Astronomy

Front 1

What exactly is a supernova?

Back 1

A supernova is what happens when a very massive star runs out of the fuel in its core. Stars run on nuclear fusion, which converts lighter elements to heavier ones + energy. Eventually the core gets full of iron, which soaks up energy rather than releasing it.

Front 2

If the Big Bang really did take place, where is the center of the Universe?

Back 2

According to the Big Bang theory, the Universe did not even exist before the explosion occurred. And space itself did not exist until the Big Bang made it and carried it along outwards into the expanding Universe. So, in a sense, the center of the Universe, or the "place" where the Big Bang happened, is spread out throughout the Universe.

This is a fairly hard concept for us three-dimensional beings to really grasp. It is outside our everyday experience and common sense. But an analogy might help: Imagine that there is a two-dimensional creature confined to the surface of a slowly expanding balloon, which is its universe. (This is a very special balloon -- its skin can stretch a very long way without breaking and it doesn't need a blow hole.)

Front 3

What is the location of the Sun in the Milky Way?

Back 3

The Milky Way galaxy is a 'spiral galaxy', meaning that it is a fairly flat disk shape, with bright spiral arms coming out of the center (much like the shape made by the water coming out of a garden sprinkler). The Sun is located in one of these spiral arms, about two-thirds of the way out from the center of the galaxy.

Front 4

what's the difference between a star and a planet?

Back 4

Physically: Planets are cool objects, sometimes with a hard crust, that orbit stars. Our Earth is a planet. Mars is a planet with a hard crust, like Earth is. Jupiter and Saturn are planets that have no hard surface - they are called "gas giants" because their atmospheres continue far down into the depths of each planet. (Earth's atmosphere is only about 100 km thick, which is about 1.5% of the Earth's radius). Most planets in our own solar system are "only" a few hundred million kilometers from the Earth. Earth is a smallish but fairly typically-sized planet.

Stars are much much bigger than planets. The Sun, the closest star to us, is about a million times larger than the Earth. Stars are so big that deep inside them, high pressures and temperatures cause nuclear fusion to occur. (Here on Earth, nuclear fusion only normally occurs inside hydrogen bombs!) The nuclear fusion heats stars, which then get so hot that they shine with their own light. Most stars (the Sun is the only exception) are more than a thousand times farther away than the planets are.

Front 5

How much energy is left in the Sun, and when is it expected to burn out?

Back 5

stars are vast clumps of hydrogen gas. Stars shine by fusing hydrogen into helium in their hot, dense cores. When a star of the Sun's size runs out of usable hydrogen in its core, it will "puff out" its outer layers. This will create a bubble-like planetary nebula. The Sun's interior will collapse into a tiny white dwarf star. This white dwarf will be very dense -- a 0.1 inch cube of white dwarf material would weigh half a ton on Earth. White dwarfs (dwarves?) glow with leftover heat, but they generate no new energy by themselves.

Stars much larger than the Sun can explode as a supernova. The remnants of a supernova collapse beyond the white dwarf stage and into a neutron star, or even a black hole.

Now, how much time does the Sun have left? Astronomers have never actually watched a star go through all the phases of its life cycle -- but they have made models and simulations of stars. They can also observe many stars in various stages of their evolution. Based on stellar models, we know that a star like the Sun (that is, with the Sun's brightness and mass) should shine for about nine or ten billion years.

Astronomers think the Sun is about halfway through those nine billion years. We know this through several sources: those solar models, the hydrogen- to-helium ratio of the Sun, and the geologic age of the Earth and of Moon rocks. (We think the Earth and the other planets formed at about the same time the Sun began to shine.)

So the Sun should burn for another 4 to 5 billion years. I am not sure how much energy it will consume during that time. We could make a rough guess of this energy by estimating how much mass is in the Sun's core. Solar fusion turns mass into energy via Einstein's famous E=mc2 equation.

(Now, there are some complications: the Sun will not fuse all of its core hydrogen before it stops. And it is fairly inefficient in the meantime, too!)