Derek Shirlaw

Derek Shirlaw | Astronomy | Stars

The Sun is our closet star, but, it's just one of an intangible number of stars in our observable Universe.

Stars are like giant nuclear reactors, burning gases like hydrogen and helium as their fuel, and producing massive amounts of energy in the process.

Stars are born, grow old and die. They effectively recycle matter.

Our local star, the Sun, is a fairly average middle-aged star (a main sequence star) that is about halfway through its life at about 5,000 million years old. Don't worry too much then about what's going to happen to us when it dies as we won't be around!

In its glittering finale, our Sun, after having used up just about all of its fuel, will expand to be a red giant, in all likliehood engulfing the planets of Mercury, Venus and Earth (curtains for us if we're still clinging on to this planet!). The outer layers of hot gas will evenutally disperse leaving behind a white dwarf that'll continue to cool and fade over billions of years.

Other stars are destined for even greater death throws and impressive endings. These are stars with masses about 10times as great as that of our Sun. These will swell into red supergiants that simply can't sustain such a massive core and explode in what is termed as a supernova. What happens next depends of the size of the core. If it's of relatively small mass it'll be crushed into a region spanning just 15km across known as a neutron star. If the core is about twice the mass of the Sun, its own gravity will collapse it further into a black hole.

Derek Shirlaw | Astronomy | Planets

There are 8 planets in our Solar System, however, Scientists have spotted over 250 more orbitting other stars in our galaxy, the Milky Way.

Our planet, Earth, is the only one we know of that supports life.

But what makes a planet, a planet?

A planet is described by the International Astronimcal Union (IAU) as a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion in its inner core, and has cleared its neighbouring region of planetesimals (small bits of rock and dust less than about 1km across in size).

This definition means that Pluto is no longer a planet, but a dwarf planet, along with Ceres and Eris. The eight planets of our solar sytem are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune and Uranus.

We often split the planets of the solar sytem up further. The inner four are rocky 'terrestrials', whilst the outer four are 'gas giants'. The dwarf planets can be either 'terrestrial' or 'ice dwarfs'.

Derek Shirlaw | Astronomy | Galaxies

These can be thought of as 'cities of stars' that come in a variety of shapes and sizes. It's reckoned that there may be as many as one hundred billion galaxies in the observable universe!

Our Sun is a fairly insignificant star in the Milky Way galaxy. The Milky Way is about 100,000 light years in diameter and thought to be spiral in shape. We estimate that we are about two thirds of the way out from the centre towards the rim on one of the galaxy's spiral arms.

Almost all the stars we see in the sky are within the Milky Way. Those nearest to us appear in all directions in the sky, however because the Milky Way is flattened in shape, we see those stars furthest away as a hazy band of light stretching across the sky.

We can however see other galaxies from Earth. The Andromeda Galaxy (around 2.4million light years away), for example is visible with the naked eye. It is the largest of a cluster of about 30 galaxies that together form what scientists call 'The Local Group'.

Scientists class galaxies depending on their shape and nature. There are spirals, barred spirals, ellipticals, and irregulars.

At the heart of a spiral galaxy is 'bulge' of old stars. Branching and curving out from the centre are arms consisting of younger stars, gas clouds and dust.

Barred spirals have a central bar from which the spiral arms extend.

Elliptical galaxies consist of very old stars and have no arms and little gas and dust. These are the largest galaxies, with perhaps ten times as much mass as a galaxy of the Milky Way. It is thought that some of the ellipticals may have been formed by the merger of smaller galaxies.

Irregulars have no defined shape or structure.

The Universe contains some objects that emit as much energy from a area not much bigger than our Solar System, as an entire galaxy does. These are known as quasars and are thought to be galaxies with massive black holes at their centre.

Derek Shirlaw | Astronomy | Nebulae

The starting place for star formation. Gas, dust and other matter begins to clump together here and eventually stars are born. Leftover bits are thought to form planets, comets and other solar system objects.

Found within galaxies, bright diffuse nebulae are regions of hydrogen gas where new stars are being formed. Protostars begin to emerge as areas of the nebulae begin to shrink as a result of the pull of it's own gravity.

There are two types of nebulae associated with the late stages of star formation. Planetary nebulae are composed of gas thrown off by red giant stars. Supernova remnants are the leftovers of exploded massive stars.

Those nebulae that appear dark have no stars illuminating them and can only be seen when silhouetted against a bright background.