Stars and nebulae

Introduction

This chapter covers the various types of stars and nebulae. It gives an overview of the formation of stars and their life-span. This chapter also looks at apparent and absolute magnitude. It also looks at the Parallax effect.

What is a star?

A star is a celestial body that at some time in its life generates light and heat through nuclear reactions, specifically by the fusion of hydrogen into helium under conditions of enormous temperature and density. A star is made of hot plasma. Plasma is an extremely hot gas composed of free-floating positive ions (atomic nuclei minus some electrons), also called ionised gas.

Stars are made of the same chemical elements as those found on Earth, though not in the same proportions. Most stars are made almost entirely of hydrogen and helium. Elements like oxygen carbon, neon, iron and nitrogen make up a small portion of a star.

Most stars appear in the sky as shiny dots. The sun is the closest star to us. Multi-star systems consist of two or more stars that are bound by gravity. These stars move around each other in stable orbits. A pattern of stars in the sky, named for a person, animal or object is called a constellation. The science that studies the stars is called stellar astronomy.

Nebulae

Reflection nebulae are clouds of dust which simply reflect the light of a nearby star or stars. Reflection nebulae are also usually the sites of star formation.

Dark nebulae are clouds of dust that block the light from other sources.

Planetary nebulae are clouds of gas thrown out by stars near the end of their lives.

A 'baby star' - protostar

Stars form from nebulae. The nebulae matter is compressed by the blast waves from exploding stars. The chemical elements hydrogen and helium are parts of giant molecular clouds of nebulae. When hydrogen atoms merge to create the next heaviest element, helium, mass is lost. The mass (M) is converted to energy (E) through Einstein's equation E = mc2, where 'c' is the speed of light.

As the nebula cloud collapses, its density and temperature increases. Temperature and density are highest at the centre of the cloud, where a new star will eventually form. The object that is initially formed at the centre of the nebula is called a protostar. A protostar is covered with a cloud of gas and dust so it is difficult to detect. Any visible light that it emits is absorbed by surrounding 'rubbish'. Only during the later stages, when a protostar is hot enough for its radiation to blow away most of the material surrounding it, can it be seen in visible light. Protostars which are starting to blow away the gas and dust surrounding them are called T-Tauri stars.

The life and death of a star

Stars have a limited amount of energy available within their hot cores. Because higher-mass stars use their energy faster than lower-mass stars, they die faster. The most massive stars live only a couple of million years.

As the dead helium core of a high mass star contracts, the star expands, reaching the size of the orbits of the outer planets. These stars become what are known as red 'supergiants'. When the star's energy sources are exhausted, gravity shrinks the star, turning it into a white dwarf - a cooling down super-dense star that has run out of its nuclear fuel. White dwarfs are the remains of stars that no longer have any source of energy. The cooling time is so long, however, that all white dwarfs ever formed are still visible.

A white dwarf collapses further and becomes a neutron star - an extremely dense star with a powerful gravitational pull. The energy released in this collapse turns the neutron star into a supernova - an exploding star. This explosion can be as bright as an entire galaxy of stars. The part of the star that has exploded is so hot that its nuclear reactions produce all the chemical elements, including a tenth of a solar mass of iron, which then blends with the gasses of interstellar space, out of which new stars are formed. Supernovae are very rare. They occur in our galaxy only two or three times a century. People have not seen one since Kepler's Star of 1604 which was so bright that it was visible in daylight. Our knowledge of supernovae comes almost entirely from observing them in other galaxies. When very large supernovae collapse completely they become black holes. See image 2.

Apparent and absolute magnitude

The brightness of stars is specified with the magnitude system. Some stars are very bright and others are very faint.The term 'magnitude' was coined by Greek astronomer Hipparchus. He looked at the stars in the sky and classified them by how bright they appeared. The brightest stars were 'magnitude 1', the next brightest were 'magnitude 2' and so on down to 'magnitude 6', which were the faintest stars he could see. By using digital cameras and CCDs (Charged Coupled Device that converts light into electrical current) modern scientists established that the brightness ratios (the comparison of two numbers by division) between different magnitude stars are a constant, which equals 2.5. The modern magnitude scale is based on the star Vega, which is defined as having a magnitude of 0.

Parallax effect

Parallax is an optical illusion. It is the perceived movement of a distant object, such as a moon, planet, or star, due to the movement of the Earth. Observable parallax in fixed stars is proof of the rotation of the Earth around the sun. By observing parallax, measuring angles, and using geometry, one can determine the distance to various objects.