Stars
Stars Colour Magnitudes Planets Life of a small star Life of a massive star Neutron stars Black holes White dwarfs
Stars: Stars are spherical hot bodies of gas that continually combust and react (through nuclear fusion) thus generating heat, light and radiation. Stars have a life span, a birth from cosmic clouds to become a fiery ball of gas then to a cold, hard white dwarf, sometimes turning into neutron stars or even black holes. Stars range dramatically in size, temperature and mass. Their diameter ranges from about 450 times smaller than our sun to over 1000 times larger than our sun, surface temperatures ranges from about 3000oc to over 50 000oc, masses range from a twentieth to 50 solar masses just to give an example.
Colour: Hotter stars tends to look a bluish colour, cooler stars are dark red. The colours in descending order of temperature: blue, white, yellow, orange and then red. Our sun has a surface temperature of 5 500oc and appears yellow so it is between those extremes. There are some stars which are 50 times bigger than our sun; thus when a star of such magnitude dies, it collapses on itself and forms a black hole.
Magnitudes: The light emitted from stars is measured in magnitudes; apparent magnitude and absolute magnitude. Apparent magnitude is the degree of brightness that the star appears to possess when seen from Earth, whilst the absolute magnitude is the degree of the light that is actually emitted by the star. Brighter stars have lower magnitudes while darker stars have higher magnitudes. Stars with apparent magnitudes of higher than 5.5 cannot be seen by the naked eye (on Earth). For example, our sun has an apparent magnitude of -26.7 but an absolute magnitude of 4.8. Rigel; one of the stars that can be seen in the night sky, has an apparent magnitude of 0.1 but possess an absolute magnitude of -7.1. This means that even through Rigel is in actual fact much brighter than our sun (absolute magnitude), it is seen to be darker (apparent magnitude) due to a variety of factors, including distance.
Planets:. As yet, we only know that one of the planets, Earth, contains life, and intelligent life at that. Probes have been sent to nearly all of the planets in order to measure the topology, atmosphere, surrounds and to determine if life exists or did exist on that planet. As yet, all attempts have been largely unsuccessful in finding life but speculations still remain that Mars and Jupiter’s ice blanketed moon called Europa may have (had) life roaming its surface, or under its surface in Europa’s case. Scientists have also turned to the stars beyond the solar system, looking for likely candidates that might host life. So far, several star systems resemble our own but further study of them is required to determine whether life may exist.
Life of a small star (one solar mass):
A Nebula
0: A Nebula which is essentially a cool cloud of gas, hydrogen and dust which gradually condenses to form a protostar.
Duration 50 million years: A protostar is a glowing ball of gas, mainly hydrogen which blows off its shell of dust with its radiation. If it contains enough matter and the core temperature reaches 15 millionoc then nuclear fusion starts, the ball contracts even further which causes it to shine, becoming a main sequence star.
A Protostar
A Main Sequence Star
Duration 10 billion years: A main sequence star (which signifies that its core undergoes nuclear fusion, converting hydrogen into helium) which produces energy using nuclear fusion. Its diameter is about 1.4 million km across. When the core has been completely converted to helium, it contracts and nuclear reactions continue to occur in the shell around the core. The core becomes hot enough for the helium to fuse to form carbon while the outer layers expand, cool, and shines less brightly. Now the star is known as a red giant.
Duration 100 million years: A red giant expands from its previous size (1.4million km across) to at least 70 million km and the surface temperature of the star drops but the core temperature rises. When the helium in its core runs out, the outer layers of the star may drift off as an expanding gas could called a planetary nebula. The remaining 80% of the star, the core, becomes a white dwarf.
A Red Giant
A White Dwarf
A cooling white dwarf
A Black Dwarf
Enclosed within the planetary nebula shell, lies a white dwarf with a diameter of about 13 000km of which a teaspoon would weigh about five tonnes. It gradually cools and its core glows a dim red, then finally when it has stopped shinning altogether the dead star becomes a black dwarf, and so ends its billions of years of legacy.
Life of a massive star (10 solar masses):
A Nebula
0: A Nebula which is essentially a cool cloud of gas, hydrogen and dust which gradually condenses to form a protostar.
Duration of a few hundred thousand years: A protostar is a glowing ball of gas, mainly hydrogen which blows off its shell of dust with its radiation. If it contains enough matter and the core temperature reaches 15 millionoc then nuclear fusion starts, the ball contracts even further which causes it to shine, becoming a main sequence star.
A Protostar
A Main Sequence Star
Duration 10 million years: A main sequence star (which signifies that its core undergoes nuclear fusion, converting hydrogen into helium) which produces energy using nuclear fusion. Its diameter is about 3 million km across. When the core has been completely converted to helium, it contracts and nuclear reactions continue to occur in the shell around the core. The core becomes hot enough for the helium to fuse to form carbon while the outer layers expand, cool, and shines less brightly. Now the star is known as a red giant.
Duration 4 million: years: A red giant expands from its previous size (3 million km across) to about 100 million km across. During the next few million years, a series of reactions takes place around the iron core. The core eventually collapses, taking less than a second to do so, and a supernova takes place.
A Red Giant
A Neutron Star
A Black Hole
Duration 1-2 years (visible): A supernova is a massive explosion that results from a star’s core collapsing on itself. The shockwaves from the explosion blows away the outer layers of the star. Supernovae may shine brighter than entire galaxies, emitting light energy of a billion stars, though for a short while only. If the surviving core is between 1.5 and 3 solar masses it becomes a tiny neutron star which is around 10 km in diameter. A teaspoon of which weighs about a billion tonnes. If the core is greater than 3 solar masses then it contracts to become a black hole; sucking in everything within its vicinity, even light.
Neutron stars:
Neutron stars result after massive stars explode that leave a core with a solar mass of between 1.5 and 3. The core contracts to form the neutron star. Usually only 10km in diameter, it comprises almost entirely of neutrons. A teaspoon of it would weigh around a billion tonnes. Neutrons are sometimes called pulsars due to the fact that they emit two beams of radio waves that sweep across space when the star rotates and are detected on Earth as short pulses.
Black holes:
Black holes result from demised stars which a core of more than 3 solar masses. After the star explodes, forming a supernova, the core (if it survives) contracts to form a black hole. Even though they are invisible (thus hard to detect), black holes possess incredibly strong gravity that try to suck in everything within grasp; ranging from a neighbouring star to light. Studies have even shown that black holes reside in some galaxies, for example M87.
White dwarfs:
White dwarfs are very small and dense cores of stars that are left when a small star dies and its outer layers of gas drifts off. White dwarfs have masses similar to that of the sun but sizes of planets about the size of the earth. A cubic centimetre would weigh many tonnes. Whites dwarfs are composed of broken down atoms where the electrons and protons are packed together as tightly as physically possible. White dwarfs do not react and produce heat, instead they merely cool down. When first formed, white dwarfs are very hot, with surface temperatures up to 100 000oc but are very faintly lit. Over billions of years white dwarfs gradually cool and fade to invisibility into the darkness of space.
