicus developed a model that placed the Sun at the center of the Solar System.

Until that point, astronomers had developed very complicated models that tried to explain the motions of the planets. At times they appeared to move backwards in the sky, and then go forwards again. Astronomers had developed the thought that there were spheres within spheres that could explain these motions. Copernicus simplified things, and showed that all the planets were orbiting the Sun, and the strange motions of the planets was then easy to understand as the Earth caught up and then passed them in orbit.

In 1610, Galileo used his first rudimentary telescope to observe that Venus went in phases just like the Moon. This went against the theory that everything orbited the Earth, and was further evidence that it goes around the Sun. Galileo also observed how Jupiter has 4 major moons that orbit it. This broke the previous believe that all objects orbit the Earth.

More precise measurements followed, and Johannes Kepler created his three laws that explained that the planets were actually following elliptical orbits around the Sun. He was the first astronomer to accurately predict a transit of Venus, where the planet was seen to pass directly in front of the Sun.

The motion of the Earth as it goes around the Sun is well calculated today. Space agencies use these calculations to launch spacecraft to explore the other planets in the Solar System. If everything went around the Earth, we’d know by now.

References:
NASA: Heliocentric Solar System
NASA Earth Observatory: Planetary Motion

There are so many beautiful pics of the Sun, it’s almost too difficult to know where to start.

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This is a picture of the Sun captured by NASA’s SOHO spacecraft. It would be a typical day on the Sun, except for the enormous coronal mass ejection blasting out of the upper right-hand side of the Sun. When the Sun is at its most active state, it can release 5-6 of these a day.

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This photograph of the Sun was one of the first captured by NASA’s STEREO mission. These twin spacecraft were launched in 2006. One is leading the Earth in orbit, while the other has fallen behind. With both observing the Sun, scientists are given a 3-dimensional view of the Sun.

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This pic of the Sun shows our star on a calm day, believe it or not. When you look close, this is what the surface of the Sun is doing all the time. The TRACE spacecraft was launched in 1997, and helps scientists study the Sun’s magnetic field – and to take beautiful photos like this.

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This picture of the Sun was captured by the EIT instrument on board the NASA/ESA SOHO spacecraft. It reveals the normally invisible ultraviolet radiation streaming from the Sun. It’s actually a composite of three different Sun photos captured at different parts of the ultraviolet spectrum and then merged together.

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You’re going to need a set of 3-D glasses to get the most out of this Sun photograph. It’s an image of Sun captured by NASA’s twin STEREO spacecraft. Images like this help scientists understand how the Sun interacts with its local environment, and better predict space weather.

We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.