Stars are balls of mainly hydrogen and helium gas. Nuclear reactions in the heart of stars generate heat and light. The heart of a star reaches 16 million°C. A grain of sand this hot would kill someone 150 km away. The gas in stars is in a special hot state called plasma, which is made of atoms stripped of electrons. In the core of a star, hydrogen nuclei fuse (join together) to form helium. This nuclear reaction is called a proton-proton chain. Stars twinkle because we see them through the wafting of the Earth’s atmosphere.

Interesting facts about Stars

• Astronomers work out how big a star is from its brightness and its temperature.
• The size and brightness of a star depend on its mass – that is, how much gas it is made of. Our Sun is a medium-sized star, and no star has more than 100 times the Sun’s mass or less than 6-7 percent of its mass.
• The coolest star, such as Arcturus and Antares, glow reddest. Hotter stars are yellow and white. The hottest are blue-white.
• The blue supergiant Zeta Puppis has a surface temperature of 40,000°C, while Rigel’s is 10,000°C.
• Stars start life in clouds of gas and dust called nebulae.
• Inside nebulae, gravity creates dark clumps called dark nebulae, each clump containing the seeds of a family of stars.
• As gravity squeezes the clumps in dark nebulae, they become hot.
• Smaller clumps never get very hot and eventually fizzle out. Even if they start burning, they lose surface gas and shrink to wizened, old white dwarf stars.
• If a larger clump reaches 10 million °C, hydrogen atoms in its core begin to join together in nuclear reactions, and the baby star starts to glow.
• In a medium-sized star like our Sun, the heat of burning hydrogen pushes gas out as fiercely as gravity pulls inwards, and the star becomes stable (steady).
• Medium-sized stars burn steadily until all of their hydrogen fuel is used up.
• Plotting the positions of the stars in the sky is a complex business because there is a vast number of them, all at hugely different distances.
• The first modern star charts were the German Bonner Durchmusterung charts of 1859, which show positions of 324,189 stars.
• The AGK1 chart of the German Astronomical was completed in 1912 and showed 454,000 stars.
• The AGK charts are now on version AGK3 and remain the standard star chart. They are compiled from photographs.
• The measurements of accurate places for huge numbers of star depends on the careful determination of 1535 stars in the Fundamental Catalog (FK3).
• Photometric catalogues map the star by magnitude and colour and position.
• Three main atlases are popular with astronomers – Norton’s Star Atlas, which plots all stars visible to the naked eye; the Tirion Sky Atlas; and the photographic Photographischer Stern-Atlas. FASCINATING FACT. Astronomers still divide the sky into 88 constellations – many of the names are the mythical ones given to them by the astronomers of ancient Greece.
• The map of the sky shows the 88 constellations that are visible during the year from each hemisphere (half) of the world.
• Star brightness is worked out on a scale of magnitude (amount) that was first devised in 150Bc by the Ancient Greek astronomer Hipparchus.
• The brightest star Hipparchus could see was Antares, and he described it as magnitude.
• The brightest-looking star from Earth is Sirius, the Dog Star, with a magnitude of -1.4.
• The magnitude scale only describes how bright a star looks from Earth compared to other stars. This is its relative magnitude.
• The further away a star is, the dimmer it looks and the smaller its relative magnitude is, regardless of how bright it really is.
• A star’s absolute magnitude describes how bright a star really is.
• The star Deneb is 60,000 times brighter than the Sun. But because it is 1800 light-years away, it looks dimmer than Sirius.
• Giant stars are 10 to 100 times as big as the Sun, and 10 to 1000 times as bright.
• Giant star have burned all their hydrogen, and so burn helium, fusing (joining) helium atoms to make carbon.
• The biggest star go on swelling after they become red giants, and grow into supergiants.
• Supergiant stars are up to 500 times as big as the Sun, with absolute magnitudes of -5 to -10.
• The Pressure in the heart of a supergiant is enough to fuse carbon atoms together to make iron.
• Our Sun is alone in space, but most stars have one, two or more starry companions.
• True binary star are two stars held together by one another’s gravity, which spend their lives whirling around together like a pair of dancers.
• Eclipsing binaries are true binary stars that spin round in exactly the same line of sight from Earth. This means they keep blocking each another’s light.
• Stars in the star Epsilon in the constellation of Lyra is called the Double because it is a pair of binaries.
• Mizar, in the Great Bear, was the first binary star to be discovered. Mizar’s companion Alcor is an optical binary star.
• Albireo in Cygnus is an optical binary visible to the naked eye — one star looks gold, the other, blue.

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Cells are the basic building blocks of all living things. The human body is composed of trillions of cells. They provide structure for the body, take in nutrients from food, convert those nutrients into energy, and carry out specialized functions. Cells also contain the body’s hereditary material and can make copies of themselves.

Do you know facts about cells that cells have many parts, each with a different function. Some of these parts, called organelles, are specialized structures that perform certain tasks within the cell.

Useful Facts About Cells

• Blood has two main kinds of cell – red cells and white cells – plus pieces of cell called platelets (see blood).
• Red cells are button-shaped and they contain mainly a red protein called haemoglobin.
• Haemoglobin is what allows red blood cells to ferry oxygen around your body.
• Red cells also contain enzymes which the body uses to make certain chemical processes happen.
• White blood cells are big cells called leucocytes and most types are involved in fighting infections.
• Most white cells contain tiny little grains and are called granulocytes.
• Most granulocytes are giant white cells called neutrophils. They are the blood’s cleaners, and their task is to eat up invaders.
• Eosinophils and basophils are granulocytes that are involved in fighting disease. Some release antibodies that help fight infection.
• Lymphocytes are also types of white cells. Each red blood cell contains more than 200 million molecules of haemoglobin.
• Cells are the basic building blocks of your body. Most are so tiny you would need 10,000 to cover a pinhead.
• There are over 200 different kinds of cell in your body, including nerve cells, skin cells, blood cells, bone cells, fat cells, muscle cells and many more.
• A cell is basically a little parcel of organic (life) chemicals with a thin membrane (casing) of protein and fat. The membrane holds the cell together, but lets nutrients in and waste out.
• Inside the cell is a liquid called cytoplasm, and floating in this are various minute structures called organelles.
• At the center of the cell is the nucleus — this is the cell’s control center and it contains the amazing molecule DNA  (genes). DNA not only has all the instructions the cell needs to function, but also has the pattern for new human life.
• Each cell is a dynamic chemical factory, and the cell’s team of organelles is continually busy — ferrying chemicals to and fro, breaking up unwanted chemicals and putting together new ones.
• Do you know facts about cells that the biggest cells in the body can be nerve cells. Although the main nucleus of nerve cells is microscopic, the tails of some cells can extend for a meter or more through the body, and be seen even without a microscope.
• Among the smallest cells in the body are red blood cells. These are just 0.0075 mm across and have no nucleus since nearly their only task is ferrying oxygen.
• Most body cells live a very short time and are continually being replaced by new ones. The main exceptions are nerve cells — these are long-lived, but rarely replaced.
• Mitochondria arc the cell’s power stations, turning chemical fuel supplied by the blood as glucose into energy packs of the chemical ATP (see muscle movement)
• The endoplasmic reticulum is the cell’s main chemical factory, where proteins are built under instruction from the nucleus
• The Golgi bodies arc the cell’s dispatch center, where chemicals arc bagged up inside tiny membranes to send where they are needed. The lysosomes are the cell’s dustbins, breaking up any unwanted material
• The ribosomes are the individual chemical assembly lines, where proteins are put together from basic chemicals called amino acids (see diet)
• The nucleus is the cell’s control center, sending out instructions via a chemical called messenger RNA whenever a new chemical is needed
• Do you know facts about cells that there are 75 trillion cells in your body!
• The instructions come from the nucleus in the cell’s control center, but every kind of organelle has its own task.

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