Answers to the informal quiz, Part 4: The Universe, etc.

What, you say? Over six months since the last post on Dear Planetary Astronomer Mike? Surely that can't be right. Moving on...the final answers to the informal quiz!

• What's the difference between the Solar System, the Galaxy, and the Universe?

These are terms that to the layman might seem interchangeable as simply "big places that the Earth is a part of", but to astronomers these divisions are of paramount importance.

Our solar system consists of one star - the Sun - and all the objects that orbit it. From largest to smallest, these objects include:

- 2 gas giant planets (Jupiter and Saturn)
- 2 ice giant planets (Uranus and Neptune)
- 4 terrestrial planets (Mercury, Venus, Earth, and Mars)
- 5 dwarf planets (Pluto, Eris, Ceres, Haumea and Makemake)
- Several hundred thousand rocky asteroids
- Many thousands (?) of icy/rocky objects in the Kuiper belt
- Millions (?) of comets
- A whole lot of dust

There's no real hard limit to where our solar system ends, although as you travel farther and farther from the center, at some point you're no longer gravitationally bound to the Sun and begin feeling the gravitational pull of other nearby stars. This limit is generally placed somewhere around the 1 light-year mark (the next closest star is 4.2 light years away) and roughly marks the outer edge of the Oort cloud, a massive hypothesized reservoir of our solar system's comets.

Next up: the galaxy. From a very dark location away from city lights, you can often make out the structure of our galaxy - the Milky Way - as a faint band of light across the sky. The Romans named this the "Via Lactea", literally the "Way of Milk", and forms the root of the word galaxy.

Our Sun is just one star of roughly 300 billion found in the Milky Way Galaxy. In addition to all those billions of stars - each of which could have many planets - there's another several billion solar masses worth of gas and dust from which new stars are constantly forming, and old stars are constantly replenishing. Lying at the exact center of this giant "star city" is a supermassive black hole, calculated to be roughly 3 million times more massive than our own Sun.

Even more massive than all of our galaxy's stars, gas, dust, and the central black hole put together, though, is our galaxy's supply of dark matter. As stated in a previous post, we don't really know what dark matter is exactly, but we know it's there. The mass of our galaxy's dark matter is currently estimated to be at least 1 trillion times the mass of our Sun.

Finally, the universe. It's everything...literally. Anything that exists, exists within our universe. We know there exist many, many billions of galaxies - each with many billions of stars - which stretch out across a cosmic web-like structure. Between these web-like filaments, each made of thousands of galaxies, are gigantic voids where little matter exists at all. The assumption is this void-and-filament structure came initially from microscopic density fluctuations just a few seconds after the Big Bang which has been ballooning outwards ever since.

• What is a star?

A star is nothing more than a ball of gas which is massive enough to produce sufficient internal pressure to start hydrogen fusion. As mentioned in the last post, our Sun is somewhat average on the mass scale of stars, though there tends to be a whole lot more small stars than large stars. For a more detailed examination of the life of stars, see this post.

• How are planets different than stars?

The big difference here is that, going by the above answer, planets *don't* have enough mass to produce sufficient internal pressure to start hydrogen fusion. Our solar system's largest planet, Jupiter, is still quite far from being a star. In fact, Jupiter would need to be 80 times more massive to produce enough internal pressure to start hydrogen fusion at its core.

Now, there is an intermediate group of objects known as "brown dwarfs", which aren't quite stars, and aren't quite planets, either. If Jupiter were only 13 times more massive it could fuse deuterium, an uncommon isotope of hydrogen (even though it still couldn't fuse regular old hydrogen).

So, a brown dwarf can shine like a star for a little while, but the problem is deuterium is uncommon. Once a brown dwarf uses up what little deuterium it has in a matter of a couple million years, that's it...it just cools down like a planet from then on. (Note that a couple million years is nothing compared to the several billion years our Sun will last, or even the trillions of years some small red stars will last.)

• Where do the stars go during the day?

Why, they're still there, of course! Just because the sky is lit up with sunlight during the day doesn't mean that the stars have "gone" anywhere. If you carefully point a telescope at the brighter stars in the middle of the day, you can actually make them out in the clear blue sky! (Cautionary because-our-lawyers-told-us-we'd-better note: do not ever, ever, ever point a telescope at the Sun!).

On days with very clear blue skies, you can even spot the planet Venus completely unaided without any telescope. It looks like a little white dot hanging in the daylit sky...the trick is to know exactly where to look.

• What's the farthest human beings have ever traveled in space?

In spite of all the sci-fi you may have seen, humans just haven't traveled that far from the planet that brought them into existence. To this day, the Moon is the most distant object humans have ever reached - only about 250,000 miles away. Compare that to the nearest planet, Venus, which even at its closest approach to Earth is over 100 times farther than the Moon.