A star is a vast sustained nuclear explosion at the centre of an enormous sphere of superheated gas (or plasma).  The Sun is the nearest star to Earth.  Massive clouds of hydrogen gas (often laced with trace amounts of heavier elements) start condensing through various mechanisms until they’re dense enough for gravity to start drawing the cloud in upon itself.  As it contracts, the temperature and pressure begin to rise.  After a while the cloud will be so dense and … Continue reading →

There are about 6000 stars in the night sky that are visible to a healthy adult standing in a properly dark location.  Of course, you can never see more than half the sky at a time, so to see all of them you would need to live in the tropics and spend some time looking upwards regularly — it takes a full year for the entire sky to pass overhead.  So at any one time, there’s something between 2500 and … Continue reading →

You need to know that the Earth’s axis is not nicely aligned with the Earth’s orbit – it’s tilted from the vertical by about 23 degrees.  So as the Earth moves around the Sun, one of the poles is leaning towards the Sun.  For six months it’s the north pole, and for six months it’s the south pole (This affects how much sunlight each side of the world gets, and causes the seasons).  The day of the year when each … Continue reading →