A star is born!

What is a star?

A star's mass is predominantly hydrogen gas. By far, the most abundant element in the universe is hydrogen simply because there are so many stars. It is estimated that for every grain of sand on Earth there could be up to 100 stars. The only way to describe that is astronomical! Our galaxy is the Milky Way, which contains billions of stars, yet this is only one of billions of galaxies, all containing billions of stars!

The hydrogen gas that makes up a star's mass undergoes nuclear fusion that turns hydrogen into helium and other heavier elements. By undergoing this nuclear fusion the star becomes a self-luminous object that radiates energy in the form of heat, light and radiation. It is this energy from our own star, the Sun, which enables life to exist on Earth.

Nuclear fusion requires immense heat and pressure in order to combine the nuclei of atoms which, in turn, create more energy than was required to undergo fusion. The surplus energy is emitted and the reactions become self-sustaining.

There are 23,000 stars plotted on this Hertzsprung-Russell diagram by Richard Powell. Stars tend to fall only into certain regions on the diagram. The most predominant is the diagonal, going from the upper-left (hot and bright) to the lower-right (cooler and less bright), called the Main Sequence. In the lower-left is where white dwarfs are found, and above the main sequence are the subgiants, red giants and supergiants. The Sun is found on the main sequence at luminosity 1 (absolute magnitude 4.8) and temperature 5,780K. — With permission under the terms of the GNU Free Documentation License

Star types

Our Sun is an average, Main-Sequence star (see the diagram to the LEFT). It is middle-aged, not too bright and not too big. This means that it is in the middle of its lifecycle and is steadily undergoing nuclear fusion. The size of a star determines its lifespan.

Red Giant stars are large, unstable stars that swell and pulsate. They have undergone too much fusion of H to He and now are fusing He to heavier elements which requires more energy and therefore they swell as a result. Our Sun will eventually become a Red Giant. Blue giants are a lot rarer than red giants and are also unstable with relatively short life-cycles. They are much hotter than red-giants and much of their light is emitted in the ultraviolet spectrum.

Supergiants can be red or blue, depending on their temperature. Red supergiants are cooler than blue supergiants. Blue supergiants are the hottest and brightest stars in the universe. A famous blue supergiant is Rigel, which is in the constellation Orion. It also has a red supergiant in its constellation known as Betelgeuse.

Dwarf stars are smaller than our Sun. Red dwarf stars are in the main sequence and are small, cool stars. White dwarf stars are what medium sized stars, such as our Sun, will become after they have gone through the red giant phase. Eventually they will explode and become a nova. A white dwarf is the core of an exploded star and is extremely hot. Because this star is no longer undergoing nuclear fusion, it will eventually lose its heat and fade into a black dwarf star.

The Hertzsprung-Russell diagram

Stars vary in temperature and this is reflected in their color. Orange and red stars are cooler stars, whereas blue and white stars are the hottest. They also vary in their brightness, which is generally related to their size. Their luminosity or brightness is a value of their absolute magnitude.

When we see stars in the sky they vary in brightness and some supergiant stars may not appear that bright. That's because of their relative distance. The twin stars Proxima Centauri and Alpha Centauri are the closest to Earth. They are approximately 4.2 light years away and due to their closeness they seem very bright.

A light year by the way is a measure of distance. It is the distance that light travels in one year; nearly 10 million million kilometers!

In contrast Rigel, the blue supergiant, is thousands of times brighter than Alpha and Proxima Centauri combined, but it is approximately 800 light years away and therefore appears dimmer when observed from Earth. The discrepancy in brightness over distance is known as the apparent magnitude. The actual brightness is known as the absolute magnitude and the bigger the negative value, the brighter the star. The absolute magnitude of Alpha Centauri is 4.7, whereas Rigel has an absolute magnitude of -6.7. In order to put it in perspective, our Sun has an absolute magnitude of 4.8.

The Hertzsprung-Russell diagram, created in the early 1900's by scientists Ejnar Hertzsprung and Henry Norris Russell, is a plot of stars on a diagram based on their absolute magnitude against their color. Running diagonally from top left to bottom right is where most stars are found, including our Sun. This is why they are called main-sequence stars.

The very top left is where blue giants and blue supergiants are found. Red supergiants are found more in the top middle to top right of the diagram. This is because they are very luminous, but not as hot as the blue stars.

White dwarf stars are found in the bottom of the diagram because they are not very bright and have high positive absolute magnitudes. But because they are very hot, they are found to the left of the diagram.

A day in the life

Like most living things, stars have a life cycle: They are born, they live, they mature and eventually they die. A star is 'born' in a cloud of interstellar space dust, hydrogen gas and plasma known as a nebula. When the collection of dust and gas becomes dense enough, heat and pressure increase until there is enough energy to begin nuclear fusion. At his point, the mass becomes a star.

Main sequence stars live for about 10 billion years. The majority of this time is spent turning H into He. Our Sun is about 5 billion years old. Eventually, after billions of years, main sequence stars gradually convert themselves into red giants, marking the end of their life cycle. After perhaps millions more years of unstable swelling and pulsating, it explodes and becomes a nova. The outer layers are expelled into space. They form nebulae. When the core of the nebulae shrinks and contracts, it becomes a white dwarf.

If the main sequence star is a very large star, it may become a red or blue supergiant. Red or blue supergiants are also entering the final stages of their lives. When they explode, it is on incredible scale - often brighter than whole galaxies with billions of stars. This is what's call a supernova.

Supernovas form heavier elements such as iron, and the heaviest natural element, uranium. As with novas, the outer layers of supernovas are blasted into space, whereas the core has so much matter that after the explosion it collapses and can become a neutron star or in some cases, a black hole.

You, too, are a star

We can therefore assume that everything in our solar system was formed from the debris of a supernova. This includes our Sun, our planet and even the elements of which humans are comprised. With that in mind then, you can safely say with absolute certainty that you, too, are a star!