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
- 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.
this a "typical planet"? Are planets common in the
- 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
- 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.
- 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,
- 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
- 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,
- 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