Overview of
Astronomical Objects
External Links page.

  The Earth
  • What is it? A mostly rocky planet, 13,000 km across, with most of the surface covered by a layer of water and a large diversity of life forms.
  • Around it: A 100-km thick atmosphere, and a much larger magnetosphere.
  • Further out: It has one natural satellite, the Moon (about 1/4 the size, 380,000 km away), and they both orbit a star, the Sun.
  • Is this a "typical planet"? Are planets common in the universe?
The Solar System
  • What is it? Our "immediate family" in space. It consists of the Sun and many objects revolving around it.
  • Main objects: Eight planets. Four inner, smaller, rocky planets; and four outer, larger, gas planets.
  • Smaller objects: Asteroids, comets, many objects in the Kuiper Belt (including Pluto) and the Oort Cloud.
  • And finally: Solar wind, leftover dust, and stuff from outside...
  • How many are there? With the naked eye, we could see several thousands, but there are billions in our galaxy alone.
  • What are they? A star is an object that is so large that the temperature in the core ignites nuclear fusion reactions, producing huge amounts of energy. The Sun is a medium-small middle-aged star.
  • Do they form a pattern? On scales of thousands of light years, they start to appear concentrated in thicker bands along the plane of the Milky Way, where there are more clouds of gas and dust.
The Milky Way
  • What is it? From here, it looks like a band of clouds that extends across the sky. What we are seeing in a region with a very high concentration of stars, and many nebulas. It is the edge-on view of the galaxy we live in.
  • What does it really look like? It is a disk-shaped concentration of 100 billion stars and other matter, revolving around a central black hole every 250 million years. Around it, is a thinner halo with some star clusters, hot gas, and satellite galaxies further out.
  • How many are there? If we could spend enough time with the best telescopes, we would probably see something like 100 billion of them.
  • What are they like? There are different kinds: spirals like the Milky Way or M31, ellipticals, and irregulars.
  • Do they form patterns? We are in the Local Group of 30-40 galaxies spread over a few million light years; Most galaxies are in clusters, and clusters form larger structures.
The Universe
  • Is the universe changing? We see all galaxies, except for some nearby ones, moving away. The further they are, the faster they move! The whole universe is stretching, expanding.
  • Did it have a beginning? We believe it did come from a stage where everything much packed in a tiny fraction of its volume and was extremely hot, because the theory makes many correct predictions.
  • Do we basically understand the universe? No! We don't even know what most of the stuff in it is! But there is a lot left to see...


page by luca bombelli <bombelli at olemiss.edu>, modified 29 sep 2012