Author: Fraser Cain

Everything’s orbiting something it seems. The Moon goes around the Earth, and the Earth orbits the Sun. But did you know that the Sun orbits the Milky Way galaxy?

Astronomers have calculated that it takes the Sun 226 million years to completely orbit around the center of the Milky Way. In other words, that last time that the Sun was in its current position in space around the Milky Way, dinosaurs ruled the Earth. in fact, this Sun orbit has only happened 20.4 times since the Sun itself formed 4.6 billion years ago.

Since the Sun is 26,000 light-years from the center of the Milky Way, it has to travel at an astonishing speed of 782,000 km/hour in a circular orbit around the Milky Way center. Just for comparison, the Earth is rotating at a speed of 1,770 km/h, and it’s moving at a speed of 108,000 km/h around the Sun.

It’s estimated that the Sun will continue fusing hydrogen for another 7 billon years or so. In other words, it only has another 31 orbits it can make before it runs out of fuel.

Are you interested in more articles about the Sun? We have written plenty for Universe Today. Here’s an article that shows how some stars take an erratic journey around the Milky Way, and another article about a ring of stars orbiting the Milky Way.

Here’s an article that describes the process astronomers used to determine the orbit around the Milky Way.

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

References:
NASA Imagine the Universe!
NASA Spacemath
NASA Solar System Exploration Page

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We owe everything we have to the Sun. If it weren’t for the Sun, there’d be no life on Earth. The relationship between Sun and Earth has gone back for 4.6 billion years, and should last for another 7 billion years or so.

As you probably know, the Sun is just a giant sphere of gas. At the core of the Sun, huge quantities of hydrogen are squished together in the intense pressure and temperature of this extreme environment. Hydrogen is converted to helium, and this reaction releases a tremendous amount of energy.

How much energy? Astronomers calculate that there are 600 million tons of hydrogen fused every second. 4 million tons of matter is converted to pure energy every second. This releases 3.86×10²⁶ joules of energy every second. Although most of this energy heads off into space, plenty still falls onto the Earth. In fact, there’s enough energy coming from the Sun to deposit 342 Watts of energy onto every square meter of the Earth (averaged over the year, over the whole planet).

From our perspective, Sun and Earth go hand in hand. This energy from the Sun heats up the planet, preventing us from cooling down to near absolute zero temperatures of space. Our atmosphere traps the energy as heat, keeping the whole planet a nice comfortable temperature.

Plants have been soaking up this energy for millions of years. When you burn gasoline in your car, it comes from oil, which is energy from the Sun that planets have been storing for millions of years.

Sun and Earth are locked in a gravitational dance as well. The mass of the Sun is 2 × 10³⁰ kilograms. This is enough to reach out across space and keep the Earth (and the rest of the planets) locked in orbit around it. We even experience tides from the gravity of the Sun.

Were you wondering how far away the Earth is from the Sun? And the Sun isn’t always trying to help us. Sometimes it’s throwing monster flares at us as well.

Here’s NASA’s Solar System Exploration Guide on the Sun. And here’s the homepage for NASA’s STEREO mission, which is taking amazing pictures and videos of the Sun.

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

References:
NASA Sun Earth Day
NASA Cosmicopia: Sun

Think you know everything there is to know about the Sun? Think again. Here are 10 facts about the Sun, collected in no particular order. Some you might already know, and others will be totally new to you.

1. The Sun is the Solar System
We live on the planet, so we think it’s an equal member of the Solar System. But that couldn’t be further from the truth. The reality is that the mass of the Sun accounts for 99.8% of the mass of the Solar System. And most of that final 0.2% comes from Jupiter. So the mass of the Earth is a fraction of a fraction of the mass of the Solar System. Really, we barely exist.

2. And the Sun is mostly hydrogen and helium
If you could take apart the Sun and pile up its different elements, you’d find that 74% of its mass comes from hydrogen. with 24% helium. The remaining 2% is includes trace amounts of iron, nickel, oxygen, and all the other elements we have in the Solar System. In other words, the Solar System is mostly made of hydrogen.

3. The Sun is pretty bright.
We know of some amazingly large and bright stars, like Eta Carina and Betelgeuse. But they’re incredibly far away. Our own Sun is a relatively bright star. If you could take the 50 closest stars within 17 light-years of the Earth, the Sun would be the 4th brightest star in absolute terms. Not bad at all.

4. The Sun is huge, but tiny
With a diameter of 109 times the size the Earth, the Sun makes a really big sphere. You could fit 1.3 million Earths inside the Sun. Or you could flatten out 11,990 Earths to cover the surface of the Sun. That’s big, but there are some much bigger stars out there. For example, the biggest star that we know of would almost reach Saturn if it were placed inside the Solar System.

5. The Sun is middle aged
Astronomers think that the Sun (and the planets) formed from the solar nebula about 4.59 billion years ago. The Sun is in the main sequence stage right now, slowly using up its hydrogen fuel. But at some point, in about 5 billion years from now, the Sun will enter the red giant phase, where it swells up to consume the inner planets – including Earth (probably). It will slough off its outer layers, and then shrink back down to a relatively tiny white dwarf.

6. The Sun has layers
The Sun looks like a burning ball of fire, but it actually has an internal structure. The visible surface we can see is called the photosphere, and heats up to a temperature of about 6,000 degrees Kelvin. Beneath that is the convective zone, where heat moves slowly from the inner Sun to the surface, and cooled material falls back down in columns. This region starts at 70% of the radius of the Sun. Beneath the convection zone is the radiative zone. In this zone, heat can only travel through radiation. The core of the Sun extends from the center of the Sun to a distance of 0.2 solar radii. This is where temperatures reach 13.6 million degrees Kelvin, and molecules of hydrogen are fused into helium.

7. The Sun is heating up, and will kill all life on Earth
It feels like the Sun has been around forever, unchanging, but that’s not true. The Sun is actually slowly heating up. It’s becoming 10% more luminous every billion years. In fact, within just a billion years, the heat from the Sun will be so intense that liquid water won’t exist on the surface of the Earth. Life on Earth as we know it will be gone forever. Bacteria might still live on underground, but the surface of the planet will be scorched and uninhabited. It’ll take another 7 billion years for the Sun to reach its red giant phase before it actually expands to the point that it engulfs the Earth and destroys the entire planet.

8. Different parts of the Sun rotate at different speeds
Unlike the planets, the Sun is great big sphere of hydrogen gas. Because of this, different parts of the Sun rotate at different speeds. You can see how fast the surface is rotating by tracking the movement of sunspots across the surface. Regions at the equator take 25 days to complete one rotation, while features at the poles can take 36 days. And the inside of the Sun seems to take about 27 days.

9. The outer atmosphere is hotter than the surface
The surface of the Sun reaches temperatures of 6,000 Kelvin. But this is actually much less than the Sun’s atmosphere. Above the surface of the Sun is a region of the atmosphere called the chromosphere, where temperatures can reach 100,000 K. But that’s nothing. There’s an even more distant region called the corona, which extends to a volume even larger than the Sun itself. Temperatures in the corona can reach 1 million K.

10. There are spacecraft observing the Sun right now.
The most famous spacecraft sent to observe the Sun is the Solar and Heliospheric Observatory, built by NASA and ESA, and launched in December, 1995. SOHO has been continuously observing the Sun since then, and sent back countless images. A more recent mission is NASA’s STEREO spacecraft. This was actually two spacecraft, launched in October 2006. These twin spacecraft were designed to watch the same activity on the Sun from two different vantage points, to give a 3-D perspective of the Sun’s activity, and allow astronomers to better predict space weather.

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

References:
NASA Science
NASA SOHO
NASA Stereo

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The Sun and the Moon are the two objects in the Solar System that influence Earth the most. Let’s take a look at all the different was we experience these two objects, how they’re similar, and how they’re mostly different.

The Size of the Sun and the Moon

In absolute terms, the Sun and the Moon couldn’t be more different in size. The Sun measures 1.4 million km across, while the Moon is a mere 3,474 km across. In other words, the Sun is roughly 400 times larger than the Moon. But the Sun also happens to be 400 times further away than the Moon, and this has created an amazing coincidence.

From our perspective, the Sun and the Moon look almost exactly the same size. This is why we can have solar eclipses, where the Moon passes in front of the Sun, just barely obscuring it from our view.

And this is just a coincidence. The gravitational interaction between the Moon and the Earth (the tides) are causing the Moon to slowly drift away from the Earth at a rate of 3.8 centimeters per year. In the ancient past, the Moon would have looked much larger than the Sun. And in the far future, the Moon will look much smaller. It’s just a happy coincidence that they look the same size from our perspective.

Gravity from the Sun and the Moon

Once again, the Sun is much larger and has a tremendous amount of mass. The mass of the Sun is about 27 million times more than the mass of the Moon. It’s this gravitational interaction that gives the Earth its orbit around the Sun, and the tiny pull of the Moon just causes the Earth to wobble a bit in its movements.

When the Sun and the Moon are pulling on the Earth from the same direction, their gravity adds up, and we get the largest spring tides. And then, when they’re on opposite sides of the Earth, their forces cancel out somewhat, and we get neap tides.

Light from the Sun and the Moon

This is a bit of a trick, since the Sun is the only object in the Solar System actually giving out light. With its enormous mass, the Sun is able to fuse hydrogen into helium at its core, generating heat and light. This light shines in the Solar System, and bounces off the Moon so we can see it in the sky.

Astronomers measure brightness using a measurement called magnitude. The star Vega was considered 0 magnitude, and the faintest star you can see with the unaided eye is about 6.5 magnitude. Venus can get as bright as -3.7, the full Moon is -12.6, and the Sun is -26.73. These numbers sound similar, but it’s a logarithmic scale, where each value is twice the amount of the previous one. 1 is twice as bright as 2, etc.

So the Sun is actually 450,000 times brighter than the Moon. From our perspective.

Composition of the Sun and the Moon

Now here’s where the Sun and the Moon differ. The Sun is almost entirely composed of hydrogen and helium. The Moon, on the other hand, was formed when a Mars-sized object crashed into the Earth billions of years ago. Lighter material from the collision collected into an object in orbit – the Moon. The Moon’s crust is primarily oxygen, silicon, magnesium, iron, calcium, and aluminium. Astronomers think the core is metallic iron with small amounts of sulfur and nickel. And it’s at least partly molten.

Here’s an article about the distance from the Earth to the Sun, and here’s a view of the Earth and the Moon, seen from Mars.

Have you ever seen that picture of the Moon and the Sun “from the North Pole”, where the Moon looks huge? It’s actually a hoax, here’s more information from Astronomy Picture of the Day.

References:
NASA SOHO
NASA Starchild: Earth’s Natural Satellites
NASA Eclipse: Measuring the Moon’s Distance
NASA: Stellar Magnitude Scale