The Structure of the Universe Nearly all these objects are galaxies. This image represents 1/13 000 000 of the whole sky.

Galaxy A cluster of stars (107-1012) gravitationally bound. The previous image suggests there may be up to 125 billion galaxies +/- 20%. Number of stars is estimated as 1010 x 125 x 109 = 1021.

An elliptical galaxy as seen by Hubble telescope.

A spiral galaxy as seen by Hubble telescope.

The ‘pinwheel’ galaxy M-101

Galaxies tend to have a ‘galactic bulge’ where density of stars is much greater.

Galaxies rotate around the common centre of mass. Galaxies contain: Stars Planets Interstellar gas Radiation and charged particles Dark matter – poorly understood but crucial in determining the fate of the Universe.

Dark matter Undetectable by its emitted radiation but its presence can be inferred by its effect on visible matter. In other words, the universe seems more massive than expected.

Why dark matter? Galaxies rotate much faster than expected. Line A predicted, B observed. Dark matter is believed to account for this discrepancy. Galaxies don’t obey Laws of Kepler.

Position of the Sun As our galaxy rotates, the Sun orbits the galaxy. The ‘milky way’ is the galactic bulge.

The milky way

Artist’s impression

Rotation speed vs distance from galactic centre. Sun’s position marked Rotation speed vs distance from galactic centre. Sun’s position marked. Predicted in red, observed blue.

Problem Our galaxy is believed to be approximately 100 000 light-years across and rotating at approximately 200 kms-1. Estimate the time to make one complete orbit.

The Solar System

Mercury 0.46 to 0.31 AU From Sun (elliptical.) 88 day orbit. Temp. at equator 100 to 700K. No atmosphere

Venus ~ AU near Circular orbit. 225 day orbit. Temp. 735 K with little variation with latitude. Geologically, Earth’s twin.

False-colour radar image of Venus Venus as it would appear without its thick atmosphere.

Earth Earth- No info Needed.

Mars 1.66-1.48 AU 689 day Orbital period Of 689d. Temp. 186 To 268K.

Earth and Mars shown to scale.

Typical surface of Mars as captured by the Pathfinder mission

Mars’ ice cap Is a mixture of CO2 and water ice.

Mars The ice cap can be seen by a basic telescope from Earth.

The asteroid belt between Mars and Jupiter

The Dwarf Planet Ceres is the belt’s only minor planet

The first Asteroid photographed by a spacecraft.

Jupiter Jupiter’s immense gravitational field prevents the asteroids forming a rocky planet. There is a large tidal force effect between Jupiter pulling one way and the Sun the other.

Jupiter 2.5 times the mass of the rest of the solar system combined – excluding the sun (obviously) Has a rocky inner core but gaseous exterior. Many moons.

Jupiter 67 000 km radius. Orbits approx 5 AU from the Sun. Can you use this info to deduce the mass and hence the density of Jupiter?

Jupiter’s moons 63 satellites of Jupiter including Io Europa Ganymede Callisto all observed by Gallileo ..what a legend.

Io 3600 km diameter Highly volcanic (perhaps due to the large tidal forces acting on it.)

Europa 6th moon The one with An ocean Below its icy surface. A very good candidate For supporting Simple life.

Ganymede

Callisto

Saturn

Saturn About 10 AU from the Sun Has at least 61 satellites not including its ring system. The largest of these is Titan. It composes 90% of the mass in orbit around Saturn.

Titan – no high resolution images available

Uranus

Uranus 21 AU from Sun 1st planet discovered by telescope in 1881 by William Herschel – what a legend. Exceedingly cold – c.50K

Uranus Has 27 moons but they are not thought to be of much significance.

Neptune

Neptune 30 AU from the Sun 55K at upper atmosphere 13 moons of which the most known is Triton which was discovered about 2 weeks after Neptune. It took 100 years to find the next one.

Triton photographed by Voyager

Pluto – elliptical orbit, 30-49 AU

The Kuiper belt Beyond Neptune Consists of rocks, metals and ices very thinly distributed Home to 3 dwarf planets – Pluto, Haumea and Makemake.

The Oort Cloud Hypothesised to lie up to 50 000 AU and represents the edge of the solar system Thought to be the origin of many long period comets

Comets

In summary The solar system is extremely big. It contains 8 main planets. Most of these planets have moons. Moons are often more intriguing than the planets. We know an awful lot but there is still much more to find and learn about.