Category: Guide to Space

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While Jupiter and Saturn are mostly composed of hydrogen and helium, the ice giant Uranus is much different. Instead, it is mostly composed of various ices, like water, ammonia and methane. The mass of Uranus is roughly 14.5 times the mass of the Earth. Astronomers think that between 9.3 and 13.5 Earth masses of this is made up of these ices. Hydrogen and helium only account for about 0.5 to 1.5 Earth masses. The rest of the material in Uranus is probably rocky material.

Uranus probably has three layers inside it: a rocky core at the center, an icy mantle surrounding that, and an outer gas envelope of hydrogen and helium. The core of Uranus is very small, with only half the mass of the Earth. The largest portion of Uranus is the icy mantle. This mantle isn’t exactly ice as we understand it, but it’s actually a hot dense fluid consisting of water, ammonia and other substances. Astronomers sometimes refer to the mantle as a water-ammonia ocean.

We have done many articles about Uranus. Here’s an article about a dark spot in Uranus’ clouds, and here’s a view of Uranus with its rings seen edge on.

Want more information on Uranus? Here’s NASA’s Solar System Exploration page, and here’s NASA’s Uranus fact sheet.

We have recorded an episode of Astronomy Cast just about Uranus. You can access it here: Episode 62: Uranus.

In all of those beautiful images of Uranus captured by Hubble and the Voyager, it’s got a blue-green color. How did Uranus get this color?

The color of Uranus comes from its atmosphere. Just like Jupiter and Saturn, Uranus is composed mostly of hydrogen and helium, with trace amounts of other elements and molecules. The third most common molecule in the atmosphere of Uranus is methane (CH₄). This substance causes the blue-green color of Uranus.

Here’s how it works. Although it looks white, the light from the Sun actually contains all the colors in the spectrum, from red and yellow to blue and green. Sunlight hits Uranus and is absorbed by its atmosphere. Some of the light is reflected by the clouds and bounces back into space. The methane in the clouds of Uranus is more likely to absorb colors at the red end of spectrum, and more likely to reflect back light at the blue-green end of the spectrum. And that’s why Uranus has its blue color.

We have written many stories about Uranus on Universe Today. Here’s an article about recent Hubble images of Uranus and Neptune.

This photograph from NASA has one of the best true-color images of Uranus. And here’s more information on Uranus from Hubblesite.

We have recorded an episode of Astronomy Cast just about Uranus. You can access it here: Episode 62: Uranus.

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Seen from space, Uranus looks bland, enshrouded in blue clouds. This blue-green color of the planet comes from the fact that the atmosphere of Uranus absorbs the red wavelengths of the visible spectrum, and prevents it from bouncing back out into space. All we can see are the blue-green photons reflected into space.

The atmosphere of Uranus is composted mainly of molecular hydrogen and helium. The third most abundant molecule after hydrogen and helium is methane (CH₄). It’s the methane in Uranus’ atmosphere that absorbs the red spectrum visible light and gives it the blue-green color.

Uranus (and Neptune) have different atmospheres from the larger Jupiter and Saturn. Although their atmospheres are mostly hydrogen and helium, they have a higher proportion of ices, like water, ammonia and methane. This is why astronomers call Uranus and Neptune “ice giants”.

Astronomers believe that the atmosphere of Uranus can be broken up into three layers: the troposphere (-500 km and 50 km); the stratosphere (50 and 4000 km) and the thermosphere/corona extending from 4,000 km to as high as 50,000 km from the surface.

We have written many stories about Uranus on Universe Today. Here’s an article about how Uranus can be stormy, and another about a dark spot on Uranus.

Want more information? Here’s an article from Windows on the Universe about the atmosphere of Uranus. And here’s a Hubble photograph of Uranus’ atmosphere.

We have recorded an episode of Astronomy Cast just about Uranus. You can access it here: Episode 62: Uranus.

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Like many planets, Neptune’s orbit isn’t exactly circular. Instead, Neptune orbits the Sun in an elliptical orbit. At its closest point, Neptune gets within 4.45 billion km, and then orbits out to a distance of 4.55 billion km. It takes almost 165 years to complete one orbit around the Sun.

It’s a shame Pluto isn’t a planet any more, because it’s really far. Pluto gets as close as 4.44 billion km. But its orbit is so elliptical that it gets out to a distance of 7.38 billion km. In fact, there are times in Pluto’s orbit when Neptune passes it. Then Neptune really is the farthest planet from the Sun, whether or not you think Pluto is a planet.

What’s farthest object from the Sun? Astronomers think that the long period comets come from a region of the Solar System known as the Oort cloud. It’s possible that this region extends out from the Sun to a distance of 50,000 astronomical units (1 AU is the distance from the Earth to the Sun).

Here’s an article that lists the distances to all the planets.

And here’s an article from Solar Views that talks about the Oort Cloud.

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

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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