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An Introduction to Astronomy Part XI: The Birth and Death of Stars. Lambert E. Murray, Ph.D. Professor of Physics. Interstellar Gas and Dust. In the late 1700’s Henry Herschell discovered “holes in the heavens” where there appeared to be fewer stars than normal.
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An Introduction to AstronomyPart XI: The Birth and Death of Stars Lambert E. Murray, Ph.D. Professor of Physics
Interstellar Gas and Dust • In the late 1700’s Henry Herschell discovered “holes in the heavens” where there appeared to be fewer stars than normal. • In the late 1800’s Edward Barnard’s photographs of these regions lead some to believe that they were clouds of material blocking out the starlight.
Barnard 86 – a Dark Nebula Barnard 86 is a good example of one of Herschell’s “holes in the heavens”.
The Constellation Orion Region of Horsehead Nebula
Evidence of Dark Nebula • The Horsehead nebula is clearly a case where dark material is obscuring the brighter emission nebula behind it. • The close-up actually reveals dim stars that are behind the dark nebula. • This seems to be clear evidence for dark material in interstellar regions which can obscure the light from stars. • Indeed, the reddish emission nebula behind the Horsehead is direct evidence of interstellar gas.
CO Radar Mapping (2.6 mm)of Orion – Monoceros Region • The following image is a radar map of the Orion – Monoceros region of the sky taken at a wavelength of 2.6 mm. • The 2.6 mm wavelength radar image maps CO concentrations. The concentrations of hydrogen are typically four orders of magnitude greater than CO in interstellar space, but this gives a measure of the amount of gas in interstellar space regions. • You can tell that there are large concentrations of gas in the regions of the horsehead and Orion nebulae. These are believed to be rich star formation regions.
M More Dark Nebula
Emission and Reflection Nebula • The term nebula is used to denote a cloud of interstellar gas and dust. • An emission nebula is one that glows because of the emission of specific spectral lines arising from the excitation of atoms within the interstellar gas cloud. • A reflection nebula is one that glows because of scattered light from a star – this scattered light is typically bluish in color (like the blue in our atmosphere). • As white light passed through a dust cloud (much like smoke) blue light is scattered and, if the cloud is not too thick, the spectrum of the light passing through the cloud becomes more reddish (interstellar reddening).
An Example of an Emission and Reflection Nebula Reflection Nebula NGC 6589 NGC 6590 Emission Nebula IC 1283-4
Extinction and Reddening • Due to the interstellar nebula, light from distant stars does not appear as bright as they would – the process is called interstellar extinction. • Similarly, the interstellar nebula cause the light from distance stars to appear more reddish – interstellar reddening. • Measurements of interstellar extinction and reddening in various directions indicates that most of the interstellar nebula are confined to the regions of the Milky Way – the faint band of hazy myriad stars which stretches across the night sky.
Looking Toward the Nucleus of Our Galaxy– Through Dark Nebula
The Formation of Protostars • Astronomers believe that stars are born from the gravitational collapse of large, cold regions of dark nebular material. This collapse may be triggered by the explosion of nearby stars creating compression waves in the nebula. • The picture on the next slide is a region of space where astronomers believe this process may be occurring.
The Orion Nebula Protostar Region?
Pre-Main-Sequence Evolution • As large regions of gas collapse under the influence of gravity, they heat up. • Initially this heat is in the form of infrared radiation. • Eventually the gas heats up enough to radiate in the visible. As the gas cloud shrinks greater internal energy is released, causing the temperature to rise more.
A Stellar Nursery H II region of the Swan in visible wavelengths Same region in infrared wavelengths. Notice the large number of “cool” stars, or protostars, on the right-hand-side. These are visible in the infrared because that wavelength can penetrate the dust and gas.
Pre-Main-Sequence Evolution Tracks of Protostars Notice that this model gives results similar to the mass-luminosity data plot.
Variations in Luminosity with Stellar Mass • The rising temperature coupled with the decreasing size causes protostars of mass greater than about 5 solar masses to maintain a relatively constant luminosity. • Protostars less massive decrease in size more rapidly than the increase in surface temperature, and the luminosity decreases.
A Star is Born • Once the protostar heats up to the point where thermonuclear fusion occurs, the radiation pressure will counteract the gravitational pressure and the star will become stable as a main-sequence star. • Stars arising from larger mass clouds become very luminous stars, while stars arising from less massive clouds become less luminous. • Gas clouds with a mass of less than about 0.08 solar masses can never heat up sufficiently for nuclear fusion to occur, and the failed star becomes a hydrogen-rich brown dwarf (something like Jupiter).
Young Stellar Disks and Jets • From the discussion of our solar system, we postulated the formation of a solar system from the gravitational collapse of a dust and gas cloud, and the development of a disk of material rotating rapidly around the young star. • Similar features have been recently observe with the Hubble Space Telescope. • In the slide that follows, the knobby jets of material appears to be emitted along the polar axes of the star in what is called bi-polar outflow. These small nebula are known as Herbig-Haro objects. • Such young, gas-ejecting stars are known as T Tauri stars, the first example having been discovered in the constellation Taurus.
Stellar Disks in the Orion Nebula • The following images were taken by the HST of the Orion Nebula. • In these pictures you will see evidence of stellar formation and of the presence of disk-like structures surrounding these new stars. • The four massive stars that dominate this region are emitting radiation and gasses which are interacting with the smaller young stars being formed. These stellar winds may prevent the formation of possible solar systems.
Main Sequence Stars • Once a star reaches the main sequence, it will remain on the main sequence for about 90% of its lifetime. For our Sun (a moderately sized star), this means approximately 10 billion years. • Larger, more luminous stars burn up their fuel much more rapidly, and remain on the main sequence for a shorter period of time. • As we will see later, when a star leaves the main sequence, it expands and moves toward the region of giant and supergiant stars.
Star Clusters Provide Evidence for our Model • High-mass stars evolve more rapidly than low-mass stars. • An association of hot, massive stars (an OB association) will emit vast quantities of uv radiation into the nebula from which it was born. • This high-energy uv radiation actually ionizes the hydrogen gas of the nebula. These free protons then combine with free electrons and emit the characteristic red color associated with these emission nebula (so-called H II regions). • We want to examine the HR diagram for the young stars associated with an H II region (NGC 2264).
HR Diagram of the Young Star Cluster NGC 2264 Note that the more massive, luminous stars in the cluster (at the upper left end) have already reached the main sequence, while the less massive protostars (at the lower end) have not yet joined the main sequence.
The Pleiades Star Cluster • In contrast to the young star cluster NGC 2264, an HR diagram of the Pleiades star cluster shows that these stars have already reached the main sequence, and the older stars are actually beginning to leave. • The Pleiades star cluster is a cluster of very bright, blue (hot) stars. • Both star clusters we have examined are known as open clusters or galactic clusters, possessing barely enough mass to hold themselves together in a cluster.
Other Stellar Nurseries • The radiation pressure from very bright stars may create compression waves in a surrounding nebula, as in the Rosette Nebula. • Likewise, when a star dies, it may generate a massive explosion which can send large compression waves out into the interstellar medium, compressing the gasses that are there. • This compression of interstellar gasses may be the breeding places for new stars. • The following images illustrate some of these massive compressional waves generated by radiation pressure and by exploding stars.
The Sun Expands in Old Age • Once a star like our Sun becomes a main sequence star, it remains stable for about 10 billion years. • At the end of that time, the hydrogen fuel in the center of the Sun will become depleted; there is too much helium to efficiently continue the thermonuclear fusion process at the core. • When that happens, the radiation pressure from the center of the Sun will be reduced and the core will collapse toward the center due to gravity. • The region just outsider the core will heat up and begin to “burn” hydrogen and this will cause the Sun to expand. This process continues as the Sun expands out to the size of the Earth’s orbit, creating a red giant.
Helium Core Burning • Once a star becomes a red giant, it will remain a red giant as its outer regions continue to burn the available hydrogen. • During this time, helium ash continues to accumulate in the center of the star as gravity pulls this heavier material to the center, heating up the core. • Eventually, the star’s core will re-ignite when the temperature of the core gets hot enough for helium to “burn”. • The ash of helium burning is both oxygen and carbon. • For low-mass stars (less than 3 solar masses) the onset of helium burning produces an explosive “helium flash”. • After the helium flash the star settles down to burning helium and becomes a smaller, hotter star.
Post-Main-Sequence Evolution Core helium burning begins where the evolutionary tracks make a sharp downward turn in the red giant region of the diagram.
