The Lives of Stars, Chap. 11

Lecture Outline

I. Protostars and Pre-Main Sequence Stars

II. Main Sequence and Giant Stars

III. Variable Stars

 Lecture Notes

Stars lose mass and their chemical makeup evolves. Major stages in the life of a star can last for millions or even billions of years. By observing stars with different temperatures, brightness, and chemical compositions, we have come to understand stellar evolution.

I. Protostars and Pre-Main Sequence Stars

Stars form out of enormous volumes of gas and dust

Supernova explosions in nebulae trigger the birth of stars

When a protostar becomes a pre-main sequence star

The evolutionary track of a pre-main sequence star

Young clusters are found in H II regions

Plotting a star cluster on an H-R diagram

Stars form out of enormous volumes of gas and dust

• Stars condense from clouds of gas and dust lying between existing stars called interstellar medium. Interstellar medium contains about 10% of all the known mass in our Galaxy.
• Astronomers map interstellar gas in order to determine where new stars are forming.
• Astronomers have come to realize that vast amounts of interstellar gas are concentrated in giant molecular clouds.

Supernova explosions in nebulae trigger the birth of stars

• A supernova is a violent detonation that ends the life of a massive star.
• Astronomers find many remains of such dead stars across the sky; they are called supernova remnants.
• During a supernova, a shell of gas passes through the surrounding interstellar medium, which excites the atoms, causing the gas to glow. If the expanding shell of a supernova remnant encounters a giant molecular cloud, it can squeeze the cloud, stimulating star birth.
• A simple collision between two interstellar clouds can also create new stars, since compression must occur at the boundary between the clouds.
• Once a giant molecular cloud compresses and cools enough, gravitational attraction causes small regions of gas and dust in it to collapse until they become stars. (Example is a balloon).
• Inside many interstellar regions are dense clouds. These dense clouds are destined to form stars. Often a giant molecular cloud will have several hundred or even a few thousand dense cores. In this case, hundreds or thousands of stars form together. Such stellar nurseries are called open clusters.
• The process of increasing mass in the central region is called accretion, and the newly forming object at the center is called a protostar. Although fusion has not begun, a protostar glows from the heat generated by the compression of the gas it contains.
• If a dense core is not spinning, it collapses into a sphere, which ultimately becomes an isolated star. If it is spinning, it collapses into a disk, which may then condense into a few stars. Or, if the disk has a low enough mass, it may become a single star but with orbiting protoplanets. (See Figure II-4, Beta Pictoris).

When a protostar becomes a pre-main sequence star

• When mass accretion stops, the protostar is called a pre-main sequence star.
• A pre-main sequence star contracts slowly, unlike the rapid collapse of a protostar.
• When the temperature in the core reaches 10 million degrees kelvin, hydrogen fusion begins. The outpouring of energy from hydrogen fusion creates pressure inside the pre-main sequence star sufficient to finally halt its gravitational contraction.
• In the final stages of pre-main sequence evolution, the outer shell of gas and dust finally dissipates. For the first time the star is directly revealed to the outside universe.

The evolutionary track of a pre-main sequence star

• An evolutionary track represents changes in a star’s temperature and luminosity, not its motion is space.
• Astronomers use high speed computers to develop models of stellar evolution as stars move on the H-R diagram.
• Pre-main sequence stars that do not have enough gravitational force compressing in their cores to initiate fusion contract to become planet-like objects called brown dwarfs. Brown dwarfs are larger than Jupiter. The first confirmed existence of a brown dwarf was made in 1995, when Hubble Space Telescope imaged Gliese 229B (Figure 11-8).

Young clusters are found in H II regions

• The nebulosity around a newborn star cluster often shines with a distinctive reddish hue. This is called an emission nebula. Because these nebulae are predominantly ionized hydrogen, they are also called H II regions.
• An H II region is a small, bright "hot spot" in a giant molecular cloud that consists of a few hot, bright O and B stars near the core of the nebula.
• In H II regions, shock waves pass through hydrogen gas which stimulates a new round of star birth.

Plotting a star cluster on an H-R diagram

• A young open cluster is a rich source of information about stars in their infancy.
• By measuring each star’s magnitude, color index, and distance, an astronomer can deduce its luminosity and surface temperature.
• Gas-ejecting stars in spectral classes G and cooler (G, K, M) are called T Tauri stars.
• Some astronomers suggest that the onset of hydrogen fusion is preceded by vigorous chromospheric activity marked by enormous spicules and flares that propel the star’s outermost layers back into space. In fact, an infant star going though its T Tauri stage can lose as much as 0.4Mo of matter and also shed its shell while still a pre-main sequence star.
• Distinctly bluish color of the nebulosity (around the Pleiades) is called reflection nebula. It is caused by fine grains of interstellar dust that efficiently scatter and reflect blue light. Indeed, reflection nebulosity is bluer for the very same reason Earth’s sky is blue: Particles scatter short-wave length light much more efficiently than longer-wavelength radiation. Blue light is therefore bounced around and scattered aback toward us much more intensely than is light of any other color.
• Open clusters (Pleiades) possess barely enough mass to hold themselves together.

II. Main Sequence and Giant Stars

Stars spend most of their lives on the main sequence

When a main sequence star becomes a giant

Helium fusion begins at the center of a giant

As stars evolve, they move on the H-R diagram

Globular clusters are bound groups of old stars

Stars spend most of their lives on the main sequence

• Once the outward force caused by thermal pressure is balanced by the inward force of gravity (a condition known as hydrostatic equilibrium), a pre-main sequence star ceases to collapse and a star is born.
• Main sequence stars are those stars fusing hydrogen into helium in their cores.
• The zero-age main sequence (ZAMS) is the location on the H-R diagram where a pre-main sequence star fusing hydrogen in its core first becomes a stable object, neither shrinking nor expanding.
• The more massive a star is, the faster it goes through its main sequence phase. Because of the greater mass, gravity presses down with greater force on the star’s core. This tremendous pressure creates such an accelerated rate of fusion the some O and B stars consume all their core in a few million years. Stars of low mass take hundreds of billions of years to convert their cores from hydrogen to helium.

When a main sequence star becomes a giant

• When the hydrogen in a star’s core is completely converted into helium, fusion ceases and the star can no longer support the weight of its outer layers.
• The enormous weight pressing inward from all sides compresses the star’s helium core, which causes it to heat further.
• Because the core is contracting, you might think that the outer envelope of the star also shrinks, but this does not happen. In fact, the star expands to become a giant.
• Photons created by fusion do not have as far to travel to reach the photosphere and so they lose less energy on the way up. Consequently, they heat the outer layers of the star more, causing the star’s outer atmosphere to swell farther and farther into space.
• The cooler, less massive stars, glowing red, are often called red giants. Our Sun will swell to a giant with a diameter of 1 AU. The first four planets will be destroyed.
• Giant stars are so enormous that their bloated outer layers constantly leak gases into space. At times, this mass loss is quite significant.

Helium fusion begins at the center of a giant

• As the hydrogen-fusing shell moves outward in the star, it adds mass to the helium core. The core slowly contracts, forcing the star’s central temperature to climb resulting in core helium fusion. The energy released in core helium fusion re-establishes thermal equilibrium.
• While fusion is again occurring in the core, further gravitational contraction of the star ceases.
• In low mass stars, helium fusion begins explosively and suddenly, in an event called the helium flash.
• The electrons in the core of the low-mass star are so closely crowded together that any further compression would violate the Pauli exclusion principle (two identical particles cannot exist in the same place at the same time).
• Now the helium-rich core of a low-mass giant is supported by electron degeneracy pressure. The pressure of these densely packed atoms now sustain the core and keep it from collapsing further. It is not dependent upon temperature.
• As helium fusion begins, temperatures rise to over 100 million degrees kelvin, and they can only continue to rise. Within a few seconds, the explosively rising temperatures causes the helium to fuse at an ever-increasing rate: the helium flash.
• After the helium flash, the star’s energy output declines, and so its outer layer again contracts. The low mass giant is left smaller, dimmer, and hotter. It has left the main sequence forever.

As stars evolve, they move on the H-R diagram

• After each star reaches the main sequence, its evolutionary track slowly inches away from the ZAMS location as the hydrogen-fusing core grows in search of fresh fuel.

Globular clusters are bound groups of old stars

• Unlike open clusters, globular clusters are gravitationally bound groups of stars that do not disperse. Astronomers know that such clusters are old because they contain no high-mass main sequence stars: If you measure the luminosity and surface temperature of many stars in a globular cluster and plot data on a color-magnitude diagram, you will find that the upper half of the main sequence is missing. (See Figure 11-20).
• All the high-mass main sequence stars have evolved long ago into giants, leaving behind only lower-mass, slowly evolving stars still undergoing core hydrogen fusion.
• An H-R diagram of a cluster can also be used to determine the age of the universe.
• As a cluster gets older, like the Pleiades, stars begin to leave the main sequence.
• Over the years, the main sequence in a cluster gets shorter and shorter. The top of the surviving portion of the main sequence is called the turnoff point.
• Stars at the turnoff point are just beginning to exhaust the hydrogen in their cores, and their main sequence lifetime is equal to the age of the cluster.
• The youngest clusters are open clusters in the disk of our Galaxy, where star formation is an ongoing process. Stars in these young clusters are said to be metal-rich because their spectra contain many prominent spectral line of heavy elements.
• This material originally cam e from dead stars that exploded long-ago, enriching the interstellar gases with the heavy elements formed in their cores.
• The Sun is an example of a young, metal-rich star. Such stars are also called population I stars.
• The oldest clusters are globular clusters. Such clusters are generally located above or below the disk of our Galaxy. These ancient stars are said to be metal-poor. They were created long ago from interstellar gases that had not yet been substantially enriched with heavy elements. They are called population II stars.

III. Variable Stars

Cepheid variables pulsate because they alternately expand and contract

Cepheids enable astronomers to estimate vast distances

Cepheid variables pulsate because they alternately expand and contract

• After core helium fusion begins, a star can become unstable and pulsate. These so-called variable stars can be easily identified by their changes in brightness amid a field of stars of constant luminosity.
• RR Lyrae variables, all have periods shorter than one day.
• High mass stars that vary in brightness are called Cepheid variables.
• A cepheid variable is characterized by the way in which its light output varies: rapid brightening followed by gradual dimming.
• When a Cepheid variable pulsates, star’s gases alternately heat up and cool down.
• Just as a bouncing ball eventually comes to rest, a pulsating star would soon stop pulsating without some sort of mechanism to keep its oscillations going.
• Type I Cepheids are brighter, metal-rich stars.
• Type II Cepheids, are dimmer, metal-poor stars.

Cepheids enable astronomers to estimate vast distances

• Cepheids are very important to astronomers because there is a direct relationship between a Cepheid’s period of pulsation and its average luminosity. This is called the period-luminosity relation.
• Because the changes in brightness of Cepheids can be seen even in distant galaxies where other techniques for measuring distance fail, the period-luminosity relation plays an important role in determining the overall size and structure of the universe.