Astrophysics E1 Introduction to the Universe

Introduction to the Universe The Solar System Kuiper belt Asteroid belt - The distance from Sun to the Earth is 1 astronomical unit (1AU = 1.5 x m)

Definition of a Planet Decided in 2006 by the IAU: i.It must orbit the Sun ii.It must have sufficient gravity to overcome rigid forces and form an approximate spherical shape iii.It must clear the neighbourhood around its orbit Planets beyond the solar system are called exoplanets.

Orbits Planets orbit the Sun and moons orbit the planets. All planets have elliptical (not circular) orbits.

The Asteroid Belt Between the rocky planets and gas giants (around 2 to 3.5 AU from the Sun). Ceres (480km): it was the first asteroid to be seen. Now it’s a dwarf planet. Mathilde (52km) 243 Ida and its satellite Dactyl Link – more info

Comparative Sizes of Planets and Stars See this linklink Some stars...

Constellation This is a group of stars in a recognizable pattern that appear to be near each other when viewed from Earth, even though they may be very far apart.

Ursa major (the plough) Ursa minor

Stellar Cluster This is a group of stars that were formed from the collapse of the same cloud of gas and dust. As a result they are physically close together, unlike a constellation. The ‘Pleiades’ cluster. Photo taken by the Hubble space telescope

Comets In the solar system, Comets also orbit the Sun although their orbits are highly elliptical. Comets originate in the Kuiper belt or beyond it, in the ‘Oort cloud’. Comets have a nucleus of small rocks, dust and ice (and other frozen gases such as methane). The tail or comma is a stream of ions evaporated from the comet by the Sun and pushed away by the solar wind (due to its magnetic field). Hale-Bopp (1997)

Galaxies Approximate distances... Star to star:1 light year Galaxy diameter:10 3 – 10 5 ly Galaxy to galaxy:10 6 ly Spiral galaxies have a central core of stars, surrounded by spiral arms. The Sun lies approximately 1/3 of the distance from the centre of the Milky way. One light year = the distance travelled by light in one year = 9.5 x m

Herschel's sketch of the Milky Way (1785): Image of current understanding of the Milky Way:

Motion of Stars Just like the Sun, other stars appear to rise in the East and set in the West in the sky (following an arc) due to the Earth spinning on its own axis. The stars we see change gradually over the year (by about 1° per night) due to Earth’s movement around the Sun.

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