Stars and Planets: July

Midsummer's Day has focused attention on our local star - and whether you're a druid or a Wimbledon fan, you can't ignore it. But how well do we understand the Sun? Its brilliant but bland surface has long hidden its secrets. In the past couple of years, astronomers have begun to understand what makes the Sun tick - and explode - thanks to the ever- vigilant satellite SOHO, the Solar and Heliospheric Observatory. This joint mission of ESA and NASA was launched in 1995.

SOHO's specialised telescopes observe the Sun's faint outer atmosphere, the corona. They have followed its ever changing shape, and revealed huge sections of it lifting off the Sun altogether, and blasting outwards into space. These "coronal mass ejections" rank among the mightiest of the Sun's storms, and are probably responsible for some of the worst damage the Sun has inflicted on the Earth - disabling satellites in orbit and causing extensive power cuts.

One of the biggest mysteries about the corona is why it is hundreds of times hotter than the Sun's surface. Now SOHO may have found the answer: a constantly-moving "magnetic carpet" at the corona's base. Innumerable thin loops of magnetic flux appear through the Sun's surface, move, break up and disappear again in millions of continuous short-circuits. In one day, these titanic sparks release as much energy as the United States consumes in 100 years - more than enough to heat the Sun's corona to a million degrees.

Magnetism is the driving force behind the Sun's "weather". Its most obvious manifestation is a rash of dark spots on the Sun's surface, each up to 100,000 miles across.

If you could stand on a sunspot and look up into the Sun's atmosphere, you'd see that it is at the base of a towering edifice of magnetism - an "active region". Hot gases trace the structure of this magnetic tower. The most violent goings-on take place at the top of the active region. Here, in the corona, vast magnetic loops are carrying millions of amperes of current. They suddenly connect and short-circuit, in the most explosive outbursts in the Solar System. These solar flares blast highly energetic particles, X-rays and gamma rays into space.

In the last month, news has emerged that flares have a profound affect on the Sun itself. Just as the blast from a rocket engine exacts its toll on the launch pad, so the backlash from a flare strikes back at the Sun - to cause sunquakes. This discovery, made by the SOHO satellite working with another satellite sensitive to gamma rays, puts earthquakes firmly in the shade. A recent flare-triggered sunquake was 40,000 times more powerful than the earthquake that devastated San Francisco in 1906. The flare raised seismic waves two miles high, which travelled at 250,000 miles an hour across the surface. The sound of the quake on the Sun must have been awesome.

What's up

With the exception of Mercury, which is visible for about an hour after sunset early in the month, all the planets are congregating in the morning sky. Jupiter and Saturn both put in an appearance just after midnight. Venus - currently a brilliant "morning star" - now rises two hours before the Sun at about 3am, followed by Mars at 4am. Unusually - because of the way the Moon's cycle falls this month - there are five, rather than four, Moon phases this July.

Diary

1st 7.42pm BST Moon at first quarter.

4th Earth at aphelion (furthest distance from Sun: 152m km).

9th 5.01pm Full Moon.

16th 4.14pm Moon at last quarter.

23rd 2.44pm New Moon.

31st 1.05pm Moon at first quarter.