# Star

A star is a sphere_ of plasma_ held together by its own gravity.

A star is called a sun if it is the center of a planetary system.

Most stars in the universe are invisible to the naked eye from Earth. Most are invisible even through the most powerful telescopes.

# 1   Distance

We can calculate the distance to a distant start by taking advantage of parallax_ and the orbit of the Earth. If we view a star in January, and again six month later in July, we can draw a triangle. The base is the diameter of Earth's orbit. The opposite angle is how far the star appears to shift. Then the distance can be calculated using trigonometry.

# 2   Questions

• How do we know the distance to the sun?

Our solar system formed from a nebulous cloud of swirling gas and dust which coalesced due to gravity. However, this nebula started off as a shapeless blob. So how does the solar system end up with all the planets and their moons orbiting in a flat disk? Why don't planets orbit the sun in multiple dimensions?

Our solar system isn't unique. Many star systems are flat, a lot of galaxies are flat, black hole accretion discs are flat, Saturn's rings are flat. So why does the universe have this preference for flatness?

The answer has to do with two things: collisions and the fact that we live in three dimensions. Any time a bunch of objects are held together by gravity, their individual paths are nearly impossible to predict. But collected together they have a single total amount of spin about their center of mass. It may be hard to figure out exactly what direction that rotation is in, but mathematics implies there must be some plane in which the cloud taken as a whole spins in two dimensions.

In three dimensions, even though the rotation of the cloud is given by one plane, particles can whiz around far up and down from that plane as the particles bump into each other. All the up-and down motion tends to cancel out its energy lost in crashing and clumping, yet the whole mass must continue spinning because the total amount of spinning in any isolated system always stays the same. Over time, through collisions and crashes, the cloud flattens into a spinning roughly two dimensional shape like a solar system or spiral galaxy.

The Sun loses about 6 million tons of mass every second due to nuclear fusion and the solar wind. Despite long that much material, it has only lost 0.05% of its ordinal mass over the past 4.5 billion years.