The Circumsolar Habitable Zone

Like the Milky Way Galaxy, our Sun has a habitable zone too, but for different reasons.  Well, one reason, really - heat.

Our Sun radiates huge amounts of light.  The closer you are, the less the light between you and it is diffused by trace amounts of dust and gas, as well as by the distance the light must travel.

When the light hits an object, some of it is absorbed and re-radiated at a different wavelength -  infrared.   Infrared radiation is also known as heat.  So, the closer you are (generally speaking), the hotter you are.

Other variables include how reflective the surface of the body is - icy bodies are much colder than rocky bodies in similar orbits around the sun because they absorb far less light.  The density and composition of a body's atmosphere plays a role as well.  For example, Venus's atmosphere is mainly carbon dioxide - a greenhouse gas that absorbs heat.   Add to that a thick cloud layer that reflects the heat back to the surface faster than the atmosphere can let it radiate back into space, and you end up with a planet surface hundreds of degrees hotter than that of Mercury's, even though it's about 35 million miles further from the Sun.

As things are right now, Earth is comfortably in the middle of the habitable zone, which starts a little bit beyond the orbit of Venus, and ends just before the orbit of Mars.  But it wasn't always there, and it won't be there forever.

When the sun first burned into existence, it burned quite a bit hotter for a short time, and then cooled down.  At the time the Earth was more or less fully formed, the Sun shone with about 70% of today's luminosity.  So why didn't the Earth start out as a popsicle?  The are a few reasons.

The first answer is that as the Earth is formed from billions and billions of small, rocky bodies colliding with each other.  The particles started out quite small; not even pebble sized; but as they grew bigger, they began attracting each other.  Soon, you had kilometer wide - and larger  - rocks slamming into each other at tremendous speeds.  This, of course, released a good chunk of heat, which warmed the growing Earth.  Even as it grew large enough that the core, mantel and crust could separate, the surface was under heavy bombardment from lumps of rock several tons in weight.

Second, as the Earth grew larger and larger, the interior of the planet was placed under tremendous pressure.  This pressure kept the core and mantel melted; the heat escaped into the crust and kept it thawed.

Third, at the same time, you also had radioactive elements decaying inside the planet.  This decay also generated a lot of heat, which helped keep the planet from cooling too much.

And fourth, you had the early atmosphere, which had large amounts of carbon dioxide and other greenhouse gases.  This trapped the heat given to us by the sun in the atmosphere, which kept the surface of the Earth at temperatures suitable for liquid water.  And since it was a readily available food source, the earliest life on Earth ate these gases, releasing oxygen; so as the Sun warmed and bathed the Earth in more heat, less heat was trapped by the atmosphere, and the water didn't boil away.

Which is good for us, since showering is key to a successful social life, and water is key to showering.  And life, as we know it, in general.