What is a star? We are all pretty familiar with stars. We see them on most clear nights as tiny shining light spots of light in the sky. Stars are the topic of countless poems, stories, and children’s songs. But, exactly, what is a star?

A star is a ball of gas, which gives out light. The gas is mostly made up of H₂ and He, held together by its own gravity. Nuclear fusion(核聚变)reactions in its center part support the star against gravity and produce photons(光子)and heat, as well as small amounts of heavier elements. The Sun is the closest star to Earth.

According to present theory, stars are born as a huge ball within huge gas clouds that fall in suddenly themselves. The cloud’s material heats up as it falls inward under the force of its own gravity. When the gas reaches about 10 million K(18 million℉), H₂ nucleus(原子核)begin to fuse into He nucleus, and the star is born. Energy from nuclear fusion radiates outward from the center of the rapid-growing star, and gradually prevents the gas cloud from falling in.

A star’s color relies on its temperature: hotter stars give out bluer light and cooler stars give out redder light. Temperature is also related to weight. Red dwarf stars have as little as 0.075 solar and the surface temperature is less than 4,000 K. The heaviest star known at present is R136a1, a Wolf-Rayet star 265 of times the Sun—its surface temperature keeps at a red-hot 50,000 K.

The heaviest and hottest stars exhaust their energy within a few million years, while tiny and cool red dwarf stars can keep on burning for many billions of years.

Bath or shower? How do you prefer to keep yourself clean? Whatever your daily ablutions, we know it’s important to have good personal hygiene. Nobody wants to smell, and nobody wants to catch your odour! A shower can bring you certain other benefits, but how often should we wash ourselves under a watery spray?

Although you might think taking a daily shower should be part of your cleaning ritual, there’s no hard and fast rule, although dermatologists agree that as a society, we shower too often. In fact, too much washing can deplete the vital oils in our skin. It can waste water as well. However, if you have a fitness routine that causes you to sweat, a more regular sprinkle of water is recommended to stop you ponging!

How often you shower is, of course, a personal choice. There’s nothing better than a hot, soapy scrub under the shower, and this doesn’t just have to be to clean yourself. Some people enjoy a shower to help them relax, warm up or to soothe any aches and pains. It’s also a good opportunity to wash your hair at the same time. Again, though, experts say we shouldn’t shower too often because, like your skin, your hair contains essential oils that you don’t want to lose through excessive scrubbing.

If the idea of a hot, steaming shower appeals, how about a freezing cold one instead? This might be a necessity if your boiler has broken, but some people choose this option because it has many health benefits, some experts claim. There is increasing evidence that braving an icy shower in the morning could keep illness at bay and may even help your brain stay sharp. A BBC podcast called Just One Thing with Michael Mosley, found the shock of being doused with cold water could have a positive impact on your body and brain.