Space Rumors

The stars in this image are a part of stellar cluster in which a supernova exploded, and the matter in it blows these stars at high velocities. This has been taken from NASA's x-ray observatory and spitzer space telescope shows the dusty remains of a collapsed star. Here x-ray data is shown in blue, data from spitzer in green (a shorter wavelength) and red yellow a longer one. The white color in the middle is a neutron star, or pulsar, all that remains of a core collapse supernova explosion.

Not all stars end their lives as quietly as the white dwarfs. Massive stars, with much more matter than the sun, continue a more complex evolution and finish their existence in a far more spectacular way. The nucleus of these stars is so compressed and hot that more nuclear reactions can occur. When such a star has used up all of its hydrogen, the nucleus becomes compressed and heats up until the carbon react bringing about heavier elements. When the carbon has run out, a similar process begins. These different phases happen quickly, because the new nuclear reactions produce less energy each time. Toward the end, the star begins to acquire a structure of layers, with the nucleus being made up of iron. When it is no longer possible to obtain more energy from the iron, the stars center collapses in on itself and the whole star explodes in one great, unimaginable bang. This explosion can produce a glow of more than one hundred million suns. We call such an explosion a Supernova. Most of the stars matter disperses into space. The explosion produces a rapidly expanding nebula. The Crab nebula in the constellation Taurus is the remains of a supernova that Chinese astronomers saw in 1054.

Stars finish their fully developed life when they have consumed all the hydrogen in their center. This happens when they have used up all their energy supply, which up to this point maintains the stars stability. The stars nucleus can no longer resist the rest of the stars weight, and begins to contract. Such compression makes the temperature of the nucleus rise and causes nuclear reactions in the layer surrounding the nucleus. The energy produced outside the center of the star makes the stars different layers dilate and cool. The star swells and then becomes reddish. This is what we call a red giant. A typical red giant is about one hundred times bigger than our sun.

When the sun becomes a red star, in about 5 billion years, it will expand out to a maximum radius of roughly 1 AU (150,000,000 km), 250 times its present size. As a giant, the Sun will lose roughly 30% of its current mass and it will grow so big that it will reach and burn up the nearest planets, mercury, Venus, and probably Earth. Many stars in the red giant stage undergo periodic changes in brightness. This is because they expand and shrink rhythmically. They are called Pulsating Variables. When the hydrogen is exhausted, the star stops being a fully developed star and becomes an old star. The core is compressed enough to start helium fusion, and the star now gradually shrinks in radius and increases its surface temperature. For larger stars, the core region transitions directly from fusing hydrogen to fusing helium.

After the star has consumed the helium at the core, fusion continues in a shell around a hot core of carbon and oxygen. The star then follows an evolutionary path that parallels the original red giant phase, but at a higher surface temperature. The life span of the stars depends on their mass, that is, on the amount of matter in the star. The greater the amount of matter from which the star formed, the more condensed and hot its nucleus is. The star is brighter and hotter; its surface is bluish. A blue star uses up its hydrogen in only some millions of years, a very short time compared to the sun. This is why we know that bright, blue stars like the Pleiades are young.

A star once it forms and begins to shine is a stable body that remains practically unchanged for many years. In the nucleus of a star, near its center, hydrogen is being converted into helium. The energy produced makes the interior of the star extraordinarily hot. This energy escapes from the surface of the star in the form of light and heat. The gas formed by the star tends to expand, and is capable of supporting the weight of the star itself, stopping its contraction. This situation lasts as long as there is enough hydrogen in the center of the star. Stars spend about 90% of their lifetime fusing hydrogen to produce helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence and are called dwarf stars, the Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion years ago. Every star generates a stellar wind of particles that causes a continual outflow of gas into space. For most stars, the amount of mass lost is negligible.

The Sun loses 10−14 solar masses every year, or about 0.01% of its total mass over its entire lifespan. However very massive stars can lose 10−7 to 10−5 solar masses each year, significantly affecting their evolution. Stars that begin with more than 50 solar masses can lose over half their total mass while they remain on the main sequence. The time for the main sequence depends on the amount of the fuel its having to fuse and the rate at which it fuses the fuel i.e. its initial mass and its luminosity. Large stars consume their fuel very rapidly and were short lived, but the small stars (red dwarfs) consume their mass very slowly and live for billions of years and at the final stage they become dimmer and dimmer. Since the age of the universe is 13.7 billion years, no red dwarfs have reached to that position.

The birth of the Stars:We know that stars must have been formed at some time in the past, but we don’t know with what exactly they were formed with. The only reasonable answer is from existing gas and dust among the stars in the galaxy. This is called interstellar matter. Under normal conditions, interstellar matter is not visible, but when illuminated by a hot, luminous star, it forms bright nebula's that have a rosy color. The force governing this whole process of formation and, in fact, the star’s subsequent life, is gravity. According to one theory, when a cloud of
A star forming cloud in Cepheus, NASA image
interstellar matter crosses a spiral arm of the galaxy, it begins to condense and the internal gravitational force increases. This makes the cloud contract more rapidly. As the matter condenses, it breaks into pieces and gets hotter. The center of any very large piece reaches temperatures over a million degrees, giving rise to a protostar. Because of this high temperature, a nuclear fusion reaction starts among the hydrogen nuclei.

The energy produced at the center of the protostar stops the contraction and a new star has been formed. The star's internal pressure prevents it from collapsing further under its own gravity. The remnants of the initial cloud form a proto planetary disc that revolves around the star. This matter can end up condensing and forming the planets that accompany the newly formed star. The period of gravitational contraction lasts for about 10–15 million years. Early stars of less than 2 solar masses are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly born stars emit jets of gas along their axis of rotation, producing small patches of nebulosity known as Herbig-Haro objects.