Transcript: Japanese theorist Chushiro Hayashi did the detailed calculations to show how stars change their properties as they evolve towards the main sequence when they are still pre-main sequence stars. In terms of the HR diagram they start at regions of high luminosity and low effective temperature, few thousand Kelvin. The time taken to collapse onto the main sequence depends on the stellar mass. The highest mass stars take a shorter amount of time than the lowest mass stars. For...

Transcript: After the freefall gravitational collapse phase, a pre-main sequence star emerges. The collapse is inside out which is to say that the material near the center of the cloud collapses first followed by material further out. After only a few thousand years gravitational contraction releases sufficient energy to raise the temperature of the cloud, still not yet a star, to a few thousand Kelvin, and thus it can emit light. Convection also helps transmit energy to the outer regions...

Transcript: A protostar is a cloud of interstellar gas and dust that’s dense enough and cool enough to contract gravitationally to form a star. An interstellar cloud may be close to the state of collapse for millions of years and be triggered by a nearby disturbance such as the death or birth of a nearby star. The collapse occurs extremely quickly on an astronomical timescale in about a hundred thousand years. This is a hundred thousand times less than the lifetime of the Sun. In the...

Transcript: Star formation is very complex. Thus when astronomers talk about the theory of star formation they’re talking about a theory that is not yet highly refined where many details remain to be worked out. Star formation occurs in dense regions from which light cannot emerge. There are four basic stages. In the first, cores form within a molecular cloud. A giant molecular cloud could potentially form hundreds or thousands of stars. In the second stage the cores collapse to form...

Transcript: Over sixty different types of molecules have been found in interstellar space. Most of them are simple molecules with two or three atoms. However some of them are complex, and they include organic materials that make people very interested in the possibilities of life in deep interstellar space. For example the amino acid glycine, NH2CH2COOH, has been found, and acetone CH3CH3CO, and ethyl alcohol CH3CH2OH. Astronomer Ben Zuckerman calculated that the Sagittarius B2 molecular...

Transcript: The birth of radio astronomy and the development of millimeter and submillimeter astronomy has opened up the capability of astronomers to detect molecules in space. Dozens of molecular species are routinely detected in molecular clouds in the interstellar medium. Radio astronomy is necessary because most of the energy transitions in molecules have low energies and therefore produce spectral features in the submillimeter or far infrared regions of the electromagnetic spectrum. ...

Transcript: The typical environment of the space between stars is a very thin and very cold gas. The interstellar medium typically has about a million particles per cubic meter, and the temperatures are only ten to twenty degrees Kelvin. However there are some regions with densities a thousand or ten thousand times higher than this. In these regions collisions can occur between the atoms and the temperatures are low enough so the atoms will stick together to form molecules. These regions...

Transcript: Ancient astronomers once thought that stars were eternal and unchanging. We now know that stars evolve. They are born, they live, and they die. Well with stellar lifetimes so much longer than a human lifetime how do we actually know that stars evolve? The Sun is 4.6 billion years old, and astronomers have evidence that the system of stars in which the Sun sits, the Milky Way, is much older, perhaps ten or eleven billion years old. Thus the Sun is not the first generation of...

Transcript: Astronomers are interested in the true population of stars in the sky. However the visibility of stars is affected by their luminosity. The more luminous stars can be seen to larger distances than the less luminous stars, and this causes them to be overrepresented in catalogs that are limited by apparent brightness. An example will make this clear. Consider a star that’s the luminosity of the Sun and one that’s five times more luminous than the Sun. Suppose we can see stars...

Transcript: When we look at the lists of the brightest stars in the sky and the nearest stars to the Sun we see that there are almost no stars in common. In fact, among the first few dozen stars in each list only Sirius is in common. Why are the lists so different? When we look at the brightest stars in the sky we are looking preferentially at high luminosity stars. Hertzsprung called these the whales among the fishes. So when we search for the apparent brightness of a star we...

Transcript: The brightest stars in the night sky are not exactly like the Sun. In fact almost all of them are hotter, more massive, and more luminous. Most of the brightest stars in the sky are luminous main sequence stars, red giants, or supergiants. Each of the seventeen brightest stars in the sky is more luminous than the Sun. The brightest ten in order are Sirius, Canopus, Arcturus, Alpha Centauri, Vega, Capella, Rigel, Procyon, Achernar, and Hadar.

Transcript: Surveying the volume around the Sun for the nearest stars reveals something interesting. If we look in the volume around the Sun, the nearest few lightyears, for the nearest one hundred stars most of them are actually not like the Sun at all. Almost all of them are cooler, less massive, and less luminous. In fact only five are more massive and more luminous than the Sun. This means that the Sun is not typical of the nearest stars. The very nearest star to the Sun is Alpha...

Transcript: There’s a simple formula to give an approximation for the lifetime of a main sequence star. The lifetime is the mass divided by the luminosity of the star, both in solar units, times nine billion years. However the lifetime does not depend exclusively on mass. As shown by theorists Russell and Vogt in the 1920s the final state of a main sequence star depends both on its mass and its chemical composition. For example a one solar mass star of three quarter hydrogen one quarter...

Transcript: Intuitively we might expect a more massive star to last longer than a less massive star because it has more hydrogen to consume in the fusion process, but intuition does not work for stars because of the very steep and sharp relationship between luminosity and mass. If we want to use the analogy of a fuel tank, a large mass star does indeed have a larger fuel tank than a low mass star. However the efficiency or the rate of using that fuel is much, much faster and so the more...

Transcript: The steep relationship between mass and luminosity for main sequence stars has an important consequence for the lifetime of the stars. Consider a star that’s a tenth the mass of the Sun. In round numbers the luminosity is ten to the minus four times the luminosity of the Sun. Thus the size of the fuel reservoir is ten times smaller, but the rate of evolution is ten thousand times smaller. This means the star will last about a thousand times longer than the Sun. Instead of a...

Transcript: The Sun is a typical main sequence star by which astronomers mean that the Suns properties lie in the middle of the range of stellar properties on the main sequence. It’s intermediate in mass, in size, in temperature, and luminosity compared to the most and least massive main sequence stars. The Sun has a spectral type G2 which gives it a photosphere temperature of fifty-seven hundred Kelvin. Spectral types O, B, A, F, G, K, and M are subdivided on a decimal scale running from...

Transcript: Main sequence stars are classified according to the system of spectral types developed almost a hundred years ago. Going from the hottest to the coolest stars there are O type main sequence stars whose mass is about fifty times that of the Sun, radius about twenty times, a temperature of forty thousand degrees, and a luminosity a million times that of the Sun. B stars have masses twenty times that of the Sun and radii seven times that, photospheres are thirty thousand Kelvin and...

Transcript: Main sequence stars have several basic properties. All main sequence stars are converting hydrogen to helium by the fusion process, and that’s responsible for the energy release and the radiation that leaves the stars. Main sequence stars are all stable with their internal structure governed by the principle of hydrostatic equilibrium. The most massive main sequence stars are more luminous, physically larger, and have hotter photospheres than low mass main sequence stars. Main...

Transcript: A star is a ball of gas held together by gravity where the temperature in the interior is sufficient to release energy by fusion reactions. The Sun is a typical star, but what sets the mass range of stars? Is it possible for example to have a star a thousand times less massive than the Sun or a thousand times more massive? The answer is no. Stellar physics dictates a particular range in which stars can exist. It spans a factor of about a thousand. Less than 0.08 times the...

Transcript: Mass is the fundamental quantity that controls stellar evolution. The main sequence on an HR diagram is a correlation between the properties of luminosity and effective temperature. The main sequence runs from high luminosity hot stars down to low luminosity cool stars, but the underlying variable on the main sequence is the mass of the star. High luminosity hot stars are high mass and low luminosity cool stars are low mass. The principles of stellar evolution were first...