You get a bonus post today because how can you talk about things astronomical without talking about gravity?
Gravity is one of the four known fundamental interactions* (the other three being electromagnetism, strong nuclear force and weak nuclear force) that causes things with mass and/or energy to come together. Since mass and energy are interchangeable (as in E=MC^2) they are both affected by gravity. Time is also affected by gravity. (Also, the more mass there is together, the stronger the gravity, and the more affect on time it has.)
So, pretend for now the universe is a great big empty nothingness containing only two atoms of hydrogen as far apart as they could possibly be. Because they have mass, they will be attracted to each other, regardless of distance. This attraction is gravity. The closer together they get, the stronger the gravitational attraction.
Now, imagine a universe full of hydrogen. No matter how spread out it is, every hydrogen atom is going to gravitationally interact with every other hydrogen atom. They closer they are, the greater the attraction.
In the early days of the Universe (for those who subscribe to the Big Bang theory), it was full of mostly hydrogen. For whatever reason, certain atoms of hydrogen got attracted to certain other atoms of hydrogen and they all came together. If you get enough hydrogen (a Jeans mass' worth) in a close enough area (a Jeans length's worth), then gravity will collapse it down enough for fusion to start and a star is born. Come back on J-day and we'll talk more about this.
Gravity is what makes fluffy clouds of hydrogen collapse down into stars. If it wasn't for gravity, nothing would bother interacting.
Remember about ten years ago when Mike Brown (aka @plutokiller) suggested the demotion of Pluto to the status of dwarf planet? Much of the current definition of a planet all involve gravity.
1. In orbit around the Sun? Requires gravitational attraction between the planet and the Sun. Planet has enough velocity so it doesn't fall into the Sun, yet insufficient velocity to escape the Sun's gravity well. (Check out the inverse-square law.)
2. Round in shape? Again, sufficient mass to have enough gravity to overcome mechanical structure and assume a round shape. Astronomers call this hydrostatic equilibrium.
3. Clear the neighbourhood around its orbit? As a planet takes it yearly trip around the Sun, it gravitationally affects stuff around it, either flinging it out of the way or dragging it in and either acquiring more small moons or new surface craters. (This is where they claim Pluto and Ceres fall down, because they're kept company by a whole bunch of other rocks in their orbits.)
So yeah. We know what gravity does. It's effect on all things (baryonic and dark matter) is quite noticeable. However, we don't know exactly what gravity is. How is it transmitted? How does it communicate? How do those two hydrogen atoms in our hypothesised empty universe know to be attracted to one another? We've been able to detect gravitational waves in our quest to figure out what this is. Does gravity travel via gravitons, the way light travels by photons? Or is there something else we have yet to discover?
No hardcore stuff for you today, as I think I've included sufficient linkage to fry your brain if you so choose
*Einstein says gravity isn't a "force" per se, but rather a consequence of the curve of spacetime and the uneven distribution of mass and energy. Don't worry too much about that, as you can see the results of two masses experiencing gravitational attraction every time you fall down.
Gravity in our everyday life: friend or foe? You tell me.
Her Grace is wondering if she should be including mathematics in some of her posts.