Star Formation And Evolution

Star Formation and Evolution There is wide variation in the luminosities, spectral types, and temperatures of the stars. One of the great astronomical discoveries of the 20th century was understanding the connection between the luminosity and temperature of the stars. Over a star’s lifetime, these properties change as the star evolves to a final fate, determined by its initial mass.

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The Hertzsprung-Russell (HR) Diagram

In the first decade of the 20th century, the Danish astronomer Ejnar Hertzsprung and American astronomer Henry Russell made the first study comparing the luminosity (absolute magnitude) with the spectral types of the stars. In 1914 they published what has come to be known as the Hertzsprung-Russell (HR) diagram. For nearly a century, the HR diagram has been a powerful visual tool for understanding the properties and evolution of stars. The HR diagram is a graph with the absolute magnitude on the vertical axis and the spectral type on the horizontal axis. The spectral sequence is equivalent to a temperature sequence, with the hot O and B stars on the left, and the cool K and M stars on the right. For most stars, there is direct relationship between spectral type and luminosity. The HR diagram features a broad diagonal band known as the main sequence. It shows a direct connection between the luminosity and the spectral type or temperature. The most luminous stars are very hot with spectral types of O, B and A; examples are Rigel in Orion and Murzim in Canis Major. These stars are in the upper left of the HR diagram. Average stars like the Sun are of spectral type G in the middle of the graph. The dim K and M stars, such as Barnard’s Star and Proxima Centauri, are cool and red at the bottom right of the main sequence. In general, stars have increasing luminosity with increasing temperature. The Hertzprung-Russell Diagram. There are many stars not on the main sequence. In the upper right corner of the H-R diagram is a group known as red giants. Some of the more famous stars in the sky are red giants: Betelgeuse in Orion, Antares in Scorpius, Delta Cephei, and Aldebaran in Taurus. In the lower left corner of the HR diagram is a unique group of low luminosity stars known as white dwarfs. This is the “stellar graveyard”. A relatively low mass star like the Sun will eventually become a white dwarf.

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Stellar Mass and Evolution

Modern astronomy has revealed that the initial mass of a star is the key to understanding a star’s evolution. A star’s mass is its fuel reservoir. The most massive stars burn hydrogen at a prodigious rate. They are brilliant, but their lives are short - no more than a few million years. Lower mass stars like the Sun are far less luminous, but they can shine for billions of years. The masses of stars are found from binary stars with well-known parallaxes like Alpha Centauri. Using the parallax, i.e. the distance, to a binary star system, the true size of its orbit in space can be calculated from the size of its apparent orbit in the sky. Then, given the orbital period, the masses of the individual component stars can be calculated. The known range of stellar masses is less than 0.01 to over 1000 times that of the Sun. We can represent the formation and evolution of a star by an evolutionary track on the H-R diagram. Note that a star’s position on the H-R diagram shows its physical properties, not its position in the sky. A track on the H-R diagram represents the changes in a star’s luminosity and temperature throughout its lifetime, not its motion through space!

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Star Birth and the Main Sequence

Stars form inside clouds of gas and dust in interstellar space. If the cloud is visible, we call it a nebula. Interstellar clouds can be extremely large and massive, up to thousand of light years in diameter, and contain from 10 to 1000 times the mass of the sun. But the density of a nebula is very low. And it contains mostly hydrogen. Star-forming region at the heart of the Eagle Nebula, M16, seen by the Hubble Space Telescope. If undisturbed, the interstellar cloud will not change. However, disturbances do occur. Such disturbances may be caused by the collisions of galaxies, a density wave in the spiral arms of the galaxy, the shock wave of a supernova, or even the birth of a new star nearby. A slight change in density will trigger a contraction of the cloud due to its own gravity. A sphere known as protostar is then formed. Models of protostars show that they will have accretion disks and jets. The jets are not long-lived, and last only about 100,000 years. If the gas and dust still cover the young star, we may not be able to see the star but we might see the two clouds produced by the jets. Upon contraction under its own gravity, the protostar heats up. Since there are as yet no nuclear reactions inside a protostar, a protostar is not a star yet. If it is massive enough (the lower mass limit is thought to be about 0.1 solar mass), the gas in the protostar continues to heat up until the central portion becomes hot and dense enough for the hydrogen atoms to overcome their mutual electrical repulsion. Nuclear fusion then takes place, and a star is finally born. The light and heat generated by the star will push out the surrounding gas and dust. The accretion disk remains and becomes the protoplanetary disk, where the planets are formed later on. The first direct observation of a protoplanetary disk around another star (Beta Pictoris) was made in 1984. Diagram of a protostar (left); image of the accretion disk around Beta Pictoris (right) in infrared light. It takes anywhere from 10,000 to 100 million years for the cloud to become a star. A protostar with a mass of 10 to 30 solar masses contracts to approximately the size of our solar system in only 10,000 years or so, and becomes an O- or B-type star. Less massive protostars will eventually become stars of spectral types G, K, or M. If the protostar is not massive enough to burn its nuclear fuel, it becomes a brown dwarf, which is very dim and hence very difficult to find. Before the protostar contracts, it is very cold and dim. Thus, it is represented by a point on the lower right in the H-R diagram. Upon contraction under its own gravity, the protostar heats up, and thus moves to the left in the diagram. Its luminosity per surface area increases because the temperature increases. But if a star is not very massive, its luminosity will drop because the size of the protostar decreases much faster. The evolutionary tracks of protostars of 1, 5, and 9 solar masses. After a star enters the main sequence stage, its energy comes from nuclear fusion, the combination of several hydrogen atoms into a helium atom. The main sequence is the region of stability on the H-R diagram. Stars live out the majority of their lives with a balance between gravitation and the radiation pressure from the nuclear reactions in their interior. Main sequence stars have stable luminosities and sizes. They are quite “boring” compared to other stars.