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.
Astronomers work out how big a star is from its brightness and its temperature.
The size and brightness of a star depends 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 stars, 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 are being born and dying all over the Universe, and by looking at stars in different stages of their life, astronomers have worked out their life stories.
Medium-sized stars last for about ten billion years. Small stars may last for 200 billion years.
Big stars have short, fierce lives of ten million years.
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 are 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 stars depends on the careful determination of 1535 stars in the Fundamental Catalog (FK3).
Photometric catalogues map the stars by magnitude and colour, and position.
Photographic atlases do not plot positions of stars on paper, but include photos of them in place.
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. This picture shows the northern constellations visible in December.
Celestial coordinates are the figures that plot a star’s position on a ball-shaped graph. The altazimuth system of coordinates gives a star’s position by its altitude (its angle in degrees from the horizon) and its azimuth (its angle in degrees clockwise around the horizon, starting from north). The ecliptic system does the same, using the ecliptic as a starting point. The equatorial system depends on the celestial equator, and gives figures called right ascensions and declination, just like latitude and longitude on Earth.
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 1. He described the faintest star he could see as magnitude 6.
Using telescopes and binoculars, astronomers can now see much fainter stars than Hipparchus could.
Good binoculars show magnitude 9 stars, while a home telescope will show magnitude 10 stars.
Brighter stars than Antares have been identified with magnitudes of less than 1, and even minus numbers. Betelgeuse is 0.8, Vega is 0.0, and the Sun is -26.7.
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. 80 Stars
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.
Red giants are stars that have swollen 10 to 100 times their size, as they reach the last stages of their life and their outer gas layers cool and expand.
Giant stars have burned all their hydrogen, and so burn helium, fusing (joining) helium atoms to make carbon.
The biggest stars 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 (see star brightness).
Pressure in the heart of a supergiant is enough to fuse carbon atoms together to make iron.
All the iron in the Universe was made in the heart of supergiant stars.
There is a limit to the brightness of supergiants, so they can be used as distance markers by comparing how bright they look to how bright they are (see distances).
Our Sun is alone in space, but most stars have one, two or more starry companions.
Binaries are double stars, and there are various kinds.
True binary stars are two stars held together by one another’s gravity, which spend their lives whirling around together like a pair of dancers.
Optical binaries are not really binaries at all. They are simply two stars that look as if they are together because they Binary system with similar sized are in roughly the stars. The stars may be close together same line of sight or millions of kilometers apart. from the Earth.
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.
Spectroscopic binaries are true binaries that spin so closely together that the only way we can tell there are two stars is by changes in colour. Stars in The star Epsilon in the constellation of Lyra is called the Double 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.